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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. | |
4 | @c See the file elisp.texi for copying conditions. | |
5 | @setfilename ../info/objects | |
6 | @node Types of Lisp Object, Numbers, Introduction, Top | |
7 | @chapter Lisp Data Types | |
8 | @cindex object | |
9 | @cindex Lisp object | |
10 | @cindex type | |
11 | @cindex data type | |
12 | ||
13 | A Lisp @dfn{object} is a piece of data used and manipulated by Lisp | |
14 | programs. For our purposes, a @dfn{type} or @dfn{data type} is a set of | |
15 | possible objects. | |
16 | ||
17 | Every object belongs to at least one type. Objects of the same type | |
18 | have similar structures and may usually be used in the same contexts. | |
19 | Types can overlap, and objects can belong to two or more types. | |
20 | Consequently, we can ask whether an object belongs to a particular type, | |
21 | but not for ``the'' type of an object. | |
22 | ||
23 | @cindex primitive type | |
24 | A few fundamental object types are built into Emacs. These, from | |
25 | which all other types are constructed, are called @dfn{primitive | |
26 | types}. Each object belongs to one and only one primitive type. These | |
27 | types include @dfn{integer}, @dfn{float}, @dfn{cons}, @dfn{symbol}, | |
28 | @dfn{string}, @dfn{vector}, @dfn{subr}, @dfn{byte-code function}, and | |
29 | several special types, such as @dfn{buffer}, that are related to | |
30 | editing. (@xref{Editing Types}.) | |
31 | ||
32 | Each primitive type has a corresponding Lisp function that checks | |
33 | whether an object is a member of that type. | |
34 | ||
35 | Note that Lisp is unlike many other languages in that Lisp objects are | |
36 | @dfn{self-typing}: the primitive type of the object is implicit in the | |
37 | object itself. For example, if an object is a vector, nothing can treat | |
38 | it as a number; Lisp knows it is a vector, not a number. | |
39 | ||
40 | In most languages, the programmer must declare the data type of each | |
41 | variable, and the type is known by the compiler but not represented in | |
42 | the data. Such type declarations do not exist in Emacs Lisp. A Lisp | |
43 | variable can have any type of value, and remembers the type of any value | |
44 | you store in it. | |
45 | ||
46 | This chapter describes the purpose, printed representation, and read | |
47 | syntax of each of the standard types in GNU Emacs Lisp. Details on how | |
48 | to use these types can be found in later chapters. | |
49 | ||
50 | @menu | |
51 | * Printed Representation:: How Lisp objects are represented as text. | |
52 | * Comments:: Comments and their formatting conventions. | |
53 | * Programming Types:: Types found in all Lisp systems. | |
54 | * Editing Types:: Types specific to Emacs. | |
55 | * Type Predicates:: Tests related to types. | |
56 | * Equality Predicates:: Tests of equality between any two objects. | |
57 | @end menu | |
58 | ||
59 | @node Printed Representation | |
60 | @comment node-name, next, previous, up | |
61 | @section Printed Representation and Read Syntax | |
62 | @cindex printed representation | |
63 | @cindex read syntax | |
64 | ||
65 | The @dfn{printed representation} of an object is the format of the | |
66 | output generated by the Lisp printer (the function @code{prin1}) for | |
67 | that object. The @dfn{read syntax} of an object is the format of the | |
68 | input accepted by the Lisp reader (the function @code{read}) for that | |
69 | object. Most objects have more than one possible read syntax. Some | |
70 | types of object have no read syntax; except for these cases, the printed | |
71 | representation of an object is also a read syntax for it. | |
72 | ||
73 | In other languages, an expression is text; it has no other form. In | |
74 | Lisp, an expression is primarily a Lisp object and only secondarily the | |
75 | text that is the object's read syntax. Often there is no need to | |
76 | emphasize this distinction, but you must keep it in the back of your | |
77 | mind, or you will occasionally be very confused. | |
78 | ||
79 | @cindex hash notation | |
80 | Every type has a printed representation. Some types have no read | |
81 | syntax, since it may not make sense to enter objects of these types | |
82 | directly in a Lisp program. For example, the buffer type does not have | |
83 | a read syntax. Objects of these types are printed in @dfn{hash | |
84 | notation}: the characters @samp{#<} followed by a descriptive string | |
85 | (typically the type name followed by the name of the object), and closed | |
86 | with a matching @samp{>}. Hash notation cannot be read at all, so the | |
87 | Lisp reader signals the error @code{invalid-read-syntax} whenever it | |
88 | encounters @samp{#<}. | |
89 | @kindex invalid-read-syntax | |
90 | ||
91 | @example | |
92 | (current-buffer) | |
93 | @result{} #<buffer objects.texi> | |
94 | @end example | |
95 | ||
96 | When you evaluate an expression interactively, the Lisp interpreter | |
97 | first reads the textual representation of it, producing a Lisp object, | |
98 | and then evaluates that object (@pxref{Evaluation}). However, | |
99 | evaluation and reading are separate activities. Reading returns the | |
100 | Lisp object represented by the text that is read; the object may or may | |
101 | not be evaluated later. @xref{Input Functions}, for a description of | |
102 | @code{read}, the basic function for reading objects. | |
103 | ||
104 | @node Comments | |
105 | @comment node-name, next, previous, up | |
106 | @section Comments | |
107 | @cindex comments | |
108 | @cindex @samp{;} in comment | |
109 | ||
110 | A @dfn{comment} is text that is written in a program only for the sake | |
111 | of humans that read the program, and that has no effect on the meaning | |
112 | of the program. In Lisp, a semicolon (@samp{;}) starts a comment if it | |
113 | is not within a string or character constant. The comment continues to | |
114 | the end of line. The Lisp reader discards comments; they do not become | |
115 | part of the Lisp objects which represent the program within the Lisp | |
116 | system. | |
117 | ||
118 | @xref{Comment Tips}, for conventions for formatting comments. | |
119 | ||
120 | @node Programming Types | |
121 | @section Programming Types | |
122 | @cindex programming types | |
123 | ||
124 | There are two general categories of types in Emacs Lisp: those having | |
125 | to do with Lisp programming, and those having to do with editing. The | |
126 | former exist in many Lisp implementations, in one form or another. The | |
127 | latter are unique to Emacs Lisp. | |
128 | ||
129 | @menu | |
130 | * Integer Type:: Numbers without fractional parts. | |
131 | * Floating Point Type:: Numbers with fractional parts and with a large range. | |
132 | * Character Type:: The representation of letters, numbers and | |
133 | control characters. | |
134 | * Sequence Type:: Both lists and arrays are classified as sequences. | |
135 | * List Type:: Lists gave Lisp its name (not to mention reputation). | |
136 | * Array Type:: Arrays include strings and vectors. | |
137 | * String Type:: An (efficient) array of characters. | |
138 | * Vector Type:: One-dimensional arrays. | |
139 | * Symbol Type:: A multi-use object that refers to a function, | |
140 | variable, property list, or itself. | |
141 | * Lisp Function Type:: A piece of executable code you can call from elsewhere. | |
142 | * Lisp Macro Type:: A method of expanding an expression into another | |
143 | expression, more fundamental but less pretty. | |
144 | * Primitive Function Type:: A function written in C, callable from Lisp. | |
145 | * Byte-Code Type:: A function written in Lisp, then compiled. | |
146 | * Autoload Type:: A type used for automatically loading seldom-used | |
147 | functions. | |
148 | @end menu | |
149 | ||
150 | @node Integer Type | |
151 | @subsection Integer Type | |
152 | ||
153 | Integers were the only kind of number in Emacs version 18. The range | |
154 | of values for integers is @minus{}8388608 to 8388607 (24 bits; i.e., | |
155 | @ifinfo | |
156 | -2**23 | |
157 | @end ifinfo | |
158 | @tex | |
159 | $-2^{23}$ | |
160 | @end tex | |
161 | to | |
162 | @ifinfo | |
163 | 2**23 - 1) | |
164 | @end ifinfo | |
165 | @tex | |
166 | $2^{23}-1$) | |
167 | @end tex | |
