Commit | Line | Data |
---|---|---|
1621af1e RS |
1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. | |
4 | @c See the file elisp.texi for copying conditions. | |
5 | @setfilename ../info/symbols | |
6 | @node Symbols, Evaluation, Sequences Arrays Vectors, Top | |
7 | @chapter Symbols | |
8 | @cindex symbol | |
9 | ||
10 | A @dfn{symbol} is an object with a unique name. This chapter | |
11 | describes symbols, their components, their property lists, and how they | |
12 | are created and interned. Separate chapters describe the use of symbols | |
13 | as variables and as function names; see @ref{Variables}, and | |
14 | @ref{Functions}. For the precise read syntax for symbols, see | |
15 | @ref{Symbol Type}. | |
16 | ||
17 | You can test whether an arbitrary Lisp object is a symbol | |
18 | with @code{symbolp}: | |
19 | ||
20 | @defun symbolp object | |
21 | This function returns @code{t} if @var{object} is a symbol, @code{nil} | |
22 | otherwise. | |
23 | @end defun | |
24 | ||
25 | @menu | |
26 | * Symbol Components:: Symbols have names, values, function definitions | |
27 | and property lists. | |
28 | * Definitions:: A definition says how a symbol will be used. | |
29 | * Creating Symbols:: How symbols are kept unique. | |
30 | * Property Lists:: Each symbol has a property list | |
31 | for recording miscellaneous information. | |
32 | @end menu | |
33 | ||
34 | @node Symbol Components, Definitions, Symbols, Symbols | |
35 | @section Symbol Components | |
36 | @cindex symbol components | |
37 | ||
38 | Each symbol has four components (or ``cells''), each of which | |
39 | references another object: | |
40 | ||
41 | @table @asis | |
42 | @item Print name | |
43 | @cindex print name cell | |
2b3fc6c3 | 44 | The @dfn{print name cell} holds a string that names the symbol for |
1621af1e RS |
45 | reading and printing. See @code{symbol-name} in @ref{Creating Symbols}. |
46 | ||
47 | @item Value | |
48 | @cindex value cell | |
49 | The @dfn{value cell} holds the current value of the symbol as a | |
50 | variable. When a symbol is used as a form, the value of the form is the | |
51 | contents of the symbol's value cell. See @code{symbol-value} in | |
52 | @ref{Accessing Variables}. | |
53 | ||
54 | @item Function | |
55 | @cindex function cell | |
56 | The @dfn{function cell} holds the function definition of the symbol. | |
57 | When a symbol is used as a function, its function definition is used in | |
58 | its place. This cell is also used to make a symbol stand for a keymap | |
59 | or a keyboard macro, for editor command execution. Because each symbol | |
60 | has separate value and function cells, variables and function names do | |
61 | not conflict. See @code{symbol-function} in @ref{Function Cells}. | |
62 | ||
63 | @item Property list | |
64 | @cindex property list cell | |
65 | The @dfn{property list cell} holds the property list of the symbol. See | |
66 | @code{symbol-plist} in @ref{Property Lists}. | |
67 | @end table | |
68 | ||
69 | The print name cell always holds a string, and cannot be changed. The | |
70 | other three cells can be set individually to any specified Lisp object. | |
71 | ||
72 | The print name cell holds the string that is the name of the symbol. | |
73 | Since symbols are represented textually by their names, it is important | |
74 | not to have two symbols with the same name. The Lisp reader ensures | |
75 | this: every time it reads a symbol, it looks for an existing symbol with | |
76 | the specified name before it creates a new one. (In GNU Emacs Lisp, | |
77 | this lookup uses a hashing algorithm and an obarray; see @ref{Creating | |
78 | Symbols}.) | |
79 | ||
80 | In normal usage, the function cell usually contains a function or | |
81 | macro, as that is what the Lisp interpreter expects to see there | |
82 | (@pxref{Evaluation}). Keyboard macros (@pxref{Keyboard Macros}), | |
83 | keymaps (@pxref{Keymaps}) and autoload objects (@pxref{Autoloading}) are | |
84 | also sometimes stored in the function cell of symbols. We often refer | |
85 | to ``the function @code{foo}'' when we really mean the function stored | |
86 | in the function cell of the symbol @code{foo}. We make the distinction | |
87 | only when necessary. | |
88 | ||
89 | The property list cell normally should hold a correctly formatted | |
90 | property list (@pxref{Property Lists}), as a number of functions expect | |
91 | to see a property list there. | |
92 | ||
93 | The function cell or the value cell may be @dfn{void}, which means | |
94 | that the cell does not reference any object. (This is not the same | |
