diff options
Diffstat (limited to 'lispref/objects.texi')
| -rw-r--r-- | lispref/objects.texi | 1468 |
1 files changed, 0 insertions, 1468 deletions
diff --git a/lispref/objects.texi b/lispref/objects.texi deleted file mode 100644 index ab2fe39a10c..00000000000 --- a/lispref/objects.texi +++ /dev/null @@ -1,1468 +0,0 @@ -@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/objects -@node Types of Lisp Object, Numbers, Introduction, Top -@chapter Lisp Data Types -@cindex object -@cindex Lisp object -@cindex type -@cindex data type - - A Lisp @dfn{object} is a piece of data used and manipulated by Lisp -programs. For our purposes, a @dfn{type} or @dfn{data type} is a set of -possible objects. - - Every object belongs to at least one type. Objects of the same type -have similar structures and may usually be used in the same contexts. -Types can overlap, and objects can belong to two or more types. -Consequently, we can ask whether an object belongs to a particular type, -but not for ``the'' type of an object. - -@cindex primitive type - A few fundamental object types are built into Emacs. These, from -which all other types are constructed, are called @dfn{primitive -types}. Each object belongs to one and only one primitive type. These -types include @dfn{integer}, @dfn{float}, @dfn{cons}, @dfn{symbol}, -@dfn{string}, @dfn{vector}, @dfn{subr}, @dfn{byte-code function}, and -several special types, such as @dfn{buffer}, that are related to -editing. (@xref{Editing Types}.) - - Each primitive type has a corresponding Lisp function that checks -whether an object is a member of that type. - - Note that Lisp is unlike many other languages in that Lisp objects are -@dfn{self-typing}: the primitive type of the object is implicit in the -object itself. For example, if an object is a vector, nothing can treat -it as a number; Lisp knows it is a vector, not a number. - - In most languages, the programmer must declare the data type of each -variable, and the type is known by the compiler but not represented in -the data. Such type declarations do not exist in Emacs Lisp. A Lisp -variable can have any type of value, and remembers the type of any value -you store in it. - - This chapter describes the purpose, printed representation, and read -syntax of each of the standard types in GNU Emacs Lisp. Details on how -to use these types can be found in later chapters. - -@menu -* Printed Representation:: How Lisp objects are represented as text. -* Comments:: Comments and their formatting conventions. -* Programming Types:: Types found in all Lisp systems. -* Editing Types:: Types specific to Emacs. -* Type Predicates:: Tests related to types. -* Equality Predicates:: Tests of equality between any two objects. -@end menu - -@node Printed Representation -@comment node-name, next, previous, up -@section Printed Representation and Read Syntax -@cindex printed representation -@cindex read syntax - - The @dfn{printed representation} of an object is the format of the -output generated by the Lisp printer (the function @code{prin1}) for -that object. The @dfn{read syntax} of an object is the format of the -input accepted by the Lisp reader (the function @code{read}) for that -object. Most objects have more than one possible read syntax. Some -types of object have no read syntax; except for these cases, the printed -representation of an object is also a read syntax for it. - - In other languages, an expression is text; it has no other form. In -Lisp, an expression is primarily a Lisp object and only secondarily the -text that is the object's read syntax. Often there is no need to -emphasize this distinction, but you must keep it in the back of your -mind, or you will occasionally be very confused. - -@cindex hash notation - Every type has a printed representation. Some types have no read -syntax, since it may not make sense to enter objects of these types -directly in a Lisp program. For example, the buffer type does not have -a read syntax. Objects of these types are printed in @dfn{hash -notation}: the characters @samp{#<} followed by a descriptive string -(typically the type name followed by the name of the object), and closed -with a matching @samp{>}. Hash notation cannot be read at all, so the -Lisp reader signals the error @code{invalid-read-syntax} whenever it -encounters @samp{#<}. -@kindex invalid-read-syntax - -@example -(current-buffer) - @result{} #<buffer objects.texi> -@end example - - When you evaluate an expression interactively, the Lisp interpreter -first reads the textual representation of it, producing a Lisp object, -and then evaluates that object (@pxref{Evaluation}). However, -evaluation and reading are separate activities. Reading returns the -Lisp object represented by the text that is read; the object may or may -not be evaluated later. @xref{Input Functions}, for a description of -@code{read}, the basic function for reading objects. - -@node Comments -@comment node-name, next, previous, up -@section Comments -@cindex comments -@cindex @samp{;} in comment - - A @dfn{comment} is text that is written in a program only for the sake -of humans that read the program, and that has no effect on the meaning -of the program. In Lisp, a semicolon (@samp{;}) starts a comment if it -is not within a string or character constant. The comment continues to -the end of line. The Lisp reader discards comments; they do not become -part of the Lisp objects which represent the program within the Lisp -system. - - @xref{Comment Tips}, for conventions for formatting comments. - -@node Programming Types -@section Programming Types -@cindex programming types - - There are two general categories of types in Emacs Lisp: those having -to do with Lisp programming, and those having to do with editing. The -former exist in many Lisp implementations, in one form or another. The -latter are unique to Emacs Lisp. - -@menu -* Integer Type:: Numbers without fractional parts. -* Floating Point Type:: Numbers with fractional parts and with a large range. -* Character Type:: The representation of letters, numbers and - control characters. -* Sequence Type:: Both lists and arrays are classified as sequences. -* List Type:: Lists gave Lisp its name (not to mention reputation). -* Array Type:: Arrays include strings and vectors. -* String Type:: An (efficient) array of characters. -* Vector Type:: One-dimensional arrays. -* Symbol Type:: A multi-use object that refers to a function, - variable, property list, or itself. -* Lisp Function Type:: A piece of executable code you can call from elsewhere. -* Lisp Macro Type:: A method of expanding an expression into another - expression, more fundamental but less pretty. -* Primitive Function Type:: A function written in C, callable from Lisp. -* Byte-Code Type:: A function written in Lisp, then compiled. -* Autoload Type:: A type used for automatically loading seldom-used - functions. -@end menu - -@node Integer Type -@subsection Integer Type - - Integers were the only kind of number in Emacs version 18. The range -of values for integers is @minus{}8388608 to 8388607 (24 bits; i.e., -@ifinfo --2**23 -@end ifinfo -@tex -$-2^{23}$ -@end tex -to -@ifinfo -2**23 - 1) -@end ifinfo -@tex -$2^{23}-1$) -@end tex -on most machines, but is 25 or 26 bits on some systems. It is important -to note that the Emacs Lisp arithmetic functions do not check for -overflow. Thus @code{(1+ 8388607)} is @minus{}8388608 on 24-bit -implementations.