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[Emacs-diffs] Changes to objects.texi
|
From: |
Glenn Morris |
|
Subject: |
[Emacs-diffs] Changes to objects.texi |
|
Date: |
Thu, 06 Sep 2007 04:22:29 +0000 |
CVSROOT: /sources/emacs
Module name: emacs
Changes by: Glenn Morris <gm> 07/09/06 04:22:29
Index: objects.texi
===================================================================
RCS file: objects.texi
diff -N objects.texi
--- /dev/null 1 Jan 1970 00:00:00 -0000
+++ objects.texi 6 Sep 2007 04:22:29 -0000 1.1
@@ -0,0 +1,2036 @@
address@hidden -*-texinfo-*-
address@hidden This is part of the GNU Emacs Lisp Reference Manual.
address@hidden Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999,
2001,
address@hidden 2002, 2003, 2004, 2005, 2006, 2007 Free Software Foundation,
Inc.
address@hidden See the file elisp.texi for copying conditions.
address@hidden ../info/objects
address@hidden Lisp Data Types, Numbers, Introduction, Top
address@hidden Lisp Data Types
address@hidden object
address@hidden Lisp object
address@hidden type
address@hidden 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.
+
address@hidden 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},
address@hidden, @dfn{vector}, @dfn{hash-table}, @dfn{subr}, and
address@hidden function}, plus several special types, such as
address@hidden, 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
address@hidden: 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.
address@hidden 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.
+
address@hidden
+* 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.
address@hidden menu
+
address@hidden Printed Representation
address@hidden node-name, next, previous, up
address@hidden Printed Representation and Read Syntax
address@hidden printed representation
address@hidden 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}.
+
address@hidden 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:
+
address@hidden
+(current-buffer)
+ @result{} #<buffer objects.texi>
address@hidden example
+
address@hidden
+Hash notation cannot be read at all, so the Lisp reader signals the
+error @code{invalid-read-syntax} whenever it encounters @samp{#<}.
address@hidden 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
address@hidden, the basic function for reading objects.
+
address@hidden Comments
address@hidden node-name, next, previous, up
address@hidden Comments
address@hidden comments
address@hidden @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.
+
address@hidden Programming Types
address@hidden Programming Types
address@hidden 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.
+
address@hidden
+* 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.
address@hidden menu
+
address@hidden Integer Type
address@hidden Integer Type
+
+ The range of values for integers in Emacs Lisp is @minus{}268435456 to
+268435455 (29 bits; i.e.,
address@hidden
+-2**28
address@hidden ifnottex
address@hidden
address@hidden
address@hidden tex
+to
address@hidden
+2**28 - 1)
address@hidden ifnottex
address@hidden
address@hidden)
address@hidden 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{.}.
+
address@hidden
address@hidden
+-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.}
address@hidden group
address@hidden example
+
+ @xref{Numbers}, for more information.
+
address@hidden Floating Point Type
address@hidden 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},
address@hidden, 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.
+
address@hidden Character Type
address@hidden Character Type
address@hidden @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
address@hidden codes; the rest are address@hidden
+(@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}.
+
address@hidden
+* Basic Char Syntax::
+* General Escape Syntax::
+* Ctl-Char Syntax::
+* Meta-Char Syntax::
+* Other Char Bits::
address@hidden menu
+
address@hidden Basic Char Syntax
address@hidden Basic Char Syntax
address@hidden read syntax for characters
address@hidden printed representation for characters
address@hidden syntax for characters
address@hidden @samp{?} in character constant
address@hidden 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
address@hidden, @samp{?B} for the character @kbd{B}, and @samp{?a} for the
+character @kbd{a}.
+
+ For example:
+
address@hidden
+?Q @result{} 81 ?q @result{} 113
address@hidden 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{?\\}.
+
address@hidden whitespace
address@hidden bell character
address@hidden @samp{\a}
address@hidden backspace
address@hidden @samp{\b}
address@hidden tab (ASCII character)
address@hidden @samp{\t}
address@hidden vertical tab
address@hidden @samp{\v}
address@hidden formfeed
address@hidden @samp{\f}
address@hidden newline
address@hidden @samp{\n}
address@hidden return (ASCII character)
address@hidden @samp{\r}
address@hidden escape (ASCII character)
address@hidden @samp{\e}
address@hidden space (ASCII character)
address@hidden @samp{\s}
+ You can express the characters control-g, backspace, tab, newline,
+vertical tab, formfeed, space, return, del, and escape as @samp{?\a},
address@hidden, @samp{?\t}, @samp{?\n}, @samp{?\v}, @samp{?\f},
address@hidden, @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,
+
address@hidden
+?\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}}
address@hidden example
+
address@hidden escape sequence
+ These sequences which start with backslash are also known as
address@hidden 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
address@hidden()\|;'`"#.,} 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.)
