@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 Lisp Data Types, 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 it remembers whatever value you store in it, type and all. 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{} # @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. The @samp{#@@@var{count}} construct, which skips the next @var{count} characters, is useful for program-generated comments containing binary data. The Emacs Lisp byte compiler uses this in its output files (@pxref{Byte Compilation}). It isn't meant for source files, however. @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. * Symbol Type:: A multi-use object that refers to a function, variable, or property list, and has a unique identity. * Sequence Type:: Both lists and arrays are classified as sequences. * Cons Cell Type:: Cons cells, and lists (which are made from cons cells). * Array Type:: Arrays include strings and vectors. * String Type:: An (efficient) array of characters. * Vector Type:: One-dimensional arrays. * Function Type:: A piece of executable code you can call from elsewhere. * 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 The range of values for integers in Emacs Lisp is @minus{}134217728 to 134217727 (28 bits; i.e., @ifinfo -2**27 @end ifinfo @tex $-2^{27}$ @end tex to @ifinfo 2**27 - 1) @end ifinfo @tex $2^{28}-1$) @end tex on most machines. (Some machines may provide a wider range.) It is important to note that the Emacs Lisp arithmetic functions do not check for overflow. Thus @code{(1+ 134217727)} is @minus{}134217728 on most machines. The read syntax for integers 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.} 268435457 ; @r{Also the integer 1!} ; @r{ (on a 28-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{\} so that the Emacs commands for editing Lisp code don't get 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 @iftex $2^{26}$ @end iftex @ifinfo 2**26 @end ifinfo 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. For historical reasons, Emacs treats the @key{DEL} character as the control equivalent of @kbd{?}: @example ?\^? @result{} 127 ?\C-? @result{} 127 @end example @noindent As a result, it is currently not possible to represent the character @kbd{Control-?}, which is a meaningful input character under X. It is not easy to change this as various Lisp files refer to @key{DEL} in this way. 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 @iftex $2^{27}$ @end iftex @ifinfo 2**27 @end ifinfo 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 @iftex $2^{7}$ @end iftex @ifinfo 2**7 @end ifinfo 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 character codes (see below), with @samp{\C-}, or with 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 @iftex $2^{25}$ @end iftex @ifinfo 2**25 @end ifinfo 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}. @iftex Numerically, the bit values are $2^{22}$ for alt, $2^{23}$ for super and $2^{24}$ for hyper. @end iftex @ifinfo Numerically, the bit values are 2**22 for alt, 2**23 for super and 2**24 for hyper. @end ifinfo @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 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 simply quotes the single character that follows the backslash. 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. In Emacs Lisp, 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 sixth 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 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 like characters in a buffer (@pxref{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 Cons Cell Type @subsection Cons Cell and List Types @cindex address field of register @cindex decrement field of register 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. A @dfn{list} is a series of cons cells, linked together so that the @sc{cdr} of each cons cell points either to another cons cell or to the empty list. @xref{Lists}, for functions that work on lists. Because most cons cells are used as part of lists, the phrase @dfn{list structure} has come to refer to any structure made out of cons cells. 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 of the list, 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 by both humans and computers, the box illustrations can be understood only 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 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 @iftex $2^{7}$ @end iftex @ifinfo 2**7 @end ifinfo 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. The printed representation of a string consists of a double-quote, the characters it contains, and another double-quote. However, you must escape any backslash or double-quote characters in the string with a backslash, like this: @code{"this \" is an embedded quote"}. The newline character is not special in the read syntax for strings; if you write a new line between the double-quotes, it becomes a character in the string. 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 while reading a string. @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 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 Function Type @subsection 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 Macro Type @subsection 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{Function Type}, for information about the functions written in Lisp. 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{} # (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 and read syntax 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 of the standard Emacs 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}). @item a list of overlays (@pxref{Overlays}). @item text properties for the text in the buffer (@pxref{Text Properties}). @end itemize @noindent The local keymap and variable list contain entries that individually override global bindings or values. These are used to customize the behavior of programs in different buffers, without actually changing the programs. A buffer may be @dfn{indirect}, which means it shares the text of another buffer. @xref{Indirect Buffers}. Buffers have no read syntax. They print in hash notation, showing the buffer name. @example @group (current-buffer) @result{} # @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{} # @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{} # @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{} # @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; their print syntax looks like @samp{#}. @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{} (#) @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{Read and Print}, for a description of functions related to streams, including 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 used only 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 262. @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. They have no read syntax, and print in hash notation, giving the buffer name and range of positions. @xref{Overlays}, for how to create and use overlays. @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{+} that 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 If you want your program to handle different types differently, you must do explicit type checking. The most common way to check the type of an object is to call a @dfn{type predicate} function. Emacs has a type predicate for each type, as well as some predicates for combinations of types. A type predicate function takes one argument; it returns @code{t} if the argument belongs to the appropriate type, and @code{nil} otherwise. Following a general Lisp convention for predicate functions, most type predicates' names end with @samp{p}. Here is an example which uses the predicates @code{listp} to check for a list and @code{symbolp} to check for a symbol. @example (defun add-on (x) (cond ((symbolp x) ;; If X is a symbol, put it on LIST. (setq list (cons x list))) ((listp x) ;; If X is a list, add its elements to LIST. (setq list (append x list))) @need 3000 (t ;; We only handle symbols and lists. (error "Invalid argument %s in add-on" x)))) @end example 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 wholenump @xref{Predicates on Numbers, wholenump}. @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 The most general way to check the type of an object is to call the function @code{type-of}. Recall that each object belongs to one and only one primitive type; @code{type-of} tells you which one (@pxref{Lisp Data Types}). But @code{type-of} knows nothing about non-primitive types. In most cases, it is more convenient to use type predicates than @code{type-of}. @defun type-of object This function returns a symbol naming the primitive type of @var{object}. The value is one of the symbols @code{symbol}, @code{integer}, @code{float}, @code{string}, @code{cons}, @code{vector}, @code{marker}, @code{overlay}, @code{window}, @code{buffer}, @code{subr}, @code{compiled-function}, @code{process}, or @code{window-configuration}. @example (type-of 1) @result{} integer (type-of 'nil) @result{} symbol (type-of '()) ; @r{@code{()} is @code{nil}.} @result{} symbol (type-of '(x)) @result{} cons @end example @end defun @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. For these predicates, see the appropriate chapter describing the data type. @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 is case-sensitive and takes account of text properties as well as the characters in the strings. To compare two strings' characters without comparing their text properties, use @code{string=} (@pxref{Text Comparison}). @example @group (equal "asdf" "ASDF") @result{} nil @end group @end example Two distinct buffers are never @code{equal}, even if their contents are the same. @end defun The test for equality is implemented recursively, and circular lists may therefore cause infinite recursion (leading to an error).