168 | on most machines, but is 25 or 26 bits on some systems. It is important | |
169 | to note that the Emacs Lisp arithmetic functions do not check for | |
170 | overflow. Thus @code{(1+ 8388607)} is @minus{}8388608 on 24-bit | |
171 | implementations.@refill | |
172 | ||
173 | The read syntax for numbers is a sequence of (base ten) digits with an | |
174 | optional sign at the beginning and an optional period at the end. The | |
175 | printed representation produced by the Lisp interpreter never has a | |
176 | leading @samp{+} or a final @samp{.}. | |
177 | ||
178 | @example | |
179 | @group | |
180 | -1 ; @r{The integer -1.} | |
181 | 1 ; @r{The integer 1.} | |
182 | 1. ; @r{Also The integer 1.} | |
183 | +1 ; @r{Also the integer 1.} | |
184 | 16777217 ; @r{Also the integer 1!} | |
185 | ; @r{ (on a 24-bit or 25-bit implementation)} | |
186 | @end group | |
187 | @end example | |
188 | ||
189 | @xref{Numbers}, for more information. | |
190 | ||
191 | @node Floating Point Type | |
192 | @subsection Floating Point Type | |
193 | ||
194 | Emacs version 19 supports floating point numbers (though there is a | |
195 | compilation option to disable them). The precise range of floating | |
196 | point numbers is machine-specific. | |
197 | ||
198 | The printed representation for floating point numbers requires either | |
199 | a decimal point (with at least one digit following), an exponent, or | |
200 | both. For example, @samp{1500.0}, @samp{15e2}, @samp{15.0e2}, | |
201 | @samp{1.5e3}, and @samp{.15e4} are five ways of writing a floating point | |
202 | number whose value is 1500. They are all equivalent. | |
203 | ||
204 | @xref{Numbers}, for more information. | |
205 | ||
206 | @node Character Type | |
207 | @subsection Character Type | |
208 | @cindex @sc{ASCII} character codes | |
209 | ||
210 | A @dfn{character} in Emacs Lisp is nothing more than an integer. In | |
211 | other words, characters are represented by their character codes. For | |
212 | example, the character @kbd{A} is represented as the @w{integer 65}. | |
213 | ||
214 | Individual characters are not often used in programs. It is far more | |
215 | common to work with @emph{strings}, which are sequences composed of | |
216 | characters. @xref{String Type}. | |
217 | ||
218 | Characters in strings, buffers, and files are currently limited to the | |
219 | range of 0 to 255---eight bits. If you store a larger integer into a | |
220 | string, buffer or file, it is truncated to that range. Characters that | |
221 | represent keyboard input have a much wider range. | |
222 | ||
223 | @cindex read syntax for characters | |
224 | @cindex printed representation for characters | |
225 | @cindex syntax for characters | |
226 | Since characters are really integers, the printed representation of a | |
227 | character is a decimal number. This is also a possible read syntax for | |
228 | a character, but writing characters that way in Lisp programs is a very | |
229 | bad idea. You should @emph{always} use the special read syntax formats | |
230 | that Emacs Lisp provides for characters. These syntax formats start | |
231 | with a question mark. | |
232 | ||
233 | The usual read syntax for alphanumeric characters is a question mark | |
234 | followed by the character; thus, @samp{?A} for the character | |
235 | @kbd{A}, @samp{?B} for the character @kbd{B}, and @samp{?a} for the | |
236 | character @kbd{a}. | |
237 | ||
238 | For example: | |
239 | ||
240 | @example | |
241 | ?Q @result{} 81 ?q @result{} 113 | |
242 | @end example | |
243 | ||
244 | You can use the same syntax for punctuation characters, but it is | |
245 | often a good idea to add a @samp{\} to prevent Lisp mode from getting | |
246 | confused. For example, @samp{?\ } is the way to write the space | |
247 | character. If the character is @samp{\}, you @emph{must} use a second | |
248 | @samp{\} to quote it: @samp{?\\}. | |
249 | ||
250 | @cindex whitespace | |
251 | @cindex bell character | |
252 | @cindex @samp{\a} | |
253 | @cindex backspace | |
254 | @cindex @samp{\b} | |
255 | @cindex tab | |
256 | @cindex @samp{\t} | |
257 | @cindex vertical tab | |
258 | @cindex @samp{\v} | |
259 | @cindex formfeed | |
260 | @cindex @samp{\f} | |
261 | @cindex newline | |
262 | @cindex @samp{\n} | |
263 | @cindex return | |
264 | @cindex @samp{\r} | |
265 | @cindex escape | |
266 | @cindex @samp{\e} | |
267 | You can express the characters Control-g, backspace, tab, newline, | |
268 | vertical tab, formfeed, return, and escape as @samp{?\a}, @samp{?\b}, | |
269 | @samp{?\t}, @samp{?\n}, @samp{?\v}, @samp{?\f}, @samp{?\r}, @samp{?\e}, | |
270 | respectively. Those values are 7, 8, 9, 10, 11, 12, 13, and 27 in | |
271 | decimal. Thus, | |
272 | ||
273 | @example | |
274 | ?\a @result{} 7 ; @r{@kbd{C-g}} | |
275 | ?\b @result{} 8 ; @r{backspace, @key{BS}, @kbd{C-h}} | |
276 | ?\t @result{} 9 ; @r{tab, @key{TAB}, @kbd{C-i}} | |
277 | ?\n @result{} 10 ; @r{newline, @key{LFD}, @kbd{C-j}} | |
278 | ?\v @result{} 11 ; @r{vertical tab, @kbd{C-k}} | |
279 | ?\f @result{} 12 ; @r{formfeed character, @kbd{C-l}} | |
280 | ?\r @result{} 13 ; @r{carriage return, @key{RET}, @kbd{C-m}} | |
281 | ?\e @result{} 27 ; @r{escape character, @key{ESC}, @kbd{C-[}} | |
282 | ?\\ @result{} 92 ; @r{backslash character, @kbd{\}} | |
283 | @end example | |
284 | ||
285 | @cindex escape sequence | |
286 | These sequences which start with backslash are also known as | |
287 | @dfn{escape sequences}, because backslash plays the role of an escape | |
288 | character; this usage has nothing to do with the character @key{ESC}. | |
289 | ||
290 | @cindex control characters | |
291 | Control characters may be represented using yet another read syntax. | |
292 | This consists of a question mark followed by a backslash, caret, and the | |
293 | corresponding non-control character, in either upper or lower case. For | |
294 | example, both @samp{?\^I} and @samp{?\^i} are valid read syntax for the | |
295 | character @kbd{C-i}, the character whose value is 9. | |
296 | ||
297 | Instead of the @samp{^}, you can use @samp{C-}; thus, @samp{?\C-i} is | |
298 | equivalent to @samp{?\^I} and to @samp{?\^i}: | |
299 | ||
300 | @example | |
301 | ?\^I @result{} 9 ?\C-I @result{} 9 | |
302 | @end example | |
303 | ||
304 | For use in strings and buffers, you are limited to the control | |
305 | characters that exist in @sc{ASCII}, but for keyboard input purposes, | |
306 | you can turn any character into a control character with @samp{C-}. The | |
307 | character codes for these non-@sc{ASCII} control characters include the | |
308 | 2**22 bit as well as the code for the corresponding non-control | |
309 | character. Ordinary terminals have no way of generating non-@sc{ASCII} | |
310 | control characters, but you can generate them straightforwardly using an | |
311 | X terminal. | |
312 | ||
313 | You can think of the @key{DEL} character as @kbd{Control-?}: | |
314 | ||
315 | @example | |
316 | ?\^? @result{} 127 ?\C-? @result{} 127 | |
317 | @end example | |
318 | ||
319 | For representing control characters to be found in files or strings, | |
320 | we recommend the @samp{^} syntax; for control characters in keyboard | |
321 | input, we prefer the @samp{C-} syntax. This does not affect the meaning | |
322 | of the program, but may guide the understanding of people who read it. | |
323 | ||
324 | @cindex meta characters | |
325 | A @dfn{meta character} is a character typed with the @key{META} | |
326 | modifier key. The integer that represents such a character has the | |
327 | 2**23 bit set (which on most machines makes it a negative number). We | |
328 | use high bits for this and other modifiers to make possible a wide range | |
329 | of basic character codes. | |
330 | ||
331 | In a string, the 2**7 bit indicates a meta character, so the meta | |
332 | characters that can fit in a string have codes in the range from 128 to | |
333 | 255, and are the meta versions of the ordinary @sc{ASCII} characters. | |
334 | (In Emacs versions 18 and older, this convention was used for characters | |
335 | outside of strings as well.) | |
336 | ||
337 | The read syntax for meta characters uses @samp{\M-}. For example, | |
338 | @samp{?\M-A} stands for @kbd{M-A}. You can use @samp{\M-} together with | |
339 | octal codes, @samp{\C-}, or any other syntax for a character. Thus, you | |