+
address@hidden General Escape Syntax
address@hidden 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.
+
address@hidden unicode character escape
+ For instance, you can specify characters by their Unicode values.
address@hidden@var{nnnn}} represents a character that maps to the Unicode
+code point @address@hidden There is a slightly different syntax
+for specifying characters with code points above @code{#xFFFF};
address@hidden@var{nnnnnn}} represents the character whose Unicode code
+point is @address@hidden, 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.
+
address@hidden @samp{\} in character constant
address@hidden backslash in character constant
address@hidden 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},
address@hidden for the character @kbd{C-a}, and @code{?\002} for the
+character @kbd{C-b}. Although this syntax can represent any
address@hidden character, it is preferred only when the precise octal
+value is more important than the @acronym{ASCII} representation.
+
address@hidden
address@hidden
+?\012 @result{} 10 ?\n @result{} 10 ?\C-j @result{} 10
+?\101 @result{} 65 ?A @result{} 65
address@hidden group
address@hidden 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
address@hidden
address@hidden@`a}.
address@hidden iftex
address@hidden
address@hidden with grave accent.
address@hidden ifnottex
+
address@hidden Ctl-Char Syntax
address@hidden Control-Character Syntax
+
address@hidden 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}:
+
address@hidden
+?\^I @result{} 9 ?\C-I @result{} 9
address@hidden 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 address@hidden control characters include the
address@hidden
address@hidden
address@hidden tex
address@hidden
+2**26
address@hidden ifnottex
+bit as well as the code for the corresponding non-control
+character. Ordinary terminals have no way of generating address@hidden
+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{?}:
+
address@hidden
+?\^? @result{} 127 ?\C-? @result{} 127
address@hidden example
+
address@hidden
+As a result, it is currently not possible to represent the character
address@hidden, which is a meaningful input character under X, using
address@hidden 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.
+
address@hidden Meta-Char Syntax
address@hidden Meta-Character Syntax
+
address@hidden 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
address@hidden
address@hidden
address@hidden tex
address@hidden
+2**27
address@hidden 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
address@hidden
address@hidden
address@hidden tex
address@hidden
+2**7
address@hidden 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,
address@hidden 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
address@hidden, @samp{?\C-\M-b}, or @samp{?\M-\002}.
+
address@hidden Other Char Bits
address@hidden 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
address@hidden
address@hidden
address@hidden tex
address@hidden
+2**25
address@hidden 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.
+
address@hidden hyper characters
address@hidden super characters
address@hidden alt characters
+ The X Window System defines three other
address@hidden 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
address@hidden (Note that @samp{\s} with no following @samp{-}
+represents the space character.)
address@hidden
+Numerically, the bit values are @math{2^{22}} for alt, @math{2^{23}}
+for super and @math{2^{24}} for hyper.
address@hidden tex
address@hidden
+Numerically, the
+bit values are 2**22 for alt, 2**23 for super and 2**24 for hyper.
address@hidden ifnottex
+
address@hidden Symbol Type
address@hidden 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
address@hidden symbol}. These symbols automatically act as constants, and
+are normally used only by comparing an unknown symbol with a few
+specific alternatives.
+
address@hidden @samp{\} in symbols
address@hidden backslash in symbols
+ A symbol name can contain any characters whatever. Most symbol names
+are written with letters, digits, and the punctuation characters
address@hidden/}. 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{_~!@@$%^&:<>@address@hidden
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.
+
address@hidden CL note---case of letters
address@hidden
address@hidden 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.
address@hidden 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.
+
address@hidden
address@hidden
+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}.}
address@hidden group
address@hidden
+1+ ; @r{A symbol named @samp{1+}}
+ ; @r{(not @samp{+1}, which is an integer).}
address@hidden group
address@hidden
+\+1 ; @r{A symbol named @samp{+1}}
+ ; @r{(not a very readable name).}
address@hidden group
address@hidden
+\(*\ 1\ 2\) ; @r{A symbol named @samp{(* 1 2)} (a worse name).}
address@hidden the @'s in this next line use up three characters, hence the
address@hidden apparent misalignment of the comment.