@refill - - The read syntax for numbers is a sequence of (base ten) digits with an -optional sign at the beginning and an optional period at the end. The -printed representation produced by the Lisp interpreter never has a -leading @samp{+} or a final @samp{.}. - -@example -@group --1 ; @r{The integer -1.} -1 ; @r{The integer 1.} -1. ; @r{Also The integer 1.} -+1 ; @r{Also the integer 1.} -16777217 ; @r{Also the integer 1!} - ; @r{ (on a 24-bit or 25-bit implementation)} -@end group -@end example - - @xref{Numbers}, for more information. - -@node Floating Point Type -@subsection Floating Point Type - - Emacs version 19 supports floating point numbers (though there is a -compilation option to disable them). The precise range of floating -point numbers is machine-specific. - - The printed representation for floating point numbers requires either -a decimal point (with at least one digit following), an exponent, or -both. For example, @samp{1500.0}, @samp{15e2}, @samp{15.0e2}, -@samp{1.5e3}, and @samp{.15e4} are five ways of writing a floating point -number whose value is 1500. They are all equivalent. - - @xref{Numbers}, for more information. - -@node Character Type -@subsection Character Type -@cindex @sc{ASCII} character codes - - A @dfn{character} in Emacs Lisp is nothing more than an integer. In -other words, characters are represented by their character codes. For -example, the character @kbd{A} is represented as the @w{integer 65}. - - Individual characters are not often used in programs. It is far more -common to work with @emph{strings}, which are sequences composed of -characters. @xref{String Type}. - - Characters in strings, buffers, and files are currently limited to the -range of 0 to 255---eight bits. If you store a larger integer into a -string, buffer or file, it is truncated to that range. Characters that -represent keyboard input have a much wider range. - -@cindex read syntax for characters -@cindex printed representation for characters -@cindex syntax for characters - Since characters are really integers, the printed representation of a -character is a decimal number. This is also a possible read syntax for -a character, but writing characters that way in Lisp programs is a very -bad idea. You should @emph{always} use the special read syntax formats -that Emacs Lisp provides for characters. These syntax formats start -with a question mark. - - The usual read syntax for alphanumeric characters is a question mark -followed by the character; thus, @samp{?A} for the character -@kbd{A}, @samp{?B} for the character @kbd{B}, and @samp{?a} for the -character @kbd{a}. - - For example: - -@example -?Q @result{} 81 ?q @result{} 113 -@end example - - You can use the same syntax for punctuation characters, but it is -often a good idea to add a @samp{\} to prevent Lisp mode from getting -confused. For example, @samp{?\ } is the way to write the space -character. If the character is @samp{\}, you @emph{must} use a second -@samp{\} to quote it: @samp{?\\}. - -@cindex whitespace -@cindex bell character -@cindex @samp{\a} -@cindex backspace -@cindex @samp{\b} -@cindex tab -@cindex @samp{\t} -@cindex vertical tab -@cindex @samp{\v} -@cindex formfeed -@cindex @samp{\f} -@cindex newline -@cindex @samp{\n} -@cindex return -@cindex @samp{\r} -@cindex escape -@cindex @samp{\e} - You can express the characters Control-g, backspace, tab, newline, -vertical tab, formfeed, return, and escape as @samp{?\a}, @samp{?\b}, -@samp{?\t}, @samp{?\n}, @samp{?\v}, @samp{?\f}, @samp{?\r}, @samp{?\e}, -respectively. Those values are 7, 8, 9, 10, 11, 12, 13, and 27 in -decimal. Thus, - -@example -?\a @result{} 7 ; @r{@kbd{C-g}} -?\b @result{} 8 ; @r{backspace, @key{BS}, @kbd{C-h}} -?\t @result{} 9 ; @r{tab, @key{TAB}, @kbd{C-i}} -?\n @result{} 10 ; @r{newline, @key{LFD}, @kbd{C-j}} -?\v @result{} 11 ; @r{vertical tab, @kbd{C-k}} -?\f @result{} 12 ; @r{formfeed character, @kbd{C-l}} -?\r @result{} 13 ; @r{carriage return, @key{RET}, @kbd{C-m}} -?\e @result{} 27 ; @r{escape character, @key{ESC}, @kbd{C-[}} -?\\ @result{} 92 ; @r{backslash character, @kbd{\}} -@end example - -@cindex escape sequence - These sequences which start with backslash are also known as -@dfn{escape sequences}, because backslash plays the role of an escape -character; this usage has nothing to do with the character @key{ESC}. - -@cindex control characters - Control characters may be represented using yet another read syntax. -This consists of a question mark followed by a backslash, caret, and the -corresponding non-control character, in either upper or lower case. For -example, both @samp{?\^I} and @samp{?\^i} are valid read syntax for the -character @kbd{C-i}, the character whose value is 9. - - Instead of the @samp{^}, you can use @samp{C-}; thus, @samp{?\C-i} is -equivalent to @samp{?\^I} and to @samp{?\^i}: - -@example -?\^I @result{} 9 ?\C-I @result{} 9 -@end example - - For use in strings and buffers, you are limited to the control -characters that exist in @sc{ASCII}, but for keyboard input purposes, -you can turn any character into a control character with @samp{C-}. The -character codes for these non-@sc{ASCII} control characters include the -2**22 bit as well as the code for the corresponding non-control -character. Ordinary terminals have no way of generating non-@sc{ASCII} -control characters, but you can generate them straightforwardly using an -X terminal. - - You can think of the @key{DEL} character as @kbd{Control-?}: - -@example -?\^? @result{} 127 ?\C-? @result{} 127 -@end example - - For representing control characters to be found in files or strings, -we recommend the @samp{^} syntax; for control characters in keyboard -input, we prefer the @samp{C-} syntax. This does not affect the meaning -of the program, but may guide the understanding of people who read it. - -@cindex meta characters - A @dfn{meta character} is a character typed with the @key{META} -modifier key. The integer that represents such a character has the -2**23 bit set (which on most machines makes it a negative number). We -use high bits for this and other modifiers to make possible a wide range -of basic character codes. - - In a string, the 2**7 bit indicates a meta character, so the meta -characters that can fit in a string have codes in the range from 128 to -255, and are the meta versions of the ordinary @sc{ASCII} characters. -(In Emacs versions 18 and older, this convention was used for characters -outside of strings as well.) - - The read syntax for meta characters uses @samp{\M-}. For example, -@samp{?