340 | can write @kbd{M-A} as @samp{?\M-A}, or as @samp{?\M-\101}. Likewise, | |
341 | you can write @kbd{C-M-b} as @samp{?\M-\C-b}, @samp{?\C-\M-b}, or | |
342 | @samp{?\M-\002}. | |
343 | ||
344 | The case of an ordinary letter is indicated by its character code as | |
345 | part of @sc{ASCII}, but @sc{ASCII} has no way to represent whether a | |
346 | control character is upper case or lower case. Emacs uses the 2**21 bit | |
347 | to indicate that the shift key was used for typing a control character. | |
348 | This distinction is possible only when you use X terminals or other | |
349 | special terminals; ordinary terminals do not indicate the distinction to | |
350 | the computer in any way. | |
351 | ||
352 | @cindex hyper characters | |
353 | @cindex super characters | |
354 | @cindex alt characters | |
355 | The X Window System defines three other modifier bits that can be set | |
356 | in a character: @dfn{hyper}, @dfn{super} and @dfn{alt}. The syntaxes | |
357 | for these bits are @samp{\H-}, @samp{\s-} and @samp{\A-}. Thus, | |
358 | @samp{?\H-\M-\A-x} represents @kbd{Alt-Hyper-Meta-x}. Numerically, the | |
359 | bit values are 2**18 for alt, 2**19 for super and 2**20 for hyper. | |
360 | ||
361 | @cindex @samp{?} in character constant | |
362 | @cindex question mark in character constant | |
363 | @cindex @samp{\} in character constant | |
364 | @cindex backslash in character constant | |
365 | @cindex octal character code | |
366 | Finally, the most general read syntax consists of a question mark | |
367 | followed by a backslash and the character code in octal (up to three | |
368 | octal digits); thus, @samp{?\101} for the character @kbd{A}, | |
369 | @samp{?\001} for the character @kbd{C-a}, and @code{?\002} for the | |
370 | character @kbd{C-b}. Although this syntax can represent any @sc{ASCII} | |
371 | character, it is preferred only when the precise octal value is more | |
372 | important than the @sc{ASCII} representation. | |
373 | ||
374 | @example | |
375 | @group | |
376 | ?\012 @result{} 10 ?\n @result{} 10 ?\C-j @result{} 10 | |
377 | ?\101 @result{} 65 ?A @result{} 65 | |
378 | @end group | |
379 | @end example | |
380 | ||
381 | A backslash is allowed, and harmless, preceding any character without | |
382 | a special escape meaning; thus, @samp{?\+} is equivalent to @samp{?+}. | |
383 | There is no reason to add a backslash before most characters. However, | |
384 | you should add a backslash before any of the characters | |
385 | @samp{()\|;'`"#.,} to avoid confusing the Emacs commands for editing | |
386 | Lisp code. Also add a backslash before whitespace characters such as | |
387 | space, tab, newline and formfeed. However, it is cleaner to use one of | |
388 | the easily readable escape sequences, such as @samp{\t}, instead of an | |
389 | actual whitespace character such as a tab. | |
390 | ||
391 | @node Sequence Type | |
392 | @subsection Sequence Types | |
393 | ||
394 | A @dfn{sequence} is a Lisp object that represents an ordered set of | |
395 | elements. There are two kinds of sequence in Emacs Lisp, lists and | |
396 | arrays. Thus, an object of type list or of type array is also | |
397 | considered a sequence. | |
398 | ||
399 | Arrays are further subdivided into strings and vectors. Vectors can | |
400 | hold elements of any type, but string elements must be characters in the | |
401 | range from 0 to 255. However, the characters in a string can have text | |
402 | properties; vectors do not support text properties even when their | |
403 | elements happen to be characters. | |
404 | ||
405 | Lists, strings and vectors are different, but they have important | |
406 | similarities. For example, all have a length @var{l}, and all have | |
407 | elements which can be indexed from zero to @var{l} minus one. Also, | |
408 | several functions, called sequence functions, accept any kind of | |
409 | sequence. For example, the function @code{elt} can be used to extract | |
410 | an element of a sequence, given its index. @xref{Sequences Arrays | |
411 | Vectors}. | |
412 | ||
413 | It is impossible to read the same sequence twice, since sequences are | |
414 | always created anew upon reading. If you read the read syntax for a | |
415 | sequence twice, you get two sequences with equal contents. There is one | |
416 | exception: the empty list @code{()} always stands for the same object, | |
417 | @code{nil}. | |
418 | ||
419 | @node List Type | |
420 | @subsection List Type | |
421 | @cindex address field of register | |
422 | @cindex decrement field of register | |
423 | ||
424 | A @dfn{list} is a series of cons cells, linked together. A @dfn{cons | |
425 | cell} is an object comprising two pointers named the @sc{car} and the | |
426 | @sc{cdr}. Each of them can point to any Lisp object, but when the cons | |
427 | cell is part of a list, the @sc{cdr} points either to another cons cell | |
428 | or to the empty list. @xref{Lists}, for functions that work on lists. | |
429 | ||
430 | The names @sc{car} and @sc{cdr} have only historical meaning now. The | |
431 | original Lisp implementation ran on an @w{IBM 704} computer which | |
432 | divided words into two parts, called the ``address'' part and the | |
433 | ``decrement''; @sc{car} was an instruction to extract the contents of | |
434 | the address part of a register, and @sc{cdr} an instruction to extract | |
435 | the contents of the decrement. By contrast, ``cons cells'' are named | |
436 | for the function @code{cons} that creates them, which in turn is named | |
437 | for its purpose, the construction of cells. | |
438 | ||
439 | @cindex atom | |
440 | Because cons cells are so central to Lisp, we also have a word for | |
441 | ``an object which is not a cons cell''. These objects are called | |
442 | @dfn{atoms}. | |
443 | ||
444 | @cindex parenthesis | |
445 | The read syntax and printed representation for lists are identical, and | |
446 | consist of a left parenthesis, an arbitrary number of elements, and a | |
447 | right parenthesis. | |
448 | ||
449 | Upon reading, each object inside the parentheses becomes an element | |
450 | of the list. That is, a cons cell is made for each element. The | |
451 | @sc{car} of the cons cell points to the element, and its @sc{cdr} points | |
452 | to the next cons cell which holds the next element in the list. The | |
453 | @sc{cdr} of the last cons cell is set to point to @code{nil}. | |
454 | ||
455 | @cindex box diagrams, for lists | |
456 | @cindex diagrams, boxed, for lists | |
457 | A list can be illustrated by a diagram in which the cons cells are | |
458 | shown as pairs of boxes. (The Lisp reader cannot read such an | |
459 | illustration; unlike the textual notation, which can be understood both | |
460 | humans and computers, the box illustrations can only be understood by | |
461 | humans.) The following represents the three-element list @code{(rose | |
462 | violet buttercup)}: | |
463 | ||
464 | @example | |
465 | @group | |
466 | ___ ___ ___ ___ ___ ___ | |
467 | |___|___|--> |___|___|--> |___|___|--> nil | |
468 | | | | | |
469 | | | | | |
470 | --> rose --> violet --> buttercup | |
471 | @end group | |
472 | @end example | |
473 | ||
474 | In this diagram, each box represents a slot that can refer to any Lisp | |
475 | object. Each pair of boxes represents a cons cell. Each arrow is a | |
476 | reference to a Lisp object, either an atom or another cons cell. | |
477 | ||
478 | In this example, the first box, the @sc{car} of the first cons cell, | |
479 | refers to or ``contains'' @code{rose} (a symbol). The second box, the | |
480 | @sc{cdr} of the first cons cell, refers to the next pair of boxes, the | |
481 | second cons cell. The @sc{car} of the second cons cell refers to | |
482 | @code{violet} and the @sc{cdr} refers to the third cons cell. The | |
483 | @sc{cdr} of the third (and last) cons cell refers to @code{nil}. | |
484 | ||
485 | Here is another diagram of the same list, @code{(rose violet | |
486 | buttercup)}, sketched in a different manner: | |
487 | ||
488 | @smallexample | |
489 | @group | |
490 | --------------- ---------------- ------------------- | |
491 | | car | cdr | | car | cdr | | car | cdr | | |
492 | | rose | o-------->| violet | o-------->| buttercup | nil | | |
493 | | | | | | | | | | | |
494 | --------------- ---------------- ------------------- | |
