2 @c This is part of the GNU Emacs Lisp Reference Manual.
3 @c Copyright (C) 1990-1995, 1998-1999, 2001-2011
4 @c Free Software Foundation, Inc.
5 @c See the file elisp.texi for copying conditions.
6 @setfilename ../../info/objects
7 @node Lisp Data Types, Numbers, Introduction, Top
8 @chapter Lisp Data Types
14 A Lisp @dfn{object} is a piece of data used and manipulated by Lisp
15 programs. For our purposes, a @dfn{type} or @dfn{data type} is a set of
18 Every object belongs to at least one type. Objects of the same type
19 have similar structures and may usually be used in the same contexts.
20 Types can overlap, and objects can belong to two or more types.
21 Consequently, we can ask whether an object belongs to a particular type,
22 but not for ``the'' type of an object.
24 @cindex primitive type
25 A few fundamental object types are built into Emacs. These, from
26 which all other types are constructed, are called @dfn{primitive types}.
27 Each object belongs to one and only one primitive type. These types
28 include @dfn{integer}, @dfn{float}, @dfn{cons}, @dfn{symbol},
29 @dfn{string}, @dfn{vector}, @dfn{hash-table}, @dfn{subr}, and
30 @dfn{byte-code function}, plus several special types, such as
31 @dfn{buffer}, that are related to editing. (@xref{Editing Types}.)
33 Each primitive type has a corresponding Lisp function that checks
34 whether an object is a member of that type.
36 Lisp is unlike many other languages in that its objects are
37 @dfn{self-typing}: the primitive type of each object is implicit in
38 the object itself. For example, if an object is a vector, nothing can
39 treat it as a number; Lisp knows it is a vector, not a number.
41 In most languages, the programmer must declare the data type of each
42 variable, and the type is known by the compiler but not represented in
43 the data. Such type declarations do not exist in Emacs Lisp. A Lisp
44 variable can have any type of value, and it remembers whatever value
45 you store in it, type and all. (Actually, a small number of Emacs
46 Lisp variables can only take on values of a certain type.
47 @xref{Variables with Restricted Values}.)
49 This chapter describes the purpose, printed representation, and read
50 syntax of each of the standard types in GNU Emacs Lisp. Details on how
51 to use these types can be found in later chapters.
54 * Printed Representation:: How Lisp objects are represented as text.
55 * Comments:: Comments and their formatting conventions.
56 * Programming Types:: Types found in all Lisp systems.
57 * Editing Types:: Types specific to Emacs.
58 * Circular Objects:: Read syntax for circular structure.
59 * Type Predicates:: Tests related to types.
60 * Equality Predicates:: Tests of equality between any two objects.
63 @node Printed Representation
64 @comment node-name, next, previous, up
65 @section Printed Representation and Read Syntax
66 @cindex printed representation
69 The @dfn{printed representation} of an object is the format of the
70 output generated by the Lisp printer (the function @code{prin1}) for
71 that object. Every data type has a unique printed representation.
72 The @dfn{read syntax} of an object is the format of the input accepted
73 by the Lisp reader (the function @code{read}) for that object. This
74 is not necessarily unique; many kinds of object have more than one
75 syntax. @xref{Read and Print}.
78 In most cases, an object's printed representation is also a read
79 syntax for the object. However, some types have no read syntax, since
80 it does not make sense to enter objects of these types as constants in
81 a Lisp program. These objects are printed in @dfn{hash notation},
82 which consists of the characters @samp{#<}, a descriptive string
83 (typically the type name followed by the name of the object), and a
84 closing @samp{>}. For example:
88 @result{} #<buffer objects.texi>
92 Hash notation cannot be read at all, so the Lisp reader signals the
93 error @code{invalid-read-syntax} whenever it encounters @samp{#<}.
94 @kindex invalid-read-syntax
96 In other languages, an expression is text; it has no other form. In
97 Lisp, an expression is primarily a Lisp object and only secondarily the
98 text that is the object's read syntax. Often there is no need to
99 emphasize this distinction, but you must keep it in the back of your
100 mind, or you will occasionally be very confused.
102 When you evaluate an expression interactively, the Lisp interpreter
103 first reads the textual representation of it, producing a Lisp object,
104 and then evaluates that object (@pxref{Evaluation}). However,
105 evaluation and reading are separate activities. Reading returns the
106 Lisp object represented by the text that is read; the object may or may
107 not be evaluated later. @xref{Input Functions}, for a description of
108 @code{read}, the basic function for reading objects.
111 @comment node-name, next, previous, up
114 @cindex @samp{;} in comment
116 A @dfn{comment} is text that is written in a program only for the sake
117 of humans that read the program, and that has no effect on the meaning
118 of the program. In Lisp, a semicolon (@samp{;}) starts a comment if it
119 is not within a string or character constant. The comment continues to
120 the end of line. The Lisp reader discards comments; they do not become
121 part of the Lisp objects which represent the program within the Lisp
124 The @samp{#@@@var{count}} construct, which skips the next @var{count}
125 characters, is useful for program-generated comments containing binary
126 data. The Emacs Lisp byte compiler uses this in its output files
127 (@pxref{Byte Compilation}). It isn't meant for source files, however.
129 @xref{Comment Tips}, for conventions for formatting comments.
131 @node Programming Types
132 @section Programming Types
133 @cindex programming types
135 There are two general categories of types in Emacs Lisp: those having
136 to do with Lisp programming, and those having to do with editing. The
137 former exist in many Lisp implementations, in one form or another. The
138 latter are unique to Emacs Lisp.
141 * Integer Type:: Numbers without fractional parts.
142 * Floating Point Type:: Numbers with fractional parts and with a large range.
143 * Character Type:: The representation of letters, numbers and
145 * Symbol Type:: A multi-use object that refers to a function,
146 variable, or property list, and has a unique identity.
147 * Sequence Type:: Both lists and arrays are classified as sequences.
148 * Cons Cell Type:: Cons cells, and lists (which are made from cons cells).
149 * Array Type:: Arrays include strings and vectors.
150 * String Type:: An (efficient) array of characters.
151 * Vector Type:: One-dimensional arrays.
152 * Char-Table Type:: One-dimensional sparse arrays indexed by characters.
153 * Bool-Vector Type:: One-dimensional arrays of @code{t} or @code{nil}.
154 * Hash Table Type:: Super-fast lookup tables.
155 * Function Type:: A piece of executable code you can call from elsewhere.
156 * Macro Type:: A method of expanding an expression into another
157 expression, more fundamental but less pretty.
158 * Primitive Function Type:: A function written in C, callable from Lisp.
159 * Byte-Code Type:: A function written in Lisp, then compiled.
160 * Autoload Type:: A type used for automatically loading seldom-used
165 @subsection Integer Type
167 The range of values for integers in Emacs Lisp is @minus{}536870912 to
168 536870911 (30 bits; i.e.,
182 on most machines. (Some machines may provide a wider range.) It is
183 important to note that the Emacs Lisp arithmetic functions do not check
184 for overflow. Thus @code{(1+ 536870911)} is @minus{}536870912 on most
187 The read syntax for integers is a sequence of (base ten) digits with an
188 optional sign at the beginning and an optional period at the end. The
189 printed representation produced by the Lisp interpreter never has a
190 leading @samp{+} or a final @samp{.}.
194 -1 ; @r{The integer -1.}
195 1 ; @r{The integer 1.}
196 1. ; @r{Also the integer 1.}
197 +1 ; @r{Also the integer 1.}
198 1073741825 ; @r{Also the integer 1 on a 30-bit implementation.}
203 As a special exception, if a sequence of digits specifies an integer
204 too large or too small to be a valid integer object, the Lisp reader
205 reads it as a floating-point number (@pxref{Floating Point Type}).
206 For instance, on most machines @code{536870912} is read as the
207 floating-point number @code{536870912.0}.
209 @xref{Numbers}, for more information.
211 @node Floating Point Type
212 @subsection Floating Point Type
214 Floating point numbers are the computer equivalent of scientific
215 notation; you can think of a floating point number as a fraction
216 together with a power of ten. The precise number of significant
217 figures and the range of possible exponents is machine-specific; Emacs
218 uses the C data type @code{double} to store the value, and internally
219 this records a power of 2 rather than a power of 10.
221 The printed representation for floating point numbers requires either
222 a decimal point (with at least one digit following), an exponent, or
223 both. For example, @samp{1500.0}, @samp{15e2}, @samp{15.0e2},
224 @samp{1.5e3}, and @samp{.15e4} are five ways of writing a floating point
225 number whose value is 1500. They are all equivalent.
