@c -*-texinfo-*- @c This is part of the GNU Emacs Lisp Reference Manual. @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2001, @c 2002, 2003, 2004, 2005, 2006, 2007 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{hash-table}, @dfn{subr}, and @dfn{byte-code function}, plus 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. (Actually, a small number of Emacs Lisp variables can only take on values of a certain type. @xref{Variables with Restricted Values}.) 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. * Circular Objects:: Read syntax for circular structure. * 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. Every data type has a unique printed representation. 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. This is not necessarily unique; many kinds of object have more than one syntax. @xref{Read and Print}. @cindex hash notation In most cases, an object's printed representation is also a read syntax for the object. However, some types have no read syntax, since it does not make sense to enter objects of these types as constants in a Lisp program. These objects are printed in @dfn{hash notation}, which consists of the characters @samp{#<}, a descriptive string (typically the type name followed by the name of the object), and a closing @samp{>}. For example: @example (current-buffer) @result{} # @end example @noindent 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 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. 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. * Char-Table Type:: One-dimensional sparse arrays indexed by characters. * Bool-Vector Type:: One-dimensional arrays of @code{t} or @code{nil}. * Hash Table Type:: Super-fast lookup tables. * 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{}268435456 to 268435455 (29 bits; i.e., @ifnottex -2**28 @end ifnottex @tex @math{-2^{28}} @end tex to @ifnottex 2**28 - 1) @end ifnottex @tex @math{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+ 268435455)} is @minus{}268435456 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.} 536870913 ; @r{Also the integer 1 on a 29-bit implementation.} @end group @end example @xref{Numbers}, for more information. @node Floating Point Type @subsection Floating Point Type Floating point numbers are the computer equivalent of scientific notation; you can think of a floating point number as a fraction together with a power of ten. The precise number of significant figures and the range of possible exponents is machine-specific; Emacs uses the C data type @code{double} to store the value, and internally this records a power of 2 rather than a power of 10. 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 @acronym{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 used occasionally in programs, but it is 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 524287---nineteen bits. But not all values in that range are valid character codes. Codes 0 through 127 are @acronym{ASCII} codes; the rest are non-@acronym{ASCII} (@pxref{Non-ASCII Characters}). Characters that represent keyboard input have a much wider range, to encode modifier keys such as Control, Meta and Shift. There are special functions for producing a human-readable textual description of a character for the sake of messages. @xref{Describing Characters}. @menu * Basic Char Syntax:: * General Escape Syntax:: * Ctl-Char Syntax:: * Meta-Char Syntax:: * Other Char Bits:: @end menu @node Basic Char Syntax @subsubsection Basic Char Syntax @cindex read syntax for characters @cindex printed representation for characters @cindex syntax for characters @cindex @samp{?} in character constant @cindex question mark in character constant 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 not clear programming. 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 open-paren 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 (ASCII character) @cindex @samp{\t} @cindex vertical tab @cindex @samp{\v} @cindex formfeed @cindex @samp{\f} @cindex newline @cindex @samp{\n} @cindex return (ASCII character) @cindex @samp{\r} @cindex escape (ASCII character) @cindex @samp{\e} @cindex space (ASCII character) @cindex @samp{\s} You can express the characters control-g, backspace, tab, newline, vertical tab, formfeed, space, return, del, and escape as @samp{?\a}, @samp{?\b}, @samp{?\t}, @samp{?\n}, @samp{?\v}, @samp{?\f}, @samp{?\s}, @samp{?\r}, @samp{?\d}, and @samp{?\e}, respectively. (@samp{?\s} followed by a dash has a different meaning---it applies the ``super'' modifier to the following character.) Thus, @example ?\a @result{} 7 ; @r{control-g, @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, @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-[}} ?\s @result{} 32 ; @r{space character, @key{SPC}} ?\\ @result{} 92 ; @r{backslash character, @kbd{\}} ?\d @result{} 127 ; @r{delete character, @key{DEL}} @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 terminology has nothing to do with the character @key{ESC}. @samp{\s} is meant for use in character constants; in string constants, just write the space. 