\M-A} stands for @kbd{M-A}. You can use @samp{\M-} together with -octal codes, @samp{\C-}, or any other syntax for a character. Thus, you -can write @kbd{M-A} as @samp{?\M-A}, or as @samp{?\M-\101}. Likewise, -you can write @kbd{C-M-b} as @samp{?\M-\C-b}, @samp{?\C-\M-b}, or -@samp{?\M-\002}. - - The case of an ordinary letter is indicated by its character code as -part of @sc{ASCII}, but @sc{ASCII} has no way to represent whether a -control character is upper case or lower case. Emacs uses the 2**21 bit -to indicate that the shift key was used for typing a control character. -This distinction is possible only when you use X terminals or other -special terminals; ordinary terminals do not indicate the distinction to -the computer in any way. - -@cindex hyper characters -@cindex super characters -@cindex alt characters - The X Window System defines three other modifier bits that can be set -in a character: @dfn{hyper}, @dfn{super} and @dfn{alt}. The syntaxes -for these bits are @samp{\H-}, @samp{\s-} and @samp{\A-}. Thus, -@samp{?\H-\M-\A-x} represents @kbd{Alt-Hyper-Meta-x}. Numerically, the -bit values are 2**18 for alt, 2**19 for super and 2**20 for hyper. - -@cindex @samp{?} in character constant -@cindex question mark in character constant -@cindex @samp{\} in character constant -@cindex backslash in character constant -@cindex octal character code - Finally, the most general read syntax consists of a question mark -followed by a backslash and the character code in octal (up to three -octal digits); thus, @samp{?\101} for the character @kbd{A}, -@samp{?\001} for the character @kbd{C-a}, and @code{?\002} for the -character @kbd{C-b}. Although this syntax can represent any @sc{ASCII} -character, it is preferred only when the precise octal value is more -important than the @sc{ASCII} representation. - -@example -@group -?\012 @result{} 10 ?\n @result{} 10 ?\C-j @result{} 10 -?\101 @result{} 65 ?A @result{} 65 -@end group -@end example - - A backslash is allowed, and harmless, preceding any character without -a special escape meaning; thus, @samp{?\+} is equivalent to @samp{?+}. -There is no reason to add a backslash before most characters. However, -you should add a backslash before any of the characters -@samp{()\|;'`"#.,} to avoid confusing the Emacs commands for editing -Lisp code. Also add a backslash before whitespace characters such as -space, tab, newline and formfeed. However, it is cleaner to use one of -the easily readable escape sequences, such as @samp{\t}, instead of an -actual whitespace character such as a tab. - -@node Sequence Type -@subsection Sequence Types - - A @dfn{sequence} is a Lisp object that represents an ordered set of -elements. There are two kinds of sequence in Emacs Lisp, lists and -arrays. Thus, an object of type list or of type array is also -considered a sequence. - - Arrays are further subdivided into strings and vectors. Vectors can -hold elements of any type, but string elements must be characters in the -range from 0 to 255. However, the characters in a string can have text -properties; vectors do not support text properties even when their -elements happen to be characters. - - Lists, strings and vectors are different, but they have important -similarities. For example, all have a length @var{l}, and all have -elements which can be indexed from zero to @var{l} minus one. Also, -several functions, called sequence functions, accept any kind of -sequence. For example, the function @code{elt} can be used to extract -an element of a sequence, given its index. @xref{Sequences Arrays -Vectors}. - - It is impossible to read the same sequence twice, since sequences are -always created anew upon reading. If you read the read syntax for a -sequence twice, you get two sequences with equal contents. There is one -exception: the empty list @code{()} always stands for the same object, -@code{nil}. - -@node List Type -@subsection List Type -@cindex address field of register -@cindex decrement field of register - - A @dfn{list} is a series of cons cells, linked together. A @dfn{cons -cell} is an object comprising two pointers named the @sc{car} and the -@sc{cdr}. Each of them can point to any Lisp object, but when the cons -cell is part of a list, the @sc{cdr} points either to another cons cell -or to the empty list. @xref{Lists}, for functions that work on lists. - - The names @sc{car} and @sc{cdr} have only historical meaning now. The -original Lisp implementation ran on an @w{IBM 704} computer which -divided words into two parts, called the ``address'' part and the -``decrement''; @sc{car} was an instruction to extract the contents of -the address part of a register, and @sc{cdr} an instruction to extract -the contents of the decrement. By contrast, ``cons cells'' are named -for the function @code{cons} that creates them, which in turn is named -for its purpose, the construction of cells. - -@cindex atom - Because cons cells are so central to Lisp, we also have a word for -``an object which is not a cons cell''. These objects are called -@dfn{atoms}. - -@cindex parenthesis - The read syntax and printed representation for lists are identical, and -consist of a left parenthesis, an arbitrary number of elements, and a -right parenthesis. - - Upon reading, each object inside the parentheses becomes an element -of the list. That is, a cons cell is made for each element. The -@sc{car} of the cons cell points to the element, and its @sc{cdr} points -to the next cons cell which holds the next element in the list. The -@sc{cdr} of the last cons cell is set to point to @code{nil}. - -@cindex box diagrams, for lists -@cindex diagrams, boxed, for lists - A list can be illustrated by a diagram in which the cons cells are -shown as pairs of boxes. (The Lisp reader cannot read such an -illustration; unlike the textual notation, which can be understood both -humans and computers, the box illustrations can only be understood by -humans.) The following represents the three-element list @code{(rose -violet buttercup)}: - -@example -@group - ___ ___ ___ ___ ___ ___ - |___|___|--> |___|___|--> |___|___|--> nil - | | | - | | | - --> rose --> violet --> buttercup -@end group -@end example - - In this diagram, each box represents a slot that can refer to any Lisp -object. Each pair of boxes represents a cons cell. Each arrow is a -reference to a Lisp