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| author | Robert J. Chassell <bob@rattlesnake.com> | 2006-10-31 21:49:55 +0000 |
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| committer | Robert J. Chassell <bob@rattlesnake.com> | 2006-10-31 21:49:55 +0000 |
| commit | 6a3f3de849030c21b5d22487c4ddceff19706464 (patch) | |
| tree | 753e02a4fade3c90fdb2098e80f2d645ec0d3540 /info | |
| parent | b096c3a92d0b7c38f6ab44ecb389d533a10c2ee9 (diff) | |
| download | emacs-6a3f3de849030c21b5d22487c4ddceff19706464.tar.gz | |
Removed Info files wrongly committed.
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| -rw-r--r-- | info/eintr-1 | 7277 | ||||
| -rw-r--r-- | info/eintr-2 | 7486 | ||||
| -rw-r--r-- | info/eintr-3 | 2798 |
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diff --git a/info/eintr b/info/eintr deleted file mode 100644 index 37ba238faba..00000000000 --- a/info/eintr +++ /dev/null @@ -1,351 +0,0 @@ -This is ../info/eintr, produced by makeinfo version 4.8 from -emacs-lisp-intro.texi. - -INFO-DIR-SECTION Emacs -START-INFO-DIR-ENTRY -* Emacs Lisp Intro: (eintr). - A simple introduction to Emacs Lisp programming. -END-INFO-DIR-ENTRY - -This is an `Introduction to Programming in Emacs Lisp', for people who -are not programmers. - -Edition 3.01, 2006 Oct 31 - -Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1997, 2001, 2002, -2003, 2004, 2005, 2006 Free Software Foundation, Inc. - -Published by the: - - GNU Press, Website: http://www.gnupress.org - a division of the General: press@gnu.org - Free Software Foundation, Inc. Orders: sales@gnu.org - 51 Franklin Street, Fifth Floor Tel: +1 (617) 542-5942 - Boston, MA 02110-1301 USA Fax: +1 (617) 542-2652 - - -ISBN 1-882114-43-4 - -Permission is granted to copy, distribute and/or modify this document -under the terms of the GNU Free Documentation License, Version 1.2 or -any later version published by the Free Software Foundation; there -being no Invariant Section, with the Front-Cover Texts being "A GNU -Manual", and with the Back-Cover Texts as in (a) below. A copy of the -license is included in the section entitled "GNU Free Documentation -License". - -(a) The FSF's Back-Cover Text is: "You have freedom to copy and modify -this GNU Manual, like GNU software. Copies published by the Free -Software Foundation raise funds for GNU development." - - -Indirect: -eintr-1: 1466 -eintr-2: 299621 -eintr-3: 597715 - -Tag Table: -(Indirect) -Node: Top1466 -Node: Preface20343 -Node: Why21442 -Node: On Reading this Text22083 -Node: Who You Are24247 -Node: Lisp History26946 -Node: Note for Novices27697 -Node: Thank You30156 -Node: List Processing30637 -Ref: List Processing-Footnote-131530 -Node: Lisp Lists31733 -Node: Numbers Lists32443 -Node: Lisp Atoms33470 -Node: Whitespace in Lists37132 -Node: Typing Lists38373 -Node: Run a Program39453 -Node: Making Errors41865 -Node: Names & Definitions46779 -Node: Lisp Interpreter48613 -Node: Complications49666 -Node: Byte Compiling51210 -Node: Evaluation52253 -Node: Evaluating Inner Lists53909 -Node: Variables56370 -Node: fill-column Example57723 -Node: Void Function59202 -Node: Void Variable60309 -Node: Arguments62088 -Ref: Arguments-Footnote-163329 -Node: Data types64141 -Node: Args as Variable or List66105 -Node: Variable Number of Arguments67625 -Node: Wrong Type of Argument68430 -Ref: Wrong Type of Argument-Footnote-171807 -Node: message71874 -Ref: message-Footnote-175667 -Node: set & setq75841 -Node: Using set76423 -Node: Using setq78662 -Node: Counting80779 -Node: Summary82720 -Node: Error Message Exercises84493 -Node: Practicing Evaluation84990 -Node: How to Evaluate85821 -Node: Buffer Names87320 -Node: Getting Buffers92913 -Ref: Getting Buffers-Footnote-195385 -Node: Switching Buffers95603 -Ref: Switching Buffers-Footnote-198858 -Ref: Switching Buffers-Footnote-299130 -Node: Buffer Size & Locations99787 -Node: Evaluation Exercise101772 -Node: Writing Defuns102043 -Node: Primitive Functions102890 -Node: defun104059 -Node: Install110503 -Node: Effect of installation111767 -Node: Change a defun112560 -Node: Interactive114227 -Node: Interactive multiply-by-seven115224 -Node: multiply-by-seven in detail117032 -Node: Interactive Options119649 -Node: Permanent Installation122700 -Node: let124780 -Node: Prevent confusion126047 -Node: Parts of let Expression127911 -Node: Sample let Expression129430 -Ref: Sample let Expression-Footnote-1131160 -Node: Uninitialized let Variables131408 -Node: if132938 -Node: if in more detail133732 -Node: type-of-animal in detail136440 -Node: else138860 -Node: Truth & Falsehood141641 -Node: nil explained142489 -Node: save-excursion144489 -Node: Point and mark145177 -Node: Template for save-excursion148134 -Node: Review149366 -Node: defun Exercises157102 -Node: Buffer Walk Through157517 -Node: Finding More158301 -Node: simplified-beginning-of-buffer162161 -Node: mark-whole-buffer166501 -Node: mark-whole-buffer overview167255 -Node: Body of mark-whole-buffer168614 -Node: append-to-buffer171615 -Node: append-to-buffer overview172158 -Node: append interactive174813 -Node: append-to-buffer body177164 -Node: append save-excursion179204 -Node: Buffer Related Review183970 -Node: Buffer Exercises185899 -Node: More Complex186332 -Node: copy-to-buffer187033 -Node: insert-buffer189608 -Node: insert-buffer code190606 -Node: insert-buffer interactive191605 -Node: Read-only buffer192010 -Node: b for interactive192622 -Node: insert-buffer body193717 -Node: if & or194859 -Node: Insert or197830 -Node: Insert let200029 -Node: New insert-buffer203657 -Node: beginning-of-buffer204808 -Node: Optional Arguments206294 -Node: beginning-of-buffer opt arg209529 -Node: Disentangle beginning-of-buffer210362 -Node: Large buffer case211606 -Node: Small buffer case214037 -Node: beginning-of-buffer complete215650 -Node: Second Buffer Related Review218513 -Node: optional Exercise220137 -Node: Narrowing & Widening220597 -Node: Narrowing advantages221055 -Node: save-restriction222933 -Node: what-line224816 -Node: narrow Exercise228818 -Node: car cdr & cons229777 -Node: Strange Names230437 -Node: car & cdr231436 -Node: cons235619 -Node: Build a list236470 -Ref: Build a list-Footnote-1237958 -Node: length238135 -Node: nthcdr239613 -Node: nth242405 -Node: setcar243724 -Node: setcdr245925 -Node: cons Exercise247381 -Node: Cutting & Storing Text247741 -Node: Storing Text248710 -Node: zap-to-char250722 -Node: Complete zap-to-char251583 -Node: zap-to-char interactive253483 -Node: zap-to-char body254955 -Node: search-forward256539 -Node: progn259479 -Node: Summing up zap-to-char261124 -Node: kill-region262201 -Node: Complete kill-region263060 -Node: condition-case267176 -Node: Lisp macro269650 -Node: copy-region-as-kill271701 -Node: Complete copy-region-as-kill272487 -Node: copy-region-as-kill body275238 -Node: last-command & this-command276205 -Node: kill-append function278454 -Node: kill-new function282423 -Node: Digression into C292644 -Ref: Digression into C-Footnote-1297997 -Node: defvar298169 -Node: See variable current value299621 -Node: defvar and asterisk301483 -Node: cons & search-fwd Review303252 -Node: search Exercises305766 -Node: List Implementation306488 -Node: Lists diagrammed307499 -Node: Symbols as Chest313466 -Node: List Exercise315452 -Node: Yanking315779 -Node: Kill Ring Overview316978 -Node: kill-ring-yank-pointer318625 -Node: yank nthcdr Exercises320974 -Node: Loops & Recursion321673 -Ref: Loops & Recursion-Footnote-1322633 -Node: while323132 -Node: Looping with while324220 -Node: Loop Example325976 -Node: print-elements-of-list329264 -Node: Incrementing Loop331710 -Node: Incrementing Example333229 -Node: Inc Example parts335595 -Node: Inc Example altogether339440 -Node: Decrementing Loop342855 -Node: Decrementing Example344127 -Node: Dec Example parts345603 -Node: Dec Example altogether347554 -Node: dolist dotimes349737 -Node: dolist350459 -Node: dotimes353504 -Node: Recursion355024 -Node: Building Robots355909 -Node: Recursive Definition Parts357616 -Node: Recursion with list359996 -Node: Recursive triangle function363524 -Node: Recursive Example arg of 1 or 2364792 -Node: Recursive Example arg of 3 or 4366996 -Node: Recursion with cond369924 -Node: Recursive Patterns372199 -Node: Every372596 -Node: Accumulate375030 -Node: Keep376263 -Node: No Deferment378116 -Node: No deferment solution380740 -Ref: No deferment solution-Footnote-1384606 -Ref: No deferment solution-Footnote-2384707 -Node: Looping exercise385199 -Node: Regexp Search386374 -Node: sentence-end388060 -Node: re-search-forward391709 -Node: forward-sentence394152 -Node: Complete forward-sentence394799 -Node: fwd-sentence while loops398175 -Node: fwd-sentence re-search401641 -Node: forward-paragraph403711 -Node: forward-paragraph in brief405275 -Node: fwd-para let406716 -Node: fwd-para while411864 -Node: etags420038 -Node: Regexp Review424307 -Node: re-search Exercises426629 -Node: Counting Words427380 -Node: Why Count Words427914 -Node: count-words-region428854 -Node: Design count-words-region429962 -Node: Whitespace Bug436373 -Node: recursive-count-words445647 -Node: Counting Exercise457450 -Node: Words in a defun457809 -Node: Divide and Conquer458940 -Node: Words and Symbols460162 -Node: Syntax462096 -Node: count-words-in-defun465666 -Node: Several defuns471673 -Node: Find a File473989 -Node: lengths-list-file476472 -Node: Several files481616 -Node: lengths-list-many-files482216 -Node: append484903 -Node: Several files recursively485495 -Node: Prepare the data488243 -Node: Sorting489915 -Node: Files List491510 -Node: Counting function definitions498739 -Node: Readying a Graph507767 -Node: Columns of a graph508867 -Node: graph-body-print522150 -Node: recursive-graph-body-print526746 -Node: Printed Axes529384 -Node: Line Graph Exercise530094 -Node: Emacs Initialization530271 -Node: Default Configuration531049 -Ref: Default Configuration-Footnote-1532997 -Node: Site-wide Init533294 -Node: defcustom535228 -Node: Beginning a .emacs File540679 -Node: Text and Auto-fill542993 -Node: Mail Aliases546786 -Node: Indent Tabs Mode547662 -Node: Keybindings548577 -Node: Keymaps551969 -Node: Loading Files554127 -Node: Autoload556951 -Node: Simple Extension559748 -Ref: Simple Extension-Footnote-1562990 -Node: X11 Colors563249 -Ref: X11 Colors-Footnote-1565065 -Node: Miscellaneous565216 -Node: Mode Line569316 -Node: Debugging574689 -Node: debug575254 -Node: debug-on-entry578250 -Node: debug-on-quit581316 -Node: edebug582277 -Node: Debugging Exercises586273 -Node: Conclusion587931 -Node: the-the593082 -Node: Kill Ring595798 -Node: current-kill597715 -Node: Understanding current-kill599878 -Node: Digression concerning error603322 -Node: Determining the Element604490 -Node: yank607913 -Node: yank-pop610822 -Node: ring file613218 -Node: Full Graph613559 -Node: Labelled Example614136 -Node: print-graph Varlist616110 -Node: print-Y-axis617757 -Node: Height of label618930 -Node: Compute a Remainder620889 -Node: Y Axis Element624619 -Node: Y-axis-column628160 -Node: print-Y-axis Penultimate629737 -Node: print-X-axis631580 -Node: Similarities differences632691 -Node: X Axis Tic Marks633491 -Node: Print Whole Graph640329 -Node: The final version641067 -Node: Test print-graph645290 -Node: Graphing words in defuns646868 -Node: lambda649415 -Node: mapcar651051 -Node: Another Bug653005 -Node: Final printed graph658447 -Node: Free Software and Free Manuals659359 -Node: GNU Free Documentation License665605 -Node: Index688028 -Node: About the Author722782 - -End Tag Table diff --git a/info/eintr-1 b/info/eintr-1 deleted file mode 100644 index 1845645c391..00000000000 --- a/info/eintr-1 +++ /dev/null @@ -1,7277 +0,0 @@ -This is ../info/eintr, produced by makeinfo version 4.8 from -emacs-lisp-intro.texi. - -INFO-DIR-SECTION Emacs -START-INFO-DIR-ENTRY -* Emacs Lisp Intro: (eintr). - A simple introduction to Emacs Lisp programming. -END-INFO-DIR-ENTRY - -This is an `Introduction to Programming in Emacs Lisp', for people who -are not programmers. - -Edition 3.01, 2006 Oct 31 - -Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1997, 2001, 2002, -2003, 2004, 2005, 2006 Free Software Foundation, Inc. - -Published by the: - - GNU Press, Website: http://www.gnupress.org - a division of the General: press@gnu.org - Free Software Foundation, Inc. Orders: sales@gnu.org - 51 Franklin Street, Fifth Floor Tel: +1 (617) 542-5942 - Boston, MA 02110-1301 USA Fax: +1 (617) 542-2652 - - -ISBN 1-882114-43-4 - -Permission is granted to copy, distribute and/or modify this document -under the terms of the GNU Free Documentation License, Version 1.2 or -any later version published by the Free Software Foundation; there -being no Invariant Section, with the Front-Cover Texts being "A GNU -Manual", and with the Back-Cover Texts as in (a) below. A copy of the -license is included in the section entitled "GNU Free Documentation -License". - -(a) The FSF's Back-Cover Text is: "You have freedom to copy and modify -this GNU Manual, like GNU software. Copies published by the Free -Software Foundation raise funds for GNU development." - - -File: eintr, Node: Top, Next: Preface, Prev: (dir), Up: (dir) - -An Introduction to Programming in Emacs Lisp -******************************************** - -This is an `Introduction to Programming in Emacs Lisp', for people who -are not programmers. - -Edition 3.01, 2006 Oct 31 - -Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1997, 2001, 2002, -2003, 2004, 2005, 2006 Free Software Foundation, Inc. - -Published by the: - - GNU Press, Website: http://www.gnupress.org - a division of the General: press@gnu.org - Free Software Foundation, Inc. Orders: sales@gnu.org - 51 Franklin Street, Fifth Floor Tel: +1 (617) 542-5942 - Boston, MA 02110-1301 USA Fax: +1 (617) 542-2652 - - -ISBN 1-882114-43-4 - -Permission is granted to copy, distribute and/or modify this document -under the terms of the GNU Free Documentation License, Version 1.2 or -any later version published by the Free Software Foundation; there -being no Invariant Section, with the Front-Cover Texts being "A GNU -Manual", and with the Back-Cover Texts as in (a) below. A copy of the -license is included in the section entitled "GNU Free Documentation -License". - -(a) The FSF's Back-Cover Text is: "You have freedom to copy and modify -this GNU Manual, like GNU software. Copies published by the Free -Software Foundation raise funds for GNU development." - -This master menu first lists each chapter and index; then it lists -every node in every chapter. - -* Menu: - -* Preface:: What to look for. -* List Processing:: What is Lisp? -* Practicing Evaluation:: Running several programs. -* Writing Defuns:: How to write function definitions. -* Buffer Walk Through:: Exploring a few buffer-related functions. -* More Complex:: A few, even more complex functions. -* Narrowing & Widening:: Restricting your and Emacs attention to - a region. -* car cdr & cons:: Fundamental functions in Lisp. -* Cutting & Storing Text:: Removing text and saving it. -* List Implementation:: How lists are implemented in the computer. -* Yanking:: Pasting stored text. -* Loops & Recursion:: How to repeat a process. -* Regexp Search:: Regular expression searches. -* Counting Words:: A review of repetition and regexps. -* Words in a defun:: Counting words in a `defun'. -* Readying a Graph:: A prototype graph printing function. -* Emacs Initialization:: How to write a `.emacs' file. -* Debugging:: How to run the Emacs Lisp debuggers. -* Conclusion:: Now you have the basics. -* the-the:: An appendix: how to find reduplicated words. -* Kill Ring:: An appendix: how the kill ring works. -* Full Graph:: How to create a graph with labelled axes. -* Free Software and Free Manuals:: -* GNU Free Documentation License:: -* Index:: -* About the Author:: - - --- The Detailed Node Listing --- - -Preface - -* Why:: Why learn Emacs Lisp? -* On Reading this Text:: Read, gain familiarity, pick up habits.... -* Who You Are:: For whom this is written. -* Lisp History:: -* Note for Novices:: You can read this as a novice. -* Thank You:: - -List Processing - -* Lisp Lists:: What are lists? -* Run a Program:: Any list in Lisp is a program ready to run. -* Making Errors:: Generating an error message. -* Names & Definitions:: Names of symbols and function definitions. -* Lisp Interpreter:: What the Lisp interpreter does. -* Evaluation:: Running a program. -* Variables:: Returning a value from a variable. -* Arguments:: Passing information to a function. -* set & setq:: Setting the value of a variable. -* Summary:: The major points. -* Error Message Exercises:: - -Lisp Lists - -* Numbers Lists:: List have numbers, other lists, in them. -* Lisp Atoms:: Elemental entities. -* Whitespace in Lists:: Formatting lists to be readable. -* Typing Lists:: How GNU Emacs helps you type lists. - -The Lisp Interpreter - -* Complications:: Variables, Special forms, Lists within. -* Byte Compiling:: Specially processing code for speed. - -Evaluation - -* Evaluating Inner Lists:: Lists within lists... - -Variables - -* fill-column Example:: -* Void Function:: The error message for a symbol - without a function. -* Void Variable:: The error message for a symbol without a value. - -Arguments - -* Data types:: Types of data passed to a function. -* Args as Variable or List:: An argument can be the value - of a variable or list. -* Variable Number of Arguments:: Some functions may take a - variable number of arguments. -* Wrong Type of Argument:: Passing an argument of the wrong type - to a function. -* message:: A useful function for sending messages. - -Setting the Value of a Variable - -* Using set:: Setting values. -* Using setq:: Setting a quoted value. -* Counting:: Using `setq' to count. - -Practicing Evaluation - -* How to Evaluate:: Typing editing commands or C-x C-e - causes evaluation. -* Buffer Names:: Buffers and files are different. -* Getting Buffers:: Getting a buffer itself, not merely its name. -* Switching Buffers:: How to change to another buffer. -* Buffer Size & Locations:: Where point is located and the size of - the buffer. -* Evaluation Exercise:: - -How To Write Function Definitions - -* Primitive Functions:: -* defun:: The `defun' special form. -* Install:: Install a function definition. -* Interactive:: Making a function interactive. -* Interactive Options:: Different options for `interactive'. -* Permanent Installation:: Installing code permanently. -* let:: Creating and initializing local variables. -* if:: What if? -* else:: If--then--else expressions. -* Truth & Falsehood:: What Lisp considers false and true. -* save-excursion:: Keeping track of point, mark, and buffer. -* Review:: -* defun Exercises:: - -Install a Function Definition - -* Effect of installation:: -* Change a defun:: How to change a function definition. - -Make a Function Interactive - -* Interactive multiply-by-seven:: An overview. -* multiply-by-seven in detail:: The interactive version. - -`let' - -* Prevent confusion:: -* Parts of let Expression:: -* Sample let Expression:: -* Uninitialized let Variables:: - -The `if' Special Form - -* if in more detail:: -* type-of-animal in detail:: An example of an `if' expression. - -Truth and Falsehood in Emacs Lisp - -* nil explained:: `nil' has two meanings. - -`save-excursion' - -* Point and mark:: A review of various locations. -* Template for save-excursion:: - -A Few Buffer--Related Functions - -* Finding More:: How to find more information. -* simplified-beginning-of-buffer:: Shows `goto-char', - `point-min', and `push-mark'. -* mark-whole-buffer:: Almost the same as `beginning-of-buffer'. -* append-to-buffer:: Uses `save-excursion' and - `insert-buffer-substring'. -* Buffer Related Review:: Review. -* Buffer Exercises:: - -The Definition of `mark-whole-buffer' - -* mark-whole-buffer overview:: -* Body of mark-whole-buffer:: Only three lines of code. - -The Definition of `append-to-buffer' - -* append-to-buffer overview:: -* append interactive:: A two part interactive expression. -* append-to-buffer body:: Incorporates a `let' expression. -* append save-excursion:: How the `save-excursion' works. - -A Few More Complex Functions - -* copy-to-buffer:: With `set-buffer', `get-buffer-create'. -* insert-buffer:: Read-only, and with `or'. -* beginning-of-buffer:: Shows `goto-char', - `point-min', and `push-mark'. -* Second Buffer Related Review:: -* optional Exercise:: - -The Definition of `insert-buffer' - -* insert-buffer code:: -* insert-buffer interactive:: When you can read, but not write. -* insert-buffer body:: The body has an `or' and a `let'. -* if & or:: Using an `if' instead of an `or'. -* Insert or:: How the `or' expression works. -* Insert let:: Two `save-excursion' expressions. -* New insert-buffer:: - -The Interactive Expression in `insert-buffer' - -* Read-only buffer:: When a buffer cannot be modified. -* b for interactive:: An existing buffer or else its name. - -Complete Definition of `beginning-of-buffer' - -* Optional Arguments:: -* beginning-of-buffer opt arg:: Example with optional argument. -* beginning-of-buffer complete:: - -`beginning-of-buffer' with an Argument - -* Disentangle beginning-of-buffer:: -* Large buffer case:: -* Small buffer case:: - -Narrowing and Widening - -* Narrowing advantages:: The advantages of narrowing -* save-restriction:: The `save-restriction' special form. -* what-line:: The number of the line that point is on. -* narrow Exercise:: - -`car', `cdr', `cons': Fundamental Functions - -* Strange Names:: An historical aside: why the strange names? -* car & cdr:: Functions for extracting part of a list. -* cons:: Constructing a list. -* nthcdr:: Calling `cdr' repeatedly. -* nth:: -* setcar:: Changing the first element of a list. -* setcdr:: Changing the rest of a list. -* cons Exercise:: - -`cons' - -* Build a list:: -* length:: How to find the length of a list. - -Cutting and Storing Text - -* Storing Text:: Text is stored in a list. -* zap-to-char:: Cutting out text up to a character. -* kill-region:: Cutting text out of a region. -* copy-region-as-kill:: A definition for copying text. -* Digression into C:: Minor note on C programming language macros. -* defvar:: How to give a variable an initial value. -* cons & search-fwd Review:: -* search Exercises:: - -`zap-to-char' - -* Complete zap-to-char:: The complete implementation. -* zap-to-char interactive:: A three part interactive expression. -* zap-to-char body:: A short overview. -* search-forward:: How to search for a string. -* progn:: The `progn' special form. -* Summing up zap-to-char:: Using `point' and `search-forward'. - -`kill-region' - -* Complete kill-region:: The function definition. -* condition-case:: Dealing with a problem. -* Lisp macro:: - -`copy-region-as-kill' - -* Complete copy-region-as-kill:: The complete function definition. -* copy-region-as-kill body:: The body of `copy-region-as-kill'. - -The Body of `copy-region-as-kill' - -* last-command & this-command:: -* kill-append function:: -* kill-new function:: - -Initializing a Variable with `defvar' - -* See variable current value:: -* defvar and asterisk:: - -How Lists are Implemented - -* Lists diagrammed:: -* Symbols as Chest:: Exploring a powerful metaphor. -* List Exercise:: - -Yanking Text Back - -* Kill Ring Overview:: -* kill-ring-yank-pointer:: The kill ring is a list. -* yank nthcdr Exercises:: The `kill-ring-yank-pointer' variable. - -Loops and Recursion - -* while:: Causing a stretch of code to repeat. -* dolist dotimes:: -* Recursion:: Causing a function to call itself. -* Looping exercise:: - -`while' - -* Looping with while:: Repeat so long as test returns true. -* Loop Example:: A `while' loop that uses a list. -* print-elements-of-list:: Uses `while', `car', `cdr'. -* Incrementing Loop:: A loop with an incrementing counter. -* Decrementing Loop:: A loop with a decrementing counter. - -A Loop with an Incrementing Counter - -* Incrementing Example:: Counting pebbles in a triangle. -* Inc Example parts:: The parts of the function definition. -* Inc Example altogether:: Putting the function definition together. - -Loop with a Decrementing Counter - -* Decrementing Example:: More pebbles on the beach. -* Dec Example parts:: The parts of the function definition. -* Dec Example altogether:: Putting the function definition together. - -Save your time: `dolist' and `dotimes' - -* dolist:: -* dotimes:: - -Recursion - -* Building Robots:: Same model, different serial number ... -* Recursive Definition Parts:: Walk until you stop ... -* Recursion with list:: Using a list as the test whether to recurse. -* Recursive triangle function:: -* Recursion with cond:: -* Recursive Patterns:: Often used templates. -* No Deferment:: Don't store up work ... -* No deferment solution:: - -Recursion in Place of a Counter - -* Recursive Example arg of 1 or 2:: -* Recursive Example arg of 3 or 4:: - -Recursive Patterns - -* Every:: -* Accumulate:: -* Keep:: - -Regular Expression Searches - -* sentence-end:: The regular expression for `sentence-end'. -* re-search-forward:: Very similar to `search-forward'. -* forward-sentence:: A straightforward example of regexp search. -* forward-paragraph:: A somewhat complex example. -* etags:: How to create your own `TAGS' table. -* Regexp Review:: -* re-search Exercises:: - -`forward-sentence' - -* Complete forward-sentence:: -* fwd-sentence while loops:: Two `while' loops. -* fwd-sentence re-search:: A regular expression search. - -`forward-paragraph': a Goldmine of Functions - -* forward-paragraph in brief:: Key parts of the function definition. -* fwd-para let:: The `let*' expression. -* fwd-para while:: The forward motion `while' loop. - -Counting: Repetition and Regexps - -* Why Count Words:: -* count-words-region:: Use a regexp, but find a problem. -* recursive-count-words:: Start with case of no words in region. -* Counting Exercise:: - -The `count-words-region' Function - -* Design count-words-region:: The definition using a `while' loop. -* Whitespace Bug:: The Whitespace Bug in `count-words-region'. - -Counting Words in a `defun' - -* Divide and Conquer:: -* Words and Symbols:: What to count? -* Syntax:: What constitutes a word or symbol? -* count-words-in-defun:: Very like `count-words'. -* Several defuns:: Counting several defuns in a file. -* Find a File:: Do you want to look at a file? -* lengths-list-file:: A list of the lengths of many definitions. -* Several files:: Counting in definitions in different files. -* Several files recursively:: Recursively counting in different files. -* Prepare the data:: Prepare the data for display in a graph. - -Count Words in `defuns' in Different Files - -* lengths-list-many-files:: Return a list of the lengths of defuns. -* append:: Attach one list to another. - -Prepare the Data for Display in a Graph - -* Sorting:: Sorting lists. -* Files List:: Making a list of files. -* Counting function definitions:: - -Readying a Graph - -* Columns of a graph:: -* graph-body-print:: How to print the body of a graph. -* recursive-graph-body-print:: -* Printed Axes:: -* Line Graph Exercise:: - -Your `.emacs' File - -* Default Configuration:: -* Site-wide Init:: You can write site-wide init files. -* defcustom:: Emacs will write code for you. -* Beginning a .emacs File:: How to write a `.emacs file'. -* Text and Auto-fill:: Automatically wrap lines. -* Mail Aliases:: Use abbreviations for email addresses. -* Indent Tabs Mode:: Don't use tabs with TeX -* Keybindings:: Create some personal keybindings. -* Keymaps:: More about key binding. -* Loading Files:: Load (i.e., evaluate) files automatically. -* Autoload:: Make functions available. -* Simple Extension:: Define a function; bind it to a key. -* X11 Colors:: Colors in X. -* Miscellaneous:: -* Mode Line:: How to customize your mode line. - -Debugging - -* debug:: How to use the built-in debugger. -* debug-on-entry:: Start debugging when you call a function. -* debug-on-quit:: Start debugging when you quit with C-g. -* edebug:: How to use Edebug, a source level debugger. -* Debugging Exercises:: - -Handling the Kill Ring - -* current-kill:: -* yank:: Paste a copy of a clipped element. -* yank-pop:: Insert element pointed to. -* ring file:: - -The `current-kill' Function - -* Understanding current-kill:: - -`current-kill' in Outline - -* Digression concerning error:: How to mislead humans, but not computers. -* Determining the Element:: - -A Graph with Labelled Axes - -* Labelled Example:: -* print-graph Varlist:: `let' expression in `print-graph'. -* print-Y-axis:: Print a label for the vertical axis. -* print-X-axis:: Print a horizontal label. -* Print Whole Graph:: The function to print a complete graph. - -The `print-Y-axis' Function - -* Height of label:: What height for the Y axis? -* Compute a Remainder:: How to compute the remainder of a division. -* Y Axis Element:: Construct a line for the Y axis. -* Y-axis-column:: Generate a list of Y axis labels. -* print-Y-axis Penultimate:: A not quite final version. - -The `print-X-axis' Function - -* Similarities differences:: Much like `print-Y-axis', but not exactly. -* X Axis Tic Marks:: Create tic marks for the horizontal axis. - -Printing the Whole Graph - -* The final version:: A few changes. -* Test print-graph:: Run a short test. -* Graphing words in defuns:: Executing the final code. -* lambda:: How to write an anonymous function. -* mapcar:: Apply a function to elements of a list. -* Another Bug:: Yet another bug ... most insidious. -* Final printed graph:: The graph itself! - - -File: eintr, Node: Preface, Next: List Processing, Prev: Top, Up: Top - -Preface -******* - -Most of the GNU Emacs integrated environment is written in the -programming language called Emacs Lisp. The code written in this -programming language is the software--the sets of instructions--that -tell the computer what to do when you give it commands. Emacs is -designed so that you can write new code in Emacs Lisp and easily -install it as an extension to the editor. - -(GNU Emacs is sometimes called an "extensible editor", but it does much -more than provide editing capabilities. It is better to refer to Emacs -as an "extensible computing environment". However, that phrase is -quite a mouthful. It is easier to refer to Emacs simply as an editor. -Moreover, everything you do in Emacs--find the Mayan date and phases of -the moon, simplify polynomials, debug code, manage files, read letters, -write books--all these activities are kinds of editing in the most -general sense of the word.) - -* Menu: - -* Why:: -* On Reading this Text:: -* Who You Are:: -* Lisp History:: -* Note for Novices:: -* Thank You:: - - -File: eintr, Node: Why, Next: On Reading this Text, Prev: Preface, Up: Preface - -Why Study Emacs Lisp? -===================== - -Although Emacs Lisp is usually thought of in association only with -Emacs, it is a full computer programming language. You can use Emacs -Lisp as you would any other programming language. - -Perhaps you want to understand programming; perhaps you want to extend -Emacs; or perhaps you want to become a programmer. This introduction to -Emacs Lisp is designed to get you started: to guide you in learning the -fundamentals of programming, and more importantly, to show you how you -can teach yourself to go further. - - -File: eintr, Node: On Reading this Text, Next: Who You Are, Prev: Why, Up: Preface - -On Reading this Text -==================== - -All through this document, you will see little sample programs you can -run inside of Emacs. If you read this document in Info inside of GNU -Emacs, you can run the programs as they appear. (This is easy to do and -is explained when the examples are presented.) Alternatively, you can -read this introduction as a printed book while sitting beside a computer -running Emacs. (This is what I like to do; I like printed books.) If -you don't have a running Emacs beside you, you can still read this book, -but in this case, it is best to treat it as a novel or as a travel guide -to a country not yet visited: interesting, but not the same as being -there. - -Much of this introduction is dedicated to walk-throughs or guided tours -of code used in GNU Emacs. These tours are designed for two purposes: -first, to give you familiarity with real, working code (code you use -every day); and, second, to give you familiarity with the way Emacs -works. It is interesting to see how a working environment is -implemented. Also, I hope that you will pick up the habit of browsing -through source code. You can learn from it and mine it for ideas. -Having GNU Emacs is like having a dragon's cave of treasures. - -In addition to learning about Emacs as an editor and Emacs Lisp as a -programming language, the examples and guided tours will give you an -opportunity to get acquainted with Emacs as a Lisp programming -environment. GNU Emacs supports programming and provides tools that -you will want to become comfortable using, such as `M-.' (the key which -invokes the `find-tag' command). You will also learn about buffers and -other objects that are part of the environment. Learning about these -features of Emacs is like learning new routes around your home town. - -Finally, I hope to convey some of the skills for using Emacs to learn -aspects of programming that you don't know. You can often use Emacs to -help you understand what puzzles you or to find out how to do something -new. This self-reliance is not only a pleasure, but an advantage. - - -File: eintr, Node: Who You Are, Next: Lisp History, Prev: On Reading this Text, Up: Preface - -For Whom This is Written -======================== - -This text is written as an elementary introduction for people who are -not programmers. If you are a programmer, you may not be satisfied with -this primer. The reason is that you may have become expert at reading -reference manuals and be put off by the way this text is organized. - -An expert programmer who reviewed this text said to me: - - I prefer to learn from reference manuals. I "dive into" each - paragraph, and "come up for air" between paragraphs. - - When I get to the end of a paragraph, I assume that that subject is - done, finished, that I know everything I need (with the possible - exception of the case when the next paragraph starts talking about - it in more detail). I expect that a well written reference manual - will not have a lot of redundancy, and that it will have excellent - pointers to the (one) place where the information I want is. - -This introduction is not written for this person! - -Firstly, I try to say everything at least three times: first, to -introduce it; second, to show it in context; and third, to show it in a -different context, or to review it. - -Secondly, I hardly ever put all the information about a subject in one -place, much less in one paragraph. To my way of thinking, that imposes -too heavy a burden on the reader. Instead I try to explain only what -you need to know at the time. (Sometimes I include a little extra -information so you won't be surprised later when the additional -information is formally introduced.) - -When you read this text, you are not expected to learn everything the -first time. Frequently, you need only make, as it were, a `nodding -acquaintance' with some of the items mentioned. My hope is that I have -structured the text and given you enough hints that you will be alert to -what is important, and concentrate on it. - -You will need to "dive into" some paragraphs; there is no other way to -read them. But I have tried to keep down the number of such -paragraphs. This book is intended as an approachable hill, rather than -as a daunting mountain. - -This introduction to `Programming in Emacs Lisp' has a companion -document, *Note The GNU Emacs Lisp Reference Manual: (elisp)Top. The -reference manual has more detail than this introduction. In the -reference manual, all the information about one topic is concentrated -in one place. You should turn to it if you are like the programmer -quoted above. And, of course, after you have read this `Introduction', -you will find the `Reference Manual' useful when you are writing your -own programs. - - -File: eintr, Node: Lisp History, Next: Note for Novices, Prev: Who You Are, Up: Preface - -Lisp History -============ - -Lisp was first developed in the late 1950s at the Massachusetts -Institute of Technology for research in artificial intelligence. The -great power of the Lisp language makes it superior for other purposes as -well, such as writing editor commands and integrated environments. - -GNU Emacs Lisp is largely inspired by Maclisp, which was written at MIT -in the 1960s. It is somewhat inspired by Common Lisp, which became a -standard in the 1980s. However, Emacs Lisp is much simpler than Common -Lisp. (The standard Emacs distribution contains an optional extensions -file, `cl.el', that adds many Common Lisp features to Emacs Lisp.) - - -File: eintr, Node: Note for Novices, Next: Thank You, Prev: Lisp History, Up: Preface - -A Note for Novices -================== - -If you don't know GNU Emacs, you can still read this document -profitably. However, I recommend you learn Emacs, if only to learn to -move around your computer screen. You can teach yourself how to use -Emacs with the on-line tutorial. To use it, type `C-h t'. (This means -you press and release the <CTRL> key and the `h' at the same time, and -then press and release `t'.) - -Also, I often refer to one of Emacs' standard commands by listing the -keys which you press to invoke the command and then giving the name of -the command in parentheses, like this: `M-C-\' (`indent-region'). What -this means is that the `indent-region' command is customarily invoked -by typing `M-C-\'. (You can, if you wish, change the keys that are -typed to invoke the command; this is called "rebinding". *Note -Keymaps: Keymaps.) The abbreviation `M-C-\' means that you type your -<META> key, <CTRL> key and <\> key all at the same time. (On many -modern keyboards the <META> key is labelled <ALT>.) Sometimes a -combination like this is called a keychord, since it is similar to the -way you play a chord on a piano. If your keyboard does not have a -<META> key, the <ESC> key prefix is used in place of it. In this case, -`M-C-\' means that you press and release your <ESC> key and then type -the <CTRL> key and the <\> key at the same time. But usually `M-C-\' -means press the <CTRL> key along with the key that is labelled <ALT> -and, at the same time, press the <\> key. - -In addition to typing a lone keychord, you can prefix what you type -with `C-u', which is called the `universal argument'. The `C-u' -keychord passes an argument to the subsequent command. Thus, to indent -a region of plain text by 6 spaces, mark the region, and then type -`C-u 6 M-C-\'. (If you do not specify a number, Emacs either passes -the number 4 to the command or otherwise runs the command differently -than it would otherwise.) *Note Numeric Arguments: (emacs)Arguments. - -If you are reading this in Info using GNU Emacs, you can read through -this whole document just by pressing the space bar, <SPC>. (To learn -about Info, type `C-h i' and then select Info.) - -A note on terminology: when I use the word Lisp alone, I often am -referring to the various dialects of Lisp in general, but when I speak -of Emacs Lisp, I am referring to GNU Emacs Lisp in particular. - - -File: eintr, Node: Thank You, Prev: Note for Novices, Up: Preface - -Thank You -========= - -My thanks to all who helped me with this book. My especial thanks to -Jim Blandy, Noah Friedman, Jim Kingdon, Roland McGrath, Frank Ritter, -Randy Smith, Richard M. Stallman, and Melissa Weisshaus. My thanks -also go to both Philip Johnson and David Stampe for their patient -encouragement. My mistakes are my own. - - Robert J. Chassell - - -File: eintr, Node: List Processing, Next: Practicing Evaluation, Prev: Preface, Up: Top - -1 List Processing -***************** - -To the untutored eye, Lisp is a strange programming language. In Lisp -code there are parentheses everywhere. Some people even claim that the -name stands for `Lots of Isolated Silly Parentheses'. But the claim is -unwarranted. Lisp stands for LISt Processing, and the programming -language handles _lists_ (and lists of lists) by putting them between -parentheses. The parentheses mark the boundaries of the list. -Sometimes a list is preceded by a single apostrophe or quotation mark, -`''(1) Lists are the basis of Lisp. - -* Menu: - -* Lisp Lists:: -* Run a Program:: -* Making Errors:: -* Names & Definitions:: -* Lisp Interpreter:: -* Evaluation:: -* Variables:: -* Arguments:: -* set & setq:: -* Summary:: -* Error Message Exercises:: - ----------- Footnotes ---------- - -(1) The single apostrophe or quotation mark is an abbreviation for the -function `quote'; you need not think about functions now; functions are -defined in *Note Generate an Error Message: Making Errors. - - -File: eintr, Node: Lisp Lists, Next: Run a Program, Prev: List Processing, Up: List Processing - -1.1 Lisp Lists -============== - -In Lisp, a list looks like this: `'(rose violet daisy buttercup)'. -This list is preceded by a single apostrophe. It could just as well be -written as follows, which looks more like the kind of list you are -likely to be familiar with: - - '(rose - violet - daisy - buttercup) - -The elements of this list are the names of the four different flowers, -separated from each other by whitespace and surrounded by parentheses, -like flowers in a field with a stone wall around them. - -* Menu: - -* Numbers Lists:: -* Lisp Atoms:: -* Whitespace in Lists:: -* Typing Lists:: - - -File: eintr, Node: Numbers Lists, Next: Lisp Atoms, Prev: Lisp Lists, Up: Lisp Lists - -Numbers, Lists inside of Lists ------------------------------- - -Lists can also have numbers in them, as in this list: `(+ 2 2)'. This -list has a plus-sign, `+', followed by two `2's, each separated by -whitespace. - -In Lisp, both data and programs are represented the same way; that is, -they are both lists of words, numbers, or other lists, separated by -whitespace and surrounded by parentheses. (Since a program looks like -data, one program may easily serve as data for another; this is a very -powerful feature of Lisp.) (Incidentally, these two parenthetical -remarks are _not_ Lisp lists, because they contain `;' and `.' as -punctuation marks.) - -Here is another list, this time with a list inside of it: - - '(this list has (a list inside of it)) - -The components of this list are the words `this', `list', `has', and -the list `(a list inside of it)'. The interior list is made up of the -words `a', `list', `inside', `of', `it'. - - -File: eintr, Node: Lisp Atoms, Next: Whitespace in Lists, Prev: Numbers Lists, Up: Lisp Lists - -1.1.1 Lisp Atoms ----------------- - -In Lisp, what we have been calling words are called "atoms". This term -comes from the historical meaning of the word atom, which means -`indivisible'. As far as Lisp is concerned, the words we have been -using in the lists cannot be divided into any smaller parts and still -mean the same thing as part of a program; likewise with numbers and -single character symbols like `+'. On the other hand, unlike an -ancient atom, a list can be split into parts. (*Note `car' `cdr' & -`cons' Fundamental Functions: car cdr & cons.) - -In a list, atoms are separated from each other by whitespace. They can -be right next to a parenthesis. - -Technically speaking, a list in Lisp consists of parentheses surrounding -atoms separated by whitespace or surrounding other lists or surrounding -both atoms and other lists. A list can have just one atom in it or -have nothing in it at all. A list with nothing in it looks like this: -`()', and is called the "empty list". Unlike anything else, an empty -list is considered both an atom and a list at the same time. - -The printed representation of both atoms and lists are called "symbolic -expressions" or, more concisely, "s-expressions". The word -"expression" by itself can refer to either the printed representation, -or to the atom or list as it is held internally in the computer. -Often, people use the term "expression" indiscriminately. (Also, in -many texts, the word "form" is used as a synonym for expression.) - -Incidentally, the atoms that make up our universe were named such when -they were thought to be indivisible; but it has been found that physical -atoms are not indivisible. Parts can split off an atom or it can -fission into two parts of roughly equal size. Physical atoms were named -prematurely, before their truer nature was found. In Lisp, certain -kinds of atom, such as an array, can be separated into parts; but the -mechanism for doing this is different from the mechanism for splitting a -list. As far as list operations are concerned, the atoms of a list are -unsplittable. - -As in English, the meanings of the component letters of a Lisp atom are -different from the meaning the letters make as a word. For example, -the word for the South American sloth, the `ai', is completely -different from the two words, `a', and `i'. - -There are many kinds of atom in nature but only a few in Lisp: for -example, "numbers", such as 37, 511, or 1729, and "symbols", such as -`+', `foo', or `forward-line'. The words we have listed in the -examples above are all symbols. In everyday Lisp conversation, the -word "atom" is not often used, because programmers usually try to be -more specific about what kind of atom they are dealing with. Lisp -programming is mostly about symbols (and sometimes numbers) within -lists. (Incidentally, the preceding three word parenthetical remark is -a proper list in Lisp, since it consists of atoms, which in this case -are symbols, separated by whitespace and enclosed by parentheses, -without any non-Lisp punctuation.) - -In addition, text between double quotation marks--even sentences or -paragraphs--is an atom. Here is an example: - - '(this list includes "text between quotation marks.") - -In Lisp, all of the quoted text including the punctuation mark and the -blank spaces is a single atom. This kind of atom is called a "string" -(for `string of characters') and is the sort of thing that is used for -messages that a computer can print for a human to read. Strings are a -different kind of atom than numbers or symbols and are used differently. - - -File: eintr, Node: Whitespace in Lists, Next: Typing Lists, Prev: Lisp Atoms, Up: Lisp Lists - -1.1.2 Whitespace in Lists -------------------------- - -The amount of whitespace in a list does not matter. From the point of -view of the Lisp language, - - '(this list - looks like this) - -is exactly the same as this: - - '(this list looks like this) - -Both examples show what to Lisp is the same list, the list made up of -the symbols `this', `list', `looks', `like', and `this' in that order. - -Extra whitespace and newlines are designed to make a list more readable -by humans. When Lisp reads the expression, it gets rid of all the extra -whitespace (but it needs to have at least one space between atoms in -order to tell them apart.) - -Odd as it seems, the examples we have seen cover almost all of what Lisp -lists look like! Every other list in Lisp looks more or less like one -of these examples, except that the list may be longer and more complex. -In brief, a list is between parentheses, a string is between quotation -marks, a symbol looks like a word, and a number looks like a number. -(For certain situations, square brackets, dots and a few other special -characters may be used; however, we will go quite far without them.) - - -File: eintr, Node: Typing Lists, Prev: Whitespace in Lists, Up: Lisp Lists - -1.1.3 GNU Emacs Helps You Type Lists ------------------------------------- - -When you type a Lisp expression in GNU Emacs using either Lisp -Interaction mode or Emacs Lisp mode, you have available to you several -commands to format the Lisp expression so it is easy to read. For -example, pressing the <TAB> key automatically indents the line the -cursor is on by the right amount. A command to properly indent the -code in a region is customarily bound to `M-C-\'. Indentation is -designed so that you can see which elements of a list belong to which -list--elements of a sub-list are indented more than the elements of the -enclosing list. - -In addition, when you type a closing parenthesis, Emacs momentarily -jumps the cursor back to the matching opening parenthesis, so you can -see which one it is. This is very useful, since every list you type in -Lisp must have its closing parenthesis match its opening parenthesis. -(*Note Major Modes: (emacs)Major Modes, for more information about -Emacs' modes.) - - -File: eintr, Node: Run a Program, Next: Making Errors, Prev: Lisp Lists, Up: List Processing - -1.2 Run a Program -================= - -A list in Lisp--any list--is a program ready to run. If you run it -(for which the Lisp jargon is "evaluate"), the computer will do one of -three things: do nothing except return to you the list itself; send you -an error message; or, treat the first symbol in the list as a command -to do something. (Usually, of course, it is the last of these three -things that you really want!) - -The single apostrophe, `'', that I put in front of some of the example -lists in preceding sections is called a "quote"; when it precedes a -list, it tells Lisp to do nothing with the list, other than take it as -it is written. But if there is no quote preceding a list, the first -item of the list is special: it is a command for the computer to obey. -(In Lisp, these commands are called _functions_.) The list `(+ 2 2)' -shown above did not have a quote in front of it, so Lisp understands -that the `+' is an instruction to do something with the rest of the -list: add the numbers that follow. - -If you are reading this inside of GNU Emacs in Info, here is how you can -evaluate such a list: place your cursor immediately after the right -hand parenthesis of the following list and then type `C-x C-e': - - (+ 2 2) - -You will see the number `4' appear in the echo area. (In the jargon, -what you have just done is "evaluate the list." The echo area is the -line at the bottom of the screen that displays or "echoes" text.) Now -try the same thing with a quoted list: place the cursor right after -the following list and type `C-x C-e': - - '(this is a quoted list) - -You will see `(this is a quoted list)' appear in the echo area. - -In both cases, what you are doing is giving a command to the program -inside of GNU Emacs called the "Lisp interpreter"--giving the -interpreter a command to evaluate the expression. The name of the Lisp -interpreter comes from the word for the task done by a human who comes -up with the meaning of an expression--who "interprets" it. - -You can also evaluate an atom that is not part of a list--one that is -not surrounded by parentheses; again, the Lisp interpreter translates -from the humanly readable expression to the language of the computer. -But before discussing this (*note Variables::), we will discuss what the -Lisp interpreter does when you make an error. - - -File: eintr, Node: Making Errors, Next: Names & Definitions, Prev: Run a Program, Up: List Processing - -1.3 Generate an Error Message -============================= - -Partly so you won't worry if you do it accidentally, we will now give a -command to the Lisp interpreter that generates an error message. This -is a harmless activity; and indeed, we will often try to generate error -messages intentionally. Once you understand the jargon, error messages -can be informative. Instead of being called "error" messages, they -should be called "help" messages. They are like signposts to a -traveller in a strange country; deciphering them can be hard, but once -understood, they can point the way. - -The error message is generated by a built-in GNU Emacs debugger. We -will `enter the debugger'. You get out of the debugger by typing `q'. - -What we will do is evaluate a list that is not quoted and does not have -a meaningful command as its first element. Here is a list almost -exactly the same as the one we just used, but without the single-quote -in front of it. Position the cursor right after it and type `C-x C-e': - - (this is an unquoted list) - -What you see depends on which version of Emacs you are running. GNU -Emacs version 22 provides more information than version 20 and before. -First, the more recent result of generating an error; then the earlier, -version 20 result. - -In GNU Emacs version 22, a `*Backtrace*' window will open up and you -will see the following in it: - - ---------- Buffer: *Backtrace* ---------- - Debugger entered--Lisp error: (void-function this) - (this is an unquoted list) - eval((this is an unquoted list)) - eval-last-sexp-1(nil) - eval-last-sexp(nil) - call-interactively(eval-last-sexp) - ---------- Buffer: *Backtrace* ---------- - -Your cursor will be in this window (you may have to wait a few seconds -before it becomes visible). To quit the debugger and make the debugger -window go away, type: - - q - -Please type `q' right now, so you become confident that you can get out -of the debugger. Then, type `C-x C-e' again to re-enter it. - -Based on what we already know, we can almost read this error message. - -You read the `*Backtrace*' buffer from the bottom up; it tells you what -Emacs did. When you typed `C-x C-e', you made an interactive call to -the command `eval-last-sexp'. `eval' is an abbreviation for `evaluate' -and `sexp' is an abbreviation for `symbolic expression'. The command -means `evaluate last symbolic expression', which is the expression just -before your cursor. - -Each line above tells you what the Lisp interpreter evaluated next. -The most recent action is at the top. The buffer is called the -`*Backtrace*' buffer because it enables you to track Emacs backwards. - -At the top of the `*Backtrace*' buffer, you see the line: - - Debugger entered--Lisp error: (void-function this) - -The Lisp interpreter tried to evaluate the first atom of the list, the -word `this'. It is this action that generated the error message -`void-function this'. - -The message contains the words `void-function' and `this'. - -The word `function' was mentioned once before. It is a very important -word. For our purposes, we can define it by saying that a "function" -is a set of instructions to the computer that tell the computer to do -something. - -Now we can begin to understand the error message: `void-function this'. -The function (that is, the word `this') does not have a definition of -any set of instructions for the computer to carry out. - -The slightly odd word, `void-function', is designed to cover the way -Emacs Lisp is implemented, which is that when a symbol does not have a -function definition attached to it, the place that should contain the -instructions is `void'. - -On the other hand, since we were able to add 2 plus 2 successfully, by -evaluating `(+ 2 2)', we can infer that the symbol `+' must have a set -of instructions for the computer to obey and those instructions must be -to add the numbers that follow the `+'. - -In GNU Emacs version 20, and in earlier versions, you will see only one -line of error message; it will appear in the echo area and look like -this: - - Symbol's function definition is void: this - -(Also, your terminal may beep at you--some do, some don't; and others -blink. This is just a device to get your attention.) The message goes -away as soon as you type another key, even just to move the cursor. - -We know the meaning of the word `Symbol'. It refers to the first atom -of the list, the word `this'. The word `function' refers to the -instructions that tell the computer what to do. (Technically, the -symbol tells the computer where to find the instructions, but this is a -complication we can ignore for the moment.) - -The error message can be understood: `Symbol's function definition is -void: this'. The symbol (that is, the word `this') lacks instructions -for the computer to carry out. - - -File: eintr, Node: Names & Definitions, Next: Lisp Interpreter, Prev: Making Errors, Up: List Processing - -1.4 Symbol Names and Function Definitions -========================================= - -We can articulate another characteristic of Lisp based on what we have -discussed so far--an important characteristic: a symbol, like `+', is -not itself the set of instructions for the computer to carry out. -Instead, the symbol is used, perhaps temporarily, as a way of locating -the definition or set of instructions. What we see is the name through -which the instructions can be found. Names of people work the same -way. I can be referred to as `Bob'; however, I am not the letters `B', -`o', `b' but am, or was, the consciousness consistently associated with -a particular life-form. The name is not me, but it can be used to -refer to me. - -In Lisp, one set of instructions can be attached to several names. For -example, the computer instructions for adding numbers can be linked to -the symbol `plus' as well as to the symbol `+' (and are in some -dialects of Lisp). Among humans, I can be referred to as `Robert' as -well as `Bob' and by other words as well. - -On the other hand, a symbol can have only one function definition -attached to it at a time. Otherwise, the computer would be confused as -to which definition to use. If this were the case among people, only -one person in the world could be named `Bob'. However, the function -definition to which the name refers can be changed readily. (*Note -Install a Function Definition: Install.) - -Since Emacs Lisp is large, it is customary to name symbols in a way -that identifies the part of Emacs to which the function belongs. Thus, -all the names for functions that deal with Texinfo start with -`texinfo-' and those for functions that deal with reading mail start -with `rmail-'. - - -File: eintr, Node: Lisp Interpreter, Next: Evaluation, Prev: Names & Definitions, Up: List Processing - -1.5 The Lisp Interpreter -======================== - -Based on what we have seen, we can now start to figure out what the -Lisp interpreter does when we command it to evaluate a list. First, it -looks to see whether there is a quote before the list; if there is, the -interpreter just gives us the list. On the other hand, if there is no -quote, the interpreter looks at the first element in the list and sees -whether it has a function definition. If it does, the interpreter -carries out the instructions in the function definition. Otherwise, -the interpreter prints an error message. - -This is how Lisp works. Simple. There are added complications which we -will get to in a minute, but these are the fundamentals. Of course, to -write Lisp programs, you need to know how to write function definitions -and attach them to names, and how to do this without confusing either -yourself or the computer. - -* Menu: - -* Complications:: -* Byte Compiling:: - - -File: eintr, Node: Complications, Next: Byte Compiling, Prev: Lisp Interpreter, Up: Lisp Interpreter - -Complications -------------- - -Now, for the first complication. In addition to lists, the Lisp -interpreter can evaluate a symbol that is not quoted and does not have -parentheses around it. The Lisp interpreter will attempt to determine -the symbol's value as a "variable". This situation is described in the -section on variables. (*Note Variables::.) - -The second complication occurs because some functions are unusual and do -not work in the usual manner. Those that don't are called "special -forms". They are used for special jobs, like defining a function, and -there are not many of them. In the next few chapters, you will be -introduced to several of the more important special forms. - -The third and final complication is this: if the function that the Lisp -interpreter is looking at is not a special form, and if it is part of a -list, the Lisp interpreter looks to see whether the list has a list -inside of it. If there is an inner list, the Lisp interpreter first -figures out what it should do with the inside list, and then it works on -the outside list. If there is yet another list embedded inside the -inner list, it works on that one first, and so on. It always works on -the innermost list first. The interpreter works on the innermost list -first, to evaluate the result of that list. The result may be used by -the enclosing expression. - -Otherwise, the interpreter works left to right, from one expression to -the next. - - -File: eintr, Node: Byte Compiling, Prev: Complications, Up: Lisp Interpreter - -1.5.1 Byte Compiling --------------------- - -One other aspect of interpreting: the Lisp interpreter is able to -interpret two kinds of entity: humanly readable code, on which we will -focus exclusively, and specially processed code, called "byte compiled" -code, which is not humanly readable. Byte compiled code runs faster -than humanly readable code. - -You can transform humanly readable code into byte compiled code by -running one of the compile commands such as `byte-compile-file'. Byte -compiled code is usually stored in a file that ends with a `.elc' -extension rather than a `.el' extension. You will see both kinds of -file in the `emacs/lisp' directory; the files to read are those with -`.el' extensions. - -As a practical matter, for most things you might do to customize or -extend Emacs, you do not need to byte compile; and I will not discuss -the topic here. *Note Byte Compilation: (elisp)Byte Compilation, for a -full description of byte compilation. - - -File: eintr, Node: Evaluation, Next: Variables, Prev: Lisp Interpreter, Up: List Processing - -1.6 Evaluation -============== - -When the Lisp interpreter works on an expression, the term for the -activity is called "evaluation". We say that the interpreter -`evaluates the expression'. I've used this term several times before. -The word comes from its use in everyday language, `to ascertain the -value or amount of; to appraise', according to `Webster's New -Collegiate Dictionary'. - -After evaluating an expression, the Lisp interpreter will most likely -"return" the value that the computer produces by carrying out the -instructions it found in the function definition, or perhaps it will -give up on that function and produce an error message. (The interpreter -may also find itself tossed, so to speak, to a different function or it -may attempt to repeat continually what it is doing for ever and ever in -what is called an `infinite loop'. These actions are less common; and -we can ignore them.) Most frequently, the interpreter returns a value. - -At the same time the interpreter returns a value, it may do something -else as well, such as move a cursor or copy a file; this other kind of -action is called a "side effect". Actions that we humans think are -important, such as printing results, are often "side effects" to the -Lisp interpreter. The jargon can sound peculiar, but it turns out that -it is fairly easy to learn to use side effects. - -In summary, evaluating a symbolic expression most commonly causes the -Lisp interpreter to return a value and perhaps carry out a side effect; -or else produce an error. - -* Menu: - -* Evaluating Inner Lists:: - - -File: eintr, Node: Evaluating Inner Lists, Prev: Evaluation, Up: Evaluation - -1.6.1 Evaluating Inner Lists ----------------------------- - -If evaluation applies to a list that is inside another list, the outer -list may use the value returned by the first evaluation as information -when the outer list is evaluated. This explains why inner expressions -are evaluated first: the values they return are used by the outer -expressions. - -We can investigate this process by evaluating another addition example. -Place your cursor after the following expression and type `C-x C-e': - - (+ 2 (+ 3 3)) - -The number 8 will appear in the echo area. - -What happens is that the Lisp interpreter first evaluates the inner -expression, `(+ 3 3)', for which the value 6 is returned; then it -evaluates the outer expression as if it were written `(+ 2 6)', which -returns the value 8. Since there are no more enclosing expressions to -evaluate, the interpreter prints that value in the echo area. - -Now it is easy to understand the name of the command invoked by the -keystrokes `C-x C-e': the name is `eval-last-sexp'. The letters `sexp' -are an abbreviation for `symbolic expression', and `eval' is an -abbreviation for `evaluate'. The command means `evaluate last symbolic -expression'. - -As an experiment, you can try evaluating the expression by putting the -cursor at the beginning of the next line immediately following the -expression, or inside the expression. - -Here is another copy of the expression: - - (+ 2 (+ 3 3)) - -If you place the cursor at the beginning of the blank line that -immediately follows the expression and type `C-x C-e', you will still -get the value 8 printed in the echo area. Now try putting the cursor -inside the expression. If you put it right after the next to last -parenthesis (so it appears to sit on top of the last parenthesis), you -will get a 6 printed in the echo area! This is because the command -evaluates the expression `(+ 3 3)'. - -Now put the cursor immediately after a number. Type `C-x C-e' and you -will get the number itself. In Lisp, if you evaluate a number, you get -the number itself--this is how numbers differ from symbols. If you -evaluate a list starting with a symbol like `+', you will get a value -returned that is the result of the computer carrying out the -instructions in the function definition attached to that name. If a -symbol by itself is evaluated, something different happens, as we will -see in the next section. - - -File: eintr, Node: Variables, Next: Arguments, Prev: Evaluation, Up: List Processing - -1.7 Variables -============= - -In Emacs Lisp, a symbol can have a value attached to it just as it can -have a function definition attached to it. The two are different. The -function definition is a set of instructions that a computer will obey. -A value, on the other hand, is something, such as number or a name, -that can vary (which is why such a symbol is called a variable). The -value of a symbol can be any expression in Lisp, such as a symbol, -number, list, or string. A symbol that has a value is often called a -"variable". - -A symbol can have both a function definition and a value attached to it -at the same time. Or it can have just one or the other. The two are -separate. This is somewhat similar to the way the name Cambridge can -refer to the city in Massachusetts and have some information attached -to the name as well, such as "great programming center". - -Another way to think about this is to imagine a symbol as being a chest -of drawers. The function definition is put in one drawer, the value in -another, and so on. What is put in the drawer holding the value can be -changed without affecting the contents of the drawer holding the -function definition, and vice-verse. - -* Menu: - -* fill-column Example:: -* Void Function:: -* Void Variable:: - - -File: eintr, Node: fill-column Example, Next: Void Function, Prev: Variables, Up: Variables - -`fill-column', an Example Variable ----------------------------------- - -The variable `fill-column' illustrates a symbol with a value attached -to it: in every GNU Emacs buffer, this symbol is set to some value, -usually 72 or 70, but sometimes to some other value. To find the value -of this symbol, evaluate it by itself. If you are reading this in Info -inside of GNU Emacs, you can do this by putting the cursor after the -symbol and typing `C-x C-e': - - fill-column - -After I typed `C-x C-e', Emacs printed the number 72 in my echo area. -This is the value for which `fill-column' is set for me as I write -this. It may be different for you in your Info buffer. Notice that -the value returned as a variable is printed in exactly the same way as -the value returned by a function carrying out its instructions. From -the point of view of the Lisp interpreter, a value returned is a value -returned. What kind of expression it came from ceases to matter once -the value is known. - -A symbol can have any value attached to it or, to use the jargon, we can -"bind" the variable to a value: to a number, such as 72; to a string, -`"such as this"'; to a list, such as `(spruce pine oak)'; we can even -bind a variable to a function definition. - -A symbol can be bound to a value in several ways. *Note Setting the -Value of a Variable: set & setq, for information about one way to do -this. - - -File: eintr, Node: Void Function, Next: Void Variable, Prev: fill-column Example, Up: Variables - -1.7.1 Error Message for a Symbol Without a Function ---------------------------------------------------- - -When we evaluated `fill-column' to find its value as a variable, we did -not place parentheses around the word. This is because we did not -intend to use it as a function name. - -If `fill-column' were the first or only element of a list, the Lisp -interpreter would attempt to find the function definition attached to -it. But `fill-column' has no function definition. Try evaluating this: - - (fill-column) - -In GNU Emacs version 22, you will create a `*Backtrace*' buffer that -says: - - ---------- Buffer: *Backtrace* ---------- - Debugger entered--Lisp error: (void-function fill-column) - (fill-column) - eval((fill-column)) - eval-last-sexp-1(nil) - eval-last-sexp(nil) - call-interactively(eval-last-sexp) - ---------- Buffer: *Backtrace* ---------- - -(Remember, to quit the debugger and make the debugger window go away, -type `q' in the `*Backtrace*' buffer.) - - -File: eintr, Node: Void Variable, Prev: Void Function, Up: Variables - -1.7.2 Error Message for a Symbol Without a Value ------------------------------------------------- - -If you attempt to evaluate a symbol that does not have a value bound to -it, you will receive an error message. You can see this by -experimenting with our 2 plus 2 addition. In the following expression, -put your cursor right after the `+', before the first number 2, type -`C-x C-e': - - (+ 2 2) - -In GNU Emacs 22, you will create a `*Backtrace*' buffer that says: - - ---------- Buffer: *Backtrace* ---------- - Debugger entered--Lisp error: (void-variable +) - eval(+) - eval-last-sexp-1(nil) - eval-last-sexp(nil) - call-interactively(eval-last-sexp) - ---------- Buffer: *Backtrace* ---------- - -(As with the other times we entered the debugger, you can quit by -typing `q' in the `*Backtrace*' buffer.) - -This backtrace is different from the very first error message we saw, -which said, `Debugger entered--Lisp error: (void-function this)'. In -this case, the function does not have a value as a variable; while in -the other error message, the function (the word `this') did not have a -definition. - -In this experiment with the `+', what we did was cause the Lisp -interpreter to evaluate the `+' and look for the value of the variable -instead of the function definition. We did this by placing the cursor -right after the symbol rather than after the parenthesis of the -enclosing list as we did before. As a consequence, the Lisp interpreter -evaluated the preceding s-expression, which in this case was the `+' by -itself. - -Since `+' does not have a value bound to it, just the function -definition, the error message reported that the symbol's value as a -variable was void. - - -File: eintr, Node: Arguments, Next: set & setq, Prev: Variables, Up: List Processing - -1.8 Arguments -============= - -To see how information is passed to functions, let's look again at our -old standby, the addition of two plus two. In Lisp, this is written as -follows: - - (+ 2 2) - -If you evaluate this expression, the number 4 will appear in your echo -area. What the Lisp interpreter does is add the numbers that follow -the `+'. - -The numbers added by `+' are called the "arguments" of the function -`+'. These numbers are the information that is given to or "passed" to -the function. - -The word `argument' comes from the way it is used in mathematics and -does not refer to a disputation between two people; instead it refers to -the information presented to the function, in this case, to the `+'. -In Lisp, the arguments to a function are the atoms or lists that follow -the function. The values returned by the evaluation of these atoms or -lists are passed to the function. Different functions require -different numbers of arguments; some functions require none at all.(1) - -* Menu: - -* Data types:: -* Args as Variable or List:: -* Variable Number of Arguments:: -* Wrong Type of Argument:: -* message:: - ----------- Footnotes ---------- - -(1) It is curious to track the path by which the word `argument' came -to have two different meanings, one in mathematics and the other in -everyday English. According to the `Oxford English Dictionary', the -word derives from the Latin for `to make clear, prove'; thus it came to -mean, by one thread of derivation, `the evidence offered as proof', -which is to say, `the information offered', which led to its meaning in -Lisp. But in the other thread of derivation, it came to mean `to -assert in a manner against which others may make counter assertions', -which led to the meaning of the word as a disputation. (Note here that -the English word has two different definitions attached to it at the -same time. By contrast, in Emacs Lisp, a symbol cannot have two -different function definitions at the same time.) - - -File: eintr, Node: Data types, Next: Args as Variable or List, Prev: Arguments, Up: Arguments - -1.8.1 Arguments' Data Types ---------------------------- - -The type of data that should be passed to a function depends on what -kind of information it uses. The arguments to a function such as `+' -must have values that are numbers, since `+' adds numbers. Other -functions use different kinds of data for their arguments. - -For example, the `concat' function links together or unites two or more -strings of text to produce a string. The arguments are strings. -Concatenating the two character strings `abc', `def' produces the -single string `abcdef'. This can be seen by evaluating the following: - - (concat "abc" "def") - -The value produced by evaluating this expression is `"abcdef"'. - -A function such as `substring' uses both a string and numbers as -arguments. The function returns a part of the string, a substring of -the first argument. This function takes three arguments. Its first -argument is the string of characters, the second and third arguments are -numbers that indicate the beginning and end of the substring. The -numbers are a count of the number of characters (including spaces and -punctuations) from the beginning of the string. - -For example, if you evaluate the following: - - (substring "The quick brown fox jumped." 16 19) - -you will see `"fox"' appear in the echo area. The arguments are the -string and the two numbers. - -Note that the string passed to `substring' is a single atom even though -it is made up of several words separated by spaces. Lisp counts -everything between the two quotation marks as part of the string, -including the spaces. You can think of the `substring' function as a -kind of `atom smasher' since it takes an otherwise indivisible atom and -extracts a part. However, `substring' is only able to extract a -substring from an argument that is a string, not from another type of -atom such as a number or symbol. - - -File: eintr, Node: Args as Variable or List, Next: Variable Number of Arguments, Prev: Data types, Up: Arguments - -1.8.2 An Argument as the Value of a Variable or List ----------------------------------------------------- - -An argument can be a symbol that returns a value when it is evaluated. -For example, when the symbol `fill-column' by itself is evaluated, it -returns a number. This number can be used in an addition. - -Position the cursor after the following expression and type `C-x C-e': - - (+ 2 fill-column) - -The value will be a number two more than what you get by evaluating -`fill-column' alone. For me, this is 74, because my value of -`fill-column' is 72. - -As we have just seen, an argument can be a symbol that returns a value -when evaluated. In addition, an argument can be a list that returns a -value when it is evaluated. For example, in the following expression, -the arguments to the function `concat' are the strings `"The "' and -`" red foxes."' and the list `(number-to-string (+ 2 fill-column))'. - - (concat "The " (number-to-string (+ 2 fill-column)) " red foxes.") - -If you evaluate this expression--and if, as with my Emacs, -`fill-column' evaluates to 72--`"The 74 red foxes."' will appear in the -echo area. (Note that you must put spaces after the word `The' and -before the word `red' so they will appear in the final string. The -function `number-to-string' converts the integer that the addition -function returns to a string. `number-to-string' is also known as -`int-to-string'.) - - -File: eintr, Node: Variable Number of Arguments, Next: Wrong Type of Argument, Prev: Args as Variable or List, Up: Arguments - -1.8.3 Variable Number of Arguments ----------------------------------- - -Some functions, such as `concat', `+' or `*', take any number of -arguments. (The `*' is the symbol for multiplication.) This can be -seen by evaluating each of the following expressions in the usual way. -What you will see in the echo area is printed in this text after `=>', -which you may read as `evaluates to'. - -In the first set, the functions have no arguments: - - (+) => 0 - - (*) => 1 - -In this set, the functions have one argument each: - - (+ 3) => 3 - - (* 3) => 3 - -In this set, the functions have three arguments each: - - (+ 3 4 5) => 12 - - (* 3 4 5) => 60 - - -File: eintr, Node: Wrong Type of Argument, Next: message, Prev: Variable Number of Arguments, Up: Arguments - -1.8.4 Using the Wrong Type Object as an Argument ------------------------------------------------- - -When a function is passed an argument of the wrong type, the Lisp -interpreter produces an error message. For example, the `+' function -expects the values of its arguments to be numbers. As an experiment we -can pass it the quoted symbol `hello' instead of a number. Position -the cursor after the following expression and type `C-x C-e': - - (+ 2 'hello) - -When you do this you will generate an error message. What has happened -is that `+' has tried to add the 2 to the value returned by `'hello', -but the value returned by `'hello' is the symbol `hello', not a number. -Only numbers can be added. So `+' could not carry out its addition. - -In GNU Emacs version 22, you will create and enter a `*Backtrace*' -buffer that says: - - - ---------- Buffer: *Backtrace* ---------- - Debugger entered--Lisp error: - (wrong-type-argument number-or-marker-p hello) - +(2 hello) - eval((+ 2 (quote hello))) - eval-last-sexp-1(nil) - eval-last-sexp(nil) - call-interactively(eval-last-sexp) - ---------- Buffer: *Backtrace* ---------- - -As usual, the error message tries to be helpful and makes sense after -you learn how to read it.(1) - -The first part of the error message is straightforward; it says `wrong -type argument'. Next comes the mysterious jargon word -`number-or-marker-p'. This word is trying to tell you what kind of -argument the `+' expected. - -The symbol `number-or-marker-p' says that the Lisp interpreter is -trying to determine whether the information presented it (the value of -the argument) is a number or a marker (a special object representing a -buffer position). What it does is test to see whether the `+' is being -given numbers to add. It also tests to see whether the argument is -something called a marker, which is a specific feature of Emacs Lisp. -(In Emacs, locations in a buffer are recorded as markers. When the -mark is set with the `C-@' or `C-<SPC>' command, its position is kept -as a marker. The mark can be considered a number--the number of -characters the location is from the beginning of the buffer.) In Emacs -Lisp, `+' can be used to add the numeric value of marker positions as -numbers. - -The `p' of `number-or-marker-p' is the embodiment of a practice started -in the early days of Lisp programming. The `p' stands for `predicate'. -In the jargon used by the early Lisp researchers, a predicate refers -to a function to determine whether some property is true or false. So -the `p' tells us that `number-or-marker-p' is the name of a function -that determines whether it is true or false that the argument supplied -is a number or a marker. Other Lisp symbols that end in `p' include -`zerop', a function that tests whether its argument has the value of -zero, and `listp', a function that tests whether its argument is a list. - -Finally, the last part of the error message is the symbol `hello'. -This is the value of the argument that was passed to `+'. If the -addition had been passed the correct type of object, the value passed -would have been a number, such as 37, rather than a symbol like -`hello'. But then you would not have got the error message. - ----------- Footnotes ---------- - -(1) `(quote hello)' is an expansion of the abbreviation `'hello'. - - -File: eintr, Node: message, Prev: Wrong Type of Argument, Up: Arguments - -1.8.5 The `message' Function ----------------------------- - -Like `+', the `message' function takes a variable number of arguments. -It is used to send messages to the user and is so useful that we will -describe it here. - -A message is printed in the echo area. For example, you can print a -message in your echo area by evaluating the following list: - - (message "This message appears in the echo area!") - -The whole string between double quotation marks is a single argument -and is printed in toto. (Note that in this example, the message itself -will appear in the echo area within double quotes; that is because you -see the value returned by the `message' function. In most uses of -`message' in programs that you write, the text will be printed in the -echo area as a side-effect, without the quotes. *Note -`multiply-by-seven' in detail: multiply-by-seven in detail, for an -example of this.) - -However, if there is a `%s' in the quoted string of characters, the -`message' function does not print the `%s' as such, but looks to the -argument that follows the string. It evaluates the second argument and -prints the value at the location in the string where the `%s' is. - -You can see this by positioning the cursor after the following -expression and typing `C-x C-e': - - (message "The name of this buffer is: %s." (buffer-name)) - -In Info, `"The name of this buffer is: *info*."' will appear in the -echo area. The function `buffer-name' returns the name of the buffer -as a string, which the `message' function inserts in place of `%s'. - -To print a value as an integer, use `%d' in the same way as `%s'. For -example, to print a message in the echo area that states the value of -the `fill-column', evaluate the following: - - (message "The value of fill-column is %d." fill-column) - -On my system, when I evaluate this list, `"The value of fill-column is -72."' appears in my echo area(1). - -If there is more than one `%s' in the quoted string, the value of the -first argument following the quoted string is printed at the location -of the first `%s' and the value of the second argument is printed at -the location of the second `%s', and so on. - -For example, if you evaluate the following, - - (message "There are %d %s in the office!" - (- fill-column 14) "pink elephants") - -a rather whimsical message will appear in your echo area. On my system -it says, `"There are 58 pink elephants in the office!"'. - -The expression `(- fill-column 14)' is evaluated and the resulting -number is inserted in place of the `%d'; and the string in double -quotes, `"pink elephants"', is treated as a single argument and -inserted in place of the `%s'. (That is to say, a string between -double quotes evaluates to itself, like a number.) - -Finally, here is a somewhat complex example that not only illustrates -the computation of a number, but also shows how you can use an -expression within an expression to generate the text that is substituted -for `%s': - - (message "He saw %d %s" - (- fill-column 32) - (concat "red " - (substring - "The quick brown foxes jumped." 