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. You can 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} or @samp{\s}, instead of an actual whitespace character such as a tab or a space. (If you do write backslash followed by a space, you should write an extra space after the character constant to separate it from the following text.) @node General Escape Syntax @subsubsection General Escape Syntax In addition to the specific excape sequences for special important control characters, Emacs provides general categories of escape syntax that you can use to specify non-ASCII text characters. @cindex unicode character escape For instance, you can specify characters by their Unicode values. @code{?\u@var{nnnn}} represents a character that maps to the Unicode code point @samp{U+@var{nnnn}}. There is a slightly different syntax for specifying characters with code points above @code{#xFFFF}; @code{\U00@var{nnnnnn}} represents the character whose Unicode code point is @samp{U+@var{nnnnnn}}, if such a character is supported by Emacs. If the corresponding character is not supported, Emacs signals an error. This peculiar and inconvenient syntax was adopted for compatibility with other programming languages. Unlike some other languages, Emacs Lisp supports this syntax in only character literals and strings. @cindex @samp{\} in character constant @cindex backslash in character constant @cindex octal character code The most general read syntax for a character represents the character code in either octal or hex. To use octal, write a question mark followed by a backslash and the octal character code (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 @acronym{ASCII} character, it is preferred only when the precise octal value is more important than the @acronym{ASCII} representation. @example @group ?\012 @result{} 10 ?\n @result{} 10 ?\C-j @result{} 10 ?\101 @result{} 65 ?A @result{} 65 @end group @end example To use hex, write a question mark followed by a backslash, @samp{x}, and the hexadecimal character code. You can use any number of hex digits, so you can represent any character code in this way. Thus, @samp{?\x41} for the character @kbd{A}, @samp{?\x1} for the character @kbd{C-a}, and @code{?\x8e0} for the Latin-1 character @iftex @samp{@`a}. @end iftex @ifnottex @samp{a} with grave accent. @end ifnottex @node Ctl-Char Syntax @subsubsection Control-Character Syntax @cindex control characters Control characters can 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 In strings and buffers, the only control characters allowed are those that exist in @acronym{ASCII}; but for keyboard input purposes, you can turn any character into a control character with @samp{C-}. The character codes for these non-@acronym{ASCII} control characters include the @tex @math{2^{26}} @end tex @ifnottex 2**26 @end ifnottex bit as well as the code for the corresponding non-control character. Ordinary terminals have no way of generating non-@acronym{ASCII} control characters, but you can generate them straightforwardly using X and other window systems. 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, using @samp{\C-}. 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. Which one you use does not affect the meaning of the program, but may guide the understanding of people who read it. @node Meta-Char Syntax @subsubsection Meta-Character Syntax @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 @tex @math{2^{27}} @end tex @ifnottex 2**27 @end ifnottex bit set. We use high bits for this and other modifiers to make possible a wide range of basic character codes. In a string, the @tex @math{2^{7}} @end tex @ifnottex 2**7 @end ifnottex bit attached to an @acronym{ASCII} character indicates a meta character; thus, 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 @acronym{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}. @node Other Char Bits @subsubsection Other Character Modifier Bits The case of a graphic character is indicated by its character code; for example, @acronym{ASCII} distinguishes between the characters @samp{a} and @samp{A}. But @acronym{ASCII} has no way to represent whether a control character is upper case or lower case. Emacs uses the @tex @math{2^{25}} @end tex @ifnottex 2**25 @end ifnottex bit to indicate that the shift key was used in typing a control character. This distinction is possible only when you use X terminals or other special terminals; ordinary terminals do not report the distinction to the computer in any way. The Lisp syntax for the shift bit is @samp{\S-}; thus, @samp{?\C-\S-o} or @samp{?\C-\S-O} represents the shifted-control-o character. @cindex hyper characters @cindex super characters @cindex alt characters The X Window System defines three other @anchor{modifier bits}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-}. (Case is significant in these prefixes.) Thus, @samp{?