227 @xref{Numbers}, for more information.
230 @subsection Character Type
231 @cindex @acronym{ASCII} character codes
233 A @dfn{character} in Emacs Lisp is nothing more than an integer. In
234 other words, characters are represented by their character codes. For
235 example, the character @kbd{A} is represented as the @w{integer 65}.
237 Individual characters are used occasionally in programs, but it is
238 more common to work with @emph{strings}, which are sequences composed
239 of characters. @xref{String Type}.
241 Characters in strings and buffers are currently limited to the range
242 of 0 to 4194303---twenty two bits (@pxref{Character Codes}). Codes 0
243 through 127 are @acronym{ASCII} codes; the rest are
244 non-@acronym{ASCII} (@pxref{Non-ASCII Characters}). Characters that
245 represent keyboard input have a much wider range, to encode modifier
246 keys such as Control, Meta and Shift.
248 There are special functions for producing a human-readable textual
249 description of a character for the sake of messages. @xref{Describing
253 * Basic Char Syntax:: Syntax for regular characters.
254 * General Escape Syntax:: How to specify characters by their codes.
255 * Ctl-Char Syntax:: Syntax for control characters.
256 * Meta-Char Syntax:: Syntax for meta-characters.
257 * Other Char Bits:: Syntax for hyper-, super-, and alt-characters.
260 @node Basic Char Syntax
261 @subsubsection Basic Char Syntax
262 @cindex read syntax for characters
263 @cindex printed representation for characters
264 @cindex syntax for characters
265 @cindex @samp{?} in character constant
266 @cindex question mark in character constant
268 Since characters are really integers, the printed representation of
269 a character is a decimal number. This is also a possible read syntax
270 for a character, but writing characters that way in Lisp programs is
271 not clear programming. You should @emph{always} use the special read
272 syntax formats that Emacs Lisp provides for characters. These syntax
273 formats start with a question mark.
275 The usual read syntax for alphanumeric characters is a question mark
276 followed by the character; thus, @samp{?A} for the character
277 @kbd{A}, @samp{?B} for the character @kbd{B}, and @samp{?a} for the
283 ?Q @result{} 81 ?q @result{} 113
286 You can use the same syntax for punctuation characters, but it is
287 often a good idea to add a @samp{\} so that the Emacs commands for
288 editing Lisp code don't get confused. For example, @samp{?\(} is the
289 way to write the open-paren character. If the character is @samp{\},
290 you @emph{must} use a second @samp{\} to quote it: @samp{?\\}.
293 @cindex bell character
297 @cindex tab (ASCII character)
305 @cindex return (ASCII character)
307 @cindex escape (ASCII character)
309 @cindex space (ASCII character)
311 You can express the characters control-g, backspace, tab, newline,
312 vertical tab, formfeed, space, return, del, and escape as @samp{?\a},
313 @samp{?\b}, @samp{?\t}, @samp{?\n}, @samp{?\v}, @samp{?\f},
314 @samp{?\s}, @samp{?\r}, @samp{?\d}, and @samp{?\e}, respectively.
315 (@samp{?\s} followed by a dash has a different meaning---it applies
316 the ``super'' modifier to the following character.) Thus,
319 ?\a @result{} 7 ; @r{control-g, @kbd{C-g}}
320 ?\b @result{} 8 ; @r{backspace, @key{BS}, @kbd{C-h}}
321 ?\t @result{} 9 ; @r{tab, @key{TAB}, @kbd{C-i}}
322 ?\n @result{} 10 ; @r{newline, @kbd{C-j}}
323 ?\v @result{} 11 ; @r{vertical tab, @kbd{C-k}}
324 ?\f @result{} 12 ; @r{formfeed character, @kbd{C-l}}
325 ?\r @result{} 13 ; @r{carriage return, @key{RET}, @kbd{C-m}}
326 ?\e @result{} 27 ; @r{escape character, @key{ESC}, @kbd{C-[}}
327 ?\s @result{} 32 ; @r{space character, @key{SPC}}
328 ?\\ @result{} 92 ; @r{backslash character, @kbd{\}}
329 ?\d @result{} 127 ; @r{delete character, @key{DEL}}
332 @cindex escape sequence
333 These sequences which start with backslash are also known as
334 @dfn{escape sequences}, because backslash plays the role of an
335 ``escape character''; this terminology has nothing to do with the
336 character @key{ESC}. @samp{\s} is meant for use in character
337 constants; in string constants, just write the space.
339 A backslash is allowed, and harmless, preceding any character without
340 a special escape meaning; thus, @samp{?\+} is equivalent to @samp{?+}.
341 There is no reason to add a backslash before most characters. However,
342 you should add a backslash before any of the characters
343 @samp{()\|;'`"#.,} to avoid confusing the Emacs commands for editing
344 Lisp code. You can also add a backslash before whitespace characters such as
345 space, tab, newline and formfeed. However, it is cleaner to use one of
346 the easily readable escape sequences, such as @samp{\t} or @samp{\s},
347 instead of an actual whitespace character such as a tab or a space.
348 (If you do write backslash followed by a space, you should write
349 an extra space after the character constant to separate it from the
352 @node General Escape Syntax
353 @subsubsection General Escape Syntax
355 In addition to the specific escape sequences for special important
356 control characters, Emacs provides several types of escape syntax that
357 you can use to specify non-ASCII text characters.
359 @cindex unicode character escape
360 You can specify characters by their Unicode values.
361 @code{?\u@var{nnnn}} represents a character that maps to the Unicode
362 code point @samp{U+@var{nnnn}} (by convention, Unicode code points are
363 given in hexadecimal). There is a slightly different syntax for
364 specifying characters with code points higher than
365 @code{U+@var{ffff}}: @code{\U00@var{nnnnnn}} represents the character
366 whose code point is @samp{U+@var{nnnnnn}}. The Unicode Standard only
367 defines code points up to @samp{U+@var{10ffff}}, so if you specify a
368 code point higher than that, Emacs signals an error.
370 This peculiar and inconvenient syntax was adopted for compatibility
371 with other programming languages. Unlike some other languages, Emacs
372 Lisp supports this syntax only in character literals and strings.
374 @cindex @samp{\} in character constant
375 @cindex backslash in character constant
376 @cindex octal character code
377 The most general read syntax for a character represents the
378 character code in either octal or hex. To use octal, write a question
379 mark followed by a backslash and the octal character code (up to three
380 octal digits); thus, @samp{?\101} for the character @kbd{A},
381 @samp{?\001} for the character @kbd{C-a}, and @code{?\002} for the
382 character @kbd{C-b}. Although this syntax can represent any
383 @acronym{ASCII} character, it is preferred only when the precise octal
384 value is more important than the @acronym{ASCII} representation.
388 ?\012 @result{} 10 ?\n @result{} 10 ?\C-j @result{} 10
389 ?\101 @result{} 65 ?A @result{} 65
393 To use hex, write a question mark followed by a backslash, @samp{x},
394 and the hexadecimal character code. You can use any number of hex
395 digits, so you can represent any character code in this way.
396 Thus, @samp{?\x41} for the character @kbd{A}, @samp{?\x1} for the
397 character @kbd{C-a}, and @code{?\x8e0} for the Latin-1 character
402 @samp{a} with grave accent.
405 @node Ctl-Char Syntax
406 @subsubsection Control-Character Syntax
408 @cindex control characters
409 Control characters can be represented using yet another read syntax.
410 This consists of a question mark followed by a backslash, caret, and the
411 corresponding non-control character, in either upper or lower case. For
412 example, both @samp{?\^I} and @samp{?\^i} are valid read syntax for the
413 character @kbd{C-i}, the character whose value is 9.
415 Instead of the @samp{^}, you can use @samp{C-}; thus, @samp{?\C-i} is
416 equivalent to @samp{?\^I} and to @samp{?\^i}:
419 ?\^I @result{} 9 ?\C-I @result{} 9
422 In strings and buffers, the only control characters allowed are those
423 that exist in @acronym{ASCII}; but for keyboard input purposes, you can turn
424 any character into a control character with @samp{C-}. The character
425 codes for these non-@acronym{ASCII} control characters include the
432 bit as well as the code for the corresponding non-control
433 character. Ordinary terminals have no way of generating non-@acronym{ASCII}
434 control characters, but you can generate them straightforwardly using X
435 and other window systems.
437 For historical reasons, Emacs treats the @key{DEL} character as
438 the control equivalent of @kbd{?}:
441 ?\^? @result{} 127 ?\C-? @result{} 127
445 As a result, it is currently not possible to represent the character
446 @kbd{Control-?}, which is a meaningful input character under X, using
447 @samp{\C-}. It is not easy to change this, as various Lisp files refer
448 to @key{DEL} in this way.
