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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, 1995, 1998, 1999, 2001, | |
114f9c96 | 4 | @c 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 |
0cc8c85a | 5 | @c Free Software Foundation, Inc. |
b8d4c8d0 | 6 | @c See the file elisp.texi for copying conditions. |
6336d8c3 | 7 | @setfilename ../../info/objects |
b8d4c8d0 GM |
8 | @node Lisp Data Types, Numbers, Introduction, Top |
9 | @chapter Lisp Data Types | |
10 | @cindex object | |
11 | @cindex Lisp object | |
12 | @cindex type | |
13 | @cindex data type | |
14 | ||
15 | A Lisp @dfn{object} is a piece of data used and manipulated by Lisp | |
16 | programs. For our purposes, a @dfn{type} or @dfn{data type} is a set of | |
17 | possible objects. | |
18 | ||
19 | Every object belongs to at least one type. Objects of the same type | |
20 | have similar structures and may usually be used in the same contexts. | |
21 | Types can overlap, and objects can belong to two or more types. | |
22 | Consequently, we can ask whether an object belongs to a particular type, | |
23 | but not for ``the'' type of an object. | |
24 | ||
25 | @cindex primitive type | |
26 | A few fundamental object types are built into Emacs. These, from | |
27 | which all other types are constructed, are called @dfn{primitive types}. | |
28 | Each object belongs to one and only one primitive type. These types | |
29 | include @dfn{integer}, @dfn{float}, @dfn{cons}, @dfn{symbol}, | |
30 | @dfn{string}, @dfn{vector}, @dfn{hash-table}, @dfn{subr}, and | |
31 | @dfn{byte-code function}, plus several special types, such as | |
32 | @dfn{buffer}, that are related to editing. (@xref{Editing Types}.) | |
33 | ||
34 | Each primitive type has a corresponding Lisp function that checks | |
35 | whether an object is a member of that type. | |
36 | ||
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37 | Lisp is unlike many other languages in that its objects are |
38 | @dfn{self-typing}: the primitive type of each object is implicit in | |
39 | the object itself. For example, if an object is a vector, nothing can | |
40 | treat it as a number; Lisp knows it is a vector, not a number. | |
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41 | |
42 | In most languages, the programmer must declare the data type of each | |
43 | variable, and the type is known by the compiler but not represented in | |
44 | the data. Such type declarations do not exist in Emacs Lisp. A Lisp | |
45 | variable can have any type of value, and it remembers whatever value | |
46 | you store in it, type and all. (Actually, a small number of Emacs | |
47 | Lisp variables can only take on values of a certain type. | |
48 | @xref{Variables with Restricted Values}.) | |
49 | ||
50 | This chapter describes the purpose, printed representation, and read | |
51 | syntax of each of the standard types in GNU Emacs Lisp. Details on how | |
52 | to use these types can be found in later chapters. | |
53 | ||
54 | @menu | |
55 | * Printed Representation:: How Lisp objects are represented as text. | |
56 | * Comments:: Comments and their formatting conventions. | |
57 | * Programming Types:: Types found in all Lisp systems. | |
58 | * Editing Types:: Types specific to Emacs. | |
59 | * Circular Objects:: Read syntax for circular structure. | |
60 | * Type Predicates:: Tests related to types. | |
61 | * Equality Predicates:: Tests of equality between any two objects. | |
62 | @end menu | |
63 | ||
64 | @node Printed Representation | |
65 | @comment node-name, next, previous, up | |
66 | @section Printed Representation and Read Syntax | |
67 | @cindex printed representation | |
68 | @cindex read syntax | |
69 | ||
70 | The @dfn{printed representation} of an object is the format of the | |
71 | output generated by the Lisp printer (the function @code{prin1}) for | |
72 | that object. Every data type has a unique printed representation. | |
73 | The @dfn{read syntax} of an object is the format of the input accepted | |
74 | by the Lisp reader (the function @code{read}) for that object. This | |
75 | is not necessarily unique; many kinds of object have more than one | |
76 | syntax. @xref{Read and Print}. | |
77 | ||
78 | @cindex hash notation | |
79 | In most cases, an object's printed representation is also a read | |
80 | syntax for the object. However, some types have no read syntax, since | |
81 | it does not make sense to enter objects of these types as constants in | |
82 | a Lisp program. These objects are printed in @dfn{hash notation}, | |
83 | which consists of the characters @samp{#<}, a descriptive string | |
84 | (typically the type name followed by the name of the object), and a | |
85 | closing @samp{>}. For example: | |
86 | ||
87 | @example | |
88 | (current-buffer) | |
89 | @result{} #<buffer objects.texi> | |
90 | @end example | |
91 | ||
92 | @noindent | |
93 | Hash notation cannot be read at all, so the Lisp reader signals the | |
94 | error @code{invalid-read-syntax} whenever it encounters @samp{#<}. | |
95 | @kindex invalid-read-syntax | |
96 | ||
97 | In other languages, an expression is text; it has no other form. In | |
98 | Lisp, an expression is primarily a Lisp object and only secondarily the | |
99 | text that is the object's read syntax. Often there is no need to | |
100 | emphasize this distinction, but you must keep it in the back of your | |
101 | mind, or you will occasionally be very confused. | |
102 | ||
103 | When you evaluate an expression interactively, the Lisp interpreter | |
104 | first reads the textual representation of it, producing a Lisp object, | |
105 | and then evaluates that object (@pxref{Evaluation}). However, | |
106 | evaluation and reading are separate activities. Reading returns the | |
107 | Lisp object represented by the text that is read; the object may or may | |
108 | not be evaluated later. @xref{Input Functions}, for a description of | |
109 | @code{read}, the basic function for reading objects. | |
110 | ||
111 | @node Comments | |
112 | @comment node-name, next, previous, up | |
113 | @section Comments | |
114 | @cindex comments | |
115 | @cindex @samp{;} in comment | |
116 | ||
117 | A @dfn{comment} is text that is written in a program only for the sake | |
118 | of humans that read the program, and that has no effect on the meaning | |
119 | of the program. In Lisp, a semicolon (@samp{;}) starts a comment if it | |
120 | is not within a string or character constant. The comment continues to | |
121 | the end of line. The Lisp reader discards comments; they do not become | |
122 | part of the Lisp objects which represent the program within the Lisp | |
123 | system. | |
124 | ||
125 | The @samp{#@@@var{count}} construct, which skips the next @var{count} | |
126 | characters, is useful for program-generated comments containing binary | |
127 | data. The Emacs Lisp byte compiler uses this in its output files | |
128 | (@pxref{Byte Compilation}). It isn't meant for source files, however. | |
129 | ||
130 | @xref{Comment Tips}, for conventions for formatting comments. | |
131 | ||
132 | @node Programming Types | |
133 | @section Programming Types | |
134 | @cindex programming types | |
135 | ||
136 | There are two general categories of types in Emacs Lisp: those having | |
137 | to do with Lisp programming, and those having to do with editing. The | |
138 | former exist in many Lisp implementations, in one form or another. The | |
139 | latter are unique to Emacs Lisp. | |
140 | ||
141 | @menu | |
142 | * Integer Type:: Numbers without fractional parts. | |
143 | * Floating Point Type:: Numbers with fractional parts and with a large range. | |
144 | * Character Type:: The representation of letters, numbers and | |
145 | control characters. | |
146 | * Symbol Type:: A multi-use object that refers to a function, | |
147 | variable, or property list, and has a unique identity. | |
148 | * Sequence Type:: Both lists and arrays are classified as sequences. | |
149 | * Cons Cell Type:: Cons cells, and lists (which are made from cons cells). | |
150 | * Array Type:: Arrays include strings and vectors. | |
151 | * String Type:: An (efficient) array of characters. | |
152 | * Vector Type:: One-dimensional arrays. | |
153 | * Char-Table Type:: One-dimensional sparse arrays indexed by characters. | |
154 | * Bool-Vector Type:: One-dimensional arrays of @code{t} or @code{nil}. | |
155 | * Hash Table Type:: Super-fast lookup tables. | |
156 | * Function Type:: A piece of executable code you can call from elsewhere. | |
157 | * Macro Type:: A method of expanding an expression into another | |
158 | expression, more fundamental but less pretty. | |
159 | * Primitive Function Type:: A function written in C, callable from Lisp. | |
160 | * Byte-Code Type:: A function written in Lisp, then compiled. | |
161 | * Autoload Type:: A type used for automatically loading seldom-used | |
162 | functions. | |
163 | @end menu | |
164 | ||
165 | @node Integer Type | |
166 | @subsection Integer Type | |
167 | ||
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168 | The range of values for integers in Emacs Lisp is @minus{}536870912 to |
169 | 536870911 (30 bits; i.e., | |
b8d4c8d0 | 170 | @ifnottex |
1ddd6622 | 171 | -2**29 |
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172 | @end ifnottex |
173 | @tex | |
1ddd6622 | 174 | @math{-2^{29}} |
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175 | @end tex |
176 | to | |
177 | @ifnottex | |
1ddd6622 | 178 | 2**29 - 1) |
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179 | @end ifnottex |
180 | @tex | |
1ddd6622 | 181 | @math{2^{29}-1}) |
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182 | @end tex |
183 | on most machines. (Some machines may provide a wider range.) It is | |
184 | important to note that the Emacs Lisp arithmetic functions do not check | |
1ddd6622 | 185 | for overflow. Thus @code{(1+ 536870911)} is @minus{}536870912 on most |
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186 | machines. |
187 | ||
188 | The read syntax for integers is a sequence of (base ten) digits with an | |
189 | optional sign at the beginning and an optional period at the end. The | |
190 | printed representation produced by the Lisp interpreter never has a | |
191 | leading @samp{+} or a final @samp{.}. | |
192 | ||
193 | @example | |
194 | @group | |
195 | -1 ; @r{The integer -1.} | |
196 | 1 ; @r{The integer 1.} | |
197 | 1. ; @r{Also the integer 1.} | |
198 | +1 ; @r{Also the integer 1.} | |
1ddd6622 | 199 | 1073741825 ; @r{Also the integer 1 on a 30-bit implementation.} |
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200 | @end group |
201 | @end example | |
202 | ||
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203 | @noindent |
204 | As a special exception, if a sequence of digits specifies an integer | |
205 | too large or too small to be a valid integer object, the Lisp reader | |
206 | reads it as a floating-point number (@pxref{Floating Point Type}). | |
207 | For instance, on most machines @code{536870912} is read as the | |
208 | floating-point number @code{536870912.0}. | |
209 | ||
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210 | @xref{Numbers}, for more information. |
211 | ||
212 | @node Floating Point Type | |
213 | @subsection Floating Point Type | |
214 | ||
215 | Floating point numbers are the computer equivalent of scientific | |
216 | notation; you can think of a floating point number as a fraction | |
217 | together with a power of ten. The precise number of significant | |
218 | figures and the range of possible exponents is machine-specific; Emacs | |
219 | uses the C data type @code{double} to store the value, and internally | |
220 | this records a power of 2 rather than a power of 10. | |
221 | ||
222 | The printed representation for floating point numbers requires either | |
223 | a decimal point (with at least one digit following), an exponent, or | |
224 | both. For example, @samp{1500.0}, @samp{15e2}, @samp{15.0e2}, | |
225 | @samp{1.5e3}, and @samp{.15e4} are five ways of writing a floating point | |