++-*/_~!@@$%^&=:<>@address@hidden ; @r{A symbol named
@samp{+-*/_~!@@$%^&=:<>@address@hidden
+ ; @r{These characters need not be escaped.}
address@hidden group
address@hidden example
+
address@hidden
address@hidden This uses ``colon'' instead of a literal `:' because Info cannot
address@hidden cope with a `:' in a menu
address@hidden @address@hidden read syntax
address@hidden ifinfo
address@hidden
address@hidden @samp{#:} read syntax
address@hidden ifnotinfo
+ Normally the Lisp reader interns all symbols (@pxref{Creating
+Symbols}). To prevent interning, you can write @samp{#:} before the
+name of the symbol.
+
address@hidden Sequence Type
address@hidden 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}.
+
address@hidden Cons Cell Type
address@hidden Cons Cell and List Types
address@hidden address field of register
address@hidden decrement field of register
address@hidden pointers
+
+ A @dfn{cons cell} is an object that consists of two slots, called the
address@hidden slot and the @sc{cdr} slot. Each slot can @dfn{hold} or
address@hidden 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}.
+
address@hidden
+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.
address@hidden quotation
+
+ A @dfn{list} is a series of cons cells, linked together so that the
address@hidden slot of each cons cell holds either the next cons cell or the
+empty list. The empty list is actually the symbol @code{nil}.
address@hidden, 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.
+
address@hidden 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
address@hidden
+
address@hidden parenthesis
address@hidden @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:
+
address@hidden
+(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).}
address@hidden 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
address@hidden 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.
+
address@hidden
+* Box Diagrams:: Drawing pictures of lists.
+* Dotted Pair Notation:: A general syntax for cons cells.
+* Association List Type:: A specially constructed list.
address@hidden menu
+
address@hidden Box Diagrams
address@hidden Drawing Lists as Box Diagrams
address@hidden box diagrams, for lists
address@hidden 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)}:
+
address@hidden
address@hidden
+ --- --- --- --- --- ---
+ | | |--> | | |--> | | |--> nil
+ --- --- --- --- --- ---
+ | | |
+ | | |
+ --> rose --> violet --> buttercup
address@hidden group
address@hidden 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
address@hidden 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:
+
address@hidden
address@hidden
+ --------------- ---------------- -------------------
+| car | cdr | | car | cdr | | car | cdr |
+| rose | o-------->| violet | o-------->| buttercup | nil |
+| | | | | | | | |
+ --------------- ---------------- -------------------
address@hidden group
address@hidden smallexample
+
address@hidden @code{nil} as a list
address@hidden 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:
+
address@hidden
address@hidden
+ --- --- --- ---
+ | | |--> | | |--> nil
+ --- --- --- ---
+ | |
+ | |
+ --> A --> nil
address@hidden group
address@hidden example
+
+ Here is a more complex illustration, showing the three-element list,
address@hidden((pine needles) oak maple)}, the first element of which is a
+two-element list:
+
address@hidden
address@hidden
+ --- --- --- --- --- ---
+ | | |--> | | |--> | | |--> nil
+ --- --- --- --- --- ---
+ | | |
+ | | |
+ | --> oak --> maple
+ |
+ | --- --- --- ---
+ --> | | |--> | | |--> nil
+ --- --- --- ---
+ | |
+ | |
+ --> pine --> needles
address@hidden group
address@hidden example
+
+ The same list represented in the second box notation looks like this:
+
address@hidden
address@hidden
+ -------------- -------------- --------------
+| car | cdr | | car | cdr | | car | cdr |
+| o | o------->| oak | o------->| maple | nil |
+| | | | | | | | | |
+ -- | --------- -------------- --------------
+ |
+ |
+ | -------------- ----------------
+ | | car | cdr | | car | cdr |
+ ------>| pine | o------->| needles | nil |
+ | | | | | |
+ -------------- ----------------
address@hidden group
address@hidden example
+
address@hidden Dotted Pair Notation
address@hidden Dotted Pair Notation
address@hidden dotted pair notation
address@hidden @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,
address@hidden(@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
address@hidden(1 2 3)} is written as @samp{(1 . (2 . (3 . nil)))}. For
address@hidden 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)}:
+
address@hidden
address@hidden
+ --- ---
+ | | |--> violet
+ --- ---
+ |
+ |
+ --> rose
address@hidden group
address@hidden example
+
+ You can combine dotted pair notation with list notation to represent
+conveniently a chain of cons cells with a address@hidden final @sc{cdr}.