450 For representing control characters to be found in files or strings,
451 we recommend the @samp{^} syntax; for control characters in keyboard
452 input, we prefer the @samp{C-} syntax. Which one you use does not
453 affect the meaning of the program, but may guide the understanding of
456 @node Meta-Char Syntax
457 @subsubsection Meta-Character Syntax
459 @cindex meta characters
460 A @dfn{meta character} is a character typed with the @key{META}
461 modifier key. The integer that represents such a character has the
468 bit set. We use high bits for this and other modifiers to make
469 possible a wide range of basic character codes.
478 bit attached to an @acronym{ASCII} character indicates a meta
479 character; thus, the meta characters that can fit in a string have
480 codes in the range from 128 to 255, and are the meta versions of the
481 ordinary @acronym{ASCII} characters. @xref{Strings of Events}, for
482 details about @key{META}-handling in strings.
484 The read syntax for meta characters uses @samp{\M-}. For example,
485 @samp{?\M-A} stands for @kbd{M-A}. You can use @samp{\M-} together with
486 octal character codes (see below), with @samp{\C-}, or with any other
487 syntax for a character. Thus, you can write @kbd{M-A} as @samp{?\M-A},
488 or as @samp{?\M-\101}. Likewise, you can write @kbd{C-M-b} as
489 @samp{?\M-\C-b}, @samp{?\C-\M-b}, or @samp{?\M-\002}.
491 @node Other Char Bits
492 @subsubsection Other Character Modifier Bits
494 The case of a graphic character is indicated by its character code;
495 for example, @acronym{ASCII} distinguishes between the characters @samp{a}
496 and @samp{A}. But @acronym{ASCII} has no way to represent whether a control
497 character is upper case or lower case. Emacs uses the
504 bit to indicate that the shift key was used in typing a control
505 character. This distinction is possible only when you use X terminals
506 or other special terminals; ordinary terminals do not report the
507 distinction to the computer in any way. The Lisp syntax for
508 the shift bit is @samp{\S-}; thus, @samp{?\C-\S-o} or @samp{?\C-\S-O}
509 represents the shifted-control-o character.
511 @cindex hyper characters
512 @cindex super characters
513 @cindex alt characters
514 The X Window System defines three other
515 @anchor{modifier bits}modifier bits that can be set
516 in a character: @dfn{hyper}, @dfn{super} and @dfn{alt}. The syntaxes
517 for these bits are @samp{\H-}, @samp{\s-} and @samp{\A-}. (Case is
518 significant in these prefixes.) Thus, @samp{?\H-\M-\A-x} represents
519 @kbd{Alt-Hyper-Meta-x}. (Note that @samp{\s} with no following @samp{-}
520 represents the space character.)
522 Numerically, the bit values are @math{2^{22}} for alt, @math{2^{23}}
523 for super and @math{2^{24}} for hyper.
527 bit values are 2**22 for alt, 2**23 for super and 2**24 for hyper.
531 @subsection Symbol Type
533 A @dfn{symbol} in GNU Emacs Lisp is an object with a name. The
534 symbol name serves as the printed representation of the symbol. In
535 ordinary Lisp use, with one single obarray (@pxref{Creating Symbols}),
536 a symbol's name is unique---no two symbols have the same name.
538 A symbol can serve as a variable, as a function name, or to hold a
539 property list. Or it may serve only to be distinct from all other Lisp
540 objects, so that its presence in a data structure may be recognized
541 reliably. In a given context, usually only one of these uses is
542 intended. But you can use one symbol in all of these ways,
545 A symbol whose name starts with a colon (@samp{:}) is called a
546 @dfn{keyword symbol}. These symbols automatically act as constants, and
547 are normally used only by comparing an unknown symbol with a few
548 specific alternatives.
550 @cindex @samp{\} in symbols
551 @cindex backslash in symbols
552 A symbol name can contain any characters whatever. Most symbol names
553 are written with letters, digits, and the punctuation characters
554 @samp{-+=*/}. Such names require no special punctuation; the characters
555 of the name suffice as long as the name does not look like a number.
556 (If it does, write a @samp{\} at the beginning of the name to force
557 interpretation as a symbol.) The characters @samp{_~!@@$%^&:<>@{@}?} are
558 less often used but also require no special punctuation. Any other
559 characters may be included in a symbol's name by escaping them with a
560 backslash. In contrast to its use in strings, however, a backslash in
561 the name of a symbol simply quotes the single character that follows the
562 backslash. For example, in a string, @samp{\t} represents a tab
563 character; in the name of a symbol, however, @samp{\t} merely quotes the
564 letter @samp{t}. To have a symbol with a tab character in its name, you
565 must actually use a tab (preceded with a backslash). But it's rare to
568 @cindex CL note---case of letters
570 @b{Common Lisp note:} In Common Lisp, lower case letters are always
571 ``folded'' to upper case, unless they are explicitly escaped. In Emacs
572 Lisp, upper case and lower case letters are distinct.
575 Here are several examples of symbol names. Note that the @samp{+} in
576 the fifth example is escaped to prevent it from being read as a number.
577 This is not necessary in the fourth example because the rest of the name
578 makes it invalid as a number.
582 foo ; @r{A symbol named @samp{foo}.}
583 FOO ; @r{A symbol named @samp{FOO}, different from @samp{foo}.}
586 1+ ; @r{A symbol named @samp{1+}}
587 ; @r{(not @samp{+1}, which is an integer).}
590 \+1 ; @r{A symbol named @samp{+1}}
591 ; @r{(not a very readable name).}
594 \(*\ 1\ 2\) ; @r{A symbol named @samp{(* 1 2)} (a worse name).}
595 @c the @'s in this next line use up three characters, hence the
596 @c apparent misalignment of the comment.
597 +-*/_~!@@$%^&=:<>@{@} ; @r{A symbol named @samp{+-*/_~!@@$%^&=:<>@{@}}.}
598 ; @r{These characters need not be escaped.}
603 @c This uses ``colon'' instead of a literal `:' because Info cannot
604 @c cope with a `:' in a menu
605 @cindex @samp{#@var{colon}} read syntax
608 @cindex @samp{#:} read syntax
610 Normally the Lisp reader interns all symbols (@pxref{Creating
611 Symbols}). To prevent interning, you can write @samp{#:} before the
615 @subsection Sequence Types
617 A @dfn{sequence} is a Lisp object that represents an ordered set of
618 elements. There are two kinds of sequence in Emacs Lisp, lists and
619 arrays. Thus, an object of type list or of type array is also
620 considered a sequence.
622 Arrays are further subdivided into strings, vectors, char-tables and
623 bool-vectors. Vectors can hold elements of any type, but string
624 elements must be characters, and bool-vector elements must be @code{t}
625 or @code{nil}. Char-tables are like vectors except that they are
626 indexed by any valid character code. The characters in a string can
627 have text properties like characters in a buffer (@pxref{Text
628 Properties}), but vectors do not support text properties, even when
629 their elements happen to be characters.
631 Lists, strings and the other array types are different, but they have
632 important similarities. For example, all have a length @var{l}, and all
633 have elements which can be indexed from zero to @var{l} minus one.
634 Several functions, called sequence functions, accept any kind of
635 sequence. For example, the function @code{elt} can be used to extract
636 an element of a sequence, given its index. @xref{Sequences Arrays
639 It is generally impossible to read the same sequence twice, since
640 sequences are always created anew upon reading. If you read the read
641 syntax for a sequence twice, you get two sequences with equal contents.
642 There is one exception: the empty list @code{()} always stands for the
643 same object, @code{nil}.
646 @subsection Cons Cell and List Types
647 @cindex address field of register
648 @cindex decrement field of register
651 A @dfn{cons cell} is an object that consists of two slots, called the
652 @sc{car} slot and the @sc{cdr} slot. Each slot can @dfn{hold} or
653 @dfn{refer to} any Lisp object. We also say that ``the @sc{car} of
654 this cons cell is'' whatever object its @sc{car} slot currently holds,
655 and likewise for the @sc{cdr}.
658 A note to C programmers: in Lisp, we do not distinguish between
659 ``holding'' a value and ``pointing to'' the value, because pointers in
663 A @dfn{list} is a series of cons cells, linked together so that the
664 @sc{cdr} slot of each cons cell holds either the next cons cell or the
665 empty list. The empty list is actually the symbol @code{nil}.
