Commit | Line | Data |
---|---|---|
e6512bcf 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/variables | |
6 | @node Variables, Functions, Control Structures, Top | |
7 | @chapter Variables | |
8 | @cindex variable | |
9 | ||
10 | A @dfn{variable} is a name used in a program to stand for a value. | |
11 | Nearly all programming languages have variables of some sort. In the | |
12 | text of a Lisp program, variables are written using the syntax for | |
13 | symbols. | |
14 | ||
15 | In Lisp, unlike most programming languages, programs are represented | |
16 | primarily as Lisp objects and only secondarily as text. The Lisp | |
17 | objects used for variables are symbols: the symbol name is the variable | |
18 | name, and the variable's value is stored in the value cell of the | |
19 | symbol. The use of a symbol as a variable is independent of its use as | |
20 | a function name. @xref{Symbol Components}. | |
21 | ||
22 | The Lisp objects that constitute a Lisp program determine the textual | |
f57ddf67 | 23 | form of the program---it is simply the read syntax for those Lisp |
e6512bcf RS |
24 | objects. This is why, for example, a variable in a textual Lisp program |
25 | is written using the read syntax for the symbol that represents the | |
26 | variable. | |
27 | ||
28 | @menu | |
29 | * Global Variables:: Variable values that exist permanently, everywhere. | |
30 | * Constant Variables:: Certain "variables" have values that never change. | |
31 | * Local Variables:: Variable values that exist only temporarily. | |
32 | * Void Variables:: Symbols that lack values. | |
33 | * Defining Variables:: A definition says a symbol is used as a variable. | |
34 | * Accessing Variables:: Examining values of variables whose names | |
35 | are known only at run time. | |
36 | * Setting Variables:: Storing new values in variables. | |
37 | * Variable Scoping:: How Lisp chooses among local and global values. | |
38 | * Buffer-Local Variables:: Variable values in effect only in one buffer. | |
39 | @end menu | |
40 | ||
41 | @node Global Variables | |
42 | @section Global Variables | |
43 | @cindex global variable | |
44 | ||
45 | The simplest way to use a variable is @dfn{globally}. This means that | |
46 | the variable has just one value at a time, and this value is in effect | |
47 | (at least for the moment) throughout the Lisp system. The value remains | |
48 | in effect until you specify a new one. When a new value replaces the | |
49 | old one, no trace of the old value remains in the variable. | |
50 | ||
51 | You specify a value for a symbol with @code{setq}. For example, | |
52 | ||
53 | @example | |
54 | (setq x '(a b)) | |
55 | @end example | |
56 | ||
57 | @noindent | |
58 | gives the variable @code{x} the value @code{(a b)}. Note that | |
59 | @code{setq} does not evaluate its first argument, the name of the | |
60 | variable, but it does evaluate the second argument, the new value. | |
61 | ||
62 | Once the variable has a value, you can refer to it by using the symbol | |
63 | by itself as an expression. Thus, | |
64 | ||
65 | @example | |
66 | @group | |
67 | x @result{} (a b) | |
68 | @end group | |
69 | @end example | |
70 | ||
71 | @noindent | |
72 | assuming the @code{setq} form shown above has already been executed. | |
73 | ||
74 | If you do another @code{setq}, the new value replaces the old one: | |
75 | ||
76 | @example | |
77 | @group | |
78 | x | |
79 | @result{} (a b) | |
80 | @end group | |
81 | @group | |
82 | (setq x 4) | |
83 | @result{} 4 | |
84 | @end group | |
85 | @group | |
86 | x | |
87 | @result{} 4 | |
88 | @end group | |
89 | @end example | |
90 | ||
91 | @node Constant Variables | |
92 | @section Variables That Never Change | |
93 | @vindex nil | |
94 | @vindex t | |
95 | @kindex setting-constant | |
96 | ||