16 21) - " leaping.")) - -In this example, `message' has three arguments: the string, `"He saw %d -%s"', the expression, `(- fill-column 32)', and the expression -beginning with the function `concat'. The value resulting from the -evaluation of `(- fill-column 32)' is inserted in place of the `%d'; -and the value returned by the expression beginning with `concat' is -inserted in place of the `%s'. - -When your fill column is 70 and you evaluate the expression, the -message `"He saw 38 red foxes leaping."' appears in your echo area. - ----------- Footnotes ---------- - -(1) Actually, you can use `%s' to print a number. It is non-specific. -`%d' prints only the part of a number left of a decimal point, and not -anything that is not a number. - - -File: eintr, Node: set & setq, Next: Summary, Prev: Arguments, Up: List Processing - -1.9 Setting the Value of a Variable -=================================== - -There are several ways by which a variable can be given a value. One of -the ways is to use either the function `set' or the function `setq'. -Another way is to use `let' (*note let::). (The jargon for this -process is to "bind" a variable to a value.) - -The following sections not only describe how `set' and `setq' work but -also illustrate how arguments are passed. - -* Menu: - -* Using set:: -* Using setq:: -* Counting:: - - -File: eintr, Node: Using set, Next: Using setq, Prev: set & setq, Up: set & setq - -1.9.1 Using `set' ------------------ - -To set the value of the symbol `flowers' to the list `'(rose violet -daisy buttercup)', evaluate the following expression by positioning the -cursor after the expression and typing `C-x C-e'. - - (set 'flowers '(rose violet daisy buttercup)) - -The list `(rose violet daisy buttercup)' will appear in the echo area. -This is what is _returned_ by the `set' function. As a side effect, -the symbol `flowers' is bound to the list; that is, the symbol -`flowers', which can be viewed as a variable, is given the list as its -value. (This process, by the way, illustrates how a side effect to the -Lisp interpreter, setting the value, can be the primary effect that we -humans are interested in. This is because every Lisp function must -return a value if it does not get an error, but it will only have a -side effect if it is designed to have one.) - -After evaluating the `set' expression, you can evaluate the symbol -`flowers' and it will return the value you just set. Here is the -symbol. Place your cursor after it and type `C-x C-e'. - - flowers - -When you evaluate `flowers', the list `(rose violet daisy buttercup)' -appears in the echo area. - -Incidentally, if you evaluate `'flowers', the variable with a quote in -front of it, what you will see in the echo area is the symbol itself, -`flowers'. Here is the quoted symbol, so you can try this: - - 'flowers - -Note also, that when you use `set', you need to quote both arguments to -`set', unless you want them evaluated. Since we do not want either -argument evaluated, neither the variable `flowers' nor the list `(rose -violet daisy buttercup)', both are quoted. (When you use `set' without -quoting its first argument, the first argument is evaluated before -anything else is done. If you did this and `flowers' did not have a -value already, you would get an error message that the `Symbol's value -as variable is void'; on the other hand, if `flowers' did return a -value after it was evaluated, the `set' would attempt to set the value -that was returned. There are situations where this is the right thing -for the function to do; but such situations are rare.) - - -File: eintr, Node: Using setq, Next: Counting, Prev: Using set, Up: set & setq - -1.9.2 Using `setq' ------------------- - -As a practical matter, you almost always quote the first argument to -`set'. The combination of `set' and a quoted first argument is so -common that it has its own name: the special form `setq'. This special -form is just like `set' except that the first argument is quoted -automatically, so you don't need to type the quote mark yourself. -Also, as an added convenience, `setq' permits you to set several -different variables to different values, all in one expression. - -To set the value of the variable `carnivores' to the list `'(lion tiger -leopard)' using `setq', the following expression is used: - - (setq carnivores '(lion tiger leopard)) - -This is exactly the same as using `set' except the first argument is -automatically quoted by `setq'. (The `q' in `setq' means `quote'.) - -With `set', the expression would look like this: - - (set 'carnivores '(lion tiger leopard)) - -Also, `setq' can be used to assign different values to different -variables. The first argument is bound to the value of the second -argument, the third argument is bound to the value of the fourth -argument, and so on. For example, you could use the following to -assign a list of trees to the symbol `trees' and a list of herbivores -to the symbol `herbivores': - - (setq trees '(pine fir oak maple) - herbivores '(gazelle antelope zebra)) - -(The expression could just as well have been on one line, but it might -not have fit on a page; and humans find it easier to read nicely -formatted lists.) - -Although I have been using the term `assign', there is another way of -thinking about the workings of `set' and `setq'; and that is to say -that `set' and `setq' make the symbol _point_ to the list. This latter -way of thinking is very common and in forthcoming chapters we shall -come upon at least one symbol that has `pointer' as part of its name. -The name is chosen because the symbol has a value, specifically a list, -attached to it; or, expressed another way, the symbol is set to "point" -to the list. - - -File: eintr, Node: Counting, Prev: Using setq, Up: set & setq - -1.9.3 Counting --------------- - -Here is an example that shows how to use `setq' in a counter. You -might use this to count how many times a part of your program repeats -itself. First set a variable to zero; then add one to the number each -time the program repeats itself. To do this, you need a variable that -serves as a counter, and two expressions: an initial `setq' expression -that sets the counter variable to zero; and a second `setq' expression -that increments the counter each time it is evaluated. - - (setq counter 0) ; Let's call this the initializer. - - (setq counter (+ counter 1)) ; This is the incrementer. - - counter ; This is the counter. - -(The text following the `;' are comments. *Note Change a Function -Definition: Change a defun.) - -If you evaluate the first of these expressions, the initializer, `(setq -counter 0)', and then evaluate the third expression, `counter', the -number `0' will appear in the echo area. If you then evaluate the -second expression, the incrementer, `(setq counter (+ counter 1))', the -counter will get the value 1. So if you again evaluate `counter', the -number `1' will appear in the echo area. Each time you evaluate the -second expression, the value of the counter will be incremented. - -When you evaluate the incrementer, `(setq counter (+ counter 1))', the -Lisp interpreter first evaluates the innermost list; this is the -addition. In order to evaluate this list, it must evaluate the variable -`counter' and the number `1'. When it evaluates the variable -`counter', it receives its current value. It passes this value and the -number `1' to the `+' which adds them together. The sum is then -returned as the value of the inner list and passed to the `setq' which -sets the variable `counter' to this new value. Thus, the value of the -variable, `counter', is changed. - - -File: eintr, Node: Summary, Next: Error Message Exercises, Prev: set & setq, Up: List Processing - -1.10 Summary -============ - -Learning Lisp is like climbing a hill in which the first part is the -steepest. You have now climbed the most difficult part; what remains -becomes easier as you progress onwards. - -In summary, - - * Lisp programs are made up of expressions, which are lists or - single atoms. - - * Lists are made up of zero or more atoms or inner lists, separated - by whitespace and surrounded by parentheses. A list can be empty. - - * Atoms are multi-character symbols, like `forward-paragraph', single - character symbols like `+', strings of characters between double - quotation marks, or numbers. - - * A number evaluates to itself. - - * A string between double quotes also evaluates to itself. - - * When you evaluate a symbol by itself, its value is returned. - - * When you evaluate a list, the Lisp interpreter looks at the first - symbol in the list and then at the function definition bound to - that symbol. Then the instructions in the function definition are - carried out. - - * A single quotation mark, ' , tells the Lisp interpreter that it - should return the following expression as written, and not - evaluate it as it would if the quote were not there. - - * Arguments are the information passed to a function. The arguments - to a function are computed by evaluating the rest of the elements - of the list of which the function is the first element. - - * A function always returns a value when it is evaluated (unless it - gets an error); in addition, it may also carry out some action - called a "side effect". In many cases, a function's primary - purpose is to create a side effect. - - -File: eintr, Node: Error Message Exercises, Prev: Summary, Up: List Processing - -1.11 Exercises -============== - -A few simple exercises: - - * Generate an error message by evaluating an appropriate symbol that - is not within parentheses. - - * Generate an error message by evaluating an appropriate symbol that - is between parentheses. - - * Create a counter that increments by two rather than one. - - * Write an expression that prints a message in the echo area when - evaluated. - - -File: eintr, Node: Practicing Evaluation, Next: Writing Defuns, Prev: List Processing, Up: Top - -2 Practicing Evaluation -*********************** - -Before learning how to write a function definition in Emacs Lisp, it is -useful to spend a little time evaluating various expressions that have -already been written. These expressions will be lists with the -functions as their first (and often only) element. Since some of the -functions associated with buffers are both simple and interesting, we -will start with those. In this section, we will evaluate a few of -these. In another section, we will study the code of several other -buffer-related functions, to see how they were written. - -* Menu: - -* How to Evaluate:: -* Buffer Names:: -* Getting Buffers:: -* Switching Buffers:: -* Buffer Size & Locations:: -* Evaluation Exercise:: - - -File: eintr, Node: How to Evaluate, Next: Buffer Names, Prev: Practicing Evaluation, Up: Practicing Evaluation - -How to Evaluate -=============== - -Whenever you give an editing command to Emacs Lisp, such as the command -to move the cursor or to scroll the screen, you are evaluating an -expression, the first element of which is a function. This is how -Emacs works. - -When you type keys, you cause the Lisp interpreter to evaluate an -expression and that is how you get your results. Even typing plain text -involves evaluating an Emacs Lisp function, in this case, one that uses -`self-insert-command', which simply inserts the character you typed. -The functions you evaluate by typing keystrokes are called -"interactive" functions, or "commands"; how you make a function -interactive will be illustrated in the chapter on how to write function -definitions. *Note Making a Function Interactive: Interactive. - -In addition to typing keyboard commands, we have seen a second way to -evaluate an expression: by positioning the cursor after a list and -typing `C-x C-e'. This is what we will do in the rest of this section. -There are other ways to evaluate an expression as well; these will be -described as we come to them. - -Besides being used for practicing evaluation, the functions shown in the -next few sections are important in their own right. A study of these -functions makes clear the distinction between buffers and files, how to -switch to a buffer, and how to determine a location within it. - - -File: eintr, Node: Buffer Names, Next: Getting Buffers, Prev: How to Evaluate, Up: Practicing Evaluation - -2.1 Buffer Names -================ - -The two functions, `buffer-name' and `buffer-file-name', show the -difference between a file and a buffer. When you evaluate the -following expression, `(buffer-name)', the name of the buffer appears -in the echo area. When you evaluate `(buffer-file-name)', the name of -the file to which the buffer refers appears in the echo area. Usually, -the name returned by `(buffer-name)' is the same as the name of the -file to which it refers, and the name returned by `(buffer-file-name)' -is the full path-name of the file. - -A file and a buffer are two different entities. A file is information -recorded permanently in the computer (unless you delete it). A buffer, -on the other hand, is information inside of Emacs that will vanish at -the end of the editing session (or when you kill the buffer). Usually, -a buffer contains information that you have copied from a file; we say -the buffer is "visiting" that file. This copy is what you work on and -modify. Changes to the buffer do not change the file, until you save -the buffer. When you save the buffer, the buffer is copied to the file -and is thus saved permanently. - -If you are reading this in Info inside of GNU Emacs, you can evaluate -each of the following expressions by positioning the cursor after it and -typing `C-x C-e'. - - (buffer-name) - - (buffer-file-name) - -When I do this in Info, the value returned by evaluating -`(buffer-name)' is `"*info*"', and the value returned by evaluating -`(buffer-file-name)' is `nil'. - -On the other hand, while I am writing this Introduction, the value -returned by evaluating `(buffer-name)' is `"introduction.texinfo"', and -the value returned by evaluating `(buffer-file-name)' is -`"/gnu/work/intro/introduction.texinfo"'. - -The former is the name of the buffer and the latter is the name of the -file. In Info, the buffer name is `"*info*"'. Info does not point to -any file, so the result of evaluating `(buffer-file-name)' is `nil'. -The symbol `nil' is from the Latin word for `nothing'; in this case, it -means that the buffer is not associated with any file. (In Lisp, `nil' -is also used to mean `false' and is a synonym for the empty list, `()'.) - -When I am writing, the name of my buffer is `"introduction.texinfo"'. -The name of the file to which it points is -`"/gnu/work/intro/introduction.texinfo"'. - -(In the expressions, the parentheses tell the Lisp interpreter to treat -`buffer-name' and `buffer-file-name' as functions; without the -parentheses, the interpreter would attempt to evaluate the symbols as -variables. *Note Variables::.) - -In spite of the distinction between files and buffers, you will often -find that people refer to a file when they mean a buffer and vice-verse. -Indeed, most people say, "I am editing a file," rather than saying, "I -am editing a buffer which I will soon save to a file." It is almost -always clear from context what people mean. When dealing with computer -programs, however, it is important to keep the distinction in mind, -since the computer is not as smart as a person. - -The word `buffer', by the way, comes from the meaning of the word as a -cushion that deadens the force of a collision. In early computers, a -buffer cushioned the interaction between files and the computer's -central processing unit. The drums or tapes that held a file and the -central processing unit were pieces of equipment that were very -different from each other, working at their own speeds, in spurts. The -buffer made it possible for them to work together effectively. -Eventually, the buffer grew from being an intermediary, a temporary -holding place, to being the place where work is done. This -transformation is rather like that of a small seaport that grew into a -great city: once it was merely the place where cargo was warehoused -temporarily before being loaded onto ships; then it became a business -and cultural center in its own right. - -Not all buffers are associated with files. For example, a `*scratch*' -buffer does not visit any file. Similarly, a `*Help*' buffer is not -associated with any file. - -In the old days, when you lacked a `~/.emacs' file and started an Emacs -session by typing the command `emacs' alone, without naming any files, -Emacs started with the `*scratch*' buffer visible. Nowadays, you will -see a splash screen. You can follow one of the commands suggested on -the splash screen, visit a file, or press the spacebar to reach the -`*scratch*' buffer. - -If you switch to the `*scratch*' buffer, type `(buffer-name)', position -the cursor after it, and then type `C-x C-e' to evaluate the -expression. The name `"*scratch*"' will be returned and will appear in -the echo area. `"*scratch*"' is the name of the buffer. When you type -`(buffer-file-name)' in the `*scratch*' buffer and evaluate that, `nil' -will appear in the echo area, just as it does when you evaluate -`(buffer-file-name)' in Info. - -Incidentally, if you are in the `*scratch*' buffer and want the value -returned by an expression to appear in the `*scratch*' buffer itself -rather than in the echo area, type `C-u C-x C-e' instead of `C-x C-e'. -This causes the value returned to appear after the expression. The -buffer will look like this: - - (buffer-name)"*scratch*" - -You cannot do this in Info since Info is read-only and it will not allow -you to change the contents of the buffer. But you can do this in any -buffer you can edit; and when you write code or documentation (such as -this book), this feature is very useful. - - -File: eintr, Node: Getting Buffers, Next: Switching Buffers, Prev: Buffer Names, Up: Practicing Evaluation - -2.2 Getting Buffers -=================== - -The `buffer-name' function returns the _name_ of the buffer; to get the -buffer _itself_, a different function is needed: the `current-buffer' -function. If you use this function in code, what you get is the buffer -itself. - -A name and the object or entity to which the name refers are different -from each other. You are not your name. You are a person to whom -others refer by name. If you ask to speak to George and someone hands -you a card with the letters `G', `e', `o', `r', `g', and `e' written on -it, you might be amused, but you would not be satisfied. You do not -want to speak to the name, but to the person to whom the name refers. -A buffer is similar: the name of the scratch buffer is `*scratch*', but -the name is not the buffer. To get a buffer itself, you need to use a -function such as `current-buffer'. - -However, there is a slight complication: if you evaluate -`current-buffer' in an expression on its own, as we will do here, what -you see is a printed representation of the name of the buffer without -the contents of the buffer. Emacs works this way for two reasons: the -buffer may be thousands of lines long--too long to be conveniently -displayed; and, another buffer may have the same contents but a -different name, and it is important to distinguish between them. - -Here is an expression containing the function: - - (current-buffer) - -If you evaluate this expression in Info in Emacs in the usual way, -`#<buffer *info*>' will appear in the echo area. The special format -indicates that the buffer itself is being returned, rather than just -its name. - -Incidentally, while you can type a number or symbol into a program, you -cannot do that with the printed representation of a buffer: the only way -to get a buffer itself is with a function such as `current-buffer'. - -A related function is `other-buffer'. This returns the most recently -selected buffer other than the one you are in currently, not a printed -representation of its name. If you have recently switched back and -forth from the `*scratch*' buffer, `other-buffer' will return that -buffer. - -You can see this by evaluating the expression: - - (other-buffer) - -You should see `#<buffer *scratch*>' appear in the echo area, or the -name of whatever other buffer you switched back from most recently(1). - ----------- Footnotes ---------- - -(1) Actually, by default, if the buffer from which you just switched is -visible to you in another window, `other-buffer' will choose the most -recent buffer that you cannot see; this is a subtlety that I often -forget. - - -File: eintr, Node: Switching Buffers, Next: Buffer Size & Locations, Prev: Getting Buffers, Up: Practicing Evaluation - -2.3 Switching Buffers -===================== - -The `other-buffer' function actually provides a buffer when it is used -as an argument to a function that requires one. We can see this by -using `other-buffer' and `switch-to-buffer' to switch to a different -buffer. - -But first, a brief introduction to the `switch-to-buffer' function. -When you switched back and forth from Info to the `*scratch*' buffer to -evaluate `(buffer-name)', you most likely typed `C-x b' and then typed -`*scratch*'(1) when prompted in the minibuffer for the name of the -buffer to which you wanted to switch. The keystrokes, `C-x b', cause -the Lisp interpreter to evaluate the interactive function -`switch-to-buffer'. As we said before, this is how Emacs works: -different keystrokes call or run different functions. For example, -`C-f' calls `forward-char', `M-e' calls `forward-sentence', and so on. - -By writing `switch-to-buffer' in an expression, and giving it a buffer -to switch to, we can switch buffers just the way `C-x b' does. - -Here is the Lisp expression: - - (switch-to-buffer (other-buffer)) - -The symbol `switch-to-buffer' is the first element of the list, so the -Lisp interpreter will treat it as a function and carry out the -instructions that are attached to it. But before doing that, the -interpreter will note that `other-buffer' is inside parentheses and -work on that symbol first. `other-buffer' is the first (and in this -case, the only) element of this list, so the Lisp interpreter calls or -runs the function. It returns another buffer. Next, the interpreter -runs `switch-to-buffer', passing to it, as an argument, the other -buffer, which is what Emacs will switch to. If you are reading this in -Info, try this now. Evaluate the expression. (To get back, type `C-x -b <RET>'.)(2) - -In the programming examples in later sections of this document, you will -see the function `set-buffer' more often than `switch-to-buffer'. This -is because of a difference between computer programs and humans: humans -have eyes and expect to see the buffer on which they are working on -their computer terminals. This is so obvious, it almost goes without -saying. However, programs do not have eyes. When a computer program -works on a buffer, that buffer does not need to be visible on the -screen. - -`switch-to-buffer' is designed for humans and does two different -things: it switches the buffer to which Emacs' attention is directed; -and it switches the buffer displayed in the window to the new buffer. -`set-buffer', on the other hand, does only one thing: it switches the -attention of the computer program to a different buffer. The buffer on -the screen remains unchanged (of course, normally nothing happens there -until the command finishes running). - -Also, we have just introduced another jargon term, the word "call". -When you evaluate a list in which the first symbol is a function, you -are calling that function. The use of the term comes from the notion of -the function as an entity that can do something for you if you `call' -it--just as a plumber is an entity who can fix a leak if you call him -or her. - ----------- Footnotes ---------- - -(1) Or rather, to save typing, you probably only typed `RET' if the -default buffer was `*scratch*', or if it was different, then you typed -just part of the name, such as `*sc', pressed your `TAB' key to cause -it to expand to the full name, and then typed your `RET' key. - -(2) Remember, this expression will move you to your most recent other -buffer that you cannot see. If you really want to go to your most -recently selected buffer, even if you can still see it, you need to -evaluate the following more complex expression: - - (switch-to-buffer (other-buffer (current-buffer) t)) - -In this case, the first argument to `other-buffer' tells it which -buffer to skip--the current one--and the second argument tells -`other-buffer' it is OK to switch to a visible buffer. In regular use, -`switch-to-buffer' takes you to an invisible window since you would -most likely use `C-x o' (`other-window') to go to another visible -buffer. - - -File: eintr, Node: Buffer Size & Locations, Next: Evaluation Exercise, Prev: Switching Buffers, Up: Practicing Evaluation - -2.4 Buffer Size and the Location of Point -========================================= - -Finally, let's look at several rather simple functions, `buffer-size', -`point', `point-min', and `point-max'. These give information about -the size of a buffer and the location of point within it. - -The function `buffer-size' tells you the size of the current buffer; -that is, the function returns a count of the number of characters in -the buffer. - - (buffer-size) - -You can evaluate this in the usual way, by positioning the cursor after -the expression and typing `C-x C-e'. - -In Emacs, the current position of the cursor is called "point". The -expression `(point)' returns a number that tells you where the cursor -is located as a count of the number of characters from the beginning of -the buffer up to point. - -You can see the character count for point in this buffer by evaluating -the following expression in the usual way: - - (point) - -As I write this, the value of `point' is 65724. The `point' function -is frequently used in some of the examples later in this book. - -The value of point depends, of course, on its location within the -buffer. If you evaluate point in this spot, the number will be larger: - - (point) - -For me, the value of point in this location is 66043, which means that -there are 319 characters (including spaces) between the two expressions. - -The function `point-min' is somewhat similar to `point', but it returns -the value of the minimum permissible value of point in the current -buffer. This is the number 1 unless "narrowing" is in effect. -(Narrowing is a mechanism whereby you can restrict yourself, or a -program, to operations on just a part of a buffer. *Note Narrowing and -Widening: Narrowing & Widening.) Likewise, the function `point-max' -returns the value of the maximum permissible value of point in the -current buffer. - - -File: eintr, Node: Evaluation Exercise, Prev: Buffer Size & Locations, Up: Practicing Evaluation - -2.5 Exercise -============ - -Find a file with which you are working and move towards its middle. -Find its buffer name, file name, length, and your position in the file. - - -File: eintr, Node: Writing Defuns, Next: Buffer Walk Through, Prev: Practicing Evaluation, Up: Top - -3 How To Write Function Definitions -*********************************** - -When the Lisp interpreter evaluates a list, it looks to see whether the -first symbol on the list has a function definition attached to it; or, -put another way, whether the symbol points to a function definition. If -it does, the computer carries out the instructions in the definition. A -symbol that has a function definition is called, simply, a function -(although, properly speaking, the definition is the function and the -symbol refers to it.) - -* Menu: - -* Primitive Functions:: -* defun:: -* Install:: -* Interactive:: -* Interactive Options:: -* Permanent Installation:: -* let:: -* if:: -* else:: -* Truth & Falsehood:: -* save-excursion:: -* Review:: -* defun Exercises:: - - -File: eintr, Node: Primitive Functions, Next: defun, Prev: Writing Defuns, Up: Writing Defuns - -An Aside about Primitive Functions -================================== - -All functions are defined in terms of other functions, except for a few -"primitive" functions that are written in the C programming language. -When you write functions' definitions, you will write them in Emacs -Lisp and use other functions as your building blocks. Some of the -functions you will use will themselves be written in Emacs Lisp (perhaps -by you) and some will be primitives written in C. The primitive -functions are used exactly like those written in Emacs Lisp and behave -like them. They are written in C so we can easily run GNU Emacs on any -computer that has sufficient power and can run C. - -Let me re-emphasize this: when you write code in Emacs Lisp, you do not -distinguish between the use of functions written in C and the use of -functions written in Emacs Lisp. The difference is irrelevant. I -mention the distinction only because it is interesting to know. Indeed, -unless you investigate, you won't know whether an already-written -function is written in Emacs Lisp or C. - - -File: eintr, Node: defun, Next: Install, Prev: Primitive Functions, Up: Writing Defuns - -3.1 The `defun' Special Form -============================ - -In Lisp, a symbol such as `mark-whole-buffer' has code attached to it -that tells the computer what to do when the function is called. This -code is called the "function definition" and is created by evaluating a -Lisp expression that starts with the symbol `defun' (which is an -abbreviation for _define function_). Because `defun' does not evaluate -its arguments in the usual way, it is called a "special form". - -In subsequent sections, we will look at function definitions from the -Emacs source code, such as `mark-whole-buffer'. In this section, we -will describe a simple function definition so you can see how it looks. -This function definition uses arithmetic because it makes for a simple -example. Some people dislike examples using arithmetic; however, if -you are such a person, do not despair. Hardly any of the code we will -study in the remainder of this introduction involves arithmetic or -mathematics. The examples mostly involve text in one way or another. - -A function definition has up to five parts following the word `defun': - - 1. The name of the symbol to which the function definition should be - attached. - - 2. A list of the arguments that will be passed to the function. If no - arguments will be passed to the function, this is an empty list, - `()'. - - 3. Documentation describing the function. (Technically optional, but - strongly recommended.) - - 4. Optionally, an expression to make the function interactive so you - can use it by typing `M-x' and then the name of the function; or by - typing an appropriate key or keychord. - - 5. The code that instructs the computer what to do: the "body" of the - function definition. - -It is helpful to think of the five parts of a function definition as -being organized in a template, with slots for each part: - - (defun FUNCTION-NAME (ARGUMENTS...) - "OPTIONAL-DOCUMENTATION..." - (interactive ARGUMENT-PASSING-INFO) ; optional - BODY...) - -As an example, here is the code for a function that multiplies its -argument by 7. (This example is not interactive. *Note Making a -Function Interactive: Interactive, for that information.) - - (defun multiply-by-seven (number) - "Multiply NUMBER by seven." - (* 7 number)) - -This definition begins with a parenthesis and the symbol `defun', -followed by the name of the function. - -The name of the function is followed by a list that contains the -arguments that will be passed to the function. This list is called the -"argument list". In this example, the list has only one element, the -symbol, `number'. When the function is used, the symbol will be bound -to the value that is used as the argument to the function. - -Instead of choosing the word `number' for the name of the argument, I -could have picked any other name. For example, I could have chosen the -word `multiplicand'. I picked the word `number' because it tells what -kind of value is intended for this slot; but I could just as well have -chosen the word `multiplicand' to indicate the role that the value -placed in this slot will play in the workings of the function. I could -have called it `foogle', but that would have been a bad choice because -it would not tell humans what it means. The choice of name is up to -the programmer and should be chosen to make the meaning of the function -clear. - -Indeed, you can choose any name you wish for a symbol in an argument -list, even the name of a symbol used in some other function: the name -you use in an argument list is private to that particular definition. -In that definition, the name refers to a different entity than any use -of the same name outside the function definition. Suppose you have a -nick-name `Shorty' in your family; when your family members refer to -`Shorty', they mean you. But outside your family, in a movie, for -example, the name `Shorty' refers to someone else. Because a name in an -argument list is private to the function definition, you can change the -value of such a symbol inside the body of a function without changing -its value outside the function. The effect is similar to that produced -by a `let' expression. (*Note `let': let.) - -The argument list is followed by the documentation string that -describes the function. This is what you see when you type `C-h f' and -the name of a function. Incidentally, when you write a documentation -string like this, you should make the first line a complete sentence -since some commands, such as `apropos', print only the first line of a -multi-line documentation string. Also, you should not indent the -second line of a documentation string, if you have one, because that -looks odd when you use `C-h f' (`describe-function'). The -documentation string is optional, but it is so useful, it should be -included in almost every function you write. - -The third line of the example consists of the body of the function -definition. (Most functions' definitions, of course, are longer than -this.) In this function, the body is the list, `(* 7 number)', which -says to multiply the value of NUMBER by 7. (In Emacs Lisp, `*' is the -function for multiplication, just as `+' is the function for addition.) - -When you use the `multiply-by-seven' function, the argument `number' -evaluates to the actual number you want used. Here is an example that -shows how `multiply-by-seven' is used; but don't try to evaluate this -yet! - - (multiply-by-seven 3) - -The symbol `number', specified in the function definition in the next -section, is given or "bound to" the value 3 in the actual use of the -function. Note that although `number' was inside parentheses in the -function definition, the argument passed to the `multiply-by-seven' -function is not in parentheses. The parentheses are written in the -function definition so the computer can figure out where the argument -list ends and the rest of the function definition begins. - -If you evaluate this example, you are likely to get an error message. -(Go ahead, try it!) This is because we have written the function -definition, but not yet told the computer about the definition--we have -not yet installed (or `loaded') the function definition in Emacs. -Installing a function is the process that tells the Lisp interpreter the -definition of the function. Installation is described in the next -section. - - -File: eintr, Node: Install, Next: Interactive, Prev: defun, Up: Writing Defuns - -3.2 Install a Function Definition -================================= - -If you are reading this inside of Info in Emacs, you can try out the -`multiply-by-seven' function by first evaluating the function -definition and then evaluating `(multiply-by-seven 3)'. A copy of the -function definition follows. Place the cursor after the last -parenthesis of the function definition and type `C-x C-e'. When you do -this, `multiply-by-seven' will appear in the echo area. (What this -means is that when a function definition is evaluated, the value it -returns is the name of the defined function.) At the same time, this -action installs the function definition. - - (defun multiply-by-seven (number) - "Multiply NUMBER by seven." - (* 7 number)) - -By evaluating this `defun', you have just installed `multiply-by-seven' -in Emacs. The function is now just as much a part of Emacs as -`forward-word' or any other editing function you use. -(`multiply-by-seven' will stay installed until you quit Emacs. To -reload code automatically whenever you start Emacs, see *Note -Installing Code Permanently: Permanent Installation.) - -* Menu: - -* Effect of installation:: -* Change a defun:: - - -File: eintr, Node: Effect of installation, Next: Change a defun, Prev: Install, Up: Install - -The effect of installation --------------------------- - -You can see the effect of installing `multiply-by-seven' by evaluating -the following sample. Place the cursor after the following expression -and type `C-x C-e'. The number 21 will appear in the echo area. - - (multiply-by-seven 3) - -If you wish, you can read the documentation for the function by typing -`C-h f' (`describe-function') and then the name of the function, -`multiply-by-seven'. When you do this, a `*Help*' window will appear -on your screen that says: - - multiply-by-seven is a Lisp function. - (multiply-by-seven NUMBER) - - Multiply NUMBER by seven. - -(To return to a single window on your screen, type `C-x 1'.) - - -File: eintr, Node: Change a defun, Prev: Effect of installation, Up: Install - -3.2.1 Change a Function Definition ----------------------------------- - -If you want to change the code in `multiply-by-seven', just rewrite it. -To install the new version in place of the old one, evaluate the -function definition again. This is how you modify code in Emacs. It is -very simple. - -As an example, you can change the `multiply-by-seven' function to add -the number to itself seven times instead of multiplying the number by -seven. It produces the same answer, but by a different path. At the -same time, we will add a comment to the code; a comment is text that -the Lisp interpreter ignores, but that a human reader may find useful -or enlightening. The comment is that this is the "second version". - - (defun multiply-by-seven (number) ; Second version. - "Multiply NUMBER by seven." - (+ number number number number number number number)) - -The comment follows a semicolon, `;'. In Lisp, everything on a line -that follows a semicolon is a comment. The end of the line is the end -of the comment. To stretch a comment over two or more lines, begin -each line with a semicolon. - -*Note Beginning a `.emacs' File: Beginning a .emacs File, and *Note -Comments: (elisp)Comments, for more about comments. - -You can install this version of the `multiply-by-seven' function by -evaluating it in the same way you evaluated the first function: place -the cursor after the last parenthesis and type `C-x C-e'. - -In summary, this is how you write code in Emacs Lisp: you write a -function; install it; test it; and then make fixes or enhancements and -install it again. - - -File: eintr, Node: Interactive, Next: Interactive Options, Prev: Install, Up: Writing Defuns - -3.3 Make a Function Interactive -=============================== - -You make a function interactive by placing a list that begins with the -special form `interactive' immediately after the documentation. A user -can invoke an interactive function by typing `M-x' and then the name of -the function; or by typing the keys to which it is bound, for example, -by typing `C-n' for `next-line' or `C-x h' for `mark-whole-buffer'. - -Interestingly, when you call an interactive function interactively, the -value returned is not automatically displayed in the echo area. This -is because you often call an interactive function for its side effects, -such as moving forward by a word or line, and not for the value -returned. If the returned value were displayed in the echo area each -time you typed a key, it would be very distracting. - -* Menu: - -* Interactive multiply-by-seven:: -* multiply-by-seven in detail:: - - -File: eintr, Node: Interactive multiply-by-seven, Next: multiply-by-seven in detail, Prev: Interactive, Up: Interactive - -An Interactive `multiply-by-seven', An Overview ------------------------------------------------ - -Both the use of the special form `interactive' and one way to display a -value in the echo area can be illustrated by creating an interactive -version of `multiply-by-seven'. - -Here is the code: - - (defun multiply-by-seven (number) ; Interactive version. - "Multiply NUMBER by seven." - (interactive "p") - (message "The result is %d" (* 7 number))) - -You can install this code by placing your cursor after it and typing -`C-x C-e'. The name of the function will appear in your echo area. -Then, you can use this code by typing `C-u' and a number and then -typing `M-x multiply-by-seven' and pressing <RET>. The phrase `The -result is ...' followed by the product will appear in the echo area. - -Speaking more generally, you invoke a function like this in either of -two ways: - - 1. By typing a prefix argument that contains the number to be passed, - and then typing `M-x' and the name of the function, as with `C-u 3 - M-x forward-sentence'; or, - - 2. By typing whatever key or keychord the function is bound to, as - with `C-u 3 M-e'. - -Both the examples just mentioned work identically to move point forward -three sentences. (Since `multiply-by-seven' is not bound to a key, it -could not be used as an example of key binding.) - -(*Note Some Keybindings: Keybindings, to learn how to bind a command to -a key.) - -A prefix argument is passed to an interactive function by typing the -<META> key followed by a number, for example, `M-3 M-e', or by typing -`C-u' and then a number, for example, `C-u 3 M-e' (if you type `C-u' -without a number, it defaults to 4). - - -File: eintr, Node: multiply-by-seven in detail, Prev: Interactive multiply-by-seven, Up: Interactive - -3.3.1 An Interactive `multiply-by-seven' ----------------------------------------- - -Let's look at the use of the special form `interactive' and then at the -function `message' in the interactive version of `multiply-by-seven'. -You will recall that the function definition looks like this: - - (defun multiply-by-seven (number) ; Interactive version. - "Multiply NUMBER by seven." - (interactive "p") - (message "The result is %d" (* 7 number))) - -In this function, the expression, `(interactive "p")', is a list of two -elements. The `"p"' tells Emacs to pass the prefix argument to the -function and use its value for the argument of the function. - -The argument will be a number. This means that the symbol `number' -will be bound to a number in the line: - - (message "The result is %d" (* 7 number)) - -For example, if your prefix argument is 5, the Lisp interpreter will -evaluate the line as if it were: - - (message "The result is %d" (* 7 5)) - -(If you are reading this in GNU Emacs, you can evaluate this expression -yourself.) First, the interpreter will evaluate the inner list, which -is `(* 7 5)'. This returns a value of 35. Next, it will evaluate the -outer list, passing the values of the second and subsequent elements of -the list to the function `message'. - -As we have seen, `message' is an Emacs Lisp function especially -designed for sending a one line message to a user. (*Note The -`message' function: message.) In summary, the `message' function -prints its first argument in the echo area as is, except for -occurrences of `%d' or `%s' (and various other %-sequences which we -have not mentioned). When it sees a control sequence, the function -looks to the second or subsequent arguments and prints the value of the -argument in the location in the string where the control sequence is -located. - -In the interactive `multiply-by-seven' function, the control string is -`%d', which requires a number, and the value returned by evaluating `(* -7 5)' is the number 35. Consequently, the number 35 is printed in -place of the `%d' and the message is `The result is 35'. - -(Note that when you call the function `multiply-by-seven', the message -is printed without quotes, but when you call `message', the text is -printed in double quotes. This is because the value returned by -`message' is what appears in the echo area when you evaluate an -expression whose first element is `message'; but when embedded in a -function, `message' prints the text as a side effect without quotes.) - - -File: eintr, Node: Interactive Options, Next: Permanent Installation, Prev: Interactive, Up: Writing Defuns - -3.4 Different Options for `interactive' -======================================= - -In the example, `multiply-by-seven' used `"p"' as the argument to -`interactive'. This argument told Emacs to interpret your typing -either `C-u' followed by a number or <META> followed by a number as a -command to pass that number to the function as its argument. Emacs has -more than twenty characters predefined for use with `interactive'. In -almost every case, one of these options will enable you to pass the -right information interactively to a function. (*Note Code Characters -for `interactive': (elisp)Interactive Codes.) - -Consider the function `zap-to-char'. Its interactive expression is - - (interactive "p\ncZap to char: ") - -The first part of the argument to `interactive' is `p', with which you -are already familiar. This argument tells Emacs to interpret a -`prefix', as a number to be passed to the function. You can specify a -prefix either by typing `C-u' followed by a number or by typing <META> -followed by a number. The prefix is the number of specified -characters. Thus, if your prefix is three and the specified character -is `x', then you will delete all the text up to and including the third -next `x'. If you do not set a prefix, then you delete all the text up -to and including the specified character, but no more. - -The `c' tells the function the name of the character to which to delete. - -More formally, a function with two or more arguments can have -information passed to each argument by adding parts to the string that -follows `interactive'. When you do this, the information is passed to -each argument in the same order it is specified in the `interactive' -list. In the string, each part is separated from the next part by a -`\n', which is a newline. For example, you can follow `p' with a `\n' -and an `cZap to char: '. This causes Emacs to pass the value of the -prefix argument (if there is one) and the character. - -In this case, the function definition looks like the following, where -`arg' and `char' are the symbols to which `interactive' binds the -prefix argument and the specified character: - - (defun NAME-OF-FUNCTION (arg char) - "DOCUMENTATION..." - (interactive "p\ncZap to char: ") - BODY-OF-FUNCTION...) - -(The space after the colon in the prompt makes it look better when you -are prompted. *Note The Definition of `copy-to-buffer': -copy-to-buffer, for an example.) - -When a function does not take arguments, `interactive' does not require -any. Such a function contains the simple expression `(interactive)'. -The `mark-whole-buffer' function is like this. - -Alternatively, if the special letter-codes are not right for your -application, you can pass your own arguments to `interactive' as a list. - -*Note The Definition of `append-to-buffer': append-to-buffer, for an -example. *Note Using `Interactive': (elisp)Using Interactive, for a -more complete explanation about this technique. - - -File: eintr, Node: Permanent Installation, Next: let, Prev: Interactive Options, Up: Writing Defuns - -3.5 Install Code Permanently -============================ - -When you install a function definition by evaluating it, it will stay -installed until you quit Emacs. The next time you start a new session -of Emacs, the function will not be installed unless you evaluate the -function definition again. - -At some point, you may want to have code installed automatically -whenever you start a new session of Emacs. There are several ways of -doing this: - - * If you have code that is just for yourself, you can put the code - for the function definition in your `.emacs' initialization file. - When you start Emacs, your `.emacs' file is automatically - evaluated and all the function definitions within it are installed. - *Note Your `.emacs' File: Emacs Initialization. - - * Alternatively, you can put the function definitions that you want - installed in one or more files of their own and use the `load' - function to cause Emacs to evaluate and thereby install each of the - functions in the files. *Note Loading Files: Loading Files. - - * Thirdly, if you have code that your whole site will use, it is - usual to put it in a file called `site-init.el' that is loaded when - Emacs is built. This makes the code available to everyone who uses - your machine. (See the `INSTALL' file that is part of the Emacs - distribution.) - -Finally, if you have code that everyone who uses Emacs may want, you -can post it on a computer network or send a copy to the Free Software -Foundation. (When you do this, please license the code and its -documentation under a license that permits other people to run, copy, -study, modify, and redistribute the code and which protects you from -having your work taken from you.) If you send a copy of your code to -the Free Software Foundation, and properly protect yourself and others, -it may be included in the next release of Emacs. In large part, this -is how Emacs has grown over the past years, by donations. - - -File: eintr, Node: let, Next: if, Prev: Permanent Installation, Up: Writing Defuns - -3.6 `let' -========= - -The `let' expression is a special form in Lisp that you will need to -use in most function definitions. - -`let' is used to attach or bind a symbol to a value in such a way that -the Lisp interpreter will not confuse the variable with a variable of -the same name that is not part of the function. - -To understand why the `let' special form is necessary, consider the -situation in which you own a home that you generally refer to as `the -house', as in the sentence, "The house needs painting." If you are -visiting a friend and your host refers to `the house', he is likely to -be referring to _his_ house, not yours, that is, to a different house. - -If your friend is referring to his house and you think he is referring -to your house, you may be in for some confusion. The same thing could -happen in Lisp if a variable that is used inside of one function has -the same name as a variable that is used inside of another function, -and the two are not intended to refer to the same value. The `let' -special form prevents this kind of confusion. - -* Menu: - -* Prevent confusion:: -* Parts of let Expression:: -* Sample let Expression:: -* Uninitialized let Variables:: - - -File: eintr, Node: Prevent confusion, Next: Parts of let Expression, Prev: let, Up: let - -`let' Prevents Confusion ------------------------- - -The `let' special form prevents confusion. `let' creates a name for a -"local variable" that overshadows any use of the same name outside the -`let' expression. This is like understanding that whenever your host -refers to `the house', he means his house, not yours. (Symbols used in -argument lists work the same way. *Note The `defun' Special Form: -defun.) - -Local variables created by a `let' expression retain their value _only_ -within the `let' expression itself (and within expressions called -within the `let' expression); the local variables have no effect -outside the `let' expression. - -Another way to think about `let' is that it is like a `setq' that is -temporary and local. The values set by `let' are automatically undone -when the `let' is finished. The setting only affects expressions that -are inside the bounds of the `let' expression. In computer science -jargon, we would say "the binding of a symbol is visible only in -functions called in the `let' form; in Emacs Lisp, scoping is dynamic, -not lexical." - -`let' can create more than one variable at once. Also, `let' gives -each variable it creates an initial value, either a value specified by -you, or `nil'. (In the jargon, this is called `binding the variable to -the value'.) After `let' has created and bound the variables, it -executes the code in the body of the `let', and returns the value of -the last expression in the body, as the value of the whole `let' -expression. (`Execute' is a jargon term that means to evaluate a list; -it comes from the use of the word meaning `to give practical effect to' -(`Oxford English Dictionary'). Since you evaluate an expression to -perform an action, `execute' has evolved as a synonym to `evaluate'.) - - -File: eintr, Node: Parts of let Expression, Next: Sample let Expression, Prev: Prevent confusion, Up: let - -3.6.1 The Parts of a `let' Expression -------------------------------------- - -A `let' expression is a list of three parts. The first part is the -symbol `let'. The second part is a list, called a "varlist", each -element of which is either a symbol by itself or a two-element list, -the first element of which is a symbol. The third part of the `let' -expression is the body of the `let'. The body usually consists of one -or more lists. - -A template for a `let' expression looks like this: - - (let VARLIST BODY...) - -The symbols in the varlist are the variables that are given initial -values by the `let' special form. Symbols by themselves are given the -initial value of `nil'; and each symbol that is the first element of a -two-element list is bound to the value that is returned when the Lisp -interpreter evaluates the second element. - -Thus, a varlist might look like this: `(thread (needles 3))'. In this -case, in a `let' expression, Emacs binds the symbol `thread' to an -initial value of `nil', and binds the symbol `needles' to an initial -value of 3. - -When you write a `let' expression, what you do is put the appropriate -expressions in the slots of the `let' expression template. - -If the varlist is composed of two-element lists, as is often the case, -the template for the `let' expression looks like this: - - (let ((VARIABLE VALUE) - (VARIABLE VALUE) - ...) - BODY...) - - -File: eintr, Node: Sample let Expression, Next: Uninitialized let Variables, Prev: Parts of let Expression, Up: let - -3.6.2 Sample `let' Expression ------------------------------ - -The following expression creates and gives initial values to the two -variables `zebra' and `tiger'. The body of the `let' expression is a -list which calls the `message' function. - - (let ((zebra 'stripes) - (tiger 'fierce)) - (message "One kind of animal has %s and another is %s." - zebra tiger)) - -Here, the varlist is `((zebra 'stripes) (tiger 'fierce))'. - -The two variables are `zebra' and `tiger'. Each variable is the first -element of a two-element list and each value is the second element of -its two-element list. In the varlist, Emacs binds the variable `zebra' -to the value `stripes'(1), and binds the variable `tiger' to the value -`fierce'. In this example, both values are symbols preceded by a -quote. The values could just as well have been another list or a -string. The body of the `let' follows after the list holding the -variables. In this example, the body is a list that uses the `message' -function to print a string in the echo area. - -You may evaluate the example in the usual fashion, by placing the -cursor after the last parenthesis and typing `C-x C-e'. When you do -this, the following will appear in the echo area: - - "One kind of animal has stripes and another is fierce." - -As we have seen before, the `message' function prints its first -argument, except for `%s'. In this example, the value of the variable -`zebra' is printed at the location of the first `%s' and the value of -the variable `tiger' is printed at the location of the second `%s'. - ----------- Footnotes ---------- - -(1) According to Jared Diamond in `Guns, Germs, and Steel', "... zebras -become impossibly dangerous as they grow older" but the claim here is -that they do not become fierce like a tiger. (1997, W. W. Norton and -Co., ISBN 0-393-03894-2, page 171) - - -File: eintr, Node: Uninitialized let Variables, Prev: Sample let Expression, Up: let - -3.6.3 Uninitialized Variables in a `let' Statement --------------------------------------------------- - -If you do not bind the variables in a `let' statement to specific -initial values, they will automatically be bound to an initial value of -`nil', as in the following expression: - - (let ((birch 3) - pine - fir - (oak 'some)) - (message - "Here are %d variables with %s, %s, and %s value." - birch pine fir oak)) - -Here, the varlist is `((birch 3) pine fir (oak 'some))'. - -If you evaluate this expression in the usual way, the following will -appear in your echo area: - - "Here are 3 variables with nil, nil, and some value." - -In this example, Emacs binds the symbol `birch' to the number 3, binds -the symbols `pine' and `fir' to `nil', and binds the symbol `oak' to -the value `some'. - -Note that in the first part of the `let', the variables `pine' and -`fir' stand alone as atoms that are not surrounded by parentheses; this -is because they are being bound to `nil', the empty list. But `oak' is -bound to `some' and so is a part of the list `(oak 'some)'. Similarly, -`birch' is bound to the number 3 and so is in a list with that number. -(Since a number evaluates to itself, the number does not need to be -quoted. Also, the number is printed in the message using a `%d' rather -than a `%s'.) The four variables as a group are put into a list to -delimit them from the body of the `let'. - - -File: eintr, Node: if, Next: else, Prev: let, Up: Writing Defuns - -3.7 The `if' Special Form -========================= - -A third special form, in addition to `defun' and `let', is the -conditional `if'. This form is used to instruct the computer to make -decisions. You can write function definitions without using `if', but -it is used often enough, and is important enough, to be included here. -It is used, for example, in the code for the function -`beginning-of-buffer'. - -The basic idea behind an `if', is that "_if_ a test is true, _then_ an -expression is evaluated." If the test is not true, the expression is -not evaluated. For example, you might make a decision such as, "if it -is warm and sunny, then go to the beach!" - -* Menu: - -* if in more detail:: -* type-of-animal in detail:: - - -File: eintr, Node: if in more detail, Next: type-of-animal in detail, Prev: if, Up: if - -`if' in more detail -------------------- - -An `if' expression written in Lisp does not use the word `then'; the -test and the action are the second and third elements of the list whose -first element is `if'. Nonetheless, the test part of an `if' -expression is often called the "if-part" and the second argument is -often called the "then-part". - -Also, when an `if' expression is written, the true-or-false-test is -usually written on the same line as the symbol `if', but the action to -carry out if the test is true, the "then-part", is written on the -second and subsequent lines. This makes the `if' expression easier to -read. - - (if TRUE-OR-FALSE-TEST - ACTION-TO-CARRY-OUT-IF-TEST-IS-TRUE) - -The true-or-false-test will be an expression that is evaluated by the -Lisp interpreter. - -Here is an example that you can evaluate in the usual manner. The test -is whether the number 5 is greater than the number 4. Since it is, the -message `5 is greater than 4!' will be printed. - - (if (> 5 4) ; if-part - (message "5 is greater than 4!")) ; then-part - -(The function `>' tests whether its first argument is greater than its -second argument and returns true if it is.) - -Of course, in actual use, the test in an `if' expression will not be -fixed for all time as it is by the expression `(> 5 4)'. Instead, at -least one of the variables used in the test will be bound to a value -that is not known ahead of time. (If the value were known ahead of -time, we would not need to run the test!) - -For example, the value may be bound to an argument of a function -definition. In the following function definition, the character of the -animal is a value that is passed to the function. If the value bound to -`characteristic' is `fierce', then the message, `It's a tiger!' will be -printed; otherwise, `nil' will be returned. - - (defun type-of-animal (characteristic) - "Print message in echo area depending on CHARACTERISTIC. - If the CHARACTERISTIC is the symbol `fierce', - then warn of a tiger." - (if (equal characteristic 'fierce) - (message "It's a tiger!"))) - -If you are reading this inside of GNU Emacs, you can evaluate the -function definition in the usual way to install it in Emacs, and then -you can evaluate the following two expressions to see the results: - - (type-of-animal 'fierce) - - (type-of-animal 'zebra) - -When you evaluate `(type-of-animal 'fierce)', you will see the -following message printed in the echo area: `"It's a tiger!"'; and when -you evaluate `(type-of-animal 'zebra)' you will see `nil' printed in -the echo area. - - -File: eintr, Node: type-of-animal in detail, Prev: if in more detail, Up: if - -3.7.1 The `type-of-animal' Function in Detail ---------------------------------------------- - -Let's look at the `type-of-animal' function in detail. - -The function definition for `type-of-animal' was written by filling the -slots of two templates, one for a function definition as a whole, and a -second for an `if' expression. - -The template for every function that is not interactive is: - - (defun NAME-OF-FUNCTION (ARGUMENT-LIST) - "DOCUMENTATION..." - BODY...) - -The parts of the function that match this template look like this: - - (defun type-of-animal (characteristic) - "Print message in echo area depending on CHARACTERISTIC. - If the CHARACTERISTIC is the symbol `fierce', - then warn of a tiger." - BODY: THE `if' EXPRESSION) - -The name of function is `type-of-animal'; it is passed the value of one -argument. The argument list is followed by a multi-line documentation -string. The documentation string is included in the example because it -is a good habit to write documentation string for every function -definition. The body of the function definition consists of the `if' -expression. - -The template for an `if' expression looks like this: - - (if TRUE-OR-FALSE-TEST - ACTION-TO-CARRY-OUT-IF-THE-TEST-RETURNS-TRUE) - -In the `type-of-animal' function, the code for the `if' looks like this: - - (if (equal characteristic 'fierce) - (message "It's a tiger!"))) - -Here, the true-or-false-test is the expression: - - (equal characteristic 'fierce) - -In Lisp, `equal' is a function that determines whether its first -argument is equal to its second argument. The second argument is the -quoted symbol `'fierce' and the first argument is the value of the -symbol `characteristic'--in other words, the argument passed to this -function. - -In the first exercise of `type-of-animal', the argument `fierce' is -passed to `type-of-animal'. Since `fierce' is equal to `fierce', the -expression, `(equal characteristic 'fierce)', returns a value of true. -When this happens, the `if' evaluates the second argument or then-part -of the `if': `(message "It's tiger!")'. - -On the other hand, in the second exercise of `type-of-animal', the -argument `zebra' is passed to `type-of-animal'. `zebra' is not equal -to `fierce', so the then-part is not evaluated and `nil' is returned by -the `if' expression. - - -File: eintr, Node: else, Next: Truth & Falsehood, Prev: if, Up: Writing Defuns - -3.8 If-then-else Expressions -============================ - -An `if' expression may have an optional third argument, called the -"else-part", for the case when the true-or-false-test returns false. -When this happens, the second argument or then-part of the overall `if' -expression is _not_ evaluated, but the third or else-part _is_ -evaluated. You might think of this as the cloudy day alternative for -the decision "if it is warm and sunny, then go to the beach, else read -a book!". - -The word "else" is not written in the Lisp code; the else-part of an -`if' expression comes after the then-part. In the written Lisp, the -else-part is usually written to start on a line of its own and is -indented less than the then-part: - - (if TRUE-OR-FALSE-TEST - ACTION-TO-CARRY-OUT-IF-THE-TEST-RETURNS-TRUE - ACTION-TO-CARRY-OUT-IF-THE-TEST-RETURNS-FALSE) - -For example, the following `if' expression prints the message `4 is not -greater than 5!' when you evaluate it in the usual way: - - (if (> 4 5) ; if-part - (message "5 is greater than 4!") ; then-part - (message "4 is not greater than 5!")) ; else-part - -Note that the different levels of indentation make it easy to -distinguish the then-part from the else-part. (GNU Emacs has several -commands that automatically indent `if' expressions correctly. *Note -GNU Emacs Helps You Type Lists: Typing Lists.) - -We can extend the `type-of-animal' function to include an else-part by -simply incorporating an additional part to the `if' expression. - -You can see the consequences of doing this if you evaluate the following -version of the `type-of-animal' function definition to install it and -then evaluate the two subsequent expressions to pass different -arguments to the function. - - (defun type-of-animal (characteristic) ; Second version. - "Print message in echo area depending on CHARACTERISTIC. - If the CHARACTERISTIC is the symbol `fierce', - then warn of a tiger; - else say it's not fierce." - (if (equal characteristic 'fierce) - (message "It's a tiger!") - (message "It's not fierce!"))) - - - (type-of-animal 'fierce) - - (type-of-animal 'zebra) - -When you evaluate `(type-of-animal 'fierce)', you will see the -following message printed in the echo area: `"It's a tiger!"'; but when -you evaluate `(type-of-animal 'zebra)', you will see `"It's not -fierce!"'. - -(Of course, if the CHARACTERISTIC were `ferocious', the message `"It's -not fierce!"' would be printed; and it would be misleading! When you -write code, you need to take into account the possibility that some -such argument will be tested by the `if' and write your program -accordingly.) - - -File: eintr, Node: Truth & Falsehood, Next: save-excursion, Prev: else, Up: Writing Defuns - -3.9 Truth and Falsehood in Emacs Lisp -===================================== - -There is an important aspect to the truth test in an `if' expression. -So far, we have spoken of `true' and `false' as values of predicates as -if they were new kinds of Emacs Lisp objects. In fact, `false' is just -our old friend `nil'. Anything else--anything at all--is `true'. - -The expression that tests for truth is interpreted as "true" if the -result of evaluating it is a value that is not `nil'. In other words, -the result of the test is considered true if the value returned is a -number such as 47, a string such as `"hello"', or a symbol (other than -`nil') such as `flowers', or a list (so long as it is not empty), or -even a buffer! - -* Menu: - -* nil explained:: - - -File: eintr, Node: nil explained, Prev: Truth & Falsehood, Up: Truth & Falsehood - -An explanation of `nil' ------------------------ - -Before illustrating a test for truth, we need an explanation of `nil'. - -In Emacs Lisp, the symbol `nil' has two meanings. First, it means the -empty list. Second, it means false and is the value returned when a -true-or-false-test tests false. `nil' can be written as an empty list, -`()', or as `nil'. As far as the Lisp interpreter is concerned, `()' -and `nil' are the same. Humans, however, tend to use `nil' for false -and `()' for the empty list. - -In Emacs Lisp, any value that is not `nil'--is not the empty list--is -considered true. This means that if an evaluation returns something -that is not an empty list, an `if' expression will test true. For -example, if a number is put in the slot for the test, it will be -evaluated and will return itself, since that is what numbers do when -evaluated. In this conditional, the `if' expression will test true. -The expression tests false only when `nil', an empty list, is returned -by evaluating the expression. - -You can see this by evaluating the two expressions in the following -examples. - -In the first example, the number 4 is evaluated as the test in the `if' -expression and returns itself; consequently, the then-part of the -expression is evaluated and returned: `true' appears in the echo area. -In the second example, the `nil' indicates false; consequently, the -else-part of the expression is evaluated and returned: `false' appears -in the echo area. - - (if 4 - 'true - 'false) - - (if nil - 'true - 'false) - -Incidentally, if some other useful value is not available for a test -that returns true, then the Lisp interpreter will return the symbol `t' -for true. For example, the expression `(> 5 4)' returns `t' when -evaluated, as you can see by evaluating it in the usual way: - - (> 5 4) - -On the other hand, this function returns `nil' if the test is false. - - (> 4 5) - - -File: eintr, Node: save-excursion, Next: Review, Prev: Truth & Falsehood, Up: Writing Defuns - -3.10 `save-excursion' -===================== - -The `save-excursion' function is the fourth and final special form that -we will discuss in this chapter. - -In Emacs Lisp programs used for editing, the `save-excursion' function -is very common. It saves the location of point and mark, executes the -body of the function, and then restores point and mark to their -previous positions if their locations were changed. Its primary -purpose is to keep the user from being surprised and disturbed by -unexpected movement of point or mark. - -* Menu: - -* Point and mark:: -* Template for save-excursion:: - - -File: eintr, Node: Point and mark, Next: Template for save-excursion, Prev: save-excursion, Up: save-excursion - -Point and Mark --------------- - -Before discussing `save-excursion', however, it may be useful first to -review what point and mark are in GNU Emacs. "Point" is the current -location of the cursor. Wherever the cursor is, that is point. More -precisely, on terminals where the cursor appears to be on top of a -character, point is immediately before the character. In Emacs Lisp, -point is an integer. The first character in a buffer is number one, -the second is number two, and so on. The function `point' returns the -current position of the cursor as a number. Each buffer has its own -value for point. - -The "mark" is another position in the buffer; its value can be set with -a command such as `C-<SPC>' (`set-mark-command'). If a mark has been -set, you can use the command `C-x C-x' (`exchange-point-and-mark') to -cause the cursor to jump to the mark and set the mark to be the -previous position of point. In addition, if you set another mark, the -position of the previous mark is saved in the mark ring. Many mark -positions can be saved this way. You can jump the cursor to a saved -mark by typing `C-u C-<SPC>' one or more times. - -The part of the buffer between point and mark is called "the region". -Numerous commands work on the region, including `center-region', -`count-lines-region', `kill-region', and `print-region'. - -The `save-excursion' special form saves the locations of point and mark -and restores those positions after the code within the body of the -special form is evaluated by the Lisp interpreter. Thus, if point were -in the beginning of a piece of text and some code moved point to the end -of the buffer, the `save-excursion' would put point back to where it -was before, after the expressions in the body of the function were -evaluated. - -In Emacs, a function frequently moves point as part of its internal -workings even though a user would not expect this. For example, -`count-lines-region' moves point. To prevent the user from being -bothered by jumps that are both unexpected and (from the user's point of -view) unnecessary, `save-excursion' is often used to keep point and -mark in the location expected by the user. The use of `save-excursion' -is good housekeeping. - -To make sure the house stays clean, `save-excursion' restores the -values of point and mark even if something goes wrong in the code inside -of it (or, to be more precise and to use the proper jargon, "in case of -abnormal exit"). This feature is very helpful. - -In addition to recording the values of point and mark, `save-excursion' -keeps track of the current buffer, and restores it, too. This means -you can write code that will change the buffer and have -`save-excursion' switch you back to the original buffer. This is how -`save-excursion' is used in `append-to-buffer'. (*Note The Definition -of `append-to-buffer': append-to-buffer.) - - -File: eintr, Node: Template for save-excursion, Prev: Point and mark, Up: save-excursion - -3.10.1 Template for a `save-excursion' Expression -------------------------------------------------- - -The template for code using `save-excursion' is simple: - - (save-excursion - BODY...) - -The body of the function is one or more expressions that will be -evaluated in sequence by the Lisp interpreter. If there is more than -one expression in the body, the value of the last one will be returned -as the value of the `save-excursion' function. The other expressions -in the body are evaluated only for their side effects; and -`save-excursion' itself is used only for its side effect (which is -restoring the positions of point and mark). - -In more detail, the template for a `save-excursion' expression looks -like this: - - (save-excursion - FIRST-EXPRESSION-IN-BODY - SECOND-EXPRESSION-IN-BODY - THIRD-EXPRESSION-IN-BODY - ... - LAST-EXPRESSION-IN-BODY) - -An expression, of course, may be a symbol on its own or a list. - -In Emacs Lisp code, a `save-excursion' expression often occurs within -the body of a `let' expression. It looks like this: - - (let VARLIST - (save-excursion - BODY...)) - - -File: eintr, Node: Review, Next: defun Exercises, Prev: save-excursion, Up: Writing Defuns - -3.11 Review -=========== - -In the last few chapters we have introduced a fair number of functions -and special forms. Here they are described in brief, along with a few -similar functions that have not been mentioned yet. - -`eval-last-sexp' - Evaluate the last symbolic expression before the current location - of point. The value is printed in the echo area unless the - function is invoked with an argument; in that case, the output is - printed in the current buffer. This command is normally bound to - `C-x C-e'. - -`defun' - Define function. This special form has up to five parts: the name, - a template for the arguments that will be passed to the function, - documentation, an optional interactive declaration, and the body - of the definition. - - For example, in an early version of Emacs, the function definition - was as follows. (It is slightly more complex now that it seeks - the first non-whitespace character rather than the first visible - character.) - - (defun back-to-indentation () - "Move point to first visible character on line." - (interactive) - (beginning-of-line 1) - (skip-chars-forward " \t")) - -`interactive' - Declare to the interpreter that the function can be used - interactively. This special form may be followed by a string with - one or more parts that pass the information to the arguments of the - function, in sequence. These parts may also tell the interpreter - to prompt for information. Parts of the string are separated by - newlines, `\n'. - - Common code characters are: - - `b' - The name of an existing buffer. - - `f' - The name of an existing file. - - `p' - The numeric prefix argument. (Note that this `p' is lower - case.) - - `r' - Point and the mark, as two numeric arguments, smallest first. - This is the only code letter that specifies two successive - arguments rather than one. - - *Note Code Characters for `interactive': (elisp)Interactive Codes, - for a complete list of code characters. - -`let' - Declare that a list of variables is for use within the body of the - `let' and give them an initial value, either `nil' or a specified - value; then evaluate the rest of the expressions in the body of - the `let' and return the value of the last one. Inside the body - of the `let', the Lisp interpreter does not see the values of the - variables of the same names that are bound outside of the `let'. - - For example, - - (let ((foo (buffer-name)) - (bar (buffer-size))) - (message - "This buffer is %s and has %d characters." - foo bar)) - -`save-excursion' - Record the values of point and mark and the current buffer before - evaluating the body of this special form. Restore the values of - point and mark and buffer afterward. - - For example, - - (message "We are %d characters into this buffer." - (- (point) - (save-excursion - (goto-char (point-min)) (point)))) - -`if' - Evaluate the first argument to the function; if it is true, - evaluate the second argument; else evaluate the third argument, if - there is one. - - The `if' special form is called a "conditional". There are other - conditionals in Emacs Lisp, but `if' is perhaps the most commonly - used. - - For example, - - (if (string-equal - (number-to-string 22) - (substring (emacs-version) 10 12)) - (message "This is version 22 Emacs") - (message "This is not version 22 Emacs")) - -`equal' -`eq' - Test whether two objects are the same. `equal' uses one meaning - of the word `same' and `eq' uses another: `equal' returns true if - the two objects have a similar structure and contents, such as two - copies of the same book. On the other hand, `eq', returns true if - both arguments are actually the same object. - -`<' -`>' -`<=' -`>=' - The `<' function tests whether its first argument is smaller than - its second argument. A corresponding function, `>', tests whether - the first argument is greater than the second. Likewise, `<=' - tests whether the first argument is less than or equal to the - second and `>=' tests whether the first argument is greater than - or equal to the second. In all cases, both arguments must be - numbers or markers (markers indicate positions in buffers). - -`=' - The `=' function tests whether two arguments, , both numbers or - markers, are equal. - -`string<' -`string-lessp' -`string=' -`string-equal' - The `string-lessp' function tests whether its first argument is - smaller than the second argument. A shorter, alternative name for - the same function (a `defalias') is `string<'. - - The arguments to `string-lessp' must be strings or symbols; the - ordering is lexicographic, so case is significant. The print - names of symbols are used instead of the symbols themselves. - - An empty string, `""', a string with no characters in it, is - smaller than any string of characters. - - `string-equal' provides the corresponding test for equality. Its - shorter, alternative name is `string='. There are no string test - functions that correspond to >, `>=', or `<='. - -`message' - Print a message in the echo area. The first argument is a string - that can contain `%s', `%d', or `%c' to print the value of - arguments that follow the string. The argument used by `%s' must - be a string or a symbol; the argument used by `%d' must be a - number. The argument used by `%c' must be an ASCII code number; - it will be printed as the character with that ASCII code. - (Various other %-sequences have not been mentioned.) - -`setq' -`set' - The `setq' function sets the value of its first argument to the - value of the second argument. The first argument is automatically - quoted by `setq'. It does the same for succeeding pairs of - arguments. Another function, `set', takes only two arguments and - evaluates both of them before setting the value returned by its - first argument to the value returned by its second argument. - -`buffer-name' - Without an argument, return the name of the buffer, as a string. - -`buffer-file-name' - Without an argument, return the name of the file the buffer is - visiting. - -`current-buffer' - Return the buffer in which Emacs is active; it may not be the - buffer that is visible on the screen. - -`other-buffer' - Return the most recently selected buffer (other than the buffer - passed to `other-buffer' as an argument and other than the current - buffer). - -`switch-to-buffer' - Select a buffer for Emacs to be active in and display it in the - current window so users can look at it. Usually bound to `C-x b'. - -`set-buffer' - Switch Emacs' attention to a buffer on which programs will run. - Don't alter what the window is showing. - -`buffer-size' - Return the number of characters in the current buffer. - -`point' - Return the value of the current position of the cursor, as an - integer counting the number of characters from the beginning of the - buffer. - -`point-min' - Return the minimum permissible value of point in the current - buffer. This is 1, unless narrowing is in effect. - -`point-max' - Return the value of the maximum permissible value of point in the - current buffer. This is the end of the buffer, unless narrowing - is in effect. - - -File: eintr, Node: defun Exercises, Prev: Review, Up: Writing Defuns - -3.12 Exercises -============== - - * Write a non-interactive function that doubles the value of its - argument, a number. Make that function interactive. - - * Write a function that tests whether the current value of - `fill-column' is greater than the argument passed to the function, - and if so, prints an appropriate message. - - -File: eintr, Node: Buffer Walk Through, Next: More Complex, Prev: Writing Defuns, Up: Top - -4 A Few Buffer-Related Functions -******************************** - -In this chapter we study in detail several of the functions used in GNU -Emacs. This is called a "walk-through". These functions are used as -examples of Lisp code, but are not imaginary examples; with the -exception of the first, simplified function definition, these functions -show the actual code used in GNU Emacs. You can learn a great deal from -these definitions. The functions described here are all related to -buffers. Later, we will study other functions. - -* Menu: - -* Finding More:: -* simplified-beginning-of-buffer:: -* mark-whole-buffer:: -* append-to-buffer:: -* Buffer Related Review:: -* Buffer Exercises:: - - -File: eintr, Node: Finding More, Next: simplified-beginning-of-buffer, Prev: Buffer Walk Through, Up: Buffer Walk Through - -4.1 Finding More Information -============================ - -In this walk-through, I will describe each new function as we come to -it, sometimes in detail and sometimes briefly. If you are interested, -you can get the full documentation of any Emacs Lisp function at any -time by typing `C-h f' and then the name of the function (and then -<RET>). Similarly, you can get the full documentation for a variable -by typing `C-h v' and then the name of the variable (and then <RET>). - -When a function is written in Emacs Lisp, `describe-function' will also -tell you the location of the function definition. - -Put point into the name of the file that contains the function and -press the <RET> key. In this case, <RET> means `push-button' rather -than `return' or `enter'. Emacs will take you directly to the function -definition. - -More generally, if you want to see a function in its original source -file, you can use the `find-tags' function to jump to it. `find-tags' -works with a wide variety of languages, not just Lisp, and C, and it -works with non-programming text as well. For example, `find-tags' will -jump to the various nodes in the Texinfo source file of this document. - -The `find-tags' function depends on `tags tables' that record the -locations of the functions, variables, and other items to which -`find-tags' jumps. - -To use the `find-tags' command, type `M-.' (i.e., press the period key -while holding down the <META> key, or else type the <ESC> key and then -type the period key), and then, at the prompt, type in the name of the -function whose source code you want to see, such as -`mark-whole-buffer', and then type <RET>. Emacs will switch buffers -and display the source code for the function on your screen. To switch -back to your current buffer, type `C-x b <RET>'. (On some keyboards, -the <META> key is labelled <ALT>.) - -Depending on how the initial default values of your copy of Emacs are -set, you may also need to specify the location of your `tags table', -which is a file called `TAGS'. For example, if you are interested in -Emacs sources, the tags table you will most likely want, if it has -already been created for you, will be in a subdirectory of the -`/usr/local/share/emacs/' directory; thus you would use the `M-x -visit-tags-table' command and specify a pathname such as -`/usr/local/share/emacs/22.0.100/lisp/TAGS'. If the tags table has not -already been created, you will have to create it yourself. It will in -a file such as `/usr/local/src/emacs/src/TAGS'. - -To create a `TAGS' file in a specific directory, switch to that -directory in Emacs using `M-x cd' command, or list the directory with -`C-x d' (`dired'). Then run the compile command, with `etags *.el' as -the command to execute: - - M-x compile RET etags *.el RET - -For more information, see *Note Create Your Own `TAGS' File: etags. - -After you become more familiar with Emacs Lisp, you will find that you -will frequently use `find-tags' to navigate your way around source code; -and you will create your own `TAGS' tables. - -Incidentally, the files that contain Lisp code are conventionally -called "libraries". The metaphor is derived from that of a specialized -library, such as a law library or an engineering library, rather than a -general library. Each library, or file, contains functions that relate -to a particular topic or activity, such as `abbrev.el' for handling -abbreviations and other typing shortcuts, and `help.el' for on-line -help. (Sometimes several libraries provide code for a single activity, -as the various `rmail...' files provide code for reading electronic -mail.) In `The GNU Emacs Manual', you will see sentences such as "The -`C-h p' command lets you search the standard Emacs Lisp libraries by -topic keywords." - - -File: eintr, Node: simplified-beginning-of-buffer, Next: mark-whole-buffer, Prev: Finding More, Up: Buffer Walk Through - -4.2 A Simplified `beginning-of-buffer' Definition -================================================= - -The `beginning-of-buffer' command is a good function to start with -since you are likely to be familiar with it and it is easy to -understand. Used as an interactive command, `beginning-of-buffer' -moves the cursor to