666 @xref{Lists}, for functions that work on lists. Because most cons
667 cells are used as part of lists, the phrase @dfn{list structure} has
668 come to refer to any structure made out of cons cells.
671 Because cons cells are so central to Lisp, we also have a word for
672 ``an object which is not a cons cell.'' These objects are called
676 @cindex @samp{(@dots{})} in lists
677 The read syntax and printed representation for lists are identical, and
678 consist of a left parenthesis, an arbitrary number of elements, and a
679 right parenthesis. Here are examples of lists:
682 (A 2 "A") ; @r{A list of three elements.}
683 () ; @r{A list of no elements (the empty list).}
684 nil ; @r{A list of no elements (the empty list).}
685 ("A ()") ; @r{A list of one element: the string @code{"A ()"}.}
686 (A ()) ; @r{A list of two elements: @code{A} and the empty list.}
687 (A nil) ; @r{Equivalent to the previous.}
688 ((A B C)) ; @r{A list of one element}
689 ; @r{(which is a list of three elements).}
692 Upon reading, each object inside the parentheses becomes an element
693 of the list. That is, a cons cell is made for each element. The
694 @sc{car} slot of the cons cell holds the element, and its @sc{cdr}
695 slot refers to the next cons cell of the list, which holds the next
696 element in the list. The @sc{cdr} slot of the last cons cell is set to
699 The names @sc{car} and @sc{cdr} derive from the history of Lisp. The
700 original Lisp implementation ran on an @w{IBM 704} computer which
701 divided words into two parts, called the ``address'' part and the
702 ``decrement''; @sc{car} was an instruction to extract the contents of
703 the address part of a register, and @sc{cdr} an instruction to extract
704 the contents of the decrement. By contrast, ``cons cells'' are named
705 for the function @code{cons} that creates them, which in turn was named
706 for its purpose, the construction of cells.
709 * Box Diagrams:: Drawing pictures of lists.
710 * Dotted Pair Notation:: A general syntax for cons cells.
711 * Association List Type:: A specially constructed list.
715 @subsubsection Drawing Lists as Box Diagrams
716 @cindex box diagrams, for lists
717 @cindex diagrams, boxed, for lists
719 A list can be illustrated by a diagram in which the cons cells are
720 shown as pairs of boxes, like dominoes. (The Lisp reader cannot read
721 such an illustration; unlike the textual notation, which can be
722 understood by both humans and computers, the box illustrations can be
723 understood only by humans.) This picture represents the three-element
724 list @code{(rose violet buttercup)}:
728 --- --- --- --- --- ---
729 | | |--> | | |--> | | |--> nil
730 --- --- --- --- --- ---
733 --> rose --> violet --> buttercup
737 In this diagram, each box represents a slot that can hold or refer to
738 any Lisp object. Each pair of boxes represents a cons cell. Each arrow
739 represents a reference to a Lisp object, either an atom or another cons
742 In this example, the first box, which holds the @sc{car} of the first
743 cons cell, refers to or ``holds'' @code{rose} (a symbol). The second
744 box, holding the @sc{cdr} of the first cons cell, refers to the next
745 pair of boxes, the second cons cell. The @sc{car} of the second cons
746 cell is @code{violet}, and its @sc{cdr} is the third cons cell. The
747 @sc{cdr} of the third (and last) cons cell is @code{nil}.
749 Here is another diagram of the same list, @code{(rose violet
750 buttercup)}, sketched in a different manner:
754 --------------- ---------------- -------------------
755 | car | cdr | | car | cdr | | car | cdr |
756 | rose | o-------->| violet | o-------->| buttercup | nil |
758 --------------- ---------------- -------------------
762 @cindex @code{nil} as a list
764 A list with no elements in it is the @dfn{empty list}; it is identical
765 to the symbol @code{nil}. In other words, @code{nil} is both a symbol
768 Here is the list @code{(A ())}, or equivalently @code{(A nil)},
769 depicted with boxes and arrows:
774 | | |--> | | |--> nil
782 Here is a more complex illustration, showing the three-element list,
783 @code{((pine needles) oak maple)}, the first element of which is a
788 --- --- --- --- --- ---
789 | | |--> | | |--> | | |--> nil
790 --- --- --- --- --- ---
796 --> | | |--> | | |--> nil
804 The same list represented in the second box notation looks like this:
808 -------------- -------------- --------------
809 | car | cdr | | car | cdr | | car | cdr |
810 | o | o------->| oak | o------->| maple | nil |
812 -- | --------- -------------- --------------
815 | -------------- ----------------
816 | | car | cdr | | car | cdr |
817 ------>| pine | o------->| needles | nil |
819 -------------- ----------------
823 @node Dotted Pair Notation
824 @subsubsection Dotted Pair Notation
825 @cindex dotted pair notation
826 @cindex @samp{.} in lists
828 @dfn{Dotted pair notation} is a general syntax for cons cells that
829 represents the @sc{car} and @sc{cdr} explicitly. In this syntax,
830 @code{(@var{a} .@: @var{b})} stands for a cons cell whose @sc{car} is
831 the object @var{a} and whose @sc{cdr} is the object @var{b}. Dotted
832 pair notation is more general than list syntax because the @sc{cdr}
833 does not have to be a list. However, it is more cumbersome in cases
834 where list syntax would work. In dotted pair notation, the list
835 @samp{(1 2 3)} is written as @samp{(1 . (2 . (3 . nil)))}. For
836 @code{nil}-terminated lists, you can use either notation, but list
837 notation is usually clearer and more convenient. When printing a
838 list, the dotted pair notation is only used if the @sc{cdr} of a cons
841 Here's an example using boxes to illustrate dotted pair notation.
842 This example shows the pair @code{(rose . violet)}:
855 You can combine dotted pair notation with list notation to represent
856 conveniently a chain of cons cells with a non-@code{nil} final @sc{cdr}.
857 You write a dot after the last element of the list, followed by the
858 @sc{cdr} of the final cons cell. For example, @code{(rose violet
859 . buttercup)} is equivalent to @code{(rose . (violet . buttercup))}.
860 The object looks like this:
865 | | |--> | | |--> buttercup
873 The syntax @code{(rose .@: violet .@: buttercup)} is invalid because
874 there is nothing that it could mean. If anything, it would say to put
875 @code{buttercup} in the @sc{cdr} of a cons cell whose @sc{cdr} is already
876 used for @code{violet}.
878 The list @code{(rose violet)} is equivalent to @code{(rose . (violet))},
884 | | |--> | | |--> nil
892 Similarly, the three-element list @code{(rose violet buttercup)}
893 is equivalent to @code{(rose . (violet . (buttercup)))}.
899 --- --- --- --- --- ---
900 | | |--> | | |--> | | |--> nil
901 --- --- --- --- --- ---
904 --> rose --> violet --> buttercup
909 @node Association List Type
910 @comment node-name, next, previous, up
911 @subsubsection Association List Type
913 An @dfn{association list} or @dfn{alist} is a specially-constructed
914 list whose elements are cons cells. In each element, the @sc{car} is
915 considered a @dfn{key}, and the @sc{cdr} is considered an
916 @dfn{associated value}. (In some cases, the associated value is stored
917 in the @sc{car} of the @sc{cdr}.) Association lists are often used as
918 stacks, since it is easy to add or remove associations at the front of
924 (setq alist-of-colors
925 '((rose . red) (lily . white) (buttercup . yellow)))
929 sets the variable @code{alist-of-colors} to an alist of three elements. In the
930 first element, @code{rose} is the key and @code{red} is the value.
932 @xref{Association Lists}, for a further explanation of alists and for
933 functions that work on alists. @xref{Hash Tables}, for another kind of
934 lookup table, which is much faster for handling a large number of keys.
937 @subsection Array Type
939 An @dfn{array} is composed of an arbitrary number of slots for
940 holding or referring to other Lisp objects, arranged in a contiguous block of
941 memory. Accessing any element of an array takes approximately the same
942 amount of time. In contrast, accessing an element of a list requires
943 time proportional to the position of the element in the list. (Elements
944 at the end of a list take longer to access than elements at the
945 beginning of a list.)
947 Emacs defines four types of array: strings, vectors, bool-vectors, and
950 A string is an array of characters and a vector is an array of
951 arbitrary objects. A bool-vector can hold only @code{t} or @code{nil}.
952 These kinds of array may have any length up to the largest integer.
953 Char-tables are sparse arrays indexed by any valid character code; they
954 can hold arbitrary objects.
956 The first element of an array has index zero, the second element has
957 index 1, and so on. This is called @dfn{zero-origin} indexing. For
958 example, an array of four elements has indices 0, 1, 2, @w{and 3}. The
959 largest possible index value is one less than the length of the array.
