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author | Richard M. Stallman <rms@gnu.org> | 1994-03-21 07:49:21 +0000 |
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committer | Richard M. Stallman <rms@gnu.org> | 1994-03-21 07:49:21 +0000 |
commit | 2bcaad75da19d0f1fb313ee73b6bed54f7df95f7 (patch) | |
tree | 232ae994f2575eae08120029c44568fb676b92fd /lispref/modes.texi | |
parent | ace5eb097b20b13ef099e897228e58c0c3e1d8a4 (diff) | |
download | emacs-2bcaad75da19d0f1fb313ee73b6bed54f7df95f7.tar.gz |
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diff --git a/lispref/modes.texi b/lispref/modes.texi new file mode 100644 index 00000000000..c15ca5a9cd0 --- /dev/null +++ b/lispref/modes.texi @@ -0,0 +1,1364 @@ +@c -*-texinfo-*- +@c This is part of the GNU Emacs Lisp Reference Manual. +@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. +@c See the file elisp.texi for copying conditions. +@setfilename ../info/modes +@node Modes, Documentation, Keymaps, Top +@chapter Major and Minor Modes +@cindex mode + + A @dfn{mode} is a set of definitions that customize Emacs and can be +turned on and off while you edit. There are two varieties of modes: +@dfn{major modes}, which are mutually exclusive and used for editing +particular kinds of text, and @dfn{minor modes}, which provide features +that users can enable individually. + + This chapter describes how to write both major and minor modes, how to +indicate them in the mode line, and how they run hooks supplied by the +user. For related topics such as keymaps and syntax tables, see +@ref{Keymaps}, and @ref{Syntax Tables}. + +@menu +* Major Modes:: Defining major modes. +* Minor Modes:: Defining minor modes. +* Mode Line Format:: Customizing the text that appears in the mode line. +* Hooks:: How to use hooks; how to write code that provides hooks. +@end menu + +@node Major Modes +@section Major Modes +@cindex major mode +@cindex Fundamental mode + + Major modes specialize Emacs for editing particular kinds of text. +Each buffer has only one major mode at a time. + + The least specialized major mode is called @dfn{Fundamental mode}. +This mode has no mode-specific definitions or variable settings, so each +Emacs command behaves in its default manner, and each option is in its +default state. All other major modes redefine various keys and options. +For example, Lisp Interaction mode provides special key bindings for +@key{LFD} (@code{eval-print-last-sexp}), @key{TAB} +(@code{lisp-indent-line}), and other keys. + + When you need to write several editing commands to help you perform a +specialized editing task, creating a new major mode is usually a good +idea. In practice, writing a major mode is easy (in contrast to +writing a minor mode, which is often difficult). + + If the new mode is similar to an old one, it is often unwise to modify +the old one to serve two purposes, since it may become harder to use and +maintain. Instead, copy and rename an existing major mode definition +and alter the copy---or define a @dfn{derived mode} (@pxref{Derived +Modes}). For example, Rmail Edit mode, which is in +@file{emacs/lisp/rmailedit.el}, is a major mode that is very similar to +Text mode except that it provides three additional commands. Its +definition is distinct from that of Text mode, but was derived from it. + + Rmail Edit mode is an example of a case where one piece of text is put +temporarily into a different major mode so it can be edited in a +different way (with ordinary Emacs commands rather than Rmail). In such +cases, the temporary major mode usually has a command to switch back to +the buffer's usual mode (Rmail mode, in this case). You might be +tempted to present the temporary redefinitions inside a recursive edit +and restore the usual ones when the user exits; but this is a bad idea +because it constrains the user's options when it is done in more than +one buffer: recursive edits must be exited most-recently-entered first. +Using alternative major modes avoids this limitation. @xref{Recursive +Editing}. + + The standard GNU Emacs Lisp library directory contains the code for +several major modes, in files including @file{text-mode.el}, +@file{texinfo.el}, @file{lisp-mode.el}, @file{c-mode.el}, and +@file{rmail.el}. You can look at these libraries to see how modes are +written. Text mode is perhaps the simplest major mode aside from +Fundamental mode. Rmail mode is a complicated and specialized mode. + +@menu +* Major Mode Conventions:: Coding conventions for keymaps, etc. +* Example Major Modes:: Text mode and Lisp modes. +* Auto Major Mode:: How Emacs chooses the major mode automatically. +* Mode Help:: Finding out how to use a mode. +* Derived Modes:: Defining a new major mode based on another major + mode. +@end menu + +@node Major Mode Conventions +@subsection Major Mode Conventions + + The code for existing major modes follows various coding conventions, +including conventions for local keymap and syntax table initialization, +global names, and hooks. Please follow these conventions when you +define a new major mode: + +@itemize @bullet +@item +Define a command whose name ends in @samp{-mode}, with no arguments, +that switches to the new mode in the current buffer. This command +should set up the keymap, syntax table, and local variables in an +existing buffer without changing the buffer's text. + +@item +Write a documentation string for this command which describes the +special commands available in this mode. @kbd{C-h m} +(@code{describe-mode}) in your mode will display this string. + +The documentation string may include the special documentation +substrings, @samp{\[@var{command}]}, @samp{\@{@var{keymap}@}}, and +@samp{\<@var{keymap}>}, that enable the documentation to adapt +automatically to the user's own key bindings. @xref{Keys in +Documentation}. + +@item +The major mode command should start by calling +@code{kill-all-local-variables}. This is what gets rid of the local +variables of the major mode previously in effect. + +@item +The major mode command should set the variable @code{major-mode} to the +major mode command symbol. This is how @code{describe-mode} discovers +which documentation to print. + +@item +The major mode command should set the variable @code{mode-name} to the +``pretty'' name of the mode, as a string. This appears in the mode +line. + +@item +@cindex functions in modes +Since all global names are in the same name space, all the global +variables, constants, and functions that are part