+You write a dot after the last element of the list, followed by the
address@hidden of the final cons cell. For example, @code{(rose violet
+. buttercup)} is equivalent to @code{(rose . (violet . buttercup))}.
+The object looks like this:
+
address@hidden
address@hidden
+ --- --- --- ---
+ | | |--> | | |--> buttercup
+ --- --- --- ---
+ | |
+ | |
+ --> rose --> violet
address@hidden group
address@hidden example
+
+ The syntax @code{(rose .@: violet .@: buttercup)} is invalid because
+there is nothing that it could mean. If anything, it would say to put
address@hidden 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:
+
address@hidden
address@hidden
+ --- --- --- ---
+ | | |--> | | |--> nil
+ --- --- --- ---
+ | |
+ | |
+ --> rose --> violet
address@hidden group
address@hidden example
+
+ Similarly, the three-element list @code{(rose violet buttercup)}
+is equivalent to @code{(rose . (violet . (buttercup)))}.
address@hidden
+It looks like this:
+
address@hidden
address@hidden
+ --- --- --- --- --- ---
+ | | |--> | | |--> | | |--> nil
+ --- --- --- --- --- ---
+ | | |
+ | | |
+ --> rose --> violet --> buttercup
address@hidden group
address@hidden example
address@hidden ifnottex
+
address@hidden Association List Type
address@hidden node-name, next, previous, up
address@hidden 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
address@hidden 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,
+
address@hidden
+(setq alist-of-colors
+ '((rose . red) (lily . white) (buttercup . yellow)))
address@hidden example
+
address@hidden
+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.
+
address@hidden Array Type
address@hidden 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.
+
address@hidden String Type
address@hidden 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.
+
address@hidden
+* Syntax for Strings::
+* Non-ASCII in Strings::
+* Nonprinting Characters::
+* Text Props and Strings::
address@hidden menu
+
address@hidden Syntax for Strings
address@hidden Syntax for Strings
+
address@hidden @samp{"} in strings
address@hidden double-quote in strings
address@hidden @samp{\} in strings
address@hidden 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"}.
+
address@hidden 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
address@hidden@samp{\ }} is likewise ignored.
+
address@hidden
+"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."
address@hidden example
+
address@hidden Non-ASCII in Strings
address@hidden address@hidden Characters in Strings
+
+ You can include a address@hidden international character in a string
+constant by writing it literally. There are two text representations
+for address@hidden 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 address@hidden character with its
+character code: use a hex escape, @address@hidden, with as many
+digits as necessary. (Multibyte address@hidden 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 @address@hidden }} (backslash and space) to
+terminate the hex escape---for example, @address@hidden }} represents
+one character, @samp{a} with grave accent. @address@hidden }} 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 address@hidden 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.
+
address@hidden Nonprinting Characters
address@hidden 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
address@hidden
address@hidden
address@hidden tex
address@hidden
+2**7
address@hidden ifnottex
+bit of the character in the string. If the string is used in
address@hidden 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.
+
address@hidden Text Props and Strings
address@hidden 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:
+
address@hidden
+#("@var{characters}" @var{property-data}...)
address@hidden example
+
address@hidden
+where @var{property-data} consists of zero or more elements, in groups
+of three as follows:
+
address@hidden
address@hidden @var{end} @var{plist}
address@hidden example
+
address@hidden
+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,
+
address@hidden
+#("foo bar" 0 3 (face bold) 3 4 nil 4 7 (face italic))
address@hidden example
+
address@hidden
+represents a string whose textual contents are @samp{foo bar}, in which
+the first three characters have a @code{face} property with value
address@hidden, and the last three have a @code{face} property with value
address@hidden (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.)
+
address@hidden Vector Type
address@hidden 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.
+
address@hidden
+[1 "two" (three)] ; @r{A vector of three elements.}
+ @result{} [1 "two" (three)]
address@hidden example
+
+ @xref{Vectors}, for functions that work with vectors.
+
address@hidden Char-Table Type
address@hidden 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:
+
address@hidden @bullet
address@hidden
+Case tables (@pxref{Case Tables}).
+
address@hidden
+Character category tables (@pxref{Categories}).