97 | Emacs Lisp has two special symbols, @code{nil} and @code{t}, that | |
98 | always evaluate to themselves. These symbols cannot be rebound, nor can | |
99 | their value cells be changed. An attempt to change the value of | |
100 | @code{nil} or @code{t} signals a @code{setting-constant} error. | |
101 | ||
102 | @example | |
103 | @group | |
104 | nil @equiv{} 'nil | |
105 | @result{} nil | |
106 | @end group | |
107 | @group | |
108 | (setq nil 500) | |
109 | @error{} Attempt to set constant symbol: nil | |
110 | @end group | |
111 | @end example | |
112 | ||
113 | @node Local Variables | |
114 | @section Local Variables | |
115 | @cindex binding local variables | |
116 | @cindex local variables | |
117 | @cindex local binding | |
118 | @cindex global binding | |
119 | ||
120 | Global variables have values that last until explicitly superseded | |
121 | with new values. Sometimes it is useful to create variable values that | |
122 | exist temporarily---only while within a certain part of the program. | |
123 | These values are called @dfn{local}, and the variables so used are | |
124 | called @dfn{local variables}. | |
125 | ||
126 | For example, when a function is called, its argument variables receive | |
127 | new local values that last until the function exits. The @code{let} | |
128 | special form explicitly establishes new local values for specified | |
129 | variables; these last until exit from the @code{let} form. | |
130 | ||
131 | @cindex shadowing of variables | |
132 | Establishing a local value saves away the previous value (or lack of | |
133 | one) of the variable. When the life span of the local value is over, | |
134 | the previous value is restored. In the mean time, we say that the | |
135 | previous value is @dfn{shadowed} and @dfn{not visible}. Both global and | |
136 | local values may be shadowed (@pxref{Scope}). | |
137 | ||
138 | If you set a variable (such as with @code{setq}) while it is local, | |
139 | this replaces the local value; it does not alter the global value, or | |
140 | previous local values that are shadowed. To model this behavior, we | |
141 | speak of a @dfn{local binding} of the variable as well as a local value. | |
142 | ||
143 | The local binding is a conceptual place that holds a local value. | |
144 | Entry to a function, or a special form such as @code{let}, creates the | |
145 | local binding; exit from the function or from the @code{let} removes the | |
146 | local binding. As long as the local binding lasts, the variable's value | |
147 | is stored within it. Use of @code{setq} or @code{set} while there is a | |
148 | local binding stores a different value into the local binding; it does | |
149 | not create a new binding. | |
150 | ||
151 | We also speak of the @dfn{global binding}, which is where | |
152 | (conceptually) the global value is kept. | |
153 | ||
154 | @cindex current binding | |
155 | A variable can have more than one local binding at a time (for | |
156 | example, if there are nested @code{let} forms that bind it). In such a | |
157 | case, the most recently created local binding that still exists is the | |
158 | @dfn{current binding} of the variable. (This is called @dfn{dynamic | |
159 | scoping}; see @ref{Variable Scoping}.) If there are no local bindings, | |
160 | the variable's global binding is its current binding. We also call the | |
161 | current binding the @dfn{most-local existing binding}, for emphasis. | |
162 | Ordinary evaluation of a symbol always returns the value of its current | |
163 | binding. | |
164 | ||
165 | The special forms @code{let} and @code{let*} exist to create | |
166 | local bindings. | |
167 | ||
168 | @defspec let (bindings@dots{}) forms@dots{} | |
f57ddf67 | 169 | This special form binds variables according to @var{bindings} and then |
e6512bcf RS |