object, either an atom or another cons cell. - - In this example, the first box, the @sc{car} of the first cons cell, -refers to or ``contains'' @code{rose} (a symbol). The second box, the -@sc{cdr} of the first cons cell, refers to the next pair of boxes, the -second cons cell. The @sc{car} of the second cons cell refers to -@code{violet} and the @sc{cdr} refers to the third cons cell. The -@sc{cdr} of the third (and last) cons cell refers to @code{nil}. - -Here is another diagram of the same list, @code{(rose violet -buttercup)}, sketched in a different manner: - -@smallexample -@group - --------------- ---------------- ------------------- -| car | cdr | | car | cdr | | car | cdr | -| rose | o-------->| violet | o-------->| buttercup | nil | -| | | | | | | | | - --------------- ---------------- ------------------- -@end group -@end smallexample - -@cindex @samp{(@dots{})} in lists -@cindex @code{nil} in lists -@cindex empty list - A list with no elements in it is the @dfn{empty list}; it is identical -to the symbol @code{nil}. In other words, @code{nil} is both a symbol -and a list. - - Here are examples of lists written in Lisp syntax: - -@example -(A 2 "A") ; @r{A list of three elements.} -() ; @r{A list of no elements (the empty list).} -nil ; @r{A list of no elements (the empty list).} -("A ()") ; @r{A list of one element: the string @code{"A ()"}.} -(A ()) ; @r{A list of two elements: @code{A} and the empty list.} -(A nil) ; @r{Equivalent to the previous.} -((A B C)) ; @r{A list of one element} - ; @r{(which is a list of three elements).} -@end example - - Here is the list @code{(A ())}, or equivalently @code{(A nil)}, -depicted with boxes and arrows: - -@example -@group - ___ ___ ___ ___ - |___|___|--> |___|___|--> nil - | | - | | - --> A --> nil -@end group -@end example - -@menu -* Dotted Pair Notation:: An alternative syntax for lists. -* Association List Type:: A specially constructed list. -@end menu - -@node Dotted Pair Notation -@comment node-name, next, previous, up -@subsubsection Dotted Pair Notation -@cindex dotted pair notation -@cindex @samp{.} in lists - - @dfn{Dotted pair notation} is an alternative syntax for cons cells -that represents the @sc{car} and @sc{cdr} explicitly. In this syntax, -@code{(@var{a} .@: @var{b})} stands for a cons cell whose @sc{car} is -the object @var{a}, and whose @sc{cdr} is the object @var{b}. Dotted -pair notation is therefore more general than list syntax. In the dotted -pair notation, the list @samp{(1 2 3)} is written as @samp{(1 . (2 . (3 -. nil)))}. For @code{nil}-terminated lists, the two notations produce -the same result, but list notation is usually clearer and more -convenient when it is applicable. When printing a list, the dotted pair -notation is only used if the @sc{cdr} of a cell is not a list. - - Here's how box notation can illustrate dotted pairs. This example -shows the pair @code{(rose . violet)}: - -@example -@group - ___ ___ - |___|___|--> violet - | - | - --> rose -@end group -@end example - - Dotted pair notation can be combined with list notation to represent a -chain of cons cells with a non-@code{nil} final @sc{cdr}. For example, -@code{(rose violet . buttercup)} is equivalent to @code{(rose . (violet -. buttercup))}. The object looks like this: - -@example -@group - ___ ___ ___ ___ - |___|___|--> |___|___|--> buttercup - | | - | | - --> rose --> violet -@end group -@end example - - These diagrams make it evident why @w{@code{(rose .@: violet .@: -buttercup)}} is invalid syntax; it would require a cons cell that has -three parts rather than two. - - The list @code{(rose violet)} is equivalent to @code{(rose . (violet))} -and looks like this: - -@example -@group - ___ ___ ___ ___ - |___|___|--> |___|___|--> nil - | | - | | - --> rose --> violet -@end group -@end example - - Similarly, the three-element list @code{(rose violet buttercup)} -is equivalent to @code{(rose . (violet . (buttercup)))}. -@ifinfo -It looks like this: - -@example -@group - ___ ___ ___ ___ ___ ___ - |___|___|--> |___|___|--> |___|___|--> nil - | | | - | | | - --> rose --> violet --> buttercup -@end group -@end example -@end ifinfo - -@node Association List Type -@comment node-name, next, previous, up -@subsubsection Association List Type - - An @dfn{association list} or @dfn{alist} is a specially-constructed -list whose elements are cons cells. In each element, the @sc{car} is -considered a @dfn{key}, and the @sc{cdr} is considered an -@dfn{associated value}. (In some cases, the associated value is stored -in the @sc{car} of the @sc{cdr}.) Association lists are often used as -stacks, since it is easy to add or remove associations at the front of -the list. - - For example, - -@example -(setq alist-of-colors - '((rose . red) (lily . white) (buttercup . yellow))) -@end example - -@noindent -sets the variable @code{alist-of-colors} to an alist of three elements. In the -first element, @code{rose} is the key and @code{red} is the value. - - @xref{Association Lists}, for a further explanation of alists and for -functions that work on alists. - -@node Array Type -@subsection Array Type - - An @dfn{array} is composed of an arbitrary number of slots for -referring to other Lisp objects, arranged in a contiguous block of -memory. Accessing any element of an array takes a the same amount of -time. In contrast, accessing an element of a list requires time -proportional to the position of the element in the list. (Elements at -the end of a list take longer to access than elements at the beginning -of a list.) - - Emacs defines two types of array, strings and vectors. A string is an -array of characters and a vector is an array of arbitrary objects. Both -are one-dimensional. (Most other programming languages support -multidimensional arrays, but they are not essential; you can get the -same effect with an array of arrays.) Each type of array has its own -read syntax; see @ref{String Type}, and @ref{Vector Type}. - - An array may have any length up to the largest integer; but once -created, it has a fixed size. The first element of an array has index -zero, the second element has index 1, and so on. This is called -@dfn{zero-origin} indexing. For example, an array of four elements has -indices 0, 1, 2, @w{and 3}. - - The array type is contained in the sequence type and contains both the -string type and the vector type. - -@node String Type -@subsection String Type - - A @dfn{string} is an array of characters. Strings are used for many -purposes in Emacs, as can be expected in a text editor; for example, as -the names of Lisp symbols, as messages for the user, and to represent -text extracted from buffers. Strings in Lisp are constants: evaluation -of a string returns the same string. - -@cindex @samp{"} in strings -@cindex double-quote in