+
address@hidden
+Display tables (@pxref{Display Tables}).
+
address@hidden
+Syntax tables (@pxref{Syntax Tables}).
address@hidden itemize
+
address@hidden Bool-Vector Type
address@hidden 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.
+
address@hidden
+(make-bool-vector 3 t)
+ @result{} #&3"^G"
+(make-bool-vector 3 nil)
+ @result{} #&3"^@@"
address@hidden example
+
address@hidden
+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:
+
address@hidden
+(equal #&3"\377" #&3"\007")
+ @result{} t
address@hidden example
+
address@hidden Hash Table Type
address@hidden 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.
address@hidden Tables}, for functions that operate on hash tables.
+
address@hidden
+(make-hash-table)
+ @result{} #<hash-table 'eql nil 0/65 0x83af980>
address@hidden example
+
address@hidden Function Type
address@hidden 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
address@hidden (@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}.
+
address@hidden Macro Type
address@hidden 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
address@hidden 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.
+
address@hidden Primitive Function Type
address@hidden Primitive Function Type
address@hidden 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})address@hidden
+
+ 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.
+
address@hidden
address@hidden
+(symbol-function 'car) ; @r{Access the function cell}
+ ; @r{of the symbol.}
+ @result{} #<subr car>
+(subrp (symbol-function 'car)) ; @r{Is this a primitive function?}
+ @result{} t ; @r{Yes.}
address@hidden group
address@hidden example
+
address@hidden Byte-Code Type
address@hidden 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{[}.
+
address@hidden Autoload Type
address@hidden Autoload Type
+
+ An @dfn{autoload object} is a list whose first element is the symbol
address@hidden 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
address@hidden, which stores the object in the function cell of a
+symbol. @xref{Autoload}, for more details.
+
address@hidden Editing Types
address@hidden Editing Types
address@hidden 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.
+
address@hidden
+* 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.
address@hidden menu
+
address@hidden Buffer Type
address@hidden 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:
+
address@hidden @bullet
address@hidden
+a local syntax table (@pxref{Syntax Tables});
+
address@hidden
+a local keymap (@pxref{Keymaps}); and,
+
address@hidden
+a list of buffer-local variable bindings (@pxref{Buffer-Local Variables}).
+
address@hidden
+overlays (@pxref{Overlays}).
+
address@hidden
+text properties for the text in the buffer (@pxref{Text Properties}).
address@hidden itemize
+
address@hidden
+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.
+
address@hidden
address@hidden
+(current-buffer)
+ @result{} #<buffer objects.texi>
address@hidden group
address@hidden example
+
address@hidden Marker Type
address@hidden 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.
+
address@hidden
address@hidden
+(point-marker)
+ @result{} #<marker at 10779 in objects.texi>
address@hidden group
address@hidden example
+
address@hidden, for information on how to test, create, copy, and move
+markers.
+
address@hidden Window Type
address@hidden 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.
+
address@hidden
address@hidden
+(selected-window)
+ @result{} #<window 1 on objects.texi>
address@hidden group
address@hidden example
+
+ @xref{Windows}, for a description of the functions that work on windows.
+
address@hidden Frame Type
address@hidden 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).
+
address@hidden
address@hidden
+(selected-frame)
+ @result{} #<frame emacs@@psilocin.gnu.org 0xdac80>
address@hidden group
address@hidden example
+
+ @xref{Frames}, for a description of the functions that work on frames.
+
address@hidden Window Configuration Type
address@hidden Window Configuration Type
address@hidden 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{#<window-configuration>}. @xref{Window
+Configurations}, for a description of several functions related to
+window configurations.
+
address@hidden Frame Configuration Type
address@hidden Frame Configuration Type
address@hidden screen layout
address@hidden 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
address@hidden 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.
+
address@hidden Process Type
address@hidden 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:
+
address@hidden
address@hidden
+(process-list)
+ @result{} (#<process shell>)
address@hidden group
address@hidden example
+
address@hidden, for information about functions that create, delete,
+return information about, send input or signals to, and receive output
+from processes.
+
address@hidden Stream Type
address@hidden 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
address@hidden 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
address@hidden or @code{standard-output}. Also, the object
address@hidden 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.
+
address@hidden Keymap Type
address@hidden 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.
+
address@hidden Overlay Type
address@hidden 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.