\H-\M-\A-x} represents @kbd{Alt-Hyper-Meta-x}. (Note that @samp{\s} with no following @samp{-} represents the space character.) @tex Numerically, the bit values are @math{2^{22}} for alt, @math{2^{23}} for super and @math{2^{24}} for hyper. @end tex @ifnottex Numerically, the bit values are 2**22 for alt, 2**23 for super and 2**24 for hyper. @end ifnottex @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 Lisp use, with one single obarray (@pxref{Creating Symbols}, a symbol's 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. A symbol whose name starts with a colon (@samp{:}) is called a @dfn{keyword symbol}. These symbols automatically act as constants, and are normally used only by comparing an unknown symbol with a few specific alternatives. @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 @samp{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 fourth 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 @ifinfo @c This uses ``colon'' instead of a literal `:' because Info cannot @c cope with a `:' in a menu @cindex @samp{#@var{colon}} read syntax @end ifinfo @ifnotinfo @cindex @samp{#:} read syntax @end ifnotinfo Normally the Lisp reader interns all symbols (@pxref{Creating Symbols}). To prevent interning, you can write @samp{#:} before the name of the symbol. @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, vectors, char-tables and bool-vectors. Vectors can hold elements of any type, but string elements must be characters, and bool-vector elements must be @code{t} or @code{nil}. Char-tables are like vectors except that they are indexed by any valid character code. The characters in a string can have text properties like characters in a buffer (@pxref{Text Properties}), but vectors do not support text properties, even when their elements happen to be characters. Lists, strings and the other array types 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. 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 generally 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 @cindex pointers A @dfn{cons cell} is an object that consists of two slots, called the @sc{car} slot and the @sc{cdr} slot. Each slot can @dfn{hold} or @dfn{refer to} any Lisp object. We also say that ``the @sc{car} of this cons cell is'' whatever object its @sc{car} slot currently holds, and likewise for the @sc{cdr}. @quotation A note to C programmers: in Lisp, we do not distinguish between ``holding'' a value and ``pointing to'' the value, because pointers in Lisp are implicit. @end quotation A @dfn{list} is a series of cons cells, linked together so that the @sc{cdr} slot of each cons cell holds either the next cons cell or the empty list. The empty list is actually the symbol @code{nil}. @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. @cindex atoms 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 @cindex @samp{(@dots{})} in lists 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. Here are examples of lists: @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 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} slot of the cons cell holds the element, and its @sc{cdr} slot refers to the next cons cell of the list, which holds the next element in the list. The @sc{cdr} slot of the last cons cell is set to hold @code{nil}. The names @sc{car} and @sc{cdr} derive from the history of Lisp. 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 was named for its purpose, the construction of cells. @menu * Box Diagrams:: Drawing pictures of lists. * Dotted Pair Notation:: A general syntax for cons cells. * Association List Type:: A specially constructed list. @end menu @node Box Diagrams @subsubsection Drawing Lists as Box Diagrams @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, like dominoes. (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.) This picture 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 hold or refer to any Lisp object. Each pair of boxes represents a cons cell. Each arrow represents a reference to a Lisp object, either an atom or another cons cell. In this example, the first box, which holds the @sc{car} of the first cons cell, refers to or ``holds'' @code{rose} (a symbol). The second box, holding 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 is @code{violet}, and its @sc{cdr} is the third cons cell. The @sc{cdr} of the third (and last) cons cell is @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 @code{nil} as a list @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 is the list @code{(A ())}, or equivalently @code{(A nil)}, depicted with boxes and arrows: @example @group --- --- --- --- | | |--> | | |--> nil --- --- --- --- | | | | --> A --> nil @end group @end example Here is a more complex illustration, showing the three-element list, @code{((pine needles) oak maple)}, the first element of which is a two-element list: @example @group --- --- --- --- --- --- | | |--> | | |--> | | |--> nil --- --- --- --- --- --- | | | | | | | --> oak --> maple | | --- --- --- --- --> | | |--> | | |--> nil --- --- --- --- | | | | --> pine --> needles @end group @end example The same list represented in the second box notation looks like this: @example @group -------------- -------------- -------------- | car | cdr | | car | cdr | | car | cdr | | o | o------->| oak | o------->| maple | nil | | | | | | | | | | | -- | --------- -------------- -------------- | | | -------------- ---------------- | | car | cdr | | car | cdr | ------>| pine | o------->| needles | nil | | | | | | | -------------- ---------------- @end group @end example @node Dotted Pair Notation @subsubsection Dotted Pair Notation @cindex dotted pair notation @cindex @samp{.