strings -@cindex @samp{\} in strings -@cindex backslash in strings - The read syntax for strings is a double-quote, an arbitrary number of -characters, and another double-quote, @code{"like this"}. The Lisp -reader accepts the same formats for reading the characters of a string -as it does for reading single characters (without the question mark that -begins a character literal). You can enter a nonprinting character such -as tab, @kbd{C-a} or @kbd{M-C-A} using the convenient escape sequences, -like this: @code{"\t, \C-a, \M-\C-a"}. You can include a double-quote -in a string by preceding it with a backslash; thus, @code{"\""} is a -string containing just a single double-quote character. -(@xref{Character Type}, for a description of the read syntax for -characters.) - - If you use the @samp{\M-} syntax to indicate a meta character in a -string constant, this sets the 2**7 bit of the character in the string. -This is not the same representation that the meta modifier has in a -character on its own (not inside a string). @xref{Character Type}. - - Strings cannot hold characters that have the hyper, super or alt -modifiers; they can hold @sc{ASCII} control characters, but no others. -They do not distinguish case in @sc{ASCII} control characters. - - In contrast with the C programming language, Emacs Lisp allows -newlines in string literals. But an escaped newline---one that is -preceded by @samp{\}---does not become part of the string; i.e., the -Lisp reader ignores an escaped newline in a string literal. -@cindex newline in strings - -@example -"It is useful to include newlines -in documentation strings, -but the newline is \ -ignored if escaped." - @result{} "It is useful to include newlines -in documentation strings, -but the newline is ignored if escaped." -@end example - - The printed representation of a string consists of a double-quote, the -characters it contains, and another double-quote. However, any -backslash or double-quote characters in the string are preceded with a -backslash like this: @code{"this \" is an embedded quote"}. - - A string can hold properties of the text it contains, in addition to -the characters themselves. This enables programs that copy text between -strings and buffers to preserve the properties with no special effort. -@xref{Text Properties}. Strings with text properties have a special -read and print syntax: - -@example -#("@var{characters}" @var{property-data}...) -@end example - -@noindent -where @var{property-data} consists of zero or more elements, in groups -of three as follows: - -@example -@var{beg} @var{end} @var{plist} -@end example - -@noindent -The elements @var{beg} and @var{end} are integers, and together specify -a range of indices in the string; @var{plist} is the property list for -that range. - - @xref{Strings and Characters}, for functions that work on strings. - -@node Vector Type -@subsection Vector Type - - A @dfn{vector} is a one-dimensional array of elements of any type. It -takes a constant amount of time to access any element of a vector. (In -a list, the access time of an element is proportional to the distance of -the element from the beginning of the list.) - - The printed representation of a vector consists of a left square -bracket, the elements, and a right square bracket. This is also the -read syntax. Like numbers and strings, vectors are considered constants -for evaluation. - -@example -[1 "two" (three)] ; @r{A vector of three elements.} - @result{} [1 "two" (three)] -@end example - - @xref{Vectors}, for functions that work with vectors. - -@node Symbol Type -@subsection Symbol Type - - A @dfn{symbol} in GNU Emacs Lisp is an object with a name. The symbol -name serves as the printed representation of the symbol. In ordinary -use, the name is unique---no two symbols have the same name. - - A symbol can serve as a variable, as a function name, or to hold a -property list. Or it may serve only to be distinct from all other Lisp -objects, so that its presence in a data structure may be recognized -reliably. In a given context, usually only one of these uses is -intended. But you can use one symbol in all of these ways, -independently. - -@cindex @samp{\} in symbols -@cindex backslash in symbols - A symbol name can contain any characters whatever. Most symbol names -are written with letters, digits, and the punctuation characters -@samp{-+=*/}. Such names require no special punctuation; the characters -of the name suffice as long as the name does not look like a number. -(If it does, write a @samp{\} at the beginning of the name to force -interpretation as a symbol.) The characters @samp{_~!@@$%^&:<>@{@}} are -less often used but also require no special punctuation. Any other -characters may be included in a symbol's name by escaping them with a -backslash. In contrast to its use in strings, however, a backslash in -the name of a symbol quotes the single character that follows the -backslash, without conversion. For example, in a string, @samp{\t} -represents a tab character; in the name of a symbol, however, @samp{\t} -merely quotes the letter @kbd{t}. To have a symbol with a tab character -in its name, you must actually use a tab (preceded with a backslash). -But it's rare to do such a thing. - -@cindex CL note---case of letters -@quotation -@b{Common Lisp note:} in Common Lisp, lower case letters are always -``folded'' to upper case, unless they are explicitly escaped. This is -in contrast to Emacs Lisp, in which upper case and lower case letters -are distinct. -@end quotation - - Here are several examples of symbol names. Note that the @samp{+} in -the fifth example is escaped to prevent it from being read as a number. -This is not necessary in the last example because the rest of the name -makes it invalid as a number. - -@example -@group -foo ; @r{A symbol named @samp{foo}.} -FOO ; @r{A symbol named @samp{FOO}, different from @samp{foo}.} -char-to-string ; @r{A symbol named @samp{char-to-string}.} -@end group -@group -1+ ; @r{A symbol named @samp{1+}} - ; @r{(not @samp{+1}, which is an integer).} -@end group -@group -\+1 ; @r{A symbol named @samp{+1}} - ; @r{(not a very readable name).} -@end group -@group -\(*\ 1\ 2\) ; @r{A symbol named @samp{(* 1 2)} (a worse name).} -@c the @'s in this next line use up three characters, hence the -@c apparent misalignment of the comment. -+-*/_~!@@$%^&=:<>@{@} ; @r{A symbol named @samp{+-*/_~!@@$%^&=:<>@{@}}.} - ; @r{These characters need not be escaped.