95 | thing as holding the symbol @code{void}, nor the same as holding the | |
2b3fc6c3 | 96 | symbol @code{nil}.) Examining a cell that is void results in an error, |
1621af1e RS |
97 | such as @samp{Symbol's value as variable is void}. |
98 | ||
99 | The four functions @code{symbol-name}, @code{symbol-value}, | |
100 | @code{symbol-plist}, and @code{symbol-function} return the contents of | |
101 | the four cells of a symbol. Here as an example we show the contents of | |
102 | the four cells of the symbol @code{buffer-file-name}: | |
103 | ||
104 | @example | |
105 | (symbol-name 'buffer-file-name) | |
106 | @result{} "buffer-file-name" | |
107 | (symbol-value 'buffer-file-name) | |
108 | @result{} "/gnu/elisp/symbols.texi" | |
109 | (symbol-plist 'buffer-file-name) | |
110 | @result{} (variable-documentation 29529) | |
111 | (symbol-function 'buffer-file-name) | |
112 | @result{} #<subr buffer-file-name> | |
113 | @end example | |
114 | ||
115 | @noindent | |
116 | Because this symbol is the variable which holds the name of the file | |
117 | being visited in the current buffer, the value cell contents we see are | |
118 | the name of the source file of this chapter of the Emacs Lisp Manual. | |
119 | The property list cell contains the list @code{(variable-documentation | |
120 | 29529)} which tells the documentation functions where to find the | |
121 | documentation string for the variable @code{buffer-file-name} in the | |
122 | @file{DOC} file. (29529 is the offset from the beginning of the | |
123 | @file{DOC} file to where that documentation string begins.) The | |
124 | function cell contains the function for returning the name of the file. | |
125 | @code{buffer-file-name} names a primitive function, which has no read | |
126 | syntax and prints in hash notation (@pxref{Primitive Function Type}). A | |
127 | symbol naming a function written in Lisp would have a lambda expression | |
128 | (or a byte-code object) in this cell. | |
129 | ||
130 | @node Definitions, Creating Symbols, Symbol Components, Symbols | |
131 | @section Defining Symbols | |
132 | @cindex definition of a symbol | |
133 | ||
134 | A @dfn{definition} in Lisp is a special form that announces your | |
135 | intention to use a certain symbol in a particular way. In Emacs Lisp, | |
136 | you can define a symbol as a variable, or define it as a function (or | |
137 | macro), or both independently. | |
138 | ||
139 | A definition construct typically specifies a value or meaning for the | |
140 | symbol for one kind of use, plus documentation for its meaning when used | |
141 | in this way. Thus, when you define a symbol as a variable, you can | |
142 | supply an initial value for the variable, plus documentation for the | |
143 | variable. | |
144 | ||
145 | @code{defvar} and @code{defconst} are special forms that define a | |
146 | symbol as a global variable. They are documented in detail in | |
147 | @ref{Defining Variables}. | |
148 | ||
149 | @code{defun} defines a symbol as a function, creating a lambda | |
150 | expression and storing it in the function cell of the symbol. This | |
151 | lambda expression thus becomes the function definition of the symbol. | |
152 | (The term ``function definition'', meaning the contents of the function | |
153 | cell, is derived from the idea that @code{defun} gives the symbol its | |
154 | definition as a function.) @xref{Functions}. | |
155 | ||
156 | @code{defmacro} defines a symbol as a macro. It creates a macro | |
157 | object and stores it in the function cell of the symbol. Note that a | |
158 | given symbol can be a macro or a function, but not both at once, because | |
159 | both macro and function definitions are kept in the function cell, and | |
160 | that cell can hold only one Lisp object at any given time. | |
161 | @xref{Macros}. | |
162 | ||
163 | In GNU Emacs Lisp, a definition is not required in order to use a | |
164 | symbol as a variable or function. Thus, you can make a symbol a global | |
165 | variable with @code{setq}, whether you define it first or not. The real | |
166 | purpose of definitions is to guide programmers and programming tools. | |
167 | They inform programmers who read the code that certain symbols are | |
168 | @emph{intended} to be used as variables, or as functions. In addition, | |
169 | utilities such as @file{etags} and @file{make-docfile} recognize | |
170 | definitions, and add appropriate information to tag tables and the | |
171 | @file{emacs/etc/DOC-@var{version}} file. @xref{Accessing Documentation}. | |
172 | ||