} in lists @dfn{Dotted pair notation} is a general 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 more general than list syntax because the @sc{cdr} does not have to be a list. However, it is more cumbersome in cases where list syntax would work. In dotted pair notation, the list @samp{(1 2 3)} is written as @samp{(1 . (2 . (3 . nil)))}. For @code{nil}-terminated lists, you can use either notation, but list notation is usually clearer and more convenient. When printing a list, the dotted pair notation is only used if the @sc{cdr} of a cons cell is not a list. Here's an example using boxes to illustrate dotted pair notation. This example shows the pair @code{(rose . violet)}: @example @group --- --- | | |--> violet --- --- | | --> rose @end group @end example You can combine dotted pair notation with list notation to represent conveniently a chain of cons cells with a non-@code{nil} final @sc{cdr}. You write a dot after the last element of the list, followed by the @sc{cdr} of the final cons cell. 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 The syntax @code{(rose .@: violet .@: buttercup)} is invalid because there is nothing that it could mean. If anything, it would say to put @code{buttercup} in the @sc{cdr} of a cons cell whose @sc{cdr} is already used for @code{violet}. 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)))}. @ifnottex It looks like this: @example @group --- --- --- --- --- --- | | |--> | | |--> | | |--> nil --- --- --- --- --- --- | | | | | | --> rose --> violet --> buttercup @end group @end example @end ifnottex @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. @xref{Hash Tables}, for another kind of lookup table, which is much faster for handling a large number of keys. @node Array Type @subsection Array Type An @dfn{array} is composed of an arbitrary number of slots for holding or referring to other Lisp objects, arranged in a contiguous block of memory. Accessing any element of an array takes approximately 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 four types of array: strings, vectors, bool-vectors, and char-tables. A string is an array of characters and a vector is an array of arbitrary objects. A bool-vector can hold only @code{t} or @code{nil}. These kinds of array may have any length up to the largest integer. Char-tables are sparse arrays indexed by any valid character code; they can hold arbitrary objects. 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 largest possible index value is one less than the length of the array. Once an array is created, its length is fixed. All Emacs Lisp arrays are one-dimensional. (Most other programming languages support multidimensional arrays, but they are not essential; you can get the same effect with nested one-dimensional arrays.) Each type of array has its own read syntax; see the following sections for details. The array type is a subset of the sequence type, and contains the string type, the vector type, the bool-vector type, and the char-table 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. @xref{Strings and Characters}, for functions that operate on strings. @menu * Syntax for Strings:: * Non-ASCII in Strings:: * Nonprinting Characters:: * Text Props and Strings:: @end menu @node Syntax for Strings @subsubsection Syntax for Strings @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"}. To include a double-quote in a string, precede it with a backslash; thus, @code{"\""} is a string containing just a single double-quote character. Likewise, you can include a backslash by preceding it with another backslash, like this: @code{"this \\ is a single embedded backslash"}. @cindex newline in strings 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. An escaped space @w{@samp{\ }} is likewise ignored. @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 @node Non-ASCII in Strings @subsubsection Non-@acronym{ASCII} Characters in Strings You can include a non-@acronym{ASCII} international character in a string constant by writing it literally. There are two text representations for non-@acronym{ASCII} characters in Emacs strings (and in buffers): unibyte and multibyte. If the string constant is read from a multibyte source, such as a multibyte buffer or string, or a file that