the beginning of the buffer, leaving the mark at the -previous position. It is generally bound to `M-<'. - -In this section, we will discuss a shortened version of the function -that shows how it is most frequently used. This shortened function -works as written, but it does not contain the code for a complex option. -In another section, we will describe the entire function. (*Note -Complete Definition of `beginning-of-buffer': beginning-of-buffer.) - -Before looking at the code, let's consider what the function definition -has to contain: it must include an expression that makes the function -interactive so it can be called by typing `M-x beginning-of-buffer' or -by typing a keychord such as `M-<'; it must include code to leave a -mark at the original position in the buffer; and it must include code -to move the cursor to the beginning of the buffer. - -Here is the complete text of the shortened version of the function: - - (defun simplified-beginning-of-buffer () - "Move point to the beginning of the buffer; - leave mark at previous position." - (interactive) - (push-mark) - (goto-char (point-min))) - -Like all function definitions, this definition has five parts following -the special form `defun': - - 1. The name: in this example, `simplified-beginning-of-buffer'. - - 2. A list of the arguments: in this example, an empty list, `()', - - 3. The documentation string. - - 4. The interactive expression. - - 5. The body. - -In this function definition, the argument list is empty; this means that -this function does not require any arguments. (When we look at the -definition for the complete function, we will see that it may be passed -an optional argument.) - -The interactive expression tells Emacs that the function is intended to -be used interactively. In this example, `interactive' does not have an -argument because `simplified-beginning-of-buffer' does not require one. - -The body of the function consists of the two lines: - - (push-mark) - (goto-char (point-min)) - -The first of these lines is the expression, `(push-mark)'. When this -expression is evaluated by the Lisp interpreter, it sets a mark at the -current position of the cursor, wherever that may be. The position of -this mark is saved in the mark ring. - -The next line is `(goto-char (point-min))'. This expression jumps the -cursor to the minimum point in the buffer, that is, to the beginning of -the buffer (or to the beginning of the accessible portion of the buffer -if it is narrowed. *Note Narrowing and Widening: Narrowing & Widening.) - -The `push-mark' command sets a mark at the place where the cursor was -located before it was moved to the beginning of the buffer by the -`(goto-char (point-min))' expression. Consequently, you can, if you -wish, go back to where you were originally by typing `C-x C-x'. - -That is all there is to the function definition! - -When you are reading code such as this and come upon an unfamiliar -function, such as `goto-char', you can find out what it does by using -the `describe-function' command. To use this command, type `C-h f' and -then type in the name of the function and press <RET>. The -`describe-function' command will print the function's documentation -string in a `*Help*' window. For example, the documentation for -`goto-char' is: - - Set point to POSITION, a number or marker. - Beginning of buffer is position (point-min), end is (point-max). - -The function's one argument is the desired position. - -(The prompt for `describe-function' will offer you the symbol under or -preceding the cursor, so you can save typing by positioning the cursor -right over or after the function and then typing `C-h f <RET>'.) - -The `end-of-buffer' function definition is written in the same way as -the `beginning-of-buffer' definition except that the body of the -function contains the expression `(goto-char (point-max))' in place of -`(goto-char (point-min))'. - - -File: eintr, Node: mark-whole-buffer, Next: append-to-buffer, Prev: simplified-beginning-of-buffer, Up: Buffer Walk Through - -4.3 The Definition of `mark-whole-buffer' -========================================= - -The `mark-whole-buffer' function is no harder to understand than the -`simplified-beginning-of-buffer' function. In this case, however, we -will look at the complete function, not a shortened version. - -The `mark-whole-buffer' function is not as commonly used as the -`beginning-of-buffer' function, but is useful nonetheless: it marks a -whole buffer as a region by putting point at the beginning and a mark -at the end of the buffer. It is generally bound to `C-x h'. - -* Menu: - -* mark-whole-buffer overview:: -* Body of mark-whole-buffer:: - - -File: eintr, Node: mark-whole-buffer overview, Next: Body of mark-whole-buffer, Prev: mark-whole-buffer, Up: mark-whole-buffer - -An overview of `mark-whole-buffer' ----------------------------------- - -In GNU Emacs 22, the code for the complete function looks like this: - - (defun mark-whole-buffer () - "Put point at beginning and mark at end of buffer. - You probably should not use this function in Lisp programs; - it is usually a mistake for a Lisp function to use any subroutine - that uses or sets the mark." - (interactive) - (push-mark (point)) - (push-mark (point-max) nil t) - (goto-char (point-min))) - -Like all other functions, the `mark-whole-buffer' function fits into -the template for a function definition. The template looks like this: - - (defun NAME-OF-FUNCTION (ARGUMENT-LIST) - "DOCUMENTATION..." - (INTERACTIVE-EXPRESSION...) - BODY...) - -Here is how the function works: the name of the function is -`mark-whole-buffer'; it is followed by an empty argument list, `()', -which means that the function does not require arguments. The -documentation comes next. - -The next line is an `(interactive)' expression that tells Emacs that -the function will be used interactively. These details are similar to -the `simplified-beginning-of-buffer' function described in the previous -section. - - -File: eintr, Node: Body of mark-whole-buffer, Prev: mark-whole-buffer overview, Up: mark-whole-buffer - -4.3.1 Body of `mark-whole-buffer' ---------------------------------- - -The body of the `mark-whole-buffer' function consists of three lines of -code: - - (push-mark (point)) - (push-mark (point-max) nil t) - (goto-char (point-min)) - -The first of these lines is the expression, `(push-mark (point))'. - -This line does exactly the same job as the first line of the body of -the `simplified-beginning-of-buffer' function, which is written -`(push-mark)'. In both cases, the Lisp interpreter sets a mark at the -current position of the cursor. - -I don't know why the expression in `mark-whole-buffer' is written -`(push-mark (point))' and the expression in `beginning-of-buffer' is -written `(push-mark)'. Perhaps whoever wrote the code did not know -that the arguments for `push-mark' are optional and that if `push-mark' -is not passed an argument, the function automatically sets mark at the -location of point by default. Or perhaps the expression was written so -as to parallel the structure of the next line. In any case, the line -causes Emacs to determine the position of point and set a mark there. - -In earlier versions of GNU Emacs, the next line of `mark-whole-buffer' -was `(push-mark (point-max))'. This expression sets a mark at the -point in the buffer that has the highest number. This will be the end -of the buffer (or, if the buffer is narrowed, the end of the accessible -portion of the buffer. *Note Narrowing and Widening: Narrowing & -Widening, for more about narrowing.) After this mark has been set, the -previous mark, the one set at point, is no longer set, but Emacs -remembers its position, just as all other recent marks are always -remembered. This means that you can, if you wish, go back to that -position by typing `C-u C-<SPC>' twice. - -In GNU Emacs 22, the `(point-max)' is slightly more complicated. The -line reads - - (push-mark (point-max) nil t) - -The expression works nearly the same as before. It sets a mark at the -highest numbered place in the buffer that it can. However, in this -version, `push-mark' has two additional arguments. The second argument -to `push-mark' is `nil'. This tells the function it _should_ display a -message that says `Mark set' when it pushes the mark. The third -argument is `t'. This tells `push-mark' to activate the mark when -Transient Mark mode is turned on. Transient Mark mode highlights the -currently active region. It is often turned off. - -Finally, the last line of the function is `(goto-char (point-min)))'. -This is written exactly the same way as it is written in -`beginning-of-buffer'. The expression moves the cursor to the minimum -point in the buffer, that is, to the beginning of the buffer (or to the -beginning of the accessible portion of the buffer). As a result of -this, point is placed at the beginning of the buffer and mark is set at -the end of the buffer. The whole buffer is, therefore, the region. - - -File: eintr, Node: append-to-buffer, Next: Buffer Related Review, Prev: mark-whole-buffer, Up: Buffer Walk Through - -4.4 The Definition of `append-to-buffer' -======================================== - -The `append-to-buffer' command is more complex than the -`mark-whole-buffer' command. What it does is copy the region (that is, -the part of the buffer between point and mark) from the current buffer -to a specified buffer. - -* Menu: - -* append-to-buffer overview:: -* append interactive:: -* append-to-buffer body:: -* append save-excursion:: - - -File: eintr, Node: append-to-buffer overview, Next: append interactive, Prev: append-to-buffer, Up: append-to-buffer - -An Overview of `append-to-buffer' ---------------------------------- - -The `append-to-buffer' command uses the `insert-buffer-substring' -function to copy the region. `insert-buffer-substring' is described by -its name: it takes a string of characters from part of a buffer, a -"substring", and inserts them into another buffer. - -Most of `append-to-buffer' is concerned with setting up the conditions -for `insert-buffer-substring' to work: the code must specify both the -buffer to which the text will go, the window it comes from and goes to, -and the region that will be copied. - -Here is the complete text of the function: - - (defun append-to-buffer (buffer start end) - "Append to specified buffer the text of the region. - It is inserted into that buffer before its point. - - When calling from a program, give three arguments: - BUFFER (or buffer name), START and END. - START and END specify the portion of the current buffer to be copied." - (interactive - (list (read-buffer "Append to buffer: " (other-buffer - (current-buffer) t)) - (region-beginning) (region-end))) - (let ((oldbuf (current-buffer))) - (save-excursion - (let* ((append-to (get-buffer-create buffer)) - (windows (get-buffer-window-list append-to t t)) - point) - (set-buffer append-to) - (setq point (point)) - (barf-if-buffer-read-only) - (insert-buffer-substring oldbuf start end) - (dolist (window windows) - (when (= (window-point window) point) - (set-window-point window (point)))))))) - -The function can be understood by looking at it as a series of -filled-in templates. - -The outermost template is for the function definition. In this -function, it looks like this (with several slots filled in): - - (defun append-to-buffer (buffer start end) - "DOCUMENTATION..." - (interactive ...) - BODY...) - -The first line of the function includes its name and three arguments. -The arguments are the `buffer' to which the text will be copied, and -the `start' and `end' of the region in the current buffer that will be -copied. - -The next part of the function is the documentation, which is clear and -complete. As is conventional, the three arguments are written in upper -case so you will notice them easily. Even better, they are described -in the same order as in the argument list. - -Note that the documentation distinguishes between a buffer and its -name. (The function can handle either.) - - -File: eintr, Node: append interactive, Next: append-to-buffer body, Prev: append-to-buffer overview, Up: append-to-buffer - -4.4.1 The `append-to-buffer' Interactive Expression ---------------------------------------------------- - -Since the `append-to-buffer' function will be used interactively, the -function must have an `interactive' expression. (For a review of -`interactive', see *Note Making a Function Interactive: Interactive.) -The expression reads as follows: - - (interactive - (list (read-buffer - "Append to buffer: " - (other-buffer (current-buffer) t)) - (region-beginning) - (region-end))) - -This expression is not one with letters standing for parts, as -described earlier. Instead, it starts a list with thee parts. - -The first part of the list is an expression to read the name of a -buffer and return it as a string. That is `read-buffer'. The function -requires a prompt as its first argument, `"Append to buffer: "'. Its -second argument tells the command what value to provide if you don't -specify anything. - -In this case that second argument is an expression containing the -function `other-buffer', an exception, and a `t', standing for true. - -The first argument to `other-buffer', the exception, is yet another -function, `current-buffer'. That is not going to be returned. The -second argument is the symbol for true, `t'. that tells `other-buffer' -that it may show visible buffers (except in this case, it will not show -the current buffer, which makes sense). - -The expression looks like this: - - (other-buffer (current-buffer) t) - -The second and third arguments to the `list' expression are -`(region-beginning)' and `(region-end)'. These two functions specify -the beginning and end of the text to be appended. - -Originally, the command used the letters `B' and `r'. The whole -`interactive' expression looked like this: - - (interactive "BAppend to buffer: \nr") - -But when that was done, the default value of the buffer switched to was -invisible. That was not wanted. - -(The prompt was separated from the second argument with a newline, -`\n'. It was followed by an `r' that told Emacs to bind the two -arguments that follow the symbol `buffer' in the function's argument -list (that is, `start' and `end') to the values of point and mark. -That argument worked fine.) - - -File: eintr, Node: append-to-buffer body, Next: append save-excursion, Prev: append interactive, Up: append-to-buffer - -4.4.2 The Body of `append-to-buffer' ------------------------------------- - -The body of the `append-to-buffer' function begins with `let'. - -As we have seen before (*note `let': let.), the purpose of a `let' -expression is to create and give initial values to one or more -variables that will only be used within the body of the `let'. This -means that such a variable will not be confused with any variable of -the same name outside the `let' expression. - -We can see how the `let' expression fits into the function as a whole -by showing a template for `append-to-buffer' with the `let' expression -in outline: - - (defun append-to-buffer (buffer start end) - "DOCUMENTATION..." - (interactive ...) - (let ((VARIABLE VALUE)) - BODY...) - -The `let' expression has three elements: - - 1. The symbol `let'; - - 2. A varlist containing, in this case, a single two-element list, - `(VARIABLE VALUE)'; - - 3. The body of the `let' expression. - -In the `append-to-buffer' function, the varlist looks like this: - - (oldbuf (current-buffer)) - -In this part of the `let' expression, the one variable, `oldbuf', is -bound to the value returned by the `(current-buffer)' expression. The -variable, `oldbuf', is used to keep track of the buffer in which you -are working and from which you will copy. - -The element or elements of a varlist are surrounded by a set of -parentheses so the Lisp interpreter can distinguish the varlist from -the body of the `let'. As a consequence, the two-element list within -the varlist is surrounded by a circumscribing set of parentheses. The -line looks like this: - - (let ((oldbuf (current-buffer))) - ... ) - -The two parentheses before `oldbuf' might surprise you if you did not -realize that the first parenthesis before `oldbuf' marks the boundary -of the varlist and the second parenthesis marks the beginning of the -two-element list, `(oldbuf (current-buffer))'. - - -File: eintr, Node: append save-excursion, Prev: append-to-buffer body, Up: append-to-buffer - -4.4.3 `save-excursion' in `append-to-buffer' --------------------------------------------- - -The body of the `let' expression in `append-to-buffer' consists of a -`save-excursion' expression. - -The `save-excursion' function saves the locations of point and mark, -and restores them to those positions after the expressions in the body -of the `save-excursion' complete execution. In addition, -`save-excursion' keeps track of the original buffer, and restores it. -This is how `save-excursion' is used in `append-to-buffer'. - -Incidentally, it is worth noting here that a Lisp function is normally -formatted so that everything that is enclosed in a multi-line spread is -indented more to the right than the first symbol. In this function -definition, the `let' is indented more than the `defun', and the -`save-excursion' is indented more than the `let', like this: - - (defun ... - ... - ... - (let... - (save-excursion - ... - -This formatting convention makes it easy to see that the lines in the -body of the `save-excursion' are enclosed by the parentheses associated -with `save-excursion', just as the `save-excursion' itself is enclosed -by the parentheses associated with the `let': - - (let ((oldbuf (current-buffer))) - (save-excursion - ... - (set-buffer ...) - (insert-buffer-substring oldbuf start end) - ...)) - -The use of the `save-excursion' function can be viewed as a process of -filling in the slots of a template: - - (save-excursion - FIRST-EXPRESSION-IN-BODY - SECOND-EXPRESSION-IN-BODY - ... - LAST-EXPRESSION-IN-BODY) - -In this function, the body of the `save-excursion' contains only one -expression, the `let*' expression. You know about a `let' function. -The `let*' function is different. It has a `*' in its name. It -enables Emacs to set each variable in its varlist in sequence, one -after another. - -Its critical feature is that variables later in the varlist can make -use of the values to which Emacs set variables earlier in the varlist. -*Note The `let*' expression: fwd-para let. - -We will skip functions like `let*' and focus on two: the `set-buffer' -function and the `insert-buffer-substring' function. - -In the old days, the `set-buffer' expression was simply - - (set-buffer (get-buffer-create buffer)) - -but now it is - - (set-buffer append-to) - -`append-to' is bound to `(get-buffer-create buffer)' earlier on in the -`let*' expression. That extra binding would not be necessary except -for that `append-to' is used later in the varlist as an argument to -`get-buffer-window-list'. - -The `append-to-buffer' function definition inserts text from the buffer -in which you are currently to a named buffer. It happens that -`insert-buffer-substring' copies text from another buffer to the -current buffer, just the reverse--that is why the `append-to-buffer' -definition starts out with a `let' that binds the local symbol `oldbuf' -to the value returned by `current-buffer'. - -The `insert-buffer-substring' expression looks like this: - - (insert-buffer-substring oldbuf start end) - -The `insert-buffer-substring' function copies a string _from_ the -buffer specified as its first argument and inserts the string into the -present buffer. In this case, the argument to -`insert-buffer-substring' is the value of the variable created and -bound by the `let', namely the value of `oldbuf', which was the current -buffer when you gave the `append-to-buffer' command. - -After `insert-buffer-substring' has done its work, `save-excursion' -will restore the action to the original buffer and `append-to-buffer' -will have done its job. - -Written in skeletal form, the workings of the body look like this: - - (let (BIND-`oldbuf'-TO-VALUE-OF-`current-buffer') - (save-excursion ; Keep track of buffer. - CHANGE-BUFFER - INSERT-SUBSTRING-FROM-`oldbuf'-INTO-BUFFER) - - CHANGE-BACK-TO-ORIGINAL-BUFFER-WHEN-FINISHED - LET-THE-LOCAL-MEANING-OF-`oldbuf'-DISAPPEAR-WHEN-FINISHED - -In summary, `append-to-buffer' works as follows: it saves the value of -the current buffer in the variable called `oldbuf'. It gets the new -buffer (creating one if need be) and switches Emacs' attention to it. -Using the value of `oldbuf', it inserts the region of text from the old -buffer into the new buffer; and then using `save-excursion', it brings -you back to your original buffer. - -In looking at `append-to-buffer', you have explored a fairly complex -function. It shows how to use `let' and `save-excursion', and how to -change to and come back from another buffer. Many function definitions -use `let', `save-excursion', and `set-buffer' this way. - - -File: eintr, Node: Buffer Related Review, Next: Buffer Exercises, Prev: append-to-buffer, Up: Buffer Walk Through - -4.5 Review -========== - -Here is a brief summary of the various functions discussed in this -chapter. - -`describe-function' -`describe-variable' - Print the documentation for a function or variable. - Conventionally bound to `C-h f' and `C-h v'. - -`find-tag' - Find the file containing the source for a function or variable and - switch buffers to it, positioning point at the beginning of the - item. Conventionally bound to `M-.' (that's a period following the - <META> key). - -`save-excursion' - Save the location of point and mark and restore their values after - the arguments to `save-excursion' have been evaluated. Also, - remember the current buffer and return to it. - -`push-mark' - Set mark at a location and record the value of the previous mark - on the mark ring. The mark is a location in the buffer that will - keep its relative position even if text is added to or removed - from the buffer. - -`goto-char' - Set point to the location specified by the value of the argument, - which can be a number, a marker, or an expression that returns - the number of a position, such as `(point-min)'. - -`insert-buffer-substring' - Copy a region of text from a buffer that is passed to the function - as an argument and insert the region into the current buffer. - -`mark-whole-buffer' - Mark the whole buffer as a region. Normally bound to `C-x h'. - -`set-buffer' - Switch the attention of Emacs to another buffer, but do not change - the window being displayed. Used when the program rather than a - human is to work on a different buffer. - -`get-buffer-create' -`get-buffer' - Find a named buffer or create one if a buffer of that name does not - exist. The `get-buffer' function returns `nil' if the named - buffer does not exist. - - -File: eintr, Node: Buffer Exercises, Prev: Buffer Related Review, Up: Buffer Walk Through - -4.6 Exercises -============= - - * Write your own `simplified-end-of-buffer' function definition; - then test it to see whether it works. - - * Use `if' and `get-buffer' to write a function that prints a - message telling you whether a buffer exists. - - * Using `find-tag', find the source for the `copy-to-buffer' - function. - - -File: eintr, Node: More Complex, Next: Narrowing & Widening, Prev: Buffer Walk Through, Up: Top - -5 A Few More Complex Functions -****************************** - -In this chapter, we build on what we have learned in previous chapters -by looking at more complex functions. The `copy-to-buffer' function -illustrates use of two `save-excursion' expressions in one definition, -while the `insert-buffer' function illustrates use of an asterisk in an -`interactive' expression, use of `or', and the important distinction -between a name and the object to which the name refers. - -* Menu: - -* copy-to-buffer:: -* insert-buffer:: -* beginning-of-buffer:: -* Second Buffer Related Review:: -* optional Exercise:: - - -File: eintr, Node: copy-to-buffer, Next: insert-buffer, Prev: More Complex, Up: More Complex - -5.1 The Definition of `copy-to-buffer' -====================================== - -After understanding how `append-to-buffer' works, it is easy to -understand `copy-to-buffer'. This function copies text into a buffer, -but instead of adding to the second buffer, it replaces all the -previous text in the second buffer. - -The body of `copy-to-buffer' looks like this, - - ... - (interactive "BCopy to buffer: \nr") - (let ((oldbuf (current-buffer))) - (with-current-buffer (get-buffer-create buffer) - (barf-if-buffer-read-only) - (erase-buffer) - (save-excursion - (insert-buffer-substring oldbuf start end))))) - -The `copy-to-buffer' function has a simpler `interactive' expression -than `append-to-buffer'. - -The definition then says - - (with-current-buffer (get-buffer-create buffer) ... - -First, look at the earliest inner expression; that is evaluated first. -That expression starts with `get-buffer-create buffer'. The function -tells the computer to use the buffer with the name specified as the one -to which you are copying, or if such a buffer does not exist, to create -it. Then, the `with-current-buffer' function evaluates its body with -that buffer temporarily current. - -(This demonstrates another way to shift the computer's attention but -not the user's. The `append-to-buffer' function showed how to do the -same with `save-excursion' and `set-buffer'. `with-current-buffer' is -a newer, and arguably easier, mechanism.) - -The `barf-if-buffer-read-only' function sends you an error message -saying the buffer is read-only if you cannot modify it. - -The next line has the `erase-buffer' function as its sole contents. -That function erases the buffer. - -Finally, the last two lines contain the `save-excursion' expression -with `insert-buffer-substring' as its body. The -`insert-buffer-substring' expression copies the text from the buffer -you are in (and you have not seen the computer shift its attention, so -you don't know that that buffer is now called `oldbuf'). - -Incidentally, this is what is meant by `replacement'. To replace text, -Emacs erases the previous text and then inserts new text. - -In outline, the body of `copy-to-buffer' looks like this: - - (let (BIND-`oldbuf'-TO-VALUE-OF-`current-buffer') - (WITH-THE-BUFFER-YOU-ARE-COPYING-TO - (BUT-DO-NOT-ERASE-OR-COPY-TO-A-READ-ONLY-BUFFER) - (erase-buffer) - (save-excursion - INSERT-SUBSTRING-FROM-`oldbuf'-INTO-BUFFER))) - - -File: eintr, Node: insert-buffer, Next: beginning-of-buffer, Prev: copy-to-buffer, Up: More Complex - -5.2 The Definition of `insert-buffer' -===================================== - -`insert-buffer' is yet another buffer-related function. This command -copies another buffer _into_ the current buffer. It is the reverse of -`append-to-buffer' or `copy-to-buffer', since they copy a region of -text _from_ the current buffer to another buffer. - -Here is a discussion based on the original code. The code was -simplified in 2003 and is harder to understand. - -*Note New Body for `insert-buffer': New insert-buffer, to see a -discussion of the new body.) - -In addition, this code illustrates the use of `interactive' with a -buffer that might be "read-only" and the important distinction between -the name of an object and the object actually referred to. - -* Menu: - -* insert-buffer code:: -* insert-buffer interactive:: -* insert-buffer body:: -* if & or:: -* Insert or:: -* Insert let:: -* New insert-buffer :: - - -File: eintr, Node: insert-buffer code, Next: insert-buffer interactive, Prev: insert-buffer, Up: insert-buffer - -The Code for `insert-buffer' ----------------------------- - -Here is the earlier code: - - (defun insert-buffer (buffer) - "Insert after point the contents of BUFFER. - Puts mark after the inserted text. - BUFFER may be a buffer or a buffer name." - (interactive "*bInsert buffer: ") - (or (bufferp buffer) - (setq buffer (get-buffer buffer))) - (let (start end newmark) - (save-excursion - (save-excursion - (set-buffer buffer) - (setq start (point-min) end (point-max))) - (insert-buffer-substring buffer start end) - (setq newmark (point))) - (push-mark newmark))) - -As with other function definitions, you can use a template to see an -outline of the function: - - (defun insert-buffer (buffer) - "DOCUMENTATION..." - (interactive "*bInsert buffer: ") - BODY...) - - -File: eintr, Node: insert-buffer interactive, Next: insert-buffer body, Prev: insert-buffer code, Up: insert-buffer - -5.2.1 The Interactive Expression in `insert-buffer' ---------------------------------------------------- - -In `insert-buffer', the argument to the `interactive' declaration has -two parts, an asterisk, `*', and `bInsert buffer: '. - -* Menu: - -* Read-only buffer:: -* b for interactive:: - - -File: eintr, Node: Read-only buffer, Next: b for interactive, Prev: insert-buffer interactive, Up: insert-buffer interactive - -A Read-only Buffer -.................. - -The asterisk is for the situation when the current buffer is a -read-only buffer--a buffer that cannot be modified. If `insert-buffer' -is called when the current buffer is read-only, a message to this -effect is printed in the echo area and the terminal may beep or blink -at you; you will not be permitted to insert anything into current -buffer. The asterisk does not need to be followed by a newline to -separate it from the next argument. - - -File: eintr, Node: b for interactive, Prev: Read-only buffer, Up: insert-buffer interactive - -`b' in an Interactive Expression -................................ - -The next argument in the interactive expression starts with a lower -case `b'. (This is different from the code for `append-to-buffer', -which uses an upper-case `B'. *Note The Definition of -`append-to-buffer': append-to-buffer.) The lower-case `b' tells the -Lisp interpreter that the argument for `insert-buffer' should be an -existing buffer or else its name. (The upper-case `B' option provides -for the possibility that the buffer does not exist.) Emacs will prompt -you for the name of the buffer, offering you a default buffer, with -name completion enabled. If the buffer does not exist, you receive a -message that says "No match"; your terminal may beep at you as well. - -The new and simplified code generates a list for `interactive'. It -uses the `barf-if-buffer-read-only' and `read-buffer' functions with -which we are already familiar and the `progn' special form with which -we are not. (It will be described later.) - - -File: eintr, Node: insert-buffer body, Next: if & or, Prev: insert-buffer interactive, Up: insert-buffer - -5.2.2 The Body of the `insert-buffer' Function ----------------------------------------------- - -The body of the `insert-buffer' function has two major parts: an `or' -expression and a `let' expression. The purpose of the `or' expression -is to ensure that the argument `buffer' is bound to a buffer and not -just the name of a buffer. The body of the `let' expression contains -the code which copies the other buffer into the current buffer. - -In outline, the two expressions fit into the `insert-buffer' function -like this: - - (defun insert-buffer (buffer) - "DOCUMENTATION..." - (interactive "*bInsert buffer: ") - (or ... - ... - (let (VARLIST) - BODY-OF-`let'... ) - -To understand how the `or' expression ensures that the argument -`buffer' is bound to a buffer and not to the name of a buffer, it is -first necessary to understand the `or' function. - -Before doing this, let me rewrite this part of the function using `if' -so that you can see what is done in a manner that will be familiar. - - -File: eintr, Node: if & or, Next: Insert or, Prev: insert-buffer body, Up: insert-buffer - -5.2.3 `insert-buffer' With an `if' Instead of an `or' ------------------------------------------------------ - -The job to be done is to make sure the value of `buffer' is a buffer -itself and not the name of a buffer. If the value is the name, then -the buffer itself must be got. - -You can imagine yourself at a conference where an usher is wandering -around holding a list with your name on it and looking for you: the -usher is "bound" to your name, not to you; but when the usher finds you -and takes your arm, the usher becomes "bound" to you. - -In Lisp, you might describe this situation like this: - - (if (not (holding-on-to-guest)) - (find-and-take-arm-of-guest)) - -We want to do the same thing with a buffer--if we do not have the -buffer itself, we want to get it. - -Using a predicate called `bufferp' that tells us whether we have a -buffer (rather than its name), we can write the code like this: - - (if (not (bufferp buffer)) ; if-part - (setq buffer (get-buffer buffer))) ; then-part - -Here, the true-or-false-test of the `if' expression is -`(not (bufferp buffer))'; and the then-part is the expression -`(setq buffer (get-buffer buffer))'. - -In the test, the function `bufferp' returns true if its argument is a -buffer--but false if its argument is the name of the buffer. (The last -character of the function name `bufferp' is the character `p'; as we -saw earlier, such use of `p' is a convention that indicates that the -function is a predicate, which is a term that means that the function -will determine whether some property is true or false. *Note Using the -Wrong Type Object as an Argument: Wrong Type of Argument.) - -The function `not' precedes the expression `(bufferp buffer)', so the -true-or-false-test looks like this: - - (not (bufferp buffer)) - -`not' is a function that returns true if its argument is false and -false if its argument is true. So if `(bufferp buffer)' returns true, -the `not' expression returns false and vice-verse: what is "not true" -is false and what is "not false" is true. - -Using this test, the `if' expression works as follows: when the value -of the variable `buffer' is actually a buffer rather than its name, the -true-or-false-test returns false and the `if' expression does not -evaluate the then-part. This is fine, since we do not need to do -anything to the variable `buffer' if it really is a buffer. - -On the other hand, when the value of `buffer' is not a buffer itself, -but the name of a buffer, the true-or-false-test returns true and the -then-part of the expression is evaluated. In this case, the then-part -is `(setq buffer (get-buffer buffer))'. This expression uses the -`get-buffer' function to return an actual buffer itself, given its -name. The `setq' then sets the variable `buffer' to the value of the -buffer itself, replacing its previous value (which was the name of the -buffer). - - -File: eintr, Node: Insert or, Next: Insert let, Prev: if & or, Up: insert-buffer - -5.2.4 The `or' in the Body --------------------------- - -The purpose of the `or' expression in the `insert-buffer' function is -to ensure that the argument `buffer' is bound to a buffer and not just -to the name of a buffer. The previous section shows how the job could -have been done using an `if' expression. However, the `insert-buffer' -function actually uses `or'. To understand this, it is necessary to -understand how `or' works. - -An `or' function can have any number of arguments. It evaluates each -argument in turn and returns the value of the first of its arguments -that is not `nil'. Also, and this is a crucial feature of `or', it -does not evaluate any subsequent arguments after returning the first -non-`nil' value. - -The `or' expression looks like this: - - (or (bufferp buffer) - (setq buffer (get-buffer buffer))) - -The first argument to `or' is the expression `(bufferp buffer)'. This -expression returns true (a non-`nil' value) if the buffer is actually a -buffer, and not just the name of a buffer. In the `or' expression, if -this is the case, the `or' expression returns this true value and does -not evaluate the next expression--and this is fine with us, since we do -not want to do anything to the value of `buffer' if it really is a -buffer. - -On the other hand, if the value of `(bufferp buffer)' is `nil', which -it will be if the value of `buffer' is the name of a buffer, the Lisp -interpreter evaluates the next element of the `or' expression. This is -the expression `(setq buffer (get-buffer buffer))'. This expression -returns a non-`nil' value, which is the value to which it sets the -variable `buffer'--and this value is a buffer itself, not the name of a -buffer. - -The result of all this is that the symbol `buffer' is always bound to a -buffer itself rather than to the name of a buffer. All this is -necessary because the `set-buffer' function in a following line only -works with a buffer itself, not with the name to a buffer. - -Incidentally, using `or', the situation with the usher would be written -like this: - - (or (holding-on-to-guest) (find-and-take-arm-of-guest)) - - -File: eintr, Node: Insert let, Next: New insert-buffer, Prev: Insert or, Up: insert-buffer - -5.2.5 The `let' Expression in `insert-buffer' ---------------------------------------------- - -After ensuring that the variable `buffer' refers to a buffer itself and -not just to the name of a buffer, the `insert-buffer function' -continues with a `let' expression. This specifies three local -variables, `start', `end', and `newmark' and binds them to the initial -value `nil'. These variables are used inside the remainder of the -`let' and temporarily hide any other occurrence of variables of the -same name in Emacs until the end of the `let'. - -The body of the `let' contains two `save-excursion' expressions. -First, we will look at the inner `save-excursion' expression in detail. -The expression looks like this: - - (save-excursion - (set-buffer buffer) - (setq start (point-min) end (point-max))) - -The expression `(set-buffer buffer)' changes Emacs' attention from the -current buffer to the one from which the text will copied. In that -buffer, the variables `start' and `end' are set to the beginning and -end of the buffer, using the commands `point-min' and `point-max'. -Note that we have here an illustration of how `setq' is able to set two -variables in the same expression. The first argument of `setq' is set -to the value of its second, and its third argument is set to the value -of its fourth. - -After the body of the inner `save-excursion' is evaluated, the -`save-excursion' restores the original buffer, but `start' and `end' -remain set to the values of the beginning and end of the buffer from -which the text will be copied. - -The outer `save-excursion' expression looks like this: - - (save-excursion - (INNER-`save-excursion'-EXPRESSION - (GO-TO-NEW-BUFFER-AND-SET-`start'-AND-`end') - (insert-buffer-substring buffer start end) - (setq newmark (point))) - -The `insert-buffer-substring' function copies the text _into_ the -current buffer _from_ the region indicated by `start' and `end' in -`buffer'. Since the whole of the second buffer lies between `start' -and `end', the whole of the second buffer is copied into the buffer you -are editing. Next, the value of point, which will be at the end of the -inserted text, is recorded in the variable `newmark'. - -After the body of the outer `save-excursion' is evaluated, point and -mark are relocated to their original places. - -However, it is convenient to locate a mark at the end of the newly -inserted text and locate point at its beginning. The `newmark' -variable records the end of the inserted text. In the last line of the -`let' expression, the `(push-mark newmark)' expression function sets a -mark to this location. (The previous location of the mark is still -accessible; it is recorded on the mark ring and you can go back to it -with `C-u C-<SPC>'.) Meanwhile, point is located at the beginning of -the inserted text, which is where it was before you called the insert -function, the position of which was saved by the first `save-excursion'. - -The whole `let' expression looks like this: - - (let (start end newmark) - (save-excursion - (save-excursion - (set-buffer buffer) - (setq start (point-min) end (point-max))) - (insert-buffer-substring buffer start end) - (setq newmark (point))) - (push-mark newmark)) - -Like the `append-to-buffer' function, the `insert-buffer' function uses -`let', `save-excursion', and `set-buffer'. In addition, the function -illustrates one way to use `or'. All these functions are building -blocks that we will find and use again and again. - - -File: eintr, Node: New insert-buffer, Prev: Insert let, Up: insert-buffer - -5.2.6 New Body for `insert-buffer' ----------------------------------- - -The body in the GNU Emacs 22 version is more confusing than the -original. - -It consists of two expressions, - - (push-mark - (save-excursion - (insert-buffer-substring (get-buffer buffer)) - (point))) - - nil - -except, and this is what confuses novices, very important work is done -inside the `push-mark' expression. - -The `get-buffer' function returns a buffer with the name provided. You -will note that the function is _not_ called `get-buffer-create'; it -does not create a buffer if one does not already exist. The buffer -returned by `get-buffer', an existing buffer, is passed to -`insert-buffer-substring', which inserts the whole of the buffer (since -you did not specify anything else). - -The location into which the buffer is inserted is recorded by -`push-mark'. Then the function returns `nil', the value of its last -command. Put another way, the `insert-buffer' function exists only to -produce a side effect, inserting another buffer, not to return any -value. - - -File: eintr, Node: beginning-of-buffer, Next: Second Buffer Related Review, Prev: insert-buffer, Up: More Complex - -5.3 Complete Definition of `beginning-of-buffer' -================================================ - -The basic structure of the `beginning-of-buffer' function has already -been discussed. (*Note A Simplified `beginning-of-buffer' Definition: -simplified-beginning-of-buffer.) This section describes the complex -part of the definition. - -As previously described, when invoked without an argument, -`beginning-of-buffer' moves the cursor to the beginning of the buffer -(in truth, the accessible portion of the buffer), leaving the mark at -the previous position. However, when the command is invoked with a -number between one and ten, the function considers that number to be a -fraction of the length of the buffer, measured in tenths, and Emacs -moves the cursor that fraction of the way from the beginning of the -buffer. Thus, you can either call this function with the key command -`M-<', which will move the cursor to the beginning of the buffer, or -with a key command such as `C-u 7 M-<' which will move the cursor to a -point 70% of the way through the buffer. If a number bigger than ten -is used for the argument, it moves to the end of the buffer. - -The `beginning-of-buffer' function can be called with or without an -argument. The use of the argument is optional. - -* Menu: - -* Optional Arguments:: -* beginning-of-buffer opt arg:: -* beginning-of-buffer complete:: - - -File: eintr, Node: Optional Arguments, Next: beginning-of-buffer opt arg, Prev: beginning-of-buffer, Up: beginning-of-buffer - -5.3.1 Optional Arguments ------------------------- - -Unless told otherwise, Lisp expects that a function with an argument in -its function definition will be called with a value for that argument. -If that does not happen, you get an error and a message that says -`Wrong number of arguments'. - -However, optional arguments are a feature of Lisp: a particular -"keyword" is used to tell the Lisp interpreter that an argument is -optional. The keyword is `&optional'. (The `&' in front of `optional' -is part of the keyword.) In a function definition, if an argument -follows the keyword `&optional', no value need be passed to that -argument when the function is called. - -The first line of the function definition of `beginning-of-buffer' -therefore looks like this: - - (defun beginning-of-buffer (&optional arg) - -In outline, the whole function looks like this: - - (defun beginning-of-buffer (&optional arg) - "DOCUMENTATION..." - (interactive "P") - (or (IS-THE-ARGUMENT-A-CONS-CELL arg) - (and ARE-BOTH-TRANSIENT-MARK-MODE-AND-MARK-ACTIVE-TRUE) - (push-mark)) - (let (DETERMINE-SIZE-AND-SET-IT) - (goto-char - (IF-THERE-IS-AN-ARGUMENT - FIGURE-OUT-WHERE-TO-GO - ELSE-GO-TO - (point-min)))) - DO-NICETY - -The function is similar to the `simplified-beginning-of-buffer' -function except that the `interactive' expression has `"P"' as an -argument and the `goto-char' function is followed by an if-then-else -expression that figures out where to put the cursor if there is an -argument that is not a cons cell. - -(Since I do not explain a cons cell for many more chapters, please -consider ignoring the function `consp'. *Note How Lists are -Implemented: List Implementation, and *Note Cons Cell and List Types: -(elisp)Cons Cell Type.) - -The `"P"' in the `interactive' expression tells Emacs to pass a prefix -argument, if there is one, to the function in raw form. A prefix -argument is made by typing the <META> key followed by a number, or by -typing `C-u' and then a number. (If you don't type a number, `C-u' -defaults to a cons cell with a 4. A lowercase `"p"' in the -`interactive' expression causes the function to convert a prefix arg to -a number.) - -The true-or-false-test of the `if' expression looks complex, but it is -not: it checks whether `arg' has a value that is not `nil' and whether -it is a cons cell. (That is what `consp' does; it checks whether its -argument is a cons cell.) If `arg' has a value that is not `nil' (and -is not a cons cell), which will be the case if `beginning-of-buffer' is -called with a numeric argument, then this true-or-false-test will -return true and the then-part of the `if' expression will be evaluated. -On the other hand, if `beginning-of-buffer' is not called with an -argument, the value of `arg' will be `nil' and the else-part of the -`if' expression will be evaluated. The else-part is simply -`point-min', and when this is the outcome, the whole `goto-char' -expression is `(goto-char (point-min))', which is how we saw the -`beginning-of-buffer' function in its simplified form. - - -File: eintr, Node: beginning-of-buffer opt arg, Next: beginning-of-buffer complete, Prev: Optional Arguments, Up: beginning-of-buffer - -5.3.2 `beginning-of-buffer' with an Argument --------------------------------------------- - -When `beginning-of-buffer' is called with an argument, an expression is -evaluated which calculates what value to pass to `goto-char'. This -expression is rather complicated at first sight. It includes an inner -`if' expression and much arithmetic. It looks like this: - - (if (> (buffer-size) 10000) - ;; Avoid overflow for large buffer sizes! - (* (prefix-numeric-value arg) - (/ size 10)) - (/ - (+ 10 - (* - size (prefix-numeric-value arg))) 10))) - -* Menu: - -* Disentangle beginning-of-buffer:: -* Large buffer case:: -* Small buffer case:: - - -File: eintr, Node: Disentangle beginning-of-buffer, Next: Large buffer case, Prev: beginning-of-buffer opt arg, Up: beginning-of-buffer opt arg - -Disentangle `beginning-of-buffer' -................................. - -Like other complex-looking expressions, the conditional expression -within `beginning-of-buffer' can be disentangled by looking at it as -parts of a template, in this case, the template for an if-then-else -expression. In skeletal form, the expression looks like this: - - (if (BUFFER-IS-LARGE - DIVIDE-BUFFER-SIZE-BY-10-AND-MULTIPLY-BY-ARG - ELSE-USE-ALTERNATE-CALCULATION - -The true-or-false-test of this inner `if' expression checks the size of -the buffer. The reason for this is that the old Version 18 Emacs used -numbers that are no bigger than eight million or so and in the -computation that followed, the programmer feared that Emacs might try -to use over-large numbers if the buffer were large. The term -`overflow', mentioned in the comment, means numbers that are over -large. Version 21 Emacs uses larger numbers, but this code has not -been touched, if only because people now look at buffers that are far, -far larger than ever before. - -There are two cases: if the buffer is large and if it is not. - - -File: eintr, Node: Large buffer case, Next: Small buffer case, Prev: Disentangle beginning-of-buffer, Up: beginning-of-buffer opt arg - -What happens in a large buffer -.............................. - -In `beginning-of-buffer', the inner `if' expression tests whether the -size of the buffer is greater than 10,000 characters. To do this, it -uses the `>' function and the computation of `size' that comes from the -let expression. - -In the old days, the function `buffer-size' was used. Not only was -that function called several times, it gave the size of the whole -buffer, not the accessible part. The computation makes much more sense -when it handles just the accessible part. (*Note Narrowing and -Widening: Narrowing & Widening, for more information on focusing -attention to an `accessible' part.) - -The line looks like this: - - (if (> size 10000) - -When the buffer is large, the then-part of the `if' expression is -evaluated. It reads like this (after formatting for easy reading): - - (* - (prefix-numeric-value arg) - (/ size 10)) - -This expression is a multiplication, with two arguments to the function -`*'. - -The first argument is `(prefix-numeric-value arg)'. When `"P"' is used -as the argument for `interactive', the value passed to the function as -its argument is passed a "raw prefix argument", and not a number. (It -is a number in a list.) To perform the arithmetic, a conversion is -necessary, and `prefix-numeric-value' does the job. - -The second argument is `(/ size 10)'. This expression divides the -numeric value by ten -- the numeric value of the size of the accessible -portion of the buffer. This produces a number that tells how many -characters make up one tenth of the buffer size. (In Lisp, `/' is used -for division, just as `*' is used for multiplication.) - -In the multiplication expression as a whole, this amount is multiplied -by the value of the prefix argument--the multiplication looks like this: - - (* NUMERIC-VALUE-OF-PREFIX-ARG - NUMBER-OF-CHARACTERS-IN-ONE-TENTH-OF-THE-ACCESSIBLE-BUFFER) - -If, for example, the prefix argument is `7', the one-tenth value will -be multiplied by 7 to give a position 70% of the way through. - -The result of all this is that if the accessible portion of the buffer -is large, the `goto-char' expression reads like this: - - (goto-char (* (prefix-numeric-value arg) - (/ size 10))) - -This puts the cursor where we want it. - - -File: eintr, Node: Small buffer case, Prev: Large buffer case, Up: beginning-of-buffer opt arg - -What happens in a small buffer -.............................. - -If the buffer contains fewer than 10,000 characters, a slightly -different computation is performed. You might think this is not -necessary, since the first computation could do the job. However, in a -small buffer, the first method may not put the cursor on exactly the -desired line; the second method does a better job. - -The code looks like this: - - (/ (+ 10 (* size (prefix-numeric-value arg))) 10)) - -This is code in which you figure out what happens by discovering how the -functions are embedded in parentheses. It is easier to read if you -reformat it with each expression indented more deeply than its -enclosing expression: - - (/ - (+ 10 - (* - size - (prefix-numeric-value arg))) - 10)) - -Looking at parentheses, we see that the innermost operation is -`(prefix-numeric-value arg)', which converts the raw argument to a -number. In the following expression, this number is multiplied by the -size of the accessible portion of the buffer: - - (* size (prefix-numeric-value arg)) - -This multiplication creates a number that may be larger than the size of -the buffer--seven times larger if the argument is 7, for example. Ten -is then added to this number and finally the large number is divided by -ten to provide a value that is one character larger than the percentage -position in the buffer. - -The number that results from all this is passed to `goto-char' and the -cursor is moved to that point. - - -File: eintr, Node: beginning-of-buffer complete, Prev: beginning-of-buffer opt arg, Up: beginning-of-buffer - -5.3.3 The Complete `beginning-of-buffer' ----------------------------------------- - -Here is the complete text of the `beginning-of-buffer' function: - - (defun beginning-of-buffer (&optional arg) - "Move point to the beginning of the buffer; - leave mark at previous position. - With \\[universal-argument] prefix, - do not set mark at previous position. - With numeric arg N, - put point N/10 of the way from the beginning. - - If the buffer is narrowed, - this command uses the beginning and size - of the accessible part of the buffer. - - Don't use this command in Lisp programs! - \(goto-char (point-min)) is faster - and avoids clobbering the mark." - (interactive "P") - (or (consp arg) - (and transient-mark-mode mark-active) - (push-mark)) - (let ((size (- (point-max) (point-min)))) - (goto-char (if (and arg (not (consp arg))) - (+ (point-min) - (if (> size 10000) - ;; Avoid overflow for large buffer sizes! - (* (prefix-numeric-value arg) - (/ size 10)) - (/ (+ 10 (* size (prefix-numeric-value arg))) 10))) - (point-min)))) - (if arg (forward-line 1))) - -Except for two small points, the previous discussion shows how this -function works. The first point deals with a detail in the -documentation string, and the second point concerns the last line of -the function. - -In the documentation string, there is reference to an expression: - - \\[universal-argument] - -A `\\' is used before the first square bracket of this expression. -This `\\' tells the Lisp interpreter to substitute whatever key is -currently bound to the `[...]'. In the case of `universal-argument', -that is usually `C-u', but it might be different. (*Note Tips for -Documentation Strings: (elisp)Documentation Tips, for more information.) - -Finally, the last line of the `beginning-of-buffer' command says to -move point to the beginning of the next line if the command is invoked -with an argument: - - (if arg (forward-line 1))) - -This puts the cursor at the beginning of the first line after the -appropriate tenths position in the buffer. This is a flourish that -means that the cursor is always located _at least_ the requested tenths -of the way through the buffer, which is a nicety that is, perhaps, not -necessary, but which, if it did not occur, would be sure to draw -complaints. - -On the other hand, it also means that if you specify the command with a -`C-u', but without a number, that is to say, if the `raw prefix -argument' is simply a cons cell, then the command puts you at the -beginning of the second line ... I don't know whether this is intended -or whether no one has dealt with the code to avoid this happening. - - -File: eintr, Node: Second Buffer Related Review, Next: optional Exercise, Prev: beginning-of-buffer, Up: More Complex - -5.4 Review -========== - -Here is a brief summary of some of the topics covered in this chapter. - -`or' - Evaluate each argument in sequence, and return the value of the - first argument that is not `nil'; if none return a value that is - not `nil', return `nil'. In brief, return the first true value of - the arguments; return a true value if one _or_ any of the others - are true. - -`and' - Evaluate each argument in sequence, and if any are `nil', return - `nil'; if none are `nil', return the value of the last argument. - In brief, return a true value only if all the arguments are true; - return a true value if one _and_ each of the others is true. - -`&optional' - A keyword used to indicate that an argument to a function - definition is optional; this means that the function can be - evaluated without the argument, if desired. - -`prefix-numeric-value' - Convert the `raw prefix argument' produced by `(interactive "P")' - to a numeric value. - -`forward-line' - Move point forward to the beginning of the next line, or if the - argument is greater than one, forward that many lines. If it - can't move as far forward as it is supposed to, `forward-line' - goes forward as far as it can and then returns a count of the - number of additional lines it was supposed to move but couldn't. - -`erase-buffer' - Delete the entire contents of the current buffer. - -`bufferp' - Return `t' if its argument is a buffer; otherwise return `nil'. - - -File: eintr, Node: optional Exercise, Prev: Second Buffer Related Review, Up: More Complex - -5.5 `optional' Argument Exercise -================================ - -Write an interactive function with an optional argument that tests -whether its argument, a number, is greater than or equal to, or else, -less than the value of `fill-column', and tells you which, in a -message. However, if you do not pass an argument to the function, use -56 as a default value. - - -File: eintr, Node: Narrowing & Widening, Next: car cdr & cons, Prev: More Complex, Up: Top - -6 Narrowing and Widening -************************ - -Narrowing is a feature of Emacs that makes it possible for you to focus -on a specific part of a buffer, and work without accidentally changing -other parts. Narrowing is normally disabled since it can confuse -novices. - -* Menu: - -* Narrowing advantages:: -* save-restriction:: -* what-line:: -* narrow Exercise:: - - -File: eintr, Node: Narrowing advantages, Next: save-restriction, Prev: Narrowing & Widening, Up: Narrowing & Widening - -The Advantages of Narrowing -=========================== - -With narrowing, the rest of a buffer is made invisible, as if it weren't -there. This is an advantage if, for example, you want to replace a word -in one part of a buffer but not in another: you narrow to the part you -want and the replacement is carried out only in that section, not in -the rest of the buffer. Searches will only work within a narrowed -region, not outside of one, so if you are fixing a part of a document, -you can keep yourself from accidentally finding parts you do not need -to fix by narrowing just to the region you want. (The key binding for -`narrow-to-region' is `C-x n n'.) - -However, narrowing does make the rest of the buffer invisible, which -can scare people who inadvertently invoke narrowing and think they have -deleted a part of their file. Moreover, the `undo' command (which is -usually bound to `C-x u') does not turn off narrowing (nor should it), -so people can become quite desperate if they do not know that they can -return the rest of a buffer to visibility with the `widen' command. -(The key binding for `widen' is `C-x n w'.) - -Narrowing is just as useful to the Lisp interpreter as to a human. -Often, an Emacs Lisp function is designed to work on just part of a -buffer; or conversely, an Emacs Lisp function needs to work on all of a -buffer that has been narrowed. The `what-line' function, for example, -removes the narrowing from a buffer, if it has any narrowing and when -it has finished its job, restores the narrowing to what it was. On the -other hand, the `count-lines' function, which is called by `what-line', -uses narrowing to restrict itself to just that portion of the buffer in -which it is interested and then restores the previous situation. - - -File: eintr, Node: save-restriction, Next: what-line, Prev: Narrowing advantages, Up: Narrowing & Widening - -6.1 The `save-restriction' Special Form -======================================= - -In Emacs Lisp, you can use the `save-restriction' special form to keep -track of whatever narrowing is in effect, if any. When the Lisp -interpreter meets with `save-restriction', it executes the code in the -body of the `save-restriction' expression, and then undoes any changes -to narrowing that the code caused. If, for example, the buffer is -narrowed and the code that follows `save-restriction' gets rid of the -narrowing, `save-restriction' returns the buffer to its narrowed region -afterwards. In the `what-line' command, any narrowing the buffer may -have is undone by the `widen' command that immediately follows the -`save-restriction' command. Any original narrowing is restored just -before the completion of the function. - -The template for a `save-restriction' expression is simple: - - (save-restriction - BODY... ) - -The body of the `save-restriction' is one or more expressions that will -be evaluated in sequence by the Lisp interpreter. - -Finally, a point to note: when you use both `save-excursion' and -`save-restriction', one right after the other, you should use -`save-excursion' outermost. If you write them in reverse order, you -may fail to record narrowing in the buffer to which Emacs switches -after calling `save-excursion'. Thus, when written together, -`save-excursion' and `save-restriction' should be written like this: - - (save-excursion - (save-restriction - BODY...)) - -In other circumstances, when not written together, the `save-excursion' -and `save-restriction' special forms must be written in the order -appropriate to the function. - -For example, - - (save-restriction - (widen) - (save-excursion - BODY...)) - - -File: eintr, Node: what-line, Next: narrow Exercise, Prev: save-restriction, Up: Narrowing & Widening - -6.2 `what-line' -=============== - -The `what-line' command tells you the number of the line in which the -cursor is located. The function illustrates the use of the -`save-restriction' and `save-excursion' commands. Here is the original -text of the function: - - (defun what-line () - "Print the current line number (in the buffer) of point." - (interactive) - (save-restriction - (widen) - (save-excursion - (beginning-of-line) - (message "Line %d" - (1+ (count-lines 1 (point))))))) - -(In recent versions of GNU Emacs, the `what-line' function has been -expanded to tell you your line number in a narrowed buffer as well as -your line number in a widened buffer. The recent version is more -complex than the version shown here. If you feel adventurous, you -might want to look at it after figuring out how this version works. -You will probably need to use `C-h f' (`describe-function'). The newer -version uses a conditional to determine whether the buffer has been -narrowed. - -(Also, it uses `line-number-at-pos', which among other simple -expressions, such as `(goto-char (point-min))', moves point to the -beginning of the current line with `(forward-line 0)' rather than -`beginning-of-line'.) - -The `what-line' function as shown here has a documentation line and is -interactive, as you would expect. The next two lines use the functions -`save-restriction' and `widen'. - -The `save-restriction' special form notes whatever narrowing is in -effect, if any, in the current buffer and restores that narrowing after -the code in the body of the `save-restriction' has been evaluated. - -The `save-restriction' special form is followed by `widen'. This -function undoes any narrowing the current buffer may have had when -`what-line' was called. (The narrowing that was there is the narrowing -that `save-restriction' remembers.) This widening makes it possible -for the line counting commands to count from the beginning of the -buffer. Otherwise, they would have been limited to counting within the -accessible region. Any original narrowing is restored just before the -completion of the function by the `save-restriction' special form. - -The call to `widen' is followed by `save-excursion', which saves the -location of the cursor (i.e., of point) and of the mark, and restores -them after the code in the body of the `save-excursion' uses the -`beginning-of-line' function to move point. - -(Note that the `(widen)' expression comes between the -`save-restriction' and `save-excursion' special forms. When you write -the two `save- ...' expressions in sequence, write `save-excursion' -outermost.) - -The last two lines of the `what-line' function are functions to count -the number of lines in the buffer and then print the number in the echo -area. - - (message "Line %d" - (1+ (count-lines 1 (point))))))) - -The `message' function prints a one-line message at the bottom of the -Emacs screen. The first argument is inside of quotation marks and is -printed as a string of characters. However, it may contain a `%d' -expression to print a following argument. `%d' prints the argument as -a decimal, so the message will say something such as `Line 243'. - -The number that is printed in place of the `%d' is computed by the last -line of the function: - - (1+ (count-lines 1 (point))) - -What this does is count the lines from the first position of the -buffer, indicated by the `1', up to `(point)', and then add one to that -number. (The `1+' function adds one to its argument.) We add one to -it because line 2 has only one line before it, and `count-lines' counts -only the lines _before_ the current line. - -After `count-lines' has done its job, and the message has been printed -in the echo area, the `save-excursion' restores point and mark to their -original positions; and `save-restriction' restores the original -narrowing, if any. - - -File: eintr, Node: narrow Exercise, Prev: what-line, Up: Narrowing & Widening - -6.3 Exercise with Narrowing -=========================== - -Write a function that will display the first 60 characters of the -current buffer, even if you have narrowed the buffer to its latter half -so that the first line is inaccessible. Restore point, mark, and -narrowing. For this exercise, you need to use a whole potpourri of -functions, including `save-restriction', `widen', `goto-char', -`point-min', `message', and `buffer-substring'. - -(`buffer-substring' is a previously unmentioned function you will have -to investigate yourself; or perhaps you will have to use -`buffer-substring-no-properties' or `filter-buffer-substring' ..., yet -other functions. Text properties are a feature otherwise not discussed -here. *Note Text Properties: (elisp)Text Properties. - -Additionally, do you really need `goto-char' or `point-min'? Or can -you write the function without them?) - - -File: eintr, Node: car cdr & cons, Next: Cutting & Storing Text, Prev: Narrowing & Widening, Up: Top - -7 `car', `cdr', `cons': Fundamental Functions -********************************************* - -In Lisp, `car', `cdr', and `cons' are fundamental functions. The -`cons' function is used to construct lists, and the `car' and `cdr' -functions are used to take them apart. - -In the walk through of the `copy-region-as-kill' function, we will see -`cons' as well as two variants on `cdr', namely, `setcdr' and `nthcdr'. -(*Note copy-region-as-kill::.) - -* Menu: - -* Strange Names:: -* car & cdr:: -* cons:: -* nthcdr:: -* nth:: -* setcar:: -* setcdr:: -* cons Exercise:: - - -File: eintr, Node: Strange Names, Next: car & cdr, Prev: car cdr & cons, Up: car cdr & cons - -Strange Names -============= - -The name of the `cons' function is not unreasonable: it is an -abbreviation of the word `construct'. The origins of the names for -`car' and `cdr', on the other hand, are esoteric: `car' is an acronym -from the phrase `Contents of the Address part of the Register'; and -`cdr' (pronounced `could-er') is an acronym from the phrase `Contents -of the Decrement part of the Register'. These phrases refer to -specific pieces of hardware on the very early computer on which the -original Lisp was developed. Besides being obsolete, the phrases have -been completely irrelevant for more than 25 years to anyone thinking -about Lisp. Nonetheless, although a few brave scholars have begun to -use more reasonable names for these functions, the old terms are still -in use. In particular, since the terms are used in the Emacs Lisp -source code, we will use them in this introduction. - - -File: eintr, Node: car & cdr, Next: cons, Prev: Strange Names, Up: car cdr & cons - -7.1 `car' and `cdr' -=================== - -The CAR of a list is, quite simply, the first item in the list. Thus -the CAR of the list `(rose violet daisy buttercup)' is `rose'. - -If you are reading this in Info in GNU Emacs, you can see this by -evaluating the following: - - (car '(rose violet daisy buttercup)) - -After evaluating the expression, `rose' will appear in the echo area. - -Clearly, a more reasonable name for the `car' function would be `first' -and this is often suggested. - -`car' does not remove the first item from the list; it only reports -what it is. After `car' has been applied to a list, the list is still -the same as it was. In the jargon, `car' is `non-destructive'. This -feature turns out to be important. - -The CDR of a list is the rest of the list, that is, the `cdr' function -returns the part of the list that follows the first item. Thus, while -the CAR of the list `'(rose violet daisy buttercup)' is `rose', the -rest of the list, the value returned by the `cdr' function, is `(violet -daisy buttercup)'. - -You can see this by evaluating the following in the usual way: - - (cdr '(rose violet daisy buttercup)) - -When you evaluate this, `(violet daisy buttercup)' will appear in the -echo area. - -Like `car', `cdr' does not remove any elements from the list--it just -returns a report of what the second and subsequent elements are. - -Incidentally, in the example, the list of flowers is quoted. If it were -not, the Lisp interpreter would try to evaluate the list by calling -`rose' as a function. In this example, we do not want to do that. - -Clearly, a more reasonable name for `cdr' would be `rest'. - -(There is a lesson here: when you name new functions, consider very -carefully what you are doing, since you may be stuck with the names for -far longer than you expect. The reason this document perpetuates these -names is that the Emacs Lisp source code uses them, and if I did not -use them, you would have a hard time reading the code; but do, please, -try to avoid using these terms yourself. The people who come after you -will be grateful to you.) - -When `car' and `cdr' are applied to a list made up of symbols, such as -the list `(pine fir oak maple)', the element of the list returned by -the function `car' is the symbol `pine' without any parentheses around -it. `pine' is the first element in the list. However, the CDR of the -list is a list itself, `(fir oak maple)', as you can see by evaluating -the following expressions in the usual way: - - (car '(pine fir oak maple)) - - (cdr '(pine fir oak maple)) - -On the other hand, in a list of lists, the first element is itself a -list. `car' returns this first element as a list. For example, the -following list contains three sub-lists, a list of carnivores, a list -of herbivores and a list of sea mammals: - - (car '((lion tiger cheetah) - (gazelle antelope zebra) - (whale dolphin seal))) - -In this example, the first element or CAR of the list is the list of -carnivores, `(lion tiger cheetah)', and the rest of the list is -`((gazelle antelope zebra) (whale dolphin seal))'. - - (cdr '((lion tiger cheetah) - (gazelle antelope zebra) - (whale dolphin seal))) - -It is worth saying again that `car' and `cdr' are non-destructive--that -is, they do not modify or change lists to which they are applied. This -is very important for how they are used. - -Also, in the first chapter, in the discussion about atoms, I said that -in Lisp, "certain kinds of atom, such as an array, can be separated -into parts; but the mechanism for doing this is different from the -mechanism for splitting a list. As far as Lisp is concerned, the atoms -of a list are unsplittable." (*Note Lisp Atoms::.) The `car' and -`cdr' functions are used for splitting lists and are considered -fundamental to Lisp. Since they cannot split or gain access to the -parts of an array, an array is considered an atom. Conversely, the -other fundamental function, `cons', can put together or construct a -list, but not an array. (Arrays are handled by array-specific -functions. *Note Arrays: (elisp)Arrays.) - - -File: eintr, Node: cons, Next: nthcdr, Prev: car & cdr, Up: car cdr & cons - -7.2 `cons' -========== - -The `cons' function constructs lists; it is the inverse of `car' and -`cdr'. For example, `cons' can be used to make a four element list -from the three element list, `(fir oak maple)': - - (cons 'pine '(fir oak maple)) - -After evaluating this list, you will see - - (pine fir oak maple) - -appear in the echo area. `cons' causes the creation of a new list in -which the element is followed by the elements of the original list. - -We often say that ``cons' puts a new element at the beginning of a -list; it attaches or pushes elements onto the list', but this phrasing -can be misleading, since `cons' does not change an existing list, but -creates a new one. - -Like `car' and `cdr', `cons' is non-destructive. - -* Menu: - -* Build a list:: -* length:: - - -File: eintr, Node: Build a list, Next: length, Prev: cons, Up: cons - -Build a list ------------- - -`cons' must have a list to attach to.