226 | number whose value is 1500. They are all equivalent. | |
227 | ||
228 | @xref{Numbers}, for more information. | |
229 | ||
230 | @node Character Type | |
231 | @subsection Character Type | |
232 | @cindex @acronym{ASCII} character codes | |
233 | ||
234 | A @dfn{character} in Emacs Lisp is nothing more than an integer. In | |
235 | other words, characters are represented by their character codes. For | |
236 | example, the character @kbd{A} is represented as the @w{integer 65}. | |
237 | ||
238 | Individual characters are used occasionally in programs, but it is | |
239 | more common to work with @emph{strings}, which are sequences composed | |
240 | of characters. @xref{String Type}. | |
241 | ||
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242 | Characters in strings and buffers are currently limited to the range |
243 | of 0 to 4194303---twenty two bits (@pxref{Character Codes}). Codes 0 | |
244 | through 127 are @acronym{ASCII} codes; the rest are | |
245 | non-@acronym{ASCII} (@pxref{Non-ASCII Characters}). Characters that | |
246 | represent keyboard input have a much wider range, to encode modifier | |
247 | keys such as Control, Meta and Shift. | |
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248 | |
249 | There are special functions for producing a human-readable textual | |
250 | description of a character for the sake of messages. @xref{Describing | |
251 | Characters}. | |
252 | ||
253 | @menu | |
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254 | * Basic Char Syntax:: Syntax for regular characters. |
255 | * General Escape Syntax:: How to specify characters by their codes. | |
256 | * Ctl-Char Syntax:: Syntax for control characters. | |
257 | * Meta-Char Syntax:: Syntax for meta-characters. | |
258 | * Other Char Bits:: Syntax for hyper-, super-, and alt-characters. | |
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259 | @end menu |
260 | ||
261 | @node Basic Char Syntax | |
262 | @subsubsection Basic Char Syntax | |
263 | @cindex read syntax for characters | |
264 | @cindex printed representation for characters | |
265 | @cindex syntax for characters | |
266 | @cindex @samp{?} in character constant | |
267 | @cindex question mark in character constant | |
268 | ||
269 | Since characters are really integers, the printed representation of | |
270 | a character is a decimal number. This is also a possible read syntax | |
271 | for a character, but writing characters that way in Lisp programs is | |
272 | not clear programming. You should @emph{always} use the special read | |
273 | syntax formats that Emacs Lisp provides for characters. These syntax | |
274 | formats start with a question mark. | |
275 | ||
276 | The usual read syntax for alphanumeric characters is a question mark | |
277 | followed by the character; thus, @samp{?A} for the character | |
278 | @kbd{A}, @samp{?B} for the character @kbd{B}, and @samp{?a} for the | |
279 | character @kbd{a}. | |
280 | ||
281 | For example: | |
282 | ||
283 | @example | |
284 | ?Q @result{} 81 ?q @result{} 113 | |
285 | @end example | |
286 | ||
287 | You can use the same syntax for punctuation characters, but it is | |
288 | often a good idea to add a @samp{\} so that the Emacs commands for | |
289 | editing Lisp code don't get confused. For example, @samp{?\(} is the | |
290 | way to write the open-paren character. If the character is @samp{\}, | |
291 | you @emph{must} use a second @samp{\} to quote it: @samp{?\\}. | |
292 | ||
293 | @cindex whitespace | |
294 | @cindex bell character | |
295 | @cindex @samp{\a} | |
296 | @cindex backspace | |
297 | @cindex @samp{\b} | |
298 | @cindex tab (ASCII character) | |
299 | @cindex @samp{\t} | |
300 | @cindex vertical tab | |
301 | @cindex @samp{\v} | |
302 | @cindex formfeed | |
303 | @cindex @samp{\f} | |
304 | @cindex newline | |
305 | @cindex @samp{\n} | |
306 | @cindex return (ASCII character) | |
307 | @cindex @samp{\r} | |
308 | @cindex escape (ASCII character) | |
309 | @cindex @samp{\e} | |
310 | @cindex space (ASCII character) | |
311 | @cindex @samp{\s} | |
312 | You can express the characters control-g, backspace, tab, newline, | |
313 | vertical tab, formfeed, space, return, del, and escape as @samp{?\a}, | |
314 | @samp{?\b}, @samp{?\t}, @samp{?\n}, @samp{?\v}, @samp{?\f}, | |
315 | @samp{?\s}, @samp{?\r}, @samp{?\d}, and @samp{?\e}, respectively. | |
316 | (@samp{?\s} followed by a dash has a different meaning---it applies | |
317 | the ``super'' modifier to the following character.) Thus, | |
318 | ||
319 | @example | |
320 | ?\a @result{} 7 ; @r{control-g, @kbd{C-g}} | |
321 | ?\b @result{} 8 ; @r{backspace, @key{BS}, @kbd{C-h}} | |
322 | ?\t @result{} 9 ; @r{tab, @key{TAB}, @kbd{C-i}} | |
323 | ?\n @result{} 10 ; @r{newline, @kbd{C-j}} | |
324 | ?\v @result{} 11 ; @r{vertical tab, @kbd{C-k}} | |
325 | ?\f @result{} 12 ; @r{formfeed character, @kbd{C-l}} | |
326 | ?\r @result{} 13 ; @r{carriage return, @key{RET}, @kbd{C-m}} | |
327 | ?\e @result{} 27 ; @r{escape character, @key{ESC}, @kbd{C-[}} | |
328 | ?\s @result{} 32 ; @r{space character, @key{SPC}} | |
329 | ?\\ @result{} 92 ; @r{backslash character, @kbd{\}} | |
330 | ?\d @result{} 127 ; @r{delete character, @key{DEL}} | |
331 | @end example | |
332 | ||
333 | @cindex escape sequence | |
334 | These sequences which start with backslash are also known as | |
335 | @dfn{escape sequences}, because backslash plays the role of an | |
336 | ``escape character''; this terminology has nothing to do with the | |
337 | character @key{ESC}. @samp{\s} is meant for use in character | |
338 | constants; in string constants, just write the space. | |
339 | ||
340 | A backslash is allowed, and harmless, preceding any character without | |
341 | a special escape meaning; thus, @samp{?\+} is equivalent to @samp{?+}. | |
342 | There is no reason to add a backslash before most characters. However, | |
343 | you should add a backslash before any of the characters | |
344 | @samp{()\|;'`"#.,} to avoid confusing the Emacs commands for editing | |
345 | Lisp code. You can also add a backslash before whitespace characters such as | |
346 | space, tab, newline and formfeed. However, it is cleaner to use one of | |
347 | the easily readable escape sequences, such as @samp{\t} or @samp{\s}, | |
348 | instead of an actual whitespace character such as a tab or a space. | |
349 | (If you do write backslash followed by a space, you should write | |
350 | an extra space after the character constant to separate it from the | |
351 | following text.) | |
352 | ||
353 | @node General Escape Syntax | |
354 | @subsubsection General Escape Syntax | |
355 | ||
49ea0074 | 356 | In addition to the specific escape sequences for special important |
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357 | control characters, Emacs provides several types of escape syntax that |
358 | you can use to specify non-ASCII text characters. | |
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359 | |
360 | @cindex unicode character escape | |
fb080b28 | 361 | You can specify characters by their Unicode values. |
b8d4c8d0 | 362 | @code{?\u@var{nnnn}} represents a character that maps to the Unicode |
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363 | code point @samp{U+@var{nnnn}} (by convention, Unicode code points are |
364 | given in hexadecimal). There is a slightly different syntax for | |
365 | specifying characters with code points higher than | |
366 | @code{U+@var{ffff}}: @code{\U00@var{nnnnnn}} represents the character | |
f14589fa | 367 | whose code point is @samp{U+@var{nnnnnn}}. The Unicode Standard only |
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368 | defines code points up to @samp{U+@var{10ffff}}, so if you specify a |
369 | code point higher than that, Emacs signals an error. | |
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370 | |
371 | This peculiar and inconvenient syntax was adopted for compatibility | |
372 | with other programming languages. Unlike some other languages, Emacs | |
47dbc044 | 373 | Lisp supports this syntax only in character literals and strings. |
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374 | |
375 | @cindex @samp{\} in character constant | |
376 | @cindex backslash in character constant | |
377 | @cindex octal character code | |
378 | The most general read syntax for a character represents the | |
379 | character code in either octal or hex. To use octal, write a question | |
380 | mark followed by a backslash and the octal character code (up to three | |
381 | octal digits); thus, @samp{?\101} for the character @kbd{A}, | |
382 | @samp{?\001} for the character @kbd{C-a}, and @code{?\002} for the | |
383 | character @kbd{C-b}. Although this syntax can represent any | |
384 | @acronym{ASCII} character, it is preferred only when the precise octal | |
385 | value is more important than the @acronym{ASCII} representation. | |
386 | ||
387 | @example | |
388 | @group | |
389 | ?\012 @result{} 10 ?\n @result{} 10 ?\C-j @result{} 10 | |
390 | ?\101 @result{} 65 ?A @result{} 65 | |
391 | @end group | |
392 | @end example | |
393 | ||
394 | To use hex, write a question mark followed by a backslash, @samp{x}, | |
395 | and the hexadecimal character code. You can use any number of hex | |
396 | digits, so you can represent any character code in this way. | |
397 | Thus, @samp{?\x41} for the character @kbd{A}, @samp{?\x1} for the | |
398 | character @kbd{C-a}, and @code{?\x8e0} for the Latin-1 character | |
399 | @iftex | |
400 | @samp{@`a}. | |
401 | @end iftex | |
402 | @ifnottex | |
403 | @samp{a} with grave accent. | |
404 | @end ifnottex | |
405 | ||
406 | @node Ctl-Char Syntax | |
407 | @subsubsection Control-Character Syntax | |
408 | ||
409 | @cindex control characters | |
410 | Control characters can be represented using yet another read syntax. | |
411 | This consists of a question mark followed by a backslash, caret, and the | |
412 | corresponding non-control character, in either upper or lower case. For | |
413 | example, both @samp{?\^I} and @samp{?\^i} are valid read syntax for the | |
414 | character @kbd{C-i}, the character whose value is 9. | |
415 | ||
416 | Instead of the @samp{^}, you can use @samp{C-}; thus, @samp{?\C-i} is | |
417 | equivalent to @samp{?\^I} and to @samp{?\^i}: | |
418 | ||
419 | @example | |
420 | ?\^I @result{} 9 ?\C-I @result{} 9 | |
421 | @end example | |
422 | ||
423 | In strings and buffers, the only control characters allowed are those | |
424 | that exist in @acronym{ASCII}; but for keyboard input purposes, you can turn | |
425 | any character into a control character with @samp{C-}. The character | |
426 | codes for these non-@acronym{ASCII} control characters include the | |
427 | @tex | |
428 | @math{2^{26}} | |
429 | @end tex | |
430 | @ifnottex | |
431 | 2**26 | |
432 | @end ifnottex | |
433 | bit as well as the code for the corresponding non-control | |
434 | character. Ordinary terminals have no way of generating non-@acronym{ASCII} | |
435 | control characters, but you can generate them straightforwardly using X | |
436 | and other window systems. | |
437 | ||
438 | For historical reasons, Emacs treats the @key{DEL} character as | |
439 | the control equivalent of @kbd{?}: | |
440 | ||
441 | @example | |
442 | ?\^? @result{} 127 ?\C-? @result{} 127 | |
443 | @end example | |
444 | ||
445 | @noindent | |
446 | As a result, it is currently not possible to represent the character | |
447 | @kbd{Control-?}, which is a meaningful input character under X, using | |
448 | @samp{\C-}. It is not easy to change this, as various Lisp files refer | |
449 | to @key{DEL} in this way. | |
450 | ||
451 | For representing control characters to be found in files or strings, | |
452 | we recommend the @samp{^} syntax; for control characters in keyboard | |
453 | input, we prefer the @samp{C-} syntax. Which one you use does not | |
454 | affect the meaning of the program, but may guide the understanding of | |
455 | people who read it. | |
456 | ||
457 | @node Meta-Char Syntax | |
458 | @subsubsection Meta-Character Syntax | |