173 | @node Creating Symbols, Property Lists, Definitions, Symbols | |
174 | @section Creating and Interning Symbols | |
175 | @cindex reading symbols | |
176 | ||
177 | To understand how symbols are created in GNU Emacs Lisp, you must know | |
178 | how Lisp reads them. Lisp must ensure that it finds the same symbol | |
179 | every time it reads the same set of characters. Failure to do so would | |
180 | cause complete confusion. | |
181 | ||
182 | @cindex symbol name hashing | |
183 | @cindex hashing | |
184 | @cindex obarray | |
185 | @cindex bucket (in obarray) | |
186 | When the Lisp reader encounters a symbol, it reads all the characters | |
187 | of the name. Then it ``hashes'' those characters to find an index in a | |
188 | table called an @dfn{obarray}. Hashing is an efficient method of | |
189 | looking something up. For example, instead of searching a telephone | |
190 | book cover to cover when looking up Jan Jones, you start with the J's | |
191 | and go from there. That is a simple version of hashing. Each element | |
192 | of the obarray is a @dfn{bucket} which holds all the symbols with a | |
193 | given hash code; to look for a given name, it is sufficient to look | |
194 | through all the symbols in the bucket for that name's hash code. | |
195 | ||
196 | @cindex interning | |
2b3fc6c3 RS |
197 | If a symbol with the desired name is found, the reader uses that |
198 | symbol. If the obarray does not contain a symbol with that name, the | |
199 | reader makes a new symbol and adds it to the obarray. Finding or adding | |
200 | a symbol with a certain name is called @dfn{interning} it, and the | |
201 | symbol is then called an @dfn{interned symbol}. | |
202 | ||
203 | Interning ensures that each obarray has just one symbol with any | |
204 | particular name. Other like-named symbols may exist, but not in the | |
205 | same obarray. Thus, the reader gets the same symbols for the same | |
206 | names, as long as you keep reading with the same obarray. | |
1621af1e RS |
207 | |
208 | @cindex symbol equality | |
209 | @cindex uninterned symbol | |
2b3fc6c3 RS |
210 | No obarray contains all symbols; in fact, some symbols are not in any |
211 | obarray. They are called @dfn{uninterned symbols}. An uninterned | |
212 | symbol has the same four cells as other symbols; however, the only way | |
213 | to gain access to it is by finding it in some other object or as the | |
214 | value of a variable. | |
1621af1e RS |
215 | |
216 | In Emacs Lisp, an obarray is actually a vector. Each element of the | |
217 | vector is a bucket; its value is either an interned symbol whose name | |
218 | hashes to that bucket, or 0 if the bucket is empty. Each interned | |
219 | symbol has an internal link (invisible to the user) to the next symbol | |
220 | in the bucket. Because these links are invisible, there is no way to | |
221 | find all the symbols in an obarray except using @code{mapatoms} (below). | |
222 | The order of symbols in a bucket is not significant. | |
223 | ||
224 | In an empty obarray, every element is 0, and you can create an obarray | |
225 | with @code{(make-vector @var{length} 0)}. @strong{This is the only | |
226 | valid way to create an obarray.} Prime numbers as lengths tend | |
227 | to result in good hashing; lengths one less than a power of two are also | |
228 | good. | |
229 | ||
230 | @strong{Do not try to put symbols in an obarray yourself.} This does | |
231 | not work---only @code{intern} can enter a symbol in an obarray properly. | |
232 | @strong{Do not try to intern one symbol in two obarrays.} This would | |
233 | garble both obarrays, because a symbol has just one slot to hold the | |
234 | following symbol in the obarray bucket. The results would be | |
235 | unpredictable. | |
236 | ||
237 | It is possible for two different symbols to have the same name in | |
238 | different obarrays; these symbols are not @code{eq} or @code{equal}. | |
239 | However, this normally happens only as part of the abbrev mechanism | |
240 | (@pxref{Abbrevs}). | |
241 | ||
242 | @cindex CL note---symbol in obarrays | |
243 | @quotation | |
ec221d13 | 244 | @b{Common Lisp note:} In Common Lisp, a single symbol may be interned in |
1621af1e RS |
245 | several obarrays. |
246 | @end quotation | |
247 | ||
248 | Most of the functions below take a name and sometimes an obarray as | |
249 | arguments. A @code{wrong-type-argument} error is signaled if the name | |
250 | is not a string, or if the obarray is not a vector. | |
251 | ||
252 | @defun symbol-name symbol | |
253 | This function returns the string that is @var{symbol}'s name. For example: | |