495 | @end group | |
496 | @end smallexample | |
497 | ||
498 | @cindex @samp{(@dots{})} in lists | |
499 | @cindex @code{nil} in lists | |
500 | @cindex empty list | |
501 | A list with no elements in it is the @dfn{empty list}; it is identical | |
502 | to the symbol @code{nil}. In other words, @code{nil} is both a symbol | |
503 | and a list. | |
504 | ||
505 | Here are examples of lists written in Lisp syntax: | |
506 | ||
507 | @example | |
508 | (A 2 "A") ; @r{A list of three elements.} | |
509 | () ; @r{A list of no elements (the empty list).} | |
510 | nil ; @r{A list of no elements (the empty list).} | |
511 | ("A ()") ; @r{A list of one element: the string @code{"A ()"}.} | |
512 | (A ()) ; @r{A list of two elements: @code{A} and the empty list.} | |
513 | (A nil) ; @r{Equivalent to the previous.} | |
514 | ((A B C)) ; @r{A list of one element} | |
515 | ; @r{(which is a list of three elements).} | |
516 | @end example | |
517 | ||
518 | Here is the list @code{(A ())}, or equivalently @code{(A nil)}, | |
519 | depicted with boxes and arrows: | |
520 | ||
521 | @example | |
522 | @group | |
523 | ___ ___ ___ ___ | |
524 | |___|___|--> |___|___|--> nil | |
525 | | | | |
526 | | | | |
527 | --> A --> nil | |
528 | @end group | |
529 | @end example | |
530 | ||
531 | @menu | |
532 | * Dotted Pair Notation:: An alternative syntax for lists. | |
533 | * Association List Type:: A specially constructed list. | |
534 | @end menu | |
535 | ||
536 | @node Dotted Pair Notation | |
537 | @comment node-name, next, previous, up | |
538 | @subsubsection Dotted Pair Notation | |
539 | @cindex dotted pair notation | |
540 | @cindex @samp{.} in lists | |
541 | ||
542 | @dfn{Dotted pair notation} is an alternative syntax for cons cells | |
543 | that represents the @sc{car} and @sc{cdr} explicitly. In this syntax, | |
544 | @code{(@var{a} .@: @var{b})} stands for a cons cell whose @sc{car} is | |
545 | the object @var{a}, and whose @sc{cdr} is the object @var{b}. Dotted | |
546 | pair notation is therefore more general than list syntax. In the dotted | |
547 | pair notation, the list @samp{(1 2 3)} is written as @samp{(1 . (2 . (3 | |
548 | . nil)))}. For @code{nil}-terminated lists, the two notations produce | |
549 | the same result, but list notation is usually clearer and more | |
550 | convenient when it is applicable. When printing a list, the dotted pair | |
551 | notation is only used if the @sc{cdr} of a cell is not a list. | |
552 | ||
553 | Here's how box notation can illustrate dotted pairs. This example | |
554 | shows the pair @code{(rose . violet)}: | |
555 | ||
556 | @example | |
557 | @group | |
558 | ___ ___ | |
559 | |___|___|--> violet | |
560 | | | |
561 | | | |
562 | --> rose | |
563 | @end group | |
564 | @end example | |
565 | ||
566 | Dotted pair notation can be combined with list notation to represent a | |
567 | chain of cons cells with a non-@code{nil} final @sc{cdr}. For example, | |
568 | @code{(rose violet . buttercup)} is equivalent to @code{(rose . (violet | |
569 | . buttercup))}. The object looks like this: | |
570 | ||
571 | @example | |
572 | @group | |
573 | ___ ___ ___ ___ | |
574 | |___|___|--> |___|___|--> buttercup | |
575 | | | | |
576 | | | | |
577 | --> rose --> violet | |
578 | @end group | |
579 | @end example | |
580 | ||
581 | These diagrams make it evident why @w{@code{(rose .@: violet .@: | |
582 | buttercup)}} is invalid syntax; it would require a cons cell that has | |
583 | three parts rather than two. | |
584 | ||
585 | The list @code{(rose violet)} is equivalent to @code{(rose . (violet))} | |
586 | and looks like this: | |
587 | ||
588 | @example | |
589 | @group | |
590 | ___ ___ ___ ___ | |
591 | |___|___|--> |___|___|--> nil | |
592 | | | | |
593 | | | | |
594 | --> rose --> violet | |
595 | @end group | |
596 | @end example | |
597 | ||
598 | Similarly, the three-element list @code{(rose violet buttercup)} | |
599 | is equivalent to @code{(rose . (violet . (buttercup)))}. | |
600 | @ifinfo | |
601 | It looks like this: | |
602 | ||
603 | @example | |
604 | @group | |
605 | ___ ___ ___ ___ ___ ___ | |
606 | |___|___|--> |___|___|--> |___|___|--> nil | |
607 | | | | | |
608 | | | | | |
609 | --> rose --> violet --> buttercup | |
610 | @end group | |
611 | @end example | |
612 | @end ifinfo | |
613 | ||
614 | @node Association List Type | |
615 | @comment node-name, next, previous, up | |
616 | @subsubsection Association List Type | |
617 | ||
618 | An @dfn{association list} or @dfn{alist} is a specially-constructed | |
619 | list whose elements are cons cells. In each element, the @sc{car} is | |
620 | considered a @dfn{key}, and the @sc{cdr} is considered an | |
621 | @dfn{associated value}. (In some cases, the associated value is stored | |
622 | in the @sc{car} of the @sc{cdr}.) Association lists are often used as | |
623 | stacks, since it is easy to add or remove associations at the front of | |
624 | the list. | |
625 | ||
626 | For example, | |
627 | ||
628 | @example | |
629 | (setq alist-of-colors | |
630 | '((rose . red) (lily . white) (buttercup . yellow))) | |
631 | @end example | |
632 | ||
633 | @noindent | |
634 | sets the variable @code{alist-of-colors} to an alist of three elements. In the | |
635 | first element, @code{rose} is the key and @code{red} is the value. | |
636 | ||
637 | @xref{Association Lists}, for a further explanation of alists and for | |
638 | functions that work on alists. | |
639 | ||
640 | @node Array Type | |
641 | @subsection Array Type | |
642 | ||
643 | An @dfn{array} is composed of an arbitrary number of slots for | |
644 | referring to other Lisp objects, arranged in a contiguous block of | |
645 | memory. Accessing any element of an array takes a the same amount of | |
646 | time. In contrast, accessing an element of a list requires time | |
647 | proportional to the position of the element in the list. (Elements at | |
648 | the end of a list take longer to access than elements at the beginning | |
649 | of a list.) | |
650 | ||
651 | Emacs defines two types of array, strings and vectors. A string is an | |
652 | array of characters and a vector is an array of arbitrary objects. Both | |
653 | are one-dimensional. (Most other programming languages support | |
654 | multidimensional arrays, but they are not essential; you can get the | |
655 | same effect with an array of arrays.) Each type of array has its own | |
656 | read syntax; see @ref{String Type}, and @ref{Vector Type}. | |
657 | ||
658 | An array may have any length up to the largest integer; but once | |
659 | created, it has a fixed size. The first element of an array has index | |
660 | zero, the second element has index 1, and so on. This is called | |
661 | @dfn{zero-origin} indexing. For example, an array of four elements has | |
662 | indices 0, 1, 2, @w{and 3}. | |
663 | ||
664 | The array type is contained in the sequence type and contains both the | |
665 | string type and the vector type. | |
666 | ||
667 | @node String Type | |
668 | @subsection String Type | |
669 | ||
670 | A @dfn{string} is an array of characters. Strings are used for many | |
671 | purposes in Emacs, as can be expected in a text editor; for example, as | |
672 | the names of Lisp symbols, as messages for the user, and to represent | |
673 | text extracted from buffers. Strings in Lisp are constants: evaluation | |
674 | of a string returns the same string. | |
675 | ||
676 | @cindex @samp{"} in strings | |
677 | @cindex double-quote in strings | |
678 | @cindex @samp{\} in strings | |
679 | @cindex backslash in strings | |
680 | The read syntax for strings is a double-quote, an arbitrary number of | |
681 | characters, and another double-quote, @code{"like this"}. The Lisp | |
682 | reader accepts the same formats for reading the characters of a string | |
683 | as it does for reading single characters (without the question mark that | |
684 | begins a character literal). You can enter a nonprinting character such | |
685 | as tab, @kbd{C-a} or @kbd{M-C-A} using the convenient escape sequences, | |
686 | like this: @code{"\t, \C-a, \M-\C-a"}. You can include a double-quote | |
687 | in a string by preceding it with a backslash; thus, @code{"\""} is a | |
688 | string containing just a single double-quote character. | |
689 | (@xref{Character Type}, for a description of the read syntax for | |