of the mode should +have names that start with the major mode name (or with an abbreviation +of it if the name is long). @xref{Style Tips}. + +@item +@cindex keymaps in modes +The major mode should usually have its own keymap, which is used as the +local keymap in all buffers in that mode. The major mode function +should call @code{use-local-map} to install this local map. +@xref{Active Keymaps}, for more information. + +This keymap should be kept in a global variable named +@code{@var{modename}-mode-map}. Normally the library that defines the +mode sets this variable. Use @code{defvar} to set the variable, so that +it is not reinitialized if it already has a value. (Such +reinitialization could discard customizations made by the user.) + +@item +@cindex syntax tables in modes +The mode may have its own syntax table or may share one with other +related modes. If it has its own syntax table, it should store this in +a variable named @code{@var{modename}-mode-syntax-table}. The reasons +for this are the same as for using a keymap variable. @xref{Syntax +Tables}. + +@item +@cindex abbrev tables in modes +The mode may have its own abbrev table or may share one with other +related modes. If it has its own abbrev table, it should store this in +a variable named @code{@var{modename}-mode-abbrev-table}. @xref{Abbrev +Tables}. + +@item +@cindex buffer-local variables in modes +To make a buffer-local binding for an Emacs customization variable, use +@code{make-local-variable} in the major mode command, not +@code{make-variable-buffer-local}. The latter function would make the +variable local to every buffer in which it is subsequently set, which +would affect buffers that do not use this mode. It is undesirable for a +mode to have such global effects. @xref{Buffer-Local Variables}. + +It's ok to use @code{make-variable-buffer-local}, if you wish, for a +variable used only within a single Lisp package. + +@item +@cindex mode hook +@cindex major mode hook +Each major mode should have a @dfn{mode hook} named +@code{@var{modename}-mode-hook}. The major mode command should run that +hook, with @code{run-hooks}, as the very last thing it +does. @xref{Hooks}. + +@item +The major mode command may also run the hooks of some more basic modes. +For example, @code{indented-text-mode} runs @code{text-mode-hook} as +well as @code{indented-text-mode-hook}. It may run these other hooks +immediately before the mode's own hook (that is, after everything else), +or it may run them earlier. + +@item +If something special should be done if the user switches a buffer from +this mode to any other major mode, the mode can set a local value for +@code{change-major-mode-hook}. + +@item +If this mode is appropriate only for specially-prepared text, then the +major mode command symbol should have a property named @code{mode-class} +with value @code{special}, put on as follows: + +@cindex @code{mode-class} property +@cindex @code{special} +@example +(put 'funny-mode 'mode-class 'special) +@end example + +@noindent +This tells Emacs that new buffers created while the current buffer has +Funny mode should not inherit Funny mode. Modes such as Dired, Rmail, +and Buffer List use this feature. + +@item +If you want to make the new mode the default for files with certain +recognizable names, add an element to @code{auto-mode-alist} to select +the mode for those file names. If you define the mode command to +autoload, you should add this element in the same file that calls +@code{autoload}. Otherwise, it is sufficient to add the element in the +file that contains the mode definition. @xref{Auto Major Mode}. + +@item +@cindex @file{.emacs} customization +In the documentation, you should provide a sample @code{autoload} form +and an example of how to add to @code{auto-mode-alist}, that users can +include in their @file{.emacs} files. + +@item +@cindex mode loading +The top level forms in the file defining the mode should be written so +that they may be evaluated more than once without adverse consequences. +Even if you never load the file more than once, someone else will. +@end itemize + +@defvar change-major-mode-hook +This normal hook is run by @code{kill-all-local-variables} before it +does anything else. This gives major modes a way to arrange for +something special to be done if the user switches to a different major +mode. For best results, make this variable buffer-local, so that it +will disappear after doing its job and will not interfere with the +subsequent major mode. +@end defvar + +@node Example Major Modes +@subsection Major Mode Examples + + Text mode is perhaps the simplest mode besides Fundamental mode. +Here are excerpts from @file{text-mode.el} that illustrate many of +the conventions listed above: + +@smallexample +@group +;; @r{Create mode-specific tables.} +(defvar text-mode-syntax-table nil + "Syntax table used while in text mode.") +@end group + +@group +(if text-mode-syntax-table + () ; @r{Do not change the table if it is already set up.} + (setq text-mode-syntax-table (make-syntax-table)) + (modify-syntax-entry ?\" ". " text-mode-syntax-table) + (modify-syntax-entry ?\\ ". " text-mode-syntax-table) + (modify-syntax-entry ?' "w " text-mode-syntax-table)) +@end group + +@group +(defvar text-mode-abbrev-table nil + "Abbrev table used while in text mode.") +(define-abbrev-table 'text-mode-abbrev-table ()) +@end group + +@group +(defvar text-mode-map nil) ; @r{Create a mode-specific keymap.} + +(if text-mode-map + () ; @r{Do not change the keymap if it is already set up.} + (setq text-mode-map (make-sparse-keymap)) + (define-key text-mode-map "\t" 'tab-to-tab-stop) + (define-key text-mode-map "\es" 'center-line) + (define-key text-mode-map "\eS" 'center-paragraph)) +@end group +@end smallexample + + Here is the complete major mode function definition for Text mode: + +@smallexample +@group +(defun text-mode () + "Major mode for editing text intended for humans to read. + Special commands: \\@{text-mode-map@} +@end group +@group +Turning on text-mode runs the hook `text-mode-hook'." + (interactive) + (kill-all-local-variables) +@end group +@group + (use-local-map text-mode-map) ; @r{This provides the local keymap.} + (setq mode-name "Text") ; @r{This name goes into the mode line.} + (setq major-mode 'text-mode) ; @r{This is how @code{describe-mode}} + ; @r{finds the doc string to print.} + (setq local-abbrev-table text-mode-abbrev-table) + (set-syntax-table text-mode-syntax-table) + (run-hooks 'text-mode-hook)) ; @r{Finally, this permits the user to} + ; @r{customize the mode with a hook.