(1) You cannot start from -absolutely nothing. If you are building a list, you need to provide at -least an empty list at the beginning. Here is a series of `cons' -expressions that build up a list of flowers. If you are reading this -in Info in GNU Emacs, you can evaluate each of the expressions in the -usual way; the value is printed in this text after `=>', which you may -read as `evaluates to'. - - (cons 'buttercup ()) - => (buttercup) - - (cons 'daisy '(buttercup)) - => (daisy buttercup) - - (cons 'violet '(daisy buttercup)) - => (violet daisy buttercup) - - (cons 'rose '(violet daisy buttercup)) - => (rose violet daisy buttercup) - -In the first example, the empty list is shown as `()' and a list made -up of `buttercup' followed by the empty list is constructed. As you -can see, the empty list is not shown in the list that was constructed. -All that you see is `(buttercup)'. The empty list is not counted as an -element of a list because there is nothing in an empty list. Generally -speaking, an empty list is invisible. - -The second example, `(cons 'daisy '(buttercup))' constructs a new, two -element list by putting `daisy' in front of `buttercup'; and the third -example constructs a three element list by putting `violet' in front of -`daisy' and `buttercup'. - ----------- Footnotes ---------- - -(1) Actually, you can `cons' an element to an atom to produce a dotted -pair. Dotted pairs are not discussed here; see *Note Dotted Pair -Notation: (elisp)Dotted Pair Notation. - - -File: eintr, Node: length, Prev: Build a list, Up: cons - -7.2.1 Find the Length of a List: `length' ------------------------------------------ - -You can find out how many elements there are in a list by using the Lisp -function `length', as in the following examples: - - (length '(buttercup)) - => 1 - - (length '(daisy buttercup)) - => 2 - - (length (cons 'violet '(daisy buttercup))) - => 3 - -In the third example, the `cons' function is used to construct a three -element list which is then passed to the `length' function as its -argument. - -We can also use `length' to count the number of elements in an empty -list: - - (length ()) - => 0 - -As you would expect, the number of elements in an empty list is zero. - -An interesting experiment is to find out what happens if you try to find -the length of no list at all; that is, if you try to call `length' -without giving it an argument, not even an empty list: - - (length ) - -What you see, if you evaluate this, is the error message - - Lisp error: (wrong-number-of-arguments length 0) - -This means that the function receives the wrong number of arguments, -zero, when it expects some other number of arguments. In this case, -one argument is expected, the argument being a list whose length the -function is measuring. (Note that _one_ list is _one_ argument, even -if the list has many elements inside it.) - -The part of the error message that says `length' is the name of the -function. - - -File: eintr, Node: nthcdr, Next: nth, Prev: cons, Up: car cdr & cons - -7.3 `nthcdr' -============ - -The `nthcdr' function is associated with the `cdr' function. What it -does is take the CDR of a list repeatedly. - -If you take the CDR of the list `(pine fir oak maple)', you will be -returned the list `(fir oak maple)'. If you repeat this on what was -returned, you will be returned the list `(oak maple)'. (Of course, -repeated CDRing on the original list will just give you the original -CDR since the function does not change the list. You need to evaluate -the CDR of the CDR and so on.) If you continue this, eventually you -will be returned an empty list, which in this case, instead of being -shown as `()' is shown as `nil'. - -For review, here is a series of repeated CDRs, the text following the -`=>' shows what is returned. - - (cdr '(pine fir oak maple)) - =>(fir oak maple) - - (cdr '(fir oak maple)) - => (oak maple) - - (cdr '(oak maple)) - =>(maple) - - (cdr '(maple)) - => nil - - (cdr 'nil) - => nil - - (cdr ()) - => nil - -You can also do several CDRs without printing the values in between, -like this: - - (cdr (cdr '(pine fir oak maple))) - => (oak maple) - -In this example, the Lisp interpreter evaluates the innermost list -first. The innermost list is quoted, so it just passes the list as it -is to the innermost `cdr'. This `cdr' passes a list made up of the -second and subsequent elements of the list to the outermost `cdr', -which produces a list composed of the third and subsequent elements of -the original list. In this example, the `cdr' function is repeated and -returns a list that consists of the original list without its first two -elements. - -The `nthcdr' function does the same as repeating the call to `cdr'. In -the following example, the argument 2 is passed to the function -`nthcdr', along with the list, and the value returned is the list -without its first two items, which is exactly the same as repeating -`cdr' twice on the list: - - (nthcdr 2 '(pine fir oak maple)) - => (oak maple) - -Using the original four element list, we can see what happens when -various numeric arguments are passed to `nthcdr', including 0, 1, and 5: - - ;; Leave the list as it was. - (nthcdr 0 '(pine fir oak maple)) - => (pine fir oak maple) - - ;; Return a copy without the first element. - (nthcdr 1 '(pine fir oak maple)) - => (fir oak maple) - - ;; Return a copy of the list without three elements. - (nthcdr 3 '(pine fir oak maple)) - => (maple) - - ;; Return a copy lacking all four elements. - (nthcdr 4 '(pine fir oak maple)) - => nil - - ;; Return a copy lacking all elements. - (nthcdr 5 '(pine fir oak maple)) - => nil - - -File: eintr, Node: nth, Next: setcar, Prev: nthcdr, Up: car cdr & cons - -7.4 `nth' -========= - -The `nthcdr' function takes the CDR of a list repeatedly. The `nth' -function takes the CAR of the result returned by `nthcdr'. It returns -the Nth element of the list. - -Thus, if it were not defined in C for speed, the definition of `nth' -would be: - - (defun nth (n list) - "Returns the Nth element of LIST. - N counts from zero. If LIST is not that long, nil is returned." - (car (nthcdr n list))) - -(Originally, `nth' was defined in Emacs Lisp in `subr.el', but its -definition was redone in C in the 1980s.) - -The `nth' function returns a single element of a list. This can be -very convenient. - -Note that the elements are numbered from zero, not one. That is to -say, the first element of a list, its CAR is the zeroth element. This -is called `zero-based' counting and often bothers people who are -accustomed to the first element in a list being number one, which is -`one-based'. - -For example: - - (nth 0 '("one" "two" "three")) - => "one" - - (nth 1 '("one" "two" "three")) - => "two" - -It is worth mentioning that `nth', like `nthcdr' and `cdr', does not -change the original list--the function is non-destructive. This is in -sharp contrast to the `setcar' and `setcdr' functions. - - -File: eintr, Node: setcar, Next: setcdr, Prev: nth, Up: car cdr & cons - -7.5 `setcar' -============ - -As you might guess from their names, the `setcar' and `setcdr' -functions set the CAR or the CDR of a list to a new value. They -actually change the original list, unlike `car' and `cdr' which leave -the original list as it was. One way to find out how this works is to -experiment. We will start with the `setcar' function. - -First, we can make a list and then set the value of a variable to the -list, using the `setq' function. Here is a list of animals: - - (setq animals '(antelope giraffe lion tiger)) - -If you are reading this in Info inside of GNU Emacs, you can evaluate -this expression in the usual fashion, by positioning the cursor after -the expression and typing `C-x C-e'. (I'm doing this right here as I -write this. This is one of the advantages of having the interpreter -built into the computing environment. Incidentally, when there is -nothing on the line after the final parentheses, such as a comment, -point can be on the next line. Thus, if your cursor is in the first -column of the next line, you do not need to move it. Indeed, Emacs -permits any amount of white space after the final parenthesis.) - -When we evaluate the variable `animals', we see that it is bound to the -list `(antelope giraffe lion tiger)': - - animals - => (antelope giraffe lion tiger) - -Put another way, the variable `animals' points to the list `(antelope -giraffe lion tiger)'. - -Next, evaluate the function `setcar' while passing it two arguments, -the variable `animals' and the quoted symbol `hippopotamus'; this is -done by writing the three element list `(setcar animals 'hippopotamus)' -and then evaluating it in the usual fashion: - - (setcar animals 'hippopotamus) - -After evaluating this expression, evaluate the variable `animals' -again. You will see that the list of animals has changed: - - animals - => (hippopotamus giraffe lion tiger) - -The first element on the list, `antelope' is replaced by `hippopotamus'. - -So we can see that `setcar' did not add a new element to the list as -`cons' would have; it replaced `antelope' with `hippopotamus'; it -_changed_ the list. - - -File: eintr, Node: setcdr, Next: cons Exercise, Prev: setcar, Up: car cdr & cons - -7.6 `setcdr' -============ - -The `setcdr' function is similar to the `setcar' function, except that -the function replaces the second and subsequent elements of a list -rather than the first element. - -(To see how to change the last element of a list, look ahead to *Note -The `kill-new' function: kill-new function, which uses the `nthcdr' and -`setcdr' functions.) - -To see how this works, set the value of the variable to a list of -domesticated animals by evaluating the following expression: - - (setq domesticated-animals '(horse cow sheep goat)) - -If you now evaluate the list, you will be returned the list `(horse cow -sheep goat)': - - domesticated-animals - => (horse cow sheep goat) - -Next, evaluate `setcdr' with two arguments, the name of the variable -which has a list as its value, and the list to which the CDR of the -first list will be set; - - (setcdr domesticated-animals '(cat dog)) - -If you evaluate this expression, the list `(cat dog)' will appear in -the echo area. This is the value returned by the function. The result -we are interested in is the "side effect", which we can see by -evaluating the variable `domesticated-animals': - - domesticated-animals - => (horse cat dog) - -Indeed, the list is changed from `(horse cow sheep goat)' to `(horse -cat dog)'. The CDR of the list is changed from `(cow sheep goat)' to -`(cat dog)'. - - -File: eintr, Node: cons Exercise, Prev: setcdr, Up: car cdr & cons - -7.7 Exercise -============ - -Construct a list of four birds by evaluating several expressions with -`cons'. Find out what happens when you `cons' a list onto itself. -Replace the first element of the list of four birds with a fish. -Replace the rest of that list with a list of other fish. - - -File: eintr, Node: Cutting & Storing Text, Next: List Implementation, Prev: car cdr & cons, Up: Top - -8 Cutting and Storing Text -************************** - -Whenever you cut or clip text out of a buffer with a `kill' command in -GNU Emacs, it is stored in a list and you can bring it back with a -`yank' command. - -(The use of the word `kill' in Emacs for processes which specifically -_do not_ destroy the values of the entities is an unfortunate -historical accident. A much more appropriate word would be `clip' since -that is what the kill commands do; they clip text out of a buffer and -put it into storage from which it can be brought back. I have often -been tempted to replace globally all occurrences of `kill' in the Emacs -sources with `clip' and all occurrences of `killed' with `clipped'.) - -* Menu: - -* Storing Text:: -* zap-to-char:: -* kill-region:: -* copy-region-as-kill:: -* Digression into C:: -* defvar:: -* cons & search-fwd Review:: -* search Exercises:: - - -File: eintr, Node: Storing Text, Next: zap-to-char, Prev: Cutting & Storing Text, Up: Cutting & Storing Text - -Storing Text in a List -====================== - -When text is cut out of a buffer, it is stored on a list. Successive -pieces of text are stored on the list successively, so the list might -look like this: - - ("a piece of text" "previous piece") - -The function `cons' can be used to create a new list from a piece of -text (an `atom', to use the jargon) and an existing list, like this: - - (cons "another piece" - '("a piece of text" "previous piece")) - -If you evaluate this expression, a list of three elements will appear in -the echo area: - - ("another piece" "a piece of text" "previous piece") - -With the `car' and `nthcdr' functions, you can retrieve whichever piece -of text you want. For example, in the following code, `nthcdr 1 ...' -returns the list with the first item removed; and the `car' returns the -first element of that remainder--the second element of the original -list: - - (car (nthcdr 1 '("another piece" - "a piece of text" - "previous piece"))) - => "a piece of text" - -The actual functions in Emacs are more complex than this, of course. -The code for cutting and retrieving text has to be written so that -Emacs can figure out which element in the list you want--the first, -second, third, or whatever. In addition, when you get to the end of -the list, Emacs should give you the first element of the list, rather -than nothing at all. - -The list that holds the pieces of text is called the "kill ring". This -chapter leads up to a description of the kill ring and how it is used -by first tracing how the `zap-to-char' function works. This function -uses (or `calls') a function that invokes a function that manipulates -the kill ring. Thus, before reaching the mountains, we climb the -foothills. - -A subsequent chapter describes how text that is cut from the buffer is -retrieved. *Note Yanking Text Back: Yanking. - - -File: eintr, Node: zap-to-char, Next: kill-region, Prev: Storing Text, Up: Cutting & Storing Text - -8.1 `zap-to-char' -================= - -The `zap-to-char' function changed a little between GNU Emacs version -19 and GNU Emacs version 22. However, `zap-to-char' calls another -function, `kill-region', which enjoyed a major rewrite. - -The `kill-region' function in Emacs 19 is complex, but does not use -code that is important at this time. We will skip it. - -The `kill-region' function in Emacs 22 is easier to read than the same -function in Emacs 19 and introduces a very important concept, that of -error handling. We will walk through the function. - -But first, let us look at the interactive `zap-to-char' function. - -* Menu: - -* Complete zap-to-char:: -* zap-to-char interactive:: -* zap-to-char body:: -* search-forward:: -* progn:: -* Summing up zap-to-char:: - - -File: eintr, Node: Complete zap-to-char, Next: zap-to-char interactive, Prev: zap-to-char, Up: zap-to-char - -The Complete `zap-to-char' Implementation ------------------------------------------ - -The GNU Emacs version 19 and version 21 implementations of the -`zap-to-char' function are nearly identical in form, and they work -alike. The function removes the text in the region between the -location of the cursor (i.e., of point) up to and including the next -occurrence of a specified character. The text that `zap-to-char' -removes is put in the kill ring; and it can be retrieved from the kill -ring by typing `C-y' (`yank'). If the command is given an argument, it -removes text through that number of occurrences. Thus, if the cursor -were at the beginning of this sentence and the character were `s', -`Thus' would be removed. If the argument were two, `Thus, if the curs' -would be removed, up to and including the `s' in `cursor'. - -If the specified character is not found, `zap-to-char' will say "Search -failed", tell you the character you typed, and not remove any text. - -In order to determine how much text to remove, `zap-to-char' uses a -search function. Searches are used extensively in code that -manipulates text, and we will focus attention on them as well as on the -deletion command. - -Here is the complete text of the version 22 implementation of the -function: - - (defun zap-to-char (arg char) - "Kill up to and including ARG'th occurrence of CHAR. - Case is ignored if `case-fold-search' is non-nil in the current buffer. - Goes backward if ARG is negative; error if CHAR not found." - (interactive "p\ncZap to char: ") - (if (char-table-p translation-table-for-input) - (setq char (or (aref translation-table-for-input char) char))) - (kill-region (point) (progn - (search-forward (char-to-string char) nil nil arg) - (point)))) - - -File: eintr, Node: zap-to-char interactive, Next: zap-to-char body, Prev: Complete zap-to-char, Up: zap-to-char - -8.1.1 The `interactive' Expression ----------------------------------- - -The interactive expression in the `zap-to-char' command looks like this: - - (interactive "p\ncZap to char: ") - -The part within quotation marks, `"p\ncZap to char: "', specifies two -different things. First, and most simply, is the `p'. This part is -separated from the next part by a newline, `\n'. The `p' means that -the first argument to the function will be passed the value of a -`processed prefix'. The prefix argument is passed by typing `C-u' and -a number, or `M-' and a number. If the function is called -interactively without a prefix, 1 is passed to this argument. - -The second part of `"p\ncZap to char: "' is `cZap to char: '. In this -part, the lower case `c' indicates that `interactive' expects a prompt -and that the argument will be a character. The prompt follows the `c' -and is the string `Zap to char: ' (with a space after the colon to make -it look good). - -What all this does is prepare the arguments to `zap-to-char' so they -are of the right type, and give the user a prompt. - -In a read-only buffer, the `zap-to-char' function copies the text to -the kill ring, but does not remove it. The echo area displays a -message saying that the buffer is read-only. Also, the terminal may -beep or blink at you. - -Let us continue with the interactive specification. - - -File: eintr, Node: zap-to-char body, Next: search-forward, Prev: zap-to-char interactive, Up: zap-to-char - -8.1.2 The Body of `zap-to-char' -------------------------------- - -The body of the `zap-to-char' function contains the code that kills -(that is, removes) the text in the region from the current position of -the cursor up to and including the specified character. - -The documentation is thorough. You do need to know the jargon meaning -of the word `kill'. - -The first part of the code looks like this: - - (if (char-table-p translation-table-for-input) - (setq char (or (aref translation-table-for-input char) char))) - (kill-region (point) (progn - (search-forward (char-to-string char) nil nil arg) - (point))) - -`char-table-p' is an hitherto unseen function. It determines whether -its argument is a character table. When it is, it sets the character -passed to `zap-to-char' to one of them, if that character exists, or to -the character itself. (This becomes important for certain characters -in non-European languages. The `aref' function extracts an element -from an array. It is an array-specific function that is not described -in this document. *Note Arrays: (elisp)Arrays.) - -`(point)' is the current position of the cursor. - -The next part of the code is an expression using `progn'. The body of -the `progn' consists of calls to `search-forward' and `point'. - -It is easier to understand how `progn' works after learning about -`search-forward', so we will look at `search-forward' and then at -`progn'. - - -File: eintr, Node: search-forward, Next: progn, Prev: zap-to-char body, Up: zap-to-char - -8.1.3 The `search-forward' Function ------------------------------------ - -The `search-forward' function is used to locate the -zapped-for-character in `zap-to-char'. If the search is successful, -`search-forward' leaves point immediately after the last character in -the target string. (In `zap-to-char', the target string is just one -character long. `zap-to-char' uses the function `char-to-string' to -ensure that the computer treats that character as a string.) If the -search is backwards, `search-forward' leaves point just before the -first character in the target. Also, `search-forward' returns `t' for -true. (Moving point is therefore a `side effect'.) - -In `zap-to-char', the `search-forward' function looks like this: - - (search-forward (char-to-string char) nil nil arg) - -The `search-forward' function takes four arguments: - - 1. The first argument is the target, what is searched for. This must - be a string, such as `"z"'. - - As it happens, the argument passed to `zap-to-char' is a single - character. Because of the way computers are built, the Lisp - interpreter may treat a single character as being different from a - string of characters. Inside the computer, a single character has - a different electronic format than a string of one character. (A - single character can often be recorded in the computer using - exactly one byte; but a string may be longer, and the computer - needs to be ready for this.) Since the `search-forward' function - searches for a string, the character that the `zap-to-char' - function receives as its argument must be converted inside the - computer from one format to the other; otherwise the - `search-forward' function will fail. The `char-to-string' - function is used to make this conversion. - - 2. The second argument bounds the search; it is specified as a - position in the buffer. In this case, the search can go to the - end of the buffer, so no bound is set and the second argument is - `nil'. - - 3. The third argument tells the function what it should do if the - search fails--it can signal an error (and print a message) or it - can return `nil'. A `nil' as the third argument causes the - function to signal an error when the search fails. - - 4. The fourth argument to `search-forward' is the repeat count--how - many occurrences of the string to look for. This argument is - optional and if the function is called without a repeat count, - this argument is passed the value 1. If this argument is - negative, the search goes backwards. - -In template form, a `search-forward' expression looks like this: - - (search-forward "TARGET-STRING" - LIMIT-OF-SEARCH - WHAT-TO-DO-IF-SEARCH-FAILS - REPEAT-COUNT) - -We will look at `progn' next. - - -File: eintr, Node: progn, Next: Summing up zap-to-char, Prev: search-forward, Up: zap-to-char - -8.1.4 The `progn' Special Form ------------------------------- - -`progn' is a special form that causes each of its arguments to be -evaluated in sequence and then returns the value of the last one. The -preceding expressions are evaluated only for the side effects they -perform. The values produced by them are discarded. - -The template for a `progn' expression is very simple: - - (progn - BODY...) - -In `zap-to-char', the `progn' expression has to do two things: put -point in exactly the right position; and return the location of point -so that `kill-region' will know how far to kill to. - -The first argument to the `progn' is `search-forward'. When -`search-forward' finds the string, the function leaves point -immediately after the last character in the target string. (In this -case the target string is just one character long.) If the search is -backwards, `search-forward' leaves point just before the first -character in the target. The movement of point is a side effect. - -The second and last argument to `progn' is the expression `(point)'. -This expression returns the value of point, which in this case will be -the location to which it has been moved by `search-forward'. (In the -source, a line that tells the function to go to the previous character, -if it is going forward, was commented out in 1999; I don't remember -whether that feature or mis-feature was ever a part of the distributed -source.) The value of `point' is returned by the `progn' expression -and is passed to `kill-region' as `kill-region''s second argument. - - -File: eintr, Node: Summing up zap-to-char, Prev: progn, Up: zap-to-char - -8.1.5 Summing up `zap-to-char' ------------------------------- - -Now that we have seen how `search-forward' and `progn' work, we can see -how the `zap-to-char' function works as a whole. - -The first argument to `kill-region' is the position of the cursor when -the `zap-to-char' command is given--the value of point at that time. -Within the `progn', the search function then moves point to just after -the zapped-to-character and `point' returns the value of this location. -The `kill-region' function puts together these two values of point, -the first one as the beginning of the region and the second one as the -end of the region, and removes the region. - -The `progn' special form is necessary because the `kill-region' command -takes two arguments; and it would fail if `search-forward' and `point' -expressions were written in sequence as two additional arguments. The -`progn' expression is a single argument to `kill-region' and returns -the one value that `kill-region' needs for its second argument. - - -File: eintr, Node: kill-region, Next: copy-region-as-kill, Prev: zap-to-char, Up: Cutting & Storing Text - -8.2 `kill-region' -================= - -The `zap-to-char' function uses the `kill-region' function. This -function clips text from a region and copies that text to the kill -ring, from which it may be retrieved. - -The Emacs 22 version of that function uses `condition-case' and -`copy-region-as-kill', both of which we will explain. `condition-case' -is an important special form. - -In essence, the `kill-region' function calls `condition-case', which -takes three arguments. In this function, the first argument does -nothing. The second argument contains the code that does the work when -all goes well. The third argument contains the code that is called in -the event of an error. - -* Menu: - -* Complete kill-region:: -* condition-case:: -* Lisp macro:: - - -File: eintr, Node: Complete kill-region, Next: condition-case, Prev: kill-region, Up: kill-region - -The Complete `kill-region' Definition -------------------------------------- - -We will go through the `condition-case' code in a moment. First, let -us look at the definition of `kill-region', with comments added: - - (defun kill-region (beg end) - "Kill (\"cut\") text between point and mark. - This deletes the text from the buffer and saves it in the kill ring. - The command \\[yank] can retrieve it from there. ... " - - ;; * Since order matters, pass point first. - (interactive (list (point) (mark))) - ;; * And tell us if we cannot cut the text. - (unless (and beg end) - (error "The mark is not set now, so there is no region")) - - ;; * `condition-case' takes three arguments. - ;; If the first argument is nil, as it is here, - ;; information about the error signal is not - ;; stored for use by another function. - (condition-case nil - - ;; * The second argument to `condition-case' tells the - ;; Lisp interpreter what to do when all goes well. - - ;; It starts with a `let' function that extracts the string - ;; and tests whether it exists. If so (that is what the - ;; `when' checks), it calls an `if' function that determines - ;; whether the previous command was another call to - ;; `kill-region'; if it was, then the new text is appended to - ;; the previous text; if not, then a different function, - ;; `kill-new', is called. - - ;; The `kill-append' function concatenates the new string and - ;; the old. The `kill-new' function inserts text into a new - ;; item in the kill ring. - - ;; `when' is an `if' without an else-part. The second `when' - ;; again checks whether the current string exists; in - ;; addition, it checks whether the previous command was - ;; another call to `kill-region'. If one or the other - ;; condition is true, then it sets the current command to - ;; be `kill-region'. - (let ((string (filter-buffer-substring beg end t))) - (when string ;STRING is nil if BEG = END - ;; Add that string to the kill ring, one way or another. - (if (eq last-command 'kill-region) - ;; - `yank-handler' is an optional argument to - ;; `kill-region' that tells the `kill-append' and - ;; `kill-new' functions how deal with properties - ;; added to the text, such as `bold' or `italics'. - (kill-append string (< end beg) yank-handler) - (kill-new string nil yank-handler))) - (when (or string (eq last-command 'kill-region)) - (setq this-command 'kill-region)) - nil) - - ;; * The third argument to `condition-case' tells the interpreter - ;; what to do with an error. - ;; The third argument has a conditions part and a body part. - ;; If the conditions are met (in this case, - ;; if text or buffer are read-only) - ;; then the body is executed. - ;; The first part of the third argument is the following: - ((buffer-read-only text-read-only) ;; the if-part - ;; ... the then-part - (copy-region-as-kill beg end) - ;; Next, also as part of the then-part, set this-command, so - ;; it will be set in an error - (setq this-command 'kill-region) - ;; Finally, in the then-part, send a message if you may copy - ;; the text to the kill ring without signally an error, but - ;; don't if you may not. - (if kill-read-only-ok - (progn (message "Read only text copied to kill ring") nil) - (barf-if-buffer-read-only) - ;; If the buffer isn't read-only, the text is. - (signal 'text-read-only (list (current-buffer))))) - - -File: eintr, Node: condition-case, Next: Lisp macro, Prev: Complete kill-region, Up: kill-region - -8.2.1 `condition-case' ----------------------- - -As we have seen earlier (*note Generate an Error Message: Making -Errors.), when the Emacs Lisp interpreter has trouble evaluating an -expression, it provides you with help; in the jargon, this is called -"signaling an error". Usually, the computer stops the program and -shows you a message. - -However, some programs undertake complicated actions. They should not -simply stop on an error. In the `kill-region' function, the most -likely error is that you will try to kill text that is read-only and -cannot be removed. So the `kill-region' function contains code to -handle this circumstance. This code, which makes up the body of the -`kill-region' function, is inside of a `condition-case' special form. - -The template for `condition-case' looks like this: - - (condition-case - VAR - BODYFORM - ERROR-HANDLER...) - -The second argument, BODYFORM, is straightforward. The -`condition-case' special form causes the Lisp interpreter to evaluate -the code in BODYFORM. If no error occurs, the special form returns the -code's value and produces the side-effects, if any. - -In short, the BODYFORM part of a `condition-case' expression determines -what should happen when everything works correctly. - -However, if an error occurs, among its other actions, the function -generating the error signal will define one or more error condition -names. - -An error handler is the third argument to `condition case'. An error -handler has two parts, a CONDITION-NAME and a BODY. If the -CONDITION-NAME part of an error handler matches a condition name -generated by an error, then the BODY part of the error handler is run. - -As you will expect, the CONDITION-NAME part of an error handler may be -either a single condition name or a list of condition names. - -Also, a complete `condition-case' expression may contain more than one -error handler. When an error occurs, the first applicable handler is -run. - -Lastly, the first argument to the `condition-case' expression, the VAR -argument, is sometimes bound to a variable that contains information -about the error. However, if that argument is nil, as is the case in -`kill-region', that information is discarded. - -In brief, in the `kill-region' function, the code `condition-case' -works like this: - - IF NO ERRORS, RUN ONLY THIS CODE - BUT, IF ERRORS, RUN THIS OTHER CODE. - - -File: eintr, Node: Lisp macro, Prev: condition-case, Up: kill-region - -8.2.2 Lisp macro ----------------- - -The part of the `condition-case' expression that is evaluated in the -expectation that all goes well has a `when'. The code uses `when' to -determine whether the `string' variable points to text that exists. - -A `when' expression is simply a programmers' convenience. It is an -`if' without the possibility of an else clause. In your mind, you can -replace `when' with `if' and understand what goes on. That is what the -Lisp interpreter does. - -Technically speaking, `when' is a Lisp macro. A Lisp "macro" enables -you to define new control constructs and other language features. It -tells the interpreter how to compute another Lisp expression which will -in turn compute the value. In this case, the `other expression' is an -`if' expression. For more about Lisp macros, see *Note Macros: -(elisp)Macros. The C programming language also provides macros. These -are different, but also useful. - -If the string has content, then another conditional expression is -executed. This is an `if' with both a then-part and an else-part. - - (if (eq last-command 'kill-region) - (kill-append string (< end beg) yank-handler) - (kill-new string nil yank-handler)) - -The then-part is evaluated if the previous command was another call to -`kill-region'; if not, the else-part is evaluated. - -`yank-handler' is an optional argument to `kill-region' that tells the -`kill-append' and `kill-new' functions how deal with properties added -to the text, such as `bold' or `italics'. - -`last-command' is a variable that comes with Emacs that we have not -seen before. Normally, whenever a function is executed, Emacs sets the -value of `last-command' to the previous command. - -In this segment of the definition, the `if' expression checks whether -the previous command was `kill-region'. If it was, - - (kill-append string (< end beg) yank-handler) - -concatenates a copy of the newly clipped text to the just previously -clipped text in the kill ring. - - -File: eintr, Node: copy-region-as-kill, Next: Digression into C, Prev: kill-region, Up: Cutting & Storing Text - -8.3 `copy-region-as-kill' -========================= - -The `copy-region-as-kill' function copies a region of text from a -buffer and (via either `kill-append' or `kill-new') saves it in the -`kill-ring'. - -If you call `copy-region-as-kill' immediately after a `kill-region' -command, Emacs appends the newly copied text to the previously copied -text. This means that if you yank back the text, you get it all, from -both this and the previous operation. On the other hand, if some other -command precedes the `copy-region-as-kill', the function copies the -text into a separate entry in the kill ring. - -* Menu: - -* Complete copy-region-as-kill:: -* copy-region-as-kill body:: - - -File: eintr, Node: Complete copy-region-as-kill, Next: copy-region-as-kill body, Prev: copy-region-as-kill, Up: copy-region-as-kill - -The complete `copy-region-as-kill' function definition ------------------------------------------------------- - -Here is the complete text of the version 22 `copy-region-as-kill' -function: - - (defun copy-region-as-kill (beg end) - "Save the region as if killed, but don't kill it. - In Transient Mark mode, deactivate the mark. - If `interprogram-cut-function' is non-nil, also save the text for a window - system cut and paste." - (interactive "r") - (if (eq last-command 'kill-region) - (kill-append (filter-buffer-substring beg end) (< end beg)) - (kill-new (filter-buffer-substring beg end))) - (if transient-mark-mode - (setq deactivate-mark t)) - nil) - -As usual, this function can be divided into its component parts: - - (defun copy-region-as-kill (ARGUMENT-LIST) - "DOCUMENTATION..." - (interactive "r") - BODY...) - -The arguments are `beg' and `end' and the function is interactive with -`"r"', so the two arguments must refer to the beginning and end of the -region. If you have been reading though this document from the -beginning, understanding these parts of a function is almost becoming -routine. - -The documentation is somewhat confusing unless you remember that the -word `kill' has a meaning different from usual. The `Transient Mark' -and `interprogram-cut-function' comments explain certain side-effects. - -After you once set a mark, a buffer always contains a region. If you -wish, you can use Transient Mark mode to highlight the region -temporarily. (No one wants to highlight the region all the time, so -Transient Mark mode highlights it only at appropriate times. Many -people turn off Transient Mark mode, so the region is never -highlighted.) - -Also, a windowing system allows you to copy, cut, and paste among -different programs. In the X windowing system, for example, the -`interprogram-cut-function' function is `x-select-text', which works -with the windowing system's equivalent of the Emacs kill ring. - -The body of the `copy-region-as-kill' function starts with an `if' -clause. What this clause does is distinguish between two different -situations: whether or not this command is executed immediately after a -previous `kill-region' command. In the first case, the new region is -appended to the previously copied text. Otherwise, it is inserted into -the beginning of the kill ring as a separate piece of text from the -previous piece. - -The last two lines of the function prevent the region from lighting up -if Transient Mark mode is turned on. - -The body of `copy-region-as-kill' merits discussion in detail. - - -File: eintr, Node: copy-region-as-kill body, Prev: Complete copy-region-as-kill, Up: copy-region-as-kill - -8.3.1 The Body of `copy-region-as-kill' ---------------------------------------- - -The `copy-region-as-kill' function works in much the same way as the -`kill-region' function. Both are written so that two or more kills in -a row combine their text into a single entry. If you yank back the -text from the kill ring, you get it all in one piece. Moreover, kills -that kill forward from the current position of the cursor are added to -the end of the previously copied text and commands that copy text -backwards add it to the beginning of the previously copied text. This -way, the words in the text stay in the proper order. - -Like `kill-region', the `copy-region-as-kill' function makes use of the -`last-command' variable that keeps track of the previous Emacs command. - -* Menu: - -* last-command & this-command:: -* kill-append function:: -* kill-new function:: - - -File: eintr, Node: last-command & this-command, Next: kill-append function, Prev: copy-region-as-kill body, Up: copy-region-as-kill body - -`last-command' and `this-command' -................................. - -Normally, whenever a function is executed, Emacs sets the value of -`this-command' to the function being executed (which in this case would -be `copy-region-as-kill'). At the same time, Emacs sets the value of -`last-command' to the previous value of `this-command'. - -In the first part of the body of the `copy-region-as-kill' function, an -`if' expression determines whether the value of `last-command' is -`kill-region'. If so, the then-part of the `if' expression is -evaluated; it uses the `kill-append' function to concatenate the text -copied at this call to the function with the text already in the first -element (the CAR) of the kill ring. On the other hand, if the value of -`last-command' is not `kill-region', then the `copy-region-as-kill' -function attaches a new element to the kill ring using the `kill-new' -function. - -The `if' expression reads as follows; it uses `eq', which is a function -we have not yet seen: - - (if (eq last-command 'kill-region) - ;; then-part - (kill-append (filter-buffer-substring beg end) (< end beg)) - ;; else-part - (kill-new (filter-buffer-substring beg end))) - -(The `filter-buffer-substring' function returns a filtered substring of -the buffer, if any. Optionally--the arguments are not here, so neither -is done--the function may delete the initial text or return the text -without its properties; this function is a replacement for the older -`buffer-substring' function, which came before text properties were -implemented.) - -The `eq' function tests whether its first argument is the same Lisp -object as its second argument. The `eq' function is similar to the -`equal' function in that it is used to test for equality, but differs -in that it determines whether two representations are actually the same -object inside the computer, but with different names. `equal' -determines whether the structure and contents of two expressions are -the same. - -If the previous command was `kill-region', then the Emacs Lisp -interpreter calls the `kill-append' function - - -File: eintr, Node: kill-append function, Next: kill-new function, Prev: last-command & this-command, Up: copy-region-as-kill body - -The `kill-append' function -.......................... - -The `kill-append' function looks like this: - - (defun kill-append (string before-p &optional yank-handler) - "Append STRING to the end of the latest kill in the kill ring. - If BEFORE-P is non-nil, prepend STRING to the kill. - ... " - (let* ((cur (car kill-ring))) - (kill-new (if before-p (concat string cur) (concat cur string)) - (or (= (length cur) 0) - (equal yank-handler (get-text-property 0 'yank-handler cur))) - yank-handler))) - -The `kill-append' function is fairly straightforward. It uses the -`kill-new' function, which we will discuss in more detail in a moment. - -(Also, the function provides an optional argument called -`yank-handler'; when invoked, this argument tells the function how to -deal with properties added to the text, such as `bold' or `italics'.) - -It has a `let*' function to set the value of the first element of the -kill ring to `cur'. (I do not know why the function does not use `let' -instead; only one value is set in the expression. Perhaps this is a -bug that produces no problems?) - -Consider the conditional that is one of the two arguments to -`kill-new'. It uses `concat' to concatenate the new text to the CAR of -the kill ring. Whether it prepends or appends the text depends on the -results of an `if' expression: - - (if before-p ; if-part - (concat string cur) ; then-part - (concat cur string)) ; else-part - -If the region being killed is before the region that was killed in the -last command, then it should be prepended before the material that was -saved in the previous kill; and conversely, if the killed text follows -what was just killed, it should be appended after the previous text. -The `if' expression depends on the predicate `before-p' to decide -whether the newly saved text should be put before or after the -previously saved text. - -The symbol `before-p' is the name of one of the arguments to -`kill-append'. When the `kill-append' function is evaluated, it is -bound to the value returned by evaluating the actual argument. In this -case, this is the expression `(< end beg)'. This expression does not -directly determine whether the killed text in this command is located -before or after the kill text of the last command; what it does is -determine whether the value of the variable `end' is less than the -value of the variable `beg'. If it is, it means that the user is most -likely heading towards the beginning of the buffer. Also, the result -of evaluating the predicate expression, `(< end beg)', will be true and -the text will be prepended before the previous text. On the other -hand, if the value of the variable `end' is greater than the value of -the variable `beg', the text will be appended after the previous text. - -When the newly saved text will be prepended, then the string with the -new text will be concatenated before the old text: - - (concat string cur) - -But if the text will be appended, it will be concatenated after the old -text: - - (concat cur string)) - -To understand how this works, we first need to review the `concat' -function. The `concat' function links together or unites two strings -of text. The result is a string. For example: - - (concat "abc" "def") - => "abcdef" - - (concat "new " - (car '("first element" "second element"))) - => "new first element" - - (concat (car - '("first element" "second element")) " modified") - => "first element modified" - -We can now make sense of `kill-append': it modifies the contents of the -kill ring. The kill ring is a list, each element of which is saved -text. The `kill-append' function uses the `kill-new' function which in -turn uses the `setcar' function. - - -File: eintr, Node: kill-new function, Prev: kill-append function, Up: copy-region-as-kill body - -The `kill-new' function -....................... - -The `kill-new' function looks like this: - - (defun kill-new (string &optional replace yank-handler) - "Make STRING the latest kill in the kill ring. - Set `kill-ring-yank-pointer' to point to it. - - If `interprogram-cut-function' is non-nil, apply it to STRING. - Optional second argument REPLACE non-nil means that STRING will replace - the front of the kill ring, rather than being added to the list. - ..." - (if (> (length string) 0) - (if yank-handler - (put-text-property 0 (length string) - 'yank-handler yank-handler string)) - (if yank-handler - (signal 'args-out-of-range - (list string "yank-handler specified for empty string")))) - (if (fboundp 'menu-bar-update-yank-menu) - (menu-bar-update-yank-menu string (and replace (car kill-ring)))) - (if (and replace kill-ring) - (setcar kill-ring string) - (push string kill-ring) - (if (> (length kill-ring) kill-ring-max) - (setcdr (nthcdr (1- kill-ring-max) kill-ring) nil))) - (setq kill-ring-yank-pointer kill-ring) - (if interprogram-cut-function - (funcall interprogram-cut-function string (not replace)))) - -(Notice that the function is not interactive.) - -As usual, we can look at this function in parts. - -The function definition has an optional `yank-handler' argument, which -when invoked tells the function how to deal with properties added to -the text, such as `bold' or `italics'. We will skip that. - -The first line of the documentation makes sense: - - Make STRING the latest kill in the kill ring. - -Let's skip over the rest of the documentation for the moment. - -Also, let's skip over the initial `if' expression and those lines of -code involving `menu-bar-update-yank-menu'. We will explain them below. - -The critical lines are these: - - (if (and replace kill-ring) - ;; then - (setcar kill-ring string) - ;; else - (push string kill-ring) - (setq kill-ring (cons string kill-ring)) - (if (> (length kill-ring) kill-ring-max) - ;; avoid overly long kill ring - (setcdr (nthcdr (1- kill-ring-max) kill-ring) nil))) - (setq kill-ring-yank-pointer kill-ring) - (if interprogram-cut-function - (funcall interprogram-cut-function string (not replace)))) - -The conditional test is `(and replace kill-ring)'. This will be true -when two conditions are met: the kill ring has something in it, and -the `replace' variable is true. - -When the `kill-append' function sets `replace' to be true and when the -kill ring has at least one item in it, the `setcar' expression is -executed: - - (setcar kill-ring string) - -The `setcar' function actually changes the first element of the -`kill-ring' list to the value of `string'. It replaces the first -element. - -On the other hand, if the kill ring is empty, or replace is false, the -else-part of the condition is executed: - - (push string kill-ring) - -`push' puts its first argument onto the second. It is the same as the -older - - (setq kill-ring (cons string kill-ring)) - -or the newer - - (add-to-list kill-ring string) - -When it is false, the expression first constructs a new version of the -kill ring by prepending `string' to the existing kill ring as a new -element (that is what the `push' does). Then it executes a second `if' -clause. This second `if' clause keeps the kill ring from growing too -long. - -Let's look at these two expressions in order. - -The `push' line of the else-part sets the new value of the kill ring to -what results from adding the string being killed to the old kill ring. - -We can see how this works with an example. - -First, - - (setq example-list '("here is a clause" "another clause")) - -After evaluating this expression with `C-x C-e', you can evaluate -`example-list' and see what it returns: - - example-list - => ("here is a clause" "another clause") - -Now, we can add a new element on to this list by evaluating the -following expression: - - (push "a third clause" example-list) - -When we evaluate `example-list', we find its value is: - - example-list - => ("a third clause" "here is a clause" "another clause") - -Thus, the third clause is added to the list by `push'. - -Now for the second part of the `if' clause. This expression keeps the -kill ring from growing too long. It looks like this: - - (if (> (length kill-ring) kill-ring-max) - (setcdr (nthcdr (1- kill-ring-max) kill-ring) nil)) - -The code checks whether the length of the kill ring is greater than the -maximum permitted length. This is the value of `kill-ring-max' (which -is 60, by default). If the length of the kill ring is too long, then -this code sets the last element of the kill ring to `nil'. It does -this by using two functions, `nthcdr' and `setcdr'. - -We looked at `setcdr' earlier (*note `setcdr': setcdr.). It sets the -CDR of a list, just as `setcar' sets the CAR of a list. In this case, -however, `setcdr' will not be setting the CDR of the whole kill ring; -the `nthcdr' function is used to cause it to set the CDR of the next to -last element of the kill ring--this means that since the CDR of the -next to last element is the last element of the kill ring, it will set -the last element of the kill ring. - -The `nthcdr' function works by repeatedly taking the CDR of a list--it -takes the CDR of the CDR of the CDR ... It does this N times and -returns the results. (*Note `nthcdr': nthcdr.) - -Thus, if we had a four element list that was supposed to be three -elements long, we could set the CDR of the next to last element to -`nil', and thereby shorten the list. (If you sent the last element to -some other value than `nil', which you could do, then you would not -have shortened the list. *Note `setcdr': setcdr.) - -You can see shortening by evaluating the following three expressions in -turn. First set the value of `trees' to `(maple oak pine birch)', then -set the CDR of its second CDR to `nil' and then find the value of -`trees': - - (setq trees '(maple oak pine birch)) - => (maple oak pine birch) - - (setcdr (nthcdr 2 trees) nil) - => nil - - trees - => (maple oak pine) - -(The value returned by the `setcdr' expression is `nil' since that is -what the CDR is set to.) - -To repeat, in `kill-new', the `nthcdr' function takes the CDR a number -of times that is one less than the maximum permitted size of the kill -ring and `setcdr' sets the CDR of that element (which will be the rest -of the elements in the kill ring) to `nil'. This prevents the kill -ring from growing too long. - -The next to last expression in the `kill-new' function is - - (setq kill-ring-yank-pointer kill-ring) - -The `kill-ring-yank-pointer' is a global variable that is set to be the -`kill-ring'. - -Even though the `kill-ring-yank-pointer' is called a `pointer', it is a -variable just like the kill ring. However, the name has been chosen to -help humans understand how the variable is used. The variable is used -in functions such as `yank' and `yank-pop' (*note Yanking Text Back: -Yanking.). - -Now, to return to an early expression in the body of the function: - - (if (fboundp 'menu-bar-update-yank-menu) - (menu-bar-update-yank-menu string (and replace (car kill-ring)))) - -It starts with an `if' expression - -In this case, the expression tests first to see whether -`menu-bar-update-yank-menu' exists as a function, and if so, calls it. -The `fboundp' function returns true if the symbol it is testing has a -function definition that `is not void'. If the symbol's function -definition were void, we would receive an error message, as we did when -we created errors intentionally (*note Generate an Error Message: -Making Errors.). - -The then-part contains an expression whose first element is the -function `and'. - -The `and' special form evaluates each of its arguments until one of the -arguments returns a value of `nil', in which case the `and' expression -returns `nil'; however, if none of the arguments returns a value of -`nil', the value resulting from evaluating the last argument is -returned. (Since such a value is not `nil', it is considered true in -Emacs Lisp.) In other words, an `and' expression returns a true value -only if all its arguments are true. (*Note Second Buffer Related -Review::.) - -The expression determines whether the second argument to -`menu-bar-update-yank-menu' is true or not. - -`menu-bar-update-yank-menu' is one of the functions that make it -possible to use the `Select and Paste' menu in the Edit item of a menu -bar; using a mouse, you can look at the various pieces of text you have -saved and select one piece to paste. - -The last expression in the `kill-new' function adds the newly copied -string to whatever facility exists for copying and pasting among -different programs running in a windowing system. In the X Windowing -system, for example, the `x-select-text' function takes the string and -stores it in memory operated by X. You can paste the string in another -program, such as an Xterm. - -The expression looks like this: - - (if interprogram-cut-function - (funcall interprogram-cut-function string (not replace)))) - -If an `interprogram-cut-function' exists, then Emacs executes -`funcall', which in turn calls its first argument as a function and -passes the remaining arguments to it. (Incidentally, as far as I can -see, this `if' expression could be replaced by an `and' expression -similar to the one in the first part of the function.) - -We are not going to discuss windowing systems and other programs -further, but merely note that this is a mechanism that enables GNU -Emacs to work easily and well with other programs. - -This code for placing text in the kill ring, either concatenated with -an existing element or as a new element, leads us to the code for -bringing back text that has been cut out of the buffer--the yank -commands. However, before discussing the yank commands, it is better -to learn how lists are implemented in a computer. This will make clear -such mysteries as the use of the term `pointer'. - - -File: eintr, Node: Digression into C, Next: defvar, Prev: copy-region-as-kill, Up: Cutting & Storing Text - -8.4 Digression into C -===================== - -The `copy-region-as-kill' function (*note `copy-region-as-kill': -copy-region-as-kill.) uses the `filter-buffer-substring' function, -which in turn uses the `delete-and-extract-region' function. It -removes the contents of a region and you cannot get them back. - -Unlike the other code discussed here, the `delete-and-extract-region' -function is not written in Emacs Lisp; it is written in C and is one of -the primitives of the GNU Emacs system. Since it is very simple, I -will digress briefly from Lisp and describe it here. - -Like many of the other Emacs primitives, `delete-and-extract-region' is -written as an instance of a C macro, a macro being a template for code. -The complete macro looks like this: - - DEFUN ("buffer-substring-no-properties", Fbuffer_substring_no_properties, - Sbuffer_substring_no_properties, 2, 2, 0, - doc: /* Return the characters of part of the buffer, - without the text properties. - The two arguments START and END are character positions; - they can be in either order. */) - (start, end) - Lisp_Object start, end; - { - register int b, e; - - validate_region (&start, &end); - b = XINT (start); - e = XINT (end); - - return make_buffer_string (b, e, 0); - } - -Without going into the details of the macro writing process, let me -point out that this macro starts with the word `DEFUN'. The word -`DEFUN' was chosen since the code serves the same purpose as `defun' -does in Lisp. (The `DEFUN' C macro is defined in `emacs/src/lisp.h'.) - -The word `DEFUN' is followed by seven parts inside of parentheses: - - * The first part is the name given to the function in Lisp, - `delete-and-extract-region'. - - * The second part is the name of the function in C, - `Fdelete_and_extract_region'. By convention, it starts with `F'. - Since C does not use hyphens in names, underscores are used - instead. - - * The third part is the name for the C constant structure that - records information on this function for internal use. It is the - name of the function in C but begins with an `S' instead of an `F'. - - * The fourth and fifth parts specify the minimum and maximum number - of arguments the function can have. This function demands exactly - 2 arguments. - - * The sixth part is nearly like the argument that follows the - `interactive' declaration in a function written in Lisp: a letter - followed, perhaps, by a prompt. The only difference from the Lisp - is when the macro is called with no arguments. Then you write a - `0' (which is a `null string'), as in this macro. - - If you were to specify arguments, you would place them between - quotation marks. The C macro for `goto-char' includes `"NGoto - char: "' in this position to indicate that the function expects a - raw prefix, in this case, a numerical location in a buffer, and - provides a prompt. - - * The seventh part is a documentation string, just like the one for a - function written in Emacs Lisp, except that every newline must be - written explicitly as `\n' followed by a backslash and carriage - return. - - Thus, the first two lines of documentation for `goto-char' are - written like this: - - "Set point to POSITION, a number or marker.\n\ - Beginning of buffer is position (point-min), end is (point-max). - -In a C macro, the formal parameters come next, with a statement of what -kind of object they are, followed by what might be called the `body' of -the macro. For `delete-and-extract-region' the `body' consists of the -following four lines: - - validate_region (&start, &end); - if (XINT (start) == XINT (end)) - return build_string (""); - return del_range_1 (XINT (start), XINT (end), 1, 1); - -The `validate_region' function checks whether the values passed as -the beginning and end of the region are the proper type and are within -range. If the beginning and end positions are the same, then return -and empty string. - -The `del_range_1' function actually deletes the text. It is a complex -function we will not look into. It updates the buffer and does other -things. However, it is worth looking at the two arguments passed to -`del_range'. These are `XINT (start)' and `XINT (end)'. - -As far as the C language is concerned, `start' and `end' are two -integers that mark the beginning and end of the region to be deleted(1). - -In early versions of Emacs, these two numbers were thirty-two bits -long, but the code is slowly being generalized to handle other lengths. -Three of the available bits are used to specify the type of -information; the remaining bits are used as `content'. - -`XINT' is a C macro that extracts the relevant number from the longer -collection of bits; the three other bits are discarded. - -The command in `delete-and-extract-region' looks like this: - - del_range_1 (XINT (start), XINT (end), 1, 1); - -It deletes the region between the beginning position, `start', and the -ending position, `end'. - -From the point of view of the person writing Lisp, Emacs is all very -simple; but hidden underneath is a great deal of complexity to make it -all work. - ----------- Footnotes ---------- - -(1) More precisely, and requiring more expert knowledge to understand, -the two integers are of type `Lisp_Object', which can also be a C union -instead of an integer type. - - -File: eintr, Node: defvar, Next: cons & search-fwd Review, Prev: Digression into C, Up: Cutting & Storing Text - -8.5 Initializing a Variable with `defvar' -========================================= - -The `copy-region-as-kill' function is written in Emacs Lisp. Two -functions within it, `kill-append' and `kill-new', copy a region in a -buffer and save it in a variable called the `kill-ring'. This section -describes how the `kill-ring' variable is created and initialized using -the `defvar' special form. - -(Again we note that the term `kill-ring' is a misnomer. The text that -is clipped out of the buffer can be brought back; it is not a ring of -corpses, but a ring of resurrectable text.) - -In Emacs Lisp, a variable such as the `kill-ring' is created and given -an initial value by using the `defvar' special form. The name comes -from "define variable". - -The `defvar' special form is similar to `setq' in that it sets the -value of a variable. It is unlike `setq' in two ways: first, it only -sets the value of the variable if the variable does not already have a -value. If the variable already has a value, `defvar' does not override -the existing value. Second, `defvar' has a documentation string. - -(Another special form, `defcustom', is designed for variables that -people customize. It has more features than `defvar'. (*Note Setting -Variables with `defcustom': defcustom.) - -* Menu: - -* See variable current value:: -* defvar and asterisk:: - diff --git a/info/eintr-2 b/info/eintr-2 deleted file mode 100644 index 2b96d6e0c42..00000000000 --- a/info/eintr-2 +++ /dev/null @@ -1,7486 +0,0 @@ -This is ../info/eintr, produced by makeinfo version 4.8 from -emacs-lisp-intro.texi. - -INFO-DIR-SECTION Emacs -START-INFO-DIR-ENTRY -* Emacs Lisp Intro: (eintr). - A simple introduction to Emacs Lisp programming. -END-INFO-DIR-ENTRY - -This is an `Introduction to Programming in Emacs Lisp', for people who -are not programmers. - -Edition 3.01, 2006 Oct 31 - -Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1997, 2001, 2002, -2003, 2004, 2005, 2006 Free Software Foundation, Inc. - -Published by the: - - GNU Press, Website: http://www.gnupress.org - a division of the General: press@gnu.org - Free Software Foundation, Inc. Orders: sales@gnu.org - 51 Franklin Street, Fifth Floor Tel: +1 (617) 542-5942 - Boston, MA 02110-1301 USA Fax: +1 (617) 542-2652 - - -ISBN 1-882114-43-4 - -Permission is granted to copy, distribute and/or modify this document -under the terms of the GNU Free Documentation License, Version 1.2 or -any later version published by the Free Software Foundation; there -being no Invariant Section, with the Front-Cover Texts being "A GNU -Manual", and with the Back-Cover Texts as in (a) below. A copy of the -license is included in the section entitled "GNU Free Documentation -License". - -(a) The FSF's Back-Cover Text is: "You have freedom to copy and modify -this GNU Manual, like GNU software. Copies published by the Free -Software Foundation raise funds for GNU development." - - -File: eintr, Node: See variable current value, Next: defvar and asterisk, Prev: defvar, Up: defvar - -Seeing the Current Value of a Variable --------------------------------------- - -You can see the current value of a variable, any variable, by using the -`describe-variable' function, which is usually invoked by typing `C-h -v'. If you type `C-h v' and then `kill-ring' (followed by <RET>) when -prompted, you will see what is in your current kill ring--this may be -quite a lot! Conversely, if you have been doing nothing this Emacs -session except read this document, you may have nothing in it. Also, -you will see the documentation for `kill-ring': - - Documentation: - List of killed text sequences. - Since the kill ring is supposed to interact nicely with cut-and-paste - facilities offered by window systems, use of this variable should - interact nicely with `interprogram-cut-function' and - `interprogram-paste-function'. The functions `kill-new', - `kill-append', and `current-kill' are supposed to implement this - interaction; you may want to use them instead of manipulating the kill - ring directly. - -The kill ring is defined by a `defvar' in the following way: - - (defvar kill-ring nil - "List of killed text sequences. - ...") - -In this variable definition, the variable is given an initial value of -`nil', which makes sense, since if you have saved nothing, you want -nothing back if you give a `yank' command. The documentation string is -written just like the documentation string of a `defun'. As with the -documentation string of the `defun', the first line of the -documentation should be a complete sentence, since some commands, like -`apropos', print only the first line of documentation. Succeeding -lines should not be indented; otherwise they look odd when you use `C-h -v' (`describe-variable'). - - -File: eintr, Node: defvar and asterisk, Prev: See variable current value, Up: defvar - -8.5.1 `defvar' and an asterisk ------------------------------- - -In the past, Emacs used the `defvar' special form both for internal -variables that you would not expect a user to change and for variables -that you do expect a user to change. Although you can still use -`defvar' for user customizable variables, please use `defcustom' -instead, since that special form provides a path into the Customization -commands. (*Note Specifying Variables using `defcustom': defcustom.) - -When you specified a variable using the `defvar' special form, you -could distinguish a readily settable variable from others by typing an -asterisk, `*', in the first column of its documentation string. For -example: - - (defvar shell-command-default-error-buffer nil - "*Buffer name for `shell-command' ... error output. - ... ") - -You could (and still can) use the `set-variable' command to change the -value of `shell-command-default-error-buffer' temporarily. However, -options set using `set-variable' are set only for the duration of your -editing session. The new values are not saved between sessions. Each -time Emacs starts, it reads the original value, unless you change the -value within your `.emacs' file, either by setting it manually or by -using `customize'. *Note Your `.emacs' File: Emacs Initialization. - -For me, the major use of the `set-variable' command is to suggest -variables that I might want to set in my `.emacs' file. There are now -more than 700 such variables -- far too many to remember readily. -Fortunately, you can press <TAB> after calling the `M-x set-variable' -command to see the list of variables. (*Note Examining and Setting -Variables: (emacs)Examining.) - - -File: eintr, Node: cons & search-fwd Review, Next: search Exercises, Prev: defvar, Up: Cutting & Storing Text - -8.6 Review -========== - -Here is a brief summary of some recently introduced functions. - -`car' -`cdr' - `car' returns the first element of a list; `cdr' returns the - second and subsequent elements of a list. - - For example: - - (car '(1 2 3 4 5 6 7)) - => 1 - (cdr '(1 2 3 4 5 6 7)) - => (2 3 4 5 6 7) - -`cons' - `cons' constructs a list by prepending its first argument to its - second argument. - - For example: - - (cons 1 '(2 3 4)) - => (1 2 3 4) - -`nthcdr' - Return the result of taking CDR `n' times on a list. The `rest of - the rest', as it were. - - For example: - - (nthcdr 3 '(1 2 3 4 5 6 7)) - => (4 5 6 7) - -`setcar' -`setcdr' - `setcar' changes the first element of a list; `setcdr' changes the - second and subsequent elements of a list. - - For example: - - (setq triple '(1 2 3)) - - (setcar triple '37) - - triple - => (37 2 3) - - (setcdr triple '("foo" "bar")) - - triple - => (37 "foo" "bar") - -`progn' - Evaluate each argument in sequence and then return the value of the - last. - - For example: - - (progn 1 2 3 4) - => 4 - -`save-restriction' - Record whatever narrowing is in effect in the current buffer, if - any, and restore that narrowing after evaluating the arguments. - -`search-forward' - Search for a string, and if the string is found, move point. - - Takes four arguments: - - 1. The string to search for. - - 2. Optionally, the limit of the search. - - 3. Optionally, what to do if the search fails, return `nil' or an - error message. - - 4. Optionally, how many times to repeat the search; if negative, - the search goes backwards. - -`kill-region' -`delete-and-extract-region' -`copy-region-as-kill' - `kill-region' cuts the text between point and mark from the buffer - and stores that text in the kill ring, so you can get it back by - yanking. - - `copy-region-as-kill' copies the text between point and mark into - the kill ring, from which you can get it by yanking. The function - does not cut or remove the text from the buffer. - -`delete-and-extract-region' removes the text between point and mark -from the buffer and throws it away. You cannot get it back. (This is -not an interactive command.) - - -File: eintr, Node: search Exercises, Prev: cons & search-fwd Review, Up: Cutting & Storing Text - -8.7 Searching Exercises -======================= - - * Write an interactive function that searches for a string. If the - search finds the string, leave point after it and display a message - that says "Found!". (Do not use `search-forward' for the name of - this function; if you do, you will overwrite the existing version - of `search-forward' that comes with Emacs. Use a name such as - `test-search' instead.) - - * Write a function that prints the third element of the kill ring in - the echo area, if any; if the kill ring does not contain a third - element, print an appropriate message. - - -File: eintr, Node: List Implementation, Next: Yanking, Prev: Cutting & Storing Text, Up: Top - -9 How Lists are Implemented -*************************** - -In Lisp, atoms are recorded in a straightforward fashion; if the -implementation is not straightforward in practice, it is, nonetheless, -straightforward in theory. The atom `rose', for example, is recorded -as the four contiguous letters `r', `o', `s', `e'. A list, on the -other hand, is kept differently. The mechanism is equally simple, but -it takes a moment to get used to the idea. A list is kept using a -series of pairs of pointers. In the series, the first pointer in each -pair points to an atom or to another list, and the second pointer in -each pair points to the next pair, or to the symbol `nil', which marks -the end of the list. - -A pointer itself is quite simply the electronic address of what is -pointed to. Hence, a list is kept as a series of electronic addresses. - -* Menu: - -* Lists diagrammed:: -* Symbols as Chest:: -* List Exercise:: - - -File: eintr, Node: Lists diagrammed, Next: Symbols as Chest, Prev: List Implementation, Up: List Implementation - -Lists diagrammed -================ - -For example, the list `(rose violet buttercup)' has three elements, -`rose', `violet', and `buttercup'. In the computer, the electronic -address of `rose' is recorded in a segment of computer memory along -with the address that gives the electronic address of where the atom -`violet' is located; and that address (the one that tells where -`violet' is located) is kept along with an address that tells where the -address for the atom `buttercup' is located. - -This sounds more complicated than it is and is easier seen in a diagram: - - ___ ___ ___ ___ ___ ___ - |___|___|--> |___|___|--> |___|___|--> nil - | | | - | | | - --> rose --> violet --> buttercup - - - -In the diagram, each box represents a word of computer memory that -holds a Lisp object, usually in the form of a memory address. The -boxes, i.e. the addresses, are in pairs. Each arrow points to what the -address is the address of, either an atom or another pair of addresses. -The first box is the electronic address of `rose' and the arrow points -to `rose'; the second box is the address of the next pair of boxes, the -first part of which is the address of `violet' and the second part of -which is the address of the next pair. The very last box points to the -symbol `nil', which marks the end of the list. - -When a variable is set to a list with a function such as `setq', it -stores the address of the first box in the variable. Thus, evaluation -of the expression - - (setq bouquet '(rose violet buttercup)) - -creates a situation like this: - - bouquet - | - | ___ ___ ___ ___ ___ ___ - --> |___|___|--> |___|___|--> |___|___|--> nil - | | | - | | | - --> rose --> violet --> buttercup - - - -In this example, the symbol `bouquet' holds the address of the first -pair of boxes. - -This same list can be illustrated in a different sort of box notation -like this: - - bouquet - | - | -------------- --------------- ---------------- - | | car | cdr | | car | cdr | | car | cdr | - -->| rose | o------->| violet | o------->| butter- | nil | - | | | | | | | cup | | - -------------- --------------- ---------------- - - - -(Symbols consist of more than pairs of addresses, but the structure of -a symbol is made up of addresses. Indeed, the symbol `bouquet' -consists of a group of address-boxes, one of which is the address of -the printed word `bouquet', a second of which is the address of a -function definition attached to the symbol, if any, a third of which is -the address of the first pair of address-boxes for the list `(rose -violet buttercup)', and so on. Here we are showing that the symbol's -third address-box points to the first pair of address-boxes for the -list.) - -If a symbol is set to the CDR of a list, the list itself is not -changed; the symbol simply has an address further down the list. (In -the jargon, CAR and CDR are `non-destructive'.) Thus, evaluation of -the following expression - - (setq flowers (cdr bouquet)) - -produces this: - - - bouquet flowers - | | - | ___ ___ | ___ ___ ___ ___ - --> | | | --> | | | | | | - |___|___|----> |___|___|--> |___|___|--> nil - | | | - | | | - --> rose --> violet --> buttercup - - - - -The value of `flowers' is `(violet buttercup)', which is to say, the -symbol `flowers' holds the address of the pair of address-boxes, the -first of which holds the address of `violet', and the second of which -holds the address of `buttercup'. - -A pair of address-boxes is called a "cons cell" or "dotted pair". -*Note Cons Cell and List Types: (elisp)Cons Cell Type, and *Note Dotted -Pair Notation: (elisp)Dotted Pair Notation, for more information about -cons cells and dotted pairs. - -The function `cons' adds a new pair of addresses to the front of a -series of addresses like that shown above. For example, evaluating the -expression - - (setq bouquet (cons 'lily bouquet)) - -produces: - - - bouquet flowers - | | - | ___ ___ ___ ___ | ___ ___ ___ ___ - --> | | | | | | --> | | | | | | - |___|___|----> |___|___|----> |___|___|---->|___|___|--> nil - | | | | - | | | | - --> lily --> rose --> violet --> buttercup - - - - -However, this does not change the value of the symbol `flowers', as you -can see by evaluating the following, - - (eq (cdr (cdr bouquet)) flowers) - -which returns `t' for true. - -Until it is reset, `flowers' still has the value `(violet buttercup)'; -that is, it has the address of the cons cell whose first address is of -`violet'. Also, this does not alter any of the pre-existing cons -cells; they are all still there. - -Thus, in Lisp, to get the CDR of a list, you just get the address of -the next cons cell in the series; to get the CAR of a list, you get the -address of the first element of the list; to `cons' a new element on a -list, you add a new cons cell to the front of the list. That is all -there is to it! The underlying structure of Lisp is brilliantly simple! - -And what does the last address in a series of cons cells refer to? It -is the address of the empty list, of `nil'. - -In summary, when a Lisp variable is set to a value, it is provided with -the address of the list to which the variable refers. - - -File: eintr, Node: Symbols as Chest, Next: List Exercise, Prev: Lists diagrammed, Up: List Implementation - -9.1 Symbols as a Chest of Drawers -================================= - -In an earlier section, I suggested that you might imagine a symbol as -being a chest of drawers. The function definition is put in one -drawer, the value in another, and so on. What is put in the drawer -holding the value can be changed without affecting the contents of the -drawer holding the function definition, and vice-verse. - -Actually, what is put in each drawer is the address of the value or -function definition. It is as if you found an old chest in the attic, -and in one of its drawers you found a map giving you directions to -where the buried treasure lies. - -(In addition to its name, symbol definition, and variable value, a -symbol has a `drawer' for a "property list" which can be used to record -other information. Property lists are not discussed here; see *Note -Property Lists: (elisp)Property Lists.) - -Here is a fanciful representation: - - - Chest of Drawers Contents of Drawers - - __ o0O0o __ - / \ - --------------------- - | directions to | [map to] - | symbol name | bouquet - | | - +---------------------+ - | directions to | - | symbol definition | [none] - | | - +---------------------+ - | directions to | [map to] - | variable value | (rose violet buttercup) - | | - +---------------------+ - | directions to | - | property list | [not described here] - | | - +---------------------+ - |/ \| - - - - - -File: eintr, Node: List Exercise, Prev: Symbols as Chest, Up: List Implementation - -9.2 Exercise -============ - -Set `flowers' to `violet' and `buttercup'. Cons two more flowers on to -this list and set this new list to `more-flowers'. Set the CAR of -`flowers' to a fish. What does the `more-flowers' list now contain? - - -File: eintr, Node: Yanking, Next: Loops & Recursion, Prev: List Implementation, Up: Top - -10 Yanking Text Back -******************** - -Whenever you cut text out of a buffer with a `kill' command in GNU -Emacs, you can bring it back with a `yank' command. The text that is -cut out of the buffer is put in the kill ring and the yank commands -insert the appropriate contents of the kill ring back into a buffer -(not necessarily the original buffer). - -A simple `C-y' (`yank') command inserts the first item from the kill -ring into the current buffer. If the `C-y' command is followed -immediately by `M-y', the first element is replaced by the second -element. Successive `M-y' commands replace the second element with the -third, fourth, or fifth element, and so on. When the last element in -the kill ring is reached, it is replaced by the first element and the -cycle is repeated. (Thus the kill ring is called a `ring' rather than -just a `list'. However, the actual data structure that holds the text -is a list. *Note Handling the Kill Ring: Kill Ring, for the details of -how the list is handled as a ring.) - -* Menu: - -* Kill Ring Overview:: -* kill-ring-yank-pointer:: -* yank nthcdr Exercises:: - - -File: eintr, Node: Kill Ring Overview, Next: kill-ring-yank-pointer, Prev: Yanking, Up: Yanking - -10.1 Kill Ring Overview -======================= - -The kill ring is a list of textual strings. This is what it looks like: - - ("some text" "a different piece of text" "yet more text") - -If this were the contents of my kill ring and I pressed `C-y', the -string of characters saying `some text' would be inserted in this -buffer where my cursor is located. - -The `yank' command is also used for duplicating text by copying it. -The copied text is not cut from the buffer, but a copy of it is put on -the kill ring and is inserted by yanking it back. - -Three functions are used for bringing text back from the kill ring: -`yank', which is usually bound to `C-y'; `yank-pop', which is usually -bound to `M-y'; and `rotate-yank-pointer', which is used by the two -other functions. - -These functions refer to the kill ring through a variable called the -`kill-ring-yank-pointer'. Indeed, the insertion code for both the -`yank' and `yank-pop' functions is: - - (insert (car kill-ring-yank-pointer)) - -(Well, no more. In GNU Emacs 22, the function has been replaced by -`insert-for-yank' which calls `insert-for-yank-1' repetitively for each -`yank-handler' segment. In turn, `insert-for-yank-1' strips text -properties from the inserted text according to -`yank-excluded-properties'. Otherwise, it is just like `insert'. We -will stick with plain `insert' since it is easier to understand.) - -To begin to understand how `yank' and `yank-pop' work, it is first -necessary to look at the `kill-ring-yank-pointer' variable and the -`rotate-yank-pointer' function. - - -File: eintr, Node: kill-ring-yank-pointer, Next: yank nthcdr Exercises, Prev: Kill Ring Overview, Up: Yanking - -10.2 The `kill-ring-yank-pointer' Variable -========================================== - -`kill-ring-yank-pointer' is a variable, just as `kill-ring' is a -variable. It points to something by being bound to the value of what -it points to, like any other Lisp variable. - -Thus, if the value of the kill ring is: - - ("some text" "a different piece of text" "yet more text") - -and the `kill-ring-yank-pointer' points to the second clause, the value -of `kill-ring-yank-pointer' is: - - ("a different piece of text" "yet more text") - -As explained in the previous chapter (*note List Implementation::), the -computer does not keep two different copies of the text being pointed to -by both the `kill-ring' and the `kill-ring-yank-pointer'. The words "a -different piece of text" and "yet more text" are not duplicated. -Instead, the two Lisp variables point to the same pieces of text. Here -is a diagram: - - kill-ring kill-ring-yank-pointer - | | - | ___ ___ | ___ ___ ___ ___ - ---> | | | --> | | | | | | - |___|___|----> |___|___|--> |___|___|--> nil - | | | - | | | - | | --> "yet more text" - | | - | --> "a different piece of text - | - --> "some text" - - - - -Both the variable `kill-ring' and the variable `kill-ring-yank-pointer' -are pointers. But the kill ring itself is usually described as if it -were actually what it is composed of. The `kill-ring' is spoken of as -if it were the list rather than that it points to the list. -Conversely, the `kill-ring-yank-pointer' is spoken of as pointing to a -list. - -These two ways of talking about the same thing sound confusing at first -but make sense on reflection. The kill ring is generally thought of as -the complete structure of data that holds the information of what has -recently been cut out of the Emacs buffers. The -`kill-ring-yank-pointer' on the other hand, serves to indicate--that -is, to `point to'--that part of the kill ring of which the first -element (the CAR) will be inserted. - - -File: eintr, Node: yank nthcdr Exercises, Prev: kill-ring-yank-pointer, Up: Yanking - -10.3 Exercises with `yank' and `nthcdr' -======================================= - - * Using `C-h v' (`describe-variable'), look at the value of your - kill ring. Add several items to your kill ring; look at its value - again. Using `M-y' (`yank-pop)', move all the way around the kill - ring. How many items were in your kill ring? Find the value of - `kill-ring-max'. Was your kill ring full, or could you have kept - more blocks of text within it? - - * Using `nthcdr' and `car', construct a series of expressions to - return the first, second, third, and fourth elements of a list. - - -File: eintr, Node: Loops & Recursion, Next: Regexp Search, Prev: Yanking, Up: Top - -11 Loops and Recursion -********************** - -Emacs Lisp has two primary ways to cause an expression, or a series of -expressions, to be evaluated repeatedly: one uses a `while' loop, and -the other uses "recursion". - -Repetition can be very valuable. For example, to move forward four -sentences, you need only write a program that will move forward one -sentence and then repeat the process four times. Since a computer does -not get bored or tired, such repetitive action does not have the -deleterious effects that excessive or the wrong kinds of repetition can -have on humans. - -People mostly write Emacs Lisp functions using `while' loops and their -kin; but you can use recursion, which provides a very powerful way to -think about and then to solve problems(1). - -* Menu: - -* while:: -* dolist dotimes:: -* Recursion:: -* Looping exercise:: - ----------- Footnotes ---------- - -(1) You can write recursive functions to be frugal or wasteful of -mental or computer resources; as it happens, methods that people find -easy--that are frugal of `mental resources'--sometimes use considerable -computer resources. Emacs was designed to run on machines that we now -consider limited and its default settings are conservative. You may -want to increase the values of `max-specpdl-size' and -`max-lisp-eval-depth'. In my `.emacs' file, I set them to 15 and 30 -times their default value. - - -File: eintr, Node: while, Next: dolist dotimes, Prev: Loops & Recursion, Up: Loops & Recursion - -11.1 `while' -============ - -The `while' special form tests whether the value returned by evaluating -its first argument is true or false. This is similar to what the Lisp -interpreter does with an `if'; what the interpreter does next, however, -is different. - -In a `while' expression, if the value returned by evaluating the first -argument is false, the Lisp interpreter skips the rest of the -expression (the "body" of the expression) and does not evaluate it. -However, if the value is true, the Lisp interpreter evaluates the body -of the expression and then again tests whether the first argument to -`while' is true or false. If the value returned by evaluating the -first argument is again true, the Lisp interpreter again evaluates the -body of the expression. - -The template for a `while' expression looks like this: - - (while TRUE-OR-FALSE-TEST - BODY...) - -* Menu: - -* Looping with while:: -* Loop Example:: -* print-elements-of-list:: -* Incrementing Loop:: -* Decrementing Loop:: - - -File: eintr, Node: Looping with while, Next: Loop Example, Prev: while, Up: while - -Looping with `while' --------------------- - -So long as the true-or-false-test of the `while' expression returns a -true value when it is evaluated, the body is repeatedly evaluated. -This process is called a loop since the Lisp interpreter repeats the -same thing again and again, like an airplane doing a loop. When the -result of evaluating the true-or-false-test is false, the Lisp -interpreter does not evaluate the rest of the `while' expression and -`exits the loop'. - -Clearly, if the value returned by evaluating the first argument to -`while' is always true, the body following will be evaluated again and -again ... and again ... forever. Conversely, if the value returned is -never true, the expressions in the body will never be evaluated. The -craft of writing a `while' loop consists of choosing a mechanism such -that the true-or-false-test returns true just the number of times that -you want the subsequent expressions to be evaluated, and then have the -test return false. - -The value returned by evaluating a `while' is the value of the -true-or-false-test. An interesting consequence of this is that a -`while' loop that evaluates without error will return `nil' or false -regardless of whether it has looped 1 or 100 times or none at all. A -`while' expression that evaluates successfully never returns a true -value! What this means is that `while' is always evaluated for its -side effects, which is to say, the consequences of evaluating the -expressions within the body of the `while' loop. This makes sense. It -is not the mere act of looping that is desired, but the consequences of -what happens when the expressions in the loop are repeatedly evaluated. - - -File: eintr, Node: Loop Example, Next: print-elements-of-list, Prev: Looping with while, Up: while - -11.1.1 A `while' Loop and a List --------------------------------- - -A common way to control a `while' loop is to test whether a list has -any elements. If it does, the loop is repeated; but if it does not, -the repetition is ended. Since this is an important technique, we will -create a short example to illustrate it. - -A simple way to test whether a list has elements is to evaluate the -list: if it has no elements, it is an empty list and will return the -empty list, `()', which is a synonym for `nil' or false. On the other -hand, a list with elements will return those elements when it is -evaluated. Since Emacs Lisp considers as true any value that is not -`nil', a list that returns elements will test true in a `while' loop. - -For example, you can set the variable `empty-list' to `nil' by -evaluating the following `setq' expression: - - (setq empty-list ()) - -After evaluating the `setq' expression, you can evaluate the variable -`empty-list' in the usual way, by placing the cursor after the symbol -and typing `C-x C-e'; `nil' will appear in your echo area: - - empty-list - -On the other hand, if you set a variable to be a list with elements, the -list will appear when you evaluate the variable, as you can see by -evaluating the following two expressions: - - (setq animals '(gazelle giraffe lion tiger)) - - animals - -Thus, to create a `while' loop that tests whether there are any items -in the list `animals', the first part of the loop will be written like -this: - - (while animals - ... - -When the `while' tests its first argument, the variable `animals' is -evaluated. It returns a list. So long as the list has elements, the -`while' considers the results of the test to be true; but when the list -is empty, it considers the results of the test to be false. - -To prevent the `while' loop from running forever, some mechanism needs -to be provided to empty the list eventually. An oft-used technique is -to have one of the subsequent forms in the `while' expression set the -value of the list to be the CDR of the list. Each time the `cdr' -function is evaluated, the list will be made shorter, until eventually -only the empty list will be left. At this point, the test of the -`while' loop will return false, and the arguments to the `while' will -no longer be evaluated. - -For example, the list of animals bound to the variable `animals' can be -set to be the CDR of the original list with the following expression: - - (setq animals (cdr animals)) - -If you have evaluated the previous expressions and then evaluate this -expression, you will see `(giraffe lion tiger)' appear in the echo -area. If you evaluate the expression again, `(lion tiger)' will appear -in the echo area. If you evaluate it again and yet again, `(tiger)' -appears and then the empty list, shown by `nil'. - -A template for a `while' loop that uses the `cdr' function repeatedly -to cause the true-or-false-test eventually to test false looks like -this: - - (while TEST-WHETHER-LIST-IS-EMPTY - BODY... - SET-LIST-TO-CDR-OF-LIST) - -This test and use of `cdr' can be put together in a function that goes -through a list and prints each element of the list on a line of its own. - - -File: eintr, Node: print-elements-of-list, Next: Incrementing Loop, Prev: Loop Example, Up: while - -11.1.2 An Example: `print-elements-of-list' -------------------------------------------- - -The `print-elements-of-list' function illustrates a `while' loop with a -list. - -The function requires several lines for its output. If you are reading -this in a recent instance of GNU Emacs, you can evaluate the following -expression inside of Info, as usual. - -If you are using an earlier version of Emacs, you need to copy the -necessary expressions to your `*scratch*' buffer and evaluate them -there. This is because the echo area had only one line in the earlier -versions. - -You can copy the expressions by marking the beginning of the region -with `C-<SPC>' (`set-mark-command'), moving the cursor to the end of -the region and then copying the region using `M-w' (`kill-ring-save', -which calls `copy-region-as-kill' and then provides visual feedback). -In the `*scratch*' buffer, you can yank the expressions back by typing -`C-y' (`yank'). - -After you have copied the expressions to the `*scratch*' buffer, -evaluate each expression in turn. Be sure to evaluate the last -expression, `(print-elements-of-list animals)', by typing `C-u C-x -C-e', that is, by giving an argument to `eval-last-sexp'. This will -cause the result of the evaluation to be printed in the `*scratch*' -buffer instead of being printed in the echo area. (Otherwise you will -see something like this in your echo area: -`^Jgazelle^J^Jgiraffe^J^Jlion^J^Jtiger^Jnil', in which each `^J' stands -for a `newline'.) - -In a recent instance of GNU Emacs, you can evaluate these expressions -directly in the Info buffer, and the echo area will grow to show the -results. - - (setq animals '(gazelle giraffe lion tiger)) - - (defun print-elements-of-list (list) - "Print each element of LIST on a line of its own." - (while list - (print (car list)) - (setq list (cdr list)))) - - (print-elements-of-list animals) - -When you evaluate the three expressions in sequence, you will see this: - - gazelle - - giraffe - - lion - - tiger - nil - -Each element of the list is printed on a line of its own (that is what -the function `print' does) and then the value returned by the function -is printed. Since the last expression in the function is the `while' -loop, and since `while' loops always return `nil', a `nil' is printed -after the last element of the list. - - -File: eintr, Node: Incrementing Loop, Next: Decrementing Loop, Prev: print-elements-of-list, Up: while - -11.1.3 A Loop with an Incrementing Counter ------------------------------------------- - -A loop is not useful unless it stops when it ought. Besides -controlling a loop with a list, a common way of stopping a loop is to -write the first argument as a test that returns false when the correct -number of repetitions are complete. This means that the loop must have -a counter--an expression that counts how many times the loop repeats -itself. - -The test can be an expression such as `(< count desired-number)' which -returns `t' for true if the value of `count' is less than the -`desired-number' of repetitions and `nil' for false if the value of -`count' is equal to or is greater than the `desired-number'. The -expression that increments the count can be a simple `setq' such as -`(setq count (1+ count))', where `1+' is a built-in function in Emacs -Lisp that adds 1 to its argument. (The expression `(1+ count)' has the -same result as `(+ count 1)', but is easier for a human to read.) - -The template for a `while' loop controlled by an incrementing counter -looks like this: - - SET-COUNT-TO-INITIAL-VALUE - (while (< count desired-number) ; true-or-false-test - BODY... - (setq count (1+ count))) ; incrementer - -Note that you need to set the initial value of `count'; usually it is -set to 1. - -* Menu: - -* Incrementing Example:: -* Inc Example parts:: -* Inc Example altogether:: - - -File: eintr, Node: Incrementing Example, Next: Inc Example parts, Prev: Incrementing Loop, Up: Incrementing Loop - -Example with incrementing counter -................................. - -Suppose you are playing on the beach and decide to make a triangle of -pebbles, putting one pebble in the first row, two in the second row, -three in the third row and so on, like this: - - - * - * * - * * * - * * * * - - -(About 2500 years ago, Pythagoras and others developed the beginnings of -number theory by considering questions such as this.) - -Suppose you want to know how many pebbles you will need to make a -triangle with 7 rows? - -Clearly, what you need to do is add up the numbers from 1 to 7. There -are two ways to do this; start with the smallest number, one, and add up -the list in sequence, 1, 2, 3, 4 and so on; or start with the largest -number and add the list going down: 7, 6, 5, 4 and so on. Because both -mechanisms illustrate common ways of writing `while' loops, we will -create two examples, one counting up and the other counting down. In -this first example, we will start with 1 and add 2, 3, 4 and so on. - -If you are just adding up a short list of numbers, the easiest way to do -it is to add up all the numbers at once. However, if you do not know -ahead of time how many numbers your list will have, or if you want to be -prepared for a very long list, then you need to design your addition so -that what you do is repeat a simple process many times instead of doing -a more complex process once. - -For example, instead of adding up all the pebbles all at once, what you -can do is add the number of pebbles in the first row, 1, to the number -in the second row, 2, and then add the total of those two rows to the -third row, 3. Then you can add the number in the fourth row, 4, to the -total of the first three rows; and so on. - -The critical characteristic of the process is that each repetitive -action is simple. In this case, at each step we add only two numbers, -the number of pebbles in the row and the total already found. This -process of adding two numbers is repeated again and again until the last -row has been added to the total of all the preceding rows. In a more -complex loop the repetitive action might not be so simple, but it will -be simpler than doing everything all at once. - - -File: eintr, Node: Inc Example parts, Next: Inc Example altogether, Prev: Incrementing Example, Up: Incrementing Loop - -The parts of the function definition -.................................... - -The preceding analysis gives us the bones of our function definition: -first, we will need a variable that we can call `total' that will be -the total number of pebbles. This will be the value returned by the -function. - -Second, we know that the function will require an argument: this -argument will be the total number of rows in the triangle. It can be -called `number-of-rows'. - -Finally, we need a variable to use as a counter. We could call this -variable `counter', but a better name is `row-number'. That is because -what the counter does in this function is count rows, and a program -should be written to be as understandable as possible. - -When the Lisp interpreter first starts evaluating the expressions in the -function, the value of `total' should be set to zero, since we have not -added anything to it. Then the function should add the number of -pebbles in the first row to the total, and then add the number of -pebbles in the second to the total, and then add the number of pebbles -in the third row to the total, and so on, until there are no more rows -left to add. - -Both `total' and `row-number' are used only inside the function, so -they can be declared as local variables with `let' and given initial -values. Clearly, the initial value for `total' should be 0. The -initial value of `row-number' should be 1, since we start with the -first row. This means that the `let' statement will look like this: - - (let ((total 0) - (row-number 1)) - BODY...) - -After the internal variables are declared and bound to their initial -values, we can begin the `while' loop. The expression that serves as -the test should return a value of `t' for true so long as the -`row-number' is less than or equal to the `number-of-rows'. (If the -expression tests true only so long as the row number is less than the -number of rows in the triangle, the last row will never be added to the -total; hence the row number has to be either less than or equal to the -number of rows.) - -Lisp provides the `<=' function that returns true if the value of its -first argument is less than or equal to the value of its second -argument and false otherwise. So the expression that the `while' will -evaluate as its test should look like this: - - (<= row-number number-of-rows) - -The total number of pebbles can be found by repeatedly adding the number -of pebbles in a row to the total already found. Since the number of -pebbles in the row is equal to the row number, the total can be found by -adding the row number to the total. (Clearly, in a more complex -situation, the number of pebbles in the row might be related to the row -number in a more complicated way; if this were the case, the row number -would be replaced by the appropriate expression.) - - (setq total (+ total row-number)) - -What this does is set the new value of `total' to be equal to the sum -of adding the number of pebbles in the row to the previous total. - -After setting the value of `total', the conditions need to be -established for the next repetition of the loop, if there is one. This -is done by incrementing the value of the `row-number' variable, which -serves as a counter. After the `row-number' variable has been -incremented, the true-or-false-test at the beginning of the `while' -loop tests whether its value is still less than or equal to the value -of the `number-of-rows' and if it is, adds the new value of the -`row-number' variable to the `total' of the previous repetition of the -loop. - -The built-in Emacs Lisp function `1+' adds 1 to a number, so the -`row-number' variable can be incremented with this expression: - - (setq row-number (1+ row-number)) - - -File: eintr, Node: Inc Example altogether, Prev: Inc Example parts, Up: Incrementing Loop - -Putting the function definition together -........................................ - -We have created the parts for the function definition; now we need to -put them together. - -First, the contents of the `while' expression: - - (while (<= row-number number-of-rows) ; true-or-false-test - (setq total (+ total row-number)) - (setq row-number (1+ row-number))) ; incrementer - -Along with the `let' expression varlist, this very nearly completes the -body of the function definition. However, it requires one final -element, the need for which is somewhat subtle. - -The final touch is to place the variable `total' on a line by itself -after the `while' expression. Otherwise, the value returned by the -whole function is the value of the last expression that is evaluated in -the body of the `let', and this is the value returned by the `while', -which is always `nil'. - -This may not be evident at first sight. It almost looks as if the -incrementing expression is the last expression of the whole function. -But that expression is part of the body of the `while'; it is the last -element of the list that starts with the symbol `while'. Moreover, the -whole of the `while' loop is a list within the body of the `let'. - -In outline, the function will look like this: - - (defun NAME-OF-FUNCTION (ARGUMENT-LIST) - "DOCUMENTATION..." - (let (VARLIST) - (while (TRUE-OR-FALSE-TEST) - BODY-OF-WHILE... ) - ... )) ; Need final expression here. - -The result of evaluating the `let' is what is going to be returned by -the `defun' since the `let' is not embedded within any containing list, -except for the `defun' as a whole. However, if the `while' is the last -element of the `let' expression, the function will always return `nil'. -This is not what we want! Instead, what we want is the value of the -variable `total'. This is returned by simply placing the symbol as the -last element of the list starting with `let'. It gets evaluated after -the preceding elements of the list are evaluated, which means it gets -evaluated after it has been assigned the correct value for the total. - -It may be easier to see this by printing the list starting with `let' -all on one line. This format makes it evident that the VARLIST and -`while' expressions are the second and third elements of the list -starting with `let', and the `total' is the last element: - - (let (VARLIST) (while (TRUE-OR-FALSE-TEST) BODY-OF-WHILE... ) total) - -Putting everything together, the `triangle' function definition looks -like this: - - (defun triangle (number-of-rows) ; Version with - ; incrementing counter. - "Add up the number of pebbles in a triangle. - The first row has one pebble, the second row two pebbles, - the third row three pebbles, and so on. - The argument is NUMBER-OF-ROWS." - (let ((total 0) - (row-number 1)) - (while (<= row-number number-of-rows) - (setq total (+ total row-number)) - (setq row-number (1+ row-number))) - total)) - -After you have installed `triangle' by evaluating the function, you can -try it out. Here are two examples: - - (triangle 4) - - (triangle 7) - -The sum of the first four numbers is 10 and the sum of the first seven -numbers is 28. - - -File: eintr, Node: Decrementing Loop, Prev: Incrementing Loop, Up: while - -11.1.4 Loop with a Decrementing Counter ---------------------------------------- - -Another common way to write a `while' loop is to write the test so that -it determines whether a counter is greater than zero. So long as the -counter is greater than zero, the loop is repeated. But when the -counter is equal to or less than zero, the loop is stopped. For this -to work, the counter has to start out greater than zero and then be -made smaller and smaller by a form that is evaluated repeatedly. - -The test will be an expression such as `(> counter 0)' which returns -`t' for true if the value of `counter' is greater than zero, and `nil' -for false if the value of `counter' is equal to or less than zero. The -expression that makes the number smaller and smaller can be a simple -`setq' such as `(setq counter (1- counter))', where `1-' is a built-in -function in Emacs Lisp that subtracts 1 from its argument. - -The template for a decrementing `while' loop looks like this: - - (while (> counter 0) ; true-or-false-test - BODY... - (setq counter (1- counter))) ; decrementer - -* Menu: - -* Decrementing Example:: -* Dec Example parts:: -* Dec Example altogether:: - - -File: eintr, Node: Decrementing Example, Next: Dec Example parts, Prev: Decrementing Loop, Up: Decrementing Loop - -Example with decrementing counter -................................. - -To illustrate a loop with a decrementing counter, we will rewrite the -`triangle' function so the counter decreases to zero. - -This is the reverse of the earlier version of the function. In this -case, to find out how many pebbles are needed to make a triangle with 3 -rows, add the number of pebbles in the third row, 3, to the number in -the preceding row, 2, and then add the total of those two rows to the -row that precedes them, which is 1. - -Likewise, to find the number of pebbles in a triangle with 7 rows, add -the number of pebbles in the seventh row, 7, to the number in the -preceding row, which is 6, and then add the total of those two rows to -the row that precedes them, which is 5, and so on. As in the previous -example, each addition only involves adding two numbers, the total of -the rows already added up and the number of pebbles in the row that is -being added to the total. This process of adding two numbers is -repeated again and again until there are no more pebbles to add. - -We know how many pebbles to start with: the number of pebbles in the -last row is equal to the number of rows. If the triangle has seven -rows, the number of pebbles in the last row is 7. Likewise, we know how -many pebbles are in the preceding row: it is one less than the number in -the row. - - -File: eintr, Node: Dec Example parts, Next: Dec Example altogether, Prev: Decrementing Example, Up: Decrementing Loop - -The parts of the function definition -.................................... - -We start with three variables: the total number of rows in the -triangle; the number of pebbles in a row; and the total number of -pebbles, which is what we want to calculate. These variables can be -named `number-of-rows', `number-of-pebbles-in-row', and `total', -respectively. - -Both `total' and `number-of-pebbles-in-row' are used only inside the -function and are declared with `let'. The initial value of `total' -should, of course, be zero. However, the initial value of -`number-of-pebbles-in-row' should be equal to the number of rows in the -triangle, since the addition will start with the longest row. - -This means that the beginning of the `let' expression will look like -this: - - (let ((total 0) - (number-of-pebbles-in-row number-of-rows)) - BODY...) - -The total number of pebbles can be found by repeatedly adding the number -of pebbles in a row to the total already found, that is, by repeatedly -evaluating the following expression: - - (setq total (+ total number-of-pebbles-in-row)) - -After the `number-of-pebbles-in-row' is added to the `total', the -`number-of-pebbles-in-row' should be decremented by one, since the next -time the loop repeats, the preceding row will be added to the total. - -The number of pebbles in a preceding row is one less than the number of -pebbles in a row, so the built-in Emacs Lisp function `1-' can be used -to compute the number of pebbles in the preceding row. This can be -done with the following expression: - - (setq number-of-pebbles-in-row - (1- number-of-pebbles-in-row)) - -Finally, we know that the `while' loop should stop making repeated -additions when there are no pebbles in a row. So the test for the -`while' loop is simply: - - (while (> number-of-pebbles-in-row 0) - - -File: eintr, Node: Dec Example altogether, Prev: Dec Example parts, Up: Decrementing Loop - -Putting the function definition together -........................................ - -We can put these expressions together to create a function definition -that works. However, on examination, we find that one of the local -variables is unneeded! - -The function definition looks like this: - - ;;; First subtractive version. - (defun triangle (number-of-rows) - "Add up the number of pebbles in a triangle." - (let ((total 0) - (number-of-pebbles-in-row number-of-rows)) - (while (> number-of-pebbles-in-row 0) - (setq total (+ total number-of-pebbles-in-row)) - (setq number-of-pebbles-in-row - (1- number-of-pebbles-in-row))) - total)) - -As written, this function works. - -However, we do not need `number-of-pebbles-in-row'. - -When the `triangle' function is evaluated, the symbol `number-of-rows' -will be bound to a number, giving it an initial value. That number can -be changed in the body of the function as if it were a local variable, -without any fear that such a change will effect the value of the -variable outside of the function. This is a very useful characteristic -of Lisp; it means that the variable `number-of-rows' can be used -anywhere in the function where `number-of-pebbles-in-row' is used. - -Here is a second version of the function written a bit more cleanly: - - (defun triangle (number) ; Second version. - "Return sum of numbers 1 through NUMBER inclusive." - (let ((total 0)) - (while (> number 0) - (setq total (+ total number)) - (setq number (1- number))) - total)) - -In brief, a properly written `while' loop will consist of three parts: - - 1. A test that will return false after the loop has repeated itself - the correct number of times. - - 2. An expression the evaluation of which will return the value desired - after being repeatedly evaluated. - - 3. An expression to change the value passed to the true-or-false-test - so that the test returns false after the loop has repeated itself - the right number of times. - - -File: eintr, Node: dolist dotimes, Next: Recursion, Prev: while, Up: Loops & Recursion - -11.2 Save your time: `dolist' and `dotimes' -=========================================== - -In addition to `while', both `dolist' and `dotimes' provide for -looping. Sometimes these are quicker to write than the equivalent -`while' loop. Both are Lisp macros. (*Note Macros: (elisp)Macros. ) - -`dolist' works like a `while' loop that `CDRs down a list': `dolist' -automatically shortens the list each time it loops--takes the CDR of -the list--and binds the CAR of each shorter version of the list to the -first of its arguments. - -`dotimes' loops a specific number of times: you specify the number. - -* Menu: - -* dolist:: -* dotimes:: - - -File: eintr, Node: dolist, Next: dotimes, Prev: dolist dotimes, Up: dolist dotimes - -The `dolist' Macro -.................. - -Suppose, for example, you want to reverse a list, so that "first" -"second" "third" becomes "third" "second" "first". - -In practice, you would use the `reverse' function, like this: - - (setq animals '(gazelle giraffe lion tiger)) - - (reverse animals) - -Here is how you could reverse the list using a `while' loop: - - (setq animals '(gazelle giraffe lion tiger)) - - (defun reverse-list-with-while (list) - "Using while, reverse the order of LIST." - (let (value) ; make sure list starts empty - (while list - (setq value (cons (car list) value)) - (setq list (cdr list))) - value)) - - (reverse-list-with-while animals) - -And here is how you could use the `dolist' macro: - - (setq animals '(gazelle giraffe lion tiger)) - - (defun reverse-list-with-dolist (list) - "Using dolist, reverse the order of LIST." - (let (value) ; make sure list starts empty - (dolist (element list value) - (setq value (cons element value))))) - - (reverse-list-with-dolist animals) - -In Info, you can place your cursor after the closing parenthesis of -each expression and type `C-x C-e'; in each case, you should see - - (tiger lion giraffe gazelle) - -in the echo area. - -For this example, the existing `reverse' function is obviously best. -The `while' loop is just like our first example (*note A `while' Loop -and a List: Loop Example.). The `while' first checks whether the list -has elements; if so, it constructs a new list by adding the first -element of the list to the existing list (which in the first iteration -of the loop is `nil'). Since the second element is prepended in front -of the first element, and the third element is prepended in front of -the second element, the list is reversed. - -In the expression using a `while' loop, the `(setq list (cdr list))' -expression shortens the list, so the `while' loop eventually stops. In -addition, it provides the `cons' expression with a new first element by -creating a new and shorter list at each repetition of the loop. - -The `dolist' expression does very much the same as the `while' -expression, except that the `dolist' macro does some of the work you -have to do when writing a `while' expression. - -Like a `while' loop, a `dolist' loops. What is different is that it -automatically shortens the list each time it loops -- it `CDRs down the -list' on its own -- and it automatically binds the CAR of each shorter -version of the list to the first of its arguments. - -In the example, the CAR of each shorter version of the list is referred -to using the symbol `element', the list itself is called `list', and -the value returned is called `value'. The remainder of the `dolist' -expression is the body. - -The `dolist' expression binds the CAR of each shorter version of the -list to `element' and then evaluates the body of the expression; and -repeats the loop. The result is returned in `value'. - - -File: eintr, Node: dotimes, Prev: dolist, Up: dolist dotimes - -The `dotimes' Macro -................... - -The `dotimes' macro is similar to `dolist', except that it loops a -specific number of times. - -The first argument to `dotimes' is assigned the numbers 0, 1, 2 and so -forth each time around the loop, and the value of the third argument is -returned. You need to provide the value of the second argument, which -is how many times the macro loops. - -For example, the following binds the numbers from 0 up to, but not -including, the number 3 to the first argument, NUMBER, and then -constructs a list of the three numbers. (The first number is 0, the -second number is 1, and the third number is 2; this makes a total of -three numbers in all, starting with zero as the first number.) - - (let (value) ; otherwise a value is a void variable - (dotimes (number 3 value) - (setq value (cons number value)))) - - => (2 1 0) - -`dotimes' returns `value', so the way to use `dotimes' is to operate on -some expression NUMBER number of times and then return the result, -either as a list or an atom. - -Here is an example of a `defun' that uses `dotimes' to add up the -number of pebbles in a triangle. - - (defun triangle-using-dotimes (number-of-rows) - "Using dotimes, add up the number of pebbles in a triangle." - (let ((total 0)) ; otherwise a total is a void variable - (dotimes (number number-of-rows total) - (setq total (+ total (1+ number)))))) - - (triangle-using-dotimes 4) - - -File: eintr, Node: Recursion, Next: Looping exercise, Prev: dolist dotimes, Up: Loops & Recursion - -11.3 Recursion -============== - -A recursive function contains code that tells the Lisp interpreter to -call a program that runs exactly like itself, but with slightly -different arguments. The code runs exactly the same because it has the -same name. However, even though the program has the same name, it is -not the same entity. It is different. In the jargon, it is a -different `instance'. - -Eventually, if the program is written correctly, the `slightly -different arguments' will become sufficiently different from the first -arguments that the final instance will stop. - -* Menu: - -* Building Robots:: -* Recursive Definition Parts:: -* Recursion with list:: -* Recursive triangle function:: -* Recursion with cond:: -* Recursive Patterns:: -* No Deferment:: -* No deferment solution:: - - -File: eintr, Node: Building Robots, Next: Recursive Definition Parts, Prev: Recursion, Up: Recursion - -11.3.1 Building Robots: Extending the Metaphor ----------------------------------------------- - -It is sometimes helpful to think of a running program as a robot that -does a job. In doing its job, a recursive function calls on a second -robot to help it. The second robot is identical to the first in every -way, except that the second robot helps the first and has been passed -different arguments than the first. - -In a recursive function, the second robot may call a third; and the -third may call a fourth, and so on. Each of these is a different -entity; but all are clones. - -Since each robot has slightly different instructions--the arguments -will differ from one robot to the next--the last robot should know when -to stop. - -Let's expand on the metaphor in which a computer program is a robot. - -A function definition provides the blueprints for a robot. When you -install a function definition, that is, when you evaluate a `defun' -special form, you install the necessary equipment to build robots. It -is as if you were in a factory, setting up an assembly line. Robots -with the same name are built according to the same blueprints. So they -have, as it were, the same `model number', but a different `serial -number'. - -We often say that a recursive function `calls itself'. What we mean is -that the instructions in a recursive function cause the Lisp -interpreter to run a different function that has the same name and does -the same job as the first, but with different arguments. - -It is important that the arguments differ from one instance to the -next; otherwise, the process will never stop. - - -File: eintr, Node: Recursive Definition Parts, Next: Recursion with list, Prev: Building Robots, Up: Recursion - -11.3.2 The Parts of a Recursive Definition ------------------------------------------- - -A recursive function typically contains a conditional expression which -has three parts: - - 1. A true-or-false-test that determines whether the function is called - again, here called the "do-again-test". - - 2. The name of the function. When this name is called, a new - instance of the function--a new robot, as it were--is created and - told what to do. - - 3. An expression that returns a different value each time the - function is called, here called the "next-step-expression". - Consequently, the argument (or arguments) passed to the new - instance of the function will be different from that passed to the - previous instance. This causes the conditional expression, the - "do-again-test", to test false after the correct number of - repetitions. - -Recursive functions can be much simpler than any other kind of -function. Indeed, when people first start to use them, they often look -so mysteriously simple as to be incomprehensible. Like riding a -bicycle, reading a recursive function definition takes a certain knack -which is hard at first but then seems simple. - -There are several different common recursive patterns. A very simple -pattern looks like this: - - (defun NAME-OF-RECURSIVE-FUNCTION (ARGUMENT-LIST) - "DOCUMENTATION..." - (if DO-AGAIN-TEST - BODY... - (NAME-OF-RECURSIVE-FUNCTION - NEXT-STEP-EXPRESSION))) - -Each time a recursive function is evaluated, a new instance of it is -created and told what to do. The arguments tell the instance what to -do. - -An argument is bound to the value of the next-step-expression. Each -instance runs with a different value of the next-step-expression. - -The value in the next-step-expression is used in the do-again-test. - -The value returned by the next-step-expression is passed to the new -instance of the function, which evaluates it (or some -transmogrification of it) to determine whether to continue or stop. -The next-step-expression is designed so that the do-again-test returns -false when the function should no longer be repeated. - -The do-again-test is sometimes called the "stop condition", since it -stops the repetitions when it tests false. - - -File: eintr, Node: Recursion with list, Next: Recursive triangle function, Prev: Recursive Definition Parts, Up: Recursion - -11.3.3 Recursion with a List ----------------------------- - -The example of a `while' loop that printed the elements of a list of -numbers can be written recursively. Here is the code, including an -expression to set the value of the variable `animals' to a list. - -If you are using GNU Emacs 20 or before, this example must be copied to -the `*scratch*' buffer and each expression must be evaluated there. -Use `C-u C-x C-e' to evaluate the `(print-elements-recursively -animals)' expression so that the results are printed in the buffer; -otherwise the Lisp interpreter will try to squeeze the results into the -one line of the echo area. - -Also, place your cursor immediately after the last closing parenthesis -of the `print-elements-recursively' function, before the comment. -Otherwise, the Lisp interpreter will try to evaluate the comment. - -If you are using a more recent version, you can evaluate this -expression directly in Info. - - (setq animals '(gazelle giraffe lion tiger)) - - (defun print-elements-recursively (list) - "Print each element of LIST on a line of its own. - Uses recursion." - (if list ; do-again-test - (progn - (print (car list)) ; body - (print-elements-recursively ; recursive call - (cdr list))))) ; next-step-expression - - (print-elements-recursively animals) - -The `print-elements-recursively' function first tests whether there is -any content in the list; if there is, the function prints the first -element of the list, the CAR of the list. Then the function `invokes -itself', but gives itself as its argument, not the whole list, but the -second and subsequent elements of the list, the CDR of the list. - -Put another way, if the list is not empty, the function invokes another -instance of code that is similar to the initial code, but is a -different thread of execution, with different arguments than the first -instance. - -Put in yet another way, if the list is not empty, the first robot -assemblies a second robot and tells it what to do; the second robot is -a different individual from the first, but is the same model. - -When the second evaluation occurs, the `if' expression is evaluated and -if true, prints the first element of the list it receives as its -argument (which is the second element of the original list). Then the -function `calls itself' with the CDR of the list it is invoked with, -which (the second time around) is the CDR of the CDR of the original -list. - -Note that although we say that the function `calls itself', what we -mean is that the Lisp interpreter assembles and instructs a new -instance of the program. The new instance is a clone of the first, but -is a separate individual. - -Each time the function `invokes itself', it invokes itself on a shorter -version of the original list. It creates a new instance that works on -a shorter list. - -Eventually, the function invokes itself on an empty list. It creates a -new instance whose argument is `nil'. The conditional expression tests -the value of `list'. Since the value of `list' is `nil', the `if' -expression tests false so the then-part is not evaluated. The function -as a whole then returns `nil'. - -When you evaluate `(print-elements-recursively animals)' in the -`*scratch*' buffer, you see this result: - - gazelle - - giraffe - - lion - - tiger - nil - - -File: eintr, Node: Recursive triangle function, Next: Recursion with cond, Prev: Recursion with list, Up: Recursion - -11.3.4 Recursion in Place of a Counter --------------------------------------- - -The `triangle' function described in a previous section can also be -written recursively. It looks like this: - - (defun triangle-recursively (number) - "Return the sum of the numbers 1 through NUMBER inclusive. - Uses recursion." - (if (= number 1) ; do-again-test - 1 ; then-part - (+ number ; else-part - (triangle-recursively ; recursive call - (1- number))))) ; next-step-expression - - (triangle-recursively 7) - -You can install this function by evaluating it and then try it by -evaluating `(triangle-recursively 7)'. (Remember to put your cursor -immediately after the last parenthesis of the function definition, -before the comment.) The function evaluates to 28. - -To understand how this function works, let's consider what happens in -the various cases when the function is passed 1, 2, 3, or 4 as the -value of its argument. - -* Menu: - -* Recursive Example arg of 1 or 2:: -* Recursive Example arg of 3 or 4:: - - -File: eintr, Node: Recursive Example arg of 1 or 2, Next: Recursive Example arg of 3 or 4, Prev: Recursive triangle function, Up: Recursive triangle function - -An argument of 1 or 2 -..................... - -First, what happens if the value of the argument is 1? - -The function has an `if' expression after the documentation string. It -tests whether the value of `number' is equal to 1; if so, Emacs -evaluates the then-part of the `if' expression, which returns the -number 1 as the value of the function. (A triangle with one row has -one pebble in it.) - -Suppose, however, that the value of the argument is 2. In this case, -Emacs evaluates the else-part of the `if' expression. - -The else-part consists of an addition, the recursive call to -`triangle-recursively' and a decrementing action; and it looks like -this: - - (+ number (triangle-recursively (1- number))) - -When Emacs evaluates this expression, the innermost expression is -evaluated first; then the other parts in sequence. Here are the steps -in detail: - -Step 1 Evaluate the innermost expression. - The innermost expression is `(1- number)' so Emacs decrements the - value of `number' from 2 to 1. - -Step 2 Evaluate the `triangle-recursively' function. - The Lisp interpreter creates an individual instance of - `triangle-recursively'. It does not matter that this function is - contained within itself. Emacs passes the result Step 1 as the - argument used by this instance of the `triangle-recursively' - function - - In this case, Emacs evaluates `triangle-recursively' with an - argument of 1. This means that this evaluation of - `triangle-recursively' returns 1. - -Step 3 Evaluate the value of `number'. - The variable `number' is the second element of the list that - starts with `+'; its value is 2. - -Step 4 Evaluate the `+' expression. - The `+' expression receives two arguments, the first from the - evaluation of `number' (Step 3) and the second from the evaluation - of `triangle-recursively' (Step 2). - - The result of the addition is the sum of 2 plus 1, and the number - 3 is returned, which is correct. A triangle with two rows has - three pebbles in it. - - -File: eintr, Node: Recursive Example arg of 3 or 4, Prev: Recursive Example arg of 1 or 2, Up: Recursive triangle function - -An argument of 3 or 4 -..................... - -Suppose that `triangle-recursively' is called with an argument of 3. - -Step 1 Evaluate the do-again-test. - The `if' expression is evaluated first. This is the do-again test - and returns false, so the else-part of the `if' expression is - evaluated. (Note that in this example, the do-again-test causes - the function to call itself when it tests false, not when it tests - true.) - -Step 2 Evaluate the innermost expression of the else-part. - The innermost expression of the else-part is evaluated, which - decrements 3 to 2. This is the next-step-expression. - -Step 3 Evaluate the `triangle-recursively' function. - The number 2 is passed to the `triangle-recursively' function. - - We know what happens when Emacs evaluates `triangle-recursively' - with an argument of 2. After going through the sequence of - actions described earlier, it returns a value of 3. So that is - what will happen here. - -Step 4 Evaluate the addition. - 3 will be passed as an argument to the addition and will be added - to the number with which the function was called, which is 3. - -The value returned by the function as a whole will be 6. - -Now that we know what will happen when `triangle-recursively' is called -with an argument of 3, it is evident what will happen if it is called -with an argument of 4: - - In the recursive call, the evaluation of - - (triangle-recursively (1- 4)) - - will return the value of evaluating - - (triangle-recursively 3) - - which is 6 and this value will be added to 4 by the addition in the - third line. - -The value returned by the function as a whole will be 10. - -Each time `triangle-recursively' is evaluated, it evaluates a version -of itself--a different instance of itself--with a smaller argument, -until the argument is small enough so that it does not evaluate itself. - -Note that this particular design for a recursive function requires that -operations be deferred. - -Before `(triangle-recursively 7)' can calculate its answer, it must -call `(triangle-recursively 6)'; and before `(triangle-recursively 6)' -can calculate its answer, it must call `(triangle-recursively 5)'; and -so on. That is to say, the calculation that `(triangle-recursively 7)' -makes must be deferred until `(triangle-recursively 6)' makes its -calculation; and `(triangle-recursively 6)' must defer until -`(triangle-recursively 5)' completes; and so on. - -If each of these instances of `triangle-recursively' are thought of as -different robots, the first robot must wait for the second to complete -its job, which must wait until the third completes, and so on. - -There is a way around this kind of waiting, which we will discuss in -*Note Recursion without Deferments: No Deferment. - - -File: eintr, Node: Recursion with cond, Next: Recursive Patterns, Prev: Recursive triangle function, Up: Recursion - -11.3.5 Recursion Example Using `cond' -------------------------------------- - -The version of `triangle-recursively' described earlier is written with -the `if' special form. It can also be written using another special -form called `cond'. The name of the special form `cond' is an -abbreviation of the word `conditional'. - -Although the `cond' special form is not used as often in the Emacs Lisp -sources as `if', it is used often enough to justify explaining it. - -The template for a `cond' expression looks like this: - - (cond - BODY...) - -where the BODY is a series of lists. - -Written out more fully, the template looks like this: - - (cond - (FIRST-TRUE-OR-FALSE-TEST FIRST-CONSEQUENT) - (SECOND-TRUE-OR-FALSE-TEST SECOND-CONSEQUENT) - (THIRD-TRUE-OR-FALSE-TEST THIRD-CONSEQUENT) - ...) - -When the Lisp interpreter evaluates the `cond' expression, it evaluates -the first element (the CAR or true-or-false-test) of the first -expression in a series of expressions within the body of the `cond'. - -If the true-or-false-test returns `nil' the rest of that expression, -the consequent, is skipped and the true-or-false-test of the next -expression is evaluated. When an expression is found whose -true-or-false-test returns a value that is not `nil', the consequent of -that expression is evaluated. The consequent can be one or more -expressions. If the consequent consists of more than one expression, -the expressions are evaluated in sequence and the value of the last one -is returned. If the expression does not have a consequent, the value -of the true-or-false-test is returned. - -If none of the true-or-false-tests test true, the `cond' expression -returns `nil'. - -Written using `cond', the `triangle' function looks like this: - - (defun triangle-using-cond (number) - (cond ((<= number 0) 0) - ((= number 1) 1) - ((> number 1) - (+ number (triangle-using-cond (1- number)))))) - -In this example, the `cond' returns 0 if the number is less than or -equal to 0, it returns 1 if the number is 1 and it evaluates `(+ number -(triangle-using-cond (1- number)))' if the number is greater than 1. - - -File: eintr, Node: Recursive Patterns, Next: No Deferment, Prev: Recursion with cond, Up: Recursion - -11.3.6 Recursive Patterns -------------------------- - -Here are three common recursive patterns. Each involves a list. -Recursion does not need to involve lists, but Lisp is designed for lists -and this provides a sense of its primal capabilities. - -* Menu: - -* Every:: -* Accumulate:: -* Keep:: - - -File: eintr, Node: Every, Next: Accumulate, Prev: Recursive Patterns, Up: Recursive Patterns - -Recursive Pattern: _every_ -.......................... - -In the `every' recursive pattern, an action is performed on every -element of a list. - -The basic pattern is: - - * If a list be empty, return `nil'. - - * Else, act on the beginning of the list (the CAR of the list) - - through a recursive call by the function on the rest (the - CDR) of the list, - - - and, optionally, combine the acted-on element, using - `cons', with the results of acting on the rest. - -Here is example: - - (defun square-each (numbers-list) - "Square each of a NUMBERS LIST, recursively." - (if (not numbers-list) ; do-again-test - nil - (cons - (* (car numbers-list) (car numbers-list)) - (square-each (cdr numbers-list))))) ; next-step-expression - - (square-each '(1 2 3)) - => (1 4 9) - -If `numbers-list' is empty, do nothing. But if it has content, -construct a list combining the square of the first number in the list -with the result of the recursive call. - -(The example follows the pattern exactly: `nil' is returned if the -numbers' list is empty. In practice, you would write the conditional -so it carries out the action when the numbers' list is not empty.) - -The `print-elements-recursively' function (*note Recursion with a List: -Recursion with list.) is another example of an `every' pattern, except -in this case, rather than bring the results together using `cons', we -print each element of output. - -The `print-elements-recursively' function looks like this: - - (setq animals '(gazelle giraffe lion tiger)) - - (defun print-elements-recursively (list) - "Print each element of LIST on a line of its own. - Uses recursion." - (if list ; do-again-test - (progn - (print (car list)) ; body - (print-elements-recursively ; recursive call - (cdr list))))) ; next-step-expression - - (print-elements-recursively animals) - -The pattern for `print-elements-recursively' is: - - * If the list be empty, do nothing. - - * But if the list has at least one element, - - act on the beginning of the list (the CAR of the list), - - - and make a recursive call on the rest (the CDR) of the - list. - - -File: eintr, Node: Accumulate, Next: Keep, Prev: Every, Up: Recursive Patterns - -Recursive Pattern: _accumulate_ -............................... - -Another recursive pattern is called the `accumulate' pattern. In the -`accumulate' recursive pattern, an action is performed on every element -of a list and the result of that action is accumulated with the results -of performing the action on the other elements. - -This is very like the `every' pattern using `cons', except that `cons' -is not used, but some other combiner. - -The pattern is: - - * If a list be empty, return zero or some other constant. - - * Else, act on the beginning of the list (the CAR of the list), - - and combine that acted-on element, using `+' or some - other combining function, with - - - a recursive call by the function on the rest (the CDR) of - the list. - -Here is an example: - - (defun add-elements (numbers-list) - "Add the elements of NUMBERS-LIST together." - (if (not numbers-list) - 0 - (+ (car numbers-list) (add-elements (cdr numbers-list))))) - - (add-elements '(1 2 3 4)) - => 10 - -*Note Making a List of Files: Files List, for an example of the -accumulate pattern. - - -File: eintr, Node: Keep, Prev: Accumulate, Up: Recursive Patterns - -Recursive Pattern: _keep_ -......................... - -A third recursive pattern is called the `keep' pattern. In the `keep' -recursive pattern, each element of a list is tested; the element is -acted on and the results are kept only if the element meets a criterion. - -Again, this is very like the `every' pattern, except the element is -skipped unless it meets a criterion. - -The pattern has three parts: - - * If a list be empty, return `nil'. - - * Else, if the beginning of the list (the CAR of the list) passes - a test - - act on that element and combine it, using `cons' with - - - a recursive call by the function on the rest (the CDR) of - the list. - - * Otherwise, if the beginning of the list (the CAR of the list) fails - the test - - skip on that element, - - - and, recursively call the function on the rest (the CDR) - of the list. - -Here is an example that uses `cond': - - (defun keep-three-letter-words (word-list) - "Keep three letter words in WORD-LIST." - (cond - ;; First do-again-test: stop-condition - ((not word-list) nil) - - ;; Second do-again-test: when to act - ((eq 3 (length (symbol-name (car word-list)))) - ;; combine acted-on element with recursive call on shorter list - (cons (car word-list) (keep-three-letter-words (cdr word-list)))) - - ;; Third do-again-test: when to skip element; - ;; recursively call shorter list with next-step expression - (t (keep-three-letter-words (cdr word-list))))) - - (keep-three-letter-words '(one two three four five six)) - => (one two six) - -It goes without saying that you need not use `nil' as the test for when -to stop; and you can, of course, combine these patterns. - - -File: eintr, Node: No Deferment, Next: No deferment solution, Prev: Recursive Patterns, Up: Recursion - -11.3.7 Recursion without Deferments ------------------------------------ - -Let's consider again what happens with the `triangle-recursively' -function. We will find that the intermediate calculations are deferred -until all can be done. - -Here is the function definition: - - (defun triangle-recursively (number) - "Return the sum of the numbers 1 through NUMBER inclusive. - Uses recursion." - (if (= number 1) ; do-again-test - 1 ; then-part - (+ number ; else-part - (triangle-recursively ; recursive call - (1- number))))) ; next-step-expression - -What happens when we call this function with a argument of 7? - -The first instance of the `triangle-recursively' function adds the -number 7 to the value returned by a second instance of -`triangle-recursively', an instance that has been passed an argument of -6. That is to say, the first calculation is: - - (+ 7 (triangle-recursively 6)) - -The first instance of `triangle-recursively'--you may want to think of -it as a little robot--cannot complete its job. It must hand off the -calculation for `(triangle-recursively 6)' to a second instance of the -program, to a second robot. This second individual is completely -different from the first one; it is, in the jargon, a `different -instantiation'. Or, put another way, it is a different robot. It is -the same model as the first; it calculates triangle numbers -recursively; but it has a different serial number. - -And what does `(triangle-recursively 6)' return? It returns the number -6 added to the value returned by evaluating `triangle-recursively' with -an argument of 5. Using the robot metaphor, it asks yet another robot -to help it. - -Now the total is: - - (+ 7 6 (triangle-recursively 5)) - -And what happens next? - - (+ 7 6 5 (triangle-recursively 4)) - -Each time `triangle-recursively' is called, except for the last time, -it creates another instance of the program--another robot--and asks it -to make a calculation. - -Eventually, the full addition is set up and performed: - - (+ 7 6 5 4 3 2 1) - -This design for the function defers the calculation of the first step -until the second can be done, and defers that until the third can be -done, and so on. Each deferment means the computer must remember what -is being waited on. This is not a problem when there are only a few -steps, as in this example. But it can be a problem when there are more -steps. - - -File: eintr, Node: No deferment solution, Prev: No Deferment, Up: Recursion - -11.3.8 No Deferment Solution ----------------------------- - -The solution to the problem of deferred operations is to write in a -manner that does not defer operations(1). This requires writing to a -different pattern, often one that involves writing two function -definitions, an `initialization' function and a `helper' function. - -The `initialization' function sets up the job; the `helper' function -does the work. - -Here are the two function definitions for adding up numbers. They are -so simple, I find them hard to understand. - - (defun triangle-initialization (number) - "Return the sum of the numbers 1 through NUMBER inclusive. - This is the `initialization' component of a two function - duo that uses recursion." - (triangle-recursive-helper 0 0 number)) - - (defun triangle-recursive-helper (sum counter number) - "Return SUM, using COUNTER, through NUMBER inclusive. - This is the `helper' component of a two function duo - that uses recursion." - (if (> counter number) - sum - (triangle-recursive-helper (+ sum counter) ; sum - (1+ counter) ; counter - number))) ; number - -Install both function definitions by evaluating them, then call -`triangle-initialization' with 2 rows: - - (triangle-initialization 2) - => 3 - -The `initialization' function calls the first instance of the `helper' -function with three arguments: zero, zero, and a number which is the -number of rows in the triangle. - -The first two arguments passed to the `helper' function are -initialization values. These values are changed when -`triangle-recursive-helper' invokes new instances.(2) - -Let's see what happens when we have a triangle that has one row. (This -triangle will have one pebble in it!) - -`triangle-initialization' will call its helper with the arguments -`0 0 1'. That function will run the conditional test whether `(> -counter number)': - - (> 0 1) - -and find that the result is false, so it will invoke the else-part of -the `if' clause: - - (triangle-recursive-helper - (+ sum counter) ; sum plus counter => sum - (1+ counter) ; increment counter => counter - number) ; number stays the same - -which will first compute: - - (triangle-recursive-helper (+ 0 0) ; sum - (1+ 0) ; counter - 1) ; number -which is: - - (triangle-recursive-helper 0 1 1) - -Again, `(> counter number)' will be false, so again, the Lisp -interpreter will evaluate `triangle-recursive-helper', creating a new -instance with new arguments. - -This new instance will be; - - (triangle-recursive-helper - (+ sum counter) ; sum plus counter => sum - (1+ counter) ; increment counter => counter - number) ; number stays the same - -which is: - - (triangle-recursive-helper 1 2 1) - -In this case, the `(> counter number)' test will be true! So the -instance will return the value of the sum, which will be 1, as expected. - -Now, let's pass `triangle-initialization' an argument of 2, to find out -how many pebbles there are in a triangle with two rows. - -That function calls `(triangle-recursive-helper 0 0 2)'. - -In stages, the instances called will be: - - sum counter number - (triangle-recursive-helper 0 1 2) - - (triangle-recursive-helper 1 2 2) - - (triangle-recursive-helper 3 3 2) - -When the last instance is called, the `(> counter number)' test will be -true, so the instance will return the value of `sum', which will be 3. - -This kind of pattern helps when you are writing functions that can use -many resources in a computer. - ----------- Footnotes ---------- - -(1) The phrase "tail recursive" is used to describe such a process, one -that uses `constant space'. - -(2) The jargon is mildly confusing: `triangle-recursive-helper' uses a -process that is iterative in a procedure that is recursive. The -process is called iterative because the computer need only record the -three values, `sum', `counter', and `number'; the procedure is -recursive because the function `calls itself'. On the other hand, both -the process and the procedure used by `triangle-recursively' are called -recursive. The word `recursive' has different meanings in the two -contexts. - - -File: eintr, Node: Looping exercise, Prev: Recursion, Up: Loops & Recursion - -11.4 Looping Exercise -===================== - - * Write a function similar to `triangle' in which each row has a - value which is the square of the row number. Use a `while' loop. - - * Write a function similar to `triangle' that multiplies instead of - adds the values. - - * Rewrite these two functions recursively. Rewrite these functions - using `cond'. - - * Write a function for Texinfo mode that creates an index entry at - the beginning of a paragraph for every `@dfn' within the paragraph. - (In a Texinfo file, `@dfn' marks a definition. This book is - written in Texinfo.) - - Many of the functions you will need are described in two of the - previous chapters, *Note Cutting and Storing Text: Cutting & - Storing Text, and *Note Yanking Text Back: Yanking. If you use - `forward-paragraph' to put the index entry at the beginning of the - paragraph, you will have to use `C-h f' (`describe-function') to - find out how to make the command go backwards. - - For more information, see *Note Indicating Definitions: - (texinfo)Indicating. - - -File: eintr, Node: Regexp Search, Next: Counting Words, Prev: Loops & Recursion, Up: Top - -12 Regular Expression Searches -****************************** - -Regular expression searches are used extensively in GNU Emacs. The two -functions, `forward-sentence' and `forward-paragraph', illustrate these -searches well. They use regular expressions to find where to move -point. The phrase `regular expression' is often written as `regexp'. - -Regular expression searches are described in *Note Regular Expression -Search: (emacs)Regexp Search, as well as in *Note Regular Expressions: -(elisp)Regular Expressions. In writing this chapter, I am presuming -that you have at least a mild acquaintance with them. The major point -to remember is that regular expressions permit you to search for -patterns as well as for literal strings of characters. For example, -the code in `forward-sentence' searches for the pattern of possible -characters that could mark the end of a sentence, and moves point to -that spot. - -Before looking at the code for the `forward-sentence' function, it is -worth considering what the pattern that marks the end of a sentence -must be. The pattern is discussed in the next section; following that -is a description of the regular expression search function, -`re-search-forward'. The `forward-sentence' function is described in -the section following. Finally, the `forward-paragraph' function is -described in the last section of this chapter. `forward-paragraph' is -a complex function that introduces several new features. - -* Menu: - -* sentence-end:: -* re-search-forward:: -* forward-sentence:: -* forward-paragraph:: -* etags:: -* Regexp Review:: -* re-search Exercises:: - - -File: eintr, Node: sentence-end, Next: re-search-forward, Prev: Regexp Search, Up: Regexp Search - -12.1 The Regular Expression for `sentence-end' -============================================== - -The symbol `sentence-end' is bound to the pattern that marks the end of -a sentence. What should this regular expression be? - -Clearly, a sentence may be ended by a period, a question mark, or an -exclamation mark. Indeed, only clauses that end with one of those three -characters should be considered the end of a sentence. This means that -the pattern should include the character set: - - [.?!] - -However, we do not want `forward-sentence' merely to jump to a period, -a question mark, or an exclamation mark, because such a character might -be used in the middle of a sentence. A period, for example, is used -after abbreviations. So other information is needed. - -According to convention, you type two spaces after every sentence, but -only one space after a period, a question mark, or an exclamation mark -in the body of a sentence. So a period, a question mark, or an -exclamation mark followed by two spaces is a good indicator of an end -of sentence. However, in a file, the two spaces may instead be a tab -or the end of a line. This means that the regular expression should -include these three items as alternatives. - -This group of alternatives will look like this: - - \\($\\| \\| \\) - ^ ^^ - TAB SPC - -Here, `$' indicates the end of the line, and I have pointed out where -the tab and two spaces are inserted in the expression. Both are -inserted by putting the actual characters into the expression. - -Two backslashes, `\\', are required before the parentheses and vertical -bars: the first backslash quotes the following backslash in Emacs; and -the second indicates that the following character, the parenthesis or -the vertical bar, is special. - -Also, a sentence may be followed by one or more carriage returns, like -this: - - [ - ]* - -Like tabs and spaces, a carriage return is inserted into a regular -expression by inserting it literally. The asterisk indicates that the -<RET> is repeated zero or more times. - -But a sentence end does not consist only of a period, a question mark or -an exclamation mark followed by appropriate space: a closing quotation -mark or a closing brace of some kind may precede the space. Indeed more -than one such mark or brace may precede the space. These require a -expression that looks like this: - - []\"')}]* - -In this expression, the first `]' is the first character in the -expression; the second character is `"', which is preceded by a `\' to -tell Emacs the `"' is _not_ special. The last three characters are -`'', `)', and `}'. - -All this suggests what the regular expression pattern for matching the -end of a sentence should be; and, indeed, if we evaluate `sentence-end' -we find that it returns the following value: - - sentence-end - => "[.?!][]\"')}]*\\($\\| \\| \\)[ - ]*" - -(Well, not in GNU Emacs 22; that is because of an effort to make the -process simpler. When its value is `nil', then use the value defined -by the function `sentence-end', and that returns a value constructed -from the variables `sentence-end-base', `sentence-end-double-space', -`sentence-end-without-period', and `sentence-end-without-space'. The -critical variable is `sentence-end-base'; its global value is similar -to the one described above but it also contains two additional -quotation marks. These have differing degrees of curliness. The -`sentence-end-without-period' variable, when true, tells Emacs that a -sentence may end without a period, such as text in Thai.) - - -File: eintr, Node: re-search-forward, Next: forward-sentence, Prev: sentence-end, Up: Regexp Search - -12.2 The `re-search-forward' Function -===================================== - -The `re-search-forward' function is very like the `search-forward' -function. (*Note The `search-forward' Function: search-forward.) - -`re-search-forward' searches for a regular expression. If the search -is successful, it leaves point immediately after the last character in -the target. If the search is backwards, it leaves point just before -the first character in the target. You may tell `re-search-forward' to -return `t' for true. (Moving point is therefore a `side effect'.) - -Like `search-forward', the `re-search-forward' function takes four -arguments: - - 1. The first argument is the regular expression that the function - searches for. The regular expression will be a string between - quotations marks. - - 2. The optional second argument limits how far the function will - search; it is a bound, which is specified as a position in the - buffer. - - 3. The optional third argument specifies how the function responds to - failure: `nil' as the third argument causes the function to signal - an error (and print a message) when the search fails; any other - value causes it to return `nil' if the search fails and `t' if the - search succeeds. - - 4. The optional fourth argument is the repeat count. A negative - repeat count causes `re-search-forward' to search backwards. - -The template for `re-search-forward' looks like this: - - (re-search-forward "REGULAR-EXPRESSION" - LIMIT-OF-SEARCH - WHAT-TO-DO-IF-SEARCH-FAILS - REPEAT-COUNT) - -The second, third, and fourth arguments are optional. However, if you -want to pass a value to either or both of the last two arguments, you -must also pass a value to all the preceding arguments. Otherwise, the -Lisp interpreter will mistake which argument you are passing the value -to. - -In the `forward-sentence' function, the regular expression will be the -value of the variable `sentence-end'. In simple form, that is: - - "[.?!][]\"')}]*\\($\\| \\| \\)[ - ]*" - -The limit of the search will be the end of the paragraph (since a -sentence cannot go beyond a paragraph). If the search fails, the -function will return `nil'; and the repeat count will be provided by -the argument to the `forward-sentence' function. - - -File: eintr, Node: forward-sentence, Next: forward-paragraph, Prev: re-search-forward, Up: Regexp Search - -12.3 `forward-sentence' -======================= - -The command to move the cursor forward a sentence is a straightforward -illustration of how to use regular expression searches in Emacs Lisp. -Indeed, the function looks longer and more complicated than it is; this -is because the function is designed to go backwards as well as forwards; -and, optionally, over more than one sentence. The function is usually -bound to the key command `M-e'. - -* Menu: - -* Complete forward-sentence:: -* fwd-sentence while loops:: -* fwd-sentence re-search:: - - -File: eintr, Node: Complete forward-sentence, Next: fwd-sentence while loops, Prev: forward-sentence, Up: forward-sentence - -Complete `forward-sentence' function definition ------------------------------------------------ - -Here is the code for `forward-sentence': - - (defun forward-sentence (&optional arg) - "Move forward to next `sentence-end'. With argument, repeat. - With negative argument, move backward repeatedly to `sentence-beginning'. - - The variable `sentence-end' is a regular expression that matches ends of - sentences. Also, every paragraph boundary terminates sentences as well." - (interactive "p") - (or arg (setq arg 1)) - (let ((opoint (point)) - (sentence-end (sentence-end))) - (while (< arg 0) - (let ((pos (point)) - (par-beg (save-excursion (start-of-paragraph-text) (point)))) - (if (and (re-search-backward sentence-end par-beg t) - (or (< (match-end 0) pos) - (re-search-backward sentence-end par-beg t))) - (goto-char (match-end 0)) - (goto-char par-beg))) - (setq arg (1+ arg))) - (while (> arg 0) - (let ((par-end (save-excursion (end-of-paragraph-text) (point)))) - (if (re-search-forward sentence-end par-end t) - (skip-chars-backward " \t\n") - (goto-char par-end))) - (setq arg (1- arg))) - (constrain-to-field nil opoint t))) - -The function looks long at first sight and it is best to look at its -skeleton first, and then its muscle. The way to see the skeleton is to -look at the expressions that start in the left-most columns: - - (defun forward-sentence (&optional arg) - "DOCUMENTATION..." - (interactive "p") - (or arg (setq arg 1)) - (let ((opoint (point)) (sentence-end (sentence-end))) - (while (< arg 0) - (let ((pos (point)) - (par-beg (save-excursion (start-of-paragraph-text) (point)))) - REST-OF-BODY-OF-WHILE-LOOP-WHEN-GOING-BACKWARDS - (while (> arg 0) - (let ((par-end (save-excursion (end-of-paragraph-text) (point)))) - REST-OF-BODY-OF-WHILE-LOOP-WHEN-GOING-FORWARDS - HANDLE-FORMS-AND-EQUIVALENT - -This looks much simpler! The function definition consists of -documentation, an `interactive' expression, an `or' expression, a `let' -expression, and `while' loops. - -Let's look at each of these parts in turn. - -We note that the documentation is thorough and understandable. - -The function has an `interactive "p"' declaration. This means that the -processed prefix argument, if any, is passed to the function as its -argument. (This will be a number.) If the function is not passed an -argument (it is optional) then the argument `arg' will be bound to 1. - -When `forward-sentence' is called non-interactively without an -argument, `arg' is bound to `nil'. The `or' expression handles this. -What it does is either leave the value of `arg' as it is, but only if -`arg' is bound to a value; or it sets the value of `arg' to 1, in the -case when `arg' is bound to `nil'. - -Next is a `let'. That specifies the values of two local variables, -`point' and `sentence-end'. The local value of point, from before the -search, is used in the `constrain-to-field' function which handles -forms and equivalents. The `sentence-end' variable is set by the -`sentence-end' function. - - -File: eintr, Node: fwd-sentence while loops, Next: fwd-sentence re-search, Prev: Complete forward-sentence, Up: forward-sentence - -The `while' loops ------------------ - -Two `while' loops follow. The first `while' has a true-or-false-test -that tests true if the prefix argument for `forward-sentence' is a -negative number. This is for going backwards. The body of this loop -is similar to the body of the second `while' clause, but it is not -exactly the same. We will skip this `while' loop and concentrate on -the second `while' loop. - -The second `while' loop is for moving point forward. Its skeleton -looks like this: - - (while (> arg 0) ; true-or-false-test - (let VARLIST - (if (TRUE-OR-FALSE-TEST) - THEN-PART - ELSE-PART - (setq arg (1- arg)))) ; `while' loop decrementer - -The `while' loop is of the decrementing kind. (*Note A Loop with a -Decrementing Counter: Decrementing Loop.) It has a true-or-false-test -that tests true so long as the counter (in this case, the variable -`arg') is greater than zero; and it has a decrementer that subtracts 1 -from the value of the counter every time the loop repeats. - -If no prefix argument is given to `forward-sentence', which is the most -common way the command is used, this `while' loop will run once, since -the value of `arg' will be 1. - -The body of the `while' loop consists of a `let' expression, which -creates and binds a local variable, and has, as its body, an `if' -expression. - -The body of the `while' loop looks like this: - - (let ((par-end - (save-excursion (end-of-paragraph-text) (point)))) - (if (re-search-forward sentence-end par-end t) - (skip-chars-backward " \t\n") - (goto-char par-end))) - -The `let' expression creates and binds the local variable `par-end'. -As we shall see, this local variable is designed to provide a bound or -limit to the regular expression search. If the search fails to find a -proper sentence ending in the paragraph, it will stop on reaching the -end of the paragraph. - -But first, let us examine how `par-end' is bound to the value of the -end of the paragraph. What happens is that the `let' sets the value of -`par-end' to the value returned when the Lisp interpreter evaluates the -expression - - (save-excursion (end-of-paragraph-text) (point)) - -In this expression, `(end-of-paragraph-text)' moves point to the end of -the paragraph, `(point)' returns the value of point, and then -`save-excursion' restores point to its original position. Thus, the -`let' binds `par-end' to the value returned by the `save-excursion' -expression, which is the position of the end of the paragraph. (The -`(end-of-paragraph-text)' function uses `forward-paragraph', which we -will discuss shortly.) - -Emacs next evaluates the body of the `let', which is an `if' expression -that looks like this: - - (if (re-search-forward sentence-end par-end t) ; if-part - (skip-chars-backward " \t\n") ; then-part - (goto-char par-end))) ; else-part - -The `if' tests whether its first argument is true and if so, evaluates -its then-part; otherwise, the Emacs Lisp interpreter evaluates the -else-part. The true-or-false-test of the `if' expression is the -regular expression search. - -It may seem odd to have what looks like the `real work' of the -`forward-sentence' function buried here, but this is a common way this -kind of operation is carried out in Lisp. - - -File: eintr, Node: fwd-sentence re-search, Prev: fwd-sentence while loops, Up: forward-sentence - -The regular expression search ------------------------------ - -The `re-search-forward' function searches for the end of the sentence, -that is, for the pattern defined by the `sentence-end' regular -expression. If the pattern is found--if the end of the sentence is -found--then the `re-search-forward' function does two things: - - 1. The `re-search-forward' function carries out a side effect, which - is to move point to the end of the occurrence found. - - 2. The `re-search-forward' function returns a value of true. This is - the value received by the `if', and means that the search was - successful. - -The side effect, the movement of point, is completed before the `if' -function is handed the value returned by the successful conclusion of -the search. - -When the `if' function receives the value of true from a successful -call to `re-search-forward', the `if' evaluates the then-part, which is -the expression `(skip-chars-backward " \t\n")'. This expression moves -backwards over any blank spaces, tabs or carriage returns until a -printed character is found and then leaves point after the character. -Since point has already been moved to the end of the pattern that marks -the end of the sentence, this action leaves point right after the -closing printed character of the sentence, which is usually a period. - -On the other hand, if the `re-search-forward' function fails to find a -pattern marking the end of the sentence, the function returns false. -The false then causes the `if' to evaluate its third argument, which is -`(goto-char par-end)': it moves point to the end of the paragraph. - -(And if the text is in a form or equivalent, and point may not move -fully, then the `constrain-to-field' function comes into play.) - -Regular expression searches are exceptionally useful and the pattern -illustrated by `re-search-forward', in which the search is the test of -an `if' expression, is handy. You will see or write code incorporating -this pattern often. - - -File: eintr, Node: forward-paragraph, Next: etags, Prev: forward-sentence, Up: Regexp Search - -12.4 `forward-paragraph': a Goldmine of Functions -================================================= - -The `forward-paragraph' function moves point forward to the end of the -paragraph. It is usually bound to `M-}' and makes use of a number of -functions that are important in themselves, including `let*', -`match-beginning', and `looking-at'. - -The function definition for `forward-paragraph' is considerably longer -than the function definition for `forward-sentence' because it works -with a paragraph, each line of which may begin with a fill prefix. - -A fill prefix consists of a string of characters that are repeated at -the beginning of each line. For example, in Lisp code, it is a -convention to start each line of a paragraph-long comment with `;;; '. -In Text mode, four blank spaces make up another common fill prefix, -creating an indented paragraph. (*Note Fill Prefix: (emacs)Fill -Prefix, for more information about fill prefixes.) - -The existence of a fill prefix means that in addition to being able to -find the end of a paragraph whose lines begin on the left-most column, -the `forward-paragraph' function must be able to find the end of a -paragraph when all or many of the lines in the buffer begin with the -fill prefix. - -Moreover, it is sometimes practical to ignore a fill prefix that -exists, especially when blank lines separate paragraphs. This is an -added complication. - -* Menu: - -* forward-paragraph in brief:: -* fwd-para let:: -* fwd-para while:: - - -File: eintr, Node: forward-paragraph in brief, Next: fwd-para let, Prev: forward-paragraph, Up: forward-paragraph - -Shortened `forward-paragraph' function definition -------------------------------------------------- - -Rather than print all of the `forward-paragraph' function, we will only -print parts of it. Read without preparation, the function can be -daunting! - -In outline, the function looks like this: - - (defun forward-paragraph (&optional arg) - "DOCUMENTATION..." - (interactive "p") - (or arg (setq arg 1)) - (let* - VARLIST - (while (and (< arg 0) (not (bobp))) ; backward-moving-code - ... - (while (and (> arg 0) (not (eobp))) ; forward-moving-code - ... - -The first parts of the function are routine: the function's argument -list consists of one optional argument. Documentation follows. - -The lower case `p' in the `interactive' declaration means that the -processed prefix argument, if any, is passed to the function. This -will be a number, and is the repeat count of how many paragraphs point -will move. The `or' expression in the next line handles the common -case when no argument is passed to the function, which occurs if the -function is called from other code rather than interactively. This -case was described earlier. (*Note The `forward-sentence' function: -forward-sentence.) Now we reach the end of the familiar part of this -function. - - -File: eintr, Node: fwd-para let, Next: fwd-para while, Prev: forward-paragraph in brief, Up: forward-paragraph - -The `let*' expression ---------------------- - -The next line of the `forward-paragraph' function begins a `let*' -expression. This is a different than `let'. The symbol is `let*' not -`let'. - -The `let*' special form is like `let' except that Emacs sets each -variable in sequence, one after another, and variables in the latter -part of the varlist can make use of the values to which Emacs set -variables in the earlier part of the varlist. - -(*Note `save-excursion' in `append-to-buffer': append save-excursion.) - -In the `let*' expression in this function, Emacs binds a total of seven -variables: `opoint', `fill-prefix-regexp', `parstart', `parsep', -`sp-parstart', `start', and `found-start'. - -The variable `parsep' appears twice, first, to remove instances of `^', -and second, to handle fill prefixes. - -The variable `opoint' is just the value of `point'. As you can guess, -it is used in a `constrain-to-field' expression, just as in -`forward-sentence'. - -The variable `fill-prefix-regexp' is set to the value returned by -evaluating the following list: - - (and fill-prefix - (not (equal fill-prefix "")) - (not paragraph-ignore-fill-prefix) - (regexp-quote fill-prefix)) - -This is an expression whose first element is the `and' special form. - -As we learned earlier (*note The `kill-new' function: kill-new -function.), the `and' special form evaluates each of its arguments -until one of the arguments returns a value of `nil', in which case the -`and' expression returns `nil'; however, if none of the arguments -returns a value of `nil', the value resulting from evaluating the last -argument is returned. (Since such a value is not `nil', it is -considered true in Lisp.) In other words, an `and' expression returns -a true value only if all its arguments are true. - -In this case, the variable `fill-prefix-regexp' is bound to a non-`nil' -value only if the following four expressions produce a true (i.e., a -non-`nil') value when they are evaluated; otherwise, -`fill-prefix-regexp' is bound to `nil'. - -`fill-prefix' - When this variable is evaluated, the value of the fill prefix, if - any, is returned. If there is no fill prefix, this variable - returns `nil'. - -`(not (equal fill-prefix "")' - This expression checks whether an existing fill prefix is an empty - string, that is, a string with no characters in it. An empty - string is not a useful fill prefix. - -`(not paragraph-ignore-fill-prefix)' - This expression returns `nil' if the variable - `paragraph-ignore-fill-prefix' has been turned on by being set to a - true value such as `t'. - -`(regexp-quote fill-prefix)' - This is the last argument to the `and' special form. If all the - arguments to the `and' are true, the value resulting from - evaluating this expression will be returned by the `and' expression - and bound to the variable `fill-prefix-regexp', - -The result of evaluating this `and' expression successfully is that -`fill-prefix-regexp' will be bound to the value of `fill-prefix' as -modified by the `regexp-quote' function. What `regexp-quote' does is -read a string and return a regular expression that will exactly match -the string and match nothing else. This means that -`fill-prefix-regexp' will be set to a value that will exactly match the -fill prefix if the fill prefix exists. Otherwise, the variable will be -set to `nil'. - -The next two local variables in the `let*' expression are designed to -remove instances of `^' from `parstart' and `parsep', the local -variables indicate the paragraph start and the paragraph separator. -The next expression sets `parsep' again. That is to handle fill -prefixes. - -This is the setting that requires the definition call `let*' rather -than `let'. The true-or-false-test for the `if' depends on whether the -variable `fill-prefix-regexp' evaluates to `nil' or some other value. - -If `fill-prefix-regexp' does not have a value, Emacs evaluates the -else-part of the `if' expression and binds `parsep' to its local value. -(`parsep' is a regular expression that matches what separates -paragraphs.) - -But if `fill-prefix-regexp' does have a value, Emacs evaluates the -then-part of the `if' expression and binds `parsep' to a regular -expression that includes the `fill-prefix-regexp' as part of the -pattern. - -Specifically, `parsep' is set to the original value of the paragraph -separate regular expression concatenated with an alternative expression -that consists of the `fill-prefix-regexp' followed by optional -whitespace to the end of the line. The whitespace is defined by -`"[ \t]*$"'.) The `\\|' defines this portion of the regexp as an -alternative to `parsep'. - -According to a comment in the code, the next local variable, -`sp-parstart', is used for searching, and then the final two, `start' -and `found-start', are set to `nil'. - -Now we get into the body of the `let*'. The first part of the body of -the `let*' deals with the case when the function is given a negative -argument and is therefore moving backwards. We will skip this section. - - -File: eintr, Node: fwd-para while, Prev: fwd-para let, Up: forward-paragraph - -The forward motion `while' loop -------------------------------- - -The second part of the body of the `let*' deals with forward motion. -It is a `while' loop that repeats itself so long as the value of `arg' -is greater than zero. In the most common use of the function, the -value of the argument is 1, so the body of the `while' loop is -evaluated exactly once, and the cursor moves forward one paragraph. - -This part handles three situations: when point is between paragraphs, -when there is a fill prefix and when there is no fill prefix. - -The `while' loop looks like this: - - ;; going forwards and not at the end of the buffer - (while (and (> arg 0) (not (eobp))) - - ;; between paragraphs - ;; Move forward over separator lines... - (while (and (not (eobp)) - (progn (move-to-left-margin) (not (eobp))) - (looking-at parsep)) - (forward-line 1)) - ;; This decrements the loop - (unless (eobp) (setq arg (1- arg))) - ;; ... and one more line. - (forward-line 1) - - (if fill-prefix-regexp - ;; There is a fill prefix; it overrides parstart; - ;; we go forward line by line - (while (and (not (eobp)) - (progn (move-to-left-margin) (not (eobp))) - (not (looking-at parsep)) - (looking-at fill-prefix-regexp)) - (forward-line 1)) - - ;; There is no fill prefix; - ;; we go forward character by character - (while (and (re-search-forward sp-parstart nil 1) - (progn (setq start (match-beginning 0)) - (goto-char start) - (not (eobp))) - (progn (move-to-left-margin) - (not (looking-at parsep))) - (or (not (looking-at parstart)) - (and use-hard-newlines - (not (get-text-property (1- start) 'hard))))) - (forward-char 1)) - - ;; and if there is no fill prefix and if we are not at the end, - ;; go to whatever was found in the regular expression search - ;; for sp-parstart - (if (< (point) (point-max)) - (goto-char start)))) - -We can see that this is a decrementing counter `while' loop, using the -expression `(setq arg (1- arg))' as the decrementer. That expression -is not far from the `while', but is hidden in another Lisp macro, an -`unless' macro. Unless we are at the end of the buffer -- that is what -the `eobp' function determines; it is an abbreviation of `End Of Buffer -P' -- we decrease the value of `arg' by one. - -(If we are at the end of the buffer, we cannot go forward any more and -the next loop of the `while' expression will test false since the test -is an `and' with `(not (eobp))'. The `not' function means exactly as -you expect; it is another name for `null', a function that returns true -when its argument is false.) - -Interestingly, the loop count is not decremented until we leave the -space between paragraphs, unless we come to the end of buffer or stop -seeing the local value of the paragraph separator. - -That second `while' also has a `(move-to-left-margin)' expression. The -function is self-explanatory. It is inside a `progn' expression and -not the last element of its body, so it is only invoked for its side -effect, which is to move point to the left margin of the current line. - -The `looking-at' function is also self-explanatory; it returns true if -the text after point matches the regular expression given as its -argument. - -The rest of the body of the loop looks difficult at first, but makes -sense as you come to understand it. - -First consider what happens if there is a fill prefix: - - (if fill-prefix-regexp - ;; There is a fill prefix; it overrides parstart; - ;; we go forward line by line - (while (and (not (eobp)) - (progn (move-to-left-margin) (not (eobp))) - (not (looking-at parsep)) - (looking-at fill-prefix-regexp)) - (forward-line 1)) - -This expression moves point forward line by line so long as four -conditions are true: - - 1. Point is not at the end of the buffer. - - 2. We can move to the left margin of the text and are not at the end - of the buffer. - - 3. The text following point does not separate paragraphs. - - 4. The pattern following point is the fill prefix regular expression. - -The last condition may be puzzling, until you remember that point was -moved to the beginning of the line early in the `forward-paragraph' -function. This means that if the text has a fill prefix, the -`looking-at' function will see it. - -Consider what happens when there is no fill prefix. - - (while (and (re-search-forward sp-parstart nil 1) - (progn (setq start (match-beginning 0)) - (goto-char start) - (not (eobp))) - (progn (move-to-left-margin) - (not (looking-at parsep))) - (or (not (looking-at parstart)) - (and use-hard-newlines - (not (get-text-property (1- start) 'hard))))) - (forward-char 1)) - -This `while' loop has us searching forward for `sp-parstart', which is -the combination of possible whitespace with a the local value of the -start of a paragraph or of a paragraph separator. (The latter two are -within an expression starting `\(?:' so that they are not referenced by -the `match-beginning' function.) - -The two expressions, - - (setq start (match-beginning 0)) - (goto-char start) - -mean go to the start of the text matched by the regular expression -search. - -The `(match-beginning 0)' expression is new. It returns a number -specifying the location of the start of the text that was matched by -the last search. - -The `match-beginning' function is used here because of a characteristic -of a forward search: a successful forward search, regardless of whether -it is a plain search or a regular expression search, moves point to the -end of the text that is found. In this case, a successful search moves -point to the end of the pattern for `sp-parstart'. - -However, we want to put point at the end of the current paragraph, not -somewhere else. Indeed, since the search possibly includes the -paragraph separator, point may end up at the beginning of the next one -unless we use an expression that includes `match-beginning'. - -When given an argument of 0, `match-beginning' returns the position -that is the start of the text matched by the most recent search. In -this case, the most recent search looks for `sp-parstart'. The -`(match-beginning 0)' expression returns the beginning position of that -pattern, rather than the end position of that pattern. - -(Incidentally, when passed a positive number as an argument, the -`match-beginning' function returns the location of point at that -parenthesized expression in the last search unless that parenthesized -expression begins with `\(?:'. I don't know why `\(?:' appears here -since the argument is 0.) - -The last expression when there is no fill prefix is - - (if (< (point) (point-max)) - (goto-char start)))) - -This says that if there is no fill prefix and if we are not at the end, -point should move to the beginning of whatever was found by the regular -expression search for `sp-parstart'. - -The full definition for the `forward-paragraph' function not only -includes code for going forwards, but also code for going backwards. - -If you are reading this inside of GNU Emacs and you want to see the -whole function, you can type `C-h f' (`describe-function') and the name -of the function. This gives you the function documentation and the -name of the library containing the function's source. Place point over -the name of the library and press the RET key; you will be taken -directly to the source. (Be sure to install your sources! Without -them, you are like a person who tries to drive a car with his eyes -shut!) - - -File: eintr, Node: etags, Next: Regexp Review, Prev: forward-paragraph, Up: Regexp Search - -12.5 Create Your Own `TAGS' File -================================ - -Besides `C-h f' (`describe-function'), another way to see the source of -a function is to type `M-.' (`find-tag') and the name of the function -when prompted for it. This is a good habit to get into. This will -take you directly to the source. If the `find-tag' function first asks -you for the name of a `TAGS' table, give it the name of a `TAGS' file -such as `/usr/local/src/emacs/src/TAGS'. (The exact path to your -`TAGS' file depends on how your copy of Emacs was installed. I just -told you the location that provides both my C and my Emacs Lisp -sources.) - -You can also create your own `TAGS' file for directories that lack one. - -The `M-.' (`find-tag') command takes you directly to the source for a -function, variable, node, or other source. The function depends on -tags tables to tell it where to go. - -You often need to build and install tags tables yourself. They are not -built automatically. A tags table is called a `TAGS' file; the name is -in upper case letters. - -You can create a `TAGS' file by calling the `etags' program that comes -as a part of the Emacs distribution. Usually, `etags' is compiled and -installed when Emacs is built. (`etags' is not an Emacs Lisp function -or a part of Emacs; it is a C program.) - -To create a `TAGS' file, first switch to the directory in which you -want to create the file. In Emacs you can do this with the `M-x cd' -command, or by visiting a file in the directory, or by listing the -directory with `C-x d' (`dired'). Then run the compile command, with -`etags *.el' as the command to execute - - M-x compile RET etags *.el RET - -to create a `TAGS' file. - -For example, if you have a large number of files in your `~/emacs' -directory, as I do--I have 137 `.el' files in it, of which I load -12--you can create a `TAGS' file for the Emacs Lisp files in that -directory. - -The `etags' program takes all the usual shell `wildcards'. For -example, if you have two directories for which you want a single `TAGS -file', type `etags *.el ../elisp/*.el', where `../elisp/' is the second -directory: - - M-x compile RET etags *.el ../elisp/*.el RET - -Type - - M-x compile RET etags --help RET - -to see a list of the options accepted by `etags' as well as a list of -supported languages. - -The `etags' program handles more than 20 languages, including Emacs -Lisp, Common Lisp, Scheme, C, C++, Ada, Fortran, Java, LaTeX, Pascal, -Perl, Python, Texinfo, makefiles, and most assemblers. The program has -no switches for specifying the language; it recognizes the language in -an input file according to its file name and contents. - -`etags' is very helpful when you are writing code yourself and want to -refer back to functions you have already written. Just run `etags' -again at intervals as you write new functions, so they become part of -the `TAGS' file. - -If you think an appropriate `TAGS' file already exists for what you -want, but do not know where it is, you can use the `locate' program to -attempt to find it. - -Type `M-x locate <RET> TAGS <RET>' and Emacs will list for you the full -path names of all your `TAGS' files. On my system, this command lists -34 `TAGS' files. On the other hand, a `plain vanilla' system I -recently installed did not contain any `TAGS' files. - -If the tags table you want has been created, you can use the `M-x -visit-tags-table' command to specify it. Otherwise, you will need to -create the tag table yourself and then use `M-x visit-tags-table'. - -Building Tags in the Emacs sources -.................................. - -The GNU Emacs sources come with a `Makefile' that contains a -sophisticated `etags' command that creates, collects, and merges tags -tables from all over the Emacs sources and puts the information into -one `TAGS' file in the `src/' directory below the top level of your -Emacs source directory. - -To build this `TAGS' file, go to the top level of your Emacs source -directory and run the compile command `make tags': - - M-x compile RET make tags RET - -(The `make tags' command works well with the GNU Emacs sources, as well -as with some other source packages.) - -For more information, see *Note Tag Tables: (emacs)Tags. - - -File: eintr, Node: Regexp Review, Next: re-search Exercises, Prev: etags, Up: Regexp Search - -12.6 Review -=========== - -Here is a brief summary of some recently introduced functions. - -`while' - Repeatedly evaluate the body of the expression so long as the first - element of the body tests true. Then return `nil'. (The - expression is evaluated only for its side effects.) - - For example: - - (let ((foo 2)) - (while (> foo 0) - (insert (format "foo is %d.