690 | characters.) | |
691 | ||
692 | If you use the @samp{\M-} syntax to indicate a meta character in a | |
693 | string constant, this sets the 2**7 bit of the character in the string. | |
694 | This is not the same representation that the meta modifier has in a | |
695 | character on its own (not inside a string). @xref{Character Type}. | |
696 | ||
697 | Strings cannot hold characters that have the hyper, super or alt | |
698 | modifiers; they can hold @sc{ASCII} control characters, but no others. | |
699 | They do not distinguish case in @sc{ASCII} control characters. | |
700 | ||
701 | In contrast with the C programming language, Emacs Lisp allows | |
702 | newlines in string literals. But an escaped newline---one that is | |
703 | preceded by @samp{\}---does not become part of the string; i.e., the | |
704 | Lisp reader ignores an escaped newline in a string literal. | |
705 | @cindex newline in strings | |
706 | ||
707 | @example | |
708 | "It is useful to include newlines | |
709 | in documentation strings, | |
710 | but the newline is \ | |
711 | ignored if escaped." | |
712 | @result{} "It is useful to include newlines | |
713 | in documentation strings, | |
714 | but the newline is ignored if escaped." | |
715 | @end example | |
716 | ||
717 | The printed representation of a string consists of a double-quote, the | |
718 | characters it contains, and another double-quote. However, any | |
719 | backslash or double-quote characters in the string are preceded with a | |
720 | backslash like this: @code{"this \" is an embedded quote"}. | |
721 | ||
722 | A string can hold properties of the text it contains, in addition to | |
723 | the characters themselves. This enables programs that copy text between | |
724 | strings and buffers to preserve the properties with no special effort. | |
725 | @xref{Text Properties}. Strings with text properties have a special | |
726 | read and print syntax: | |
727 | ||
728 | @example | |
729 | #("@var{characters}" @var{property-data}...) | |
730 | @end example | |
731 | ||
732 | @noindent | |
733 | where @var{property-data} consists of zero or more elements, in groups | |
734 | of three as follows: | |
735 | ||
736 | @example | |
737 | @var{beg} @var{end} @var{plist} | |
738 | @end example | |
739 | ||
740 | @noindent | |
741 | The elements @var{beg} and @var{end} are integers, and together specify | |
742 | a range of indices in the string; @var{plist} is the property list for | |
743 | that range. | |
744 | ||
745 | @xref{Strings and Characters}, for functions that work on strings. | |
746 | ||
747 | @node Vector Type | |
748 | @subsection Vector Type | |
749 | ||
750 | A @dfn{vector} is a one-dimensional array of elements of any type. It | |
751 | takes a constant amount of time to access any element of a vector. (In | |
752 | a list, the access time of an element is proportional to the distance of | |
753 | the element from the beginning of the list.) | |
754 | ||
755 | The printed representation of a vector consists of a left square | |
756 | bracket, the elements, and a right square bracket. This is also the | |
757 | read syntax. Like numbers and strings, vectors are considered constants | |
758 | for evaluation. | |
759 | ||
760 | @example | |
761 | [1 "two" (three)] ; @r{A vector of three elements.} | |
762 | @result{} [1 "two" (three)] | |
763 | @end example | |
764 | ||
765 | @xref{Vectors}, for functions that work with vectors. | |
766 | ||
767 | @node Symbol Type | |
768 | @subsection Symbol Type | |
769 | ||
770 | A @dfn{symbol} in GNU Emacs Lisp is an object with a name. The symbol | |
771 | name serves as the printed representation of the symbol. In ordinary | |
772 | use, the name is unique---no two symbols have the same name. | |
773 | ||
774 | A symbol can serve as a variable, as a function name, or to hold a | |
775 | property list. Or it may serve only to be distinct from all other Lisp | |
776 | objects, so that its presence in a data structure may be recognized | |
777 | reliably. In a given context, usually only one of these uses is | |
778 | intended. But you can use one symbol in all of these ways, | |
779 | independently. | |
780 | ||
781 | @cindex @samp{\} in symbols | |
782 | @cindex backslash in symbols | |
783 | A symbol name can contain any characters whatever. Most symbol names | |
784 | are written with letters, digits, and the punctuation characters | |
785 | @samp{-+=*/}. Such names require no special punctuation; the characters | |
786 | of the name suffice as long as the name does not look like a number. | |
787 | (If it does, write a @samp{\} at the beginning of the name to force | |
788 | interpretation as a symbol.) The characters @samp{_~!@@$%^&:<>@{@}} are | |
789 | less often used but also require no special punctuation. Any other | |
790 | characters may be included in a symbol's name by escaping them with a | |
791 | backslash. In contrast to its use in strings, however, a backslash in | |
792 | the name of a symbol quotes the single character that follows the | |
793 | backslash, without conversion. For example, in a string, @samp{\t} | |
794 | represents a tab character; in the name of a symbol, however, @samp{\t} | |
795 | merely quotes the letter @kbd{t}. To have a symbol with a tab character | |
796 | in its name, you must actually use a tab (preceded with a backslash). | |
797 | But it's rare to do such a thing. | |
798 | ||
799 | @cindex CL note---case of letters | |
800 | @quotation | |
801 | @b{Common Lisp note:} in Common Lisp, lower case letters are always | |
802 | ``folded'' to upper case, unless they are explicitly escaped. This is | |
803 | in contrast to Emacs Lisp, in which upper case and lower case letters | |
804 | are distinct. | |
805 | @end quotation | |
806 | ||
807 | Here are several examples of symbol names. Note that the @samp{+} in | |
808 | the fifth example is escaped to prevent it from being read as a number. | |
809 | This is not necessary in the last example because the rest of the name | |
810 | makes it invalid as a number. | |
811 | ||
812 | @example | |
813 | @group | |
814 | foo ; @r{A symbol named @samp{foo}.} | |
815 | FOO ; @r{A symbol named @samp{FOO}, different from @samp{foo}.} | |
816 | char-to-string ; @r{A symbol named @samp{char-to-string}.} | |
817 | @end group | |
818 | @group | |
819 | 1+ ; @r{A symbol named @samp{1+}} | |
820 | ; @r{(not @samp{+1}, which is an integer).} | |
821 | @end group | |
822 | @group | |
823 | \+1 ; @r{A symbol named @samp{+1}} | |
824 | ; @r{(not a very readable name).} | |
825 | @end group | |
826 | @group | |
827 | \(*\ 1\ 2\) ; @r{A symbol named @samp{(* 1 2)} (a worse name).} | |
828 | @c the @'s in this next line use up three characters, hence the | |
829 | @c apparent misalignment of the comment. | |
830 | +-*/_~!@@$%^&=:<>@{@} ; @r{A symbol named @samp{+-*/_~!@@$%^&=:<>@{@}}.} | |
831 | ; @r{These characters need not be escaped.} | |
832 | @end group | |
833 | @end example | |
834 | ||
835 | @node Lisp Function Type | |
836 | @subsection Lisp Function Type | |
837 | ||
838 | Just as functions in other programming languages are executable, | |
839 | @dfn{Lisp function} objects are pieces of executable code. However, | |
840 | functions in Lisp are primarily Lisp objects, and only secondarily the | |
841 | text which represents them. These Lisp objects are lambda expressions: | |
842 | lists whose first element is the symbol @code{lambda} (@pxref{Lambda | |
843 | Expressions}). | |
844 | ||
845 | In most programming languages, it is impossible to have a function | |
846 | without a name. In Lisp, a function has no intrinsic name. A lambda | |
847 | expression is also called an @dfn{anonymous function} (@pxref{Anonymous | |
848 | Functions}). A named function in Lisp is actually a symbol with a valid | |
849 | function in its function cell (@pxref{Defining Functions}). | |
850 | ||
851 | Most of the time, functions are called when their names are written in | |
852 | Lisp expressions in Lisp programs. However, you can construct or obtain | |
853 | a function object at run time and then call it with the primitive | |
854 | functions @code{funcall} and @code{apply}. @xref{Calling Functions}. | |
855 | ||
856 | @node Lisp Macro Type | |
857 | @subsection Lisp Macro Type | |
858 | ||
859 | A @dfn{Lisp macro} is a user-defined construct that extends the Lisp | |