} +@end group +@end smallexample + +@cindex @file{lisp-mode.el} + The three Lisp modes (Lisp mode, Emacs Lisp mode, and Lisp +Interaction mode) have more features than Text mode and the code is +correspondingly more complicated. Here are excerpts from +@file{lisp-mode.el} that illustrate how these modes are written. + +@cindex syntax table example +@smallexample +@group +;; @r{Create mode-specific table variables.} +(defvar lisp-mode-syntax-table nil "") +(defvar emacs-lisp-mode-syntax-table nil "") +(defvar lisp-mode-abbrev-table nil "") +@end group + +@group +(if (not emacs-lisp-mode-syntax-table) ; @r{Do not change the table} + ; @r{if it is already set.} + (let ((i 0)) + (setq emacs-lisp-mode-syntax-table (make-syntax-table)) +@end group + +@group + ;; @r{Set syntax of chars up to 0 to class of chars that are} + ;; @r{part of symbol names but not words.} + ;; @r{(The number 0 is @code{48} in the @sc{ASCII} character set.)} + (while (< i ?0) + (modify-syntax-entry i "_ " emacs-lisp-mode-syntax-table) + (setq i (1+ i))) + @dots{} +@end group +@group + ;; @r{Set the syntax for other characters.} + (modify-syntax-entry ? " " emacs-lisp-mode-syntax-table) + (modify-syntax-entry ?\t " " emacs-lisp-mode-syntax-table) + @dots{} +@end group +@group + (modify-syntax-entry ?\( "() " emacs-lisp-mode-syntax-table) + (modify-syntax-entry ?\) ")( " emacs-lisp-mode-syntax-table) + @dots{})) +;; @r{Create an abbrev table for lisp-mode.} +(define-abbrev-table 'lisp-mode-abbrev-table ()) +@end group +@end smallexample + + Much code is shared among the three Lisp modes. The following +function sets various variables; it is called by each of the major Lisp +mode functions: + +@smallexample +@group +(defun lisp-mode-variables (lisp-syntax) + ;; @r{The @code{lisp-syntax} argument is @code{nil} in Emacs Lisp mode,} + ;; @r{and @code{t} in the other two Lisp modes.} + (cond (lisp-syntax + (if (not lisp-mode-syntax-table) + ;; @r{The Emacs Lisp mode syntax table always exists, but} + ;; @r{the Lisp Mode syntax table is created the first time a} + ;; @r{mode that needs it is called. This is to save space.} +@end group +@group + (progn (setq lisp-mode-syntax-table + (copy-syntax-table emacs-lisp-mode-syntax-table)) + ;; @r{Change some entries for Lisp mode.} + (modify-syntax-entry ?\| "\" " + lisp-mode-syntax-table) + (modify-syntax-entry ?\[ "_ " + lisp-mode-syntax-table) + (modify-syntax-entry ?\] "_ " + lisp-mode-syntax-table))) +@end group +@group + (set-syntax-table lisp-mode-syntax-table))) + (setq local-abbrev-table lisp-mode-abbrev-table) + @dots{}) +@end group +@end smallexample + + Functions such as @code{forward-paragraph} use the value of the +@code{paragraph-start} variable. Since Lisp code is different from +ordinary text, the @code{paragraph-start} variable needs to be set +specially to handle Lisp. Also, comments are indented in a special +fashion in Lisp and the Lisp modes need their own mode-specific +@code{comment-indent-function}. The code to set these variables is the +rest of @code{lisp-mode-variables}. + +@smallexample +@group + (make-local-variable 'paragraph-start) + (setq paragraph-start (concat "^$\\|" page-delimiter)) + @dots{} +@end group +@group + (make-local-variable 'comment-indent-function) + (setq comment-indent-function 'lisp-comment-indent)) +@end group +@end smallexample + + Each of the different Lisp modes has a slightly different keymap. For +example, Lisp mode binds @kbd{C-c C-l} to @code{run-lisp}, but the other +Lisp modes do not. However, all Lisp modes have some commands in +common. The following function adds these common commands to a given +keymap. + +@smallexample +@group +(defun lisp-mode-commands (map) + (define-key map "\e\C-q" 'indent-sexp) + (define-key map "\177" 'backward-delete-char-untabify) + (define-key map "\t" 'lisp-indent-line)) +@end group +@end smallexample + + Here is an example of using @code{lisp-mode-commands} to initialize a +keymap, as part of the code for Emacs Lisp mode. First we declare a +variable with @code{defvar} to hold the mode-specific keymap. When this +@code{defvar} executes, it sets the variable to @code{nil} if it was +void. Then we set up the keymap if the variable is @code{nil}. + + This code avoids changing the keymap or the variable if it is already +set up. This lets the user customize the keymap if he or she so +wishes. + +@smallexample +@group +(defvar emacs-lisp-mode-map () "") +(if emacs-lisp-mode-map + () + (setq emacs-lisp-mode-map (make-sparse-keymap)) + (define-key emacs-lisp-mode-map "\e\C-x" 'eval-defun) + (lisp-mode-commands emacs-lisp-mode-map)) +@end group +@end smallexample + + Finally, here is the complete major mode function definition for +Emacs Lisp mode. + +@smallexample +@group +(defun emacs-lisp-mode () + "Major mode for editing Lisp code to run in Emacs. +Commands: +Delete converts tabs to spaces as it moves back. +Blank lines separate paragraphs. Semicolons start comments. +\\@{emacs-lisp-mode-map@} +@end group +@group +Entry to this mode runs the hook `emacs-lisp-mode-hook'." + (interactive) + (kill-all-local-variables) + (use-local-map emacs-lisp-mode-map) ; @r{This provides the local keymap.} + (set-syntax-table emacs-lisp-mode-syntax-table) +@end group +@group + (setq major-mode 'emacs-lisp-mode) ; @r{This is how @code{describe-mode}} + ; @r{finds out what to describe.} + (setq mode-name "Emacs-Lisp") ; @r{This goes into the mode line.} + (lisp-mode-variables nil) ; @r{This define various variables.} + (run-hooks 'emacs-lisp-mode-hook)) ; @r{This permits the user to use a} + ; @r{hook to customize the mode.