\n" foo)) - (setq foo (1- foo)))) - - => foo is 2. - foo is 1. - nil - - (The `insert' function inserts its arguments at point; the - `format' function returns a string formatted from its arguments - the way `message' formats its arguments; `\n' produces a new line.) - -`re-search-forward' - Search for a pattern, and if the pattern is found, move point to - rest just after it. - - Takes four arguments, like `search-forward': - - 1. A regular expression that specifies the pattern to search for. - (Remember to put quotation marks around this argument!) - - 2. Optionally, the limit of the search. - - 3. Optionally, what to do if the search fails, return `nil' or an - error message. - - 4. Optionally, how many times to repeat the search; if negative, - the search goes backwards. - -`let*' - Bind some variables locally to particular values, and then - evaluate the remaining arguments, returning the value of the last - one. While binding the local variables, use the local values of - variables bound earlier, if any. - - For example: - - (let* ((foo 7) - (bar (* 3 foo))) - (message "`bar' is %d." bar)) - => `bar' is 21. - -`match-beginning' - Return the position of the start of the text found by the last - regular expression search. - -`looking-at' - Return `t' for true if the text after point matches the argument, - which should be a regular expression. - -`eobp' - Return `t' for true if point is at the end of the accessible part - of a buffer. The end of the accessible part is the end of the - buffer if the buffer is not narrowed; it is the end of the - narrowed part if the buffer is narrowed. - - -File: eintr, Node: re-search Exercises, Prev: Regexp Review, Up: Regexp Search - -12.7 Exercises with `re-search-forward' -======================================= - - * Write a function to search for a regular expression that matches - two or more blank lines in sequence. - - * Write a function to search for duplicated words, such as `the the'. - *Note Syntax of Regular Expressions: (emacs)Regexps, for - information on how to write a regexp (a regular expression) to - match a string that is composed of two identical halves. You can - devise several regexps; some are better than others. The function - I use is described in an appendix, along with several regexps. - *Note `the-the' Duplicated Words Function: the-the. - - -File: eintr, Node: Counting Words, Next: Words in a defun, Prev: Regexp Search, Up: Top - -13 Counting: Repetition and Regexps -*********************************** - -Repetition and regular expression searches are powerful tools that you -often use when you write code in Emacs Lisp. This chapter illustrates -the use of regular expression searches through the construction of word -count commands using `while' loops and recursion. - -* Menu: - -* Why Count Words:: -* count-words-region:: -* recursive-count-words:: -* Counting Exercise:: - - -File: eintr, Node: Why Count Words, Next: count-words-region, Prev: Counting Words, Up: Counting Words - -Counting words -============== - -The standard Emacs distribution contains a function for counting the -number of lines within a region. However, there is no corresponding -function for counting words. - -Certain types of writing ask you to count words. Thus, if you write an -essay, you may be limited to 800 words; if you write a novel, you may -discipline yourself to write 1000 words a day. It seems odd to me that -Emacs lacks a word count command. Perhaps people use Emacs mostly for -code or types of documentation that do not require word counts; or -perhaps they restrict themselves to the operating system word count -command, `wc'. Alternatively, people may follow the publishers' -convention and compute a word count by dividing the number of -characters in a document by five. In any event, here are commands to -count words. - - -File: eintr, Node: count-words-region, Next: recursive-count-words, Prev: Why Count Words, Up: Counting Words - -13.1 The `count-words-region' Function -====================================== - -A word count command could count words in a line, paragraph, region, or -buffer. What should the command cover? You could design the command -to count the number of words in a complete buffer. However, the Emacs -tradition encourages flexibility--you may want to count words in just a -section, rather than all of a buffer. So it makes more sense to design -the command to count the number of words in a region. Once you have a -`count-words-region' command, you can, if you wish, count words in a -whole buffer by marking it with `C-x h' (`mark-whole-buffer'). - -Clearly, counting words is a repetitive act: starting from the -beginning of the region, you count the first word, then the second -word, then the third word, and so on, until you reach the end of the -region. This means that word counting is ideally suited to recursion -or to a `while' loop. - -* Menu: - -* Design count-words-region:: -* Whitespace Bug:: - - -File: eintr, Node: Design count-words-region, Next: Whitespace Bug, Prev: count-words-region, Up: count-words-region - -Designing `count-words-region' ------------------------------- - -First, we will implement the word count command with a `while' loop, -then with recursion. The command will, of course, be interactive. - -The template for an interactive function definition is, as always: - - (defun NAME-OF-FUNCTION (ARGUMENT-LIST) - "DOCUMENTATION..." - (INTERACTIVE-EXPRESSION...) - BODY...) - -What we need to do is fill in the slots. - -The name of the function should be self-explanatory and similar to the -existing `count-lines-region' name. This makes the name easier to -remember. `count-words-region' is a good choice. - -The function counts words within a region. This means that the -argument list must contain symbols that are bound to the two positions, -the beginning and end of the region. These two positions can be called -`beginning' and `end' respectively. The first line of the -documentation should be a single sentence, since that is all that is -printed as documentation by a command such as `apropos'. The -interactive expression will be of the form `(interactive "r")', since -that will cause Emacs to pass the beginning and end of the region to -the function's argument list. All this is routine. - -The body of the function needs to be written to do three tasks: first, -to set up conditions under which the `while' loop can count words, -second, to run the `while' loop, and third, to send a message to the -user. - -When a user calls `count-words-region', point may be at the beginning -or the end of the region. However, the counting process must start at -the beginning of the region. This means we will want to put point -there if it is not already there. Executing `(goto-char beginning)' -ensures this. Of course, we will want to return point to its expected -position when the function finishes its work. For this reason, the -body must be enclosed in a `save-excursion' expression. - -The central part of the body of the function consists of a `while' loop -in which one expression jumps point forward word by word, and another -expression counts those jumps. The true-or-false-test of the `while' -loop should test true so long as point should jump forward, and false -when point is at the end of the region. - -We could use `(forward-word 1)' as the expression for moving point -forward word by word, but it is easier to see what Emacs identifies as a -`word' if we use a regular expression search. - -A regular expression search that finds the pattern for which it is -searching leaves point after the last character matched. This means -that a succession of successful word searches will move point forward -word by word. - -As a practical matter, we want the regular expression search to jump -over whitespace and punctuation between words as well as over the words -themselves. A regexp that refuses to jump over interword whitespace -would never jump more than one word! This means that the regexp should -include the whitespace and punctuation that follows a word, if any, as -well as the word itself. (A word may end a buffer and not have any -following whitespace or punctuation, so that part of the regexp must be -optional.) - -Thus, what we want for the regexp is a pattern defining one or more -word constituent characters followed, optionally, by one or more -characters that are not word constituents. The regular expression for -this is: - - \w+\W* - -The buffer's syntax table determines which characters are and are not -word constituents. (*Note What Constitutes a Word or Symbol?: Syntax, -for more about syntax. Also, see *Note Syntax: (emacs)Syntax, and -*Note Syntax Tables: (elisp)Syntax Tables.) - -The search expression looks like this: - - (re-search-forward "\\w+\\W*") - -(Note that paired backslashes precede the `w' and `W'. A single -backslash has special meaning to the Emacs Lisp interpreter. It -indicates that the following character is interpreted differently than -usual. For example, the two characters, `\n', stand for `newline', -rather than for a backslash followed by `n'. Two backslashes in a row -stand for an ordinary, `unspecial' backslash, which in this case is -followed by a letter, the combination of which is important to -`re-search-forward'.) - -We need a counter to count how many words there are; this variable must -first be set to 0 and then incremented each time Emacs goes around the -`while' loop. The incrementing expression is simply: - - (setq count (1+ count)) - -Finally, we want to tell the user how many words there are in the -region. The `message' function is intended for presenting this kind of -information to the user. The message has to be phrased so that it -reads properly regardless of how many words there are in the region: we -don't want to say that "there are 1 words in the region". The conflict -between singular and plural is ungrammatical. We can solve this -problem by using a conditional expression that evaluates different -messages depending on the number of words in the region. There are -three possibilities: no words in the region, one word in the region, -and more than one word. This means that the `cond' special form is -appropriate. - -All this leads to the following function definition: - - ;;; First version; has bugs! - (defun count-words-region (beginning end) - "Print number of words in the region. - Words are defined as at least one word-constituent - character followed by at least one character that - is not a word-constituent. The buffer's syntax - table determines which characters these are." - (interactive "r") - (message "Counting words in region ... ") - - ;;; 1. Set up appropriate conditions. - (save-excursion - (goto-char beginning) - (let ((count 0)) - - ;;; 2. Run the while loop. - (while (< (point) end) - (re-search-forward "\\w+\\W*") - (setq count (1+ count))) - - ;;; 3. Send a message to the user. - (cond ((zerop count) - (message - "The region does NOT have any words.")) - ((= 1 count) - (message - "The region has 1 word.")) - (t - (message - "The region has %d words." count)))))) - -As written, the function works, but not in all circumstances. - - -File: eintr, Node: Whitespace Bug, Prev: Design count-words-region, Up: count-words-region - -13.1.1 The Whitespace Bug in `count-words-region' -------------------------------------------------- - -The `count-words-region' command described in the preceding section has -two bugs, or rather, one bug with two manifestations. First, if you -mark a region containing only whitespace in the middle of some text, -the `count-words-region' command tells you that the region contains one -word! Second, if you mark a region containing only whitespace at the -end of the buffer or the accessible portion of a narrowed buffer, the -command displays an error message that looks like this: - - Search failed: "\\w+\\W*" - -If you are reading this in Info in GNU Emacs, you can test for these -bugs yourself. - -First, evaluate the function in the usual manner to install it. Here -is a copy of the definition. Place your cursor after the closing -parenthesis and type `C-x C-e' to install it. - - ;; First version; has bugs! - (defun count-words-region (beginning end) - "Print number of words in the region. - Words are defined as at least one word-constituent character followed - by at least one character that is not a word-constituent. The buffer's - syntax table determines which characters these are." - (interactive "r") - (message "Counting words in region ... ") - - ;;; 1. Set up appropriate conditions. - (save-excursion - (goto-char beginning) - (let ((count 0)) - - ;;; 2. Run the while loop. - (while (< (point) end) - (re-search-forward "\\w+\\W*") - (setq count (1+ count))) - - ;;; 3. Send a message to the user. - (cond ((zerop count) - (message "The region does NOT have any words.")) - ((= 1 count) (message "The region has 1 word.")) - (t (message "The region has %d words." count)))))) - -If you wish, you can also install this keybinding by evaluating it: - - (global-set-key "\C-c=" 'count-words-region) - -To conduct the first test, set mark and point to the beginning and end -of the following line and then type `C-c =' (or `M-x -count-words-region' if you have not bound `C-c ='): - - one two three - -Emacs will tell you, correctly, that the region has three words. - -Repeat the test, but place mark at the beginning of the line and place -point just _before_ the word `one'. Again type the command `C-c =' (or -`M-x count-words-region'). Emacs should tell you that the region has -no words, since it is composed only of the whitespace at the beginning -of the line. But instead Emacs tells you that the region has one word! - -For the third test, copy the sample line to the end of the `*scratch*' -buffer and then type several spaces at the end of the line. Place mark -right after the word `three' and point at the end of line. (The end of -the line will be the end of the buffer.) Type `C-c =' (or `M-x -count-words-region') as you did before. Again, Emacs should tell you -that the region has no words, since it is composed only of the -whitespace at the end of the line. Instead, Emacs displays an error -message saying `Search failed'. - -The two bugs stem from the same problem. - -Consider the first manifestation of the bug, in which the command tells -you that the whitespace at the beginning of the line contains one word. -What happens is this: The `M-x count-words-region' command moves point -to the beginning of the region. The `while' tests whether the value of -point is smaller than the value of `end', which it is. Consequently, -the regular expression search looks for and finds the first word. It -leaves point after the word. `count' is set to one. The `while' loop -repeats; but this time the value of point is larger than the value of -`end', the loop is exited; and the function displays a message saying -the number of words in the region is one. In brief, the regular -expression search looks for and finds the word even though it is outside -the marked region. - -In the second manifestation of the bug, the region is whitespace at the -end of the buffer. Emacs says `Search failed'. What happens is that -the true-or-false-test in the `while' loop tests true, so the search -expression is executed. But since there are no more words in the -buffer, the search fails. - -In both manifestations of the bug, the search extends or attempts to -extend outside of the region. - -The solution is to limit the search to the region--this is a fairly -simple action, but as you may have come to expect, it is not quite as -simple as you might think. - -As we have seen, the `re-search-forward' function takes a search -pattern as its first argument. But in addition to this first, -mandatory argument, it accepts three optional arguments. The optional -second argument bounds the search. The optional third argument, if -`t', causes the function to return `nil' rather than signal an error if -the search fails. The optional fourth argument is a repeat count. (In -Emacs, you can see a function's documentation by typing `C-h f', the -name of the function, and then <RET>.) - -In the `count-words-region' definition, the value of the end of the -region is held by the variable `end' which is passed as an argument to -the function. Thus, we can add `end' as an argument to the regular -expression search expression: - - (re-search-forward "\\w+\\W*" end) - -However, if you make only this change to the `count-words-region' -definition and then test the new version of the definition on a stretch -of whitespace, you will receive an error message saying `Search failed'. - -What happens is this: the search is limited to the region, and fails as -you expect because there are no word-constituent characters in the -region. Since it fails, we receive an error message. But we do not -want to receive an error message in this case; we want to receive the -message that "The region does NOT have any words." - -The solution to this problem is to provide `re-search-forward' with a -third argument of `t', which causes the function to return `nil' rather -than signal an error if the search fails. - -However, if you make this change and try it, you will see the message -"Counting words in region ... " and ... you will keep on seeing that -message ..., until you type `C-g' (`keyboard-quit'). - -Here is what happens: the search is limited to the region, as before, -and it fails because there are no word-constituent characters in the -region, as expected. Consequently, the `re-search-forward' expression -returns `nil'. It does nothing else. In particular, it does not move -point, which it does as a side effect if it finds the search target. -After the `re-search-forward' expression returns `nil', the next -expression in the `while' loop is evaluated. This expression -increments the count. Then the loop repeats. The true-or-false-test -tests true because the value of point is still less than the value of -end, since the `re-search-forward' expression did not move point. ... -and the cycle repeats ... - -The `count-words-region' definition requires yet another modification, -to cause the true-or-false-test of the `while' loop to test false if -the search fails. Put another way, there are two conditions that must -be satisfied in the true-or-false-test before the word count variable -is incremented: point must still be within the region and the search -expression must have found a word to count. - -Since both the first condition and the second condition must be true -together, the two expressions, the region test and the search -expression, can be joined with an `and' special form and embedded in -the `while' loop as the true-or-false-test, like this: - - (and (< (point) end) (re-search-forward "\\w+\\W*" end t)) - -(*Note The `kill-new' function: kill-new function, for information -about `and'.) - -The `re-search-forward' expression returns `t' if the search succeeds -and as a side effect moves point. Consequently, as words are found, -point is moved through the region. When the search expression fails to -find another word, or when point reaches the end of the region, the -true-or-false-test tests false, the `while' loop exits, and the -`count-words-region' function displays one or other of its messages. - -After incorporating these final changes, the `count-words-region' works -without bugs (or at least, without bugs that I have found!). Here is -what it looks like: - - ;;; Final version: `while' - (defun count-words-region (beginning end) - "Print number of words in the region." - (interactive "r") - (message "Counting words in region ... ") - - ;;; 1. Set up appropriate conditions. - (save-excursion - (let ((count 0)) - (goto-char beginning) - - ;;; 2. Run the while loop. - (while (and (< (point) end) - (re-search-forward "\\w+\\W*" end t)) - (setq count (1+ count))) - - ;;; 3. Send a message to the user. - (cond ((zerop count) - (message - "The region does NOT have any words.")) - ((= 1 count) - (message - "The region has 1 word.")) - (t - (message - "The region has %d words." count)))))) - - -File: eintr, Node: recursive-count-words, Next: Counting Exercise, Prev: count-words-region, Up: Counting Words - -13.2 Count Words Recursively -============================ - -You can write the function for counting words recursively as well as -with a `while' loop. Let's see how this is done. - -First, we need to recognize that the `count-words-region' function has -three jobs: it sets up the appropriate conditions for counting to -occur; it counts the words in the region; and it sends a message to the -user telling how many words there are. - -If we write a single recursive function to do everything, we will -receive a message for every recursive call. If the region contains 13 -words, we will receive thirteen messages, one right after the other. -We don't want this! Instead, we must write two functions to do the -job, one of which (the recursive function) will be used inside of the -other. One function will set up the conditions and display the -message; the other will return the word count. - -Let us start with the function that causes the message to be displayed. -We can continue to call this `count-words-region'. - -This is the function that the user will call. It will be interactive. -Indeed, it will be similar to our previous versions of this function, -except that it will call `recursive-count-words' to determine how many -words are in the region. - -We can readily construct a template for this function, based on our -previous versions: - - ;; Recursive version; uses regular expression search - (defun count-words-region (beginning end) - "DOCUMENTATION..." - (INTERACTIVE-EXPRESSION...) - - ;;; 1. Set up appropriate conditions. - (EXPLANATORY MESSAGE) - (SET-UP FUNCTIONS... - - ;;; 2. Count the words. - RECURSIVE CALL - - ;;; 3. Send a message to the user. - MESSAGE PROVIDING WORD COUNT)) - -The definition looks straightforward, except that somehow the count -returned by the recursive call must be passed to the message displaying -the word count. A little thought suggests that this can be done by -making use of a `let' expression: we can bind a variable in the varlist -of a `let' expression to the number of words in the region, as returned -by the recursive call; and then the `cond' expression, using binding, -can display the value to the user. - -Often, one thinks of the binding within a `let' expression as somehow -secondary to the `primary' work of a function. But in this case, what -you might consider the `primary' job of the function, counting words, -is done within the `let' expression. - -Using `let', the function definition looks like this: - - (defun count-words-region (beginning end) - "Print number of words in the region." - (interactive "r") - - ;;; 1. Set up appropriate conditions. - (message "Counting words in region ... ") - (save-excursion - (goto-char beginning) - - ;;; 2. Count the words. - (let ((count (recursive-count-words end))) - - ;;; 3. Send a message to the user. - (cond ((zerop count) - (message - "The region does NOT have any words.")) - ((= 1 count) - (message - "The region has 1 word.")) - (t - (message - "The region has %d words." count)))))) - -Next, we need to write the recursive counting function. - -A recursive function has at least three parts: the `do-again-test', the -`next-step-expression', and the recursive call. - -The do-again-test determines whether the function will or will not be -called again. Since we are counting words in a region and can use a -function that moves point forward for every word, the do-again-test can -check whether point is still within the region. The do-again-test -should find the value of point and determine whether point is before, -at, or after the value of the end of the region. We can use the -`point' function to locate point. Clearly, we must pass the value of -the end of the region to the recursive counting function as an argument. - -In addition, the do-again-test should also test whether the search -finds a word. If it does not, the function should not call itself -again. - -The next-step-expression changes a value so that when the recursive -function is supposed to stop calling itself, it stops. More precisely, -the next-step-expression changes a value so that at the right time, the -do-again-test stops the recursive function from calling itself again. -In this case, the next-step-expression can be the expression that moves -point forward, word by word. - -The third part of a recursive function is the recursive call. - -Somewhere, also, we also need a part that does the `work' of the -function, a part that does the counting. A vital part! - -But already, we have an outline of the recursive counting function: - - (defun recursive-count-words (region-end) - "DOCUMENTATION..." - DO-AGAIN-TEST - NEXT-STEP-EXPRESSION - RECURSIVE CALL) - -Now we need to fill in the slots. Let's start with the simplest cases -first: if point is at or beyond the end of the region, there cannot be -any words in the region, so the function should return zero. Likewise, -if the search fails, there are no words to count, so the function -should return zero. - -On the other hand, if point is within the region and the search -succeeds, the function should call itself again. - -Thus, the do-again-test should look like this: - - (and (< (point) region-end) - (re-search-forward "\\w+\\W*" region-end t)) - -Note that the search expression is part of the do-again-test--the -function returns `t' if its search succeeds and `nil' if it fails. -(*Note The Whitespace Bug in `count-words-region': Whitespace Bug, for -an explanation of how `re-search-forward' works.) - -The do-again-test is the true-or-false test of an `if' clause. -Clearly, if the do-again-test succeeds, the then-part of the `if' -clause should call the function again; but if it fails, the else-part -should return zero since either point is outside the region or the -search failed because there were no words to find. - -But before considering the recursive call, we need to consider the -next-step-expression. What is it? Interestingly, it is the search -part of the do-again-test. - -In addition to returning `t' or `nil' for the do-again-test, -`re-search-forward' moves point forward as a side effect of a -successful search. This is the action that changes the value of point -so that the recursive function stops calling itself when point -completes its movement through the region. Consequently, the -`re-search-forward' expression is the next-step-expression. - -In outline, then, the body of the `recursive-count-words' function -looks like this: - - (if DO-AGAIN-TEST-AND-NEXT-STEP-COMBINED - ;; then - RECURSIVE-CALL-RETURNING-COUNT - ;; else - RETURN-ZERO) - -How to incorporate the mechanism that counts? - -If you are not used to writing recursive functions, a question like -this can be troublesome. But it can and should be approached -systematically. - -We know that the counting mechanism should be associated in some way -with the recursive call. Indeed, since the next-step-expression moves -point forward by one word, and since a recursive call is made for each -word, the counting mechanism must be an expression that adds one to the -value returned by a call to `recursive-count-words'. - -Consider several cases: - - * If there are two words in the region, the function should return a - value resulting from adding one to the value returned when it - counts the first word, plus the number returned when it counts the - remaining words in the region, which in this case is one. - - * If there is one word in the region, the function should return a - value resulting from adding one to the value returned when it - counts that word, plus the number returned when it counts the - remaining words in the region, which in this case is zero. - - * If there are no words in the region, the function should return - zero. - -From the sketch we can see that the else-part of the `if' returns zero -for the case of no words. This means that the then-part of the `if' -must return a value resulting from adding one to the value returned -from a count of the remaining words. - -The expression will look like this, where `1+' is a function that adds -one to its argument. - - (1+ (recursive-count-words region-end)) - -The whole `recursive-count-words' function will then look like this: - - (defun recursive-count-words (region-end) - "DOCUMENTATION..." - - ;;; 1. do-again-test - (if (and (< (point) region-end) - (re-search-forward "\\w+\\W*" region-end t)) - - ;;; 2. then-part: the recursive call - (1+ (recursive-count-words region-end)) - - ;;; 3. else-part - 0)) - -Let's examine how this works: - -If there are no words in the region, the else part of the `if' -expression is evaluated and consequently the function returns zero. - -If there is one word in the region, the value of point is less than the -value of `region-end' and the search succeeds. In this case, the -true-or-false-test of the `if' expression tests true, and the then-part -of the `if' expression is evaluated. The counting expression is -evaluated. This expression returns a value (which will be the value -returned by the whole function) that is the sum of one added to the -value returned by a recursive call. - -Meanwhile, the next-step-expression has caused point to jump over the -first (and in this case only) word in the region. This means that when -`(recursive-count-words region-end)' is evaluated a second time, as a -result of the recursive call, the value of point will be equal to or -greater than the value of region end. So this time, -`recursive-count-words' will return zero. The zero will be added to -one, and the original evaluation of `recursive-count-words' will return -one plus zero, which is one, which is the correct amount. - -Clearly, if there are two words in the region, the first call to -`recursive-count-words' returns one added to the value returned by -calling `recursive-count-words' on a region containing the remaining -word--that is, it adds one to one, producing two, which is the correct -amount. - -Similarly, if there are three words in the region, the first call to -`recursive-count-words' returns one added to the value returned by -calling `recursive-count-words' on a region containing the remaining -two words--and so on and so on. - -With full documentation the two functions look like this: - -The recursive function: - - (defun recursive-count-words (region-end) - "Number of words between point and REGION-END." - - ;;; 1. do-again-test - (if (and (< (point) region-end) - (re-search-forward "\\w+\\W*" region-end t)) - - ;;; 2. then-part: the recursive call - (1+ (recursive-count-words region-end)) - - ;;; 3. else-part - 0)) - -The wrapper: - - ;;; Recursive version - (defun count-words-region (beginning end) - "Print number of words in the region. - - Words are defined as at least one word-constituent - character followed by at least one character that is - not a word-constituent. The buffer's syntax table - determines which characters these are." - (interactive "r") - (message "Counting words in region ... ") - (save-excursion - (goto-char beginning) - (let ((count (recursive-count-words end))) - (cond ((zerop count) - (message - "The region does NOT have any words.")) - ((= 1 count) - (message "The region has 1 word.")) - (t - (message - "The region has %d words." count)))))) - - -File: eintr, Node: Counting Exercise, Prev: recursive-count-words, Up: Counting Words - -13.3 Exercise: Counting Punctuation -=================================== - -Using a `while' loop, write a function to count the number of -punctuation marks in a region--period, comma, semicolon, colon, -exclamation mark, and question mark. Do the same using recursion. - - -File: eintr, Node: Words in a defun, Next: Readying a Graph, Prev: Counting Words, Up: Top - -14 Counting Words in a `defun' -****************************** - -Our next project is to count the number of words in a function -definition. Clearly, this can be done using some variant of -`count-word-region'. *Note Counting Words: Repetition and Regexps: -Counting Words. If we are just going to count the words in one -definition, it is easy enough to mark the definition with the `C-M-h' -(`mark-defun') command, and then call `count-word-region'. - -However, I am more ambitious: I want to count the words and symbols in -every definition in the Emacs sources and then print a graph that shows -how many functions there are of each length: how many contain 40 to 49 -words or symbols, how many contain 50 to 59 words or symbols, and so -on. I have often been curious how long a typical function is, and this -will tell. - -* Menu: - -* Divide and Conquer:: -* Words and Symbols:: -* Syntax:: -* count-words-in-defun:: -* Several defuns:: -* Find a File:: -* lengths-list-file:: -* Several files:: -* Several files recursively:: -* Prepare the data:: - - -File: eintr, Node: Divide and Conquer, Next: Words and Symbols, Prev: Words in a defun, Up: Words in a defun - -Divide and Conquer -================== - -Described in one phrase, the histogram project is daunting; but divided -into numerous small steps, each of which we can take one at a time, the -project becomes less fearsome. Let us consider what the steps must be: - - * First, write a function to count the words in one definition. This - includes the problem of handling symbols as well as words. - - * Second, write a function to list the numbers of words in each - function in a file. This function can use the - `count-words-in-defun' function. - - * Third, write a function to list the numbers of words in each - function in each of several files. This entails automatically - finding the various files, switching to them, and counting the - words in the definitions within them. - - * Fourth, write a function to convert the list of numbers that we - created in step three to a form that will be suitable for printing - as a graph. - - * Fifth, write a function to print the results as a graph. - -This is quite a project! But if we take each step slowly, it will not -be difficult. - - -File: eintr, Node: Words and Symbols, Next: Syntax, Prev: Divide and Conquer, Up: Words in a defun - -14.1 What to Count? -=================== - -When we first start thinking about how to count the words in a function -definition, the first question is (or ought to be) what are we going to -count? When we speak of `words' with respect to a Lisp function -definition, we are actually speaking, in large part, of `symbols'. For -example, the following `multiply-by-seven' function contains the five -symbols `defun', `multiply-by-seven', `number', `*', and `7'. In -addition, in the documentation string, it contains the four words -`Multiply', `NUMBER', `by', and `seven'. The symbol `number' is -repeated, so the definition contains a total of ten words and symbols. - - (defun multiply-by-seven (number) - "Multiply NUMBER by seven." - (* 7 number)) - -However, if we mark the `multiply-by-seven' definition with `C-M-h' -(`mark-defun'), and then call `count-words-region' on it, we will find -that `count-words-region' claims the definition has eleven words, not -ten! Something is wrong! - -The problem is twofold: `count-words-region' does not count the `*' as -a word, and it counts the single symbol, `multiply-by-seven', as -containing three words. The hyphens are treated as if they were -interword spaces rather than intraword connectors: `multiply-by-seven' -is counted as if it were written `multiply by seven'. - -The cause of this confusion is the regular expression search within the -`count-words-region' definition that moves point forward word by word. -In the canonical version of `count-words-region', the regexp is: - - "\\w+\\W*" - -This regular expression is a pattern defining one or more word -constituent characters possibly followed by one or more characters that -are not word constituents. What is meant by `word constituent -characters' brings us to the issue of syntax, which is worth a section -of its own. - - -File: eintr, Node: Syntax, Next: count-words-in-defun, Prev: Words and Symbols, Up: Words in a defun - -14.2 What Constitutes a Word or Symbol? -======================================= - -Emacs treats different characters as belonging to different "syntax -categories". For example, the regular expression, `\\w+', is a pattern -specifying one or more _word constituent_ characters. Word constituent -characters are members of one syntax category. Other syntax categories -include the class of punctuation characters, such as the period and the -comma, and the class of whitespace characters, such as the blank space -and the tab character. (For more information, see *Note Syntax: -(emacs)Syntax, and *Note Syntax Tables: (elisp)Syntax Tables.) - -Syntax tables specify which characters belong to which categories. -Usually, a hyphen is not specified as a `word constituent character'. -Instead, it is specified as being in the `class of characters that are -part of symbol names but not words.' This means that the -`count-words-region' function treats it in the same way it treats an -interword white space, which is why `count-words-region' counts -`multiply-by-seven' as three words. - -There are two ways to cause Emacs to count `multiply-by-seven' as one -symbol: modify the syntax table or modify the regular expression. - -We could redefine a hyphen as a word constituent character by modifying -the syntax table that Emacs keeps for each mode. This action would -serve our purpose, except that a hyphen is merely the most common -character within symbols that is not typically a word constituent -character; there are others, too. - -Alternatively, we can redefine the regular expression used in the -`count-words' definition so as to include symbols. This procedure has -the merit of clarity, but the task is a little tricky. - -The first part is simple enough: the pattern must match "at least one -character that is a word or symbol constituent". Thus: - - "\\(\\w\\|\\s_\\)+" - -The `\\(' is the first part of the grouping construct that includes the -`\\w' and the `\\s_' as alternatives, separated by the `\\|'. The -`\\w' matches any word-constituent character and the `\\s_' matches any -character that is part of a symbol name but not a word-constituent -character. The `+' following the group indicates that the word or -symbol constituent characters must be matched at least once. - -However, the second part of the regexp is more difficult to design. -What we want is to follow the first part with "optionally one or more -characters that are not constituents of a word or symbol". At first, I -thought I could define this with the following: - - "\\(\\W\\|\\S_\\)*" - -The upper case `W' and `S' match characters that are _not_ word or -symbol constituents. Unfortunately, this expression matches any -character that is either not a word constituent or not a symbol -constituent. This matches any character! - -I then noticed that every word or symbol in my test region was followed -by white space (blank space, tab, or newline). So I tried placing a -pattern to match one or more blank spaces after the pattern for one or -more word or symbol constituents. This failed, too. Words and symbols -are often separated by whitespace, but in actual code parentheses may -follow symbols and punctuation may follow words. So finally, I -designed a pattern in which the word or symbol constituents are -followed optionally by characters that are not white space and then -followed optionally by white space. - -Here is the full regular expression: - - "\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*" - - -File: eintr, Node: count-words-in-defun, Next: Several defuns, Prev: Syntax, Up: Words in a defun - -14.3 The `count-words-in-defun' Function -======================================== - -We have seen that there are several ways to write a `count-word-region' -function. To write a `count-words-in-defun', we need merely adapt one -of these versions. - -The version that uses a `while' loop is easy to understand, so I am -going to adapt that. Because `count-words-in-defun' will be part of a -more complex program, it need not be interactive and it need not -display a message but just return the count. These considerations -simplify the definition a little. - -On the other hand, `count-words-in-defun' will be used within a buffer -that contains function definitions. Consequently, it is reasonable to -ask that the function determine whether it is called when point is -within a function definition, and if it is, to return the count for -that definition. This adds complexity to the definition, but saves us -from needing to pass arguments to the function. - -These considerations lead us to prepare the following template: - - (defun count-words-in-defun () - "DOCUMENTATION..." - (SET UP... - (WHILE LOOP...) - RETURN COUNT) - -As usual, our job is to fill in the slots. - -First, the set up. - -We are presuming that this function will be called within a buffer -containing function definitions. Point will either be within a -function definition or not. For `count-words-in-defun' to work, point -must move to the beginning of the definition, a counter must start at -zero, and the counting loop must stop when point reaches the end of the -definition. - -The `beginning-of-defun' function searches backwards for an opening -delimiter such as a `(' at the beginning of a line, and moves point to -that position, or else to the limit of the search. In practice, this -means that `beginning-of-defun' moves point to the beginning of an -enclosing or preceding function definition, or else to the beginning of -the buffer. We can use `beginning-of-defun' to place point where we -wish to start. - -The `while' loop requires a counter to keep track of the words or -symbols being counted. A `let' expression can be used to create a -local variable for this purpose, and bind it to an initial value of -zero. - -The `end-of-defun' function works like `beginning-of-defun' except that -it moves point to the end of the definition. `end-of-defun' can be -used as part of an expression that determines the position of the end -of the definition. - -The set up for `count-words-in-defun' takes shape rapidly: first we -move point to the beginning of the definition, then we create a local -variable to hold the count, and finally, we record the position of the -end of the definition so the `while' loop will know when to stop -looping. - -The code looks like this: - - (beginning-of-defun) - (let ((count 0) - (end (save-excursion (end-of-defun) (point)))) - -The code is simple. The only slight complication is likely to concern -`end': it is bound to the position of the end of the definition by a -`save-excursion' expression that returns the value of point after -`end-of-defun' temporarily moves it to the end of the definition. - -The second part of the `count-words-in-defun', after the set up, is the -`while' loop. - -The loop must contain an expression that jumps point forward word by -word and symbol by symbol, and another expression that counts the -jumps. The true-or-false-test for the `while' loop should test true so -long as point should jump forward, and false when point is at the end -of the definition. We have already redefined the regular expression -for this (*note Syntax::), so the loop is straightforward: - - (while (and (< (point) end) - (re-search-forward - "\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*" end t) - (setq count (1+ count))) - -The third part of the function definition returns the count of words -and symbols. This part is the last expression within the body of the -`let' expression, and can be, very simply, the local variable `count', -which when evaluated returns the count. - -Put together, the `count-words-in-defun' definition looks like this: - - (defun count-words-in-defun () - "Return the number of words and symbols in a defun." - (beginning-of-defun) - (let ((count 0) - (end (save-excursion (end-of-defun) (point)))) - (while - (and (< (point) end) - (re-search-forward - "\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*" - end t)) - (setq count (1+ count))) - count)) - -How to test this? The function is not interactive, but it is easy to -put a wrapper around the function to make it interactive; we can use -almost the same code as for the recursive version of -`count-words-region': - - ;;; Interactive version. - (defun count-words-defun () - "Number of words and symbols in a function definition." - (interactive) - (message - "Counting words and symbols in function definition ... ") - (let ((count (count-words-in-defun))) - (cond - ((zerop count) - (message - "The definition does NOT have any words or symbols.")) - ((= 1 count) - (message - "The definition has 1 word or symbol.")) - (t - (message - "The definition has %d words or symbols." count))))) - -Let's re-use `C-c =' as a convenient keybinding: - - (global-set-key "\C-c=" 'count-words-defun) - -Now we can try out `count-words-defun': install both -`count-words-in-defun' and `count-words-defun', and set the keybinding, -and then place the cursor within the following definition: - - (defun multiply-by-seven (number) - "Multiply NUMBER by seven." - (* 7 number)) - => 10 - -Success! The definition has 10 words and symbols. - -The next problem is to count the numbers of words and symbols in -several definitions within a single file. - - -File: eintr, Node: Several defuns, Next: Find a File, Prev: count-words-in-defun, Up: Words in a defun - -14.4 Count Several `defuns' Within a File -========================================= - -A file such as `simple.el' may have a hundred or more function -definitions within it. Our long term goal is to collect statistics on -many files, but as a first step, our immediate goal is to collect -statistics on one file. - -The information will be a series of numbers, each number being the -length of a function definition. We can store the numbers in a list. - -We know that we will want to incorporate the information regarding one -file with information about many other files; this means that the -function for counting definition lengths within one file need only -return the list of lengths. It need not and should not display any -messages. - -The word count commands contain one expression to jump point forward -word by word and another expression to count the jumps. The function -to return the lengths of definitions can be designed to work the same -way, with one expression to jump point forward definition by definition -and another expression to construct the lengths' list. - -This statement of the problem makes it elementary to write the function -definition. Clearly, we will start the count at the beginning of the -file, so the first command will be `(goto-char (point-min))'. Next, we -start the `while' loop; and the true-or-false test of the loop can be a -regular expression search for the next function definition--so long as -the search succeeds, point is moved forward and then the body of the -loop is evaluated. The body needs an expression that constructs the -lengths' list. `cons', the list construction command, can be used to -create the list. That is almost all there is to it. - -Here is what this fragment of code looks like: - - (goto-char (point-min)) - (while (re-search-forward "^(defun" nil t) - (setq lengths-list - (cons (count-words-in-defun) lengths-list))) - -What we have left out is the mechanism for finding the file that -contains the function definitions. - -In previous examples, we either used this, the Info file, or we -switched back and forth to some other buffer, such as the `*scratch*' -buffer. - -Finding a file is a new process that we have not yet discussed. - - -File: eintr, Node: Find a File, Next: lengths-list-file, Prev: Several defuns, Up: Words in a defun - -14.5 Find a File -================ - -To find a file in Emacs, you use the `C-x C-f' (`find-file') command. -This command is almost, but not quite right for the lengths problem. - -Let's look at the source for `find-file': - - (defun find-file (filename) - "Edit file FILENAME. - Switch to a buffer visiting file FILENAME, - creating one if none already exists." - (interactive "FFind file: ") - (switch-to-buffer (find-file-noselect filename))) - -(The most recent version of the `find-file' function definition permits -you to specify optional wildcards visit multiple files; that makes the -definition more complex and we will not discuss it here, since it is -not relevant. You can see its source using either `M-.' (`find-tag') -or `C-h f' (`describe-function').) - -The definition I am showing possesses short but complete documentation -and an interactive specification that prompts you for a file name when -you use the command interactively. The body of the definition contains -two functions, `find-file-noselect' and `switch-to-buffer'. - -According to its documentation as shown by `C-h f' (the -`describe-function' command), the `find-file-noselect' function reads -the named file into a buffer and returns the buffer. (Its most recent -version includes an optional wildcards argument, too, as well as -another to read a file literally and an other you suppress warning -messages. These optional arguments are irrelevant.) - -However, the `find-file-noselect' function does not select the buffer -in which it puts the file. Emacs does not switch its attention (or -yours if you are using `find-file-noselect') to the named buffer. That -is what `switch-to-buffer' does: it switches the buffer to which Emacs -attention is directed; and it switches the buffer displayed in the -window to the new buffer. We have discussed buffer switching -elsewhere. (*Note Switching Buffers::.) - -In this histogram project, we do not need to display each file on the -screen as the program determines the length of each definition within -it. Instead of employing `switch-to-buffer', we can work with -`set-buffer', which redirects the attention of the computer program to -a different buffer but does not redisplay it on the screen. So instead -of calling on `find-file' to do the job, we must write our own -expression. - -The task is easy: use `find-file-noselect' and `set-buffer'. - - -File: eintr, Node: lengths-list-file, Next: Several files, Prev: Find a File, Up: Words in a defun - -14.6 `lengths-list-file' in Detail -================================== - -The core of the `lengths-list-file' function is a `while' loop -containing a function to move point forward `defun by defun' and a -function to count the number of words and symbols in each defun. This -core must be surrounded by functions that do various other tasks, -including finding the file, and ensuring that point starts out at the -beginning of the file. The function definition looks like this: - - (defun lengths-list-file (filename) - "Return list of definitions' lengths within FILE. - The returned list is a list of numbers. - Each number is the number of words or - symbols in one function definition." - (message "Working on `%s' ... " filename) - (save-excursion - (let ((buffer (find-file-noselect filename)) - (lengths-list)) - (set-buffer buffer) - (setq buffer-read-only t) - (widen) - (goto-char (point-min)) - (while (re-search-forward "^(defun" nil t) - (setq lengths-list - (cons (count-words-in-defun) lengths-list))) - (kill-buffer buffer) - lengths-list))) - -The function is passed one argument, the name of the file on which it -will work. It has four lines of documentation, but no interactive -specification. Since people worry that a computer is broken if they -don't see anything going on, the first line of the body is a message. - -The next line contains a `save-excursion' that returns Emacs' attention -to the current buffer when the function completes. This is useful in -case you embed this function in another function that presumes point is -restored to the original buffer. - -In the varlist of the `let' expression, Emacs finds the file and binds -the local variable `buffer' to the buffer containing the file. At the -same time, Emacs creates `lengths-list' as a local variable. - -Next, Emacs switches its attention to the buffer. - -In the following line, Emacs makes the buffer read-only. Ideally, this -line is not necessary. None of the functions for counting words and -symbols in a function definition should change the buffer. Besides, -the buffer is not going to be saved, even if it were changed. This -line is entirely the consequence of great, perhaps excessive, caution. -The reason for the caution is that this function and those it calls -work on the sources for Emacs and it is very inconvenient if they are -inadvertently modified. It goes without saying that I did not realize -a need for this line until an experiment went awry and started to -modify my Emacs source files ... - -Next comes a call to widen the buffer if it is narrowed. This function -is usually not needed--Emacs creates a fresh buffer if none already -exists; but if a buffer visiting the file already exists Emacs returns -that one. In this case, the buffer may be narrowed and must be -widened. If we wanted to be fully `user-friendly', we would arrange to -save the restriction and the location of point, but we won't. - -The `(goto-char (point-min))' expression moves point to the beginning -of the buffer. - -Then comes a `while' loop in which the `work' of the function is -carried out. In the loop, Emacs determines the length of each -definition and constructs a lengths' list containing the information. - -Emacs kills the buffer after working through it. This is to save space -inside of Emacs. My version of GNU Emacs 19 contained over 300 source -files of interest; GNU Emacs 22 contains over a thousand source files. -Another function will apply `lengths-list-file' to each of the files. - -Finally, the last expression within the `let' expression is the -`lengths-list' variable; its value is returned as the value of the -whole function. - -You can try this function by installing it in the usual fashion. Then -place your cursor after the following expression and type `C-x C-e' -(`eval-last-sexp'). - - (lengths-list-file - "/usr/local/share/emacs/22.0.100/lisp/emacs-lisp/debug.el") - -(You may need to change the pathname of the file; the one here is for -GNU Emacs version 22.0.100. To change the expression, copy it to the -`*scratch*' buffer and edit it. - -(Also, to see the full length of the list, rather than a truncated -version, you may have to evaluate the following: - - (custom-set-variables '(eval-expression-print-length nil)) - -(*Note Specifying Variables using `defcustom': defcustom.) Then -evaluate the `lengths-list-file' expression.) - -The lengths' list for `debug.el' takes less than a second to produce -and looks like this in GNU Emacs 22: - - (83 113 105 144 289 22 30 97 48 89 25 52 52 88 28 29 77 49 43 290 232 587) - -(Using my old machine, the version 19 lengths' list for `debug.el' took -seven seconds to produce and looked like this: - - (75 41 80 62 20 45 44 68 45 12 34 235) - -(The newer version of `debug.el' contains more defuns than the earlier -one; and my new machine is much faster than the old one.) - -Note that the length of the last definition in the file is first in the -list. - - -File: eintr, Node: Several files, Next: Several files recursively, Prev: lengths-list-file, Up: Words in a defun - -14.7 Count Words in `defuns' in Different Files -=============================================== - -In the previous section, we created a function that returns a list of -the lengths of each definition in a file. Now, we want to define a -function to return a master list of the lengths of the definitions in a -list of files. - -Working on each of a list of files is a repetitious act, so we can use -either a `while' loop or recursion. - -* Menu: - -* lengths-list-many-files:: -* append:: - - -File: eintr, Node: lengths-list-many-files, Next: append, Prev: Several files, Up: Several files - -Determine the lengths of `defuns' ---------------------------------- - -The design using a `while' loop is routine. The argument passed the -function is a list of files. As we saw earlier (*note Loop Example::), -you can write a `while' loop so that the body of the loop is evaluated -if such a list contains elements, but to exit the loop if the list is -empty. For this design to work, the body of the loop must contain an -expression that shortens the list each time the body is evaluated, so -that eventually the list is empty. The usual technique is to set the -value of the list to the value of the CDR of the list each time the -body is evaluated. - -The template looks like this: - - (while TEST-WHETHER-LIST-IS-EMPTY - BODY... - SET-LIST-TO-CDR-OF-LIST) - -Also, we remember that a `while' loop returns `nil' (the result of -evaluating the true-or-false-test), not the result of any evaluation -within its body. (The evaluations within the body of the loop are done -for their side effects.) However, the expression that sets the -lengths' list is part of the body--and that is the value that we want -returned by the function as a whole. To do this, we enclose the -`while' loop within a `let' expression, and arrange that the last -element of the `let' expression contains the value of the lengths' -list. (*Note Loop Example with an Incrementing Counter: Incrementing -Example.) - -These considerations lead us directly to the function itself: - - ;;; Use `while' loop. - (defun lengths-list-many-files (list-of-files) - "Return list of lengths of defuns in LIST-OF-FILES." - (let (lengths-list) - - ;;; true-or-false-test - (while list-of-files - (setq lengths-list - (append - lengths-list - - ;;; Generate a lengths' list. - (lengths-list-file - (expand-file-name (car list-of-files))))) - - ;;; Make files' list shorter. - (setq list-of-files (cdr list-of-files))) - - ;;; Return final value of lengths' list. - lengths-list)) - -`expand-file-name' is a built-in function that converts a file name to -the absolute, long, path name form of the directory in which the -function is called. - -Thus, if `expand-file-name' is called on `debug.el' when Emacs is -visiting the `/usr/local/share/emacs/22.0.100/lisp/emacs-lisp/' -directory, - - debug.el - -becomes - - /usr/local/share/emacs/22.0.100/lisp/emacs-lisp/debug.el - -The only other new element of this function definition is the as yet -unstudied function `append', which merits a short section for itself. - - -File: eintr, Node: append, Prev: lengths-list-many-files, Up: Several files - -14.7.1 The `append' Function ----------------------------- - -The `append' function attaches one list to another. Thus, - - (append '(1 2 3 4) '(5 6 7 8)) - -produces the list - - (1 2 3 4 5 6 7 8) - -This is exactly how we want to attach two lengths' lists produced by -`lengths-list-file' to each other. The results contrast with `cons', - - (cons '(1 2 3 4) '(5 6 7 8)) - -which constructs a new list in which the first argument to `cons' -becomes the first element of the new list: - - ((1 2 3 4) 5 6 7 8) - - -File: eintr, Node: Several files recursively, Next: Prepare the data, Prev: Several files, Up: Words in a defun - -14.8 Recursively Count Words in Different Files -=============================================== - -Besides a `while' loop, you can work on each of a list of files with -recursion. A recursive version of `lengths-list-many-files' is short -and simple. - -The recursive function has the usual parts: the `do-again-test', the -`next-step-expression', and the recursive call. The `do-again-test' -determines whether the function should call itself again, which it will -do if the `list-of-files' contains any remaining elements; the -`next-step-expression' resets the `list-of-files' to the CDR of itself, -so eventually the list will be empty; and the recursive call calls -itself on the shorter list. The complete function is shorter than this -description! - - (defun recursive-lengths-list-many-files (list-of-files) - "Return list of lengths of each defun in LIST-OF-FILES." - (if list-of-files ; do-again-test - (append - (lengths-list-file - (expand-file-name (car list-of-files))) - (recursive-lengths-list-many-files - (cdr list-of-files))))) - -In a sentence, the function returns the lengths' list for the first of -the `list-of-files' appended to the result of calling itself on the -rest of the `list-of-files'. - -Here is a test of `recursive-lengths-list-many-files', along with the -results of running `lengths-list-file' on each of the files -individually. - -Install `recursive-lengths-list-many-files' and `lengths-list-file', if -necessary, and then evaluate the following expressions. You may need -to change the files' pathnames; those here work when this Info file and -the Emacs sources are located in their customary places. To change the -expressions, copy them to the `*scratch*' buffer, edit them, and then -evaluate them. - -The results are shown after the `=>'. (These results are for files -from Emacs Version 22.0.100; files from other versions of Emacs may -produce different results.) - - (cd "/usr/local/share/emacs/22.0.100/") - - (lengths-list-file "./lisp/macros.el") - => (283 263 480 90) - - (lengths-list-file "./lisp/mail/mailalias.el") - => (38 32 29 95 178 180 321 218 324) - - (lengths-list-file "./lisp/makesum.el") - => (85 181) - - (recursive-lengths-list-many-files - '("./lisp/macros.el" - "./lisp/mail/mailalias.el" - "./lisp/makesum.el")) - => (283 263 480 90 38 32 29 95 178 180 321 218 324 85 181) - -The `recursive-lengths-list-many-files' function produces the output we -want. - -The next step is to prepare the data in the list for display in a graph. - - -File: eintr, Node: Prepare the data, Prev: Several files recursively, Up: Words in a defun - -14.9 Prepare the Data for Display in a Graph -============================================ - -The `recursive-lengths-list-many-files' function returns a list of -numbers. Each number records the length of a function definition. -What we need to do now is transform this data into a list of numbers -suitable for generating a graph. The new list will tell how many -functions definitions contain less than 10 words and symbols, how many -contain between 10 and 19 words and symbols, how many contain between -20 and 29 words and symbols, and so on. - -In brief, we need to go through the lengths' list produced by the -`recursive-lengths-list-many-files' function and count the number of -defuns within each range of lengths, and produce a list of those -numbers. - -Based on what we have done before, we can readily foresee that it -should not be too hard to write a function that `CDRs' down the -lengths' list, looks at each element, determines which length range it -is in, and increments a counter for that range. - -However, before beginning to write such a function, we should consider -the advantages of sorting the lengths' list first, so the numbers are -ordered from smallest to largest. First, sorting will make it easier -to count the numbers in each range, since two adjacent numbers will -either be in the same length range or in adjacent ranges. Second, by -inspecting a sorted list, we can discover the highest and lowest -number, and thereby determine the largest and smallest length range -that we will need. - -* Menu: - -* Sorting:: -* Files List:: -* Counting function definitions:: - - -File: eintr, Node: Sorting, Next: Files List, Prev: Prepare the data, Up: Prepare the data - -14.9.1 Sorting Lists --------------------- - -Emacs contains a function to sort lists, called (as you might guess) -`sort'. The `sort' function takes two arguments, the list to be -sorted, and a predicate that determines whether the first of two list -elements is "less" than the second. - -As we saw earlier (*note Using the Wrong Type Object as an Argument: -Wrong Type of Argument.), a predicate is a function that determines -whether some property is true or false. The `sort' function will -reorder a list according to whatever property the predicate uses; this -means that `sort' can be used to sort non-numeric lists by non-numeric -criteria--it can, for example, alphabetize a list. - -The `<' function is used when sorting a numeric list. For example, - - (sort '(4 8 21 17 33 7 21 7) '<) - -produces this: - - (4 7 7 8 17 21 21 33) - -(Note that in this example, both the arguments are quoted so that the -symbols are not evaluated before being passed to `sort' as arguments.) - -Sorting the list returned by the `recursive-lengths-list-many-files' -function is straightforward; it uses the `<' function: - - (sort - (recursive-lengths-list-many-files - '("./lisp/macros.el" - "./lisp/mailalias.el" - "./lisp/makesum.el")) - '<) - -which produces: - - (29 32 38 85 90 95 178 180 181 218 263 283 321 324 480) - -(Note that in this example, the first argument to `sort' is not quoted, -since the expression must be evaluated so as to produce the list that -is passed to `sort'.) - - -File: eintr, Node: Files List, Next: Counting function definitions, Prev: Sorting, Up: Prepare the data - -14.9.2 Making a List of Files ------------------------------ - -The `recursive-lengths-list-many-files' function requires a list of -files as its argument. For our test examples, we constructed such a -list by hand; but the Emacs Lisp source directory is too large for us -to do for that. Instead, we will write a function to do the job for -us. In this function, we will use both a `while' loop and a recursive -call. - -We did not have to write a function like this for older versions of GNU -Emacs, since they placed all the `.el' files in one directory. -Instead, we were able to use the `directory-files' function, which -lists the names of files that match a specified pattern within a single -directory. - -However, recent versions of Emacs place Emacs Lisp files in -sub-directories of the top level `lisp' directory. This re-arrangement -eases navigation. For example, all the mail related files are in a -`lisp' sub-directory called `mail'. But at the same time, this -arrangement forces us to create a file listing function that descends -into the sub-directories. - -We can create this function, called `files-in-below-directory', using -familiar functions such as `car', `nthcdr', and `substring' in -conjunction with an existing function called -`directory-files-and-attributes'. This latter function not only lists -all the filenames in a directory, including the names of -sub-directories, but also their attributes. - -To restate our goal: to create a function that will enable us to feed -filenames to `recursive-lengths-list-many-files' as a list that looks -like this (but with more elements): - - ("./lisp/macros.el" - "./lisp/mail/rmail.el" - "./lisp/makesum.el") - -The `directory-files-and-attributes' function returns a list of lists. -Each of the lists within the main list consists of 13 elements. The -first element is a string that contains the name of the file - which, -in GNU/Linux, may be a `directory file', that is to say, a file with -the special attributes of a directory. The second element of the list -is `t' for a directory, a string for symbolic link (the string is the -name linked to), or `nil'. - -For example, the first `.el' file in the `lisp/' directory is -`abbrev.el'. Its name is -`/usr/local/share/emacs/22.0.100/lisp/abbrev.el' and it is not a -directory or a symbolic link. - -This is how `directory-files-and-attributes' lists that file and its -attributes: - - ("abbrev.el" - nil - 1 - 1000 - 100 - (17733 259) - (17491 28834) - (17596 62124) - 13157 - "-rw-rw-r--" - nil - 2971624 - 773) - -On the other hand, `mail/' is a directory within the `lisp/' directory. -The beginning of its listing looks like this: - - ("mail" - t - ... - ) - -(To learn about the different attributes, look at the documentation of -`file-attributes'. Bear in mind that the `file-attributes' function -does not list the filename, so its first element is -`directory-files-and-attributes''s second element.) - -We will want our new function, `files-in-below-directory', to list the -`.el' files in the directory it is told to check, and in any -directories below that directory. - -This gives us a hint on how to construct `files-in-below-directory': -within a directory, the function should add `.el' filenames to a list; -and if, within a directory, the function comes upon a sub-directory, it -should go into that sub-directory and repeat its actions. - -However, we should note that every directory contains a name that -refers to itself, called `.', ("dot") and a name that refers to its -parent directory, called `..' ("double dot"). (In `/', the root -directory, `..' refers to itself, since `/' has no parent.) Clearly, -we do not want our `files-in-below-directory' function to enter those -directories, since they always lead us, directly or indirectly, to the -current directory. - -Consequently, our `files-in-below-directory' function must do several -tasks: - - * Check to see whether it is looking at a filename that ends in - `.el'; and if so, add its name to a list. - - * Check to see whether it is looking at a filename that is the name - of a directory; and if so, - - - Check to see whether it is looking at `.' or `..'; and if so - skip it. - - - Or else, go into that directory and repeat the process. - -Let's write a function definition to do these tasks. We will use a -`while' loop to move from one filename to another within a directory, -checking what needs to be done; and we will use a recursive call to -repeat the actions on each sub-directory. The recursive pattern is -`accumulate' (*note Recursive Pattern: _accumulate_: Accumulate.), -using `append' as the combiner. - -Here is the function: - - (defun files-in-below-directory (directory) - "List the .el files in DIRECTORY and in its sub-directories." - ;; Although the function will be used non-interactively, - ;; it will be easier to test if we make it interactive. - ;; The directory will have a name such as - ;; "/usr/local/share/emacs/22.0.100/lisp/" - (interactive "DDirectory name: ") - (let (el-files-list - (current-directory-list - (directory-files-and-attributes directory t))) - ;; while we are in the current directory - (while current-directory-list - (cond - ;; check to see whether filename ends in `.el' - ;; and if so, append its name to a list. - ((equal ".el" (substring (car (car current-directory-list)) -3)) - (setq el-files-list - (cons (car (car current-directory-list)) el-files-list))) - ;; check whether filename is that of a directory - ((eq t (car (cdr (car current-directory-list)))) - ;; decide whether to skip or recurse - (if - (equal "." - (substring (car (car current-directory-list)) -1)) - ;; then do nothing since filename is that of - ;; current directory or parent, "." or ".." - () - ;; else descend into the directory and repeat the process - (setq el-files-list - (append - (files-in-below-directory - (car (car current-directory-list))) - el-files-list))))) - ;; move to the next filename in the list; this also - ;; shortens the list so the while loop eventually comes to an end - (setq current-directory-list (cdr current-directory-list))) - ;; return the filenames - el-files-list)) - -The `files-in-below-directory' `directory-files' function takes one -argument, the name of a directory. - -Thus, on my system, - - (length - (files-in-below-directory "/usr/local/share/emacs/22.0.100/lisp/")) - -tells me that my Lisp sources directory contains 1031 `.el' files. - -`files-in-below-directory' returns a list in reverse alphabetical -order. An expression to sort the list in alphabetical order looks like -this: - - (sort - (files-in-below-directory "/usr/local/share/emacs/22.0.100/lisp/") - 'string-lessp) - - -File: eintr, Node: Counting function definitions, Prev: Files List, Up: Prepare the data - -14.9.3 Counting function definitions ------------------------------------- - -Our immediate goal is to generate a list that tells us how many -function definitions contain fewer than 10 words and symbols, how many -contain between 10 and 19 words and symbols, how many contain between -20 and 29 words and symbols, and so on. - -With a sorted list of numbers, this is easy: count how many elements of -the list are smaller than 10, then, after moving past the numbers just -counted, count how many are smaller than 20, then, after moving past -the numbers just counted, count how many are smaller than 30, and so -on. Each of the numbers, 10, 20, 30, 40, and the like, is one larger -than the top of that range. We can call the list of such numbers the -`top-of-ranges' list. - -If we wished, we could generate this list automatically, but it is -simpler to write a list manually. Here it is: - - (defvar top-of-ranges - '(10 20 30 40 50 - 60 70 80 90 100 - 110 120 130 140 150 - 160 170 180 190 200 - 210 220 230 240 250 - 260 270 280 290 300) - "List specifying ranges for `defuns-per-range'.") - -To change the ranges, we edit this list. - -Next, we need to write the function that creates the list of the number -of definitions within each range. Clearly, this function must take the -`sorted-lengths' and the `top-of-ranges' lists as arguments. - -The `defuns-per-range' function must do two things again and again: it -must count the number of definitions within a range specified by the -current top-of-range value; and it must shift to the next higher value -in the `top-of-ranges' list after counting the number of definitions in -the current range. Since each of these actions is repetitive, we can -use `while' loops for the job. One loop counts the number of -definitions in the range defined by the current top-of-range value, and -the other loop selects each of the top-of-range values in turn. - -Several entries of the `sorted-lengths' list are counted for each -range; this means that the loop for the `sorted-lengths' list will be -inside the loop for the `top-of-ranges' list, like a small gear inside -a big gear. - -The inner loop counts the number of definitions within the range. It -is a simple counting loop of the type we have seen before. (*Note A -loop with an incrementing counter: Incrementing Loop.) The -true-or-false test of the loop tests whether the value from the -`sorted-lengths' list is smaller than the current value of the top of -the range. If it is, the function increments the counter and tests the -next value from the `sorted-lengths' list. - -The inner loop looks like this: - - (while LENGTH-ELEMENT-SMALLER-THAN-TOP-OF-RANGE - (setq number-within-range (1+ number-within-range)) - (setq sorted-lengths (cdr sorted-lengths))) - -The outer loop must start with the lowest value of the `top-of-ranges' -list, and then be set to each of the succeeding higher values in turn. -This can be done with a loop like this: - - (while top-of-ranges - BODY-OF-LOOP... - (setq top-of-ranges (cdr top-of-ranges))) - -Put together, the two loops look like this: - - (while top-of-ranges - - ;; Count the number of elements within the current range. - (while LENGTH-ELEMENT-SMALLER-THAN-TOP-OF-RANGE - (setq number-within-range (1+ number-within-range)) - (setq sorted-lengths (cdr sorted-lengths))) - - ;; Move to next range. - (setq top-of-ranges (cdr top-of-ranges))) - -In addition, in each circuit of the outer loop, Emacs should record the -number of definitions within that range (the value of -`number-within-range') in a list. We can use `cons' for this purpose. -(*Note `cons': cons.) - -The `cons' function works fine, except that the list it constructs will -contain the number of definitions for the highest range at its -beginning and the number of definitions for the lowest range at its -end. This is because `cons' attaches new elements of the list to the -beginning of the list, and since the two loops are working their way -through the lengths' list from the lower end first, the -`defuns-per-range-list' will end up largest number first. But we will -want to print our graph with smallest values first and the larger -later. The solution is to reverse the order of the -`defuns-per-range-list'. We can do this using the `nreverse' function, -which reverses the order of a list. - -For example, - - (nreverse '(1 2 3 4)) - -produces: - - (4 3 2 1) - -Note that the `nreverse' function is "destructive"--that is, it changes -the list to which it is applied; this contrasts with the `car' and -`cdr' functions, which are non-destructive. In this case, we do not -want the original `defuns-per-range-list', so it does not matter that -it is destroyed. (The `reverse' function provides a reversed copy of a -list, leaving the original list as is.) - -Put all together, the `defuns-per-range' looks like this: - - (defun defuns-per-range (sorted-lengths top-of-ranges) - "SORTED-LENGTHS defuns in each TOP-OF-RANGES range." - (let ((top-of-range (car top-of-ranges)) - (number-within-range 0) - defuns-per-range-list) - - ;; Outer loop. - (while top-of-ranges - - ;; Inner loop. - (while (and - ;; Need number for numeric test. - (car sorted-lengths) - (< (car sorted-lengths) top-of-range)) - - ;; Count number of definitions within current range. - (setq number-within-range (1+ number-within-range)) - (setq sorted-lengths (cdr sorted-lengths))) - - ;; Exit inner loop but remain within outer loop. - - (setq defuns-per-range-list - (cons number-within-range defuns-per-range-list)) - (setq number-within-range 0) ; Reset count to zero. - - ;; Move to next range. - (setq top-of-ranges (cdr top-of-ranges)) - ;; Specify next top of range value. - (setq top-of-range (car top-of-ranges))) - - ;; Exit outer loop and count the number of defuns larger than - ;; the largest top-of-range value. - (setq defuns-per-range-list - (cons - (length sorted-lengths) - defuns-per-range-list)) - - ;; Return a list of the number of definitions within each range, - ;; smallest to largest. - (nreverse defuns-per-range-list))) - -The function is straightforward except for one subtle feature. The -true-or-false test of the inner loop looks like this: - - (and (car sorted-lengths) - (< (car sorted-lengths) top-of-range)) - -instead of like this: - - (< (car sorted-lengths) top-of-range) - -The purpose of the test is to determine whether the first item in the -`sorted-lengths' list is less than the value of the top of the range. - -The simple version of the test works fine unless the `sorted-lengths' -list has a `nil' value. In that case, the `(car sorted-lengths)' -expression function returns `nil'. The `<' function cannot compare a -number to `nil', which is an empty list, so Emacs signals an error and -stops the function from attempting to continue to execute. - -The `sorted-lengths' list always becomes `nil' when the counter reaches -the end of the list. This means that any attempt to use the -`defuns-per-range' function with the simple version of the test will -fail. - -We solve the problem by using the `(car sorted-lengths)' expression in -conjunction with the `and' expression. The `(car sorted-lengths)' -expression returns a non-`nil' value so long as the list has at least -one number within it, but returns `nil' if the list is empty. The -`and' expression first evaluates the `(car sorted-lengths)' expression, -and if it is `nil', returns false _without_ evaluating the `<' -expression. But if the `(car sorted-lengths)' expression returns a -non-`nil' value, the `and' expression evaluates the `<' expression, and -returns that value as the value of the `and' expression. - -This way, we avoid an error. (*Note The `kill-new' function: kill-new -function, for information about `and'.) - -Here is a short test of the `defuns-per-range' function. First, -evaluate the expression that binds (a shortened) `top-of-ranges' list -to the list of values, then evaluate the expression for binding the -`sorted-lengths' list, and then evaluate the `defuns-per-range' -function. - - ;; (Shorter list than we will use later.) - (setq top-of-ranges - '(110 120 130 140 150 - 160 170 180 190 200)) - - (setq sorted-lengths - '(85 86 110 116 122 129 154 176 179 200 265 300 300)) - - (defuns-per-range sorted-lengths top-of-ranges) - -The list returned looks like this: - - (2 2 2 0 0 1 0 2 0 0 4) - -Indeed, there are two elements of the `sorted-lengths' list smaller -than 110, two elements between 110 and 119, two elements between 120 -and 129, and so on. There are four elements with a value of 200 or -larger. - - -File: eintr, Node: Readying a Graph, Next: Emacs Initialization, Prev: Words in a defun, Up: Top - -15 Readying a Graph -******************* - -Our goal is to construct a graph showing the numbers of function -definitions of various lengths in the Emacs lisp sources. - -As a practical matter, if you were creating a graph, you would probably -use a program such as `gnuplot' to do the job. (`gnuplot' is nicely -integrated into GNU Emacs.) In this case, however, we create one from -scratch, and in the process we will re-acquaint ourselves with some of -what we learned before and learn more. - -In this chapter, we will first write a simple graph printing function. -This first definition will be a "prototype", a rapidly written function -that enables us to reconnoiter this unknown graph-making territory. We -will discover dragons, or find that they are myth. After scouting the -terrain, we will feel more confident and enhance the function to label -the axes automatically. - -* Menu: - -* Columns of a graph:: -* graph-body-print:: -* recursive-graph-body-print:: -* Printed Axes:: -* Line Graph Exercise:: - - -File: eintr, Node: Columns of a graph, Next: graph-body-print, Prev: Readying a Graph, Up: Readying a Graph - -Printing the Columns of a Graph -=============================== - -Since Emacs is designed to be flexible and work with all kinds of -terminals, including character-only terminals, the graph will need to -be made from one of the `typewriter' symbols. An asterisk will do; as -we enhance the graph-printing function, we can make the choice of -symbol a user option. - -We can call this function `graph-body-print'; it will take a -`numbers-list' as its only argument. At this stage, we will not label -the graph, but only print its body. - -The `graph-body-print' function inserts a vertical column of asterisks -for each element in the `numbers-list'. The height of each line is -determined by the value of that element of the `numbers-list'. - -Inserting columns is a repetitive act; that means that this function can -be written either with a `while' loop or recursively. - -Our first challenge is to discover how to print a column of asterisks. -Usually, in Emacs, we print characters onto a screen horizontally, line -by line, by typing. We have two routes we can follow: write our own -column-insertion function or discover whether one exists in Emacs. - -To see whether there is one in Emacs, we can use the `M-x apropos' -command. This command is like the `C-h a' (`command-apropos') command, -except that the latter finds only those functions that are commands. -The `M-x apropos' command lists all symbols that match a regular -expression, including functions that are not interactive. - -What we want to look for is some command that prints or inserts -columns. Very likely, the name of the function will contain either the -word `print' or the word `insert' or the word `column'. Therefore, we -can simply type `M-x apropos RET print\|insert\|column RET' and look at -the result. On my system, this command once too takes quite some time, -and then produced a list of 79 functions and variables. Now it does -not take much time at all and produces a list of 211 functions and -variables. Scanning down the list, the only function that looks as if -it might do the job is `insert-rectangle'. - -Indeed, this is the function we want; its documentation says: - - insert-rectangle: - Insert text of RECTANGLE with upper left corner at point. - RECTANGLE's first line is inserted at point, - its second line is inserted at a point vertically under point, etc. - RECTANGLE should be a list of strings. - After this command, the mark is at the upper left corner - and point is at the lower right corner. - -We can run a quick test, to make sure it does what we expect of it. - -Here is the result of placing the cursor after the `insert-rectangle' -expression and typing `C-u C-x C-e' (`eval-last-sexp'). The function -inserts the strings `"first"', `"second"', and `"third"' at and below -point. Also the function returns `nil'. - - (insert-rectangle '("first" "second" "third"))first - second - thirdnil - -Of course, we won't be inserting the text of the `insert-rectangle' -expression itself into the buffer in which we are making the graph, but -will call the function from our program. We shall, however, have to -make sure that point is in the buffer at the place where the -`insert-rectangle' function will insert its column of strings. - -If you are reading this in Info, you can see how this works by -switching to another buffer, such as the `*scratch*' buffer, placing -point somewhere in the buffer, typing `M-:', typing the -`insert-rectangle' expression into the minibuffer at the prompt, and -then typing <RET>. This causes Emacs to evaluate the expression in the -minibuffer, but to use as the value of point the position of point in -the `*scratch*' buffer. (`M-:' is the keybinding for -`eval-expression'. Also, `nil' does not appear in the `*scratch*' -buffer since the expression is evaluated in the minibuffer.) - -We find when we do this that point ends up at the end of the last -inserted line--that is to say, this function moves point as a -side-effect. If we were to repeat the command, with point at this -position, the next insertion would be below and to the right of the -previous insertion. We don't want this! If we are going to make a bar -graph, the columns need to be