860 | language. It is represented as an object much like a function, but with | |
861 | different parameter-passing semantics. A Lisp macro has the form of a | |
862 | list whose first element is the symbol @code{macro} and whose @sc{cdr} | |
863 | is a Lisp function object, including the @code{lambda} symbol. | |
864 | ||
865 | Lisp macro objects are usually defined with the built-in | |
866 | @code{defmacro} function, but any list that begins with @code{macro} is | |
867 | a macro as far as Emacs is concerned. @xref{Macros}, for an explanation | |
868 | of how to write a macro. | |
869 | ||
870 | @node Primitive Function Type | |
871 | @subsection Primitive Function Type | |
872 | @cindex special forms | |
873 | ||
874 | A @dfn{primitive function} is a function callable from Lisp but | |
875 | written in the C programming language. Primitive functions are also | |
876 | called @dfn{subrs} or @dfn{built-in functions}. (The word ``subr'' is | |
877 | derived from ``subroutine''.) Most primitive functions evaluate all | |
878 | their arguments when they are called. A primitive function that does | |
879 | not evaluate all its arguments is called a @dfn{special form} | |
880 | (@pxref{Special Forms}).@refill | |
881 | ||
882 | It does not matter to the caller of a function whether the function is | |
883 | primitive. However, this does matter if you try to substitute a | |
884 | function written in Lisp for a primitive of the same name. The reason | |
885 | is that the primitive function may be called directly from C code. | |
886 | Calls to the redefined function from Lisp will use the new definition, | |
887 | but calls from C code may still use the built-in definition. | |
888 | ||
889 | The term @dfn{function} refers to all Emacs functions, whether written | |
890 | in Lisp or C. @xref{Lisp Function Type}, for information about the | |
891 | functions written in Lisp.@refill | |
892 | ||
893 | Primitive functions have no read syntax and print in hash notation | |
894 | with the name of the subroutine. | |
895 | ||
896 | @example | |
897 | @group | |
898 | (symbol-function 'car) ; @r{Access the function cell} | |
899 | ; @r{of the symbol.} | |
900 | @result{} #<subr car> | |
901 | (subrp (symbol-function 'car)) ; @r{Is this a primitive function?} | |
902 | @result{} t ; @r{Yes.} | |
903 | @end group | |
904 | @end example | |
905 | ||
906 | @node Byte-Code Type | |
907 | @subsection Byte-Code Function Type | |
908 | ||
909 | The byte compiler produces @dfn{byte-code function objects}. | |
910 | Internally, a byte-code function object is much like a vector; however, | |
911 | the evaluator handles this data type specially when it appears as a | |
912 | function to be called. @xref{Byte Compilation}, for information about | |
913 | the byte compiler. | |
914 | ||
915 | The printed representation for a byte-code function object is like that | |
916 | for a vector, with an additional @samp{#} before the opening @samp{[}. | |
917 | ||
918 | @node Autoload Type | |
919 | @subsection Autoload Type | |
920 | ||
921 | An @dfn{autoload object} is a list whose first element is the symbol | |
922 | @code{autoload}. It is stored as the function definition of a symbol as | |
923 | a placeholder for the real definition; it says that the real definition | |
924 | is found in a file of Lisp code that should be loaded when necessary. | |
925 | The autoload object contains the name of the file, plus some other | |
926 | information about the real definition. | |
927 | ||
928 | After the file has been loaded, the symbol should have a new function | |
929 | definition that is not an autoload object. The new definition is then | |
930 | called as if it had been there to begin with. From the user's point of | |
931 | view, the function call works as expected, using the function definition | |
932 | in the loaded file. | |
933 | ||
934 | An autoload object is usually created with the function | |
935 | @code{autoload}, which stores the object in the function cell of a | |
936 | symbol. @xref{Autoload}, for more details. | |
937 | ||
938 | @node Editing Types | |
939 | @section Editing Types | |
940 | @cindex editing types | |
941 | ||
942 | The types in the previous section are common to many Lisp dialects. | |
943 | Emacs Lisp provides several additional data types for purposes connected | |
944 | with editing. | |
945 | ||
946 | @menu | |
947 | * Buffer Type:: The basic object of editing. | |
948 | * Marker Type:: A position in a buffer. | |
949 | * Window Type:: Buffers are displayed in windows. | |
950 | * Frame Type:: Windows subdivide frames. | |
951 | * Window Configuration Type:: Recording the way a frame is subdivided. | |
952 | * Process Type:: A process running on the underlying OS. | |
953 | * Stream Type:: Receive or send characters. | |
954 | * Keymap Type:: What function a keystroke invokes. | |
955 | * Syntax Table Type:: What a character means. | |
956 | * Display Table Type:: How display tables are represented. | |
957 | * Overlay Type:: How an overlay is represented. | |
958 | @end menu | |
959 | ||
960 | @node Buffer Type | |
961 | @subsection Buffer Type | |
962 | ||
963 | A @dfn{buffer} is an object that holds text that can be edited | |
964 | (@pxref{Buffers}). Most buffers hold the contents of a disk file | |
965 | (@pxref{Files}) so they can be edited, but some are used for other | |
966 | purposes. Most buffers are also meant to be seen by the user, and | |
967 | therefore displayed, at some time, in a window (@pxref{Windows}). But a | |
968 | buffer need not be displayed in any window. | |
969 | ||
970 | The contents of a buffer are much like a string, but buffers are not | |
971 | used like strings in Emacs Lisp, and the available operations are | |
972 | different. For example, insertion of text into a buffer is very | |
973 | efficient, whereas ``inserting'' text into a string requires | |
974 | concatenating substrings, and the result is an entirely new string | |
975 | object. | |
976 | ||
977 | Each buffer has a designated position called @dfn{point} | |
978 | (@pxref{Positions}). At any time, one buffer is the @dfn{current | |
979 | buffer}. Most editing commands act on the contents of the current | |
980 | buffer in the neighborhood of point. Many other functions manipulate or | |
981 | test the characters in the current buffer; a whole chapter in this | |
982 | manual is devoted to describing these functions (@pxref{Text}). | |
983 | ||
984 | Several other data structures are associated with each buffer: | |
985 | ||
986 | @itemize @bullet | |
987 | @item | |
988 | a local syntax table (@pxref{Syntax Tables}); | |
989 | ||
990 | @item | |
991 | a local keymap (@pxref{Keymaps}); and, | |
992 | ||
993 | @item | |
994 | a local variable binding list (@pxref{Buffer-Local Variables}). | |
995 | @end itemize | |
996 | ||
997 | @noindent | |
998 | The local keymap and variable list contain entries which individually | |
999 | override global bindings or values. These are used to customize the | |
1000 | behavior of programs in different buffers, without actually changing the | |
1001 | programs. | |
1002 | ||
1003 | Buffers have no read syntax. They print in hash notation with the | |
1004 | buffer name. | |
1005 | ||
1006 | @example | |
1007 | @group | |
1008 | (current-buffer) | |
1009 | @result{} #<buffer objects.texi> | |
1010 | @end group | |
1011 | @end example | |
1012 | ||
1013 | @node Marker Type | |
1014 | @subsection Marker Type | |
1015 | ||
1016 | A @dfn{marker} denotes a position in a specific buffer. Markers | |
1017 | therefore have two components: one for the buffer, and one for the | |
1018 | position. Changes in the buffer's text automatically relocate the | |
1019 | position value as necessary to ensure that the marker always points | |
1020 | between the same two characters in the buffer. | |
1021 | ||
1022 | Markers have no read syntax. They print in hash notation, giving the | |
1023 | current character position and the name of the buffer. | |
1024 | ||
1025 | @example | |
1026 | @group | |
1027 | (point-marker) | |
1028 | @result{} #<marker at 10779 in objects.texi> | |
1029 | @end group | |
1030 | @end example | |
1031 | ||
1032 | @xref{Markers}, for information on how to test, create, copy, and move | |
1033 | markers. | |
1034 | ||
1035 | @node Window Type | |