} -@end group -@end example - -@node Lisp Function Type -@subsection Lisp Function Type - - Just as functions in other programming languages are executable, -@dfn{Lisp function} objects are pieces of executable code. However, -functions in Lisp are primarily Lisp objects, and only secondarily the -text which represents them. These Lisp objects are lambda expressions: -lists whose first element is the symbol @code{lambda} (@pxref{Lambda -Expressions}). - - In most programming languages, it is impossible to have a function -without a name. In Lisp, a function has no intrinsic name. A lambda -expression is also called an @dfn{anonymous function} (@pxref{Anonymous -Functions}). A named function in Lisp is actually a symbol with a valid -function in its function cell (@pxref{Defining Functions}). - - Most of the time, functions are called when their names are written in -Lisp expressions in Lisp programs. However, you can construct or obtain -a function object at run time and then call it with the primitive -functions @code{funcall} and @code{apply}. @xref{Calling Functions}. - -@node Lisp Macro Type -@subsection Lisp Macro Type - - A @dfn{Lisp macro} is a user-defined construct that extends the Lisp -language. It is represented as an object much like a function, but with -different parameter-passing semantics. A Lisp macro has the form of a -list whose first element is the symbol @code{macro} and whose @sc{cdr} -is a Lisp function object, including the @code{lambda} symbol. - - Lisp macro objects are usually defined with the built-in -@code{defmacro} function, but any list that begins with @code{macro} is -a macro as far as Emacs is concerned. @xref{Macros}, for an explanation -of how to write a macro. - -@node Primitive Function Type -@subsection Primitive Function Type -@cindex special forms - - A @dfn{primitive function} is a function callable from Lisp but -written in the C programming language. Primitive functions are also -called @dfn{subrs} or @dfn{built-in functions}. (The word ``subr'' is -derived from ``subroutine''.) Most primitive functions evaluate all -their arguments when they are called. A primitive function that does -not evaluate all its arguments is called a @dfn{special form} -(@pxref{Special Forms}).@refill - - It does not matter to the caller of a function whether the function is -primitive. However, this does matter if you try to substitute a -function written in Lisp for a primitive of the same name. The reason -is that the primitive function may be called directly from C code. -Calls to the redefined function from Lisp will use the new definition, -but calls from C code may still use the built-in definition. - - The term @dfn{function} refers to all Emacs functions, whether written -in Lisp or C. @xref{Lisp Function Type}, for information about the -functions written in Lisp.@refill - - Primitive functions have no read syntax and print in hash notation -with the name of the subroutine. - -@example -@group -(symbol-function 'car) ; @r{Access the function cell} - ; @r{of the symbol.} - @result{} #<subr car> -(subrp (symbol-function 'car)) ; @r{Is this a primitive function?} - @result{} t ; @r{Yes.} -@end group -@end example - -@node Byte-Code Type -@subsection Byte-Code Function Type - -The byte compiler produces @dfn{byte-code function objects}. -Internally, a byte-code function object is much like a vector; however, -the evaluator handles this data type specially when it appears as a -function to be called. @xref{Byte Compilation}, for information about -the byte compiler. - -The printed representation for a byte-code function object is like that -for a vector, with an additional @samp{#} before the opening @samp{[}. - -@node Autoload Type -@subsection Autoload Type - - An @dfn{autoload object} is a list whose first element is the symbol -@code{autoload}. It is stored as the function definition of a symbol as -a placeholder for the real definition; it says that the real definition -is found in a file of Lisp code that should be loaded when necessary. -The autoload object contains the name of the file, plus some other -information about the real definition. - - After the file has been loaded, the symbol should have a new function -definition that is not an autoload object. The new definition is then -called as if it had been there to begin with. From the user's point of -view, the function call works as expected, using the function definition -in the loaded file. - - An autoload object is usually created with the function -@code{autoload}, which stores the object in the function cell of a -symbol. @xref{Autoload}, for more details. - -@node Editing Types -@section Editing Types -@cindex editing types - - The types in the previous section are common to many Lisp dialects. -Emacs Lisp provides several additional data types for purposes connected -with editing. - -@menu -* Buffer Type:: The basic object of editing. -* Marker Type:: A position in a buffer. -* Window Type:: Buffers are displayed in windows. -* Frame Type:: Windows subdivide frames. -* Window Configuration Type:: Recording the way a frame is subdivided. -* Process Type:: A process running on the underlying OS. -* Stream Type:: Receive or send characters. -* Keymap Type:: What function a keystroke invokes. -* Syntax Table Type:: What a character means. -* Display Table Type:: How display tables are represented. -* Overlay Type:: How an overlay is represented. -@end menu - -@node Buffer Type -@subsection Buffer Type - - A @dfn{buffer} is an object that holds text that can be edited -(@pxref{Buffers}). Most buffers hold the contents of a disk file -(@pxref{Files}) so they can be edited, but some are used for other -purposes. Most buffers are also meant to be seen by the user, and -therefore displayed, at some time, in a window (@pxref{Windows}). But a -buffer need not be displayed in any window. - - The contents of a buffer are much like a string, but buffers are not -used like strings in Emacs Lisp, and the available operations are -different. For example, insertion of text into a buffer is very -efficient, whereas ``inserting'' text into a string requires -concatenating substrings, and the result is an entirely new string -object. - - Each buffer has a designated position called @dfn{point} -(@pxref{Positions}). At any time, one buffer is the @dfn{current -buffer}. Most editing commands act on the contents of the current -buffer in the neighborhood of point. Many other functions manipulate or -test the characters in the current buffer; a whole chapter in this -manual is devoted to describing these functions (@pxref{Text}). - - Several other data structures are associated with each buffer: - -@itemize @bullet -@item -a local syntax table (@pxref{Syntax Tables}); - -@item -a local keymap (@pxref{Keymaps}); and, - -@item -a local variable binding list (@pxref{Buffer-Local Variables}). -@end itemize - -@noindent -The local keymap and variable list contain entries which individually -override global bindings or values. These are used to customize the -behavior of programs in different buffers, without actually changing