459 | ||
460 | @cindex meta characters | |
461 | A @dfn{meta character} is a character typed with the @key{META} | |
462 | modifier key. The integer that represents such a character has the | |
463 | @tex | |
464 | @math{2^{27}} | |
465 | @end tex | |
466 | @ifnottex | |
467 | 2**27 | |
468 | @end ifnottex | |
469 | bit set. We use high bits for this and other modifiers to make | |
470 | possible a wide range of basic character codes. | |
471 | ||
472 | In a string, the | |
473 | @tex | |
474 | @math{2^{7}} | |
475 | @end tex | |
476 | @ifnottex | |
477 | 2**7 | |
478 | @end ifnottex | |
479 | bit attached to an @acronym{ASCII} character indicates a meta | |
480 | character; thus, the meta characters that can fit in a string have | |
481 | codes in the range from 128 to 255, and are the meta versions of the | |
417f77e6 CY |
482 | ordinary @acronym{ASCII} characters. @xref{Strings of Events}, for |
483 | details about @key{META}-handling in strings. | |
b8d4c8d0 GM |
484 | |
485 | The read syntax for meta characters uses @samp{\M-}. For example, | |
486 | @samp{?\M-A} stands for @kbd{M-A}. You can use @samp{\M-} together with | |
487 | octal character codes (see below), with @samp{\C-}, or with any other | |
488 | syntax for a character. Thus, you can write @kbd{M-A} as @samp{?\M-A}, | |
489 | or as @samp{?\M-\101}. Likewise, you can write @kbd{C-M-b} as | |
490 | @samp{?\M-\C-b}, @samp{?\C-\M-b}, or @samp{?\M-\002}. | |
491 | ||
492 | @node Other Char Bits | |
493 | @subsubsection Other Character Modifier Bits | |
494 | ||
495 | The case of a graphic character is indicated by its character code; | |
496 | for example, @acronym{ASCII} distinguishes between the characters @samp{a} | |
497 | and @samp{A}. But @acronym{ASCII} has no way to represent whether a control | |
498 | character is upper case or lower case. Emacs uses the | |
499 | @tex | |
500 | @math{2^{25}} | |
501 | @end tex | |
502 | @ifnottex | |
503 | 2**25 | |
504 | @end ifnottex | |
505 | bit to indicate that the shift key was used in typing a control | |
506 | character. This distinction is possible only when you use X terminals | |
507 | or other special terminals; ordinary terminals do not report the | |
508 | distinction to the computer in any way. The Lisp syntax for | |
509 | the shift bit is @samp{\S-}; thus, @samp{?\C-\S-o} or @samp{?\C-\S-O} | |
510 | represents the shifted-control-o character. | |
511 | ||
512 | @cindex hyper characters | |
513 | @cindex super characters | |
514 | @cindex alt characters | |
515 | The X Window System defines three other | |
516 | @anchor{modifier bits}modifier bits that can be set | |
517 | in a character: @dfn{hyper}, @dfn{super} and @dfn{alt}. The syntaxes | |
518 | for these bits are @samp{\H-}, @samp{\s-} and @samp{\A-}. (Case is | |
519 | significant in these prefixes.) Thus, @samp{?\H-\M-\A-x} represents | |
520 | @kbd{Alt-Hyper-Meta-x}. (Note that @samp{\s} with no following @samp{-} | |
521 | represents the space character.) | |
522 | @tex | |
523 | Numerically, the bit values are @math{2^{22}} for alt, @math{2^{23}} | |
524 | for super and @math{2^{24}} for hyper. | |
525 | @end tex | |
526 | @ifnottex | |
527 | Numerically, the | |
528 | bit values are 2**22 for alt, 2**23 for super and 2**24 for hyper. | |
529 | @end ifnottex | |
530 | ||
531 | @node Symbol Type | |
532 | @subsection Symbol Type | |
533 | ||
534 | A @dfn{symbol} in GNU Emacs Lisp is an object with a name. The | |
535 | symbol name serves as the printed representation of the symbol. In | |
a2afc99d | 536 | ordinary Lisp use, with one single obarray (@pxref{Creating Symbols}), |
b8d4c8d0 GM |
537 | a symbol's name is unique---no two symbols have the same name. |
538 | ||
539 | A symbol can serve as a variable, as a function name, or to hold a | |
540 | property list. Or it may serve only to be distinct from all other Lisp | |
541 | objects, so that its presence in a data structure may be recognized | |
542 | reliably. In a given context, usually only one of these uses is | |
543 | intended. But you can use one symbol in all of these ways, | |
544 | independently. | |
545 | ||
546 | A symbol whose name starts with a colon (@samp{:}) is called a | |
547 | @dfn{keyword symbol}. These symbols automatically act as constants, and | |
548 | are normally used only by comparing an unknown symbol with a few | |
549 | specific alternatives. | |
550 | ||
551 | @cindex @samp{\} in symbols | |
552 | @cindex backslash in symbols | |
553 | A symbol name can contain any characters whatever. Most symbol names | |
554 | are written with letters, digits, and the punctuation characters | |
555 | @samp{-+=*/}. Such names require no special punctuation; the characters | |
556 | of the name suffice as long as the name does not look like a number. | |
557 | (If it does, write a @samp{\} at the beginning of the name to force | |
558 | interpretation as a symbol.) The characters @samp{_~!@@$%^&:<>@{@}?} are | |
559 | less often used but also require no special punctuation. Any other | |
560 | characters may be included in a symbol's name by escaping them with a | |
561 | backslash. In contrast to its use in strings, however, a backslash in | |
562 | the name of a symbol simply quotes the single character that follows the | |
563 | backslash. For example, in a string, @samp{\t} represents a tab | |
564 | character; in the name of a symbol, however, @samp{\t} merely quotes the | |
565 | letter @samp{t}. To have a symbol with a tab character in its name, you | |
566 | must actually use a tab (preceded with a backslash). But it's rare to | |
567 | do such a thing. | |
568 | ||
569 | @cindex CL note---case of letters | |
570 | @quotation | |
571 | @b{Common Lisp note:} In Common Lisp, lower case letters are always | |
572 | ``folded'' to upper case, unless they are explicitly escaped. In Emacs | |
573 | Lisp, upper case and lower case letters are distinct. | |
574 | @end quotation | |
575 | ||
576 | Here are several examples of symbol names. Note that the @samp{+} in | |
577 | the fifth example is escaped to prevent it from being read as a number. | |
578 | This is not necessary in the fourth example because the rest of the name | |
579 | makes it invalid as a number. | |
580 | ||
581 | @example | |
582 | @group | |
583 | foo ; @r{A symbol named @samp{foo}.} | |
584 | FOO ; @r{A symbol named @samp{FOO}, different from @samp{foo}.} | |
585 | char-to-string ; @r{A symbol named @samp{char-to-string}.} | |
586 | @end group | |
587 | @group | |
588 | 1+ ; @r{A symbol named @samp{1+}} | |
589 | ; @r{(not @samp{+1}, which is an integer).} | |
590 | @end group | |
591 | @group | |
592 | \+1 ; @r{A symbol named @samp{+1}} | |
593 | ; @r{(not a very readable name).} | |
594 | @end group | |
595 | @group | |
596 | \(*\ 1\ 2\) ; @r{A symbol named @samp{(* 1 2)} (a worse name).} | |
597 | @c the @'s in this next line use up three characters, hence the | |
598 | @c apparent misalignment of the comment. | |
599 | +-*/_~!@@$%^&=:<>@{@} ; @r{A symbol named @samp{+-*/_~!@@$%^&=:<>@{@}}.} | |
600 | ; @r{These characters need not be escaped.} | |
601 | @end group | |
602 | @end example | |
603 | ||
604 | @ifinfo | |
605 | @c This uses ``colon'' instead of a literal `:' because Info cannot | |
606 | @c cope with a `:' in a menu | |
607 | @cindex @samp{#@var{colon}} read syntax | |
608 | @end ifinfo | |
609 | @ifnotinfo | |
610 | @cindex @samp{#:} read syntax | |
611 | @end ifnotinfo | |
612 | Normally the Lisp reader interns all symbols (@pxref{Creating | |
613 | Symbols}). To prevent interning, you can write @samp{#:} before the | |
614 | name of the symbol. | |
615 | ||
616 | @node Sequence Type | |
617 | @subsection Sequence Types | |
618 | ||
619 | A @dfn{sequence} is a Lisp object that represents an ordered set of | |
620 | elements. There are two kinds of sequence in Emacs Lisp, lists and | |
621 | arrays. Thus, an object of type list or of type array is also | |
622 | considered a sequence. | |
623 | ||
624 | Arrays are further subdivided into strings, vectors, char-tables and | |
625 | bool-vectors. Vectors can hold elements of any type, but string | |
626 | elements must be characters, and bool-vector elements must be @code{t} | |
627 | or @code{nil}. Char-tables are like vectors except that they are | |
628 | indexed by any valid character code. The characters in a string can | |
629 | have text properties like characters in a buffer (@pxref{Text | |
630 | Properties}), but vectors do not support text properties, even when | |
631 | their elements happen to be characters. | |
632 | ||
633 | Lists, strings and the other array types are different, but they have | |
634 | important similarities. For example, all have a length @var{l}, and all | |
635 | have elements which can be indexed from zero to @var{l} minus one. | |
636 | Several functions, called sequence functions, accept any kind of | |
637 | sequence. For example, the function @code{elt} can be used to extract | |
638 | an element of a sequence, given its index. @xref{Sequences Arrays | |
639 | Vectors}. | |
640 | ||
641 | It is generally impossible to read the same sequence twice, since | |
642 | sequences are always created anew upon reading. If you read the read | |
643 | syntax for a sequence twice, you get two sequences with equal contents. | |
644 | There is one exception: the empty list @code{()} always stands for the | |
645 | same object, @code{nil}. | |
646 | ||
647 | @node Cons Cell Type | |
648 | @subsection Cons Cell and List Types | |
649 | @cindex address field of register | |
650 | @cindex decrement field of register | |
651 | @cindex pointers | |
652 | ||
653 | A @dfn{cons cell} is an object that consists of two slots, called the | |
654 | @sc{car} slot and the @sc{cdr} slot. Each slot can @dfn{hold} or | |
655 | @dfn{refer to} any Lisp object. We also say that ``the @sc{car} of | |
656 | this cons cell is'' whatever object its @sc{car} slot currently holds, | |
657 | and likewise for the @sc{cdr}. | |
658 | ||
659 | @quotation | |
660 | A note to C programmers: in Lisp, we do not distinguish between | |
661 | ``holding'' a value and ``pointing to'' the value, because pointers in | |
662 | Lisp are implicit. | |
663 | @end quotation | |
664 | ||
665 | A @dfn{list} is a series of cons cells, linked together so that the | |
666 | @sc{cdr} slot of each cons cell holds either the next cons cell or the | |
667 | empty list. The empty list is actually the symbol @code{nil}. | |
668 | @xref{Lists}, for functions that work on lists. Because most cons | |
669 | cells are used as part of lists, the phrase @dfn{list structure} has | |
670 | come to refer to any structure made out of cons cells. | |
671 | ||
672 | @cindex atoms | |
673 | Because cons cells are so central to Lisp, we also have a word for | |
674 | ``an object which is not a cons cell.'' These objects are called | |
675 | @dfn{atoms}. | |
676 | ||
677 | @cindex parenthesis | |
678 | @cindex @samp{(@dots{})} in lists | |
679 | The read syntax and printed representation for lists are identical, and | |
680 | consist of a left parenthesis, an arbitrary number of elements, and a | |
681 | right parenthesis. Here are examples of lists: | |
682 | ||
683 | @example | |
684 | (A 2 "A") ; @r{A list of three elements.} | |
685 | () ; @r{A list of no elements (the empty list).} | |
686 | nil ; @r{A list of no elements (the empty list).} | |
687 | ("A ()") ; @r{A list of one element: the string @code{"A ()"}.} | |
688 | (A ()) ; @r{A list of two elements: @code{A} and the empty list.} | |
689 | (A nil) ; @r{Equivalent to the previous.} | |
690 | ((A B C)) ; @r{A list of one element} | |
691 | ; @r{(which is a list of three elements).} | |
692 | @end example | |
693 | ||
694 | Upon reading, each object inside the parentheses becomes an element | |
695 | of the list. That is, a cons cell is made for each element. The | |
696 | @sc{car} slot of the cons cell holds the element, and its @sc{cdr} | |
697 | slot refers to the next cons cell of the list, which holds the next | |
698 | element in the list. The @sc{cdr} slot of the last cons cell is set to | |