} +@end group +@end smallexample + +@node Auto Major Mode +@subsection How Emacs Chooses a Major Mode + + Based on information in the file name or in the file itself, Emacs +automatically selects a major mode for the new buffer when a file is +visited. + +@deffn Command fundamental-mode + Fundamental mode is a major mode that is not specialized for anything +in particular. Other major modes are defined in effect by comparison +with this one---their definitions say what to change, starting from +Fundamental mode. The @code{fundamental-mode} function does @emph{not} +run any hooks; you're not supposed to customize it. (If you want Emacs +to behave differently in Fundamental mode, change the @emph{global} +state of Emacs.) +@end deffn + +@deffn Command normal-mode &optional find-file + This function establishes the proper major mode and local variable +bindings for the current buffer. First it calls @code{set-auto-mode}, +then it runs @code{hack-local-variables} to parse, and bind or +evaluate as appropriate, any local variables. + + If the @var{find-file} argument to @code{normal-mode} is +non-@code{nil}, @code{normal-mode} assumes that the @code{find-file} +function is calling it. In this case, it may process a local variables +list at the end of the file. The variable @code{enable-local-variables} +controls whether to do so. + + If you run @code{normal-mode} interactively, the argument +@var{find-file} is normally @code{nil}. In this case, +@code{normal-mode} unconditionally processes any local variables list. +@xref{File variables, , Local Variables in Files, emacs, The GNU Emacs +Manual}, for the syntax of the local variables section of a file. + +@cindex file mode specification error + @code{normal-mode} uses @code{condition-case} around the call to the +major mode function, so errors are caught and reported as a @samp{File +mode specification error}, followed by the original error message. +@end deffn + +@defopt enable-local-variables +This variable controls processing of local variables lists in files +being visited. A value of @code{t} means process the local variables +lists unconditionally; @code{nil} means ignore them; anything else means +ask the user what to do for each file. The default value is @code{t}. +@end defopt + +@defopt enable-local-eval +This variable controls processing of @samp{Eval:} in local variables +lists in files being visited. A value of @code{t} means process them +unconditionally; @code{nil} means ignore them; anything else means ask +the user what to do for each file. The default value is @code{maybe}. +@end defopt + +@defun set-auto-mode +@cindex visited file mode + This function selects the major mode that is appropriate for the +current buffer. It may base its decision on the value of the @w{@samp{-*-}} +line, on the visited file name (using @code{auto-mode-alist}), or on the +value of a local variable). However, this function does not look for +the @samp{mode:} local variable near the end of a file; the +@code{hack-local-variables} function does that. @xref{Choosing Modes, , +How Major Modes are Chosen, emacs, The GNU Emacs Manual}. +@end defun + +@defopt default-major-mode + This variable holds the default major mode for new buffers. The +standard value is @code{fundamental-mode}. + + If the value of @code{default-major-mode} is @code{nil}, Emacs uses +the (previously) current buffer's major mode for the major mode of a new +buffer. However, if the major mode symbol has a @code{mode-class} +property with value @code{special}, then it is not used for new buffers; +Fundamental mode is used instead. The modes that have this property are +those such as Dired and Rmail that are useful only with text that has +been specially prepared. +@end defopt + +@defvar initial-major-mode +@cindex @samp{*scratch*} +The value of this variable determines the major mode of the initial +@samp{*scratch*} buffer. The value should be a symbol that is a major +mode command name. The default value is @code{lisp-interaction-mode}. +@end defvar + +@defvar auto-mode-alist +This variable contains an association list of file name patterns +(regular expressions; @pxref{Regular Expressions}) and corresponding +major mode functions. Usually, the file name patterns test for +suffixes, such as @samp{.el} and @samp{.c}, but this need not be the +case. An ordinary element of the alist looks like @code{(@var{regexp} . +@var{mode-function})}. + +For example, + +@smallexample +@group +(("^/tmp/fol/" . text-mode) + ("\\.texinfo$" . texinfo-mode) + ("\\.texi$" . texinfo-mode) +@end group +@group + ("\\.el$" . emacs-lisp-mode) + ("\\.c$" . c-mode) + ("\\.h$" . c-mode) + @dots{}) +@end group +@end smallexample + +When you visit a file whose expanded file name (@pxref{File Name +Expansion}) matches a @var{regexp}, @code{set-auto-mode} calls the +corresponding @var{mode-function}. This feature enables Emacs to select +the proper major mode for most files. + +If an element of @code{auto-mode-alist} has the form @code{(@var{regexp} +@var{function} t)}, then after calling @var{function}, Emacs searches +@code{auto-mode-alist} again for a match against the portion of the file +name that did not match before. + +This match-again feature is useful for uncompression packages: an entry +of the form @code{("\\.gz\\'" . @var{function})} can uncompress the file +and then put the uncompressed file in the proper mode according to the +name sans @samp{.gz}. + +Here is an example of how to prepend several pattern pairs to +@code{auto-mode-alist}. (You might use this sort of expression in your +@file{.emacs} file.) + +@smallexample +@group +(setq auto-mode-alist + (append + ;; @r{Filename starts with a dot.} + '(("/\\.[^/]*$" . fundamental-mode) + ;; @r{Filename has no dot.} + ("[^\\./]*$" . fundamental-mode) + ("\\.C$" . c++-mode)) + auto-mode-alist)) +@end group +@end smallexample +@end defvar + +@defvar interpreter-mode-alist +This variable specifes major modes to use for scripts that specify a +command interpreter in an @samp{!#} line. Its value is a list of +elements of the form @code{(@var{interpreter} . @var{mode})}; for +example, @code{("perl" . perl-mode)} is one element present by default. +The element says to use mode @var{mode} if the file specifies +@var{interpreter}. + +This variable is applicable only when the file name doesn't indicate +which major mode to use. +@end defvar + +@defun hack-local-variables &optional force + This function parses, and binds or evaluates as appropriate, any local +variables for the current buffer. + + The handling of @code{enable-local-variables} documented for +@code{normal-mode} actually takes place here. The argument @var{force} +reflects the argument @var{find-file} given to @code{normal-mode}. +@end defun + +@node Mode Help +@subsection Getting Help about a Major Mode +@cindex mode help +@cindex help for major mode +@cindex documentation for major mode + + The @code{describe-mode} function is used to provide information +about major modes. It is normally called with @kbd{C-h m}. The +@code{describe-mode} function uses the value of @code{major-mode}, +which is why every major mode function needs to set the +@code{major-mode} variable. + +@deffn Command describe-mode +This function displays the documentation of the current major mode. + +The @code{describe-mode} function