1036 | @subsection Window Type | |
1037 | ||
1038 | A @dfn{window} describes the portion of the terminal screen that Emacs | |
1039 | uses to display a buffer. Every window has one associated buffer, whose | |
1040 | contents appear in the window. By contrast, a given buffer may appear | |
1041 | in one window, no window, or several windows. | |
1042 | ||
1043 | Though many windows may exist simultaneously, at any time one window | |
1044 | is designated the @dfn{selected window}. This is the window where the | |
1045 | cursor is (usually) displayed when Emacs is ready for a command. The | |
1046 | selected window usually displays the current buffer, but this is not | |
1047 | necessarily the case. | |
1048 | ||
1049 | Windows are grouped on the screen into frames; each window belongs to | |
1050 | one and only one frame. @xref{Frame Type}. | |
1051 | ||
1052 | Windows have no read syntax. They print in hash notation, giving the | |
1053 | window number and the name of the buffer being displayed. The window | |
1054 | numbers exist to identify windows uniquely, since the buffer displayed | |
1055 | in any given window can change frequently. | |
1056 | ||
1057 | @example | |
1058 | @group | |
1059 | (selected-window) | |
1060 | @result{} #<window 1 on objects.texi> | |
1061 | @end group | |
1062 | @end example | |
1063 | ||
1064 | @xref{Windows}, for a description of the functions that work on windows. | |
1065 | ||
1066 | @node Frame Type | |
1067 | @subsection Frame Type | |
1068 | ||
1069 | A @var{frame} is a rectangle on the screen that contains one or more | |
1070 | Emacs windows. A frame initially contains a single main window (plus | |
1071 | perhaps a minibuffer window) which you can subdivide vertically or | |
1072 | horizontally into smaller windows. | |
1073 | ||
1074 | Frames have no read syntax. They print in hash notation, giving the | |
1075 | frame's title, plus its address in core (useful to identify the frame | |
1076 | uniquely). | |
1077 | ||
1078 | @example | |
1079 | @group | |
1080 | (selected-frame) | |
1081 | @result{} #<frame xemacs@@mole.gnu.ai.mit.edu 0xdac80> | |
1082 | @end group | |
1083 | @end example | |
1084 | ||
1085 | @xref{Frames}, for a description of the functions that work on frames. | |
1086 | ||
1087 | @node Window Configuration Type | |
1088 | @subsection Window Configuration Type | |
1089 | @cindex screen layout | |
1090 | ||
1091 | A @dfn{window configuration} stores information about the positions, | |
1092 | sizes, and contents of the windows in a frame, so you can recreate the | |
1093 | same arrangement of windows later. | |
1094 | ||
1095 | Window configurations do not have a read syntax. They print as | |
1096 | @samp{#<window-configuration>}. @xref{Window Configurations}, for a | |
1097 | description of several functions related to window configurations. | |
1098 | ||
1099 | @node Process Type | |
1100 | @subsection Process Type | |
1101 | ||
1102 | The word @dfn{process} usually means a running program. Emacs itself | |
1103 | runs in a process of this sort. However, in Emacs Lisp, a process is a | |
1104 | Lisp object that designates a subprocess created by the Emacs process. | |
1105 | Programs such as shells, GDB, ftp, and compilers, running in | |
1106 | subprocesses of Emacs, extend the capabilities of Emacs. | |
1107 | ||
1108 | An Emacs subprocess takes textual input from Emacs and returns textual | |
1109 | output to Emacs for further manipulation. Emacs can also send signals | |
1110 | to the subprocess. | |
1111 | ||
1112 | Process objects have no read syntax. They print in hash notation, | |
1113 | giving the name of the process: | |
1114 | ||
1115 | @example | |
1116 | @group | |
1117 | (process-list) | |
1118 | @result{} (#<process shell>) | |
1119 | @end group | |
1120 | @end example | |
1121 | ||
1122 | @xref{Processes}, for information about functions that create, delete, | |
1123 | return information about, send input or signals to, and receive output | |
1124 | from processes. | |
1125 | ||
1126 | @node Stream Type | |
1127 | @subsection Stream Type | |
1128 | ||
1129 | A @dfn{stream} is an object that can be used as a source or sink for | |
1130 | characters---either to supply characters for input or to accept them as | |
1131 | output. Many different types can be used this way: markers, buffers, | |
1132 | strings, and functions. Most often, input streams (character sources) | |
1133 | obtain characters from the keyboard, a buffer, or a file, and output | |
1134 | streams (character sinks) send characters to a buffer, such as a | |
1135 | @file{*Help*} buffer, or to the echo area. | |
1136 | ||
1137 | The object @code{nil}, in addition to its other meanings, may be used | |
1138 | as a stream. It stands for the value of the variable | |
1139 | @code{standard-input} or @code{standard-output}. Also, the object | |
1140 | @code{t} as a stream specifies input using the minibuffer | |
1141 | (@pxref{Minibuffers}) or output in the echo area (@pxref{The Echo | |
1142 | Area}). | |
1143 | ||
1144 | Streams have no special printed representation or read syntax, and | |
1145 | print as whatever primitive type they are. | |
1146 | ||
1147 | @xref{Streams}, for a description of various functions related to | |
1148 | streams, including various parsing and printing functions. | |
1149 | ||
1150 | @node Keymap Type | |
1151 | @subsection Keymap Type | |
1152 | ||
1153 | A @dfn{keymap} maps keys typed by the user to commands. This mapping | |
1154 | controls how the user's command input is executed. A keymap is actually | |
1155 | a list whose @sc{car} is the symbol @code{keymap}. | |
1156 | ||
1157 | @xref{Keymaps}, for information about creating keymaps, handling prefix | |
1158 | keys, local as well as global keymaps, and changing key bindings. | |
1159 | ||
1160 | @node Syntax Table Type | |
1161 | @subsection Syntax Table Type | |
1162 | ||
1163 | A @dfn{syntax table} is a vector of 256 integers. Each element of the | |
1164 | vector defines how one character is interpreted when it appears in a | |
1165 | buffer. For example, in C mode (@pxref{Major Modes}), the @samp{+} | |
1166 | character is punctuation, but in Lisp mode it is a valid character in a | |
1167 | symbol. These modes specify different interpretations by changing the | |
1168 | syntax table entry for @samp{+}, at index 43 in the syntax table. | |
1169 | ||
1170 | Syntax tables are only used for scanning text in buffers, not for | |
1171 | reading Lisp expressions. The table the Lisp interpreter uses to read | |
1172 | expressions is built into the Emacs source code and cannot be changed; | |
1173 | thus, to change the list delimiters to be @samp{@{} and @samp{@}} | |
1174 | instead of @samp{(} and @samp{)} would be impossible. | |
1175 | ||
1176 | @xref{Syntax Tables}, for details about syntax classes and how to make | |
1177 | and modify syntax tables. | |
1178 | ||
1179 | @node Display Table Type | |
1180 | @subsection Display Table Type | |
1181 | ||
1182 | A @dfn{display table} specifies how to display each character code. | |
1183 | Each buffer and each window can have its own display table. A display | |
1184 | table is actually a vector of length 261. @xref{Display Tables}. | |
1185 | ||
1186 | @node Overlay Type | |
1187 | @subsection Overlay Type | |
1188 | ||
1189 | An @dfn{overlay} specifies temporary alteration of the display | |
1190 | appearance of a part of a buffer. It contains markers delimiting a | |
1191 | range of the buffer, plus a property list (a list whose elements are | |
1192 | alternating property names and values). Overlays are used to present | |
1193 | parts of the buffer temporarily in a different display style. | |
1194 | ||
1195 | @xref{Overlays}, for how to create and use overlays. They have no | |
1196 | read syntax, and print in hash notation, giving the buffer name and | |
1197 | range of positions. | |
1198 | ||
1199 | @node Type Predicates | |
1200 | @section Type Predicates | |
1201 | @cindex predicates | |
1202 | @cindex type checking | |
1203 | @kindex wrong-type-argument | |
1204 | ||
1205 | The Emacs Lisp interpreter itself does not perform type checking on | |
1206 | the actual arguments passed to functions when they are called. It could | |
1207 | not do so, since function arguments in Lisp do not have declared data | |
1208 | types, as they do in other programming languages. It is therefore up to | |