170 | evaluates all of the @var{forms} in textual order. The @code{let}-form |
171 | returns the value of the last form in @var{forms}. | |
172 | ||
173 | Each of the @var{bindings} is either @w{(i) a} symbol, in which case | |
174 | that symbol is bound to @code{nil}; or @w{(ii) a} list of the form | |
175 | @code{(@var{symbol} @var{value-form})}, in which case @var{symbol} is | |
176 | bound to the result of evaluating @var{value-form}. If @var{value-form} | |
177 | is omitted, @code{nil} is used. | |
178 | ||
179 | All of the @var{value-form}s in @var{bindings} are evaluated in the | |
180 | order they appear and @emph{before} any of the symbols are bound. Here | |
181 | is an example of this: @code{Z} is bound to the old value of @code{Y}, | |
182 | which is 2, not the new value, 1. | |
183 | ||
184 | @example | |
185 | @group | |
186 | (setq Y 2) | |
187 | @result{} 2 | |
188 | @end group | |
189 | @group | |
190 | (let ((Y 1) | |
191 | (Z Y)) | |
192 | (list Y Z)) | |
193 | @result{} (1 2) | |
194 | @end group | |
195 | @end example | |
196 | @end defspec | |
197 | ||
198 | @defspec let* (bindings@dots{}) forms@dots{} | |
199 | This special form is like @code{let}, but it binds each variable right | |
200 | after computing its local value, before computing the local value for | |
201 | the next variable. Therefore, an expression in @var{bindings} can | |
202 | reasonably refer to the preceding symbols bound in this @code{let*} | |
203 | form. Compare the following example with the example above for | |
204 | @code{let}. | |
205 | ||
206 | @example | |
207 | @group | |
208 | (setq Y 2) | |
209 | @result{} 2 | |
210 | @end group | |
211 | @group | |
212 | (let* ((Y 1) | |
213 | (Z Y)) ; @r{Use the just-established value of @code{Y}.} | |
214 | (list Y Z)) | |
215 | @result{} (1 1) | |
216 | @end group | |
217 | @end example | |
218 | @end defspec | |
219 | ||
f57ddf67 | 220 | Here is a complete list of the other facilities that create local |
e6512bcf RS |
221 | bindings: |
222 | ||
223 | @itemize @bullet | |
224 | @item | |
225 | Function calls (@pxref{Functions}). | |
226 | ||
227 | @item | |
228 | Macro calls (@pxref{Macros}). | |
229 | ||
230 | @item | |
231 | @code{condition-case} (@pxref{Errors}). | |
232 | @end itemize | |
233 | ||
234 | @defvar max-specpdl-size | |
235 | @cindex variable limit error | |
236 | @cindex evaluation error | |
237 | @cindex infinite recursion | |
238 | This variable defines the limit on the total number of local variable | |
239 | bindings and @code{unwind-protect} cleanups (@pxref{Nonlocal Exits}) | |
240 | that are allowed before signaling an error (with data @code{"Variable | |
241 | binding depth exceeds max-specpdl-size"}). | |
242 | ||
243 | This limit, with the associated error when it is exceeded, is one way | |
244 | that Lisp avoids infinite recursion on an ill-defined function. | |
245 | ||
246 | The default value is 600. | |
247 | ||
248 | @code{max-lisp-eval-depth} provides another limit on depth of nesting. | |
249 | @xref{Eval}. | |
250 | @end defvar | |
251 | ||
252 | @node Void Variables | |
253 | @section When a Variable is ``Void'' | |
254 | @kindex void-variable | |
255 | @cindex void variable | |
256 | ||
257 | If you have never given a symbol any value as a global variable, we | |
258 | say that that symbol's global value is @dfn{void}. In other words, the | |
259 | symbol's value cell does not have any Lisp object in it. If you try to | |
260 | evaluate the symbol, you get a @code{void-variable} error rather than | |
261 | a value. | |
262 | ||
263 | Note that a value of @code{nil} is not the same as void. The symbol | |
264 | @code{nil} is a Lisp object and can be the value of a variable just as any | |
265 | other object can be; but it is @emph{a value}. A void variable does not | |