960 Once an array is created, its length is fixed.
962 All Emacs Lisp arrays are one-dimensional. (Most other programming
963 languages support multidimensional arrays, but they are not essential;
964 you can get the same effect with nested one-dimensional arrays.) Each
965 type of array has its own read syntax; see the following sections for
968 The array type is a subset of the sequence type, and contains the
969 string type, the vector type, the bool-vector type, and the char-table
973 @subsection String Type
975 A @dfn{string} is an array of characters. Strings are used for many
976 purposes in Emacs, as can be expected in a text editor; for example, as
977 the names of Lisp symbols, as messages for the user, and to represent
978 text extracted from buffers. Strings in Lisp are constants: evaluation
979 of a string returns the same string.
981 @xref{Strings and Characters}, for functions that operate on strings.
984 * Syntax for Strings:: How to specify Lisp strings.
985 * Non-ASCII in Strings:: International characters in strings.
986 * Nonprinting Characters:: Literal unprintable characters in strings.
987 * Text Props and Strings:: Strings with text properties.
990 @node Syntax for Strings
991 @subsubsection Syntax for Strings
993 @cindex @samp{"} in strings
994 @cindex double-quote in strings
995 @cindex @samp{\} in strings
996 @cindex backslash in strings
997 The read syntax for a string is a double-quote, an arbitrary number
998 of characters, and another double-quote, @code{"like this"}. To
999 include a double-quote in a string, precede it with a backslash; thus,
1000 @code{"\""} is a string containing just a single double-quote
1001 character. Likewise, you can include a backslash by preceding it with
1002 another backslash, like this: @code{"this \\ is a single embedded
1005 @cindex newline in strings
1006 The newline character is not special in the read syntax for strings;
1007 if you write a new line between the double-quotes, it becomes a
1008 character in the string. But an escaped newline---one that is preceded
1009 by @samp{\}---does not become part of the string; i.e., the Lisp reader
1010 ignores an escaped newline while reading a string. An escaped space
1011 @w{@samp{\ }} is likewise ignored.
1014 "It is useful to include newlines
1015 in documentation strings,
1016 but the newline is \
1017 ignored if escaped."
1018 @result{} "It is useful to include newlines
1019 in documentation strings,
1020 but the newline is ignored if escaped."
1023 @node Non-ASCII in Strings
1024 @subsubsection Non-@acronym{ASCII} Characters in Strings
1026 You can include a non-@acronym{ASCII} international character in a string
1027 constant by writing it literally. There are two text representations
1028 for non-@acronym{ASCII} characters in Emacs strings (and in buffers): unibyte
1029 and multibyte. If the string constant is read from a multibyte source,
1030 such as a multibyte buffer or string, or a file that would be visited as
1031 multibyte, then the character is read as a multibyte character, and that
1032 makes the string multibyte. If the string constant is read from a
1033 unibyte source, then the character is read as unibyte and that makes the
1036 You can also represent a multibyte non-@acronym{ASCII} character with its
1037 character code: use a hex escape, @samp{\x@var{nnnnnnn}}, with as many
1038 digits as necessary. (Multibyte non-@acronym{ASCII} character codes are all
1039 greater than 256.) Any character which is not a valid hex digit
1040 terminates this construct. If the next character in the string could be
1041 interpreted as a hex digit, write @w{@samp{\ }} (backslash and space) to
1042 terminate the hex escape---for example, @w{@samp{\x8e0\ }} represents
1043 one character, @samp{a} with grave accent. @w{@samp{\ }} in a string
1044 constant is just like backslash-newline; it does not contribute any
1045 character to the string, but it does terminate the preceding hex escape.
1047 You can represent a unibyte non-@acronym{ASCII} character with its
1048 character code, which must be in the range from 128 (0200 octal) to
1049 255 (0377 octal). If you write all such character codes in octal and
1050 the string contains no other characters forcing it to be multibyte,
1051 this produces a unibyte string. However, using any hex escape in a
1052 string (even for an @acronym{ASCII} character) forces the string to be
1055 You can also specify characters in a string by their numeric values
1056 in Unicode, using @samp{\u} and @samp{\U} (@pxref{Character Type}).
1058 @xref{Text Representations}, for more information about the two
1059 text representations.
1061 @node Nonprinting Characters
1062 @subsubsection Nonprinting Characters in Strings
1064 You can use the same backslash escape-sequences in a string constant
1065 as in character literals (but do not use the question mark that begins a
1066 character constant). For example, you can write a string containing the
1067 nonprinting characters tab and @kbd{C-a}, with commas and spaces between
1068 them, like this: @code{"\t, \C-a"}. @xref{Character Type}, for a
1069 description of the read syntax for characters.
1071 However, not all of the characters you can write with backslash
1072 escape-sequences are valid in strings. The only control characters that
1073 a string can hold are the @acronym{ASCII} control characters. Strings do not
1074 distinguish case in @acronym{ASCII} control characters.
1076 Properly speaking, strings cannot hold meta characters; but when a
1077 string is to be used as a key sequence, there is a special convention
1078 that provides a way to represent meta versions of @acronym{ASCII}
1079 characters in a string. If you use the @samp{\M-} syntax to indicate
1080 a meta character in a string constant, this sets the
1087 bit of the character in the string. If the string is used in
1088 @code{define-key} or @code{lookup-key}, this numeric code is translated
1089 into the equivalent meta character. @xref{Character Type}.
1091 Strings cannot hold characters that have the hyper, super, or alt
1094 @node Text Props and Strings
1095 @subsubsection Text Properties in Strings
1097 @cindex @samp{#(} read syntax
1098 @cindex text properties, read syntax
1099 A string can hold properties for the characters it contains, in
1100 addition to the characters themselves. This enables programs that copy
1101 text between strings and buffers to copy the text's properties with no
1102 special effort. @xref{Text Properties}, for an explanation of what text
1103 properties mean. Strings with text properties use a special read and
1107 #("@var{characters}" @var{property-data}...)
1111 where @var{property-data} consists of zero or more elements, in groups
1112 of three as follows:
1115 @var{beg} @var{end} @var{plist}
1119 The elements @var{beg} and @var{end} are integers, and together specify
1120 a range of indices in the string; @var{plist} is the property list for
1121 that range. For example,
1124 #("foo bar" 0 3 (face bold) 3 4 nil 4 7 (face italic))
1128 represents a string whose textual contents are @samp{foo bar}, in which
1129 the first three characters have a @code{face} property with value
1130 @code{bold}, and the last three have a @code{face} property with value
1131 @code{italic}. (The fourth character has no text properties, so its
1132 property list is @code{nil}. It is not actually necessary to mention
1133 ranges with @code{nil} as the property list, since any characters not
1134 mentioned in any range will default to having no properties.)
1137 @subsection Vector Type
1139 A @dfn{vector} is a one-dimensional array of elements of any type. It
1140 takes a constant amount of time to access any element of a vector. (In
1141 a list, the access time of an element is proportional to the distance of
1142 the element from the beginning of the list.)
1144 The printed representation of a vector consists of a left square
1145 bracket, the elements, and a right square bracket. This is also the
1146 read syntax. Like numbers and strings, vectors are considered constants
1150 [1 "two" (three)] ; @r{A vector of three elements.}
1151 @result{} [1 "two" (three)]
1154 @xref{Vectors}, for functions that work with vectors.
1156 @node Char-Table Type
1157 @subsection Char-Table Type
1159 A @dfn{char-table} is a one-dimensional array of elements of any type,
1160 indexed by character codes. Char-tables have certain extra features to
1161 make them more useful for many jobs that involve assigning information
1162 to character codes---for example, a char-table can have a parent to
1163 inherit from, a default value, and a small number of extra slots to use for
1164 special purposes. A char-table can also specify a single value for
1165 a whole character set.
1167 The printed representation of a char-table is like a vector
1168 except that there is an extra @samp{#^} at the beginning.
1170 @xref{Char-Tables}, for special functions to operate on char-tables.
1171 Uses of char-tables include:
1175 Case tables (@pxref{Case Tables}).
1178 Character category tables (@pxref{Categories}).
1181 Display tables (@pxref{Display Tables}).
1184 Syntax tables (@pxref{Syntax Tables}).
1187 @node Bool-Vector Type
1188 @subsection Bool-Vector Type
1190 A @dfn{bool-vector} is a one-dimensional array whose elements must
1191 be @code{t} or @code{nil}.