the -programs. - - Buffers have no read syntax. They print in hash notation with the -buffer name. - -@example -@group -(current-buffer) - @result{} #<buffer objects.texi> -@end group -@end example - -@node Marker Type -@subsection Marker Type - - A @dfn{marker} denotes a position in a specific buffer. Markers -therefore have two components: one for the buffer, and one for the -position. Changes in the buffer's text automatically relocate the -position value as necessary to ensure that the marker always points -between the same two characters in the buffer. - - Markers have no read syntax. They print in hash notation, giving the -current character position and the name of the buffer. - -@example -@group -(point-marker) - @result{} #<marker at 10779 in objects.texi> -@end group -@end example - -@xref{Markers}, for information on how to test, create, copy, and move -markers. - -@node Window Type -@subsection Window Type - - A @dfn{window} describes the portion of the terminal screen that Emacs -uses to display a buffer. Every window has one associated buffer, whose -contents appear in the window. By contrast, a given buffer may appear -in one window, no window, or several windows. - - Though many windows may exist simultaneously, at any time one window -is designated the @dfn{selected window}. This is the window where the -cursor is (usually) displayed when Emacs is ready for a command. The -selected window usually displays the current buffer, but this is not -necessarily the case. - - Windows are grouped on the screen into frames; each window belongs to -one and only one frame. @xref{Frame Type}. - - Windows have no read syntax. They print in hash notation, giving the -window number and the name of the buffer being displayed. The window -numbers exist to identify windows uniquely, since the buffer displayed -in any given window can change frequently. - -@example -@group -(selected-window) - @result{} #<window 1 on objects.texi> -@end group -@end example - - @xref{Windows}, for a description of the functions that work on windows. - -@node Frame Type -@subsection Frame Type - - A @var{frame} is a rectangle on the screen that contains one or more -Emacs windows. A frame initially contains a single main window (plus -perhaps a minibuffer window) which you can subdivide vertically or -horizontally into smaller windows. - - Frames have no read syntax. They print in hash notation, giving the -frame's title, plus its address in core (useful to identify the frame -uniquely). - -@example -@group -(selected-frame) - @result{} #<frame xemacs@@mole.gnu.ai.mit.edu 0xdac80> -@end group -@end example - - @xref{Frames}, for a description of the functions that work on frames. - -@node Window Configuration Type -@subsection Window Configuration Type -@cindex screen layout - - A @dfn{window configuration} stores information about the positions, -sizes, and contents of the windows in a frame, so you can recreate the -same arrangement of windows later. - - Window configurations do not have a read syntax. They print as -@samp{#<window-configuration>}. @xref{Window Configurations}, for a -description of several functions related to window configurations. - -@node Process Type -@subsection Process Type - - The word @dfn{process} usually means a running program. Emacs itself -runs in a process of this sort. However, in Emacs Lisp, a process is a -Lisp object that designates a subprocess created by the Emacs process. -Programs such as shells, GDB, ftp, and compilers, running in -subprocesses of Emacs, extend the capabilities of Emacs. - - An Emacs subprocess takes textual input from Emacs and returns textual -output to Emacs for further manipulation. Emacs can also send signals -to the subprocess. - - Process objects have no read syntax. They print in hash notation, -giving the name of the process: - -@example -@group -(process-list) - @result{} (#<process shell>) -@end group -@end example - -@xref{Processes}, for information about functions that create, delete, -return information about, send input or signals to, and receive output -from processes. - -@node Stream Type -@subsection Stream Type - - A @dfn{stream} is an object that can be used as a source or sink for -characters---either to supply characters for input or to accept them as -output. Many different types can be used this way: markers, buffers, -strings, and functions. Most often, input streams (character sources) -obtain characters from the keyboard, a buffer, or a file, and output -streams (character sinks) send characters to a buffer, such as a -@file{*Help*} buffer, or to the echo area. - - The object @code{nil}, in addition to its other meanings, may be used -as a stream. It stands for the value of the variable -@code{standard-input} or @code{standard-output}. Also, the object -@code{t} as a stream specifies input using the minibuffer -(@pxref{Minibuffers}) or output in the echo area (@pxref{The Echo -Area}). - - Streams have no special printed representation or read syntax, and -print as whatever primitive type they are. - - @xref{Streams}, for a description of various functions related to -streams, including various parsing and printing functions. - -@node Keymap Type -@subsection Keymap Type - - A @dfn{keymap} maps keys typed by the user to commands. This mapping -controls how the user's command input is executed. A keymap is actually -a list whose @sc{car} is the symbol @code{keymap}. - - @xref{Keymaps}, for information about creating keymaps, handling prefix -keys, local as well as global keymaps, and changing key bindings. - -@node Syntax Table Type -@subsection Syntax Table Type - - A @dfn{syntax table} is a vector of 256 integers. Each element of the -vector defines how one character is interpreted when it appears in a -buffer. For example, in C mode (@pxref{Major Modes}), the @samp{+} -character is punctuation, but in Lisp mode it is a valid character in a -symbol. These modes specify different interpretations by changing the -syntax table entry for @samp{+}, at index 43 in the syntax table. - - Syntax tables are only used for scanning text in buffers, not for -reading Lisp expressions. The table the Lisp interpreter uses to read -expressions is built into the Emacs source code and cannot be changed; -thus, to change the list delimiters to be @samp{@{} and @samp{@}} -instead of @samp{(} and @samp{)} would be impossible. - - @xref{Syntax Tables}, for details about syntax classes and how to make -and modify syntax tables. - -@node Display Table Type -@subsection Display Table Type - - A @dfn{display table} specifies how to display each character code. -Each buffer and each window can have its own display table. A display -table is actually a vector of length 261. @xref{Display Tables}. - -@node Overlay Type -@subsection Overlay