699 | hold @code{nil}. | |
700 | ||
701 | The names @sc{car} and @sc{cdr} derive from the history of Lisp. The | |
702 | original Lisp implementation ran on an @w{IBM 704} computer which | |
703 | divided words into two parts, called the ``address'' part and the | |
704 | ``decrement''; @sc{car} was an instruction to extract the contents of | |
705 | the address part of a register, and @sc{cdr} an instruction to extract | |
706 | the contents of the decrement. By contrast, ``cons cells'' are named | |
707 | for the function @code{cons} that creates them, which in turn was named | |
708 | for its purpose, the construction of cells. | |
709 | ||
710 | @menu | |
711 | * Box Diagrams:: Drawing pictures of lists. | |
712 | * Dotted Pair Notation:: A general syntax for cons cells. | |
713 | * Association List Type:: A specially constructed list. | |
714 | @end menu | |
715 | ||
716 | @node Box Diagrams | |
717 | @subsubsection Drawing Lists as Box Diagrams | |
718 | @cindex box diagrams, for lists | |
719 | @cindex diagrams, boxed, for lists | |
720 | ||
721 | A list can be illustrated by a diagram in which the cons cells are | |
722 | shown as pairs of boxes, like dominoes. (The Lisp reader cannot read | |
723 | such an illustration; unlike the textual notation, which can be | |
724 | understood by both humans and computers, the box illustrations can be | |
725 | understood only by humans.) This picture represents the three-element | |
726 | list @code{(rose violet buttercup)}: | |
727 | ||
728 | @example | |
729 | @group | |
730 | --- --- --- --- --- --- | |
731 | | | |--> | | |--> | | |--> nil | |
732 | --- --- --- --- --- --- | |
733 | | | | | |
734 | | | | | |
735 | --> rose --> violet --> buttercup | |
736 | @end group | |
737 | @end example | |
738 | ||
739 | In this diagram, each box represents a slot that can hold or refer to | |
740 | any Lisp object. Each pair of boxes represents a cons cell. Each arrow | |
741 | represents a reference to a Lisp object, either an atom or another cons | |
742 | cell. | |
743 | ||
744 | In this example, the first box, which holds the @sc{car} of the first | |
745 | cons cell, refers to or ``holds'' @code{rose} (a symbol). The second | |
746 | box, holding the @sc{cdr} of the first cons cell, refers to the next | |
747 | pair of boxes, the second cons cell. The @sc{car} of the second cons | |
748 | cell is @code{violet}, and its @sc{cdr} is the third cons cell. The | |
749 | @sc{cdr} of the third (and last) cons cell is @code{nil}. | |
750 | ||
751 | Here is another diagram of the same list, @code{(rose violet | |
752 | buttercup)}, sketched in a different manner: | |
753 | ||
754 | @smallexample | |
755 | @group | |
756 | --------------- ---------------- ------------------- | |
757 | | car | cdr | | car | cdr | | car | cdr | | |
758 | | rose | o-------->| violet | o-------->| buttercup | nil | | |
759 | | | | | | | | | | | |
760 | --------------- ---------------- ------------------- | |
761 | @end group | |
762 | @end smallexample | |
763 | ||
764 | @cindex @code{nil} as a list | |
765 | @cindex empty list | |
766 | A list with no elements in it is the @dfn{empty list}; it is identical | |
767 | to the symbol @code{nil}. In other words, @code{nil} is both a symbol | |
768 | and a list. | |
769 | ||
770 | Here is the list @code{(A ())}, or equivalently @code{(A nil)}, | |
771 | depicted with boxes and arrows: | |
772 | ||
773 | @example | |
774 | @group | |
775 | --- --- --- --- | |
776 | | | |--> | | |--> nil | |
777 | --- --- --- --- | |
778 | | | | |
779 | | | | |
780 | --> A --> nil | |
781 | @end group | |
782 | @end example | |
783 | ||
784 | Here is a more complex illustration, showing the three-element list, | |
785 | @code{((pine needles) oak maple)}, the first element of which is a | |
786 | two-element list: | |
787 | ||
788 | @example | |
789 | @group | |
790 | --- --- --- --- --- --- | |
791 | | | |--> | | |--> | | |--> nil | |
792 | --- --- --- --- --- --- | |
793 | | | | | |
794 | | | | | |
795 | | --> oak --> maple | |
796 | | | |
797 | | --- --- --- --- | |
798 | --> | | |--> | | |--> nil | |
799 | --- --- --- --- | |
800 | | | | |
801 | | | | |
802 | --> pine --> needles | |
803 | @end group | |
804 | @end example | |
805 | ||
806 | The same list represented in the second box notation looks like this: | |
807 | ||
808 | @example | |
809 | @group | |
810 | -------------- -------------- -------------- | |
811 | | car | cdr | | car | cdr | | car | cdr | | |
812 | | o | o------->| oak | o------->| maple | nil | | |
813 | | | | | | | | | | | | |
814 | -- | --------- -------------- -------------- | |
815 | | | |
816 | | | |
817 | | -------------- ---------------- | |
818 | | | car | cdr | | car | cdr | | |
819 | ------>| pine | o------->| needles | nil | | |
820 | | | | | | | | |
821 | -------------- ---------------- | |
822 | @end group | |
823 | @end example | |
824 | ||
825 | @node Dotted Pair Notation | |
826 | @subsubsection Dotted Pair Notation | |
827 | @cindex dotted pair notation | |
828 | @cindex @samp{.} in lists | |
829 | ||
830 | @dfn{Dotted pair notation} is a general syntax for cons cells that | |
831 | represents the @sc{car} and @sc{cdr} explicitly. In this syntax, | |
832 | @code{(@var{a} .@: @var{b})} stands for a cons cell whose @sc{car} is | |
833 | the object @var{a} and whose @sc{cdr} is the object @var{b}. Dotted | |
834 | pair notation is more general than list syntax because the @sc{cdr} | |
835 | does not have to be a list. However, it is more cumbersome in cases | |
836 | where list syntax would work. In dotted pair notation, the list | |
837 | @samp{(1 2 3)} is written as @samp{(1 . (2 . (3 . nil)))}. For | |
838 | @code{nil}-terminated lists, you can use either notation, but list | |
839 | notation is usually clearer and more convenient. When printing a | |
840 | list, the dotted pair notation is only used if the @sc{cdr} of a cons | |
841 | cell is not a list. | |
842 | ||
843 | Here's an example using boxes to illustrate dotted pair notation. | |
844 | This example shows the pair @code{(rose . violet)}: | |
845 | ||
846 | @example | |
847 | @group | |
848 | --- --- | |
849 | | | |--> violet | |
850 | --- --- | |
851 | | | |
852 | | | |
853 | --> rose | |
854 | @end group | |
855 | @end example | |
856 | ||
857 | You can combine dotted pair notation with list notation to represent | |
858 | conveniently a chain of cons cells with a non-@code{nil} final @sc{cdr}. | |
859 | You write a dot after the last element of the list, followed by the | |
860 | @sc{cdr} of the final cons cell. For example, @code{(rose violet | |
861 | . buttercup)} is equivalent to @code{(rose . (violet . buttercup))}. | |
862 | The object looks like this: | |
863 | ||
864 | @example | |
865 | @group | |
866 | --- --- --- --- | |
867 | | | |--> | | |--> buttercup | |
868 | --- --- --- --- | |
869 | | | | |
870 | | | | |
871 | --> rose --> violet | |
872 | @end group | |
873 | @end example | |
874 | ||
875 | The syntax @code{(rose .@: violet .@: buttercup)} is invalid because | |
876 | there is nothing that it could mean. If anything, it would say to put | |
877 | @code{buttercup} in the @sc{cdr} of a cons cell whose @sc{cdr} is already | |
878 | used for @code{violet}. | |
879 | ||
880 | The list @code{(rose violet)} is equivalent to @code{(rose . (violet))}, | |
881 | and looks like this: | |
882 | ||
883 | @example | |
884 | @group | |
885 | --- --- --- --- | |
886 | | | |--> | | |--> nil | |
887 | --- --- --- --- | |
888 | | | | |
889 | | | | |
890 | --> rose --> violet | |
891 | @end group | |
892 | @end example | |
893 | ||
894 | Similarly, the three-element list @code{(rose violet buttercup)} | |
895 | is equivalent to @code{(rose . (violet . (buttercup)))}. | |
896 | @ifnottex | |
897 | It looks like this: | |
898 | ||
899 | @example | |
900 | @group | |
901 | --- --- --- --- --- --- | |
902 | | | |--> | | |--> | | |--> nil | |
903 | --- --- --- --- --- --- | |
904 | | | | | |
905 | | | | | |
906 | --> rose --> violet --> buttercup | |
907 | @end group | |
908 | @end example | |
909 | @end ifnottex | |
910 | ||
911 | @node Association List Type | |
912 | @comment node-name, next, previous, up | |
913 | @subsubsection Association List Type | |
914 | ||
915 | An @dfn{association list} or @dfn{alist} is a specially-constructed | |
916 | list whose elements are cons cells. In each element, the @sc{car} is | |
917 | considered a @dfn{key}, and the @sc{cdr} is considered an | |
918 | @dfn{associated value}. (In some cases, the associated value is stored | |
919 | in the @sc{car} of the @sc{cdr}.) Association lists are often used as | |
920 | stacks, since it is easy to add or remove associations at the front of | |
921 | the list. | |
922 | ||
923 | For example, | |
924 | ||
925 | @example | |
926 | (setq alist-of-colors | |
927 | '((rose . red) (lily . white) (buttercup . yellow))) | |
928 | @end example | |
929 | ||
930 | @noindent | |
931 | sets the variable @code{alist-of-colors} to an alist of three elements. In the | |
932 | first element, @code{rose} is the key and @code{red} is the value. | |
933 | ||
934 | @xref{Association Lists}, for a further explanation of alists and for | |
935 | functions that work on alists. @xref{Hash Tables}, for another kind of | |
936 | lookup table, which is much faster for handling a large number of keys. | |
937 | ||
938 | @node Array Type | |
939 | @subsection Array Type | |
940 | ||
941 | An @dfn{array} is composed of an arbitrary number of slots for | |
942 | holding or referring to other Lisp objects, arranged in a contiguous block of | |
943 | memory. Accessing any element of an array takes approximately the same | |
944 | amount of time. In contrast, accessing an element of a list requires | |
945 | time proportional to the position of the element in the list. (Elements | |
946 | at the end of a list take longer to access than elements at the | |
947 | beginning of a list.) | |
948 | ||
949 | Emacs defines four types of array: strings, vectors, bool-vectors, and | |
950 | char-tables. | |
951 | ||
952 | A string is an array of characters and a vector is an array of | |
953 | arbitrary objects. A bool-vector can hold only @code{t} or @code{nil}. | |
954 | These kinds of array may have any length up to the largest integer. | |
955 | Char-tables are sparse arrays indexed by any valid character code; they | |
956 | can hold arbitrary objects. | |
957 | ||
958 | The first element of an array has index zero, the second element has | |
959 | index 1, and so on. This is called @dfn{zero-origin} indexing. For | |
960 | example, an array of four elements has indices 0, 1, 2, @w{and 3}. The | |
961 | largest possible index value is one less than the length of the array. | |
962 | Once an array is created, its length is fixed. | |
963 | ||
964 | All Emacs Lisp arrays are one-dimensional. (Most other programming | |
965 | languages support multidimensional arrays, but they are not essential; | |
966 | you can get the same effect with nested one-dimensional arrays.) Each | |
967 | type of array has its own read syntax; see the following sections for | |
968 | details. | |
969 | ||
970 | The array type is a subset of the sequence type, and contains the | |
971 | string type, the vector type, the bool-vector type, and the char-table | |
972 | type. | |
973 | ||
974 | @node String Type | |
975 | @subsection String Type | |
976 | ||
977 | A @dfn{string} is an array of characters. Strings are used for many | |
978 | purposes in Emacs, as can be expected in a text editor; for example, as | |
979 | the names of Lisp symbols, as messages for the user, and to represent | |
980 | text extracted from buffers. Strings in Lisp are constants: evaluation | |
981 | of a string returns the same string. | |
982 | ||
983 | @xref{Strings and Characters}, for functions that operate on strings. | |
984 | ||
985 | @menu | |
0cc8c85a GM |
986 | * Syntax for Strings:: How to specify Lisp strings. |
987 | * Non-ASCII in Strings:: International characters in strings. | |