calls the @code{documentation} +function using the value of @code{major-mode} as an argument. Thus, it +displays the documentation string of the major mode function. +(@xref{Accessing Documentation}.) +@end deffn + +@defvar major-mode +This variable holds the symbol for the current buffer's major mode. +This symbol should have a function definition which is the command to +switch to that major mode. The @code{describe-mode} function uses the +documentation string of this symbol as the documentation of the major +mode. +@end defvar + +@node Derived Modes +@subsection Defining Derived Modes + + It's often useful to define a new major mode in terms of an existing +one. An easy way to do this is to use @code{define-derived-mode}. + +@defmac define-derived-mode variant parent name doc body@dots{} +This construct defines @var{variant} as a major mode command, using +@var{name} as the string form of the mode which. + +The definition of the command is to call the function @var{parent}, then +override certain aspects of that parent mode: + +@itemize @bullet +@item +The new mode has its own keymap, named @code{@var{variant}-map}. +@code{define-derived-mode} initializes this map to inherit from +@code{@var{parent}-map}, if it is not already set. + +@item +The new mode has its own syntax table, taken from the variable +@code{@var{variant}-syntax-table}. +@code{define-derived-mode} initializes this variable by copying +@code{@var{parent}-syntax-table}, if it is not already set. + +@item +The new mode has its own abbrev table, taken from the variable +@code{@var{variant}-abbrev-table}. +@code{define-derived-mode} initializes this variable by copying +@code{@var{parent}-abbrev-table}, if it is not already set. + +@item +The new mode has its own mode hook, @code{@var{variant}-hook}, +which it runs in standard fashion as the very last thing that it does. +(The new mode also runs the mode hook of @var{parent} as part +of calling @var{parent}.) +@end itemize + +In addition, you can specify how to override other aspects of +@var{parent-mode} with @var{body}. The command @var{variant} +evaluates the forms in @var{body} after setting up all its usual +overrides, just before running @code{@var{variant}-hook}. + +The argument @var{docstring} specifies the documentation string for the +new mode. If you omit @var{docstring}, @code{define-derived-mode} +generates a documentation string. + +Here is a hypothetical example: + +@example +(define-derived-mode hypertext-mode + text-mode "Hypertext" + "Major mode for hypertext. +\\@{hypertext-mode-map@}" + (setq case-fold-search nil)) + +(define-key hypertext-mode-map + [down-mouse-3] 'do-hyper-link) +@end example +@end defmac + +@node Minor Modes +@section Minor Modes +@cindex minor mode + + A @dfn{minor mode} provides features that users may enable or disable +independently of the choice of major mode. Minor modes can be enabled +individually or in combination. Minor modes would be better named +``Generally available, optional feature modes'' except that such a name is +unwieldy. + + A minor mode is not usually a modification of single major mode. For +example, Auto Fill mode may be used in any major mode that permits text +insertion. To be general, a minor mode must be effectively independent +of the things major modes do. + + A minor mode is often much more difficult to implement than a major +mode. One reason is that you should be able to activate and deactivate +minor modes in any order. + +and restore the environment of the major mode to the state it was in +before the minor mode was activated. + + Often the biggest problem in implementing a minor mode is finding a +way to insert the necessary hook into the rest of Emacs. Minor mode +keymaps make this easier in Emacs 19 than it used to be. + +@menu +* Minor Mode Conventions:: Tips for writing a minor mode. +* Keymaps and Minor Modes:: How a minor mode can have its own keymap. +@end menu + +@node Minor Mode Conventions +@subsection Conventions for Writing Minor Modes +@cindex minor mode conventions +@cindex conventions for writing minor modes + + There are conventions for writing minor modes just as there are for +major modes. Several of the major mode conventions apply to minor +modes as well: those regarding the name of the mode initialization +function, the names of global symbols, and the use of keymaps and +other tables. + + In addition, there are several conventions that are specific to +minor modes. + +@itemize @bullet +@item +@cindex mode variable +Make a variable whose name ends in @samp{-mode} to represent the minor +mode. Its value should enable or disable the mode (@code{nil} to +disable; anything else to enable.) We call this the @dfn{mode +variable}. + +This variable is used in conjunction with the @code{minor-mode-alist} to +display the minor mode name in the mode line. It can also enable +or disable a minor mode keymap. Individual commands or hooks can also +check the variable's value. + +If you want the minor mode to be enabled separately in each buffer, +make the variable buffer-local. + +@item +Define a command whose name is the same as the mode variable. +Its job is to enable and disable the mode by setting the variable. + +The command should accept one optional argument. If the argument is +@code{nil}, it should toggle the mode (turn it on if it is off, and off +if it is on). Otherwise, it should turn the mode on if the argument is +a positive integer, a symbol other than @code{nil} or @code{-}, or a +list whose @sc{car} is such an integer or symbol; it should turn the +mode off otherwise. + +Here is an example taken from the definition of @code{overwrite-mode}. +It shows the use of @code{overwrite-mode} as a variable which enables or +disables the mode's behavior. + +@smallexample +@group +(setq overwrite-mode + (if (null arg) (not overwrite-mode) + (> (prefix-numeric-value arg) 0))) +@end group +@end smallexample + +@item +Add an element to @code{minor-mode-alist} for each minor mode +(@pxref{Mode Line Variables}). This element should be a list of the +following form: + +@smallexample +(@var{mode-variable} @var{string}) +@end smallexample + +Here @var{mode-variable} is the variable that controls enablement of the +minor mode, and @var{string} is a short string, starting with a space, +to represent the mode in the mode line. These strings must be short so +that there is room for several of them at once. + +When you add an element to @code{minor-mode-alist}, use @code{assq} to +check for an