1193 The printed representation of a bool-vector is like a string, except
1194 that it begins with @samp{#&} followed by the length. The string
1195 constant that follows actually specifies the contents of the bool-vector
1196 as a bitmap---each ``character'' in the string contains 8 bits, which
1197 specify the next 8 elements of the bool-vector (1 stands for @code{t},
1198 and 0 for @code{nil}). The least significant bits of the character
1199 correspond to the lowest indices in the bool-vector.
1202 (make-bool-vector 3 t)
1204 (make-bool-vector 3 nil)
1209 These results make sense, because the binary code for @samp{C-g} is
1210 111 and @samp{C-@@} is the character with code 0.
1212 If the length is not a multiple of 8, the printed representation
1213 shows extra elements, but these extras really make no difference. For
1214 instance, in the next example, the two bool-vectors are equal, because
1215 only the first 3 bits are used:
1218 (equal #&3"\377" #&3"\007")
1222 @node Hash Table Type
1223 @subsection Hash Table Type
1225 A hash table is a very fast kind of lookup table, somewhat like an
1226 alist in that it maps keys to corresponding values, but much faster.
1227 The printed representation of a hash table specifies its properties
1228 and contents, like this:
1232 @result{} #s(hash-table size 65 test eql rehash-size 1.5
1233 rehash-threshold 0.8 data ())
1237 @xref{Hash Tables}, for more information about hash tables.
1240 @subsection Function Type
1242 Lisp functions are executable code, just like functions in other
1243 programming languages. In Lisp, unlike most languages, functions are
1244 also Lisp objects. A non-compiled function in Lisp is a lambda
1245 expression: that is, a list whose first element is the symbol
1246 @code{lambda} (@pxref{Lambda Expressions}).
1248 In most programming languages, it is impossible to have a function
1249 without a name. In Lisp, a function has no intrinsic name. A lambda
1250 expression can be called as a function even though it has no name; to
1251 emphasize this, we also call it an @dfn{anonymous function}
1252 (@pxref{Anonymous Functions}). A named function in Lisp is just a
1253 symbol with a valid function in its function cell (@pxref{Defining
1256 Most of the time, functions are called when their names are written in
1257 Lisp expressions in Lisp programs. However, you can construct or obtain
1258 a function object at run time and then call it with the primitive
1259 functions @code{funcall} and @code{apply}. @xref{Calling Functions}.
1262 @subsection Macro Type
1264 A @dfn{Lisp macro} is a user-defined construct that extends the Lisp
1265 language. It is represented as an object much like a function, but with
1266 different argument-passing semantics. A Lisp macro has the form of a
1267 list whose first element is the symbol @code{macro} and whose @sc{cdr}
1268 is a Lisp function object, including the @code{lambda} symbol.
1270 Lisp macro objects are usually defined with the built-in
1271 @code{defmacro} function, but any list that begins with @code{macro} is
1272 a macro as far as Emacs is concerned. @xref{Macros}, for an explanation
1273 of how to write a macro.
1275 @strong{Warning}: Lisp macros and keyboard macros (@pxref{Keyboard
1276 Macros}) are entirely different things. When we use the word ``macro''
1277 without qualification, we mean a Lisp macro, not a keyboard macro.
1279 @node Primitive Function Type
1280 @subsection Primitive Function Type
1281 @cindex primitive function
1283 A @dfn{primitive function} is a function callable from Lisp but
1284 written in the C programming language. Primitive functions are also
1285 called @dfn{subrs} or @dfn{built-in functions}. (The word ``subr'' is
1286 derived from ``subroutine.'') Most primitive functions evaluate all
1287 their arguments when they are called. A primitive function that does
1288 not evaluate all its arguments is called a @dfn{special form}
1289 (@pxref{Special Forms}).@refill
1291 It does not matter to the caller of a function whether the function is
1292 primitive. However, this does matter if you try to redefine a primitive
1293 with a function written in Lisp. The reason is that the primitive
1294 function may be called directly from C code. Calls to the redefined
1295 function from Lisp will use the new definition, but calls from C code
1296 may still use the built-in definition. Therefore, @strong{we discourage
1297 redefinition of primitive functions}.
1299 The term @dfn{function} refers to all Emacs functions, whether written
1300 in Lisp or C. @xref{Function Type}, for information about the
1301 functions written in Lisp.
1303 Primitive functions have no read syntax and print in hash notation
1304 with the name of the subroutine.
1308 (symbol-function 'car) ; @r{Access the function cell}
1309 ; @r{of the symbol.}
1310 @result{} #<subr car>
1311 (subrp (symbol-function 'car)) ; @r{Is this a primitive function?}
1312 @result{} t ; @r{Yes.}
1316 @node Byte-Code Type
1317 @subsection Byte-Code Function Type
1319 The byte compiler produces @dfn{byte-code function objects}.
1320 Internally, a byte-code function object is much like a vector; however,
1321 the evaluator handles this data type specially when it appears as a
1322 function to be called. @xref{Byte Compilation}, for information about
1325 The printed representation and read syntax for a byte-code function
1326 object is like that for a vector, with an additional @samp{#} before the
1330 @subsection Autoload Type
1332 An @dfn{autoload object} is a list whose first element is the symbol
1333 @code{autoload}. It is stored as the function definition of a symbol,
1334 where it serves as a placeholder for the real definition. The autoload
1335 object says that the real definition is found in a file of Lisp code
1336 that should be loaded when necessary. It contains the name of the file,
1337 plus some other information about the real definition.
1339 After the file has been loaded, the symbol should have a new function
1340 definition that is not an autoload object. The new definition is then
1341 called as if it had been there to begin with. From the user's point of
1342 view, the function call works as expected, using the function definition
1345 An autoload object is usually created with the function
1346 @code{autoload}, which stores the object in the function cell of a
1347 symbol. @xref{Autoload}, for more details.
1350 @section Editing Types
1351 @cindex editing types
1353 The types in the previous section are used for general programming
1354 purposes, and most of them are common to most Lisp dialects. Emacs Lisp
1355 provides several additional data types for purposes connected with
1359 * Buffer Type:: The basic object of editing.
1360 * Marker Type:: A position in a buffer.
1361 * Window Type:: Buffers are displayed in windows.
1362 * Frame Type:: Windows subdivide frames.
1363 * Terminal Type:: A terminal device displays frames.
1364 * Window Configuration Type:: Recording the way a frame is subdivided.
1365 * Frame Configuration Type:: Recording the status of all frames.
1366 * Process Type:: A subprocess of Emacs running on the underlying OS.
1367 * Stream Type:: Receive or send characters.
1368 * Keymap Type:: What function a keystroke invokes.
1369 * Overlay Type:: How an overlay is represented.
1370 * Font Type:: Fonts for displaying text.
1374 @subsection Buffer Type
1376 A @dfn{buffer} is an object that holds text that can be edited
1377 (@pxref{Buffers}). Most buffers hold the contents of a disk file
1378 (@pxref{Files}) so they can be edited, but some are used for other
1379 purposes. Most buffers are also meant to be seen by the user, and
1380 therefore displayed, at some time, in a window (@pxref{Windows}). But
1381 a buffer need not be displayed in any window. Each buffer has a
1382 designated position called @dfn{point} (@pxref{Positions}); most
1383 editing commands act on the contents of the current buffer in the
1384 neighborhood of point. At any time, one buffer is the @dfn{current
1387 The contents of a buffer are much like a string, but buffers are not
1388 used like strings in Emacs Lisp, and the available operations are
1389 different. For example, you can insert text efficiently into an
1390 existing buffer, altering the buffer's contents, whereas ``inserting''
1391 text into a string requires concatenating substrings, and the result
1392 is an entirely new string object.
1394 Many of the standard Emacs functions manipulate or test the
1395 characters in the current buffer; a whole chapter in this manual is
1396 devoted to describing these functions (@pxref{Text}).
1398 Several other data structures are associated with each buffer:
1402 a local syntax table (@pxref{Syntax Tables});
1405 a local keymap (@pxref{Keymaps}); and,
1408 a list of buffer-local variable bindings (@pxref{Buffer-Local Variables}).
1411 overlays (@pxref{Overlays}).
1414 text properties for the text in the buffer (@pxref{Text Properties}).
1418 The local keymap and variable list contain entries that individually
1419 override global bindings or values. These are used to customize the
1420 behavior of programs in different buffers, without actually changing the
1423 A buffer may be @dfn{indirect}, which means it shares the text
1424 of another buffer, but presents it differently. @xref{Indirect Buffers}.