+
address@hidden Circular Objects
address@hidden Read Syntax for Circular Objects
address@hidden circular structure, read syntax
address@hidden shared structure, read syntax
address@hidden @address@hidden read syntax
address@hidden @address@hidden read syntax
+
+ To represent shared or circular structures within a complex of Lisp
+objects, you can use the reader constructs @address@hidden and
address@hidden@var{n}#}.
+
+ Use @address@hidden before an object to label it for later reference;
+subsequently, you can use @address@hidden 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:
+
address@hidden
+(#1=(a) b #1#)
address@hidden example
+
address@hidden
+This differs from ordinary syntax such as this
+
address@hidden
+((a) b (a))
address@hidden example
+
address@hidden
+which would result in a list whose first and third elements
+look alike but are not the same Lisp object. This shows the difference:
+
address@hidden
+(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
address@hidden example
+
+ You can also use the same syntax to make a circular structure, which
+appears as an ``element'' within itself. Here is an example:
+
address@hidden
+#1=(a #1#)
address@hidden example
+
address@hidden
+This makes a list whose second element is the list itself.
+Here's how you can see that it really works:
+
address@hidden
+(prog1 nil
+ (setq x '#1=(a #1#)))
+(eq x (cadr x))
+ @result{} t
address@hidden 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 address@hidden value. @xref{Output Variables}.
+
address@hidden Type Predicates
address@hidden Type Predicates
address@hidden type checking
address@hidden 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:
+
address@hidden
address@hidden
+(+ 2 'a)
+ @error{} Wrong type argument: number-or-marker-p, a
address@hidden group
address@hidden example
+
address@hidden type predicates
address@hidden 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.
+
address@hidden
+(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))))
address@hidden example
+
+ Here is a table of predefined type predicates, in alphabetical order,
+with references to further information.
+
address@hidden @code
address@hidden atom
address@hidden Predicates, atom}.
+
address@hidden arrayp
address@hidden Functions, arrayp}.
+
address@hidden bool-vector-p
address@hidden, bool-vector-p}.
+
address@hidden bufferp
address@hidden Basics, bufferp}.
+
address@hidden byte-code-function-p
address@hidden Type, byte-code-function-p}.
+
address@hidden case-table-p
address@hidden Tables, case-table-p}.
+
address@hidden char-or-string-p
address@hidden for Strings, char-or-string-p}.
+
address@hidden char-table-p
address@hidden, char-table-p}.
+
address@hidden commandp
address@hidden Call, commandp}.
+
address@hidden consp
address@hidden Predicates, consp}.
+
address@hidden display-table-p
address@hidden Tables, display-table-p}.
+
address@hidden floatp
address@hidden on Numbers, floatp}.
+
address@hidden frame-configuration-p
address@hidden Configurations, frame-configuration-p}.
+
address@hidden frame-live-p
address@hidden Frames, frame-live-p}.
+
address@hidden framep
address@hidden, framep}.
+
address@hidden functionp
address@hidden, functionp}.
+
address@hidden hash-table-p
address@hidden Hash, hash-table-p}.
+
address@hidden integer-or-marker-p
address@hidden on Markers, integer-or-marker-p}.
+
address@hidden integerp
address@hidden on Numbers, integerp}.
+
address@hidden keymapp
address@hidden Keymaps, keymapp}.
+
address@hidden keywordp
address@hidden Variables}.
+
address@hidden listp
address@hidden Predicates, listp}.
+
address@hidden markerp
address@hidden on Markers, markerp}.
+
address@hidden wholenump
address@hidden on Numbers, wholenump}.
+
address@hidden nlistp
address@hidden Predicates, nlistp}.
+
address@hidden numberp
address@hidden on Numbers, numberp}.
+
address@hidden number-or-marker-p
address@hidden on Markers, number-or-marker-p}.
+
address@hidden overlayp
address@hidden, overlayp}.
+
address@hidden processp
address@hidden, processp}.
+
address@hidden sequencep
address@hidden Functions, sequencep}.
+
address@hidden stringp
address@hidden for Strings, stringp}.
+
address@hidden subrp
address@hidden Cells, subrp}.
+
address@hidden symbolp
address@hidden, symbolp}.
+
address@hidden syntax-table-p
address@hidden Tables, syntax-table-p}.
+
address@hidden user-variable-p
address@hidden Variables, user-variable-p}.
+
address@hidden vectorp
address@hidden, vectorp}.