266 | have any value. | |
267 | ||
268 | After you have given a variable a value, you can make it void once more | |
269 | using @code{makunbound}. | |
270 | ||
271 | @defun makunbound symbol | |
272 | This function makes the current binding of @var{symbol} void. | |
273 | Subsequent attempts to use this symbol's value as a variable will signal | |
274 | the error @code{void-variable}, unless or until you set it again. | |
275 | ||
276 | @code{makunbound} returns @var{symbol}. | |
277 | ||
278 | @example | |
279 | @group | |
280 | (makunbound 'x) ; @r{Make the global value} | |
281 | ; @r{of @code{x} void.} | |
282 | @result{} x | |
283 | @end group | |
284 | @group | |
285 | x | |
286 | @error{} Symbol's value as variable is void: x | |
287 | @end group | |
288 | @end example | |
289 | ||
290 | If @var{symbol} is locally bound, @code{makunbound} affects the most | |
291 | local existing binding. This is the only way a symbol can have a void | |
292 | local binding, since all the constructs that create local bindings | |
293 | create them with values. In this case, the voidness lasts at most as | |
294 | long as the binding does; when the binding is removed due to exit from | |
295 | the construct that made it, the previous or global binding is reexposed | |
296 | as usual, and the variable is no longer void unless the newly reexposed | |
297 | binding was void all along. | |
298 | ||
299 | @smallexample | |
300 | @group | |
301 | (setq x 1) ; @r{Put a value in the global binding.} | |
302 | @result{} 1 | |
303 | (let ((x 2)) ; @r{Locally bind it.} | |
304 | (makunbound 'x) ; @r{Void the local binding.} | |
305 | x) | |
306 | @error{} Symbol's value as variable is void: x | |
307 | @end group | |
308 | @group | |
309 | x ; @r{The global binding is unchanged.} | |
310 | @result{} 1 | |
311 | ||
312 | (let ((x 2)) ; @r{Locally bind it.} | |
313 | (let ((x 3)) ; @r{And again.} | |
314 | (makunbound 'x) ; @r{Void the innermost-local binding.} | |
315 | x)) ; @r{And refer: it's void.} | |
316 | @error{} Symbol's value as variable is void: x | |
317 | @end group | |
318 | ||
319 | @group | |
320 | (let ((x 2)) | |
321 | (let ((x 3)) | |
322 | (makunbound 'x)) ; @r{Void inner binding, then remove it.} | |
323 | x) ; @r{Now outer @code{let} binding is visible.} | |
324 | @result{} 2 | |
325 | @end group | |
326 | @end smallexample | |
327 | @end defun | |
328 | ||
329 | A variable that has been made void with @code{makunbound} is | |
330 | indistinguishable from one that has never received a value and has | |
331 | always been void. | |
332 | ||
333 | You can use the function @code{boundp} to test whether a variable is | |
334 | currently void. | |
335 | ||
336 | @defun boundp variable | |
337 | @code{boundp} returns @code{t} if @var{variable} (a symbol) is not void; | |
338 | more precisely, if its current binding is not void. It returns | |
339 | @code{nil} otherwise. | |
340 | ||
341 | @smallexample | |
342 | @group | |
343 | (boundp 'abracadabra) ; @r{Starts out void.} | |
344 | @result{} nil | |
345 | @end group | |
346 | @group | |
347 | (let ((abracadabra 5)) ; @r{Locally bind it.} | |
348 | (boundp 'abracadabra)) | |
349 | @result{} t | |
350 | @end group | |
351 | @group | |
352 | (boundp 'abracadabra) ; @r{Still globally void.} | |
353 | @result{} nil | |
354 | @end group | |
355 | @group | |
356 | (setq abracadabra 5) ; @r{Make it globally nonvoid.} | |
357 | @result{} 5 | |
358 | @end group | |
359 | @group | |
360 | (boundp 'abracadabra) | |
361 | @result{} t | |
362 | @end group | |
363 | @end smallexample | |
364 | @end defun | |
365 | ||
366 | @node Defining Variables | |
367 | @section Defining Global Variables | |
f57ddf67 | 368 | @cindex variable definition |
e6512bcf RS |
369 | |
370 | You may announce your intention to use a symbol as a global variable | |