beside each other. - -So we discover that each cycle of the column-inserting `while' loop -must reposition point to the place we want it, and that place will be -at the top, not the bottom, of the column. Moreover, we remember that -when we print a graph, we do not expect all the columns to be the same -height. This means that the top of each column may be at a different -height from the previous one. We cannot simply reposition point to the -same line each time, but moved over to the right--or perhaps we can... - -We are planning to make the columns of the bar graph out of asterisks. -The number of asterisks in the column is the number specified by the -current element of the `numbers-list'. We need to construct a list of -asterisks of the right length for each call to `insert-rectangle'. If -this list consists solely of the requisite number of asterisks, then we -will have position point the right number of lines above the base for -the graph to print correctly. This could be difficult. - -Alternatively, if we can figure out some way to pass `insert-rectangle' -a list of the same length each time, then we can place point on the -same line each time, but move it over one column to the right for each -new column. If we do this, however, some of the entries in the list -passed to `insert-rectangle' must be blanks rather than asterisks. For -example, if the maximum height of the graph is 5, but the height of the -column is 3, then `insert-rectangle' requires an argument that looks -like this: - - (" " " " "*" "*" "*") - -This last proposal is not so difficult, so long as we can determine the -column height. There are two ways for us to specify the column height: -we can arbitrarily state what it will be, which would work fine for -graphs of that height; or we can search through the list of numbers and -use the maximum height of the list as the maximum height of the graph. -If the latter operation were difficult, then the former procedure would -be easiest, but there is a function built into Emacs that determines -the maximum of its arguments. We can use that function. The function -is called `max' and it returns the largest of all its arguments, which -must be numbers. Thus, for example, - - (max 3 4 6 5 7 3) - -returns 7. (A corresponding function called `min' returns the smallest -of all its arguments.) - -However, we cannot simply call `max' on the `numbers-list'; the `max' -function expects numbers as its argument, not a list of numbers. Thus, -the following expression, - - (max '(3 4 6 5 7 3)) - -produces the following error message; - - Wrong type of argument: number-or-marker-p, (3 4 6 5 7 3) - -We need a function that passes a list of arguments to a function. This -function is `apply'. This function `applies' its first argument (a -function) to its remaining arguments, the last of which may be a list. - -For example, - - (apply 'max 3 4 7 3 '(4 8 5)) - -returns 8. - -(Incidentally, I don't know how you would learn of this function -without a book such as this. It is possible to discover other -functions, like `search-forward' or `insert-rectangle', by guessing at -a part of their names and then using `apropos'. Even though its base -in metaphor is clear--`apply' its first argument to the rest--I doubt a -novice would come up with that particular word when using `apropos' or -other aid. Of course, I could be wrong; after all, the function was -first named by someone who had to invent it.) - -The second and subsequent arguments to `apply' are optional, so we can -use `apply' to call a function and pass the elements of a list to it, -like this, which also returns 8: - - (apply 'max '(4 8 5)) - -This latter way is how we will use `apply'. The -`recursive-lengths-list-many-files' function returns a numbers' list to -which we can apply `max' (we could also apply `max' to the sorted -numbers' list; it does not matter whether the list is sorted or not.) - -Hence, the operation for finding the maximum height of the graph is -this: - - (setq max-graph-height (apply 'max numbers-list)) - -Now we can return to the question of how to create a list of strings -for a column of the graph. Told the maximum height of the graph and -the number of asterisks that should appear in the column, the function -should return a list of strings for the `insert-rectangle' command to -insert. - -Each column is made up of asterisks or blanks. Since the function is -passed the value of the height of the column and the number of -asterisks in the column, the number of blanks can be found by -subtracting the number of asterisks from the height of the column. -Given the number of blanks and the number of asterisks, two `while' -loops can be used to construct the list: - - ;;; First version. - (defun column-of-graph (max-graph-height actual-height) - "Return list of strings that is one column of a graph." - (let ((insert-list nil) - (number-of-top-blanks - (- max-graph-height actual-height))) - - ;; Fill in asterisks. - (while (> actual-height 0) - (setq insert-list (cons "*" insert-list)) - (setq actual-height (1- actual-height))) - - ;; Fill in blanks. - (while (> number-of-top-blanks 0) - (setq insert-list (cons " " insert-list)) - (setq number-of-top-blanks - (1- number-of-top-blanks))) - - ;; Return whole list. - insert-list)) - -If you install this function and then evaluate the following expression -you will see that it returns the list as desired: - - (column-of-graph 5 3) - -returns - - (" " " " "*" "*" "*") - -As written, `column-of-graph' contains a major flaw: the symbols used -for the blank and for the marked entries in the column are `hard-coded' -as a space and asterisk. This is fine for a prototype, but you, or -another user, may wish to use other symbols. For example, in testing -the graph function, you many want to use a period in place of the -space, to make sure the point is being repositioned properly each time -the `insert-rectangle' function is called; or you might want to -substitute a `+' sign or other symbol for the asterisk. You might even -want to make a graph-column that is more than one display column wide. -The program should be more flexible. The way to do that is to replace -the blank and the asterisk with two variables that we can call -`graph-blank' and `graph-symbol' and define those variables separately. - -Also, the documentation is not well written. These considerations lead -us to the second version of the function: - - (defvar graph-symbol "*" - "String used as symbol in graph, usually an asterisk.") - - (defvar graph-blank " " - "String used as blank in graph, usually a blank space. - graph-blank must be the same number of columns wide - as graph-symbol.") - -(For an explanation of `defvar', see *Note Initializing a Variable with -`defvar': defvar.) - - ;;; Second version. - (defun column-of-graph (max-graph-height actual-height) - "Return MAX-GRAPH-HEIGHT strings; ACTUAL-HEIGHT are graph-symbols. - The graph-symbols are contiguous entries at the end - of the list. - The list will be inserted as one column of a graph. - The strings are either graph-blank or graph-symbol." - - (let ((insert-list nil) - (number-of-top-blanks - (- max-graph-height actual-height))) - - ;; Fill in `graph-symbols'. - (while (> actual-height 0) - (setq insert-list (cons graph-symbol insert-list)) - (setq actual-height (1- actual-height))) - - ;; Fill in `graph-blanks'. - (while (> number-of-top-blanks 0) - (setq insert-list (cons graph-blank insert-list)) - (setq number-of-top-blanks - (1- number-of-top-blanks))) - - ;; Return whole list. - insert-list)) - -If we wished, we could rewrite `column-of-graph' a third time to -provide optionally for a line graph as well as for a bar graph. This -would not be hard to do. One way to think of a line graph is that it -is no more than a bar graph in which the part of each bar that is below -the top is blank. To construct a column for a line graph, the function -first constructs a list of blanks that is one shorter than the value, -then it uses `cons' to attach a graph symbol to the list; then it uses -`cons' again to attach the `top blanks' to the list. - -It is easy to see how to write such a function, but since we don't need -it, we will not do it. But the job could be done, and if it were done, -it would be done with `column-of-graph'. Even more important, it is -worth noting that few changes would have to be made anywhere else. The -enhancement, if we ever wish to make it, is simple. - -Now, finally, we come to our first actual graph printing function. -This prints the body of a graph, not the labels for the vertical and -horizontal axes, so we can call this `graph-body-print'. - - -File: eintr, Node: graph-body-print, Next: recursive-graph-body-print, Prev: Columns of a graph, Up: Readying a Graph - -15.1 The `graph-body-print' Function -==================================== - -After our preparation in the preceding section, the `graph-body-print' -function is straightforward. The function will print column after -column of asterisks and blanks, using the elements of a numbers' list -to specify the number of asterisks in each column. This is a -repetitive act, which means we can use a decrementing `while' loop or -recursive function for the job. In this section, we will write the -definition using a `while' loop. - -The `column-of-graph' function requires the height of the graph as an -argument, so we should determine and record that as a local variable. - -This leads us to the following template for the `while' loop version of -this function: - - (defun graph-body-print (numbers-list) - "DOCUMENTATION..." - (let ((height ... - ...)) - - (while numbers-list - INSERT-COLUMNS-AND-REPOSITION-POINT - (setq numbers-list (cdr numbers-list))))) - -We need to fill in the slots of the template. - -Clearly, we can use the `(apply 'max numbers-list)' expression to -determine the height of the graph. - -The `while' loop will cycle through the `numbers-list' one element at a -time. As it is shortened by the `(setq numbers-list (cdr -numbers-list))' expression, the CAR of each instance of the list is the -value of the argument for `column-of-graph'. - -At each cycle of the `while' loop, the `insert-rectangle' function -inserts the list returned by `column-of-graph'. Since the -`insert-rectangle' function moves point to the lower right of the -inserted rectangle, we need to save the location of point at the time -the rectangle is inserted, move back to that position after the -rectangle is inserted, and then move horizontally to the next place -from which `insert-rectangle' is called. - -If the inserted columns are one character wide, as they will be if -single blanks and asterisks are used, the repositioning command is -simply `(forward-char 1)'; however, the width of a column may be -greater than one. This means that the repositioning command should be -written `(forward-char symbol-width)'. The `symbol-width' itself is -the length of a `graph-blank' and can be found using the expression -`(length graph-blank)'. The best place to bind the `symbol-width' -variable to the value of the width of graph column is in the varlist of -the `let' expression. - -These considerations lead to the following function definition: - - (defun graph-body-print (numbers-list) - "Print a bar graph of the NUMBERS-LIST. - The numbers-list consists of the Y-axis values." - - (let ((height (apply 'max numbers-list)) - (symbol-width (length graph-blank)) - from-position) - - (while numbers-list - (setq from-position (point)) - (insert-rectangle - (column-of-graph height (car numbers-list))) - (goto-char from-position) - (forward-char symbol-width) - ;; Draw graph column by column. - (sit-for 0) - (setq numbers-list (cdr numbers-list))) - ;; Place point for X axis labels. - (forward-line height) - (insert "\n") - )) - -The one unexpected expression in this function is the `(sit-for 0)' -expression in the `while' loop. This expression makes the graph -printing operation more interesting to watch than it would be -otherwise. The expression causes Emacs to `sit' or do nothing for a -zero length of time and then redraw the screen. Placed here, it causes -Emacs to redraw the screen column by column. Without it, Emacs would -not redraw the screen until the function exits. - -We can test `graph-body-print' with a short list of numbers. - - 1. Install `graph-symbol', `graph-blank', `column-of-graph', which - are in *Note Columns of a graph::, and `graph-body-print'. - - 2. Copy the following expression: - - (graph-body-print '(1 2 3 4 6 4 3 5 7 6 5 2 3)) - - 3. Switch to the `*scratch*' buffer and place the cursor where you - want the graph to start. - - 4. Type `M-:' (`eval-expression'). - - 5. Yank the `graph-body-print' expression into the minibuffer with - `C-y' (`yank)'. - - 6. Press <RET> to evaluate the `graph-body-print' expression. - -Emacs will print a graph like this: - - * - * ** - * **** - *** **** - ********* * - ************ - ************* - - -File: eintr, Node: recursive-graph-body-print, Next: Printed Axes, Prev: graph-body-print, Up: Readying a Graph - -15.2 The `recursive-graph-body-print' Function -============================================== - -The `graph-body-print' function may also be written recursively. The -recursive solution is divided into two parts: an outside `wrapper' that -uses a `let' expression to determine the values of several variables -that need only be found once, such as the maximum height of the graph, -and an inside function that is called recursively to print the graph. - -The `wrapper' is uncomplicated: - - (defun recursive-graph-body-print (numbers-list) - "Print a bar graph of the NUMBERS-LIST. - The numbers-list consists of the Y-axis values." - (let ((height (apply 'max numbers-list)) - (symbol-width (length graph-blank)) - from-position) - (recursive-graph-body-print-internal - numbers-list - height - symbol-width))) - -The recursive function is a little more difficult. It has four parts: -the `do-again-test', the printing code, the recursive call, and the -`next-step-expression'. The `do-again-test' is an `if' expression that -determines whether the `numbers-list' contains any remaining elements; -if it does, the function prints one column of the graph using the -printing code and calls itself again. The function calls itself again -according to the value produced by the `next-step-expression' which -causes the call to act on a shorter version of the `numbers-list'. - - (defun recursive-graph-body-print-internal - (numbers-list height symbol-width) - "Print a bar graph. - Used within recursive-graph-body-print function." - - (if numbers-list - (progn - (setq from-position (point)) - (insert-rectangle - (column-of-graph height (car numbers-list))) - (goto-char from-position) - (forward-char symbol-width) - (sit-for 0) ; Draw graph column by column. - (recursive-graph-body-print-internal - (cdr numbers-list) height symbol-width)))) - -After installation, this expression can be tested; here is a sample: - - (recursive-graph-body-print '(3 2 5 6 7 5 3 4 6 4 3 2 1)) - -Here is what `recursive-graph-body-print' produces: - - * - ** * - **** * - **** *** - * ********* - ************ - ************* - -Either of these two functions, `graph-body-print' or -`recursive-graph-body-print', create the body of a graph. - - -File: eintr, Node: Printed Axes, Next: Line Graph Exercise, Prev: recursive-graph-body-print, Up: Readying a Graph - -15.3 Need for Printed Axes -========================== - -A graph needs printed axes, so you can orient yourself. For a do-once -project, it may be reasonable to draw the axes by hand using Emacs' -Picture mode; but a graph drawing function may be used more than once. - -For this reason, I have written enhancements to the basic -`print-graph-body' function that automatically print labels for the -horizontal and vertical axes. Since the label printing functions do -not contain much new material, I have placed their description in an -appendix. *Note A Graph with Labelled Axes: Full Graph. - - -File: eintr, Node: Line Graph Exercise, Prev: Printed Axes, Up: Readying a Graph - -15.4 Exercise -============= - -Write a line graph version of the graph printing functions. - - -File: eintr, Node: Emacs Initialization, Next: Debugging, Prev: Readying a Graph, Up: Top - -16 Your `.emacs' File -********************* - -"You don't have to like Emacs to like it" - this seemingly paradoxical -statement is the secret of GNU Emacs. The plain, `out of the box' -Emacs is a generic tool. Most people who use it, customize it to suit -themselves. - -GNU Emacs is mostly written in Emacs Lisp; this means that by writing -expressions in Emacs Lisp you can change or extend Emacs. - -* Menu: - -* Default Configuration:: -* Site-wide Init:: -* defcustom:: -* Beginning a .emacs File:: -* Text and Auto-fill:: -* Mail Aliases:: -* Indent Tabs Mode:: -* Keybindings:: -* Keymaps:: -* Loading Files:: -* Autoload:: -* Simple Extension:: -* X11 Colors:: -* Miscellaneous:: -* Mode Line:: - - -File: eintr, Node: Default Configuration, Next: Site-wide Init, Prev: Emacs Initialization, Up: Emacs Initialization - -Emacs' Default Configuration -============================ - -There are those who appreciate Emacs' default configuration. After -all, Emacs starts you in C mode when you edit a C file, starts you in -Fortran mode when you edit a Fortran file, and starts you in -Fundamental mode when you edit an unadorned file. This all makes -sense, if you do not know who is going to use Emacs. Who knows what a -person hopes to do with an unadorned file? Fundamental mode is the -right default for such a file, just as C mode is the right default for -editing C code. (Enough programming languages have syntaxes that -enable them to share or nearly share features, so C mode is now -provided by by CC mode, the `C Collection'.) - -But when you do know who is going to use Emacs--you, yourself--then it -makes sense to customize Emacs. - -For example, I seldom want Fundamental mode when I edit an otherwise -undistinguished file; I want Text mode. This is why I customize Emacs: -so it suits me. - -You can customize and extend Emacs by writing or adapting a `~/.emacs' -file. This is your personal initialization file; its contents, written -in Emacs Lisp, tell Emacs what to do.(1) - -A `~/.emacs' file contains Emacs Lisp code. You can write this code -yourself; or you can use Emacs' `customize' feature to write the code -for you. You can combine your own expressions and auto-written -Customize expressions in your `.emacs' file. - -(I myself prefer to write my own expressions, except for those, -particularly fonts, that I find easier to manipulate using the -`customize' command. I combine the two methods.) - -Most of this chapter is about writing expressions yourself. It -describes a simple `.emacs' file; for more information, see *Note The -Init File: (emacs)Init File, and *Note The Init File: (elisp)Init File. - ----------- Footnotes ---------- - -(1) You may also add `.el' to `~/.emacs' and call it a `~/.emacs.el' -file. In the past, you were forbidden to type the extra keystrokes -that the name `~/.emacs.el' requires, but now you may. The new format -is consistent with the Emacs Lisp file naming conventions; the old -format saves typing. - - -File: eintr, Node: Site-wide Init, Next: defcustom, Prev: Default Configuration, Up: Emacs Initialization - -16.1 Site-wide Initialization Files -=================================== - -In addition to your personal initialization file, Emacs automatically -loads various site-wide initialization files, if they exist. These -have the same form as your `.emacs' file, but are loaded by everyone. - -Two site-wide initialization files, `site-load.el' and `site-init.el', -are loaded into Emacs and then `dumped' if a `dumped' version of Emacs -is created, as is most common. (Dumped copies of Emacs load more -quickly. However, once a file is loaded and dumped, a change to it -does not lead to a change in Emacs unless you load it yourself or -re-dump Emacs. *Note Building Emacs: (elisp)Building Emacs, and the -`INSTALL' file.) - -Three other site-wide initialization files are loaded automatically -each time you start Emacs, if they exist. These are `site-start.el', -which is loaded _before_ your `.emacs' file, and `default.el', and the -terminal type file, which are both loaded _after_ your `.emacs' file. - -Settings and definitions in your `.emacs' file will overwrite -conflicting settings and definitions in a `site-start.el' file, if it -exists; but the settings and definitions in a `default.el' or terminal -type file will overwrite those in your `.emacs' file. (You can prevent -interference from a terminal type file by setting `term-file-prefix' to -`nil'. *Note A Simple Extension: Simple Extension.) - -The `INSTALL' file that comes in the distribution contains descriptions -of the `site-init.el' and `site-load.el' files. - -The `loadup.el', `startup.el', and `loaddefs.el' files control loading. -These files are in the `lisp' directory of the Emacs distribution and -are worth perusing. - -The `loaddefs.el' file contains a good many suggestions as to what to -put into your own `.emacs' file, or into a site-wide initialization -file. - - -File: eintr, Node: defcustom, Next: Beginning a .emacs File, Prev: Site-wide Init, Up: Emacs Initialization - -16.2 Specifying Variables using `defcustom' -=========================================== - -You can specify variables using `defcustom' so that you and others can -then use Emacs' `customize' feature to set their values. (You cannot -use `customize' to write function definitions; but you can write -`defuns' in your `.emacs' file. Indeed, you can write any Lisp -expression in your `.emacs' file.) - -The `customize' feature depends on the `defcustom' special form. -Although you can use `defvar' or `setq' for variables that users set, -the `defcustom' special form is designed for the job. - -You can use your knowledge of `defvar' for writing the first three -arguments for `defcustom'. The first argument to `defcustom' is the -name of the variable. The second argument is the variable's initial -value, if any; and this value is set only if the value has not already -been set. The third argument is the documentation. - -The fourth and subsequent arguments to `defcustom' specify types and -options; these are not featured in `defvar'. (These arguments are -optional.) - -Each of these arguments consists of a keyword followed by a value. -Each keyword starts with the colon character `:'. - -For example, the customizable user option variable `text-mode-hook' -looks like this: - - (defcustom text-mode-hook nil - "Normal hook run when entering Text mode and many related modes." - :type 'hook - :options '(turn-on-auto-fill flyspell-mode) - :group 'data) - -The name of the variable is `text-mode-hook'; it has no default value; -and its documentation string tells you what it does. - -The `:type' keyword tells Emacs the kind of data to which -`text-mode-hook' should be set and how to display the value in a -Customization buffer. - -The `:options' keyword specifies a suggested list of values for the -variable. Currently, you can use `:options' only for a hook. The list -is only a suggestion; it is not exclusive; a person who sets the -variable may set it to other values; the list shown following the -`:options' keyword is intended to offer convenient choices to a user. - -Finally, the `:group' keyword tells the Emacs Customization command in -which group the variable is located. This tells where to find it. - -For more information, see *Note Writing Customization Definitions: -(elisp)Customization. - -Consider `text-mode-hook' as an example. - -There are two ways to customize this variable. You can use the -customization command or write the appropriate expressions yourself. - -Using the customization command, you can type: - - M-x customize - -and find that the group for editing files of data is called `data'. -Enter that group. Text Mode Hook is the first member. You can click -on its various options, such as `turn-on-auto-fill', to set the values. -After you click on the button to - - Save for Future Sessions - -Emacs will write an expression into your `.emacs' file. It will look -like this: - - (custom-set-variables - ;; custom-set-variables was added by Custom. - ;; If you edit it by hand, you could mess it up, so be careful. - ;; Your init file should contain only one such instance. - ;; If there is more than one, they won't work right. - '(text-mode-hook (quote (turn-on-auto-fill text-mode-hook-identify)))) - -(The `text-mode-hook-identify' function tells -`toggle-text-mode-auto-fill' which buffers are in Text mode. It comes -on automatically. ) - -The `custom-set-variables' function works somewhat differently than a -`setq'. While I have never learned the differences, I modify the -`custom-set-variables' expressions in my `.emacs' file by hand: I make -the changes in what appears to me to be a reasonable manner and have -not had any problems. Others prefer to use the Customization command -and let Emacs do the work for them. - -Another `custom-set-...' function is `custom-set-faces'. This function -sets the various font faces. Over time, I have set a considerable -number of faces. Some of the time, I re-set them using `customize'; -other times, I simply edit the `custom-set-faces' expression in my -`.emacs' file itself. - -The second way to customize your `text-mode-hook' is to set it yourself -in your `.emacs' file using code that has nothing to do with the -`custom-set-...' functions. - -When you do this, and later use `customize', you will see a message -that says - - CHANGED outside Customize; operating on it here may be unreliable. - -This message is only a warning. If you click on the button to - - Save for Future Sessions - -Emacs will write a `custom-set-...' expression near the end of your -`.emacs' file that will be evaluated after your hand-written -expression. It will, therefore, overrule your hand-written expression. -No harm will be done. When you do this, however, be careful to -remember which expression is active; if you forget, you may confuse -yourself. - -So long as you remember where the values are set, you will have no -trouble. In any event, the values are always set in your -initialization file, which is usually called `.emacs'. - -I myself use `customize' for hardly anything. Mostly, I write -expressions myself. - -Incidentally, `defsubst' defines an inline function. The syntax is -just like that of `defun'. `defconst' defines a symbol as a constant. -The intent is that neither programs nor users should ever change a -value set by `defconst' - - -File: eintr, Node: Beginning a .emacs File, Next: Text and Auto-fill, Prev: defcustom, Up: Emacs Initialization - -16.3 Beginning a `.emacs' File -============================== - -When you start Emacs, it loads your `.emacs' file unless you tell it -not to by specifying `-q' on the command line. (The `emacs -q' command -gives you a plain, out-of-the-box Emacs.) - -A `.emacs' file contains Lisp expressions. Often, these are no more -than expressions to set values; sometimes they are function definitions. - -*Note The Init File `~/.emacs': (emacs)Init File, for a short -description of initialization files. - -This chapter goes over some of the same ground, but is a walk among -extracts from a complete, long-used `.emacs' file--my own. - -The first part of the file consists of comments: reminders to myself. -By now, of course, I remember these things, but when I started, I did -not. - - ;;;; Bob's .emacs file - ; Robert J. Chassell - ; 26 September 1985 - -Look at that date! I started this file a long time ago. I have been -adding to it ever since. - - ; Each section in this file is introduced by a - ; line beginning with four semicolons; and each - ; entry is introduced by a line beginning with - ; three semicolons. - -This describes the usual conventions for comments in Emacs Lisp. -Everything on a line that follows a semicolon is a comment. Two, -three, and four semicolons are used as section and subsection markers. -(*Note Comments: (elisp)Comments, for more about comments.) - - ;;;; The Help Key - ; Control-h is the help key; - ; after typing control-h, type a letter to - ; indicate the subject about which you want help. - ; For an explanation of the help facility, - ; type control-h two times in a row. - -Just remember: type `C-h' two times for help. - - ; To find out about any mode, type control-h m - ; while in that mode. For example, to find out - ; about mail mode, enter mail mode and then type - ; control-h m. - -`Mode help', as I call this, is very helpful. Usually, it tells you -all you need to know. - -Of course, you don't need to include comments like these in your -`.emacs' file. I included them in mine because I kept forgetting about -Mode help or the conventions for comments--but I was able to remember -to look here to remind myself. - - -File: eintr, Node: Text and Auto-fill, Next: Mail Aliases, Prev: Beginning a .emacs File, Up: Emacs Initialization - -16.4 Text and Auto Fill Mode -============================ - -Now we come to the part that `turns on' Text mode and Auto Fill mode. - - ;;; Text mode and Auto Fill mode - ; The next two lines put Emacs into Text mode - ; and Auto Fill mode, and are for writers who - ; want to start writing prose rather than code. - - (setq default-major-mode 'text-mode) - (add-hook 'text-mode-hook 'turn-on-auto-fill) - -Here is the first part of this `.emacs' file that does something -besides remind a forgetful human! - -The first of the two lines in parentheses tells Emacs to turn on Text -mode when you find a file, _unless_ that file should go into some other -mode, such as C mode. - -When Emacs reads a file, it looks at the extension to the file name, if -any. (The extension is the part that comes after a `.'.) If the file -ends with a `.c' or `.h' extension then Emacs turns on C mode. Also, -Emacs looks at first nonblank line of the file; if the line says -`-*- C -*-', Emacs turns on C mode. Emacs possesses a list of -extensions and specifications that it uses automatically. In addition, -Emacs looks near the last page for a per-buffer, "local variables -list", if any. - -*Note How Major Modes are Chosen: (emacs)Choosing Modes. - -*Note Local Variables in Files: (emacs)File Variables. - -Now, back to the `.emacs' file. - -Here is the line again; how does it work? - - (setq default-major-mode 'text-mode) - -This line is a short, but complete Emacs Lisp expression. - -We are already familiar with `setq'. It sets the following variable, -`default-major-mode', to the subsequent value, which is `text-mode'. -The single quote mark before `text-mode' tells Emacs to deal directly -with the `text-mode' variable, not with whatever it might stand for. -*Note Setting the Value of a Variable: set & setq, for a reminder of -how `setq' works. The main point is that there is no difference -between the procedure you use to set a value in your `.emacs' file and -the procedure you use anywhere else in Emacs. - -Here is the next line: - - (add-hook 'text-mode-hook 'turn-on-auto-fill) - -In this line, the `add-hook' command adds `turn-on-auto-fill' to the -variable. - -`turn-on-auto-fill' is the name of a program, that, you guessed it!, -turns on Auto Fill mode. - -Every time Emacs turns on Text mode, Emacs runs the commands `hooked' -onto Text mode. So every time Emacs turns on Text mode, Emacs also -turns on Auto Fill mode. - -In brief, the first line causes Emacs to enter Text mode when you edit a -file, unless the file name extension, a first non-blank line, or local -variables to tell Emacs otherwise. - -Text mode among other actions, sets the syntax table to work -conveniently for writers. In Text mode, Emacs considers an apostrophe -as part of a word like a letter; but Emacs does not consider a period -or a space as part of a word. Thus, `M-f' moves you over `it's'. On -the other hand, in C mode, `M-f' stops just after the `t' of `it's'. - -The second line causes Emacs to turn on Auto Fill mode when it turns on -Text mode. In Auto Fill mode, Emacs automatically breaks a line that -is too wide and brings the excessively wide part of the line down to -the next line. Emacs breaks lines between words, not within them. - -When Auto Fill mode is turned off, lines continue to the right as you -type them. Depending on how you set the value of `truncate-lines', the -words you type either disappear off the right side of the screen, or -else are shown, in a rather ugly and unreadable manner, as a -continuation line on the screen. - -In addition, in this part of my `.emacs' file, I tell the Emacs fill -commands to insert two spaces after a colon: - - (setq colon-double-space t) - - -File: eintr, Node: Mail Aliases, Next: Indent Tabs Mode, Prev: Text and Auto-fill, Up: Emacs Initialization - -16.5 Mail Aliases -================= - -Here is a `setq' that `turns on' mail aliases, along with more -reminders. - - ;;; Mail mode - ; To enter mail mode, type `C-x m' - ; To enter RMAIL (for reading mail), - ; type `M-x rmail' - - (setq mail-aliases t) - -This `setq' command sets the value of the variable `mail-aliases' to -`t'. Since `t' means true, the line says, in effect, "Yes, use mail -aliases." - -Mail aliases are convenient short names for long email addresses or for -lists of email addresses. The file where you keep your `aliases' is -`~/.mailrc'. You write an alias like this: - - alias geo george@foobar.wiz.edu - -When you write a message to George, address it to `geo'; the mailer -will automatically expand `geo' to the full address. - - -File: eintr, Node: Indent Tabs Mode, Next: Keybindings, Prev: Mail Aliases, Up: Emacs Initialization - -16.6 Indent Tabs Mode -===================== - -By default, Emacs inserts tabs in place of multiple spaces when it -formats a region. (For example, you might indent many lines of text -all at once with the `indent-region' command.) Tabs look fine on a -terminal or with ordinary printing, but they produce badly indented -output when you use TeX or Texinfo since TeX ignores tabs. - -The following turns off Indent Tabs mode: - - ;;; Prevent Extraneous Tabs - (setq-default indent-tabs-mode nil) - -Note that this line uses `setq-default' rather than the `setq' command -that we have seen before. The `setq-default' command sets values only -in buffers that do not have their own local values for the variable. - -*Note Tabs vs. Spaces: (emacs)Just Spaces. - -*Note Local Variables in Files: (emacs)File Variables. - - -File: eintr, Node: Keybindings, Next: Keymaps, Prev: Indent Tabs Mode, Up: Emacs Initialization - -16.7 Some Keybindings -===================== - -Now for some personal keybindings: - - ;;; Compare windows - (global-set-key "\C-cw" 'compare-windows) - -`compare-windows' is a nifty command that compares the text in your -current window with text in the next window. It makes the comparison -by starting at point in each window, moving over text in each window as -far as they match. I use this command all the time. - -This also shows how to set a key globally, for all modes. - -The command is `global-set-key'. It is followed by the keybinding. In -a `.emacs' file, the keybinding is written as shown: `\C-c' stands for -`control-c', which means `press the control key and the `c' key at the -same time'. The `w' means `press the `w' key'. The keybinding is -surrounded by double quotation marks. In documentation, you would -write this as `C-c w'. (If you were binding a <META> key, such as -`M-c', rather than a <CTRL> key, you would write `\M-c'. *Note -Rebinding Keys in Your Init File: (emacs)Init Rebinding, for details.) - -The command invoked by the keys is `compare-windows'. Note that -`compare-windows' is preceded by a single quote; otherwise, Emacs would -first try to evaluate the symbol to determine its value. - -These three things, the double quotation marks, the backslash before -the `C', and the single quote mark are necessary parts of keybinding -that I tend to forget. Fortunately, I have come to remember that I -should look at my existing `.emacs' file, and adapt what is there. - -As for the keybinding itself: `C-c w'. This combines the prefix key, -`C-c', with a single character, in this case, `w'. This set of keys, -`C-c' followed by a single character, is strictly reserved for -individuals' own use. (I call these `own' keys, since these are for my -own use.) You should always be able to create such a keybinding for -your own use without stomping on someone else's keybinding. If you -ever write an extension to Emacs, please avoid taking any of these keys -for public use. Create a key like `C-c C-w' instead. Otherwise, we -will run out of `own' keys. - -Here is another keybinding, with a comment: - - ;;; Keybinding for `occur' - ; I use occur a lot, so let's bind it to a key: - (global-set-key "\C-co" 'occur) - -The `occur' command shows all the lines in the current buffer that -contain a match for a regular expression. Matching lines are shown in -a buffer called `*Occur*'. That buffer serves as a menu to jump to -occurrences. - -Here is how to unbind a key, so it does not work: - - ;;; Unbind `C-x f' - (global-unset-key "\C-xf") - -There is a reason for this unbinding: I found I inadvertently typed -`C-x f' when I meant to type `C-x C-f'. Rather than find a file, as I -intended, I accidentally set the width for filled text, almost always -to a width I did not want. Since I hardly ever reset my default width, -I simply unbound the key. - -The following rebinds an existing key: - - ;;; Rebind `C-x C-b' for `buffer-menu' - (global-set-key "\C-x\C-b" 'buffer-menu) - -By default, `C-x C-b' runs the `list-buffers' command. This command -lists your buffers in _another_ window. Since I almost always want to -do something in that window, I prefer the `buffer-menu' command, which -not only lists the buffers, but moves point into that window. - - -File: eintr, Node: Keymaps, Next: Loading Files, Prev: Keybindings, Up: Emacs Initialization - -16.8 Keymaps -============ - -Emacs uses "keymaps" to record which keys call which commands. When -you use `global-set-key' to set the keybinding for a single command in -all parts of Emacs, you are specifying the keybinding in -`current-global-map'. - -Specific modes, such as C mode or Text mode, have their own keymaps; -the mode-specific keymaps override the global map that is shared by all -buffers. - -The `global-set-key' function binds, or rebinds, the global keymap. -For example, the following binds the key `C-x C-b' to the function -`buffer-menu': - - (global-set-key "\C-x\C-b" 'buffer-menu) - -Mode-specific keymaps are bound using the `define-key' function, which -takes a specific keymap as an argument, as well as the key and the -command. For example, my `.emacs' file contains the following -expression to bind the `texinfo-insert-@group' command to `C-c C-c g': - - (define-key texinfo-mode-map "\C-c\C-cg" 'texinfo-insert-@group) - -The `texinfo-insert-@group' function itself is a little extension to -Texinfo mode that inserts `@group' into a Texinfo file. I use this -command all the time and prefer to type the three strokes `C-c C-c g' -rather than the six strokes `@ g r o u p'. (`@group' and its matching -`@end group' are commands that keep all enclosed text together on one -page; many multi-line examples in this book are surrounded by `@group -... @end group'.) - -Here is the `texinfo-insert-@group' function definition: - - (defun texinfo-insert-@group () - "Insert the string @group in a Texinfo buffer." - (interactive) - (beginning-of-line) - (insert "@group\n")) - -(Of course, I could have used Abbrev mode to save typing, rather than -write a function to insert a word; but I prefer key strokes consistent -with other Texinfo mode key bindings.) - -You will see numerous `define-key' expressions in `loaddefs.el' as well -as in the various mode libraries, such as `cc-mode.el' and -`lisp-mode.el'. - -*Note Customizing Key Bindings: (emacs)Key Bindings, and *Note Keymaps: -(elisp)Keymaps, for more information about keymaps. - - -File: eintr, Node: Loading Files, Next: Autoload, Prev: Keymaps, Up: Emacs Initialization - -16.9 Loading Files -================== - -Many people in the GNU Emacs community have written extensions to -Emacs. As time goes by, these extensions are often included in new -releases. For example, the Calendar and Diary packages are now part of -the standard GNU Emacs, as is Calc. - -You can use a `load' command to evaluate a complete file and thereby -install all the functions and variables in the file into Emacs. For -example: - - (load "~/emacs/slowsplit") - -This evaluates, i.e. loads, the `slowsplit.el' file or if it exists, -the faster, byte compiled `slowsplit.elc' file from the `emacs' -sub-directory of your home directory. The file contains the function -`split-window-quietly', which John Robinson wrote in 1989. - -The `split-window-quietly' function splits a window with the minimum of -redisplay. I installed it in 1989 because it worked well with the slow -1200 baud terminals I was then using. Nowadays, I only occasionally -come across such a slow connection, but I continue to use the function -because I like the way it leaves the bottom half of a buffer in the -lower of the new windows and the top half in the upper window. - -To replace the key binding for the default `split-window-vertically', -you must also unset that key and bind the keys to -`split-window-quietly', like this: - - (global-unset-key "\C-x2") - (global-set-key "\C-x2" 'split-window-quietly) - -If you load many extensions, as I do, then instead of specifying the -exact location of the extension file, as shown above, you can specify -that directory as part of Emacs' `load-path'. Then, when Emacs loads a -file, it will search that directory as well as its default list of -directories. (The default list is specified in `paths.h' when Emacs is -built.) - -The following command adds your `~/emacs' directory to the existing -load path: - - ;;; Emacs Load Path - (setq load-path (cons "~/emacs" load-path)) - -Incidentally, `load-library' is an interactive interface to the `load' -function. The complete function looks like this: - - (defun load-library (library) - "Load the library named LIBRARY. - This is an interface to the function `load'." - (interactive - (list (completing-read "Load library: " - 'locate-file-completion - (cons load-path (get-load-suffixes))))) - (load library)) - -The name of the function, `load-library', comes from the use of -`library' as a conventional synonym for `file'. The source for the -`load-library' command is in the `files.el' library. - -Another interactive command that does a slightly different job is -`load-file'. *Note Libraries of Lisp Code for Emacs: (emacs)Lisp -Libraries, for information on the distinction between `load-library' -and this command. - - -File: eintr, Node: Autoload, Next: Simple Extension, Prev: Loading Files, Up: Emacs Initialization - -16.10 Autoloading -================= - -Instead of installing a function by loading the file that contains it, -or by evaluating the function definition, you can make the function -available but not actually install it until it is first called. This -is called "autoloading". - -When you execute an autoloaded function, Emacs automatically evaluates -the file that contains the definition, and then calls the function. - -Emacs starts quicker with autoloaded functions, since their libraries -are not loaded right away; but you need to wait a moment when you first -use such a function, while its containing file is evaluated. - -Rarely used functions are frequently autoloaded. The `loaddefs.el' -library contains hundreds of autoloaded functions, from `bookmark-set' -to `wordstar-mode'. Of course, you may come to use a `rare' function -frequently. When you do, you should load that function's file with a -`load' expression in your `.emacs' file. - -In my `.emacs' file for Emacs version 22, I load 14 libraries that -contain functions that would otherwise be autoloaded. (Actually, it -would have been better to include these files in my `dumped' Emacs, but -I forgot. *Note Building Emacs: (elisp)Building Emacs, and the -`INSTALL' file for more about dumping.) - -You may also want to include autoloaded expressions in your `.emacs' -file. `autoload' is a built-in function that takes up to five -arguments, the final three of which are optional. The first argument -is the name of the function to be autoloaded; the second is the name of -the file to be loaded. The third argument is documentation for the -function, and the fourth tells whether the function can be called -interactively. The fifth argument tells what type of -object--`autoload' can handle a keymap or macro as well as a function -(the default is a function). - -Here is a typical example: - - (autoload 'html-helper-mode - "html-helper-mode" "Edit HTML documents" t) - -(`html-helper-mode' is an alternative to `html-mode', which is a -standard part of the distribution). - -This expression autoloads the `html-helper-mode' function. It takes it -from the `html-helper-mode.el' file (or from the byte compiled file -`html-helper-mode.elc', if it exists.) The file must be located in a -directory specified by `load-path'. The documentation says that this -is a mode to help you edit documents written in the HyperText Markup -Language. You can call this mode interactively by typing `M-x -html-helper-mode'. (You need to duplicate the function's regular -documentation in the autoload expression because the regular function -is not yet loaded, so its documentation is not available.) - -*Note Autoload: (elisp)Autoload, for more information. - - -File: eintr, Node: Simple Extension, Next: X11 Colors, Prev: Autoload, Up: Emacs Initialization - -16.11 A Simple Extension: `line-to-top-of-window' -================================================= - -Here is a simple extension to Emacs that moves the line point is on to -the top of the window. I use this all the time, to make text easier to -read. - -You can put the following code into a separate file and then load it -from your `.emacs' file, or you can include it within your `.emacs' -file. - -Here is the definition: - - ;;; Line to top of window; - ;;; replace three keystroke sequence C-u 0 C-l - (defun line-to-top-of-window () - "Move the line point is on to top of window." - (interactive) - (recenter 0)) - -Now for the keybinding. - -Nowadays, function keys as well as mouse button events and non-ASCII -characters are written within square brackets, without quotation marks. -(In Emacs version 18 and before, you had to write different function -key bindings for each different make of terminal.) - -I bind `line-to-top-of-window' to my <F6> function key like this: - - (global-set-key [f6] 'line-to-top-of-window) - -For more information, see *Note Rebinding Keys in Your Init File: -(emacs)Init Rebinding. - -If you run two versions of GNU Emacs, such as versions 21 and 22, and -use one `.emacs' file, you can select which code to evaluate with the -following conditional: - - (cond - ((string-equal (number-to-string 21) (substring (emacs-version) 10 12)) - ;; evaluate version 21 code - ( ... )) - ((string-equal (number-to-string 22) (substring (emacs-version) 10 12)) - ;; evaluate version 22 code - ( ... ))) - -For example, in contrast to version 20, version 21 blinks its cursor by -default. I hate such blinking, as well as some other features in -version 21, so I placed the following in my `.emacs' file(1): - - (if (string-equal "21" (substring (emacs-version) 10 12)) - (progn - (blink-cursor-mode 0) - ;; Insert newline when you press `C-n' (next-line) - ;; at the end of the buffer - (setq next-line-add-newlines t) - ;; Turn on image viewing - (auto-image-file-mode t) - ;; Turn on menu bar (this bar has text) - ;; (Use numeric argument to turn on) - (menu-bar-mode 1) - ;; Turn off tool bar (this bar has icons) - ;; (Use numeric argument to turn on) - (tool-bar-mode nil) - ;; Turn off tooltip mode for tool bar - ;; (This mode causes icon explanations to pop up) - ;; (Use numeric argument to turn on) - (tooltip-mode nil) - ;; If tooltips turned on, make tips appear promptly - (setq tooltip-delay 0.1) ; default is one second - )) - -(You will note that instead of typing `(number-to-string 21)', I -decided to save typing and wrote `21' as a string, `"21"', rather than -convert it from an integer to a string. In this instance, this -expression is better than the longer, but more general -`(number-to-string 21)'. However, if you do not know ahead of time -what type of information will be returned, then the `number-to-string' -function will be needed.) - ----------- Footnotes ---------- - -(1) When I start instances of Emacs that do not load my `.emacs' file -or any site file, I also turn off blinking: - - emacs -q --no-site-file -eval '(blink-cursor-mode nil)' - -Or nowadays, using an even more sophisticated set of options, - - emacs -Q - D - - -File: eintr, Node: X11 Colors, Next: Miscellaneous, Prev: Simple Extension, Up: Emacs Initialization - -16.12 X11 Colors -================ - -You can specify colors when you use Emacs with the MIT X Windowing -system. - -I dislike the default colors and specify my own. - -Here are the expressions in my `.emacs' file that set values: - - ;; Set cursor color - (set-cursor-color "white") - - ;; Set mouse color - (set-mouse-color "white") - - ;; Set foreground and background - (set-foreground-color "white") - (set-background-color "darkblue") - - ;;; Set highlighting colors for isearch and drag - (set-face-foreground 'highlight "white") - (set-face-background 'highlight "blue") - - (set-face-foreground 'region "cyan") - (set-face-background 'region "blue") - - (set-face-foreground 'secondary-selection "skyblue") - (set-face-background 'secondary-selection "darkblue") - - ;; Set calendar highlighting colors - (setq calendar-load-hook - '(lambda () - (set-face-foreground 'diary-face "skyblue") - (set-face-background 'holiday-face "slate blue") - (set-face-foreground 'holiday-face "white"))) - -The various shades of blue soothe my eye and prevent me from seeing the -screen flicker. - -Alternatively, I could have set my specifications in various X -initialization files. For example, I could set the foreground, -background, cursor, and pointer (i.e., mouse) colors in my -`~/.Xresources' file like this: - - Emacs*foreground: white - Emacs*background: darkblue - Emacs*cursorColor: white - Emacs*pointerColor: white - -In any event, since it is not part of Emacs, I set the root color of my -X window in my `~/.xinitrc' file, like this(1): - - xsetroot -solid Navy -fg white & - ----------- Footnotes ---------- - -(1) I also run more modern window managers, such as Enlightenment, -Gnome, or KDE; in those cases, I often specify an image rather than a -plain color. - - -File: eintr, Node: Miscellaneous, Next: Mode Line, Prev: X11 Colors, Up: Emacs Initialization - -16.13 Miscellaneous Settings for a `.emacs' File -================================================ - -Here are a few miscellaneous settings: - - - Set the shape and color of the mouse cursor: - - ; Cursor shapes are defined in - ; `/usr/include/X11/cursorfont.h'; - ; for example, the `target' cursor is number 128; - ; the `top_left_arrow' cursor is number 132. - - (let ((mpointer (x-get-resource "*mpointer" - "*emacs*mpointer"))) - ;; If you have not set your mouse pointer - ;; then set it, otherwise leave as is: - (if (eq mpointer nil) - (setq mpointer "132")) ; top_left_arrow - (setq x-pointer-shape (string-to-int mpointer)) - (set-mouse-color "white")) - - - Or you can set the values of a variety of features in an alist, - like this: - - (setq-default - default-frame-alist - '((cursor-color . "white") - (mouse-color . "white") - (foreground-color . "white") - (background-color . "DodgerBlue4") - ;; (cursor-type . bar) - (cursor-type . box) - (tool-bar-lines . 0) - (menu-bar-lines . 1) - (width . 80) - (height . 58) - (font . - "-Misc-Fixed-Medium-R-Normal--20-200-75-75-C-100-ISO8859-1") - )) - - - Convert `<CTRL>-h' into <DEL> and <DEL> into `<CTRL>-h'. - (Some older keyboards needed this, although I have not seen the - problem recently.) - - ;; Translate `C-h' to <DEL>. - ; (keyboard-translate ?\C-h ?\C-?) - - ;; Translate <DEL> to `C-h'. - (keyboard-translate ?\C-? ?\C-h) - - - Turn off a blinking cursor! - - (if (fboundp 'blink-cursor-mode) - (blink-cursor-mode -1)) - - or start GNU Emacs with the command `emacs -nbc'. - - - Ignore case when using `grep' - `-n' Prefix each line of output with line number - `-i' Ignore case distinctions - `-e' Protect patterns beginning with a hyphen character, `-' - - (setq grep-command "grep -n -i -e ") - - - Find an existing buffer, even if it has a different name - This avoids problems with symbolic links. - - (setq find-file-existing-other-name t) - - - Set your language environment and default input method - - (set-language-environment "latin-1") - ;; Remember you can enable or disable multilingual text input - ;; with the `toggle-input-method'' (C-\) command - (setq default-input-method "latin-1-prefix") - - If you want to write with Chinese `GB' characters, set this - instead: - - (set-language-environment "Chinese-GB") - (setq default-input-method "chinese-tonepy") - -Fixing Unpleasant Key Bindings -.............................. - -Some systems bind keys unpleasantly. Sometimes, for example, the -<CTRL> key appears in an awkward spot rather than at the far left of -the home row. - -Usually, when people fix these sorts of keybindings, they do not change -their `~/.emacs' file. Instead, they bind the proper keys on their -consoles with the `loadkeys' or `install-keymap' commands in their boot -script and then include `xmodmap' commands in their `.xinitrc' or -`.Xsession' file for X Windows. - -For a boot script: - - loadkeys /usr/share/keymaps/i386/qwerty/emacs2.kmap.gz - -or - - install-keymap emacs2 - -For a `.xinitrc' or `.Xsession' file when the <Caps Lock> key is at the -far left of the home row: - - # Bind the key labeled `Caps Lock' to `Control' - # (Such a broken user interface suggests that keyboard manufacturers - # think that computers are typewriters from 1885.) - - xmodmap -e "clear Lock" - xmodmap -e "add Control = Caps_Lock" - -In a `.xinitrc' or `.Xsession' file, to convert an <ALT> key to a -<META> key: - - # Some ill designed keyboards have a key labeled ALT and no Meta - xmodmap -e "keysym Alt_L = Meta_L Alt_L" - - -File: eintr, Node: Mode Line, Prev: Miscellaneous, Up: Emacs Initialization - -16.14 A Modified Mode Line -========================== - -Finally, a feature I really like: a modified mode line. - -When I work over a network, I forget which machine I am using. Also, I -tend to I lose track of where I am, and which line point is on. - -So I reset my mode line to look like this: - - -:-- foo.texi rattlesnake:/home/bob/ Line 1 (Texinfo Fill) Top - -I am visiting a file called `foo.texi', on my machine `rattlesnake' in -my `/home/bob' buffer. I am on line 1, in Texinfo mode, and am at the -top of the buffer. - -My `.emacs' file has a section that looks like this: - - ;; Set a Mode Line that tells me which machine, which directory, - ;; and which line I am on, plus the other customary information. - (setq default-mode-line-format - (quote - (#("-" 0 1 - (help-echo - "mouse-1: select window, mouse-2: delete others ...")) - mode-line-mule-info - mode-line-modified - mode-line-frame-identification - " " - mode-line-buffer-identification - " " - (:eval (substring - (system-name) 0 (string-match "\\..+" (system-name)))) - ":" - default-directory - #(" " 0 1 - (help-echo - "mouse-1: select window, mouse-2: delete others ...")) - (line-number-mode " Line %l ") - global-mode-string - #(" %[(" 0 6 - (help-echo - "mouse-1: select window, mouse-2: delete others ...")) - (:eval (mode-line-mode-name)) - mode-line-process - minor-mode-alist - #("%n" 0 2 (help-echo "mouse-2: widen" local-map (keymap ...))) - ")%] " - (-3 . "%P") - ;; "-%-" - ))) - -Here, I redefine the default mode line. Most of the parts are from the -original; but I make a few changes. I set the _default_ mode line -format so as to permit various modes, such as Info, to override it. - -Many elements in the list are self-explanatory: `mode-line-modified' is -a variable that tells whether the buffer has been modified, `mode-name' -tells the name of the mode, and so on. However, the format looks -complicated because of two features we have not discussed. - -The first string in the mode line is a dash or hyphen, `-'. In the old -days, it would have been specified simply as `"-"'. But nowadays, -Emacs can add properties to a string, such as highlighting or, as in -this case, a help feature. If you place your mouse cursor over the -hyphen, some help information appears (By default, you must wait -seven-tenths of a second before the information appears. You can -change that timing by changing the value of `tooltip-delay'.) - -The new string format has a special syntax: - - #("-" 0 1 (help-echo "mouse-1: select window, ...")) - -The `#(' begins a list. The first element of the list is the string -itself, just one `-'. The second and third elements specify the range -over which the fourth element applies. A range starts _after_ a -character, so a zero means the range starts just before the first -character; a 1 means that the range ends just after the first -character. The third element is the property for the range. It -consists of a property list, a property name, in this case, -`help-echo', followed by a value, in this case, a string. The second, -third, and fourth elements of this new string format can be repeated. - -*Note Text Properties: (elisp)Text Properties, and see *Note Mode Line -Format: (elisp)Mode Line Format, for more information. - -`mode-line-buffer-identification' displays the current buffer name. It -is a list beginning `(#("%12b" 0 4 ...'. The `#(' begins the list. - -The `"%12b"' displays the current buffer name, using the `buffer-name' -function with which we are familiar; the `12' specifies the maximum -number of characters that will be displayed. When a name has fewer -characters, whitespace is added to fill out to this number. (Buffer -names can and often should be longer than 12 characters; this length -works well in a typical 80 column wide window.) - -`:eval' says to evaluate the following form and use the result as a -string to display. In this case, the expression displays the first -component of the full system name. The end of the first component is a -`.' (`period'), so I use the `string-match' function to tell me the -length of the first component. The substring from the zeroth character -to that length is the name of the machine. - -This is the expression: - - (:eval (substring - (system-name) 0 (string-match "\\..+" (system-name)))) - -`%[' and `%]' cause a pair of square brackets to appear for each -recursive editing level. `%n' says `Narrow' when narrowing is in -effect. `%P' tells you the percentage of the buffer that is above the -bottom of the window, or `Top', `Bottom', or `All'. (A lower case `p' -tell you the percentage above the _top_ of the window.) `%-' inserts -enough dashes to fill out the line. - -Remember, "You don't have to like Emacs to like it" -- your own Emacs -can have different colors, different commands, and different keys than -a default Emacs. - -On the other hand, if you want to bring up a plain `out of the box' -Emacs, with no customization, type: - - emacs -q - -This will start an Emacs that does _not_ load your `~/.emacs' -initialization file. A plain, default Emacs. Nothing more. - - -File: eintr, Node: Debugging, Next: Conclusion, Prev: Emacs Initialization, Up: Top - -17 Debugging -************ - -GNU Emacs has two debuggers, `debug' and `edebug'. The first is built -into the internals of Emacs and is always with you; the second requires -that you instrument a function before you can use it. - -Both debuggers are described extensively in *Note Debugging Lisp -Programs: (elisp)Debugging. In this chapter, I will walk through a -short example of each. - -* Menu: - -* debug:: -* debug-on-entry:: -* debug-on-quit:: -* edebug:: -* Debugging Exercises:: - - -File: eintr, Node: debug, Next: debug-on-entry, Prev: Debugging, Up: Debugging - -17.1 `debug' -============ - -Suppose you have written a function definition that is intended to -return the sum of the numbers 1 through a given number. (This is the -`triangle' function discussed earlier. *Note Example with Decrementing -Counter: Decrementing Example, for a discussion.) - -However, your function definition has a bug. You have mistyped `1=' -for `1-'. Here is the broken definition: - - (defun triangle-bugged (number) - "Return sum of numbers 1 through NUMBER inclusive." - (let ((total 0)) - (while (> number 0) - (setq total (+ total number)) - (setq number (1= number))) ; Error here. - total)) - -If you are reading this in Info, you can evaluate this definition in -the normal fashion. You will see `triangle-bugged' appear in the echo -area. - -Now evaluate the `triangle-bugged' function with an argument of 4: - - (triangle-bugged 4) - -In GNU Emacs version 21, you will create and enter a `*Backtrace*' -buffer that says: - - - ---------- Buffer: *Backtrace* ---------- - Debugger entered--Lisp error: (void-function 1=) - (1= number) - (setq number (1= number)) - (while (> number 0) (setq total (+ total number)) - (setq number (1= number))) - (let ((total 0)) (while (> number 0) (setq total ...) - (setq number ...)) total) - triangle-bugged(4) - eval((triangle-bugged 4)) - eval-last-sexp-1(nil) - eval-last-sexp(nil) - call-interactively(eval-last-sexp) - ---------- Buffer: *Backtrace* ---------- - -(I have reformatted this example slightly; the debugger does not fold -long lines. As usual, you can quit the debugger by typing `q' in the -`*Backtrace*' buffer.) - -In practice, for a bug as simple as this, the `Lisp error' line will -tell you what you need to know to correct the definition. The function -`1=' is `void'. - -However, suppose you are not quite certain what is going on? You can -read the complete backtrace. - -In this case, you need to run GNU Emacs 22, which automatically starts -the debugger that puts you in the `*Backtrace*' buffer; or else, you -need to start the debugger manually as described below. - -Read the `*Backtrace*' buffer from the bottom up; it tells you what -Emacs did that led to the error. Emacs made an interactive call to -`C-x C-e' (`eval-last-sexp'), which led to the evaluation of the -`triangle-bugged' expression. Each line above tells you what the Lisp -interpreter evaluated next. - -The third line from the top of the buffer is - - (setq number (1= number)) - -Emacs tried to evaluate this expression; in order to do so, it tried to -evaluate the inner expression shown on the second line from the top: - - (1= number) - -This is where the error occurred; as the top line says: - - Debugger entered--Lisp error: (void-function 1=) - -You can correct the mistake, re-evaluate the function definition, and -then run your test again. - - -File: eintr, Node: debug-on-entry, Next: debug-on-quit, Prev: debug, Up: Debugging - -17.2 `debug-on-entry' -===================== - -GNU Emacs 22 starts the debugger automatically when your function has -an error. - -Incidentally, you can start the debugger manually for all versions of -Emacs; the advantage is that the debugger runs even if you do not have -a bug in your code. Sometimes your code will be free of bugs! - -You can enter the debugger when you call the function by calling -`debug-on-entry'. - -Type: - - M-x debug-on-entry RET triangle-bugged RET - -Now, evaluate the following: - - (triangle-bugged 5) - -All versions of Emacs will create a `*Backtrace*' buffer and tell you -that it is beginning to evaluate the `triangle-bugged' function: - - ---------- Buffer: *Backtrace* ---------- - Debugger entered--entering a function: - * triangle-bugged(5) - eval((triangle-bugged 5)) - eval-last-sexp-1(nil) - eval-last-sexp(nil) - call-interactively(eval-last-sexp) - ---------- Buffer: *Backtrace* ---------- - -In the `*Backtrace*' buffer, type `d'. Emacs will evaluate the first -expression in `triangle-bugged'; the buffer will look like this: - - ---------- Buffer: *Backtrace* ---------- - Debugger entered--beginning evaluation of function call form: - * (let ((total 0)) (while (> number 0) (setq total ...) - (setq number ...)) total) - * triangle-bugged(5) - eval((triangle-bugged 5)) - eval-last-sexp-1(nil) - eval-last-sexp(nil) - call-interactively(eval-last-sexp) - ---------- Buffer: *Backtrace* ---------- - -Now, type `d' again, eight times, slowly. Each time you type `d', -Emacs will evaluate another expression in the function definition. - -Eventually, the buffer will look like this: - - ---------- Buffer: *Backtrace* ---------- - Debugger entered--beginning evaluation of function call form: - * (setq number (1= number)) - * (while (> number 0) (setq total (+ total number)) - (setq number (1= number))) - * (let ((total 0)) (while (> number 0) (setq total ...) - (setq number ...)) total) - * triangle-bugged(5) - eval((triangle-bugged 5)) - eval-last-sexp-1(nil) - eval-last-sexp(nil) - call-interactively(eval-last-sexp) - ---------- Buffer: *Backtrace* ---------- - -Finally, after you type `d' two more times, Emacs will reach the error, -and the top two lines of the `*Backtrace*' buffer will look like this: - - ---------- Buffer: *Backtrace* ---------- - Debugger entered--Lisp error: (void-function 1=) - * (1= number) - ... - ---------- Buffer: *Backtrace* ---------- - -By typing `d', you were able to step through the function. - -You can quit a `*Backtrace*' buffer by typing `q' in it; this quits the -trace, but does not cancel `debug-on-entry'. - -To cancel the effect of `debug-on-entry', call `cancel-debug-on-entry' -and the name of the function, like this: - - M-x cancel-debug-on-entry RET triangle-bugged RET - -(If you are reading this in Info, cancel `debug-on-entry' now.) - - -File: eintr, Node: debug-on-quit, Next: edebug, Prev: debug-on-entry, Up: Debugging - -17.3 `debug-on-quit' and `(debug)' -================================== - -In addition to setting `debug-on-error' or calling `debug-on-entry', -there are two other ways to start `debug'. - -You can start `debug' whenever you type `C-g' (`keyboard-quit') by -setting the variable `debug-on-quit' to `t'. This is useful for -debugging infinite loops. - -Or, you can insert a line that says `(debug)' into your code where you -want the debugger to start, like this: - - (defun triangle-bugged (number) - "Return sum of numbers 1 through NUMBER inclusive." - (let ((total 0)) - (while (> number 0) - (setq total (+ total number)) - (debug) ; Start debugger. - (setq number (1= number))) ; Error here. - total)) - -The `debug' function is described in detail in *Note The Lisp Debugger: -(elisp)Debugger. - - -File: eintr, Node: edebug, Next: Debugging Exercises, Prev: debug-on-quit, Up: Debugging - -17.4 The `edebug' Source Level Debugger -======================================= - -Edebug is a source level debugger. Edebug normally displays the source -of the code you are debugging, with an arrow at the left that shows -which line you are currently executing. - -You can walk through the execution of a function, line by line, or run -quickly until reaching a "breakpoint" where execution stops. - -Edebug is described in *Note Edebug: (elisp)edebug. - -Here is a bugged function definition for `triangle-recursively'. *Note -Recursion in place of a counter: Recursive triangle function, for a -review of it. - - (defun triangle-recursively-bugged (number) - "Return sum of numbers 1 through NUMBER inclusive. - Uses recursion." - (if (= number 1) - 1 - (+ number - (triangle-recursively-bugged - (1= number))))) ; Error here. - -Normally, you would install this definition by positioning your cursor -after the function's closing parenthesis and typing `C-x C-e' -(`eval-last-sexp') or else by positioning your cursor within the -definition and typing `C-M-x' (`eval-defun'). (By default, the -`eval-defun' command works only in Emacs Lisp mode or in Lisp -Interactive mode.) - -However, to prepare this function definition for Edebug, you must first -"instrument" the code using a different command. You can do this by -positioning your cursor within the definition and typing - - M-x edebug-defun RET - -This will cause Emacs to load Edebug automatically if it is not already -loaded, and properly instrument the function. - -After instrumenting the function, place your cursor after the following -expression and type `C-x C-e' (`eval-last-sexp'): - - (triangle-recursively-bugged 3) - -You will be jumped back to the source for `triangle-recursively-bugged' -and the cursor positioned at the beginning of the `if' line of the -function. Also, you will see an arrowhead at the left hand side of -that line. The arrowhead marks the line where the function is -executing. (In the following examples, we show the arrowhead with -`=>'; in a windowing system, you may see the arrowhead as a solid -triangle in the window `fringe'.) - - =>-!-(if (= number 1) - -In the example, the location of point is displayed as `-!-' (in a -printed book, it is displayed with a five pointed star). - -If you now press <SPC>, point will move to the next expression to be -executed; the line will look like this: - - =>(if -!-(= number 1) - -As you continue to press <SPC>, point will move from expression to -expression. At the same time, whenever an expression returns a value, -that value will be displayed in the echo area. For example, after you -move point past `number', you will see the following: - - Result: 3 (#o3, #x3, ?\C-c) - -This means the value of `number' is 3, which is octal three, -hexadecimal three, and ASCII `control-c' (the third letter of the -alphabet, in case you need to know this information). - -You can continue moving through the code until you reach the line with -the error. Before evaluation, that line looks like this: - - => -!-(1= number))))) ; Error here. - -When you press <SPC> once again, you will produce an error message that -says: - - Symbol's function definition is void: 1= - -This is the bug. - -Press `q' to quit Edebug. - -To remove instrumentation from a function definition, simply -re-evaluate it with a command that does not instrument it. For -example, you could place your cursor after the definition's closing -parenthesis and type `C-x C-e'. - -Edebug does a great deal more than walk with you through a function. -You can set it so it races through on its own, stopping only at an -error or at specified stopping points; you can cause it to display the -changing values of various expressions; you can find out how many times -a function is called, and more. - -Edebug is described in *Note Edebug: (elisp)edebug. - - -File: eintr, Node: Debugging Exercises, Prev: edebug, Up: Debugging - -17.5 Debugging Exercises -======================== - - * Install the `count-words-region' function and then cause it to - enter the built-in debugger when you call it. Run the command on a - region containing two words. You will need to press `d' a - remarkable number of times. On your system, is a `hook' called - after the command finishes? (For information on hooks, see *Note - Command Loop Overview: (elisp)Command Overview.) - - * Copy `count-words-region' into the `*scratch*' buffer, instrument - the function for Edebug, and walk through its execution. The - function does not need to have a bug, although you can introduce - one if you wish. If the function lacks a bug, the walk-through - completes without problems. - - * While running Edebug, type `?' to see a list of all the Edebug - commands. (The `global-edebug-prefix' is usually `C-x X', i.e. - `<CTRL>-x' followed by an upper case `X'; use this prefix for - commands made outside of the Edebug debugging buffer.) - - * In the Edebug debugging buffer, use the `p' - (`edebug-bounce-point') command to see where in the region the - `count-words-region' is working. - - * Move point to some spot further down the function and then type the - `h' (`edebug-goto-here') command to jump to that location. - - * Use the `t' (`edebug-trace-mode') command to cause Edebug to walk - through the function on its own; use an upper case `T' for - `edebug-Trace-fast-mode'. - - * Set a breakpoint, then run Edebug in Trace mode until it reaches - the stopping point. - - -File: eintr, Node: Conclusion, Next: the-the, Prev: Debugging, Up: Top - -18 Conclusion -************* - -We have now reached the end of this Introduction. You have now learned -enough about programming in Emacs Lisp to set values, to write simple -`.emacs' files for yourself and your friends, and write simple -customizations and extensions to Emacs. - -This is a place to stop. Or, if you wish, you can now go onward, and -teach yourself. - -You have learned some of the basic nuts and bolts of programming. But -only some. There are a great many more brackets and hinges that are -easy to use that we have not touched. - -A path you can follow right now lies among the sources to GNU Emacs and -in *Note The GNU Emacs Lisp Reference Manual: (elisp)Top. - -The Emacs Lisp sources are an adventure. When you read the sources and -come across a function or expression that is unfamiliar, you need to -figure out or find out what it does. - -Go to the Reference Manual. It is a thorough, complete, and fairly -easy-to-read description of Emacs Lisp. It is written not only for -experts, but for people who know what you know. (The `Reference -Manual' comes with the standard GNU Emacs distribution. Like this -introduction, it comes as a Texinfo source file, so you can read it -on-line and as a typeset, printed book.) - -Go to the other on-line help that is part of GNU Emacs: the on-line -documentation for all functions and variables, and `find-tags', the -program that takes you to sources. - -Here is an example of how I explore the sources. Because of its name, -`simple.el' is the file I looked at first, a long time ago. As it -happens some of the functions in `simple.el' are complicated, or at -least look complicated at first sight. The `open-line' function, for -example, looks complicated. - -You may want to walk through this function slowly, as we did with the -`forward-sentence' function. (*Note The `forward-sentence' function: -forward-sentence.) Or you may want to skip that function and look at -another, such as `split-line'. You don't need to read all the -functions. According to `count-words-in-defun', the `split-line' -function contains 102 words and symbols. - -Even though it is short, `split-line' contains expressions we have not -studied: `skip-chars-forward', `indent-to', `current-column' and -`insert-and-inherit'. - -Consider the `skip-chars-forward' function. (It is part of the -function definition for `back-to-indentation', which is shown in *Note -Review: Review.) - -In GNU Emacs, you can find out more about `skip-chars-forward' by -typing `C-h f' (`describe-function') and the name of the function. -This gives you the function documentation. - -You may be able to guess what is done by a well named function such as -`indent-to'; or you can look it up, too. Incidentally, the -`describe-function' function itself is in `help.el'; it is one of those -long, but decipherable functions. You can look up `describe-function' -using the `C-h f' command! - -In this instance, since the code is Lisp, the `*Help*' buffer contains -the name of the library containing the function's source. You can put -point over the name of the library and press the RET key, which in this -situation is bound to `help-follow', and be taken directly to the -source, in the same way as `M-.' (`find-tag'). - -The definition for `describe-function' illustrates how to customize the -`interactive' expression without using the standard character codes; -and it shows how to create a temporary buffer. - -(The `indent-to' function is written in C rather than Emacs Lisp; it is -a `built-in' function. `help-follow' takes you to its source as does -`find-tag', when properly set up.) - -You can look at a function's source using `find-tag', which is bound to -`M-.' Finally, you can find out what the Reference Manual has to say -by visiting the manual in Info, and typing `i' (`Info-index') and the -name of the function, or by looking up the function in the index to a -printed copy of the manual. - -Similarly, you can find out what is meant by `insert-and-inherit'. - -Other interesting source files include `paragraphs.el', `loaddefs.el', -and `loadup.el'. The `paragraphs.el' file includes short, easily -understood functions as well as longer ones. The `loaddefs.el' file -contains the many standard autoloads and many keymaps. I have never -looked at it all; only at parts. `loadup.el' is the file that loads -the standard parts of Emacs; it tells you a great deal about how Emacs -is built. (*Note Building Emacs: (elisp)Building Emacs, for more about -building.) - -As I said, you have learned some nuts and bolts; however, and very -importantly, we have hardly touched major aspects of programming; I -have said nothing about how to sort information, except to use the -predefined `sort' function; I have said nothing about how to store -information, except to use variables and lists; I have said nothing -about how to write programs that write programs. These are topics for -another, and different kind of book, a different kind of learning. - -What you have done is learn enough for much practical work with GNU -Emacs. What you have done is get started. This is the end of a -beginning. - - -File: eintr, Node: the-the, Next: Kill Ring, Prev: Conclusion, Up: Top - -Appendix A The `the-the' Function -********************************* - -Sometimes when you you write text, you duplicate words--as with "you -you" near the beginning of this sentence. I find that most frequently, -I duplicate "the"; hence, I call the function for detecting duplicated -words, `the-the'. - -As a first step, you could use the following regular expression to -search for duplicates: - - \\(\\w+[ \t\n]+\\)\\1 - -This regexp matches one or more word-constituent characters followed by -one or more spaces, tabs, or newlines. However, it does not detect -duplicated words on different lines, since the ending of the first -word, the end of the line, is different from the ending of the second -word, a space. (For more information about regular expressions, see -*Note Regular Expression Searches: Regexp Search, as well as *Note -Syntax of Regular Expressions: (emacs)Regexps, and *Note Regular -Expressions: (elisp)Regular Expressions.) - -You might try searching just for duplicated word-constituent characters -but that does not work since the pattern detects doubles such as the -two occurrences of `th' in `with the'. - -Another possible regexp searches for word-constituent characters -followed by non-word-constituent characters, reduplicated. Here, -`\\w+' matches one or more word-constituent characters and `\\W*' -matches zero or more non-word-constituent characters. - - \\(\\(\\w+\\)\\W*\\)\\1 - -Again, not useful. - -Here is the pattern that I use. It is not perfect, but good enough. -`\\b' matches the empty string, provided it is at the beginning or end -of a word; `[^@ \n\t]+' matches one or more occurrences of any -characters that are _not_ an @-sign, space, newline, or tab. - - \\b\\([^@ \n\t]+\\)[ \n\t]+\\1\\b - -One can write more complicated expressions, but I found that this -expression is good enough, so I use it. - -Here is the `the-the' function, as I include it in my `.emacs' file, -along with a handy global key binding: - - (defun the-the () - "Search forward for for a duplicated word." - (interactive) - (message "Searching for for duplicated words ...") - (push-mark) - ;; This regexp is not perfect - ;; but is fairly good over all: - (if (re-search-forward - "\\b\\([^@ \n\t]+\\)[ \n\t]+\\1\\b" nil 'move) - (message "Found duplicated word.") - (message "End of buffer"))) - - ;; Bind `the-the' to C-c \ - (global-set-key "\C-c\\" 'the-the) - - -Here is test text: - - one two two three four five - five six seven - -You can substitute the other regular expressions shown above in the -function definition and try each of them on this list. - - -File: eintr, Node: Kill Ring, Next: Full Graph, Prev: the-the, Up: Top - -Appendix B Handling the Kill Ring -********************************* - -The kill ring is a list that is transformed into a ring by the workings -of the `current-kill' function. The `yank' and `yank-pop' commands use -the `current-kill' function. - -This appendix describes the `current-kill' function as well as both the -`yank' and the `yank-pop' commands, but first, consider the workings of -the kill ring. - -The kill ring has a default maximum length of sixty items; this number -is too large for an explanation. Instead, set it to four. Please -evaluate the following: - - (setq old-kill-ring-max kill-ring-max) - (setq kill-ring-max 4) - -Then, please copy each line of the following indented example into the -kill ring. You may kill each line with `C-k' or mark it and copy it -with `M-w'. - -(In a read-only buffer, such as the `*info*' buffer, the kill command, -`C-k' (`kill-line'), will not remove the text, merely copy it to the -kill ring. However, your machine may beep at you. (`kill-line' calls -`kill-region'.) Alternatively, for silence, you may copy the region of -each line with the `M-w' (`kill-ring-save') command. You must mark -each line for this command to succeed, but it does not matter at which -end you put point or mark.) - -Please invoke the calls in order, so that five elements attempt to fill -the kill ring: - - first some text - second piece of text - third line - fourth line of text - fifth bit of text - -Then find the value of `kill-ring' by evaluating - - kill-ring - -It is: - - ("fifth bit of text" "fourth line of text" - "third line" "second piece of text") - -The first element, `first some text', was dropped. - -To return to the old value for the length of the kill ring, evaluate: - - (setq kill-ring-max old-kill-ring-max) - -* Menu: - -* current-kill:: -* yank:: -* yank-pop:: -* ring file:: - diff --git a/info/eintr-3 b/info/eintr-3 deleted file mode 100644 index c172f95a389..00000000000 --- a/info/eintr-3 +++ /dev/null @@ -1,2798 +0,0 @@ -This is ../info/eintr, produced by makeinfo version 4.8 from -emacs-lisp-intro.texi. - -INFO-DIR-SECTION Emacs -START-INFO-DIR-ENTRY -* Emacs Lisp Intro: (eintr). - A simple introduction to Emacs Lisp programming. -END-INFO-DIR-ENTRY - -This is an `Introduction to Programming in Emacs Lisp', for people who -are not programmers. - -Edition 3.01, 2006 Oct 31 - -Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1997, 2001, 2002, -2003, 2004, 2005, 2006 Free Software Foundation, Inc. - -Published by the: - - GNU Press, Website: http://www.gnupress.org - a division of the General: press@gnu.org - Free Software Foundation, Inc. Orders: sales@gnu.org - 51 Franklin Street, Fifth Floor Tel: +1 (617) 542-5942 - Boston, MA 02110-1301 USA Fax: +1 (617) 542-2652 - - -ISBN 1-882114-43-4 - -Permission is granted to copy, distribute and/or modify this document -under the terms of the GNU Free Documentation License, Version 1.2 or -any later version published by the Free Software Foundation; there -being no Invariant Section, with the Front-Cover Texts being "A GNU -Manual", and with the Back-Cover Texts as in (a) below. A copy of the -license is included in the section entitled "GNU Free Documentation -License". - -(a) The FSF's Back-Cover Text is: "You have freedom to copy and modify -this GNU Manual, like GNU software. Copies published by the Free -Software Foundation raise funds for GNU development." - - -File: eintr, Node: current-kill, Next: yank, Prev: Kill Ring, Up: Kill Ring - -B.1 The `current-kill' Function -=============================== - -The `current-kill' function changes the element in the kill ring to -which `kill-ring-yank-pointer' points. (Also, the `kill-new' function -sets `kill-ring-yank-pointer' to point to the latest element of the the -kill ring.) - -The `current-kill' function is used by `yank' and by `yank-pop'. Here -is the code for `current-kill': - - (defun current-kill (n &optional do-not-move) - "Rotate the yanking point by N places, and then return that kill. - If N is zero, `interprogram-paste-function' is set, and calling it - returns a string, then that string is added to the front of the - kill ring and returned as the latest kill. - If optional arg DO-NOT-MOVE is non-nil, then don't actually move the - yanking point; just return the Nth kill forward." - (let ((interprogram-paste (and (= n 0) - interprogram-paste-function - (funcall interprogram-paste-function)))) - (if interprogram-paste - (progn - ;; Disable the interprogram cut function when we add the new - ;; text to the kill ring, so Emacs doesn't try to own the - ;; selection, with identical text. - (let ((interprogram-cut-function nil)) - (kill-new interprogram-paste)) - interprogram-paste) - (or kill-ring (error "Kill ring is empty")) - (let ((ARGth-kill-element - (nthcdr (mod (- n (length kill-ring-yank-pointer)) - (length kill-ring)) - kill-ring))) - (or do-not-move - (setq kill-ring-yank-pointer ARGth-kill-element)) - (car ARGth-kill-element))))) - -In addition, the `kill-new' function sets `kill-ring-yank-pointer' to -the latest element of the the kill ring. And indirectly so does -`kill-append', since it calls `kill-new'. In