would be visited as multibyte, then the character is read as a multibyte character, and that makes the string multibyte. If the string constant is read from a unibyte source, then the character is read as unibyte and that makes the string unibyte. You can also represent a multibyte non-@acronym{ASCII} character with its character code: use a hex escape, @samp{\x@var{nnnnnnn}}, with as many digits as necessary. (Multibyte non-@acronym{ASCII} character codes are all greater than 256.) Any character which is not a valid hex digit terminates this construct. If the next character in the string could be interpreted as a hex digit, write @w{@samp{\ }} (backslash and space) to terminate the hex escape---for example, @w{@samp{\x8e0\ }} represents one character, @samp{a} with grave accent. @w{@samp{\ }} in a string constant is just like backslash-newline; it does not contribute any character to the string, but it does terminate the preceding hex escape. You can represent a unibyte non-@acronym{ASCII} character with its character code, which must be in the range from 128 (0200 octal) to 255 (0377 octal). If you write all such character codes in octal and the string contains no other characters forcing it to be multibyte, this produces a unibyte string. However, using any hex escape in a string (even for an @acronym{ASCII} character) forces the string to be multibyte. You can also specify characters in a string by their numeric values in Unicode, using @samp{\u} and @samp{\U} (@pxref{Character Type}). @xref{Text Representations}, for more information about the two text representations. @node Nonprinting Characters @subsubsection Nonprinting Characters in Strings You can use the same backslash escape-sequences in a string constant as in character literals (but do not use the question mark that begins a character constant). For example, you can write a string containing the nonprinting characters tab and @kbd{C-a}, with commas and spaces between them, like this: @code{"\t, \C-a"}. @xref{Character Type}, for a description of the read syntax for characters. However, not all of the characters you can write with backslash escape-sequences are valid in strings. The only control characters that a string can hold are the @acronym{ASCII} control characters. Strings do not distinguish case in @acronym{ASCII} control characters. Properly speaking, strings cannot hold meta characters; but when a string is to be used as a key sequence, there is a special convention that provides a way to represent meta versions of @acronym{ASCII} characters in a string. If you use the @samp{\M-} syntax to indicate a meta character in a string constant, this sets the @tex @math{2^{7}} @end tex @ifnottex 2**7 @end ifnottex bit of the character in the string. If the string is used in @code{define-key} or @code{lookup-key}, this numeric code is translated into the equivalent meta character. @xref{Character Type}. Strings cannot hold characters that have the hyper, super, or alt modifiers. @node Text Props and Strings @subsubsection Text Properties in Strings A string can hold properties for the characters it contains, in addition to the characters themselves. This enables programs that copy text between strings and buffers to copy the text's properties with no special effort. @xref{Text Properties}, for an explanation of what text properties mean. Strings with text properties use 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. For example, @example #("foo bar" 0 3 (face bold) 3 4 nil 4 7 (face italic)) @end example @noindent represents a string whose textual contents are @samp{foo bar}, in which the first three characters have a @code{face} property with value @code{bold}, and the last three have a @code{face} property with value @code{italic}. (The fourth character has no text properties, so its property list is @code{nil}. It is not actually necessary to mention ranges with @code{nil} as the property list, since any characters not mentioned in any range will default to having no properties.) @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 Char-Table Type @subsection Char-Table Type A @dfn{char-table} is a one-dimensional array of elements of any type, indexed by character codes. Char-tables have certain extra features to make them more useful for many jobs that involve assigning information to character codes---for example, a char-table can have a parent to inherit from, a default value, and a small number of extra slots to use for special purposes. A char-table can also specify a single value for a whole character set. The printed representation of a char-table is like a vector except that there is an extra @samp{#^} at the beginning. @xref{Char-Tables}, for special functions to operate on char-tables. Uses of char-tables include: @itemize @bullet @item Case tables (@pxref{Case Tables}). @item Character category tables (@pxref{Categories}). @item Display tables (@pxref{Display Tables}). @item Syntax tables (@pxref{Syntax Tables}). @end