f57ddf67 RS |
371 | with a @dfn{variable definition}: a special form, either @code{defconst} |
372 | or @code{defvar}. | |
e6512bcf RS |
373 | |
374 | In Emacs Lisp, definitions serve three purposes. First, they inform | |
375 | people who read the code that certain symbols are @emph{intended} to be | |
376 | used a certain way (as variables). Second, they inform the Lisp system | |
377 | of these things, supplying a value and documentation. Third, they | |
378 | provide information to utilities such as @code{etags} and | |
379 | @code{make-docfile}, which create data bases of the functions and | |
380 | variables in a program. | |
381 | ||
382 | The difference between @code{defconst} and @code{defvar} is primarily | |
383 | a matter of intent, serving to inform human readers of whether programs | |
384 | will change the variable. Emacs Lisp does not restrict the ways in | |
385 | which a variable can be used based on @code{defconst} or @code{defvar} | |
f57ddf67 | 386 | declarations. However, it does make a difference for initialization: |
e6512bcf RS |
387 | @code{defconst} unconditionally initializes the variable, while |
388 | @code{defvar} initializes it only if it is void. | |
389 | ||
390 | One would expect user option variables to be defined with | |
391 | @code{defconst}, since programs do not change them. Unfortunately, this | |
392 | has bad results if the definition is in a library that is not preloaded: | |
393 | @code{defconst} would override any prior value when the library is | |
394 | loaded. Users would like to be able to set user options in their init | |
395 | files, and override the default values given in the definitions. For | |
396 | this reason, user options must be defined with @code{defvar}. | |
397 | ||
398 | @defspec defvar symbol [value [doc-string]] | |
399 | This special form defines @var{symbol} as a value and initializes it. | |
400 | The definition informs a person reading your code that @var{symbol} is | |
401 | used as a variable that programs are likely to set or change. It is | |
402 | also used for all user option variables except in the preloaded parts of | |
403 | Emacs. Note that @var{symbol} is not evaluated; the symbol to be | |
404 | defined must appear explicitly in the @code{defvar}. | |
405 | ||
406 | If @var{symbol} already has a value (i.e., it is not void), @var{value} | |
407 | is not even evaluated, and @var{symbol}'s value remains unchanged. If | |
408 | @var{symbol} is void and @var{value} is specified, @code{defvar} | |
409 | evaluates it and sets @var{symbol} to the result. (If @var{value} is | |
410 | omitted, the value of @var{symbol} is not changed in any case.) | |
411 | ||
412 | If @var{symbol} has a buffer-local binding in the current buffer, | |
413 | @code{defvar} sets the default value, not the local value. | |
414 | @xref{Buffer-Local Variables}. | |
415 | ||
416 | If the @var{doc-string} argument appears, it specifies the documentation | |
417 | for the variable. (This opportunity to specify documentation is one of | |
418 | the main benefits of defining the variable.) The documentation is | |
419 | stored in the symbol's @code{variable-documentation} property. The | |
420 | Emacs help functions (@pxref{Documentation}) look for this property. | |
421 | ||
422 | If the first character of @var{doc-string} is @samp{*}, it means that | |
423 | this variable is considered a user option. This lets users set the | |
424 | variable conventiently using the commands @code{set-variable} and | |
425 | @code{edit-options}. | |
426 | ||
427 | For example, this form defines @code{foo} but does not set its value: | |
428 | ||
429 | @example | |
430 | @group | |
431 | (defvar foo) | |
432 | @result{} foo | |
433 | @end group | |
434 | @end example | |
435 | ||