existing element, to avoid duplication. For example: + +@smallexample +@group +(or (assq 'leif-mode minor-mode-alist) + (setq minor-mode-alist + (cons '(leif-mode " Leif") minor-mode-alist))) +@end group +@end smallexample +@end itemize + +@node Keymaps and Minor Modes +@subsection Keymaps and Minor Modes + +As of Emacs version 19, each minor mode can have its own keymap which is +active when the mode is enabled. @xref{Active Keymaps}. To set up a +keymap for a minor mode, add an element to the alist +@code{minor-mode-map-alist}. + +@cindex @code{self-insert-command}, minor modes +One use of minor mode keymaps is to modify the behavior of certain +self-inserting characters so that they do something else as well as +self-insert. In general, this is the only way to do that, since the +facilities for customizing @code{self-insert-command} are limited to +special cases (designed for abbrevs and Auto Fill mode). (Do not try +substituting your own definition of @code{self-insert-command} for the +standard one. The editor command loop handles this function specially.) + +@defvar minor-mode-map-alist +This variable is an alist of elements that look like this: + +@example +(@var{variable} . @var{keymap}) +@end example + +@noindent +where @var{variable} is the variable which indicates whether the minor +mode is enabled, and @var{keymap} is the keymap. The keymap +@var{keymap} is active whenever @var{variable} has a non-@code{nil} +value. + +Note that elements of @code{minor-mode-map-alist} do not have the same +structure as elements of @code{minor-mode-alist}. The map must be the +@sc{cdr} of the element; a list with the map as the second element will +not do. + +What's more, the keymap itself must appear in the @sc{cdr}. It does not +work to store a variable in the @sc{cdr} and make the map the value of +that variable. + +When more than one minor mode keymap is active, their order of priority +is the order of @code{minor-mode-map-alist}. But you should design +minor modes so that they don't interfere with each other. If you do +this properly, the order will not matter. +@end defvar + +@node Mode Line Format +@section Mode Line Format +@cindex mode line + + Each Emacs window (aside from minibuffer windows) includes a mode line +which displays status information about the buffer displayed in the +window. The mode line contains information about the buffer such as its +name, associated file, depth of recursive editing, and the major and +minor modes of the buffer. + + This section describes how the contents of the mode line are +controlled. It is in the chapter on modes because much of the +information displayed in the mode line relates to the enabled major and +minor modes. + + @code{mode-line-format} is a buffer-local variable that holds a +template used to display the mode line of the current buffer. All +windows for the same buffer use the same @code{mode-line-format} and the +mode lines will appear the same (except for scrolling percentages and +line numbers). + + The mode line of a window is normally updated whenever a different +buffer is shown in the window, or when the buffer's modified-status +changes from @code{nil} to @code{t} or vice-versa. If you modify any of +the variables referenced by @code{mode-line-format}, you may want to +force an update of the mode line so as to display the new information. + +@c Emacs 19 feature +@defun force-mode-line-update +Force redisplay of the current buffer's mode line. +@end defun + + The mode line is usually displayed in inverse video; see +@code{mode-line-inverse-video} in @ref{Inverse Video}. + +@menu +* Mode Line Data:: The data structure that controls the mode line. +* Mode Line Variables:: Variables used in that data structure. +* %-Constructs:: Putting information into a mode line. +@end menu + +@node Mode Line Data +@subsection The Data Structure of the Mode Line +@cindex mode line construct + + The mode line contents are controlled by a data structure of lists, +strings, symbols and numbers kept in the buffer-local variable +@code{mode-line-format}. The data structure is called a @dfn{mode line +construct}, and it is built in recursive fashion out of simpler mode line +constructs. + +@defvar mode-line-format +The value of this variable is a mode line construct with overall +responsibility for the mode line format. The value of this variable +controls which other variables are used to form the mode line text, and +where they appear. +@end defvar + + A mode line construct may be as simple as a fixed string of text, but +it usually specifies how to use other variables to construct the text. +Many of these variables are themselves defined to have mode line +constructs as their values. + + The default value of @code{mode-line-format} incorporates the values +of variables such as @code{mode-name} and @code{minor-mode-alist}. +Because of this, very few modes need to alter @code{mode-line-format}. +For most purposes, it is sufficient to alter the variables referenced by +@code{mode-line-format}. + + A mode line construct may be a list, cons cell, symbol, or string. If +the value is a list, each element may be a list, a cons cell, a symbol, +or a string. + +@table @code +@cindex percent symbol in mode line +@item @var{string} +A string as a mode line construct is displayed verbatim in the mode line +except for @dfn{@code{%}-constructs}. Decimal digits after the @code{%} +specify the field width for space filling on the right (i.e., the data +is left justified). @xref{%-Constructs}. + +@item @var{symbol} +A symbol as a mode line construct stands for its value. The value of +@var{symbol} is used in place of @var{symbol} unless @var{symbol} is +@code{t} or @code{nil}, or is void, in which case @var{symbol} is +ignored. + +There is one exception: if the value of @var{symbol} is a string, it is +processed verbatim in that the @code{%}-constructs are not recognized. + +@item (@var{string} @var{rest}@dots{}) @r{or} (@var{list} @var{rest}@dots{}) +A list whose first element is a string or list, means to concatenate all +the elements. This is the most common form of mode line construct. + +@item (@var{symbol} @var{then} @var{else}) +A list whose first element is a symbol is a conditional. Its meaning +depends on the value of @var{symbol}. If the value is non-@code{nil}, +the second element of the list (@var{then}) is processed recursively as +a mode line element. But if the value of @var{symbol} is @code{nil}, +the third element of the list (if there is one) is