988 | * Nonprinting Characters:: Literal unprintable characters in strings. | |
989 | * Text Props and Strings:: Strings with text properties. | |
b8d4c8d0 GM |
990 | @end menu |
991 | ||
992 | @node Syntax for Strings | |
993 | @subsubsection Syntax for Strings | |
994 | ||
995 | @cindex @samp{"} in strings | |
996 | @cindex double-quote in strings | |
997 | @cindex @samp{\} in strings | |
998 | @cindex backslash in strings | |
2bd1f99a CY |
999 | The read syntax for a string is a double-quote, an arbitrary number |
1000 | of characters, and another double-quote, @code{"like this"}. To | |
1001 | include a double-quote in a string, precede it with a backslash; thus, | |
1002 | @code{"\""} is a string containing just a single double-quote | |
1003 | character. Likewise, you can include a backslash by preceding it with | |
1004 | another backslash, like this: @code{"this \\ is a single embedded | |
1005 | backslash"}. | |
b8d4c8d0 GM |
1006 | |
1007 | @cindex newline in strings | |
1008 | The newline character is not special in the read syntax for strings; | |
1009 | if you write a new line between the double-quotes, it becomes a | |
1010 | character in the string. But an escaped newline---one that is preceded | |
1011 | by @samp{\}---does not become part of the string; i.e., the Lisp reader | |
1012 | ignores an escaped newline while reading a string. An escaped space | |
1013 | @w{@samp{\ }} is likewise ignored. | |
1014 | ||
1015 | @example | |
1016 | "It is useful to include newlines | |
1017 | in documentation strings, | |
1018 | but the newline is \ | |
1019 | ignored if escaped." | |
1020 | @result{} "It is useful to include newlines | |
1021 | in documentation strings, | |
1022 | but the newline is ignored if escaped." | |
1023 | @end example | |
1024 | ||
1025 | @node Non-ASCII in Strings | |
1026 | @subsubsection Non-@acronym{ASCII} Characters in Strings | |
1027 | ||
1028 | You can include a non-@acronym{ASCII} international character in a string | |
1029 | constant by writing it literally. There are two text representations | |
1030 | for non-@acronym{ASCII} characters in Emacs strings (and in buffers): unibyte | |
1031 | and multibyte. If the string constant is read from a multibyte source, | |
1032 | such as a multibyte buffer or string, or a file that would be visited as | |
1033 | multibyte, then the character is read as a multibyte character, and that | |
1034 | makes the string multibyte. If the string constant is read from a | |
1035 | unibyte source, then the character is read as unibyte and that makes the | |
1036 | string unibyte. | |
1037 | ||
1038 | You can also represent a multibyte non-@acronym{ASCII} character with its | |
1039 | character code: use a hex escape, @samp{\x@var{nnnnnnn}}, with as many | |
1040 | digits as necessary. (Multibyte non-@acronym{ASCII} character codes are all | |
1041 | greater than 256.) Any character which is not a valid hex digit | |
1042 | terminates this construct. If the next character in the string could be | |
1043 | interpreted as a hex digit, write @w{@samp{\ }} (backslash and space) to | |
1044 | terminate the hex escape---for example, @w{@samp{\x8e0\ }} represents | |
1045 | one character, @samp{a} with grave accent. @w{@samp{\ }} in a string | |
1046 | constant is just like backslash-newline; it does not contribute any | |
1047 | character to the string, but it does terminate the preceding hex escape. | |
1048 | ||
1049 | You can represent a unibyte non-@acronym{ASCII} character with its | |
1050 | character code, which must be in the range from 128 (0200 octal) to | |
1051 | 255 (0377 octal). If you write all such character codes in octal and | |
1052 | the string contains no other characters forcing it to be multibyte, | |
1053 | this produces a unibyte string. However, using any hex escape in a | |
1054 | string (even for an @acronym{ASCII} character) forces the string to be | |
1055 | multibyte. | |
1056 | ||
1057 | You can also specify characters in a string by their numeric values | |
1058 | in Unicode, using @samp{\u} and @samp{\U} (@pxref{Character Type}). | |
1059 | ||
1060 | @xref{Text Representations}, for more information about the two | |
1061 | text representations. | |
1062 | ||
1063 | @node Nonprinting Characters | |
1064 | @subsubsection Nonprinting Characters in Strings | |
1065 | ||
1066 | You can use the same backslash escape-sequences in a string constant | |
1067 | as in character literals (but do not use the question mark that begins a | |
1068 | character constant). For example, you can write a string containing the | |
1069 | nonprinting characters tab and @kbd{C-a}, with commas and spaces between | |
1070 | them, like this: @code{"\t, \C-a"}. @xref{Character Type}, for a | |
1071 | description of the read syntax for characters. | |
1072 | ||
1073 | However, not all of the characters you can write with backslash | |
1074 | escape-sequences are valid in strings. The only control characters that | |
1075 | a string can hold are the @acronym{ASCII} control characters. Strings do not | |
1076 | distinguish case in @acronym{ASCII} control characters. | |
1077 | ||
1078 | Properly speaking, strings cannot hold meta characters; but when a | |
1079 | string is to be used as a key sequence, there is a special convention | |
1080 | that provides a way to represent meta versions of @acronym{ASCII} | |
1081 | characters in a string. If you use the @samp{\M-} syntax to indicate | |
1082 | a meta character in a string constant, this sets the | |
1083 | @tex | |
1084 | @math{2^{7}} | |
1085 | @end tex | |
1086 | @ifnottex | |
1087 | 2**7 | |
1088 | @end ifnottex | |
1089 | bit of the character in the string. If the string is used in | |
1090 | @code{define-key} or @code{lookup-key}, this numeric code is translated | |
1091 | into the equivalent meta character. @xref{Character Type}. | |
1092 | ||
1093 | Strings cannot hold characters that have the hyper, super, or alt | |
1094 | modifiers. | |
1095 | ||
1096 | @node Text Props and Strings | |
1097 | @subsubsection Text Properties in Strings | |
1098 | ||
6bfc8698 EZ |
1099 | @cindex @samp{#(} read syntax |
1100 | @cindex text properties, read syntax | |
b8d4c8d0 GM |
1101 | A string can hold properties for the characters it contains, in |
1102 | addition to the characters themselves. This enables programs that copy | |
1103 | text between strings and buffers to copy the text's properties with no | |
1104 | special effort. @xref{Text Properties}, for an explanation of what text | |
1105 | properties mean. Strings with text properties use a special read and | |
1106 | print syntax: | |
1107 | ||
1108 | @example | |
1109 | #("@var{characters}" @var{property-data}...) | |
1110 | @end example | |
1111 | ||
1112 | @noindent | |
1113 | where @var{property-data} consists of zero or more elements, in groups | |
1114 | of three as follows: | |
1115 | ||
1116 | @example | |
1117 | @var{beg} @var{end} @var{plist} | |
1118 | @end example | |
1119 | ||
1120 | @noindent | |
1121 | The elements @var{beg} and @var{end} are integers, and together specify | |
1122 | a range of indices in the string; @var{plist} is the property list for | |
1123 | that range. For example, | |
1124 | ||
1125 | @example | |
1126 | #("foo bar" 0 3 (face bold) 3 4 nil 4 7 (face italic)) | |
1127 | @end example | |
1128 | ||
1129 | @noindent | |
1130 | represents a string whose textual contents are @samp{foo bar}, in which | |
1131 | the first three characters have a @code{face} property with value | |
1132 | @code{bold}, and the last three have a @code{face} property with value | |
1133 | @code{italic}. (The fourth character has no text properties, so its | |
1134 | property list is @code{nil}. It is not actually necessary to mention | |
1135 | ranges with @code{nil} as the property list, since any characters not | |
1136 | mentioned in any range will default to having no properties.) | |
1137 | ||
1138 | @node Vector Type | |
1139 | @subsection Vector Type | |
1140 | ||
1141 | A @dfn{vector} is a one-dimensional array of elements of any type. It | |
1142 | takes a constant amount of time to access any element of a vector. (In | |
1143 | a list, the access time of an element is proportional to the distance of | |
1144 | the element from the beginning of the list.) | |
1145 | ||
1146 | The printed representation of a vector consists of a left square | |
1147 | bracket, the elements, and a right square bracket. This is also the | |
1148 | read syntax. Like numbers and strings, vectors are considered constants | |
1149 | for evaluation. | |
1150 | ||
1151 | @example | |
1152 | [1 "two" (three)] ; @r{A vector of three elements.} | |
1153 | @result{} [1 "two" (three)] | |
1154 | @end example | |
1155 | ||
1156 | @xref{Vectors}, for functions that work with vectors. | |
1157 | ||
1158 | @node Char-Table Type | |
1159 | @subsection Char-Table Type | |
1160 | ||
1161 | A @dfn{char-table} is a one-dimensional array of elements of any type, | |
1162 | indexed by character codes. Char-tables have certain extra features to | |
1163 | make them more useful for many jobs that involve assigning information | |
1164 | to character codes---for example, a char-table can have a parent to | |
1165 | inherit from, a default value, and a small number of extra slots to use for | |
1166 | special purposes. A char-table can also specify a single value for | |
1167 | a whole character set. | |
1168 | ||
1169 | The printed representation of a char-table is like a vector | |
1170 | except that there is an extra @samp{#^} at the beginning. | |
1171 | ||
1172 | @xref{Char-Tables}, for special functions to operate on char-tables. | |
1173 | Uses of char-tables include: | |
1174 | ||
1175 | @itemize @bullet | |
1176 | @item | |
1177 | Case tables (@pxref{Case Tables}). | |
1178 | ||
1179 | @item | |
1180 | Character category tables (@pxref{Categories}). | |
1181 | ||
1182 | @item | |
1183 | Display tables (@pxref{Display Tables}). | |
1184 | ||
1185 | @item | |
1186 | Syntax tables (@pxref{Syntax Tables}). | |
1187 | @end itemize | |
1188 | ||
1189 | @node Bool-Vector Type | |
1190 | @subsection Bool-Vector Type | |
1191 | ||
1192 | A @dfn{bool-vector} is a one-dimensional array of elements that | |
1193 | must be @code{t} or @code{nil}. | |
1194 | ||
1195 | The printed representation of a bool-vector is like a string, except | |
1196 | that it begins with @samp{#&} followed by the length. The string | |
1197 | constant that follows actually specifies the contents of the bool-vector | |
1198 | as a bitmap---each ``character'' in the string contains 8 bits, which | |
1199 | specify the next 8 elements of the bool-vector (1 stands for @code{t}, | |
1200 | and 0 for @code{nil}). The least significant bits of the character | |
1201 | correspond to the lowest indices in the bool-vector. | |
1202 | ||
1203 | @example | |
1204 | (make-bool-vector 3 t) | |
1205 | @result{} #&3"^G" | |
1206 | (make-bool-vector 3 nil) | |
1207 | @result{} #&3"^@@" | |
1208 | @end example | |
1209 | ||
1210 | @noindent | |
1211 | These results make sense, because the binary code for @samp{C-g} is | |
1212 | 111 and @samp{C-@@} is the character with code 0. | |
1213 | ||
1214 | If the length is not a multiple of 8, the printed representation | |
1215 | shows extra elements, but these extras really make no difference. For | |
1216 | instance, in the next example, the two bool-vectors are equal, because | |
1217 | only the first 3 bits are used: | |
1218 | ||
1219 | @example | |
1220 | (equal #&3"\377" #&3"\007") | |
1221 | @result{} t | |
1222 | @end example | |
1223 | ||
1224 | @node Hash Table Type | |
1225 | @subsection Hash Table Type | |
1226 | ||
1227 | A hash table is a very fast kind of lookup table, somewhat like an | |
1228 | alist in that it maps keys to corresponding values, but much faster. | |
16d1ff5f CY |
1229 | The printed representation of a hash table specifies its properties |
1230 | and contents, like this: | |
b8d4c8d0 GM |
1231 | |
1232 | @example | |
1233 | (make-hash-table) | |
16d1ff5f CY |
1234 | @result{} #s(hash-table size 65 test eql rehash-size 1.5 |
1235 | rehash-threshold 0.8 data ()) | |