436 | The following example sets the value of @code{bar} to @code{23}, and | |
437 | gives it a documentation string: | |
438 | ||
439 | @example | |
440 | @group | |
441 | (defvar bar 23 | |
442 | "The normal weight of a bar.") | |
443 | @result{} bar | |
444 | @end group | |
445 | @end example | |
446 | ||
447 | The following form changes the documentation string for @code{bar}, | |
448 | making it a user option, but does not change the value, since @code{bar} | |
449 | already has a value. (The addition @code{(1+ 23)} is not even | |
450 | performed.) | |
451 | ||
452 | @example | |
453 | @group | |
454 | (defvar bar (1+ 23) | |
455 | "*The normal weight of a bar.") | |
456 | @result{} bar | |
457 | @end group | |
458 | @group | |
459 | bar | |
460 | @result{} 23 | |
461 | @end group | |
462 | @end example | |
463 | ||
464 | Here is an equivalent expression for the @code{defvar} special form: | |
465 | ||
466 | @example | |
467 | @group | |
468 | (defvar @var{symbol} @var{value} @var{doc-string}) | |
469 | @equiv{} | |
470 | (progn | |
471 | (if (not (boundp '@var{symbol})) | |
472 | (setq @var{symbol} @var{value})) | |
473 | (put '@var{symbol} 'variable-documentation '@var{doc-string}) | |
474 | '@var{symbol}) | |
475 | @end group | |
476 | @end example | |
477 | ||
478 | The @code{defvar} form returns @var{symbol}, but it is normally used | |
479 | at top level in a file where its value does not matter. | |
480 | @end defspec | |
481 | ||
482 | @defspec defconst symbol [value [doc-string]] | |
483 | This special form defines @var{symbol} as a value and initializes it. | |
484 | It informs a person reading your code that @var{symbol} has a global | |
485 | value, established here, that will not normally be changed or locally | |
486 | bound by the execution of the program. The user, however, may be | |
487 | welcome to change it. Note that @var{symbol} is not evaluated; the | |
488 | symbol to be defined must appear explicitly in the @code{defconst}. | |
489 | ||
490 | @code{defconst} always evaluates @var{value} and sets the global value | |
491 | of @var{symbol} to the result, provided @var{value} is given. If | |
492 | @var{symbol} has a buffer-local binding in the current buffer, | |
493 | @code{defconst} sets the default value, not the local value. | |
494 | ||
b22f3a19 | 495 | @strong{Please note:} Don't use @code{defconst} for user option |
e6512bcf RS |
496 | variables in libraries that are not standardly preloaded. The user |
497 | should be able to specify a value for such a variable in the | |
498 | @file{.emacs} file, so that it will be in effect if and when the library | |
499 | is loaded later. | |
500 | ||
501 | Here, @code{pi} is a constant that presumably ought not to be changed | |
502 | by anyone (attempts by the Indiana State Legislature notwithstanding). | |
503 | As the second form illustrates, however, this is only advisory. | |
504 | ||
505 | @example | |
506 | @group | |
507 | (defconst pi 3.1415 "Pi to five places.") | |
508 | @result{} pi | |
509 | @end group | |
510 | @group | |
511 | (setq pi 3) | |
512 | @result{} pi | |
513 | @end group | |
514 | @group | |
515 | pi | |
516 | @result{} 3 | |
517 | @end group | |
518 | @end example | |
519 | @end defspec | |
520 | ||
521 | @defun user-variable-p variable | |
522 | @cindex user option | |
f57ddf67 | 523 | This function returns @code{t} if @var{variable} is a user option---a |
e6512bcf RS |
524 | variable intended to be set by the user for customization---and |
525 | @code{nil} otherwise. (Variables other than user options exist for the | |
526 | internal purposes of Lisp programs, and users need not know about them.) | |
527 | ||
528 | User option variables are distinguished from other variables by the | |
529 | first character of the @code{variable-documentation} property. If the | |