1209 | the individual function to test whether each actual argument belongs to | |
1210 | a type that the function can use. | |
1211 | ||
1212 | All built-in functions do check the types of their actual arguments | |
1213 | when appropriate, and signal a @code{wrong-type-argument} error if an | |
1214 | argument is of the wrong type. For example, here is what happens if you | |
1215 | pass an argument to @code{+} which it cannot handle: | |
1216 | ||
1217 | @example | |
1218 | @group | |
1219 | (+ 2 'a) | |
1220 | @error{} Wrong type argument: integer-or-marker-p, a | |
1221 | @end group | |
1222 | @end example | |
1223 | ||
1224 | @cindex type predicates | |
1225 | @cindex testing types | |
1226 | Lisp provides functions, called @dfn{type predicates}, to test whether | |
1227 | an object is a member of a given type. (Following a convention of long | |
1228 | standing, the names of most Emacs Lisp predicates end in @samp{p}.) | |
1229 | ||
1230 | Here is a table of predefined type predicates, in alphabetical order, | |
1231 | with references to further information. | |
1232 | ||
1233 | @table @code | |
1234 | @item atom | |
1235 | @xref{List-related Predicates, atom}. | |
1236 | ||
1237 | @item arrayp | |
1238 | @xref{Array Functions, arrayp}. | |
1239 | ||
1240 | @item bufferp | |
1241 | @xref{Buffer Basics, bufferp}. | |
1242 | ||
1243 | @item byte-code-function-p | |
1244 | @xref{Byte-Code Type, byte-code-function-p}. | |
1245 | ||
1246 | @item case-table-p | |
1247 | @xref{Case Table, case-table-p}. | |
1248 | ||
1249 | @item char-or-string-p | |
1250 | @xref{Predicates for Strings, char-or-string-p}. | |
1251 | ||
1252 | @item commandp | |
1253 | @xref{Interactive Call, commandp}. | |
1254 | ||
1255 | @item consp | |
1256 | @xref{List-related Predicates, consp}. | |
1257 | ||
1258 | @item floatp | |
1259 | @xref{Predicates on Numbers, floatp}. | |
1260 | ||
1261 | @item frame-live-p | |
1262 | @xref{Deleting Frames, frame-live-p}. | |
1263 | ||
1264 | @item framep | |
1265 | @xref{Frames, framep}. | |
1266 | ||
1267 | @item integer-or-marker-p | |
1268 | @xref{Predicates on Markers, integer-or-marker-p}. | |
1269 | ||
1270 | @item integerp | |
1271 | @xref{Predicates on Numbers, integerp}. | |
1272 | ||
1273 | @item keymapp | |
1274 | @xref{Creating Keymaps, keymapp}. | |
1275 | ||
1276 | @item listp | |
1277 | @xref{List-related Predicates, listp}. | |
1278 | ||
1279 | @item markerp | |
1280 | @xref{Predicates on Markers, markerp}. | |
1281 | ||
1282 | @item natnump | |
1283 | @xref{Predicates on Numbers, natnump}. | |
1284 | ||
1285 | @item nlistp | |
1286 | @xref{List-related Predicates, nlistp}. | |
1287 | ||
1288 | @item numberp | |
1289 | @xref{Predicates on Numbers, numberp}. | |
1290 | ||
1291 | @item number-or-marker-p | |
1292 | @xref{Predicates on Markers, number-or-marker-p}. | |
1293 | ||
1294 | @item overlayp | |
1295 | @xref{Overlays, overlayp}. | |
1296 | ||
1297 | @item processp | |
1298 | @xref{Processes, processp}. | |
1299 | ||
1300 | @item sequencep | |
1301 | @xref{Sequence Functions, sequencep}. | |
1302 | ||
1303 | @item stringp | |
1304 | @xref{Predicates for Strings, stringp}. | |
1305 | ||
1306 | @item subrp | |
1307 | @xref{Function Cells, subrp}. | |
1308 | ||
1309 | @item symbolp | |
1310 | @xref{Symbols, symbolp}. | |
1311 | ||
1312 | @item syntax-table-p | |
1313 | @xref{Syntax Tables, syntax-table-p}. | |
1314 | ||
1315 | @item user-variable-p | |
1316 | @xref{Defining Variables, user-variable-p}. | |
1317 | ||
1318 | @item vectorp | |
1319 | @xref{Vectors, vectorp}. | |
1320 | ||
1321 | @item window-configuration-p | |
1322 | @xref{Window Configurations, window-configuration-p}. | |
1323 | ||
1324 | @item window-live-p | |
1325 | @xref{Deleting Windows, window-live-p}. | |
1326 | ||
1327 | @item windowp | |
1328 | @xref{Basic Windows, windowp}. | |
1329 | @end table | |
1330 | ||
1331 | @node Equality Predicates | |
1332 | @section Equality Predicates | |
1333 | @cindex equality | |
1334 | ||
1335 | Here we describe two functions that test for equality between any two | |
1336 | objects. Other functions test equality between objects of specific | |
1337 | types, e.g., strings. See the appropriate chapter describing the data | |
1338 | type for these predicates. | |
1339 | ||
1340 | @defun eq object1 object2 | |
1341 | This function returns @code{t} if @var{object1} and @var{object2} are | |
1342 | the same object, @code{nil} otherwise. The ``same object'' means that a | |
1343 | change in one will be reflected by the same change in the other. | |
1344 | ||
1345 | @code{eq} returns @code{t} if @var{object1} and @var{object2} are | |
1346 | integers with the same value. Also, since symbol names are normally | |
1347 | unique, if the arguments are symbols with the same name, they are | |
1348 | @code{eq}. For other types (e.g., lists, vectors, strings), two | |
1349 | arguments with the same contents or elements are not necessarily | |
1350 | @code{eq} to each other: they are @code{eq} only if they are the same | |
1351 | object. | |
1352 | ||
1353 | (The @code{make-symbol} function returns an uninterned symbol that is | |
1354 | not interned in the standard @code{obarray}. When uninterned symbols | |
1355 | are in use, symbol names are no longer unique. Distinct symbols with | |
1356 | the same name are not @code{eq}. @xref{Creating Symbols}.) | |
1357 | ||
1358 | @example | |
1359 | @group | |
1360 | (eq 'foo 'foo) | |
1361 | @result{} t | |
1362 | @end group | |
1363 | ||
1364 | @group | |
1365 | (eq 456 456) | |
1366 | @result{} t | |
1367 | @end group | |
1368 | ||
1369 | @group | |
1370 | (eq "asdf" "asdf") | |
1371 | @result{} nil | |
1372 | @end group | |
1373 | ||
1374 | @group | |
1375 | (eq '(1 (2 (3))) '(1 (2 (3)))) | |
1376 | @result{} nil | |
1377 | @end group | |
1378 | ||
1379 | @group | |
1380 | (setq foo '(1 (2 (3)))) | |
1381 | @result{} (1 (2 (3))) | |
1382 | (eq foo foo) | |
1383 | @result{} t | |
1384 | (eq foo '(1 (2 (3)))) | |
1385 | @result{} nil | |
1386 | @end group | |
1387 | ||
1388 | @group | |
1389 | (eq [(1 2) 3] [(1 2) 3]) | |
1390 | @result{} nil | |
1391 | @end group | |
1392 | ||
1393 | @group | |
1394 | (eq (point-marker) (point-marker)) | |
1395 | @result{} nil | |
1396 | @end group | |
1397 | @end example | |
1398 | ||
1399 | @end defun | |
1400 | ||
1401 | @defun equal object1 object2 | |
1402 | This function returns @code{t} if @var{object1} and @var{object2} have | |
1403 | equal components, @code{nil} otherwise. Whereas @code{eq} tests if its | |
1404 | arguments are the same object, @code{equal} looks inside nonidentical | |
1405 | arguments to see if their elements are the same. So, if two objects are | |
1406 | @code{eq}, they are @code{equal}, but the converse is not always true. | |
1407 | ||
1408 | @example | |
1409 | @group | |
1410 | (equal 'foo 'foo) | |
1411 | @result{} t | |
1412 | @end group | |
1413 | ||
1414 | @group | |
1415 | (equal 456 456) | |
1416 | @result{} t | |
1417 | @end group | |
1418 | ||
1419 | @group | |
1420 | (equal "asdf" "asdf") | |
1421 | @result{} t | |
1422 | @end group | |
1423 | @group | |
1424 | (eq "asdf" "asdf") | |
1425 | @result{} nil | |
1426 | @end group | |
1427 | ||
1428 | @group | |
1429 | (equal '(1 (2 (3))) '(1 (2 (3)))) | |
1430 | @result{} t | |
1431 | @end group | |
1432 | @group | |
1433 | (eq '(1 (2 (3))) '(1 (2 (3)))) | |
1434 | @result{} nil | |
1435 | @end group | |
1436 | ||
1437 | @group | |
1438 | (equal [(1 2) 3] [(1 2) 3]) | |
1439 | @result{} t | |
1440 | @end group | |
1441 | @group | |
1442 | (eq [(1 2) 3] [(1 2) 3]) | |
1443 | @result{} nil | |
1444 | @end group | |
1445 | ||
1446 | @group | |
1447 | (equal (point-marker) (point-marker)) | |
1448 | @result{} t | |
1449 | @end group | |
1450 | ||
1451 | @group | |
1452 | (eq (point-marker) (point-marker)) | |
1453 | @result{} nil | |
1454 | @end group | |
1455 | @end example | |
1456 | ||
1457 | Comparison of strings uses @code{string=}, and is case-sensitive. | |
1458 | ||
1459 | @example | |
1460 | @group | |
1461 | (equal "asdf" "ASDF") | |
1462 | @result{} nil | |
1463 | @end group | |
1464 | @end example | |
1465 | @end defun | |
1466 | ||
1467 | The test for equality is implemented recursively, and circular lists may | |
1468 | therefore cause infinite recursion (leading to an error). |