b8d4c8d0 GM |
1236 | @end example |
1237 | ||
16d1ff5f CY |
1238 | @noindent |
1239 | @xref{Hash Tables}, for more information about hash tables. | |
1240 | ||
b8d4c8d0 GM |
1241 | @node Function Type |
1242 | @subsection Function Type | |
1243 | ||
1244 | Lisp functions are executable code, just like functions in other | |
1245 | programming languages. In Lisp, unlike most languages, functions are | |
1246 | also Lisp objects. A non-compiled function in Lisp is a lambda | |
1247 | expression: that is, a list whose first element is the symbol | |
1248 | @code{lambda} (@pxref{Lambda Expressions}). | |
1249 | ||
1250 | In most programming languages, it is impossible to have a function | |
1251 | without a name. In Lisp, a function has no intrinsic name. A lambda | |
1252 | expression can be called as a function even though it has no name; to | |
1253 | emphasize this, we also call it an @dfn{anonymous function} | |
1254 | (@pxref{Anonymous Functions}). A named function in Lisp is just a | |
1255 | symbol with a valid function in its function cell (@pxref{Defining | |
1256 | Functions}). | |
1257 | ||
1258 | Most of the time, functions are called when their names are written in | |
1259 | Lisp expressions in Lisp programs. However, you can construct or obtain | |
1260 | a function object at run time and then call it with the primitive | |
1261 | functions @code{funcall} and @code{apply}. @xref{Calling Functions}. | |
1262 | ||
1263 | @node Macro Type | |
1264 | @subsection Macro Type | |
1265 | ||
1266 | A @dfn{Lisp macro} is a user-defined construct that extends the Lisp | |
1267 | language. It is represented as an object much like a function, but with | |
1268 | different argument-passing semantics. A Lisp macro has the form of a | |
1269 | list whose first element is the symbol @code{macro} and whose @sc{cdr} | |
1270 | is a Lisp function object, including the @code{lambda} symbol. | |
1271 | ||
1272 | Lisp macro objects are usually defined with the built-in | |
1273 | @code{defmacro} function, but any list that begins with @code{macro} is | |
1274 | a macro as far as Emacs is concerned. @xref{Macros}, for an explanation | |
1275 | of how to write a macro. | |
1276 | ||
1277 | @strong{Warning}: Lisp macros and keyboard macros (@pxref{Keyboard | |
1278 | Macros}) are entirely different things. When we use the word ``macro'' | |
1279 | without qualification, we mean a Lisp macro, not a keyboard macro. | |
1280 | ||
1281 | @node Primitive Function Type | |
1282 | @subsection Primitive Function Type | |
45e46036 | 1283 | @cindex primitive function |
b8d4c8d0 GM |
1284 | |
1285 | A @dfn{primitive function} is a function callable from Lisp but | |
1286 | written in the C programming language. Primitive functions are also | |
1287 | called @dfn{subrs} or @dfn{built-in functions}. (The word ``subr'' is | |
1288 | derived from ``subroutine.'') Most primitive functions evaluate all | |
1289 | their arguments when they are called. A primitive function that does | |
1290 | not evaluate all its arguments is called a @dfn{special form} | |
1291 | (@pxref{Special Forms}).@refill | |
1292 | ||
1293 | It does not matter to the caller of a function whether the function is | |
1294 | primitive. However, this does matter if you try to redefine a primitive | |
1295 | with a function written in Lisp. The reason is that the primitive | |
1296 | function may be called directly from C code. Calls to the redefined | |
1297 | function from Lisp will use the new definition, but calls from C code | |
1298 | may still use the built-in definition. Therefore, @strong{we discourage | |
1299 | redefinition of primitive functions}. | |
1300 | ||
1301 | The term @dfn{function} refers to all Emacs functions, whether written | |
1302 | in Lisp or C. @xref{Function Type}, for information about the | |
1303 | functions written in Lisp. | |
1304 | ||
1305 | Primitive functions have no read syntax and print in hash notation | |
1306 | with the name of the subroutine. | |
1307 | ||
1308 | @example | |
1309 | @group | |
1310 | (symbol-function 'car) ; @r{Access the function cell} | |
1311 | ; @r{of the symbol.} | |
1312 | @result{} #<subr car> | |
1313 | (subrp (symbol-function 'car)) ; @r{Is this a primitive function?} | |
1314 | @result{} t ; @r{Yes.} | |
1315 | @end group | |
1316 | @end example | |
1317 | ||
1318 | @node Byte-Code Type | |
1319 | @subsection Byte-Code Function Type | |
1320 | ||
1321 | The byte compiler produces @dfn{byte-code function objects}. | |
1322 | Internally, a byte-code function object is much like a vector; however, | |
1323 | the evaluator handles this data type specially when it appears as a | |
1324 | function to be called. @xref{Byte Compilation}, for information about | |
1325 | the byte compiler. | |
1326 | ||
1327 | The printed representation and read syntax for a byte-code function | |
1328 | object is like that for a vector, with an additional @samp{#} before the | |
1329 | opening @samp{[}. | |
1330 | ||
1331 | @node Autoload Type | |
1332 | @subsection Autoload Type | |
1333 | ||
1334 | An @dfn{autoload object} is a list whose first element is the symbol | |
1335 | @code{autoload}. It is stored as the function definition of a symbol, | |
1336 | where it serves as a placeholder for the real definition. The autoload | |
1337 | object says that the real definition is found in a file of Lisp code | |
1338 | that should be loaded when necessary. It contains the name of the file, | |
1339 | plus some other information about the real definition. | |
1340 | ||
1341 | After the file has been loaded, the symbol should have a new function | |
1342 | definition that is not an autoload object. The new definition is then | |
1343 | called as if it had been there to begin with. From the user's point of | |
1344 | view, the function call works as expected, using the function definition | |
1345 | in the loaded file. | |
1346 | ||
1347 | An autoload object is usually created with the function | |
1348 | @code{autoload}, which stores the object in the function cell of a | |
1349 | symbol. @xref{Autoload}, for more details. | |
1350 | ||
1351 | @node Editing Types | |
1352 | @section Editing Types | |
1353 | @cindex editing types | |
1354 | ||
1355 | The types in the previous section are used for general programming | |
1356 | purposes, and most of them are common to most Lisp dialects. Emacs Lisp | |
1357 | provides several additional data types for purposes connected with | |
1358 | editing. | |
1359 | ||
1360 | @menu | |
1361 | * Buffer Type:: The basic object of editing. | |
1362 | * Marker Type:: A position in a buffer. | |
1363 | * Window Type:: Buffers are displayed in windows. | |
eba64e97 EZ |
1364 | * Frame Type:: Windows subdivide frames. |
1365 | * Terminal Type:: A terminal device displays frames. | |
b8d4c8d0 GM |
1366 | * Window Configuration Type:: Recording the way a frame is subdivided. |
1367 | * Frame Configuration Type:: Recording the status of all frames. | |
6fdbd4c6 | 1368 | * Process Type:: A subprocess of Emacs running on the underlying OS. |
b8d4c8d0 GM |
1369 | * Stream Type:: Receive or send characters. |
1370 | * Keymap Type:: What function a keystroke invokes. | |
1371 | * Overlay Type:: How an overlay is represented. | |
2bd1f99a | 1372 | * Font Type:: Fonts for displaying text. |
b8d4c8d0 GM |
1373 | @end menu |
1374 | ||
1375 | @node Buffer Type | |
1376 | @subsection Buffer Type | |
1377 | ||
1378 | A @dfn{buffer} is an object that holds text that can be edited | |
1379 | (@pxref{Buffers}). Most buffers hold the contents of a disk file | |
1380 | (@pxref{Files}) so they can be edited, but some are used for other | |
1381 | purposes. Most buffers are also meant to be seen by the user, and | |
2bd1f99a CY |
1382 | therefore displayed, at some time, in a window (@pxref{Windows}). But |
1383 | a buffer need not be displayed in any window. Each buffer has a | |
1384 | designated position called @dfn{point} (@pxref{Positions}); most | |
1385 | editing commands act on the contents of the current buffer in the | |
1386 | neighborhood of point. At any time, one buffer is the @dfn{current | |
1387 | buffer}. | |
b8d4c8d0 GM |
1388 | |
1389 | The contents of a buffer are much like a string, but buffers are not | |
1390 | used like strings in Emacs Lisp, and the available operations are | |
1391 | different. For example, you can insert text efficiently into an | |
1392 | existing buffer, altering the buffer's contents, whereas ``inserting'' | |
2bd1f99a CY |
1393 | text into a string requires concatenating substrings, and the result |
1394 | is an entirely new string object. | |
b8d4c8d0 | 1395 | |
2bd1f99a CY |
1396 | Many of the standard Emacs functions manipulate or test the |
1397 | characters in the current buffer; a whole chapter in this manual is | |
1398 | devoted to describing these functions (@pxref{Text}). | |
b8d4c8d0 GM |
1399 | |
1400 | Several other data structures are associated with each buffer: | |
1401 | ||
1402 | @itemize @bullet | |
1403 | @item | |
1404 | a local syntax table (@pxref{Syntax Tables}); | |
1405 | ||
1406 | @item | |
1407 | a local keymap (@pxref{Keymaps}); and, | |
1408 | ||
1409 | @item | |
1410 | a list of buffer-local variable bindings (@pxref{Buffer-Local Variables}). | |
1411 | ||
1412 | @item | |
1413 | overlays (@pxref{Overlays}). | |
1414 | ||
1415 | @item | |
1416 | text properties for the text in the buffer (@pxref{Text Properties}). | |
1417 | @end itemize | |
1418 | ||
1419 | @noindent | |
1420 | The local keymap and variable list contain entries that individually | |
1421 | override global bindings or values. These are used to customize the | |
1422 | behavior of programs in different buffers, without actually changing the | |
1423 | programs. | |
1424 | ||
1425 | A buffer may be @dfn{indirect}, which means it shares the text | |
1426 | of another buffer, but presents it differently. @xref{Indirect Buffers}. | |
1427 | ||
1428 | Buffers have no read syntax. They print in hash notation, showing the | |
1429 | buffer name. | |
1430 | ||
1431 | @example | |
1432 | @group | |
1433 | (current-buffer) | |
1434 | @result{} #<buffer objects.texi> | |
1435 | @end group | |
1436 | @end example | |
1437 | ||
1438 | @node Marker Type | |
1439 | @subsection Marker Type | |
1440 | ||
1441 | A @dfn{marker} denotes a position in a specific buffer. Markers | |
1442 | therefore have two components: one for the buffer, and one for the | |
1443 | position. Changes in the buffer's text automatically relocate the | |
1444 | position value as necessary to ensure that the marker always points | |
1445 | between the same two characters in the buffer. | |
1446 | ||
1447 | Markers have no read syntax. They print in hash notation, giving the | |
1448 | current character position and the name of the buffer. | |
1449 | ||
1450 | @example | |
1451 | @group | |
1452 | (point-marker) | |
1453 | @result{} #<marker at 10779 in objects.texi> | |
1454 | @end group | |
1455 | @end example | |
1456 | ||
1457 | @xref{Markers}, for information on how to test, create, copy, and move | |
1458 | markers. | |
1459 | ||
1460 | @node Window Type | |
1461 | @subsection Window Type | |
1462 | ||
1463 | A @dfn{window} describes the portion of the terminal screen that Emacs | |
1464 | uses to display a buffer. Every window has one associated buffer, whose | |
1465 | contents appear in the window. By contrast, a given buffer may appear | |
1466 | in one window, no window, or several windows. | |
1467 | ||
1468 | Though many windows may exist simultaneously, at any time one window | |
1469 | is designated the @dfn{selected window}. This is the window where the | |
1470 | cursor is (usually) displayed when Emacs is ready for a command. The | |
1471 | selected window usually displays the current buffer, but this is not | |
1472 | necessarily the case. | |
1473 | ||
1474 | Windows are grouped on the screen into frames; each window belongs to | |
1475 | one and only one frame. @xref{Frame Type}. | |
1476 | ||
1477 | Windows have no read syntax. They print in hash notation, giving the | |