254 | ||
255 | @example | |
256 | @group | |
257 | (symbol-name 'foo) | |
258 | @result{} "foo" | |
259 | @end group | |
260 | @end example | |
261 | ||
262 | Changing the string by substituting characters, etc, does change the | |
263 | name of the symbol, but fails to update the obarray, so don't do it! | |
264 | @end defun | |
265 | ||
266 | @defun make-symbol name | |
267 | This function returns a newly-allocated, uninterned symbol whose name is | |
268 | @var{name} (which must be a string). Its value and function definition | |
269 | are void, and its property list is @code{nil}. In the example below, | |
270 | the value of @code{sym} is not @code{eq} to @code{foo} because it is a | |
271 | distinct uninterned symbol whose name is also @samp{foo}. | |
272 | ||
273 | @example | |
274 | (setq sym (make-symbol "foo")) | |
275 | @result{} foo | |
276 | (eq sym 'foo) | |
277 | @result{} nil | |
278 | @end example | |
279 | @end defun | |
280 | ||
281 | @defun intern name &optional obarray | |
282 | This function returns the interned symbol whose name is @var{name}. If | |
283 | there is no such symbol in the obarray @var{obarray}, @code{intern} | |
284 | creates a new one, adds it to the obarray, and returns it. If | |
285 | @var{obarray} is omitted, the value of the global variable | |
286 | @code{obarray} is used. | |
287 | ||
288 | @example | |
289 | (setq sym (intern "foo")) | |
290 | @result{} foo | |
291 | (eq sym 'foo) | |
292 | @result{} t | |
293 | ||
294 | (setq sym1 (intern "foo" other-obarray)) | |
295 | @result{} foo | |
296 | (eq sym 'foo) | |
297 | @result{} nil | |
298 | @end example | |
299 | @end defun | |
300 | ||
301 | @defun intern-soft name &optional obarray | |
302 | This function returns the symbol in @var{obarray} whose name is | |
303 | @var{name}, or @code{nil} if @var{obarray} has no symbol with that name. | |
304 | Therefore, you can use @code{intern-soft} to test whether a symbol with | |
305 | a given name is already interned. If @var{obarray} is omitted, the | |
306 | value of the global variable @code{obarray} is used. | |
307 | ||
308 | @smallexample | |
309 | (intern-soft "frazzle") ; @r{No such symbol exists.} | |
310 | @result{} nil | |
311 | (make-symbol "frazzle") ; @r{Create an uninterned one.} | |
312 | @result{} frazzle | |
313 | (intern-soft "frazzle") ; @r{That one cannot be found.} | |
314 | @result{} nil | |
315 | (setq sym (intern "frazzle")) ; @r{Create an interned one.} | |
316 | @result{} frazzle | |
317 | (intern-soft "frazzle") ; @r{That one can be found!} | |
318 | @result{} frazzle | |
319 | @group | |
320 | (eq sym 'frazzle) ; @r{And it is the same one.} | |
321 | @result{} t | |
322 | @end group | |
323 | @end smallexample | |
324 | @end defun | |
325 | ||
326 | @defvar obarray | |
327 | This variable is the standard obarray for use by @code{intern} and | |
328 | @code{read}. | |
329 | @end defvar | |
330 | ||
331 | @defun mapatoms function &optional obarray | |
2b3fc6c3 | 332 | This function calls @var{function} for each symbol in the obarray |
1621af1e RS |
333 | @var{obarray}. It returns @code{nil}. If @var{obarray} is omitted, it |
334 | defaults to the value of @code{obarray}, the standard obarray for | |
335 | ordinary symbols. | |
336 | ||
337 | @smallexample | |
338 | (setq count 0) | |
339 | @result{} 0 | |
340 | (defun count-syms (s) | |
341 | (setq count (1+ count))) | |
342 | @result{} count-syms | |
343 | (mapatoms 'count-syms) | |
344 | @result{} nil | |
345 | count | |
346 | @result{} 1871 | |
347 | @end smallexample | |
348 | ||
349 | See @code{documentation} in @ref{Accessing Documentation}, for another | |
350 | example using @code{mapatoms}. | |
351 | @end defun | |
352 | ||
353 | @node Property Lists,, Creating Symbols, Symbols | |
354 | @section Property Lists | |
355 | @cindex property list | |
356 | @cindex plist | |
357 | ||
358 | A @dfn{property list} (@dfn{plist} for short) is a list of paired | |
359 | elements stored in the property list cell of a symbol. Each of the | |
360 | pairs associates a property name (usually a symbol) with a property or | |
361 | value. Property lists are generally used to record information about a | |
2b3fc6c3 RS |
362 | symbol, such as its documentation as a variable, the name of the file |
363 | where it was defined, or perhaps even the grammatical class of the | |
364 | symbol (representing a word) in a language-understanding system. | |
1621af1e RS |
365 | |
366 | Character positions in a string or buffer can also have property lists. | |