1426 Buffers have no read syntax. They print in hash notation, showing the
1432 @result{} #<buffer objects.texi>
1437 @subsection Marker Type
1439 A @dfn{marker} denotes a position in a specific buffer. Markers
1440 therefore have two components: one for the buffer, and one for the
1441 position. Changes in the buffer's text automatically relocate the
1442 position value as necessary to ensure that the marker always points
1443 between the same two characters in the buffer.
1445 Markers have no read syntax. They print in hash notation, giving the
1446 current character position and the name of the buffer.
1451 @result{} #<marker at 10779 in objects.texi>
1455 @xref{Markers}, for information on how to test, create, copy, and move
1459 @subsection Window Type
1461 A @dfn{window} describes the portion of the terminal screen that Emacs
1462 uses to display a buffer. Every window has one associated buffer, whose
1463 contents appear in the window. By contrast, a given buffer may appear
1464 in one window, no window, or several windows.
1466 Though many windows may exist simultaneously, at any time one window
1467 is designated the @dfn{selected window}. This is the window where the
1468 cursor is (usually) displayed when Emacs is ready for a command. The
1469 selected window usually displays the current buffer, but this is not
1470 necessarily the case.
1472 Windows are grouped on the screen into frames; each window belongs to
1473 one and only one frame. @xref{Frame Type}.
1475 Windows have no read syntax. They print in hash notation, giving the
1476 window number and the name of the buffer being displayed. The window
1477 numbers exist to identify windows uniquely, since the buffer displayed
1478 in any given window can change frequently.
1483 @result{} #<window 1 on objects.texi>
1487 @xref{Windows}, for a description of the functions that work on windows.
1490 @subsection Frame Type
1492 A @dfn{frame} is a screen area that contains one or more Emacs
1493 windows; we also use the term ``frame'' to refer to the Lisp object
1494 that Emacs uses to refer to the screen area.
1496 Frames have no read syntax. They print in hash notation, giving the
1497 frame's title, plus its address in core (useful to identify the frame
1503 @result{} #<frame emacs@@psilocin.gnu.org 0xdac80>
1507 @xref{Frames}, for a description of the functions that work on frames.
1510 @subsection Terminal Type
1511 @cindex terminal type
1513 A @dfn{terminal} is a device capable of displaying one or more
1514 Emacs frames (@pxref{Frame Type}).
1516 Terminals have no read syntax. They print in hash notation giving
1517 the terminal's ordinal number and its TTY device file name.
1521 (get-device-terminal nil)
1522 @result{} #<terminal 1 on /dev/tty>
1526 @c FIXME: add an xref to where terminal-related primitives are described.
1528 @node Window Configuration Type
1529 @subsection Window Configuration Type
1530 @cindex window layout in a frame
1532 A @dfn{window configuration} stores information about the positions,
1533 sizes, and contents of the windows in a frame, so you can recreate the
1534 same arrangement of windows later.
1536 Window configurations do not have a read syntax; their print syntax
1537 looks like @samp{#<window-configuration>}. @xref{Window
1538 Configurations}, for a description of several functions related to
1539 window configurations.
1541 @node Frame Configuration Type
1542 @subsection Frame Configuration Type
1543 @cindex screen layout
1544 @cindex window layout, all frames
1546 A @dfn{frame configuration} stores information about the positions,
1547 sizes, and contents of the windows in all frames. It is not a
1548 primitive type---it is actually a list whose @sc{car} is
1549 @code{frame-configuration} and whose @sc{cdr} is an alist. Each alist
1550 element describes one frame, which appears as the @sc{car} of that
1553 @xref{Frame Configurations}, for a description of several functions
1554 related to frame configurations.
1557 @subsection Process Type
1559 The word @dfn{process} usually means a running program. Emacs itself
1560 runs in a process of this sort. However, in Emacs Lisp, a process is a
1561 Lisp object that designates a subprocess created by the Emacs process.
1562 Programs such as shells, GDB, ftp, and compilers, running in
1563 subprocesses of Emacs, extend the capabilities of Emacs.
1565 An Emacs subprocess takes textual input from Emacs and returns textual
1566 output to Emacs for further manipulation. Emacs can also send signals
1569 Process objects have no read syntax. They print in hash notation,
1570 giving the name of the process:
1575 @result{} (#<process shell>)
1579 @xref{Processes}, for information about functions that create, delete,
1580 return information about, send input or signals to, and receive output
1584 @subsection Stream Type
1586 A @dfn{stream} is an object that can be used as a source or sink for
1587 characters---either to supply characters for input or to accept them as
1588 output. Many different types can be used this way: markers, buffers,
1589 strings, and functions. Most often, input streams (character sources)
1590 obtain characters from the keyboard, a buffer, or a file, and output
1591 streams (character sinks) send characters to a buffer, such as a
1592 @file{*Help*} buffer, or to the echo area.
1594 The object @code{nil}, in addition to its other meanings, may be used
1595 as a stream. It stands for the value of the variable
1596 @code{standard-input} or @code{standard-output}. Also, the object
1597 @code{t} as a stream specifies input using the minibuffer
1598 (@pxref{Minibuffers}) or output in the echo area (@pxref{The Echo
1601 Streams have no special printed representation or read syntax, and
1602 print as whatever primitive type they are.
1604 @xref{Read and Print}, for a description of functions
1605 related to streams, including parsing and printing functions.
1608 @subsection Keymap Type
1610 A @dfn{keymap} maps keys typed by the user to commands. This mapping
1611 controls how the user's command input is executed. A keymap is actually
1612 a list whose @sc{car} is the symbol @code{keymap}.
1614 @xref{Keymaps}, for information about creating keymaps, handling prefix
1615 keys, local as well as global keymaps, and changing key bindings.
1618 @subsection Overlay Type
1620 An @dfn{overlay} specifies properties that apply to a part of a
1621 buffer. Each overlay applies to a specified range of the buffer, and
1622 contains a property list (a list whose elements are alternating property
1623 names and values). Overlay properties are used to present parts of the
1624 buffer temporarily in a different display style. Overlays have no read
1625 syntax, and print in hash notation, giving the buffer name and range of
1628 @xref{Overlays}, for how to create and use overlays.
1631 @subsection Font Type
1633 A @dfn{font} specifies how to display text on a graphical terminal.
1634 There are actually three separate font types---@dfn{font objects},
1635 @dfn{font specs}, and @dfn{font entities}---each of which has slightly
1636 different properties. None of them have a read syntax; their print
1637 syntax looks like @samp{#<font-object>}, @samp{#<font-spec>}, and
1638 @samp{#<font-entity>} respectively. @xref{Low-Level Font}, for a
1639 description of these Lisp objects.
1641 @node Circular Objects
1642 @section Read Syntax for Circular Objects
1643 @cindex circular structure, read syntax
1644 @cindex shared structure, read syntax
1645 @cindex @samp{#@var{n}=} read syntax
1646 @cindex @samp{#@var{n}#} read syntax
1648 To represent shared or circular structures within a complex of Lisp
1649 objects, you can use the reader constructs @samp{#@var{n}=} and
1652 Use @code{#@var{n}=} before an object to label it for later reference;
1653 subsequently, you can use @code{#@var{n}#} to refer the same object in
1654 another place. Here, @var{n} is some integer. For example, here is how
1655 to make a list in which the first element recurs as the third element:
1662 This differs from ordinary syntax such as this
1669 which would result in a list whose first and third elements
1670 look alike but are not the same Lisp object. This shows the difference:
1674 (setq x '(#1=(a) b #1#)))
1675 (eq (nth 0 x) (nth 2 x))
1677 (setq x '((a) b (a)))
1678 (eq (nth 0 x) (nth 2 x))
1682 You can also use the same syntax to make a circular structure, which
1683 appears as an ``element'' within itself. Here is an example:
1690 This makes a list whose second element is the list itself.
1691 Here's how you can see that it really works:
1695 (setq x '#1=(a #1#)))
1700 The Lisp printer can produce this syntax to record circular and shared
1701 structure in a Lisp object, if you bind the variable @code{print-circle}
1702 to a non-@code{nil} value. @xref{Output Variables}.
1704 @node Type Predicates
1705 @section Type Predicates
1706 @cindex type checking
1707 @kindex wrong-type-argument
1709 The Emacs Lisp interpreter itself does not perform type checking on
1710 the actual arguments passed to functions when they are called. It could
1711 not do so, since function arguments in Lisp do not have declared data
1712 types, as they do in other programming languages. It is therefore up to
1713 the individual function to test whether each actual argument belongs to
1714 a type that the function can use.