1478 | window number and the name of the buffer being displayed. The window | |
1479 | numbers exist to identify windows uniquely, since the buffer displayed | |
1480 | in any given window can change frequently. | |
1481 | ||
1482 | @example | |
1483 | @group | |
1484 | (selected-window) | |
1485 | @result{} #<window 1 on objects.texi> | |
1486 | @end group | |
1487 | @end example | |
1488 | ||
1489 | @xref{Windows}, for a description of the functions that work on windows. | |
1490 | ||
1491 | @node Frame Type | |
1492 | @subsection Frame Type | |
1493 | ||
1494 | A @dfn{frame} is a screen area that contains one or more Emacs | |
1495 | windows; we also use the term ``frame'' to refer to the Lisp object | |
1496 | that Emacs uses to refer to the screen area. | |
1497 | ||
1498 | Frames have no read syntax. They print in hash notation, giving the | |
1499 | frame's title, plus its address in core (useful to identify the frame | |
1500 | uniquely). | |
1501 | ||
1502 | @example | |
1503 | @group | |
1504 | (selected-frame) | |
1505 | @result{} #<frame emacs@@psilocin.gnu.org 0xdac80> | |
1506 | @end group | |
1507 | @end example | |
1508 | ||
1509 | @xref{Frames}, for a description of the functions that work on frames. | |
1510 | ||
eba64e97 EZ |
1511 | @node Terminal Type |
1512 | @subsection Terminal Type | |
1513 | @cindex terminal type | |
1514 | ||
1515 | A @dfn{terminal} is a device capable of displaying one or more | |
1516 | Emacs frames (@pxref{Frame Type}). | |
1517 | ||
1518 | Terminals have no read syntax. They print in hash notation giving | |
1519 | the terminal's ordinal number and its TTY device file name. | |
1520 | ||
1521 | @example | |
1522 | @group | |
1523 | (get-device-terminal nil) | |
1524 | @result{} #<terminal 1 on /dev/tty> | |
1525 | @end group | |
1526 | @end example | |
1527 | ||
1528 | @c FIXME: add an xref to where terminal-related primitives are described. | |
1529 | ||
b8d4c8d0 GM |
1530 | @node Window Configuration Type |
1531 | @subsection Window Configuration Type | |
1532 | @cindex window layout in a frame | |
1533 | ||
1534 | A @dfn{window configuration} stores information about the positions, | |
1535 | sizes, and contents of the windows in a frame, so you can recreate the | |
1536 | same arrangement of windows later. | |
1537 | ||
1538 | Window configurations do not have a read syntax; their print syntax | |
1539 | looks like @samp{#<window-configuration>}. @xref{Window | |
1540 | Configurations}, for a description of several functions related to | |
1541 | window configurations. | |
1542 | ||
1543 | @node Frame Configuration Type | |
1544 | @subsection Frame Configuration Type | |
1545 | @cindex screen layout | |
1546 | @cindex window layout, all frames | |
1547 | ||
1548 | A @dfn{frame configuration} stores information about the positions, | |
2bd1f99a CY |
1549 | sizes, and contents of the windows in all frames. It is not a |
1550 | primitive type---it is actually a list whose @sc{car} is | |
1551 | @code{frame-configuration} and whose @sc{cdr} is an alist. Each alist | |
1552 | element describes one frame, which appears as the @sc{car} of that | |
1553 | element. | |
b8d4c8d0 GM |
1554 | |
1555 | @xref{Frame Configurations}, for a description of several functions | |
1556 | related to frame configurations. | |
1557 | ||
1558 | @node Process Type | |
1559 | @subsection Process Type | |
1560 | ||
1561 | The word @dfn{process} usually means a running program. Emacs itself | |
1562 | runs in a process of this sort. However, in Emacs Lisp, a process is a | |
1563 | Lisp object that designates a subprocess created by the Emacs process. | |
1564 | Programs such as shells, GDB, ftp, and compilers, running in | |
1565 | subprocesses of Emacs, extend the capabilities of Emacs. | |
1566 | ||
1567 | An Emacs subprocess takes textual input from Emacs and returns textual | |
1568 | output to Emacs for further manipulation. Emacs can also send signals | |
1569 | to the subprocess. | |
1570 | ||
1571 | Process objects have no read syntax. They print in hash notation, | |
1572 | giving the name of the process: | |
1573 | ||
1574 | @example | |
1575 | @group | |
1576 | (process-list) | |
1577 | @result{} (#<process shell>) | |
1578 | @end group | |
1579 | @end example | |
1580 | ||
1581 | @xref{Processes}, for information about functions that create, delete, | |
1582 | return information about, send input or signals to, and receive output | |
1583 | from processes. | |
1584 | ||
1585 | @node Stream Type | |
1586 | @subsection Stream Type | |
1587 | ||
1588 | A @dfn{stream} is an object that can be used as a source or sink for | |
1589 | characters---either to supply characters for input or to accept them as | |
1590 | output. Many different types can be used this way: markers, buffers, | |
1591 | strings, and functions. Most often, input streams (character sources) | |
1592 | obtain characters from the keyboard, a buffer, or a file, and output | |
1593 | streams (character sinks) send characters to a buffer, such as a | |
1594 | @file{*Help*} buffer, or to the echo area. | |
1595 | ||
1596 | The object @code{nil}, in addition to its other meanings, may be used | |
1597 | as a stream. It stands for the value of the variable | |
1598 | @code{standard-input} or @code{standard-output}. Also, the object | |
1599 | @code{t} as a stream specifies input using the minibuffer | |
1600 | (@pxref{Minibuffers}) or output in the echo area (@pxref{The Echo | |
1601 | Area}). | |
1602 | ||
1603 | Streams have no special printed representation or read syntax, and | |
1604 | print as whatever primitive type they are. | |
1605 | ||
1606 | @xref{Read and Print}, for a description of functions | |
1607 | related to streams, including parsing and printing functions. | |
1608 | ||
1609 | @node Keymap Type | |
1610 | @subsection Keymap Type | |
1611 | ||
1612 | A @dfn{keymap} maps keys typed by the user to commands. This mapping | |
1613 | controls how the user's command input is executed. A keymap is actually | |
1614 | a list whose @sc{car} is the symbol @code{keymap}. | |
1615 | ||
1616 | @xref{Keymaps}, for information about creating keymaps, handling prefix | |
1617 | keys, local as well as global keymaps, and changing key bindings. | |
1618 | ||
1619 | @node Overlay Type | |
1620 | @subsection Overlay Type | |
1621 | ||
1622 | An @dfn{overlay} specifies properties that apply to a part of a | |
1623 | buffer. Each overlay applies to a specified range of the buffer, and | |
1624 | contains a property list (a list whose elements are alternating property | |
1625 | names and values). Overlay properties are used to present parts of the | |
1626 | buffer temporarily in a different display style. Overlays have no read | |
1627 | syntax, and print in hash notation, giving the buffer name and range of | |
1628 | positions. | |
1629 | ||
1630 | @xref{Overlays}, for how to create and use overlays. | |
1631 | ||
2bd1f99a CY |
1632 | @node Font Type |
1633 | @subsection Font Type | |
1634 | ||
1635 | A @dfn{font} specifies how to display text on a graphical terminal. | |
1636 | There are actually three separate font types---@dfn{font objects}, | |
1637 | @dfn{font specs}, and @dfn{font entities}---each of which has slightly | |
1638 | different properties. None of them have a read syntax; their print | |
1639 | syntax looks like @samp{#<font-object>}, @samp{#<font-spec>}, and | |
1640 | @samp{#<font-entity>} respectively. @xref{Low-Level Font}, for a | |
1641 | description of these Lisp objects. | |
1642 | ||
b8d4c8d0 GM |
1643 | @node Circular Objects |
1644 | @section Read Syntax for Circular Objects | |
1645 | @cindex circular structure, read syntax | |
1646 | @cindex shared structure, read syntax | |
1647 | @cindex @samp{#@var{n}=} read syntax | |
1648 | @cindex @samp{#@var{n}#} read syntax | |
1649 | ||
1650 | To represent shared or circular structures within a complex of Lisp | |
1651 | objects, you can use the reader constructs @samp{#@var{n}=} and | |
1652 | @samp{#@var{n}#}. | |
1653 | ||
1654 | Use @code{#@var{n}=} before an object to label it for later reference; | |
1655 | subsequently, you can use @code{#@var{n}#} to refer the same object in | |
1656 | another place. Here, @var{n} is some integer. For example, here is how | |
1657 | to make a list in which the first element recurs as the third element: | |
1658 | ||
1659 | @example | |
1660 | (#1=(a) b #1#) | |
1661 | @end example | |
1662 | ||
1663 | @noindent | |
1664 | This differs from ordinary syntax such as this | |
1665 | ||
1666 | @example | |
1667 | ((a) b (a)) | |
1668 | @end example | |
1669 | ||
1670 | @noindent | |
1671 | which would result in a list whose first and third elements | |
1672 | look alike but are not the same Lisp object. This shows the difference: | |
1673 | ||
1674 | @example | |
1675 | (prog1 nil | |
1676 | (setq x '(#1=(a) b #1#))) | |
1677 | (eq (nth 0 x) (nth 2 x)) | |
1678 | @result{} t | |
1679 | (setq x '((a) b (a))) | |
1680 | (eq (nth 0 x) (nth 2 x)) | |
1681 | @result{} nil | |
1682 | @end example | |
1683 | ||
1684 | You can also use the same syntax to make a circular structure, which | |
1685 | appears as an ``element'' within itself. Here is an example: | |
1686 | ||
1687 | @example | |
1688 | #1=(a #1#) | |
1689 | @end example | |
1690 | ||
1691 | @noindent | |
1692 | This makes a list whose second element is the list itself. | |
1693 | Here's how you can see that it really works: | |
1694 | ||
1695 | @example | |
1696 | (prog1 nil | |
1697 | (setq x '#1=(a #1#))) | |
1698 | (eq x (cadr x)) | |
1699 | @result{} t | |
1700 | @end example | |
1701 | ||
1702 | The Lisp printer can produce this syntax to record circular and shared | |
1703 | structure in a Lisp object, if you bind the variable @code{print-circle} | |
1704 | to a non-@code{nil} value. @xref{Output Variables}. | |
1705 | ||
1706 | @node Type Predicates | |
1707 | @section Type Predicates | |
1708 | @cindex type checking | |
1709 | @kindex wrong-type-argument | |
1710 | ||
1711 | The Emacs Lisp interpreter itself does not perform type checking on | |
1712 | the actual arguments passed to functions when they are called. It could | |
1713 | not do so, since function arguments in Lisp do not have declared data | |
1714 | types, as they do in other programming languages. It is therefore up to | |
1715 | the individual function to test whether each actual argument belongs to | |
1716 | a type that the function can use. | |
1717 | ||
1718 | All built-in functions do check the types of their actual arguments | |
1719 | when appropriate, and signal a @code{wrong-type-argument} error if an | |
1720 | argument is of the wrong type. For example, here is what happens if you | |
1721 | pass an argument to @code{+} that it cannot handle: | |
1722 | ||
1723 | @example | |
1724 | @group | |
1725 | (+ 2 'a) | |
1726 | @error{} Wrong type argument: number-or-marker-p, a | |
1727 | @end group | |
1728 | @end example | |
1729 | ||
1730 | @cindex type predicates | |
1731 | @cindex testing types | |
1732 | If you want your program to handle different types differently, you | |
1733 | must do explicit type checking. The most common way to check the type | |
1734 | of an object is to call a @dfn{type predicate} function. Emacs has a | |
1735 | type predicate for each type, as well as some predicates for | |
1736 | combinations of types. | |
1737 | ||
1738 | A type predicate function takes one argument; it returns @code{t} if | |
1739 | the argument belongs to the appropriate type, and @code{nil} otherwise. | |
1740 | Following a general Lisp convention for predicate functions, most type | |
1741 | predicates' names end with @samp{p}. | |
1742 | ||
1743 | Here is an example which uses the predicates @code{listp} to check for | |
1744 | a list and @code{symbolp} to check for a symbol. | |
1745 | ||
1746 | @example | |
1747 | (defun add-on (x) | |
1748 | (cond ((symbolp x) | |
1749 | ;; If X is a symbol, put it on LIST. | |
1750 | (setq list (cons x list))) | |
1751 | ((listp x) | |
1752 | ;; If X is a list, add its elements to LIST. | |