itemize @node Bool-Vector Type @subsection Bool-Vector Type A @dfn{bool-vector} is a one-dimensional array of elements that must be @code{t} or @code{nil}. The printed representation of a bool-vector is like a string, except that it begins with @samp{#&} followed by the length. The string constant that follows actually specifies the contents of the bool-vector as a bitmap---each ``character'' in the string contains 8 bits, which specify the next 8 elements of the bool-vector (1 stands for @code{t}, and 0 for @code{nil}). The least significant bits of the character correspond to the lowest indices in the bool-vector. @example (make-bool-vector 3 t) @result{} #&3"^G" (make-bool-vector 3 nil) @result{} #&3"^@@" @end example @noindent These results make sense, because the binary code for @samp{C-g} is 111 and @samp{C-@@} is the character with code 0. If the length is not a multiple of 8, the printed representation shows extra elements, but these extras really make no difference. For instance, in the next example, the two bool-vectors are equal, because only the first 3 bits are used: @example (equal #&3"\377" #&3"\007") @result{} t @end example @node Hash Table Type @subsection Hash Table Type A hash table is a very fast kind of lookup table, somewhat like an alist in that it maps keys to corresponding values, but much faster. Hash tables have no read syntax, and print using hash notation. @xref{Hash Tables}, for functions that operate on hash tables. @example (make-hash-table) @result{} # @end example @node Function Type @subsection Function Type Lisp functions are executable code, just like functions in other programming languages. In Lisp, unlike most languages, functions are also Lisp objects. A non-compiled function in Lisp is a lambda expression: that is, a list 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 can be called as a function even though it has no name; to emphasize this, we also call it an @dfn{anonymous function} (@pxref{Anonymous Functions}). A named function in Lisp is just 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 argument-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. @strong{Warning}: Lisp macros and keyboard macros (@pxref{Keyboard Macros}) are entirely different things. When we use the word ``macro'' without qualification, we mean a Lisp macro, not a keyboard 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 redefine a primitive with a function written in Lisp. 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. Therefore, @strong{we discourage redefinition of primitive functions}. 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, where it serves as a placeholder for the real definition. The autoload object says that the real definition is found in a file of Lisp code that should be loaded when necessary. It 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 used for general programming purposes, and most of them are common to most 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. * Frame Configuration Type:: Recording the status of all frames. * Process Type:: A process running on the underlying OS. * Stream Type:: Receive or send characters. * Keymap Type:: What function a keystroke invokes. * 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, you can insert text efficiently into an existing buffer, altering the buffer's contents, 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 list of buffer-local variable bindings (@pxref{Buffer-Local Variables}). @item 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, but presents it differently. @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 @dfn{frame} is a screen area that contains one or more Emacs windows; we also use the term ``frame'' to refer to the Lisp object that Emacs uses to refer to the screen area. 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 window layout in a frame 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 Frame Configuration Type @subsection Frame Configuration Type @cindex screen layout @cindex window layout, all frames A @dfn{frame configuration} stores information about the positions, sizes, and contents of the windows in all frames. It is actually a list whose @sc{car} is @code{frame-configuration} and whose @sc{cdr} is an alist. Each alist element describes one frame, which appears as the @sc{car} of that element. @xref{Frame Configurations}, for a description of several functions related to frame 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 Overlay Type @subsection Overlay Type An @dfn{overlay} specifies properties that apply to a part of a buffer. Each overlay applies to a specified range of the buffer, and contains a property list (a list whose elements are alternating property names and values). Overlay properties are used to present parts of the buffer temporarily in a different display style. Overlays 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 Circular Objects @section Read Syntax for Circular Objects @cindex circular structure, read syntax @cindex shared structure, read syntax @cindex @samp{#@var{n}=} read syntax @cindex @samp{#@var{n}#} read syntax To represent shared or circular structures within a complex of Lisp objects, you can use the reader constructs @samp{#@var{n}=} and @samp{#@var{n}#}. Use @code{#@var{n}=} before an object to label it for later reference; subsequently, you can use @code{#@var{n}#} to refer the same object in another place. Here, @var{n} is some integer. For example, here is how to make a list in which the first element recurs as the third element: @example (#1=(a) b #1#) @end example @noindent This differs from ordinary syntax such as this @example ((a) b (a)) @end example @noindent which would result in a list whose first and third elements look alike but are not the same Lisp object. This shows the difference: @example (prog1 nil (setq x '(#1=(a) b #1#))) (eq (nth 0 x) (nth 2 x)) @result{} t (setq x '((a) b (a))) (eq (nth 0 x) (nth 2 x)) @result{} nil @end example You can also use the same syntax to make a circular structure, which appears as an ``element'' within itself. Here is an example: @example #1=(a #1#) @end example @noindent This makes a list whose second element is the list itself. Here's how you can see that it really works: @example (prog1 nil (setq x '#1=(a #1#))) (eq x (cadr x)) @result{} t @end example The Lisp printer can produce this syntax to record circular and shared structure in a Lisp object, if you bind the variable @code{print-circle} to a non-@code{nil} value. @xref{Output Variables}. @node Type Predicates @section Type 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: number-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))) (t ;; We handle only 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 bool-vector-p @xref{Bool-Vectors, bool-vector-p}. @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 Tables, case-table-p}. @item char-or-string-p @xref{Predicates for Strings, char-or-string-p}. @item char-table-p @xref{Char-Tables, char-table-p}. @item commandp @xref{Interactive Call, commandp}. @item consp @xref{List-related Predicates, consp}. @item display-table-p @xref{Display Tables, display-table-p}. @item floatp @xref{Predicates on Numbers, floatp}. @item frame-configuration-p @xref{Frame Configurations, frame-configuration-p}. @item frame-live-p @xref{Deleting Frames, frame-live-p}. @item framep @xref{Frames, framep}. @item functionp @xref{Functions, functionp}. @item hash-table-p @xref{Other Hash, hash-table-p}. @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 keywordp @xref{Constant Variables}. @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}. @item booleanp @xref{nil and t, booleanp}. @item string-or-null-p @xref{Predicates for Strings, string-or-null-p}. @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{char-table}, @code{bool-vector}, @code{hash-table}, @code{subr}, @code{compiled-function}, @code{marker}, @code{overlay}, @code{window}, @code{buffer}, @code{frame}, @code{process}, or @code{window-configuration}. @example (type-of 1) @result{} integer @group (type-of 'nil) @result{} symbol (type-of '()) ; @r{@code{()} is @code{nil}.} @result{} symbol (type-of '(x)) @result{} cons @end group @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. @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, meaning that a change in the contents of one will be reflected by the same change in the contents of the other. @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 The @code{make-symbol} function returns an uninterned symbol, distinct from the symbol that is used if you write the name in a Lisp expression. Distinct symbols with the same name are not @code{eq}. @xref{Creating Symbols}. @example @group (eq (make-symbol "foo") 'foo) @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 or contents 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, but does not take account of text properties---it compares only the characters in the strings. For technical reasons, a unibyte string and a multibyte string are @code{equal} if and only if they contain the same sequence of character codes and all these codes are either in the range 0 through 127 (@acronym{ASCII}) or 160 through 255 (@code{eight-bit-graphic}). (@pxref{Text Representations}). @example @group (equal "asdf" "ASDF") @result{} nil @end group @end example However, two distinct buffers are never considered @code{equal}, even if their textual contents are the same. @end defun The test for equality is implemented recursively; for example, given two cons cells @var{x} and @var{y}, @code{(equal @var{x} @var{y})} returns @code{t} if and only if both the expressions below return @code{t}: @example (equal (car @var{x}) (car @var{y})) (equal (cdr @var{x}) (cdr @var{y})) @end example Because of this recursive method, circular lists may therefore cause infinite recursion (leading to an error). @ignore arch-tag: 9711a66e-4749-4265-9e8c-972d55b67096 @end ignore