addition, `kill-region' -and `kill-line' call the `kill-new' function. - -Here is the line in `kill-new', which is explained in *Note The -`kill-new' function: kill-new function. - - (setq kill-ring-yank-pointer kill-ring) - -* Menu: - -* Understanding current-kill:: - - -File: eintr, Node: Understanding current-kill, Prev: current-kill, Up: current-kill - -`current-kill' in Outline -------------------------- - -The `current-kill' function looks complex, but as usual, it can be -understood by taking it apart piece by piece. First look at it in -skeletal form: - - (defun current-kill (n &optional do-not-move) - "Rotate the yanking point by N places, and then return that kill. - (let VARLIST - BODY...) - -This function takes two arguments, one of which is optional. It has a -documentation string. It is _not_ interactive. - -The body of the function definition is a `let' expression, which itself -has a body as well as a VARLIST. - -The `let' expression declares a variable that will be only usable -within the bounds of this function. This variable is called -`interprogram-paste' and is for copying to another program. It is not -for copying within this instance of GNU Emacs. Most window systems -provide a facility for interprogram pasting. Sadly, that facility -usually provides only for the lasted element. Most windowing systems -have not adopted a ring of many possibilities, even though Emacs has -provided it for decades. - -The `if' expression has two parts, one if there exists -`interprogram-paste' and one if not. - -Let us consider the `if not' or else-part of the `current-kill' -function. (The then-part uses the the `kill-new' function, which we -have already described. (*Note The `kill-new' function: kill-new -function.) - - (or kill-ring (error "Kill ring is empty")) - (let ((ARGth-kill-element - (nthcdr (mod (- n (length kill-ring-yank-pointer)) - (length kill-ring)) - kill-ring))) - (or do-not-move - (setq kill-ring-yank-pointer ARGth-kill-element)) - (car ARGth-kill-element)) - -The code first checks whether the kill ring has content; otherwise it -signals an error. - -Note that the `or' expression is very similar to writing - - (if (zerop (length kill-ring)) ; if-part - (error "Kill ring is empty")) ; then-part - ;; No else-part - -If there is not anything in the kill ring, its length must be zero and -an error message sent to the user: `Kill ring is empty'. The -`current-kill' function uses an `or' expression which is simpler. But -an `if' expression reminds us what goes on. - -This `if' expression uses the function `zerop' which returns true if -the value it is testing is zero. When `zerop' tests true, the -then-part of the `if' is evaluated. The then-part is a list starting -with the function `error', which is a function that is similar to the -`message' function (*note The `message' Function: message.), in that it -prints a one-line message in the echo area. However, in addition to -printing a message, `error' also stops evaluation of the function -within which it is embedded. This means that the rest of the function -will not be evaluated if the length of the kill ring is zero. - -Then the `current-kill' function selects the element to return. The -selection depends on the number of places that `current-kill' rotates -and on where `kill-ring-yank-pointer' points. - -Next, either the optional `do-not-move' argument is true or the current -value of `kill-ring-yank-pointer' is set to point to the list, the -first element of which is returned even if the `do-not-move' argument -is true. - -* Menu: - -* Digression concerning error:: -* Determining the Element :: - - -File: eintr, Node: Digression concerning error, Next: Determining the Element, Prev: Understanding current-kill, Up: Understanding current-kill - -Digression about the word `error' -................................. - -In my opinion, it is slightly misleading, at least to humans, to use -the term `error' as the name of the `error' function. A better term -would be `cancel'. Strictly speaking, of course, you cannot point to, -much less rotate a pointer to a list that has no length, so from the -point of view of the computer, the word `error' is correct. But a -human expects to attempt this sort of thing, if only to find out -whether the kill ring is full or empty. This is an act of exploration. - -From the human point of view, the act of exploration and discovery is -not necessarily an error, and therefore should not be labelled as one, -even in the bowels of a computer. As it is, the code in Emacs implies -that a human who is acting virtuously, by exploring his or her -environment, is making an error. This is bad. Even though the computer -takes the same steps as it does when there is an `error', a term such as -`cancel' would have a clearer connotation. - - -File: eintr, Node: Determining the Element, Prev: Digression concerning error, Up: Understanding current-kill - -Determining the Element -....................... - -Among other actions, the else-part of the `if' expression sets the -value of `kill-ring-yank-pointer' to `ARGth-kill-element' when the kill -ring has something in it and the value of `do-not-move' is `nil'. - -The code looks like this: - - (nthcdr (mod (- n (length kill-ring-yank-pointer)) - (length kill-ring)) - kill-ring))) - -This needs some examination. Unless it is not supposed to move the -pointer, the `current-kill' function changes where -`kill-ring-yank-pointer' points. That is what the -`(setq kill-ring-yank-pointer ARGth-kill-element))' expression does. -Also, clearly, `ARGth-kill-element' is being set to be equal to some -CDR of the kill ring, using the `nthcdr' function that is described in -an earlier section. (*Note copy-region-as-kill::.) How does it do -this? - -As we have seen before (*note nthcdr::), the `nthcdr' function works by -repeatedly taking the CDR of a list--it takes the CDR of the CDR of the -CDR ... - -The two following expressions produce the same result: - - (setq kill-ring-yank-pointer (cdr kill-ring)) - - (setq kill-ring-yank-pointer (nthcdr 1 kill-ring)) - -However, the `nthcdr' expression is more complicated. It uses the -`mod' function to determine which CDR to select. - -(You will remember to look at inner functions first; indeed, we will -have to go inside the `mod'.) - -The `mod' function returns the value of its first argument modulo the -second; that is to say, it returns the remainder after dividing the -first argument by the second. The value returned has the same sign as -the second argument. - -Thus, - - (mod 12 4) - => 0 ;; because there is no remainder - (mod 13 4) - => 1 - -In this case, the first argument is often smaller than the second. -That is fine. - - (mod 0 4) - => 0 - (mod 1 4) - => 1 - -We can guess what the `-' function does. It is like `+' but subtracts -instead of adds; the `-' function subtracts its second argument from -its first. Also, we already know what the `length' function does -(*note length::). It returns the length of a list. - -And `n' is the name of the required argument to the `current-kill' -function. - -So when the first argument to `nthcdr' is zero, the `nthcdr' expression -returns the whole list, as you can see by evaluating the following: - - ;; kill-ring-yank-pointer and kill-ring have a length of four - (nthcdr (mod (- 0 4) 4) ; (mod -4 4) => 0 - '("fourth line of text" - "third line" - "second piece of text" - "first some text")) - -When the first argument to the `current-kill' function is one, the -`nthcdr' expression returns the list without its first element. - - (nthcdr (mod (- 1 4) 4) - '("fourth line of text" - "third line" - "second piece of text" - "first some text")) - -Incidentally, both `kill-ring' and `kill-ring-yank-pointer' are "global -variables". That means that any expression in Emacs Lisp can access -them. They are not like the local variables set by `let' or like the -symbols in an argument list. Local variables can only be accessed -within the `let' that defines them or the function that specifies them -in an argument list (and within expressions called by them). - - -File: eintr, Node: yank, Next: yank-pop, Prev: current-kill, Up: Kill Ring - -B.2 `yank' -========== - -After learning about `current-kill', the code for the `yank' function -is almost easy. It has only one tricky part, which is the computation -of the argument to be passed to `rotate-yank-pointer'. - -The code looks like this: - - (defun yank (&optional arg) - "Reinsert (\"paste\") the last stretch of killed text. - More precisely, reinsert the stretch of killed text most recently - killed OR yanked. Put point at end, and set mark at beginning. - With just \\[universal-argument] as argument, same but put point at - beginning (and mark at end). With argument N, reinsert the Nth most - recently killed stretch of killed text. - - When this command inserts killed text into the buffer, it honors - `yank-excluded-properties' and `yank-handler' as described in the - doc string for `insert-for-yank-1', which see. - - See also the command \\[yank-pop]." - (interactive "*P") - (setq yank-window-start (window-start)) - ;; If we don't get all the way thru, make last-command indicate that - ;; for the following command. - (setq this-command t) - (push-mark (point)) - (insert-for-yank (current-kill (cond - ((listp arg) 0) - ((eq arg '-) -2) - (t (1- arg))))) - (if (consp arg) - ;; This is like exchange-point-and-mark, - ;; but doesn't activate the mark. - ;; It is cleaner to avoid activation, even though the command - ;; loop would deactivate the mark because we inserted text. - (goto-char (prog1 (mark t) - (set-marker (mark-marker) (point) (current-buffer))))) - ;; If we do get all the way thru, make this-command indicate that. - (if (eq this-command t) - (setq this-command 'yank)) - nil) - -The key expression is `insert-for-yank', which inserts the string -returned by `current-kill', but removes some text properties from it. - -However, before getting to that expression, the function set the value -of `yank-window-start' to the position returned by the `(window-start)' -expression, the position at which the display currently starts. It -also set `this-command' and pushed the mark. - -After it yanks the appropriate element, if the optional argument is a -CONS rather than a number or nothing, put point at beginning of the -yanked text and mark at its end. (The `prog1' function is like `progn' -but returns the value of its first argument rather than the value of -its last argument. Its first argument is forced to return the buffer's -mark as an integer. You can see the documentation for these functions -by placing point over them in this buffer and then typing `C-h f' -(`describe-function') followed by a `RET'; the default is the function.) - -The last part of the function tells what to do when it succeeds. - - -File: eintr, Node: yank-pop, Next: ring file, Prev: yank, Up: Kill Ring - -B.3 `yank-pop' -============== - -After understanding `yank' and `current-kill', you know how to approach -the `yank-pop' function Leaving out the documentation to save space, it -looks like this: - - (defun yank-pop (&optional arg) - "..." - (interactive "*p") - (if (not (eq last-command 'yank)) - (error "Previous command was not a yank")) - (setq this-command 'yank) - (unless arg (setq arg 1)) - (let ((inhibit-read-only t) - (before (< (point) (mark t)))) - (if before - (funcall (or yank-undo-function 'delete-region) (point) (mark t)) - (funcall (or yank-undo-function 'delete-region) (mark t) (point))) - (setq yank-undo-function nil) - (set-marker (mark-marker) (point) (current-buffer)) - (insert-for-yank (current-kill arg)) - ;; Set the window start back where it was in the yank command, - ;; if possible. - (set-window-start (selected-window) yank-window-start t) - (if before - ;; This is like exchange-point-and-mark, - ;; but doesn't activate the mark. - ;; It is cleaner to avoid activation, even though the command - ;; loop would deactivate the mark because we inserted text. - (goto-char (prog1 (mark t) - (set-marker (mark-marker) - (point) - (current-buffer)))))) - nil) - -The function is interactive with a small `p' so the prefix argument is -processed and passed to the function. The command can only be used -after a previous yank; otherwise an error message is sent. This check -uses the variable `last-command' which is set by `yank' and is -discussed elsewhere. (*Note copy-region-as-kill::.) - -The `let' clause sets the variable `before' to true or false depending -whether point is before or after mark and then the region between point -and mark is deleted. This is the region that was just inserted by the -previous yank and it is this text that will be replaced. - -`funcall' calls its first argument as a function, passing remaining -arguments to it. The first argument is whatever the `or' expression -returns. The two remaining arguments are the positions of point and -mark set by the preceding `yank' command. - -There is more, but that is the hardest part. - - -File: eintr, Node: ring file, Prev: yank-pop, Up: Kill Ring - -B.4 The `ring.el' File -====================== - -Interestingly, GNU Emacs posses a file called `ring.el' that provides -many of the features we just discussed. But functions such as -`kill-ring-yank-pointer' do not use this library, possibly because they -were written earlier. - - -File: eintr, Node: Full Graph, Next: Free Software and Free Manuals, Prev: Kill Ring, Up: Top - -Appendix C A Graph with Labelled Axes -************************************* - -Printed axes help you understand a graph. They convey scale. In an -earlier chapter (*note Readying a Graph: Readying a Graph.), we wrote -the code to print the body of a graph. Here we write the code for -printing and labelling vertical and horizontal axes, along with the -body itself. - -* Menu: - -* Labelled Example:: -* print-graph Varlist:: -* print-Y-axis:: -* print-X-axis:: -* Print Whole Graph:: - - -File: eintr, Node: Labelled Example, Next: print-graph Varlist, Prev: Full Graph, Up: Full Graph - -Labelled Example Graph -====================== - -Since insertions fill a buffer to the right and below point, the new -graph printing function should first print the Y or vertical axis, then -the body of the graph, and finally the X or horizontal axis. This -sequence lays out for us the contents of the function: - - 1. Set up code. - - 2. Print Y axis. - - 3. Print body of graph. - - 4. Print X axis. - -Here is an example of how a finished graph should look: - - 10 - - * - * * - * ** - * *** - 5 - * ******* - * *** ******* - ************* - *************** - 1 - **************** - | | | | - 1 5 10 15 - -In this graph, both the vertical and the horizontal axes are labelled -with numbers. However, in some graphs, the horizontal axis is time and -would be better labelled with months, like this: - - 5 - * - * ** * - ******* - ********** ** - 1 - ************** - | ^ | - Jan June Jan - -Indeed, with a little thought, we can easily come up with a variety of -vertical and horizontal labelling schemes. Our task could become -complicated. But complications breed confusion. Rather than permit -this, it is better choose a simple labelling scheme for our first -effort, and to modify or replace it later. - -These considerations suggest the following outline for the -`print-graph' function: - - (defun print-graph (numbers-list) - "DOCUMENTATION..." - (let ((height ... - ...)) - (print-Y-axis height ... ) - (graph-body-print numbers-list) - (print-X-axis ... ))) - -We can work on each part of the `print-graph' function definition in -turn. - - -File: eintr, Node: print-graph Varlist, Next: print-Y-axis, Prev: Labelled Example, Up: Full Graph - -C.1 The `print-graph' Varlist -============================= - -In writing the `print-graph' function, the first task is to write the -varlist in the `let' expression. (We will leave aside for the moment -any thoughts about making the function interactive or about the -contents of its documentation string.) - -The varlist should set several values. Clearly, the top of the label -for the vertical axis must be at least the height of the graph, which -means that we must obtain this information here. Note that the -`print-graph-body' function also requires this information. There is -no reason to calculate the height of the graph in two different places, -so we should change `print-graph-body' from the way we defined it -earlier to take advantage of the calculation. - -Similarly, both the function for printing the X axis labels and the -`print-graph-body' function need to learn the value of the width of -each symbol. We can perform the calculation here and change the -definition for `print-graph-body' from the way we defined it in the -previous chapter. - -The length of the label for the horizontal axis must be at least as long -as the graph. However, this information is used only in the function -that prints the horizontal axis, so it does not need to be calculated -here. - -These thoughts lead us directly to the following form for the varlist -in the `let' for `print-graph': - - (let ((height (apply 'max numbers-list)) ; First version. - (symbol-width (length graph-blank))) - -As we shall see, this expression is not quite right. - - -File: eintr, Node: print-Y-axis, Next: print-X-axis, Prev: print-graph Varlist, Up: Full Graph - -C.2 The `print-Y-axis' Function -=============================== - -The job of the `print-Y-axis' function is to print a label for the -vertical axis that looks like this: - - 10 - - - - - - 5 - - - - - 1 - - -The function should be passed the height of the graph, and then should -construct and insert the appropriate numbers and marks. - -It is easy enough to see in the figure what the Y axis label should -look like; but to say in words, and then to write a function definition -to do the job is another matter. It is not quite true to say that we -want a number and a tic every five lines: there are only three lines -between the `1' and the `5' (lines 2, 3, and 4), but four lines between -the `5' and the `10' (lines 6, 7, 8, and 9). It is better to say that -we want a number and a tic mark on the base line (number 1) and then -that we want a number and a tic on the fifth line from the bottom and -on every line that is a multiple of five. - -* Menu: - -* Height of label:: -* Compute a Remainder:: -* Y Axis Element:: -* Y-axis-column:: -* print-Y-axis Penultimate:: - - -File: eintr, Node: Height of label, Next: Compute a Remainder, Prev: print-Y-axis, Up: print-Y-axis - -What height should the label be? --------------------------------- - -The next issue is what height the label should be? Suppose the maximum -height of tallest column of the graph is seven. Should the highest -label on the Y axis be `5 -', and should the graph stick up above the -label? Or should the highest label be `7 -', and mark the peak of the -graph? Or should the highest label be `10 -', which is a multiple of -five, and be higher than the topmost value of the graph? - -The latter form is preferred. Most graphs are drawn within rectangles -whose sides are an integral number of steps long--5, 10, 15, and so on -for a step distance of five. But as soon as we decide to use a step -height for the vertical axis, we discover that the simple expression in -the varlist for computing the height is wrong. The expression is -`(apply 'max numbers-list)'. This returns the precise height, not the -maximum height plus whatever is necessary to round up to the nearest -multiple of five. A more complex expression is required. - -As usual in cases like this, a complex problem becomes simpler if it is -divided into several smaller problems. - -First, consider the case when the highest value of the graph is an -integral multiple of five--when it is 5, 10, 15, or some higher -multiple of five. We can use this value as the Y axis height. - -A fairly simply way to determine whether a number is a multiple of five -is to divide it by five and see if the division results in a remainder. -If there is no remainder, the number is a multiple of five. Thus, -seven divided by five has a remainder of two, and seven is not an -integral multiple of five. Put in slightly different language, more -reminiscent of the classroom, five goes into seven once, with a -remainder of two. However, five goes into ten twice, with no -remainder: ten is an integral multiple of five. - - -File: eintr, Node: Compute a Remainder, Next: Y Axis Element, Prev: Height of label, Up: print-Y-axis - -C.2.1 Side Trip: Compute a Remainder ------------------------------------- - -In Lisp, the function for computing a remainder is `%'. The function -returns the remainder of its first argument divided by its second -argument. As it happens, `%' is a function in Emacs Lisp that you -cannot discover using `apropos': you find nothing if you type `M-x -apropos <RET> remainder <RET>'. The only way to learn of the existence -of `%' is to read about it in a book such as this or in the Emacs Lisp -sources. - -You can try the `%' function by evaluating the following two -expressions: - - (% 7 5) - - (% 10 5) - -The first expression returns 2 and the second expression returns 0. - -To test whether the returned value is zero or some other number, we can -use the `zerop' function. This function returns `t' if its argument, -which must be a number, is zero. - - (zerop (% 7 5)) - => nil - - (zerop (% 10 5)) - => t - -Thus, the following expression will return `t' if the height of the -graph is evenly divisible by five: - - (zerop (% height 5)) - -(The value of `height', of course, can be found from `(apply 'max -numbers-list)'.) - -On the other hand, if the value of `height' is not a multiple of five, -we want to reset the value to the next higher multiple of five. This -is straightforward arithmetic using functions with which we are already -familiar. First, we divide the value of `height' by five to determine -how many times five goes into the number. Thus, five goes into twelve -twice. If we add one to this quotient and multiply by five, we will -obtain the value of the next multiple of five that is larger than the -height. Five goes into twelve twice. Add one to two, and multiply by -five; the result is fifteen, which is the next multiple of five that is -higher than twelve. The Lisp expression for this is: - - (* (1+ (/ height 5)) 5) - -For example, if you evaluate the following, the result is 15: - - (* (1+ (/ 12 5)) 5) - -All through this discussion, we have been using `five' as the value for -spacing labels on the Y axis; but we may want to use some other value. -For generality, we should replace `five' with a variable to which we -can assign a value. The best name I can think of for this variable is -`Y-axis-label-spacing'. - -Using this term, and an `if' expression, we produce the following: - - (if (zerop (% height Y-axis-label-spacing)) - height - ;; else - (* (1+ (/ height Y-axis-label-spacing)) - Y-axis-label-spacing)) - -This expression returns the value of `height' itself if the height is -an even multiple of the value of the `Y-axis-label-spacing' or else it -computes and returns a value of `height' that is equal to the next -higher multiple of the value of the `Y-axis-label-spacing'. - -We can now include this expression in the `let' expression of the -`print-graph' function (after first setting the value of -`Y-axis-label-spacing'): - - (defvar Y-axis-label-spacing 5 - "Number of lines from one Y axis label to next.") - - ... - (let* ((height (apply 'max numbers-list)) - (height-of-top-line - (if (zerop (% height Y-axis-label-spacing)) - height - ;; else - (* (1+ (/ height Y-axis-label-spacing)) - Y-axis-label-spacing))) - (symbol-width (length graph-blank)))) - ... - -(Note use of the `let*' function: the initial value of height is -computed once by the `(apply 'max numbers-list)' expression and then -the resulting value of `height' is used to compute its final value. -*Note The `let*' expression: fwd-para let, for more about `let*'.) - - -File: eintr, Node: Y Axis Element, Next: Y-axis-column, Prev: Compute a Remainder, Up: print-Y-axis - -C.2.2 Construct a Y Axis Element --------------------------------- - -When we print the vertical axis, we want to insert strings such as -`5 -' and `10 - ' every five lines. Moreover, we want the numbers and -dashes to line up, so shorter numbers must be padded with leading -spaces. If some of the strings use two digit numbers, the strings with -single digit numbers must include a leading blank space before the -number. - -To figure out the length of the number, the `length' function is used. -But the `length' function works only with a string, not with a number. -So the number has to be converted from being a number to being a -string. This is done with the `number-to-string' function. For -example, - - (length (number-to-string 35)) - => 2 - - (length (number-to-string 100)) - => 3 - -(`number-to-string' is also called `int-to-string'; you will see this -alternative name in various sources.) - -In addition, in each label, each number is followed by a string such as -` - ', which we will call the `Y-axis-tic' marker. This variable is -defined with `defvar': - - (defvar Y-axis-tic " - " - "String that follows number in a Y axis label.") - -The length of the Y label is the sum of the length of the Y axis tic -mark and the length of the number of the top of the graph. - - (length (concat (number-to-string height) Y-axis-tic))) - -This value will be calculated by the `print-graph' function in its -varlist as `full-Y-label-width' and passed on. (Note that we did not -think to include this in the varlist when we first proposed it.) - -To make a complete vertical axis label, a tic mark is concatenated with -a number; and the two together may be preceded by one or more spaces -depending on how long the number is. The label consists of three -parts: the (optional) leading spaces, the number, and the tic mark. -The function is passed the value of the number for the specific row, -and the value of the width of the top line, which is calculated (just -once) by `print-graph'. - - (defun Y-axis-element (number full-Y-label-width) - "Construct a NUMBERed label element. - A numbered element looks like this ` 5 - ', - and is padded as needed so all line up with - the element for the largest number." - (let* ((leading-spaces - (- full-Y-label-width - (length - (concat (number-to-string number) - Y-axis-tic))))) - (concat - (make-string leading-spaces ? ) - (number-to-string number) - Y-axis-tic))) - -The `Y-axis-element' function concatenates together the leading spaces, -if any; the number, as a string; and the tic mark. - -To figure out how many leading spaces the label will need, the function -subtracts the actual length of the label--the length of the number plus -the length of the tic mark--from the desired label width. - -Blank spaces are inserted using the `make-string' function. This -function takes two arguments: the first tells it how long the string -will be and the second is a symbol for the character to insert, in a -special format. The format is a question mark followed by a blank -space, like this, `? '. *Note Character Type: (elisp)Character Type, -for a description of the syntax for characters. - -The `number-to-string' function is used in the concatenation -expression, to convert the number to a string that is concatenated with -the leading spaces and the tic mark. - - -File: eintr, Node: Y-axis-column, Next: print-Y-axis Penultimate, Prev: Y Axis Element, Up: print-Y-axis - -C.2.3 Create a Y Axis Column ----------------------------- - -The preceding functions provide all the tools needed to construct a -function that generates a list of numbered and blank strings to insert -as the label for the vertical axis: - - (defun Y-axis-column (height width-of-label) - "Construct list of Y axis labels and blank strings. - For HEIGHT of line above base and WIDTH-OF-LABEL." - (let (Y-axis) - (while (> height 1) - (if (zerop (% height Y-axis-label-spacing)) - ;; Insert label. - (setq Y-axis - (cons - (Y-axis-element height width-of-label) - Y-axis)) - ;; Else, insert blanks. - (setq Y-axis - (cons - (make-string width-of-label ? ) - Y-axis))) - (setq height (1- height))) - ;; Insert base line. - (setq Y-axis - (cons (Y-axis-element 1 width-of-label) Y-axis)) - (nreverse Y-axis))) - -In this function, we start with the value of `height' and repetitively -subtract one from its value. After each subtraction, we test to see -whether the value is an integral multiple of the -`Y-axis-label-spacing'. If it is, we construct a numbered label using -the `Y-axis-element' function; if not, we construct a blank label using -the `make-string' function. The base line consists of the number one -followed by a tic mark. - - -File: eintr, Node: print-Y-axis Penultimate, Prev: Y-axis-column, Up: print-Y-axis - -C.2.4 The Not Quite Final Version of `print-Y-axis' ---------------------------------------------------- - -The list constructed by the `Y-axis-column' function is passed to the -`print-Y-axis' function, which inserts the list as a column. - - (defun print-Y-axis (height full-Y-label-width) - "Insert Y axis using HEIGHT and FULL-Y-LABEL-WIDTH. - Height must be the maximum height of the graph. - Full width is the width of the highest label element." - ;; Value of height and full-Y-label-width - ;; are passed by `print-graph'. - (let ((start (point))) - (insert-rectangle - (Y-axis-column height full-Y-label-width)) - ;; Place point ready for inserting graph. - (goto-char start) - ;; Move point forward by value of full-Y-label-width - (forward-char full-Y-label-width))) - -The `print-Y-axis' uses the `insert-rectangle' function to insert the Y -axis labels created by the `Y-axis-column' function. In addition, it -places point at the correct position for printing the body of the graph. - -You can test `print-Y-axis': - - 1. Install - - Y-axis-label-spacing - Y-axis-tic - Y-axis-element - Y-axis-column - print-Y-axis - - 2. Copy the following expression: - - (print-Y-axis 12 5) - - 3. Switch to the `*scratch*' buffer and place the cursor where you - want the axis labels to start. - - 4. Type `M-:' (`eval-expression'). - - 5. Yank the `graph-body-print' expression into the minibuffer with - `C-y' (`yank)'. - - 6. Press <RET> to evaluate the expression. - -Emacs will print labels vertically, the top one being `10 - '. (The -`print-graph' function will pass the value of `height-of-top-line', -which in this case would end up as 15.) - - -File: eintr, Node: print-X-axis, Next: Print Whole Graph, Prev: print-Y-axis, Up: Full Graph - -C.3 The `print-X-axis' Function -=============================== - -X axis labels are much like Y axis labels, except that the ticks are on -a line above the numbers. Labels should look like this: - - | | | | - 1 5 10 15 - -The first tic is under the first column of the graph and is preceded by -several blank spaces. These spaces provide room in rows above for the Y -axis labels. The second, third, fourth, and subsequent ticks are all -spaced equally, according to the value of `X-axis-label-spacing'. - -The second row of the X axis consists of numbers, preceded by several -blank spaces and also separated according to the value of the variable -`X-axis-label-spacing'. - -The value of the variable `X-axis-label-spacing' should itself be -measured in units of `symbol-width', since you may want to change the -width of the symbols that you are using to print the body of the graph -without changing the ways the graph is labelled. - -* Menu: - -* Similarities differences:: -* X Axis Tic Marks:: - - -File: eintr, Node: Similarities differences, Next: X Axis Tic Marks, Prev: print-X-axis, Up: print-X-axis - -Similarities and differences ----------------------------- - -The `print-X-axis' function is constructed in more or less the same -fashion as the `print-Y-axis' function except that it has two lines: -the line of tic marks and the numbers. We will write a separate -function to print each line and then combine them within the -`print-X-axis' function. - -This is a three step process: - - 1. Write a function to print the X axis tic marks, - `print-X-axis-tic-line'. - - 2. Write a function to print the X numbers, - `print-X-axis-numbered-line'. - - 3. Write a function to print both lines, the `print-X-axis' function, - using `print-X-axis-tic-line' and `print-X-axis-numbered-line'. - - -File: eintr, Node: X Axis Tic Marks, Prev: Similarities differences, Up: print-X-axis - -C.3.1 X Axis Tic Marks ----------------------- - -The first function should print the X axis tic marks. We must specify -the tic marks themselves and their spacing: - - (defvar X-axis-label-spacing - (if (boundp 'graph-blank) - (* 5 (length graph-blank)) 5) - "Number of units from one X axis label to next.") - -(Note that the value of `graph-blank' is set by another `defvar'. The -`boundp' predicate checks whether it has already been set; `boundp' -returns `nil' if it has not. If `graph-blank' were unbound and we did -not use this conditional construction, in GNU Emacs 21, we would enter -the debugger and see an error message saying -`Debugger entered--Lisp error: (void-variable graph-blank)'.) - -Here is the `defvar' for `X-axis-tic-symbol': - - (defvar X-axis-tic-symbol "|" - "String to insert to point to a column in X axis.") - -The goal is to make a line that looks like this: - - | | | | - -The first tic is indented so that it is under the first column, which is -indented to provide space for the Y axis labels. - -A tic element consists of the blank spaces that stretch from one tic to -the next plus a tic symbol. The number of blanks is determined by the -width of the tic symbol and the `X-axis-label-spacing'. - -The code looks like this: - - ;;; X-axis-tic-element - ... - (concat - (make-string - ;; Make a string of blanks. - (- (* symbol-width X-axis-label-spacing) - (length X-axis-tic-symbol)) - ? ) - ;; Concatenate blanks with tic symbol. - X-axis-tic-symbol) - ... - -Next, we determine how many blanks are needed to indent the first tic -mark to the first column of the graph. This uses the value of -`full-Y-label-width' passed it by the `print-graph' function. - -The code to make `X-axis-leading-spaces' looks like this: - - ;; X-axis-leading-spaces - ... - (make-string full-Y-label-width ? ) - ... - -We also need to determine the length of the horizontal axis, which is -the length of the numbers list, and the number of ticks in the -horizontal axis: - - ;; X-length - ... - (length numbers-list) - - ;; tic-width - ... - (* symbol-width X-axis-label-spacing) - - ;; number-of-X-ticks - (if (zerop (% (X-length tic-width))) - (/ (X-length tic-width)) - (1+ (/ (X-length tic-width)))) - -All this leads us directly to the function for printing the X axis tic -line: - - (defun print-X-axis-tic-line - (number-of-X-tics X-axis-leading-spaces X-axis-tic-element) - "Print ticks for X axis." - (insert X-axis-leading-spaces) - (insert X-axis-tic-symbol) ; Under first column. - ;; Insert second tic in the right spot. - (insert (concat - (make-string - (- (* symbol-width X-axis-label-spacing) - ;; Insert white space up to second tic symbol. - (* 2 (length X-axis-tic-symbol))) - ? ) - X-axis-tic-symbol)) - ;; Insert remaining ticks. - (while (> number-of-X-tics 1) - (insert X-axis-tic-element) - (setq number-of-X-tics (1- number-of-X-tics)))) - -The line of numbers is equally straightforward: - -First, we create a numbered element with blank spaces before each -number: - - (defun X-axis-element (number) - "Construct a numbered X axis element." - (let ((leading-spaces - (- (* symbol-width X-axis-label-spacing) - (length (number-to-string number))))) - (concat (make-string leading-spaces ? ) - (number-to-string number)))) - -Next, we create the function to print the numbered line, starting with -the number "1" under the first column: - - (defun print-X-axis-numbered-line - (number-of-X-tics X-axis-leading-spaces) - "Print line of X-axis numbers" - (let ((number X-axis-label-spacing)) - (insert X-axis-leading-spaces) - (insert "1") - (insert (concat - (make-string - ;; Insert white space up to next number. - (- (* symbol-width X-axis-label-spacing) 2) - ? ) - (number-to-string number))) - ;; Insert remaining numbers. - (setq number (+ number X-axis-label-spacing)) - (while (> number-of-X-tics 1) - (insert (X-axis-element number)) - (setq number (+ number X-axis-label-spacing)) - (setq number-of-X-tics (1- number-of-X-tics))))) - -Finally, we need to write the `print-X-axis' that uses -`print-X-axis-tic-line' and `print-X-axis-numbered-line'. - -The function must determine the local values of the variables used by -both `print-X-axis-tic-line' and `print-X-axis-numbered-line', and then -it must call them. Also, it must print the carriage return that -separates the two lines. - -The function consists of a varlist that specifies five local variables, -and calls to each of the two line printing functions: - - (defun print-X-axis (numbers-list) - "Print X axis labels to length of NUMBERS-LIST." - (let* ((leading-spaces - (make-string full-Y-label-width ? )) - ;; symbol-width is provided by graph-body-print - (tic-width (* symbol-width X-axis-label-spacing)) - (X-length (length numbers-list)) - (X-tic - (concat - (make-string - ;; Make a string of blanks. - (- (* symbol-width X-axis-label-spacing) - (length X-axis-tic-symbol)) - ? ) - ;; Concatenate blanks with tic symbol. - X-axis-tic-symbol)) - (tic-number - (if (zerop (% X-length tic-width)) - (/ X-length tic-width) - (1+ (/ X-length tic-width))))) - (print-X-axis-tic-line tic-number leading-spaces X-tic) - (insert "\n") - (print-X-axis-numbered-line tic-number leading-spaces))) - -You can test `print-X-axis': - - 1. Install `X-axis-tic-symbol', `X-axis-label-spacing', - `print-X-axis-tic-line', as well as `X-axis-element', - `print-X-axis-numbered-line', and `print-X-axis'. - - 2. Copy the following expression: - - (progn - (let ((full-Y-label-width 5) - (symbol-width 1)) - (print-X-axis - '(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16)))) - - 3. Switch to the `*scratch*' buffer and place the cursor where you - want the axis labels to start. - - 4. Type `M-:' (`eval-expression'). - - 5. Yank the test expression into the minibuffer with `C-y' (`yank)'. - - 6. Press <RET> to evaluate the expression. - -Emacs will print the horizontal axis like this: - - | | | | | - 1 5 10 15 20 - - -File: eintr, Node: Print Whole Graph, Prev: print-X-axis, Up: Full Graph - -C.4 Printing the Whole Graph -============================ - -Now we are nearly ready to print the whole graph. - -The function to print the graph with the proper labels follows the -outline we created earlier (*note A Graph with Labelled Axes: Full -Graph.), but with additions. - -Here is the outline: - - (defun print-graph (numbers-list) - "DOCUMENTATION..." - (let ((height ... - ...)) - (print-Y-axis height ... ) - (graph-body-print numbers-list) - (print-X-axis ... ))) - -* Menu: - -* The final version:: -* Test print-graph:: -* Graphing words in defuns:: -* lambda:: -* mapcar:: -* Another Bug:: -* Final printed graph:: - - -File: eintr, Node: The final version, Next: Test print-graph, Prev: Print Whole Graph, Up: Print Whole Graph - -Changes for the Final Version ------------------------------ - -The final version is different from what we planned in two ways: first, -it contains additional values calculated once in the varlist; second, -it carries an option to specify the labels' increment per row. This -latter feature turns out to be essential; otherwise, a graph may have -more rows than fit on a display or on a sheet of paper. - -This new feature requires a change to the `Y-axis-column' function, to -add `vertical-step' to it. The function looks like this: - - ;;; Final version. - (defun Y-axis-column - (height width-of-label &optional vertical-step) - "Construct list of labels for Y axis. - HEIGHT is maximum height of graph. - WIDTH-OF-LABEL is maximum width of label. - VERTICAL-STEP, an option, is a positive integer - that specifies how much a Y axis label increments - for each line. For example, a step of 5 means - that each line is five units of the graph." - (let (Y-axis - (number-per-line (or vertical-step 1))) - (while (> height 1) - (if (zerop (% height Y-axis-label-spacing)) - ;; Insert label. - (setq Y-axis - (cons - (Y-axis-element - (* height number-per-line) - width-of-label) - Y-axis)) - ;; Else, insert blanks. - (setq Y-axis - (cons - (make-string width-of-label ? ) - Y-axis))) - (setq height (1- height))) - ;; Insert base line. - (setq Y-axis (cons (Y-axis-element - (or vertical-step 1) - width-of-label) - Y-axis)) - (nreverse Y-axis))) - -The values for the maximum height of graph and the width of a symbol -are computed by `print-graph' in its `let' expression; so -`graph-body-print' must be changed to accept them. - - ;;; Final version. - (defun graph-body-print (numbers-list height symbol-width) - "Print a bar graph of the NUMBERS-LIST. - The numbers-list consists of the Y-axis values. - HEIGHT is maximum height of graph. - SYMBOL-WIDTH is number of each column." - (let (from-position) - (while numbers-list - (setq from-position (point)) - (insert-rectangle - (column-of-graph height (car numbers-list))) - (goto-char from-position) - (forward-char symbol-width) - ;; Draw graph column by column. - (sit-for 0) - (setq numbers-list (cdr numbers-list))) - ;; Place point for X axis labels. - (forward-line height) - (insert "\n"))) - -Finally, the code for the `print-graph' function: - - ;;; Final version. - (defun print-graph - (numbers-list &optional vertical-step) - "Print labelled bar graph of the NUMBERS-LIST. - The numbers-list consists of the Y-axis values. - - Optionally, VERTICAL-STEP, a positive integer, - specifies how much a Y axis label increments for - each line. For example, a step of 5 means that - each row is five units." - (let* ((symbol-width (length graph-blank)) - ;; `height' is both the largest number - ;; and the number with the most digits. - (height (apply 'max numbers-list)) - (height-of-top-line - (if (zerop (% height Y-axis-label-spacing)) - height - ;; else - (* (1+ (/ height Y-axis-label-spacing)) - Y-axis-label-spacing))) - (vertical-step (or vertical-step 1)) - (full-Y-label-width - (length - (concat - (number-to-string - (* height-of-top-line vertical-step)) - Y-axis-tic)))) - - (print-Y-axis - height-of-top-line full-Y-label-width vertical-step) - (graph-body-print - numbers-list height-of-top-line symbol-width) - (print-X-axis numbers-list))) - - -File: eintr, Node: Test print-graph, Next: Graphing words in defuns, Prev: The final version, Up: Print Whole Graph - -C.4.1 Testing `print-graph' ---------------------------- - -We can test the `print-graph' function with a short list of numbers: - - 1. Install the final versions of `Y-axis-column', `graph-body-print', - and `print-graph' (in addition to the rest of the code.) - - 2. Copy the following expression: - - (print-graph '(3 2 5 6 7 5 3 4 6 4 3 2 1)) - - 3. Switch to the `*scratch*' buffer and place the cursor where you - want the axis labels to start. - - 4. Type `M-:' (`eval-expression'). - - 5. Yank the test expression into the minibuffer with `C-y' (`yank)'. - - 6. Press <RET> to evaluate the expression. - -Emacs will print a graph that looks like this: - - 10 - - - - * - ** * - 5 - **** * - **** *** - * ********* - ************ - 1 - ************* - - | | | | - 1 5 10 15 - -On the other hand, if you pass `print-graph' a `vertical-step' value of -2, by evaluating this expression: - - (print-graph '(3 2 5 6 7 5 3 4 6 4 3 2 1) 2) - -The graph looks like this: - - 20 - - - - * - ** * - 10 - **** * - **** *** - * ********* - ************ - 2 - ************* - - | | | | - 1 5 10 15 - -(A question: is the `2' on the bottom of the vertical axis a bug or a -feature? If you think it is a bug, and should be a `1' instead, (or -even a `0'), you can modify the sources.) - - -File: eintr, Node: Graphing words in defuns, Next: lambda, Prev: Test print-graph, Up: Print Whole Graph - -C.4.2 Graphing Numbers of Words and Symbols -------------------------------------------- - -Now for the graph for which all this code was written: a graph that -shows how many function definitions contain fewer than 10 words and -symbols, how many contain between 10 and 19 words and symbols, how many -contain between 20 and 29 words and symbols, and so on. - -This is a multi-step process. First make sure you have loaded all the -requisite code. - -It is a good idea to reset the value of `top-of-ranges' in case you -have set it to some different value. You can evaluate the following: - - (setq top-of-ranges - '(10 20 30 40 50 - 60 70 80 90 100 - 110 120 130 140 150 - 160 170 180 190 200 - 210 220 230 240 250 - 260 270 280 290 300) - -Next create a list of the number of words and symbols in each range. - -Evaluate the following: - - (setq list-for-graph - (defuns-per-range - (sort - (recursive-lengths-list-many-files - (directory-files "/usr/local/emacs/lisp" - t ".+el$")) - '<) - top-of-ranges)) - -On my old machine, this took about an hour. It looked though 303 Lisp -files in my copy of Emacs version 19.23. After all that computing, the -`list-for-graph' had this value: - - (537 1027 955 785 594 483 349 292 224 199 166 120 116 99 - 90 80 67 48 52 45 41 33 28 26 25 20 12 28 11 13 220) - -This means that my copy of Emacs had 537 function definitions with -fewer than 10 words or symbols in them, 1,027 function definitions with -10 to 19 words or symbols in them, 955 function definitions with 20 to -29 words or symbols in them, and so on. - -Clearly, just by looking at this list we can see that most function -definitions contain ten to thirty words and symbols. - -Now for printing. We do _not_ want to print a graph that is 1,030 -lines high ... Instead, we should print a graph that is fewer than -twenty-five lines high. A graph that height can be displayed on almost -any monitor, and easily printed on a sheet of paper. - -This means that each value in `list-for-graph' must be reduced to -one-fiftieth its present value. - -Here is a short function to do just that, using two functions we have -not yet seen, `mapcar' and `lambda'. - - (defun one-fiftieth (full-range) - "Return list, each number one-fiftieth of previous." - (mapcar '(lambda (arg) (/ arg 50)) full-range)) - - -File: eintr, Node: lambda, Next: mapcar, Prev: Graphing words in defuns, Up: Print Whole Graph - -C.4.3 A `lambda' Expression: Useful Anonymity ---------------------------------------------- - -`lambda' is the symbol for an anonymous function, a function without a -name. Every time you use an anonymous function, you need to include -its whole body. - -Thus, - - (lambda (arg) (/ arg 50)) - -is a function definition that says `return the value resulting from -dividing whatever is passed to me as `arg' by 50'. - -Earlier, for example, we had a function `multiply-by-seven'; it -multiplied its argument by 7. This function is similar, except it -divides its argument by 50; and, it has no name. The anonymous -equivalent of `multiply-by-seven' is: - - (lambda (number) (* 7 number)) - -(*Note The `defun' Special Form: defun.) - -If we want to multiply 3 by 7, we can write: - - (multiply-by-seven 3) - \_______________/ ^ - | | - function argument - - - -This expression returns 21. - -Similarly, we can write: - - ((lambda (number) (* 7 number)) 3) - \____________________________/ ^ - | | - anonymous function argument - - - -If we want to divide 100 by 50, we can write: - - ((lambda (arg) (/ arg 50)) 100) - \______________________/ \_/ - | | - anonymous function argument - - - -This expression returns 2. The 100 is passed to the function, which -divides that number by 50. - -*Note Lambda Expressions: (elisp)Lambda Expressions, for more about -`lambda'. Lisp and lambda expressions derive from the Lambda Calculus. - - -File: eintr, Node: mapcar, Next: Another Bug, Prev: lambda, Up: Print Whole Graph - -C.4.4 The `mapcar' Function ---------------------------- - -`mapcar' is a function that calls its first argument with each element -of its second argument, in turn. The second argument must be a -sequence. - -The `map' part of the name comes from the mathematical phrase, `mapping -over a domain', meaning to apply a function to each of the elements in -a domain. The mathematical phrase is based on the metaphor of a -surveyor walking, one step at a time, over an area he is mapping. And -`car', of course, comes from the Lisp notion of the first of a list. - -For example, - - (mapcar '1+ '(2 4 6)) - => (3 5 7) - -The function `1+' which adds one to its argument, is executed on _each_ -element of the list, and a new list is returned. - -Contrast this with `apply', which applies its first argument to all the -remaining. (*Note Readying a Graph: Readying a Graph, for a -explanation of `apply'.) - -In the definition of `one-fiftieth', the first argument is the -anonymous function: - - (lambda (arg) (/ arg 50)) - -and the second argument is `full-range', which will be bound to -`list-for-graph'. - -The whole expression looks like this: - - (mapcar '(lambda (arg) (/ arg 50)) full-range)) - -*Note Mapping Functions: (elisp)Mapping Functions, for more about -`mapcar'. - -Using the `one-fiftieth' function, we can generate a list in which each -element is one-fiftieth the size of the corresponding element in -`list-for-graph'. - - (setq fiftieth-list-for-graph - (one-fiftieth list-for-graph)) - -The resulting list looks like this: - - (10 20 19 15 11 9 6 5 4 3 3 2 2 - 1 1 1 1 0 1 0 0 0 0 0 0 0 0 0 0 0 4) - -This, we are almost ready to print! (We also notice the loss of -information: many of the higher ranges are 0, meaning that fewer than -50 defuns had that many words or symbols--but not necessarily meaning -that none had that many words or symbols.) - - -File: eintr, Node: Another Bug, Next: Final printed graph, Prev: mapcar, Up: Print Whole Graph - -C.4.5 Another Bug ... Most Insidious ------------------------------------- - -I said `almost ready to print'! Of course, there is a bug in the -`print-graph' function ... It has a `vertical-step' option, but not a -`horizontal-step' option. The `top-of-range' scale goes from 10 to 300 -by tens. But the `print-graph' function will print only by ones. - -This is a classic example of what some consider the most insidious type -of bug, the bug of omission. This is not the kind of bug you can find -by studying the code, for it is not in the code; it is an omitted -feature. Your best actions are to try your program early and often; -and try to arrange, as much as you can, to write code that is easy to -understand and easy to change. Try to be aware, whenever you can, that -whatever you have written, _will_ be rewritten, if not soon, -eventually. A hard maxim to follow. - -It is the `print-X-axis-numbered-line' function that needs the work; -and then the `print-X-axis' and the `print-graph' functions need to be -adapted. Not much needs to be done; there is one nicety: the numbers -ought to line up under the tic marks. This takes a little thought. - -Here is the corrected `print-X-axis-numbered-line': - - (defun print-X-axis-numbered-line - (number-of-X-tics X-axis-leading-spaces - &optional horizontal-step) - "Print line of X-axis numbers" - (let ((number X-axis-label-spacing) - (horizontal-step (or horizontal-step 1))) - (insert X-axis-leading-spaces) - ;; Delete extra leading spaces. - (delete-char - (- (1- - (length (number-to-string horizontal-step))))) - (insert (concat - (make-string - ;; Insert white space. - (- (* symbol-width - X-axis-label-spacing) - (1- - (length - (number-to-string horizontal-step))) - 2) - ? ) - (number-to-string - (* number horizontal-step)))) - ;; Insert remaining numbers. - (setq number (+ number X-axis-label-spacing)) - (while (> number-of-X-tics 1) - (insert (X-axis-element - (* number horizontal-step))) - (setq number (+ number X-axis-label-spacing)) - (setq number-of-X-tics (1- number-of-X-tics))))) - -If you are reading this in Info, you can see the new versions of -`print-X-axis' `print-graph' and evaluate them. If you are reading -this in a printed book, you can see the changed lines here (the full -text is too much to print). - - (defun print-X-axis (numbers-list horizontal-step) - "Print X axis labels to length of NUMBERS-LIST. - Optionally, HORIZONTAL-STEP, a positive integer, - specifies how much an X axis label increments for - each column." - ;; Value of symbol-width and full-Y-label-width - ;; are passed by `print-graph'. - (let* ((leading-spaces - (make-string full-Y-label-width ? )) - ;; symbol-width is provided by graph-body-print - (tic-width (* symbol-width X-axis-label-spacing)) - (X-length (length numbers-list)) - (X-tic - (concat - (make-string - ;; Make a string of blanks. - (- (* symbol-width X-axis-label-spacing) - (length X-axis-tic-symbol)) - ? ) - ;; Concatenate blanks with tic symbol. - X-axis-tic-symbol)) - (tic-number - (if (zerop (% X-length tic-width)) - (/ X-length tic-width) - (1+ (/ X-length tic-width))))) - - (print-X-axis-tic-line - tic-number leading-spaces X-tic) - (insert "\n") - (print-X-axis-numbered-line - tic-number leading-spaces horizontal-step))) - - (defun print-graph - (numbers-list &optional vertical-step horizontal-step) - "Print labelled bar graph of the NUMBERS-LIST. - The numbers-list consists of the Y-axis values. - - Optionally, VERTICAL-STEP, a positive integer, - specifies how much a Y axis label increments for - each line. For example, a step of 5 means that - each row is five units. - - Optionally, HORIZONTAL-STEP, a positive integer, - specifies how much an X axis label increments for - each column." - (let* ((symbol-width (length graph-blank)) - ;; `height' is both the largest number - ;; and the number with the most digits. - (height (apply 'max numbers-list)) - (height-of-top-line - (if (zerop (% height Y-axis-label-spacing)) - height - ;; else - (* (1+ (/ height Y-axis-label-spacing)) - Y-axis-label-spacing))) - (vertical-step (or vertical-step 1)) - (full-Y-label-width - (length - (concat - (number-to-string - (* height-of-top-line vertical-step)) - Y-axis-tic)))) - (print-Y-axis - height-of-top-line full-Y-label-width vertical-step) - (graph-body-print - numbers-list height-of-top-line symbol-width) - (print-X-axis numbers-list horizontal-step))) - - -File: eintr, Node: Final printed graph, Prev: Another Bug, Up: Print Whole Graph - -C.4.6 The Printed Graph ------------------------ - -When made and installed, you can call the `print-graph' command like -this: - - (print-graph fiftieth-list-for-graph 50 10) - - -Here is the graph: - - - 1000 - * - ** - ** - ** - ** - 750 - *** - *** - *** - *** - **** - 500 - ***** - ****** - ****** - ****** - ******* - 250 - ******** - ********* * - *********** * - ************* * - 50 - ***************** * * - | | | | | | | | - 10 50 100 150 200 250 300 350 - - - -The largest group of functions contain 10 - 19 words and symbols each. - - -File: eintr, Node: Free Software and Free Manuals, Next: GNU Free Documentation License, Prev: Full Graph, Up: Top - -Appendix D Free Software and Free Manuals -***************************************** - -*by Richard M. Stallman* - -The biggest deficiency in free operating systems is not in the -software--it is the lack of good free manuals that we can include in -these systems. Many of our most important programs do not come with -full manuals. Documentation is an essential part of any software -package; when an important free software package does not come with a -free manual, that is a major gap. We have many such gaps today. - -Once upon a time, many years ago, I thought I would learn Perl. I got -a copy of a free manual, but I found it hard to read. When I asked -Perl users about alternatives, they told me that there were better -introductory manuals--but those were not free. - -Why was this? The authors of the good manuals had written them for -O'Reilly Associates, which published them with restrictive terms--no -copying, no modification, source files not available--which exclude -them from the free software community. - -That wasn't the first time this sort of thing has happened, and (to our -community's great loss) it was far from the last. Proprietary manual -publishers have enticed a great many authors to restrict their manuals -since then. Many times I have heard a GNU user eagerly tell me about a -manual that he is writing, with which he expects to help the GNU -project--and then had my hopes dashed, as he proceeded to explain that -he had signed a contract with a publisher that would restrict it so -that we cannot use it. - -Given that writing good English is a rare skill among programmers, we -can ill afford to lose manuals this way. - -(The Free Software Foundation sells printed copies of free GNU manuals -(http://www.gnu.org/doc/doc.html), too.) - -Free documentation, like free software, is a matter of freedom, not -price. The problem with these manuals was not that O'Reilly Associates -charged a price for printed copies--that in itself is fine. (The Free -Software Foundation sells printed copies of free GNU manuals, too.) -But GNU manuals are available in source code form, while these manuals -are available only on paper. GNU manuals come with permission to copy -and modify; the Perl manuals do not. These restrictions are the -problems. - -The criterion for a free manual is pretty much the same as for free -software: it is a matter of giving all users certain freedoms. -Redistribution (including commercial redistribution) must be permitted, -so that the manual can accompany every copy of the program, on-line or -on paper. Permission for modification is crucial too. - -As a general rule, I don't believe that it is essential for people to -have permission to modify all sorts of articles and books. The issues -for writings are not necessarily the same as those for software. For -example, I don't think you or I are obliged to give permission to -modify articles like this one, which describe our actions and our views. - -But there is a particular reason why the freedom to modify is crucial -for documentation for free software. When people exercise their right -to modify the software, and add or change its features, if they are -conscientious they will change the manual too--so they can provide -accurate and usable documentation with the modified program. A manual -which forbids programmers to be conscientious and finish the job, or -more precisely requires them to write a new manual from scratch if they -change the program, does not fill our community's needs. - -While a blanket prohibition on modification is unacceptable, some kinds -of limits on the method of modification pose no problem. For example, -requirements to preserve the original author's copyright notice, the -distribution terms, or the list of authors, are ok. It is also no -problem to require modified versions to include notice that they were -modified, even to have entire sections that may not be deleted or -changed, as long as these sections deal with nontechnical topics. -(Some GNU manuals have them.) - -These kinds of restrictions are not a problem because, as a practical -matter, they don't stop the conscientious programmer from adapting the -manual to fit the modified program. In other words, they don't block -the free software community from making full use of the manual. - -However, it must be possible to modify all the technical content of the -manual, and then distribute the result in all the usual media, through -all the usual channels; otherwise, the restrictions do block the -community, the manual is not free, and so we need another manual. - -Unfortunately, it is often hard to find someone to write another manual -when a proprietary manual exists. The obstacle is that many users -think that a proprietary manual is good enough--so they don't see the -need to write a free manual. They do not see that the free operating -system has a gap that needs filling. - -Why do users think that proprietary manuals are good enough? Some have -not considered the issue. I hope this article will do something to -change that. - -Other users consider proprietary manuals acceptable for the same reason -so many people consider proprietary software acceptable: they judge in -purely practical terms, not using freedom as a criterion. These people -are entitled to their opinions, but since those opinions spring from -values which do not include freedom, they are no guide for those of us -who do value freedom. - -Please spread the word about this issue. We continue to lose manuals -to proprietary publishing. If we spread the word that proprietary -manuals are not sufficient, perhaps the next person who wants to help -GNU by writing documentation will realize, before it is too late, that -he must above all make it free. - -We can also encourage commercial publishers to sell free, copylefted -manuals instead of proprietary ones. One way you can help this is to -check the distribution terms of a manual before you buy it, and prefer -copylefted manuals to non-copylefted ones. - - - -Note: The Free Software Foundation maintains a page on its Web site -that lists free books available from other publishers: -`http://www.gnu.org/doc/other-free-books.html' - - -File: eintr, Node: GNU Free Documentation License, Next: Index, Prev: Free Software and Free Manuals, Up: Top - -Appendix E GNU Free Documentation License -***************************************** - - Version 1.2, November 2002 - - Copyright (C) 2000,2001,2002 Free Software Foundation, Inc. - 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA - - Everyone is permitted to copy and distribute verbatim copies - of this license document, but changing it is not allowed. - - 0. 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In case of a - disagreement between the translation and the original version of - this License or a notice or disclaimer, the original version will - prevail. - - If a section in the Document is Entitled "Acknowledgements", - "Dedications", or "History", the requirement (section 4) to - Preserve its Title (section 1) will typically require changing the - actual title. - - 9. TERMINATION - - You may not copy, modify, sublicense, or distribute the Document - except as expressly provided for under this License. Any other - attempt to copy, modify, sublicense or distribute the Document is - void, and will automatically terminate your rights under this - License. However, parties who have received copies, or rights, - from you under this License will not have their licenses - terminated so long as such parties remain in full compliance. - - 10. FUTURE REVISIONS OF THIS LICENSE - - The Free Software Foundation may publish new, revised versions of - the GNU Free Documentation License from time to time. Such new - versions will be similar in spirit to the present version, but may - differ in detail to address new problems or concerns. See - `http://www.gnu.org/copyleft/'. - - Each version of the License is given a distinguishing version - number. If the Document specifies that a particular numbered - version of this License "or any later version" applies to it, you - have the option of following the terms and conditions either of - that specified version or of any later version that has been - published (not as a draft) by the Free Software Foundation. If - the Document does not specify a version number of this License, - you may choose any version ever published (not as a draft) by the - Free Software Foundation. - -E.0.1 ADDENDUM: How to use this License for your documents ----------------------------------------------------------- - -To use this License in a document you have written, include a copy of -the License in the document and put the following copyright and license -notices just after the title page: - - Copyright (C) YEAR YOUR NAME. - Permission is granted to copy, distribute and/or modify this document - under the terms of the GNU Free Documentation License, Version 1.2 - or any later version published by the Free Software Foundation; - with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. - A copy of the license is included in the section entitled ``GNU - Free Documentation License''. - -If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, -replace the "with...Texts." line with this: - - with the Invariant Sections being LIST THEIR TITLES, with - the Front-Cover Texts being LIST, and with the Back-Cover Texts - being LIST. - -If you have Invariant Sections without Cover Texts, or some other -combination of the three, merge those two alternatives to suit the -situation. - -If your document contains nontrivial examples of program code, we -recommend releasing these examples in parallel under your choice of -free software license, such as the GNU General Public License, to -permit their use in free software. - - -File: eintr, Node: Index, Next: About the Author, Prev: GNU Free Documentation License, Up: Top - -Index -***** - - -* Menu: - -* % (remainder function): Compute a Remainder. (line 6) -* (debug) in code: debug-on-quit. (line 13) -* * (multiplication): defun. (line 101) -* * for read-only buffer: Read-only buffer. (line 6) -* *scratch* buffer: print-elements-of-list. - (line 9) -* .emacs file: Emacs Initialization. - (line 6) -* .emacs file, beginning of: Beginning a .emacs File. - (line 6) -* / (division): Large buffer case. (line 38) -* <= (less than or equal): Inc Example parts. (line 47) -* > (greater than): if in more detail. (line 32) -* Accumulate, type of recursive pattern: Accumulate. (line 6) -* add-hook: Text and Auto-fill. (line 55) -* and <1>: fwd-para let. (line 45) -* and: kill-new function. (line 213) -* Anonymous function: lambda. (line 6) -* append-to-buffer: append-to-buffer. (line 6) -* apply: Columns of a graph. (line 141) -* apropos: Columns of a graph. (line 32) -* Argument as local variable: Dec Example altogether. - (line 27) -* argument defined: Arguments. (line 16) -* argument list defined: defun. (line 60) -* Argument, wrong type of: Wrong Type of Argument. - (line 6) -* Arguments: Arguments. (line 6) -* Arguments' data types: Data types. (line 6) -* Arguments, variable number of: Variable Number of Arguments. - (line 6) -* Asterisk for read-only buffer: Read-only buffer. (line 6) -* Auto Fill mode turned on: Text and Auto-fill. (line 55) -* autoload: Autoload. (line 6) -* Automatic mode selection: Text and Auto-fill. (line 23) -* Axis, print horizontal: print-X-axis. (line 6) -* Axis, print vertical: print-Y-axis. (line 6) -* beginning-of-buffer: beginning-of-buffer. (line 6) -* bind defined: set & setq. (line 6) -* Bindings, key, fixing unpleasant: Miscellaneous. (line 88) -* body defined: defun. (line 38) -* Body of graph: Readying a Graph. (line 6) -* Buffer size: Buffer Size & Locations. - (line 6) -* Buffer, history of word: Buffer Names. (line 66) -* buffer-file-name: Buffer Names. (line 6) -* buffer-menu, bound to key: Keybindings. (line 68) -* buffer-name: Buffer Names. (line 6) -* Bug, most insidious type: Another Bug. (line 6) -* Building robots: Building Robots. (line 6) -* Building Tags in the Emacs sources: etags. (line 87) -* Byte compiling: Byte Compiling. (line 6) -* C language primitives: Primitive Functions. (line 6) -* C, a digression into: Digression into C. (line 6) -* call defined: Switching Buffers. (line 57) -* cancel-debug-on-entry: debug-on-entry. (line 83) -* car, introduced: car cdr & cons. (line 6) -* cdr, introduced: car cdr & cons. (line 6) -* Changing a function definition: Change a defun. (line 6) -* Chest of Drawers, metaphor for a symbol: Symbols as Chest. (line 6) -* Clipping text: Cutting & Storing Text. - (line 6) -* Code installation: Permanent Installation. - (line 6) -* command defined: How to Evaluate. (line 11) -* Comments in Lisp code: Change a defun. (line 22) -* Common Lisp: Lisp History. (line 11) -* compare-windows: Keybindings. (line 11) -* concat: Data types. (line 11) -* cond: Recursion with cond. (line 6) -* condition-case: condition-case. (line 6) -* Conditional 'twixt two versions of Emacs: Simple Extension. (line 37) -* Conditional with if: if. (line 6) -* cons, introduced: cons. (line 6) -* copy-region-as-kill: copy-region-as-kill. (line 6) -* copy-to-buffer: copy-to-buffer. (line 6) -* Count words recursively: recursive-count-words. - (line 6) -* count-words-in-defun: count-words-in-defun. - (line 99) -* count-words-region: count-words-region. (line 6) -* Counting: Counting. (line 6) -* Counting words in a defun <1>: count-words-in-defun. - (line 6) -* Counting words in a defun: Words in a defun. (line 6) -* current-buffer: Getting Buffers. (line 6) -* current-kill: current-kill. (line 6) -* Customizing your .emacs file: Emacs Initialization. - (line 6) -* Cutting and storing text: Cutting & Storing Text. - (line 6) -* Data types: Data types. (line 6) -* debug: debug. (line 6) -* debug-on-entry: debug-on-entry. (line 6) -* debug-on-quit: debug-on-quit. (line 9) -* debugging: Debugging. (line 6) -* default-mode-line-format: Mode Line. (line 6) -* default.el init file: Site-wide Init. (line 6) -* defconst: defcustom. (line 127) -* defcustom: defcustom. (line 6) -* Deferment in recursion: No Deferment. (line 6) -* Defermentless solution: No deferment solution. - (line 6) -* Definition installation: Install. (line 6) -* Definition writing: Writing Defuns. (line 6) -* Definition, how to change: Change a defun. (line 6) -* defsubst: defcustom. (line 127) -* defun: defun. (line 6) -* defvar: defvar. (line 6) -* defvar for a user customizable variable: defvar and asterisk. - (line 6) -* defvar with an asterisk: defvar and asterisk. (line 6) -* delete-and-extract-region: Digression into C. (line 6) -* Deleting text: Cutting & Storing Text. - (line 6) -* describe-function: simplified-beginning-of-buffer. - (line 78) -* describe-function, introduced: Finding More. (line 6) -* Digression into C: Digression into C. (line 6) -* directory-files: Files List. (line 13) -* Division: Large buffer case. (line 38) -* dolist: dolist. (line 6) -* dotimes: dotimes. (line 6) -* Drawers, Chest of, metaphor for a symbol: Symbols as Chest. (line 6) -* Duplicated words function: the-the. (line 6) -* edebug: edebug. (line 6) -* Else: else. (line 6) -* Emacs version, choosing: Simple Extension. (line 37) -* empty list defined: Lisp Atoms. (line 18) -* empty string defined: Review. (line 143) -* eobp: fwd-para while. (line 59) -* eq: Review. (line 113) -* eq (example of use): last-command & this-command. - (line 37) -* equal: Review. (line 113) -* Erasing text: Cutting & Storing Text. - (line 6) -* error: Understanding current-kill. - (line 52) -* Error for symbol without function: Void Function. (line 6) -* Error for symbol without value: Void Variable. (line 6) -* Error message generation: Making Errors. (line 6) -* etags: etags. (line 6) -* evaluate defined: Run a Program. (line 6) -* Evaluating inner lists: Evaluating Inner Lists. - (line 6) -* Evaluation: Evaluation. (line 6) -* Evaluation practice: Practicing Evaluation. - (line 6) -* Every, type of recursive pattern: Every. (line 6) -* Example variable, fill-column: fill-column Example. (line 6) -* expression defined: Lisp Atoms. (line 25) -* Falsehood and truth in Emacs Lisp: Truth & Falsehood. (line 6) -* FDL, GNU Free Documentation License: GNU Free Documentation License. - (line 6) -* files-in-below-directory: Files List. (line 26) -* fill-column, an example variable: fill-column Example. (line 6) -* filter-buffer-substring: last-command & this-command. - (line 30) -* Find a File: Find a File. (line 6) -* Find function documentation: Finding More. (line 6) -* Find source of function: Finding More. (line 13) -* find-tags: Finding More. (line 40) -* Flowers in a field: Lisp Lists. (line 18) -* Focusing attention (narrowing): Narrowing & Widening. - (line 6) -* form defined: Lisp Atoms. (line 25) -* Formatting convention: append save-excursion. - (line 15) -* Formatting help: Typing Lists. (line 6) -* forward-paragraph: forward-paragraph. (line 6) -* forward-sentence: forward-sentence. (line 6) -* function defined: Making Errors. (line 51) -* function definition defined: defun. (line 6) -* Function definition installation: Install. (line 6) -* Function definition writing: Writing Defuns. (line 6) -* Function definition, how to change: Change a defun. (line 6) -* Functions, primitive: Primitive Functions. (line 6) -* Generate an error message: Making Errors. (line 6) -* Getting a buffer: Getting Buffers. (line 6) -* Global set key: Keybindings. (line 18) -* global variable defined: Determining the Element. - (line 88) -* global-set-key: Keybindings. (line 18) -* global-unset-key: Keybindings. (line 57) -* Graph prototype: Readying a Graph. (line 6) -* Graph, printing all: Print Whole Graph. (line 6) -* graph-body-print: graph-body-print. (line 6) -* graph-body-print Final version.: The final version. (line 53) -* Handling the kill ring: Kill Ring. (line 6) -* Help typing lists: Typing Lists. (line 6) -* Horizontal axis printing: print-X-axis. (line 6) -* if: if. (line 6) -* if-part defined: if in more detail. (line 6) -* indent-tabs-mode: Indent Tabs Mode. (line 6) -* Indentation for formatting: append save-excursion. - (line 15) -* Initialization file: Emacs Initialization. - (line 6) -* Initializing a variable: defvar. (line 6) -* Inner list evaluation: Evaluating Inner Lists. - (line 6) -* insert-buffer: insert-buffer. (line 6) -* insert-buffer, new version body: New insert-buffer. (line 6) -* insert-buffer-substring: append-to-buffer overview. - (line 6) -* Insidious type of bug: Another Bug. (line 6) -* Install a Function Definition: Install. (line 6) -* Install code permanently: Permanent Installation. - (line 6) -* interactive: Interactive. (line 6) -* interactive function defined: How to Evaluate. (line 11) -* Interactive functions: Interactive. (line 6) -* Interactive options: Interactive Options. (line 6) -* interactive, example use of: insert-buffer interactive. - (line 6) -* Interpreter, Lisp, explained: Run a Program. (line 39) -* Interpreter, what it does: Lisp Interpreter. (line 6) -* Keep, type of recursive pattern: Keep. (line 6) -* Key bindings, fixing: Miscellaneous. (line 88) -* Key setting globally: Keybindings. (line 18) -* Key unbinding: Keybindings. (line 57) -* Keymaps: Keymaps. (line 6) -* Keyword: Optional Arguments. (line 11) -* Kill ring handling: Kill Ring. (line 6) -* Kill ring overview: Kill Ring Overview. (line 6) -* kill-append: kill-append function. - (line 6) -* kill-new: kill-new function. (line 6) -* kill-region: kill-region. (line 6) -* Killing text: Cutting & Storing Text. - (line 6) -* lambda: lambda. (line 6) -* length: length. (line 6) -* lengths-list-file: lengths-list-file. (line 11) -* lengths-list-many-files: lengths-list-many-files. - (line 33) -* let: let. (line 6) -* let expression sample: Sample let Expression. - (line 6) -* let expression, parts of: Parts of let Expression. - (line 6) -* let variables uninitialized: Uninitialized let Variables. - (line 6) -* Library, as term for `file': Finding More. (line 64) -* line-to-top-of-window: Simple Extension. (line 6) -* Lisp Atoms: Lisp Atoms. (line 6) -* Lisp history: Lisp History. (line 6) -* Lisp interpreter, explained: Run a Program. (line 39) -* Lisp interpreter, what it does: Lisp Interpreter. (line 6) -* Lisp Lists: Lisp Lists. (line 6) -* Lisp macro: Lisp macro. (line 6) -* list-buffers, rebound: Keybindings. (line 68) -* Lists in a computer: List Implementation. (line 6) -* load-library: Loading Files. (line 52) -* load-path: Loading Files. (line 36) -* Loading files: Loading Files. (line 6) -* local variable defined: Prevent confusion. (line 6) -* Local variables list, per-buffer,: Text and Auto-fill. (line 23) -* Location of point: Buffer Size & Locations. - (line 6) -* looking-at: fwd-para while. (line 81) -* Loops: while. (line 6) -* Loops and recursion: Loops & Recursion. (line 6) -* Maclisp: Lisp History. (line 11) -* Macro, lisp: Lisp macro. (line 6) -* Mail aliases: Mail Aliases. (line 16) -* make tags: etags. (line 87) -* make-string: Y Axis Element. (line 74) -* mapcar: mapcar. (line 6) -* mark: save-excursion. (line 6) -* mark-whole-buffer: mark-whole-buffer. (line 6) -* match-beginning: fwd-para while. (line 158) -* max: Columns of a graph. (line 129) -* message: message. (line 6) -* min: Columns of a graph. (line 129) -* Mode line format: Mode Line. (line 6) -* Mode selection, automatic: Text and Auto-fill. (line 23) -* Motion by sentence and paragraph: Regexp Search. (line 6) -* Narrowing: Narrowing & Widening. - (line 6) -* narrowing defined: Buffer Size & Locations. - (line 40) -* new version body for insert-buffer: New insert-buffer. (line 6) -* nil: Truth & Falsehood. (line 6) -* nil, history of word: Buffer Names. (line 42) -* No deferment solution: No deferment solution. - (line 6) -* nreverse: Counting function definitions. - (line 100) -* nth: nth. (line 6) -* nthcdr <1>: copy-region-as-kill. (line 6) -* nthcdr: nthcdr. (line 6) -* nthcdr, example: kill-new function. (line 149) -* number-to-string: Y Axis Element. (line 13) -* occur: Keybindings. (line 52) -* optional: Optional Arguments. (line 11) -* Optional arguments: Optional Arguments. (line 11) -* Options for interactive: Interactive Options. (line 6) -* or: Insert or. (line 13) -* other-buffer: Getting Buffers. (line 6) -* Paragraphs, movement by: Regexp Search. (line 6) -* Parts of a Recursive Definition: Recursive Definition Parts. - (line 6) -* Parts of let expression: Parts of let Expression. - (line 6) -* Passing information to functions: Arguments. (line 6) -* Pasting text: Yanking. (line 6) -* Patterns, searching for: Regexp Search. (line 6) -* Per-buffer, local variables list: Text and Auto-fill. (line 23) -* Permanent code installation: Permanent Installation. - (line 6) -* point: save-excursion. (line 6) -* point defined: Buffer Size & Locations. - (line 19) -* Point location: Buffer Size & Locations. - (line 6) -* Point, mark, buffer preservation: save-excursion. (line 6) -* Practicing evaluation: Practicing Evaluation. - (line 6) -* Preserving point, mark, and buffer: save-excursion. (line 6) -* Primitive functions: Primitive Functions. (line 6) -* Primitives written in C: Primitive Functions. (line 6) -* Print horizontal axis: print-X-axis. (line 6) -* Print vertical axis: print-Y-axis. (line 6) -* print-elements-of-list: print-elements-of-list. - (line 6) -* print-elements-recursively: Recursion with list. (line 24) -* print-graph Final version.: The final version. (line 75) -* print-graph varlist: print-graph Varlist. (line 6) -* print-X-axis: X Axis Tic Marks. (line 146) -* print-X-axis-numbered-line: X Axis Tic Marks. (line 116) -* print-X-axis-tic-line: X Axis Tic Marks. (line 82) -* print-Y-axis: print-Y-axis Penultimate. - (line 9) -* Printing the whole graph: Print Whole Graph. (line 6) -* progn: progn. (line 6) -* Program, running one: Run a Program. (line 6) -* Properties, in mode line example: Mode Line. (line 64) -* Properties, mention of buffer-substring-no-properties: narrow Exercise. - (line 13) -* Prototype graph: Readying a Graph. (line 6) -* push, example: kill-new function. (line 118) -* re-search-forward: re-search-forward. (line 6) -* Read-only buffer: Read-only buffer. (line 6) -* Readying a graph: Readying a Graph. (line 6) -* Rebinding keys: Keymaps. (line 6) -* Recursion: Recursion. (line 6) -* Recursion and loops: Loops & Recursion. (line 6) -* Recursion without Deferments: No Deferment. (line 6) -* Recursive Definition Parts: Recursive Definition Parts. - (line 6) -* Recursive pattern: accumulate: Accumulate. (line 6) -* Recursive pattern: every: Every. (line 6) -* Recursive pattern: keep: Keep. (line 6) -* Recursive Patterns: Recursive Patterns. (line 6) -* recursive-count-words: recursive-count-words. - (line 258) -* recursive-graph-body-print: recursive-graph-body-print. - (line 6) -* recursive-lengths-list-many-files: Several files recursively. - (line 17) -* Recursively counting words: recursive-count-words. - (line 6) -* regexp-quote: fwd-para let. (line 73) -* Region, what it is: save-excursion. (line 6) -* Regular expression searches: Regexp Search. (line 6) -* Regular expressions for word counting: Counting Words. (line 6) -* Remainder function, %: Compute a Remainder. (line 6) -* Repetition (loops): Loops & Recursion. (line 6) -* Repetition for word counting: Counting Words. (line 6) -* Retrieving text: Yanking. (line 6) -* reverse: Counting function definitions. - (line 115) -* Ring, making a list like a: Kill Ring. (line 6) -* ring.el file: ring file. (line 6) -* Robots, building: Building Robots. (line 6) -* rotate-yank-pointer: Yanking. (line 6) -* Run a program: Run a Program. (line 6) -* Sample let expression: Sample let Expression. - (line 6) -* save-excursion: save-excursion. (line 6) -* save-restriction: save-restriction. (line 6) -* search-forward: search-forward. (line 6) -* Searches, illustrating: Regexp Search. (line 6) -* sentence-end: sentence-end. (line 6) -* Sentences, movement by: Regexp Search. (line 6) -* set: Using set. (line 6) -* set-buffer: Switching Buffers. (line 6) -* set-variable: defvar and asterisk. (line 22) -* setcar: setcar. (line 6) -* setcdr: setcdr. (line 6) -* setcdr, example: kill-new function. (line 153) -* setq: Using setq. (line 6) -* Setting a key globally: Keybindings. (line 18) -* Setting value of variable: set & setq. (line 6) -* side effect defined: Evaluation. (line 22) -* Simple extension in .emacs file: Simple Extension. (line 6) -* simplified-beginning-of-buffer: simplified-beginning-of-buffer. - (line 6) -* site-init.el init file: Site-wide Init. (line 6) -* site-load.el init file: Site-wide Init. (line 6) -* Size of buffer: Buffer Size & Locations. - (line 6) -* Solution without deferment: No deferment solution. - (line 6) -* sort: Sorting. (line 6) -* Source level debugger: edebug. (line 6) -* Special form: Complications. (line 12) -* Special form of defun: defun. (line 6) -* Storing and cutting text: Cutting & Storing Text. - (line 6) -* string defined: Lisp Atoms. (line 64) -* switch-to-buffer: Switching Buffers. (line 6) -* Switching to a buffer: Switching Buffers. (line 6) -* Symbol names: Names & Definitions. (line 6) -* Symbol without function error: Void Function. (line 6) -* Symbol without value error: Void Variable. (line 6) -* Symbolic expressions, introduced: Lisp Atoms. (line 25) -* Symbols as a Chest of Drawers: Symbols as Chest. (line 6) -* Syntax categories and tables: Syntax. (line 6) -* Tabs, preventing: Indent Tabs Mode. (line 6) -* TAGS file, create own: etags. (line 6) -* Tags in the Emacs sources: etags. (line 87) -* TAGS table, specifying: Finding More. (line 40) -* Text between double quotation marks: Lisp Atoms. (line 60) -* Text Mode turned on: Text and Auto-fill. (line 40) -* Text retrieval: Yanking. (line 6) -* the-the: the-the. (line 6) -* then-part defined: if in more detail. (line 6) -* top-of-ranges: Counting function definitions. - (line 20) -* triangle-bugged: debug. (line 14) -* triangle-recursively: Recursive triangle function. - (line 6) -* Truth and falsehood in Emacs Lisp: Truth & Falsehood. (line 6) -* Types of data: Data types. (line 6) -* Unbinding key: Keybindings. (line 57) -* Uninitialized let variables: Uninitialized let Variables. - (line 6) -* Variable initialization: defvar. (line 6) -* Variable number of arguments: Variable Number of Arguments. - (line 6) -* Variable, example of, fill-column: fill-column Example. (line 6) -* variable, global, defined: Determining the Element. - (line 88) -* variable, local, defined: Prevent confusion. (line 6) -* Variable, setting value: set & setq. (line 6) -* Variables: Variables. (line 6) -* varlist defined: Parts of let Expression. - (line 6) -* Version of Emacs, choosing: Simple Extension. (line 37) -* Vertical axis printing: print-Y-axis. (line 6) -* what-line: what-line. (line 6) -* while: while. (line 6) -* Whitespace in lists: Whitespace in Lists. (line 6) -* Whole graph printing: Print Whole Graph. (line 6) -* Widening: Narrowing & Widening. - (line 6) -* Widening, example of: what-line. (line 6) -* Word counting in a defun: Words in a defun. (line 6) -* Words and symbols in defun: Words and Symbols. (line 6) -* Words, counted recursively: recursive-count-words. - (line 6) -* Words, duplicated: the-the. (line 6) -* Writing a function definition: Writing Defuns. (line 6) -* Wrong type of argument: Wrong Type of Argument. - (line 6) -* X axis printing: print-X-axis. (line 6) -* X-axis-element: X Axis Tic Marks. (line 105) -* Y axis printing: print-Y-axis. (line 6) -* Y-axis-column: Y-axis-column. (line 10) -* Y-axis-column Final version.: The final version. (line 15) -* Y-axis-label-spacing: Compute a Remainder. (line 79) -* Y-axis-tic: Y Axis Element. (line 32) -* yank <1>: yank. (line 6) -* yank: Yanking. (line 6) -* yank-pop: yank-pop. (line 6) -* zap-to-char: zap-to-char. (line 6) -* zerop: Understanding current-kill. - (line 52) - - -File: eintr, Node: About the Author, Prev: Index, Up: Top - -About the Author -**************** - - Robert J. Chassell has worked with GNU Emacs since 1985. He writes - and edits, teaches Emacs and Emacs Lisp, and speaks throughout the - world on software freedom. Chassell was a founding Director and - Treasurer of the Free Software Foundation, Inc. He is co-author of - the `Texinfo' manual, and has edited more than a dozen other - books. He graduated from Cambridge University, in England. He - has an abiding interest in social and economic history and flies - his own airplane. - - - - |