processed +recursively. + +@item (@var{width} @var{rest}@dots{}) +A list whose first element is an integer specifies truncation or +padding of the results of @var{rest}. The remaining elements +@var{rest} are processed recursively as mode line constructs and +concatenated together. Then the result is space filled (if +@var{width} is positive) or truncated (to @minus{}@var{width} columns, +if @var{width} is negative) on the right. + +For example, the usual way to show what percentage of a buffer is above +the top of the window is to use a list like this: @code{(-3 . "%p")}. +@end table + + If you do alter @code{mode-line-format} itself, the new value should +use all the same variables that are used by the default value, rather +than duplicating their contents or displaying the information in another +fashion. This way, customizations made by the user, by libraries (such +as @code{display-time}) and by major modes via changes to those +variables remain effective. + +@cindex Shell mode @code{mode-line-format} + Here is an example of a @code{mode-line-format} that might be +useful for @code{shell-mode} since it contains the hostname and default +directory. + +@example +@group +(setq mode-line-format + (list "" + 'mode-line-modified + "%b--" +@end group + (getenv "HOST") ; @r{One element is not constant.} + ":" + 'default-directory + " " + 'global-mode-string + " %[(" 'mode-name + 'minor-mode-alist + "%n" + 'mode-line-process + ")%]----" +@group + (line-number-mode "L%l--") + '(-3 . "%p") + "-%-")) +@end group +@end example + +@node Mode Line Variables +@subsection Variables Used in the Mode Line + + This section describes variables incorporated by the +standard value of @code{mode-line-format} into the text of the mode +line. There is nothing inherently special about these variables; any +other variables could have the same effects on the mode line if +@code{mode-line-format} were changed to use them. + +@defvar mode-line-modified + This variable holds the value of the mode-line construct that displays +whether the current buffer is modified. + + The default value of @code{mode-line-modified} is +@code{("--%1*%1*-")}. This means that the mode line displays +@samp{--**-} if the buffer is modified, @samp{-----} if the buffer is +not modified, and @samp{--%%-} if the buffer is read only. + +Changing this variable does not force an update of the mode line. +@end defvar + +@defvar mode-line-buffer-identification + This variable identifies the buffer being displayed in the window. +Its default value is @samp{Emacs: %17b}, which means that it displays +@samp{Emacs:} followed by the buffer name. You may want to change this +in modes such as Rmail that do not behave like a ``normal'' Emacs. +@end defvar + +@defvar global-mode-string +This variable holds a mode line spec that appears in the mode line by +default, just after the buffer name. The command @code{display-time} +sets @code{global-mode-string} to refer to the variable +@code{display-time-string}, which holds a string containing the time and +load information. + +The @samp{%M} construct substitutes the value of +@code{global-mode-string}, but this is obsolete, since the variable is +included directly in the mode line. +@end defvar + +@defvar mode-name + This buffer-local variable holds the ``pretty'' name of the current +buffer's major mode. Each major mode should set this variable so that the +mode name will appear in the mode line. +@end defvar + +@defvar minor-mode-alist + This variable holds an association list whose elements specify how the +mode line should indicate that a minor mode is active. Each element of +the @code{minor-mode-alist} should be a two-element list: + +@example +(@var{minor-mode-variable} @var{mode-line-string}) +@end example + +More generally, @var{mode-line-string} can be any mode line spec. It +appears in the mode line when the value of @var{minor-mode-variable} is +non-@code{nil}, and not otherwise. These strings should begin with +spaces so that they don't run together. Conventionally, the +@var{minor-mode-variable} for a specific mode is set to a non-@code{nil} +value when that minor mode is activated. + +The default value of @code{minor-mode-alist} is: + +@example +@group +minor-mode-alist +@result{} ((abbrev-mode " Abbrev") + (overwrite-mode " Ovwrt") + (auto-fill-function " Fill") + (defining-kbd-macro " Def")) +@end group +@end example + +@noindent +(In earlier Emacs versions, @code{auto-fill-function} was called +@code{auto-fill-hook}.) + + @code{minor-mode-alist} is not buffer-local. The variables mentioned +in the alist should be buffer-local if the minor mode can be enabled +separately in each buffer. +@end defvar + +@defvar mode-line-process +This buffer-local variable contains the mode line information on process +status in modes used for communicating with subprocesses. It is +displayed immediately following the major mode name, with no intervening +space. For example, its value in the @samp{*shell*} buffer is +@code{(":@: %s")}, which allows the shell to display its status along +with the major mode as: @samp{(Shell:@: run)}. Normally this variable +is @code{nil}. +@end defvar + +@defvar default-mode-line-format + This variable holds the default @code{mode-line-format} for buffers +that do not override it. This is the same as @code{(default-value +'mode-line-format)}. + + The default value of @code{default-mode-line-format} is: + +@example +@group +("" + mode-line-modified + mode-line-buffer-identification + " " + global-mode-string + " %[(" + mode-name +@end group +@group + minor-mode-alist + "%n" + mode-line-process + ")%]----" + (-3 . "%p") + "-%-") +@end group +@end example +@end defvar + +@node %-Constructs +@subsection @code{%}-Constructs in the Mode Line + + The following table lists the recognized @code{%}-constructs and what +they mean. + +@table @code +@item %b +The current buffer name, obtained with the @code{buffer-name} function. +@xref{Buffer Names}. + +@item %f +The visited file name, obtained with the @code{buffer-file-name} +function. @xref{Buffer File Name}. + +@item %* +@samp{%} if the buffer is read only (see @code{buffer-read-only}); @* +@samp{*} if the buffer is modified (see @code{buffer-modified-p}); @* +@samp{-} otherwise. @xref{Buffer Modification}. + +@item %+ +@samp{*} if the buffer is modified, and otherwise @samp{-}. + +@item %s +The status of the subprocess belonging to the current buffer, obtained with +@code{process-status}. @xref{Process Information}. + +@item %p +The percent of the buffer above the @strong{top} of window, or +@samp{Top}, @samp{Bottom} or @samp{All}. + +@item %P +The percentage of the buffer text that is above the @strong{bottom} of +the window (which includes the text visible in the window, as well as +the text above the top), plus @samp{Top} if the top of the buffer is +visible on screen; or @samp{Bottom} or @samp{All}. + +@item %n +@samp{Narrow} when narrowing is in effect; nothing otherwise (see +@code{narrow-to-region} in @ref{Narrowing}). + +@item %[ +An indication of the depth of recursive editing levels (not counting +minibuffer levels): one @samp{[} for each editing level. +@xref{Recursive Editing}. + +@item %] +One @samp{]} for each recursive editing level (not counting minibuffer +levels). + +@item %% +The character @samp{%}---this is how to include a literal @samp{%} in a +string in which @code{%}-constructs are allowed. + +@item %- +Dashes sufficient to fill the remainder of the mode line. +@end table + +The following two @code{%}-constructs are still supported, but they are +obsolete, since you can get the same results with the variables +@code{mode-name} and @code{global-mode-string}. + +@table @code +@item %m +The value of @code{mode-name}. + +@item %M +The value of @code{global-mode-string}. Currently, only +@code{display-time} modifies the value of @code{global-mode-string}. +@end table + +@node Hooks +@section Hooks +@cindex hooks + + A @dfn{hook} is a variable where you can store a function or functions +to be called on a particular occasion by an existing program. Emacs +provides hooks for the sake of customization. Most often, hooks are set +up in the @file{.emacs} file, but Lisp programs can set them also. +@xref{Standard Hooks}, for a list of standard hook variables. + + Most of the hooks in Emacs are @dfn{normal hooks}. These variables +contain lists of functions to be called with no arguments. The reason +most hooks are normal hooks is so that you can use them in a uniform +way. You can always tell when a hook is a normal hook, because its +name ends in @samp{-hook}. + + The recommended way to add a hook function to a normal hook is by +calling @code{add-hook} (see below). The hook functions may be any of +the valid kinds of functions that @code{funcall} accepts (@pxref{What Is +a Function}). Most normal hook variables are initially void; +@code{add-hook} knows how to deal with this. + + As for abnormal hooks, those whose names end in @samp{-function} have +a value which is a single function. Those whose names end in +@samp{-hooks} have a value which is a list of functions. Any hook which +is abnormal is abnormal because a normal hook won't do the job; either +the functions are called with arguments, or their values are meaningful. +The name shows you that the hook is abnormal and that you should look at +its documentation string to see how to use it properly. + + Most major modes run hooks as the last step of initialization. This +makes it easy for a user to customize the behavior of the mode, by +overriding the local variable assignments already made by the mode. But +hooks are used in other contexts too. For example, the hook +@code{suspend-hook} runs just before Emacs suspends itself +(@pxref{Suspending Emacs}). + + Here's an expression you can put in your @file{.emacs} file to turn on +Auto Fill mode when in Lisp Interaction mode: + +@example +(add-hook 'lisp-interaction-mode-hook 'turn-on-auto-fill) +@end example + + The next example shows how to use a hook to customize the way Emacs +formats C code. (People often have strong personal preferences for one +format or another.) Here the hook function is an anonymous lambda +expression. + +@cindex lambda expression in hook +@example +@group +(add-hook 'c-mode-hook + (function (lambda () + (setq c-indent-level 4 + c-argdecl-indent 0 + c-label-offset -4 +@end group +@group + c-continued-statement-indent 0 + c-brace-offset 0 + comment-column 40)))) + +(setq c++-mode-hook c-mode-hook) +@end group +@end example + + Finally, here is an example of how to use the Text mode hook to +provide a customized mode line for buffers in Text mode, displaying the +default directory in addition to the standard components of the +mode line. (This may cause the mode line to run out of space if you +have very long file names or display the time and load.) + +@example +@group +(add-hook 'text-mode-hook + (function (lambda () + (setq mode-line-format + '(mode-line-modified + "Emacs: %14b" + " " +@end group + default-directory + " " + global-mode-string + "%[(" + mode-name + minor-mode-alist +@group + "%n" + mode-line-process + ") %]---" + (-3 . "%p") + "-%-"))))) +@end group +@end example + + At the appropriate time, Emacs uses the @code{run-hooks} function to +run particular hooks. This function calls the hook functions you have +added with @code{add-hooks}. + +@defun run-hooks &rest hookvar +This function takes one or more hook variable names as arguments, and +runs each hook in turn. Each @var{hookvar} argument should be a symbol +that is a hook variable. These arguments are processed in the order +specified. + +If a hook variable has a non-@code{nil} value, that value may be a +function or a list of functions. If the value is a function (either a +lambda expression or a symbol with a function definition), it is +called. If it is a list, the elements are called, in order. +The hook functions are called with no arguments. + +For example, here's how @code{emacs-lisp-hooks} runs its mode hook: + +@example +(run-hooks 'emacs-lisp-mode-hook) +@end example +@end defun + +@defun add-hook hook function &optional append +This function is the handy way to add function @var{function} to hook +variable @var{hook}. For example, + +@example +(add-hook 'text-mode-hook 'my-text-hook-function) +@end example + +@noindent +adds @code{my-text-hook-function} to the hook called @code{text-mode-hook}. + +It is best to design your hook functions so that the order in which they +are executed does not matter. Any dependence on the order is ``asking +for trouble.'' However, the order is predictable: normally, +@var{function} goes at the front of the hook list, so it will be +executed first (barring another @code{add-hook} call). + +If the optional argument @var{append} is non-@code{nil}, the new hook +function goes at the end of the hook list and will be executed last. +@end defun + +@defun remove-hook hook function +This function removes @var{function} from the hook variable @var{hook}. +@end defun |