+
address@hidden window-configuration-p
address@hidden Configurations, window-configuration-p}.
+
address@hidden window-live-p
address@hidden Windows, window-live-p}.
+
address@hidden windowp
address@hidden Windows, windowp}.
+
address@hidden booleanp
address@hidden and t, booleanp}.
+
address@hidden string-or-null-p
address@hidden for Strings, string-or-null-p}.
address@hidden 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
address@hidden
+
address@hidden type-of object
+This function returns a symbol naming the primitive type of
address@hidden The value is one of the symbols @code{symbol},
address@hidden, @code{float}, @code{string}, @code{cons}, @code{vector},
address@hidden, @code{bool-vector}, @code{hash-table}, @code{subr},
address@hidden, @code{marker}, @code{overlay}, @code{window},
address@hidden, @code{frame}, @code{process}, or
address@hidden
+
address@hidden
+(type-of 1)
+ @result{} integer
address@hidden
+(type-of 'nil)
+ @result{} symbol
+(type-of '()) ; @address@hidden()} is @code{nil}.}
+ @result{} symbol
+(type-of '(x))
+ @result{} cons
address@hidden group
address@hidden example
address@hidden defun
+
address@hidden Equality Predicates
address@hidden Equality Predicates
address@hidden 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.
+
address@hidden eq object1 object2
+This function returns @code{t} if @var{object1} and @var{object2} are
+the same object, @code{nil} otherwise.
+
address@hidden 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
address@hidden For other types (e.g., lists, vectors, strings), two
+arguments with the same contents or elements are not necessarily
address@hidden 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.
+
address@hidden
address@hidden
+(eq 'foo 'foo)
+ @result{} t
address@hidden group
+
address@hidden
+(eq 456 456)
+ @result{} t
address@hidden group
+
address@hidden
+(eq "asdf" "asdf")
+ @result{} nil
address@hidden group
+
address@hidden
+(eq '(1 (2 (3))) '(1 (2 (3))))
+ @result{} nil
address@hidden group
+
address@hidden
+(setq foo '(1 (2 (3))))
+ @result{} (1 (2 (3)))
+(eq foo foo)
+ @result{} t
+(eq foo '(1 (2 (3))))
+ @result{} nil
address@hidden group
+
address@hidden
+(eq [(1 2) 3] [(1 2) 3])
+ @result{} nil
address@hidden group
+
address@hidden
+(eq (point-marker) (point-marker))
+ @result{} nil
address@hidden group
address@hidden 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}.
+
address@hidden
address@hidden
+(eq (make-symbol "foo") 'foo)
+ @result{} nil
address@hidden group
address@hidden example
address@hidden defun
+
address@hidden 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.
+
address@hidden
address@hidden
+(equal 'foo 'foo)
+ @result{} t
address@hidden group
+
address@hidden
+(equal 456 456)
+ @result{} t
address@hidden group
+
address@hidden
+(equal "asdf" "asdf")
+ @result{} t
address@hidden group
address@hidden
+(eq "asdf" "asdf")
+ @result{} nil
address@hidden group
+
address@hidden
+(equal '(1 (2 (3))) '(1 (2 (3))))
+ @result{} t
address@hidden group
address@hidden
+(eq '(1 (2 (3))) '(1 (2 (3))))
+ @result{} nil
address@hidden group
+
address@hidden
+(equal [(1 2) 3] [(1 2) 3])
+ @result{} t
address@hidden group
address@hidden
+(eq [(1 2) 3] [(1 2) 3])
+ @result{} nil
address@hidden group
+
address@hidden
+(equal (point-marker) (point-marker))
+ @result{} t
address@hidden group
+
address@hidden
+(eq (point-marker) (point-marker))
+ @result{} nil
address@hidden group
address@hidden 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
address@hidden 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}).
+
address@hidden
address@hidden
+(equal "asdf" "ASDF")
+ @result{} nil
address@hidden group
address@hidden example
+
+However, two distinct buffers are never considered @code{equal}, even if
+their textual contents are the same.
address@hidden 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
address@hidden:
+
address@hidden
+(equal (car @var{x}) (car @var{y}))
+(equal (cdr @var{x}) (cdr @var{y}))
address@hidden example
+
+Because of this recursive method, circular lists may therefore cause
+infinite recursion (leading to an error).
+
address@hidden
+ arch-tag: 9711a66e-4749-4265-9e8c-972d55b67096
address@hidden ignore