1753 | (setq list (append x list))) | |
1754 | (t | |
1755 | ;; We handle only symbols and lists. | |
1756 | (error "Invalid argument %s in add-on" x)))) | |
1757 | @end example | |
1758 | ||
1759 | Here is a table of predefined type predicates, in alphabetical order, | |
1760 | with references to further information. | |
1761 | ||
1762 | @table @code | |
1763 | @item atom | |
1764 | @xref{List-related Predicates, atom}. | |
1765 | ||
1766 | @item arrayp | |
1767 | @xref{Array Functions, arrayp}. | |
1768 | ||
1769 | @item bool-vector-p | |
1770 | @xref{Bool-Vectors, bool-vector-p}. | |
1771 | ||
1772 | @item bufferp | |
1773 | @xref{Buffer Basics, bufferp}. | |
1774 | ||
1775 | @item byte-code-function-p | |
1776 | @xref{Byte-Code Type, byte-code-function-p}. | |
1777 | ||
1778 | @item case-table-p | |
1779 | @xref{Case Tables, case-table-p}. | |
1780 | ||
1781 | @item char-or-string-p | |
1782 | @xref{Predicates for Strings, char-or-string-p}. | |
1783 | ||
1784 | @item char-table-p | |
1785 | @xref{Char-Tables, char-table-p}. | |
1786 | ||
1787 | @item commandp | |
1788 | @xref{Interactive Call, commandp}. | |
1789 | ||
1790 | @item consp | |
1791 | @xref{List-related Predicates, consp}. | |
1792 | ||
1793 | @item display-table-p | |
1794 | @xref{Display Tables, display-table-p}. | |
1795 | ||
1796 | @item floatp | |
1797 | @xref{Predicates on Numbers, floatp}. | |
1798 | ||
2bd1f99a CY |
1799 | @item fontp |
1800 | @xref{Low-Level Font}. | |
1801 | ||
b8d4c8d0 GM |
1802 | @item frame-configuration-p |
1803 | @xref{Frame Configurations, frame-configuration-p}. | |
1804 | ||
1805 | @item frame-live-p | |
1806 | @xref{Deleting Frames, frame-live-p}. | |
1807 | ||
1808 | @item framep | |
1809 | @xref{Frames, framep}. | |
1810 | ||
1811 | @item functionp | |
1812 | @xref{Functions, functionp}. | |
1813 | ||
1814 | @item hash-table-p | |
1815 | @xref{Other Hash, hash-table-p}. | |
1816 | ||
1817 | @item integer-or-marker-p | |
1818 | @xref{Predicates on Markers, integer-or-marker-p}. | |
1819 | ||
1820 | @item integerp | |
1821 | @xref{Predicates on Numbers, integerp}. | |
1822 | ||
1823 | @item keymapp | |
1824 | @xref{Creating Keymaps, keymapp}. | |
1825 | ||
1826 | @item keywordp | |
1827 | @xref{Constant Variables}. | |
1828 | ||
1829 | @item listp | |
1830 | @xref{List-related Predicates, listp}. | |
1831 | ||
1832 | @item markerp | |
1833 | @xref{Predicates on Markers, markerp}. | |
1834 | ||
1835 | @item wholenump | |
1836 | @xref{Predicates on Numbers, wholenump}. | |
1837 | ||
1838 | @item nlistp | |
1839 | @xref{List-related Predicates, nlistp}. | |
1840 | ||
1841 | @item numberp | |
1842 | @xref{Predicates on Numbers, numberp}. | |
1843 | ||
1844 | @item number-or-marker-p | |
1845 | @xref{Predicates on Markers, number-or-marker-p}. | |
1846 | ||
1847 | @item overlayp | |
1848 | @xref{Overlays, overlayp}. | |
1849 | ||
1850 | @item processp | |
1851 | @xref{Processes, processp}. | |
1852 | ||
1853 | @item sequencep | |
1854 | @xref{Sequence Functions, sequencep}. | |
1855 | ||
1856 | @item stringp | |
1857 | @xref{Predicates for Strings, stringp}. | |
1858 | ||
1859 | @item subrp | |
1860 | @xref{Function Cells, subrp}. | |
1861 | ||
1862 | @item symbolp | |
1863 | @xref{Symbols, symbolp}. | |
1864 | ||
1865 | @item syntax-table-p | |
1866 | @xref{Syntax Tables, syntax-table-p}. | |
1867 | ||
1868 | @item user-variable-p | |
1869 | @xref{Defining Variables, user-variable-p}. | |
1870 | ||
1871 | @item vectorp | |
1872 | @xref{Vectors, vectorp}. | |
1873 | ||
1874 | @item window-configuration-p | |
1875 | @xref{Window Configurations, window-configuration-p}. | |
1876 | ||
1877 | @item window-live-p | |
1878 | @xref{Deleting Windows, window-live-p}. | |
1879 | ||
1880 | @item windowp | |
1881 | @xref{Basic Windows, windowp}. | |
1882 | ||
1883 | @item booleanp | |
1884 | @xref{nil and t, booleanp}. | |
1885 | ||
1886 | @item string-or-null-p | |
1887 | @xref{Predicates for Strings, string-or-null-p}. | |
1888 | @end table | |
1889 | ||
1890 | The most general way to check the type of an object is to call the | |
1891 | function @code{type-of}. Recall that each object belongs to one and | |
1892 | only one primitive type; @code{type-of} tells you which one (@pxref{Lisp | |
1893 | Data Types}). But @code{type-of} knows nothing about non-primitive | |
1894 | types. In most cases, it is more convenient to use type predicates than | |
1895 | @code{type-of}. | |
1896 | ||
1897 | @defun type-of object | |
1898 | This function returns a symbol naming the primitive type of | |
2bd1f99a CY |
1899 | @var{object}. The value is one of the symbols @code{bool-vector}, |
1900 | @code{buffer}, @code{char-table}, @code{compiled-function}, | |
1901 | @code{cons}, @code{float}, @code{font-entity}, @code{font-object}, | |
1902 | @code{font-spec}, @code{frame}, @code{hash-table}, @code{integer}, | |
1903 | @code{marker}, @code{overlay}, @code{process}, @code{string}, | |
1904 | @code{subr}, @code{symbol}, @code{vector}, @code{window}, or | |
b8d4c8d0 GM |
1905 | @code{window-configuration}. |
1906 | ||
1907 | @example | |
1908 | (type-of 1) | |
1909 | @result{} integer | |
1910 | @group | |
1911 | (type-of 'nil) | |
1912 | @result{} symbol | |
1913 | (type-of '()) ; @r{@code{()} is @code{nil}.} | |
1914 | @result{} symbol | |
1915 | (type-of '(x)) | |
1916 | @result{} cons | |
1917 | @end group | |
1918 | @end example | |
1919 | @end defun | |
1920 | ||
1921 | @node Equality Predicates | |
1922 | @section Equality Predicates | |
1923 | @cindex equality | |
1924 | ||
d8c8e45a | 1925 | Here we describe functions that test for equality between any two |
f2b480f4 | 1926 | objects. Other functions test equality of contents between objects of specific |
b8d4c8d0 GM |
1927 | types, e.g., strings. For these predicates, see the appropriate chapter |
1928 | describing the data type. | |
1929 | ||
1930 | @defun eq object1 object2 | |
1931 | This function returns @code{t} if @var{object1} and @var{object2} are | |
1932 | the same object, @code{nil} otherwise. | |
1933 | ||
1934 | @code{eq} returns @code{t} if @var{object1} and @var{object2} are | |
1935 | integers with the same value. Also, since symbol names are normally | |
1936 | unique, if the arguments are symbols with the same name, they are | |
1937 | @code{eq}. For other types (e.g., lists, vectors, strings), two | |
1938 | arguments with the same contents or elements are not necessarily | |
1939 | @code{eq} to each other: they are @code{eq} only if they are the same | |
1940 | object, meaning that a change in the contents of one will be reflected | |
1941 | by the same change in the contents of the other. | |
1942 | ||
1943 | @example | |
1944 | @group | |
1945 | (eq 'foo 'foo) | |
1946 | @result{} t | |
1947 | @end group | |
1948 | ||
1949 | @group | |
1950 | (eq 456 456) | |
1951 | @result{} t | |
1952 | @end group | |
1953 | ||
1954 | @group | |
1955 | (eq "asdf" "asdf") | |
1956 | @result{} nil | |
1957 | @end group | |
1958 | ||
f2b480f4 RS |
1959 | @group |
1960 | (eq "" "") | |
1961 | @result{} t | |
1962 | ;; @r{This exception occurs because Emacs Lisp} | |
1963 | ;; @r{makes just one multibyte empty string, to save space.} | |
1964 | @end group | |
1965 | ||
b8d4c8d0 GM |
1966 | @group |
1967 | (eq '(1 (2 (3))) '(1 (2 (3)))) | |
1968 | @result{} nil | |
1969 | @end group | |
1970 | ||
1971 | @group | |
1972 | (setq foo '(1 (2 (3)))) | |
1973 | @result{} (1 (2 (3))) | |
1974 | (eq foo foo) | |
1975 | @result{} t | |
1976 | (eq foo '(1 (2 (3)))) | |
1977 | @result{} nil | |
1978 | @end group | |
1979 | ||
1980 | @group | |
1981 | (eq [(1 2) 3] [(1 2) 3]) | |
1982 | @result{} nil | |
1983 | @end group | |
1984 | ||
1985 | @group | |
1986 | (eq (point-marker) (point-marker)) | |
1987 | @result{} nil | |
1988 | @end group | |
1989 | @end example | |
1990 | ||
1991 | The @code{make-symbol} function returns an uninterned symbol, distinct | |
1992 | from the symbol that is used if you write the name in a Lisp expression. | |
1993 | Distinct symbols with the same name are not @code{eq}. @xref{Creating | |
1994 | Symbols}. | |
1995 | ||
1996 | @example | |
1997 | @group | |
1998 | (eq (make-symbol "foo") 'foo) | |
1999 | @result{} nil | |
2000 | @end group | |
2001 | @end example | |
2002 | @end defun | |
2003 | ||
2004 | @defun equal object1 object2 | |
2005 | This function returns @code{t} if @var{object1} and @var{object2} have | |
2006 | equal components, @code{nil} otherwise. Whereas @code{eq} tests if its | |
2007 | arguments are the same object, @code{equal} looks inside nonidentical | |
2008 | arguments to see if their elements or contents are the same. So, if two | |
2009 | objects are @code{eq}, they are @code{equal}, but the converse is not | |
2010 | always true. | |
2011 | ||
2012 | @example | |
2013 | @group | |
2014 | (equal 'foo 'foo) | |
2015 | @result{} t | |
2016 | @end group | |
2017 | ||
2018 | @group | |
2019 | (equal 456 456) | |
2020 | @result{} t | |
2021 | @end group | |
2022 | ||
2023 | @group | |
2024 | (equal "asdf" "asdf") | |
2025 | @result{} t | |
2026 | @end group | |
2027 | @group | |
2028 | (eq "asdf" "asdf") | |
2029 | @result{} nil | |
2030 | @end group | |
2031 | ||
2032 | @group | |
2033 | (equal '(1 (2 (3))) '(1 (2 (3)))) | |
2034 | @result{} t | |
2035 | @end group | |
2036 | @group | |
2037 | (eq '(1 (2 (3))) '(1 (2 (3)))) | |
2038 | @result{} nil | |
2039 | @end group | |
2040 | ||
2041 | @group | |
2042 | (equal [(1 2) 3] [(1 2) 3]) | |
2043 | @result{} t | |
2044 | @end group | |
2045 | @group | |
2046 | (eq [(1 2) 3] [(1 2) 3]) | |
2047 | @result{} nil | |
2048 | @end group | |
2049 | ||
2050 | @group | |
2051 | (equal (point-marker) (point-marker)) | |
2052 | @result{} t | |
2053 | @end group | |
2054 | ||
2055 | @group | |
2056 | (eq (point-marker) (point-marker)) | |
2057 | @result{} nil | |
2058 | @end group | |
2059 | @end example | |
2060 | ||
2061 | Comparison of strings is case-sensitive, but does not take account of | |
bfffe0b1 GM |
2062 | text properties---it compares only the characters in the strings. Use |
2063 | @code{equal-including-properties} to also compare text properties. For | |
b8d4c8d0 GM |
2064 | technical reasons, a unibyte string and a multibyte string are |
2065 | @code{equal} if and only if they contain the same sequence of | |
2066 | character codes and all these codes are either in the range 0 through | |
2067 | 127 (@acronym{ASCII}) or 160 through 255 (@code{eight-bit-graphic}). | |
2068 | (@pxref{Text Representations}). | |
2069 | ||
2070 | @example | |
2071 | @group | |
2072 | (equal "asdf" "ASDF") | |
2073 | @result{} nil | |
2074 | @end group | |
2075 | @end example | |
2076 | ||
2077 | However, two distinct buffers are never considered @code{equal}, even if | |
2078 | their textual contents are the same. | |
2079 | @end defun | |
2080 | ||
2081 | The test for equality is implemented recursively; for example, given | |
2082 | two cons cells @var{x} and @var{y}, @code{(equal @var{x} @var{y})} | |
2083 | returns @code{t} if and only if both the expressions below return | |
2084 | @code{t}: | |
2085 | ||
2086 | @example | |
2087 | (equal (car @var{x}) (car @var{y})) | |
2088 | (equal (cdr @var{x}) (cdr @var{y})) | |
2089 | @end example | |
2090 | ||
2091 | Because of this recursive method, circular lists may therefore cause | |
2092 | infinite recursion (leading to an error). | |
2093 | ||
46c0aa17 GM |
2094 | @defun equal-including-properties object1 object2 |
2095 | This function behaves like @code{equal} in all cases but also requires | |
2096 | that for two strings to be equal, they have the same text properties. | |
2097 | ||
2098 | @example | |
2099 | @group | |
2100 | (equal "asdf" (propertize "asdf" '(asdf t))) | |
2101 | @result{} t | |
2102 | @end group | |
2103 | @group | |
2104 | (equal-including-properties "asdf" | |
2105 | (propertize "asdf" '(asdf t))) | |
2106 | @result{} nil | |
2107 | @end group | |
2108 | @end example | |
2109 | @end defun | |
2110 | ||
b8d4c8d0 GM |
2111 | @ignore |
2112 | arch-tag: 9711a66e-4749-4265-9e8c-972d55b67096 | |
2113 | @end ignore |