Type - - An @dfn{overlay} specifies temporary alteration of the display -appearance of a part of a buffer. It contains markers delimiting a -range of the buffer, plus a property list (a list whose elements are -alternating property names and values). Overlays are used to present -parts of the buffer temporarily in a different display style. - - @xref{Overlays}, for how to create and use overlays. They have no -read syntax, and print in hash notation, giving the buffer name and -range of positions. - -@node Type Predicates -@section Type Predicates -@cindex predicates -@cindex type checking -@kindex wrong-type-argument - - The Emacs Lisp interpreter itself does not perform type checking on -the actual arguments passed to functions when they are called. It could -not do so, since function arguments in Lisp do not have declared data -types, as they do in other programming languages. It is therefore up to -the individual function to test whether each actual argument belongs to -a type that the function can use. - - All built-in functions do check the types of their actual arguments -when appropriate, and signal a @code{wrong-type-argument} error if an -argument is of the wrong type. For example, here is what happens if you -pass an argument to @code{+} which it cannot handle: - -@example -@group -(+ 2 'a) - @error{} Wrong type argument: integer-or-marker-p, a -@end group -@end example - -@cindex type predicates -@cindex testing types - Lisp provides functions, called @dfn{type predicates}, to test whether -an object is a member of a given type. (Following a convention of long -standing, the names of most Emacs Lisp predicates end in @samp{p}.) - -Here is a table of predefined type predicates, in alphabetical order, -with references to further information. - -@table @code -@item atom -@xref{List-related Predicates, atom}. - -@item arrayp -@xref{Array Functions, arrayp}. - -@item bufferp -@xref{Buffer Basics, bufferp}. - -@item byte-code-function-p -@xref{Byte-Code Type, byte-code-function-p}. - -@item case-table-p -@xref{Case Table, case-table-p}. - -@item char-or-string-p -@xref{Predicates for Strings, char-or-string-p}. - -@item commandp -@xref{Interactive Call, commandp}. - -@item consp -@xref{List-related Predicates, consp}. - -@item floatp -@xref{Predicates on Numbers, floatp}. - -@item frame-live-p -@xref{Deleting Frames, frame-live-p}. - -@item framep -@xref{Frames, framep}. - -@item integer-or-marker-p -@xref{Predicates on Markers, integer-or-marker-p}. - -@item integerp -@xref{Predicates on Numbers, integerp}. - -@item keymapp -@xref{Creating Keymaps, keymapp}. - -@item listp -@xref{List-related Predicates, listp}. - -@item markerp -@xref{Predicates on Markers, markerp}. - -@item natnump -@xref{Predicates on Numbers, natnump}. - -@item nlistp -@xref{List-related Predicates, nlistp}. - -@item numberp -@xref{Predicates on Numbers, numberp}. - -@item number-or-marker-p -@xref{Predicates on Markers, number-or-marker-p}. - -@item overlayp -@xref{Overlays, overlayp}. - -@item processp -@xref{Processes, processp}. - -@item sequencep -@xref{Sequence Functions, sequencep}. - -@item stringp -@xref{Predicates for Strings, stringp}. - -@item subrp -@xref{Function Cells, subrp}. - -@item symbolp -@xref{Symbols, symbolp}. - -@item syntax-table-p -@xref{Syntax Tables, syntax-table-p}. - -@item user-variable-p -@xref{Defining Variables, user-variable-p}. - -@item vectorp -@xref{Vectors, vectorp}. - -@item window-configuration-p -@xref{Window Configurations, window-configuration-p}. - -@item window-live-p -@xref{Deleting Windows, window-live-p}. - -@item windowp -@xref{Basic Windows, windowp}. -@end table - -@node Equality Predicates -@section Equality Predicates -@cindex equality - - Here we describe two functions that test for equality between any two -objects. Other functions test equality between objects of specific -types, e.g., strings. See the appropriate chapter describing the data -type for these predicates. - -@defun eq object1 object2 -This function returns @code{t} if @var{object1} and @var{object2} are -the same object, @code{nil} otherwise. The ``same object'' means that a -change in one will be reflected by the same change in the other. - -@code{eq} returns @code{t} if @var{object1} and @var{object2} are -integers with the same value. Also, since symbol names are normally -unique, if the arguments are symbols with the same name, they are -@code{eq}. For other types (e.g., lists, vectors, strings), two -arguments with the same contents or elements are not necessarily -@code{eq} to each other: they are @code{eq} only if they are the same -object. - -(The @code{make-symbol} function returns an uninterned symbol that is -not interned in the standard @code{obarray}. When uninterned symbols -are in use, symbol names are no longer unique. Distinct symbols with -the same name are not @code{eq}. @xref{Creating Symbols}.) - -@example -@group -(eq 'foo 'foo) - @result{} t -@end group - -@group -(eq 456 456) - @result{} t -@end group - -@group -(eq "asdf" "asdf") - @result{} nil -@end group - -@group -(eq '(1 (2 (3))) '(1 (2 (3)))) - @result{} nil -@end group - -@group -(setq foo '(1 (2 (3)))) - @result{} (1 (2 (3))) -(eq foo foo) - @result{} t -(eq foo '(1 (2 (3)))) - @result{} nil -@end group - -@group -(eq [(1 2) 3] [(1 2) 3]) - @result{} nil -@end group - -@group -(eq (point-marker) (point-marker)) - @result{} nil -@end group -@end example - -@end defun - -@defun equal object1 object2 -This function returns @code{t} if @var{object1} and @var{object2} have -equal components, @code{nil} otherwise. Whereas @code{eq} tests if its -arguments are the same object, @code{equal} looks inside nonidentical -arguments to see if their elements are the same. So, if two objects are -@code{eq}, they are @code{equal}, but the converse is not always true. - -@example -@group -(equal 'foo 'foo) - @result{} t -@end group - -@group -(equal 456 456) - @result{} t -@end group - -@group -(equal "asdf" "asdf") - @result{} t -@end group -@group -(eq "asdf" "asdf") - @result{} nil -@end group - -@group -(equal '(1 (2 (3))) '(1 (2 (3)))) - @result{} t -@end group -@group -(eq '(1 (2 (3))) '(1 (2 (3)))) - @result{} nil -@end group - -@group -(equal [(1 2) 3] [(1 2) 3]) - @result{} t -@end group -@group -(eq [(1 2) 3] [(1 2) 3]) - @result{} nil -@end group - -@group -(equal (point-marker) (point-marker)) - @result{} t -@end group - -@group -(eq (point-marker) (point-marker)) - @result{} nil -@end group -@end example - -Comparison of strings uses @code{string=}, and is case-sensitive. - -@example -@group -(equal "asdf" "ASDF") - @result{} nil -@end group -@end example -@end defun - - The test for equality is implemented recursively, and circular lists may -therefore cause infinite recursion (leading to an error). |