1716 All built-in functions do check the types of their actual arguments
1717 when appropriate, and signal a @code{wrong-type-argument} error if an
1718 argument is of the wrong type. For example, here is what happens if you
1719 pass an argument to @code{+} that it cannot handle:
1724 @error{} Wrong type argument: number-or-marker-p, a
1728 @cindex type predicates
1729 @cindex testing types
1730 If you want your program to handle different types differently, you
1731 must do explicit type checking. The most common way to check the type
1732 of an object is to call a @dfn{type predicate} function. Emacs has a
1733 type predicate for each type, as well as some predicates for
1734 combinations of types.
1736 A type predicate function takes one argument; it returns @code{t} if
1737 the argument belongs to the appropriate type, and @code{nil} otherwise.
1738 Following a general Lisp convention for predicate functions, most type
1739 predicates' names end with @samp{p}.
1741 Here is an example which uses the predicates @code{listp} to check for
1742 a list and @code{symbolp} to check for a symbol.
1747 ;; If X is a symbol, put it on LIST.
1748 (setq list (cons x list)))
1750 ;; If X is a list, add its elements to LIST.
1751 (setq list (append x list)))
1753 ;; We handle only symbols and lists.
1754 (error "Invalid argument %s in add-on" x))))
1757 Here is a table of predefined type predicates, in alphabetical order,
1758 with references to further information.
1762 @xref{List-related Predicates, atom}.
1765 @xref{Array Functions, arrayp}.
1768 @xref{Bool-Vectors, bool-vector-p}.
1771 @xref{Buffer Basics, bufferp}.
1773 @item byte-code-function-p
1774 @xref{Byte-Code Type, byte-code-function-p}.
1777 @xref{Case Tables, case-table-p}.
1779 @item char-or-string-p
1780 @xref{Predicates for Strings, char-or-string-p}.
1783 @xref{Char-Tables, char-table-p}.
1786 @xref{Interactive Call, commandp}.
1789 @xref{List-related Predicates, consp}.
1791 @item display-table-p
1792 @xref{Display Tables, display-table-p}.
1795 @xref{Predicates on Numbers, floatp}.
1798 @xref{Low-Level Font}.
1800 @item frame-configuration-p
1801 @xref{Frame Configurations, frame-configuration-p}.
1804 @xref{Deleting Frames, frame-live-p}.
1807 @xref{Frames, framep}.
1810 @xref{Functions, functionp}.
1813 @xref{Other Hash, hash-table-p}.
1815 @item integer-or-marker-p
1816 @xref{Predicates on Markers, integer-or-marker-p}.
1819 @xref{Predicates on Numbers, integerp}.
1822 @xref{Creating Keymaps, keymapp}.
1825 @xref{Constant Variables}.
1828 @xref{List-related Predicates, listp}.
1831 @xref{Predicates on Markers, markerp}.
1834 @xref{Predicates on Numbers, wholenump}.
1837 @xref{List-related Predicates, nlistp}.
1840 @xref{Predicates on Numbers, numberp}.
1842 @item number-or-marker-p
1843 @xref{Predicates on Markers, number-or-marker-p}.
1846 @xref{Overlays, overlayp}.
1849 @xref{Processes, processp}.
1852 @xref{Sequence Functions, sequencep}.
1855 @xref{Predicates for Strings, stringp}.
1858 @xref{Function Cells, subrp}.
1861 @xref{Symbols, symbolp}.
1863 @item syntax-table-p
1864 @xref{Syntax Tables, syntax-table-p}.
1866 @item user-variable-p
1867 @xref{Defining Variables, user-variable-p}.
1870 @xref{Vectors, vectorp}.
1872 @item window-configuration-p
1873 @xref{Window Configurations, window-configuration-p}.
1876 @xref{Deleting Windows, window-live-p}.
1879 @xref{Basic Windows, windowp}.
1882 @xref{nil and t, booleanp}.
1884 @item string-or-null-p
1885 @xref{Predicates for Strings, string-or-null-p}.
1888 The most general way to check the type of an object is to call the
1889 function @code{type-of}. Recall that each object belongs to one and
1890 only one primitive type; @code{type-of} tells you which one (@pxref{Lisp
1891 Data Types}). But @code{type-of} knows nothing about non-primitive
1892 types. In most cases, it is more convenient to use type predicates than
1895 @defun type-of object
1896 This function returns a symbol naming the primitive type of
1897 @var{object}. The value is one of the symbols @code{bool-vector},
1898 @code{buffer}, @code{char-table}, @code{compiled-function},
1899 @code{cons}, @code{float}, @code{font-entity}, @code{font-object},
1900 @code{font-spec}, @code{frame}, @code{hash-table}, @code{integer},
1901 @code{marker}, @code{overlay}, @code{process}, @code{string},
1902 @code{subr}, @code{symbol}, @code{vector}, @code{window}, or
1903 @code{window-configuration}.
1911 (type-of '()) ; @r{@code{()} is @code{nil}.}
1919 @node Equality Predicates
1920 @section Equality Predicates
1923 Here we describe functions that test for equality between any two
1924 objects. Other functions test equality of contents between objects of specific
1925 types, e.g., strings. For these predicates, see the appropriate chapter
1926 describing the data type.
1928 @defun eq object1 object2
1929 This function returns @code{t} if @var{object1} and @var{object2} are
1930 the same object, @code{nil} otherwise.
1932 @code{eq} returns @code{t} if @var{object1} and @var{object2} are
1933 integers with the same value. Also, since symbol names are normally
1934 unique, if the arguments are symbols with the same name, they are
1935 @code{eq}. For other types (e.g., lists, vectors, strings), two
1936 arguments with the same contents or elements are not necessarily
1937 @code{eq} to each other: they are @code{eq} only if they are the same
1938 object, meaning that a change in the contents of one will be reflected
1939 by the same change in the contents of the other.
1960 ;; @r{This exception occurs because Emacs Lisp}
1961 ;; @r{makes just one multibyte empty string, to save space.}
1965 (eq '(1 (2 (3))) '(1 (2 (3))))
1970 (setq foo '(1 (2 (3))))
1971 @result{} (1 (2 (3)))
1974 (eq foo '(1 (2 (3))))
1979 (eq [(1 2) 3] [(1 2) 3])
1984 (eq (point-marker) (point-marker))
1989 The @code{make-symbol} function returns an uninterned symbol, distinct
1990 from the symbol that is used if you write the name in a Lisp expression.
1991 Distinct symbols with the same name are not @code{eq}. @xref{Creating
1996 (eq (make-symbol "foo") 'foo)
2002 @defun equal object1 object2
2003 This function returns @code{t} if @var{object1} and @var{object2} have
2004 equal components, @code{nil} otherwise. Whereas @code{eq} tests if its
2005 arguments are the same object, @code{equal} looks inside nonidentical
2006 arguments to see if their elements or contents are the same. So, if two
2007 objects are @code{eq}, they are @code{equal}, but the converse is not
2022 (equal "asdf" "asdf")
2031 (equal '(1 (2 (3))) '(1 (2 (3))))
2035 (eq '(1 (2 (3))) '(1 (2 (3))))
2040 (equal [(1 2) 3] [(1 2) 3])
2044 (eq [(1 2) 3] [(1 2) 3])
2049 (equal (point-marker) (point-marker))
2054 (eq (point-marker) (point-marker))
2059 Comparison of strings is case-sensitive, but does not take account of
2060 text properties---it compares only the characters in the strings. Use
2061 @code{equal-including-properties} to also compare text properties. For
2062 technical reasons, a unibyte string and a multibyte string are
2063 @code{equal} if and only if they contain the same sequence of
2064 character codes and all these codes are either in the range 0 through
2065 127 (@acronym{ASCII}) or 160 through 255 (@code{eight-bit-graphic}).
2066 (@pxref{Text Representations}).
2070 (equal "asdf" "ASDF")
2075 However, two distinct buffers are never considered @code{equal}, even if
2076 their textual contents are the same.
2079 The test for equality is implemented recursively; for example, given
2080 two cons cells @var{x} and @var{y}, @code{(equal @var{x} @var{y})}
2081 returns @code{t} if and only if both the expressions below return
2085 (equal (car @var{x}) (car @var{y}))
2086 (equal (cdr @var{x}) (cdr @var{y}))
2089 Because of this recursive method, circular lists may therefore cause
2090 infinite recursion (leading to an error).
2092 @defun equal-including-properties object1 object2
2093 This function behaves like @code{equal} in all cases but also requires
2094 that for two strings to be equal, they have the same text properties.
2098 (equal "asdf" (propertize "asdf" '(asdf t)))
2102 (equal-including-properties "asdf"
2103 (propertize "asdf" '(asdf t)))