367 | @xref{Text Properties}. | |
368 | ||
369 | The property names and values in a property list can be any Lisp | |
370 | objects, but the names are usually symbols. They are compared using | |
371 | @code{eq}. Here is an example of a property list, found on the symbol | |
372 | @code{progn} when the compiler is loaded: | |
373 | ||
374 | @example | |
375 | (lisp-indent-function 0 byte-compile byte-compile-progn) | |
376 | @end example | |
377 | ||
378 | @noindent | |
379 | Here @code{lisp-indent-function} and @code{byte-compile} are property | |
380 | names, and the other two elements are the corresponding values. | |
381 | ||
382 | @cindex property lists vs association lists | |
383 | Association lists (@pxref{Association Lists}) are very similar to | |
384 | property lists. In contrast to association lists, the order of the | |
385 | pairs in the property list is not significant since the property names | |
386 | must be distinct. | |
387 | ||
388 | Property lists are better than association lists for attaching | |
389 | information to various Lisp function names or variables. If all the | |
390 | associations are recorded in one association list, the program will need | |
391 | to search that entire list each time a function or variable is to be | |
392 | operated on. By contrast, if the information is recorded in the | |
393 | property lists of the function names or variables themselves, each | |
394 | search will scan only the length of one property list, which is usually | |
395 | short. This is why the documentation for a variable is recorded in a | |
396 | property named @code{variable-documentation}. The byte compiler | |
397 | likewise uses properties to record those functions needing special | |
398 | treatment. | |
399 | ||
400 | However, association lists have their own advantages. Depending on | |
401 | your application, it may be faster to add an association to the front of | |
402 | an association list than to update a property. All properties for a | |
403 | symbol are stored in the same property list, so there is a possibility | |
404 | of a conflict between different uses of a property name. (For this | |
405 | reason, it is a good idea to choose property names that are probably | |
406 | unique, such as by including the name of the library in the property | |
407 | name.) An association list may be used like a stack where associations | |
408 | are pushed on the front of the list and later discarded; this is not | |
409 | possible with a property list. | |
410 | ||
411 | @defun symbol-plist symbol | |
412 | This function returns the property list of @var{symbol}. | |
413 | @end defun | |
414 | ||
415 | @defun setplist symbol plist | |
416 | This function sets @var{symbol}'s property list to @var{plist}. | |
417 | Normally, @var{plist} should be a well-formed property list, but this is | |
418 | not enforced. | |
419 | ||
420 | @smallexample | |
421 | (setplist 'foo '(a 1 b (2 3) c nil)) | |
422 | @result{} (a 1 b (2 3) c nil) | |
423 | (symbol-plist 'foo) | |
424 | @result{} (a 1 b (2 3) c nil) | |
425 | @end smallexample | |
426 | ||
427 | For symbols in special obarrays, which are not used for ordinary | |
428 | purposes, it may make sense to use the property list cell in a | |
429 | nonstandard fashion; in fact, the abbrev mechanism does so | |
430 | (@pxref{Abbrevs}). | |
431 | @end defun | |
432 | ||
433 | @defun get symbol property | |
434 | This function finds the value of the property named @var{property} in | |
435 | @var{symbol}'s property list. If there is no such property, @code{nil} | |
436 | is returned. Thus, there is no distinction between a value of | |
437 | @code{nil} and the absence of the property. | |
438 | ||
439 | The name @var{property} is compared with the existing property names | |
440 | using @code{eq}, so any object is a legitimate property. | |
441 | ||
442 | See @code{put} for an example. | |
443 | @end defun | |
444 | ||
445 | @defun put symbol property value | |
446 | This function puts @var{value} onto @var{symbol}'s property list under | |
447 | the property name @var{property}, replacing any previous property value. | |
448 | The @code{put} function returns @var{value}. | |
449 | ||
450 | @smallexample | |
451 | (put 'fly 'verb 'transitive) | |
452 | @result{}'transitive | |
453 | (put 'fly 'noun '(a buzzing little bug)) | |
454 | @result{} (a buzzing little bug) | |
455 | (get 'fly 'verb) | |
456 | @result{} transitive | |
457 | (symbol-plist 'fly) | |
458 | @result{} (verb transitive noun (a buzzing little bug)) | |
459 | @end smallexample | |
460 | @end defun |