530 | property exists and is a string, and its first character is @samp{*}, | |
531 | then the variable is a user option. | |
532 | @end defun | |
533 | ||
534 | If a user option variable has a @code{variable-interactive} property, | |
535 | @code{set-variable} uses that value to control reading the new value for | |
536 | the variable. The property's value is used as if it were the argument | |
537 | to @code{interactive}. | |
538 | ||
b22f3a19 | 539 | @strong{Warning:} If the @code{defconst} and @code{defvar} special |
e6512bcf RS |
540 | forms are used while the variable has a local binding, they set the |
541 | local binding's value; the global binding is not changed. This is not | |
542 | what we really want. To prevent it, use these special forms at top | |
543 | level in a file, where normally no local binding is in effect, and make | |
544 | sure to load the file before making a local binding for the variable. | |
545 | ||
546 | @node Accessing Variables | |
547 | @section Accessing Variable Values | |
548 | ||
549 | The usual way to reference a variable is to write the symbol which | |
550 | names it (@pxref{Symbol Forms}). This requires you to specify the | |
551 | variable name when you write the program. Usually that is exactly what | |
552 | you want to do. Occasionally you need to choose at run time which | |
553 | variable to reference; then you can use @code{symbol-value}. | |
554 | ||
555 | @defun symbol-value symbol | |
556 | This function returns the value of @var{symbol}. This is the value in | |
557 | the innermost local binding of the symbol, or its global value if it | |
558 | has no local bindings. | |
559 | ||
560 | @example | |
561 | @group | |
562 | (setq abracadabra 5) | |
563 | @result{} 5 | |
564 | @end group | |
565 | @group | |
566 | (setq foo 9) | |
567 | @result{} 9 | |
568 | @end group | |
569 | ||
570 | @group | |
571 | ;; @r{Here the symbol @code{abracadabra}} | |
572 | ;; @r{is the symbol whose value is examined.} | |
573 | (let ((abracadabra 'foo)) | |
574 | (symbol-value 'abracadabra)) | |
575 | @result{} foo | |
576 | @end group | |
577 | ||
578 | @group | |
579 | ;; @r{Here the value of @code{abracadabra},} | |
580 | ;; @r{which is @code{foo},} | |
581 | ;; @r{is the symbol whose value is examined.} | |
582 | (let ((abracadabra 'foo)) | |
583 | (symbol-value abracadabra)) | |
584 | @result{} 9 | |
585 | @end group | |
586 | ||
587 | @group | |
588 | (symbol-value 'abracadabra) | |
589 | @result{} 5 | |
590 | @end group | |
591 | @end example | |
592 | ||
593 | A @code{void-variable} error is signaled if @var{symbol} has neither a | |
594 | local binding nor a global value. | |
595 | @end defun | |
596 | ||
597 | @node Setting Variables | |
598 | @section How to Alter a Variable Value | |
599 | ||
600 | The usual way to change the value of a variable is with the special | |
601 | form @code{setq}. When you need to compute the choice of variable at | |
602 | run time, use the function @code{set}. | |
603 | ||
604 | @defspec setq [symbol form]@dots{} | |
605 | This special form is the most common method of changing a variable's | |
606 | value. Each @var{symbol} is given a new value, which is the result of | |
607 | evaluating the corresponding @var{form}. The most-local existing | |
608 | binding of the symbol is changed. | |
609 | ||
610 | @code{setq} does not evaluate @var{symbol}; it sets the symbol that you | |
611 | write. We say that this argument is @dfn{automatically quoted}. The | |
612 | @samp{q} in @code{setq} stands for ``quoted.'' | |
613 | ||
614 | The value of the @code{setq} form is the value of the last @var{form}. | |
615 | ||
616 | @example | |
617 | @group | |
618 | (setq x (1+ 2)) | |
619 | @result{} 3 | |
620 | @end group | |
621 | x ; @r{@code{x} now has a global value.} | |
622 | @result{} 3 | |
623 | @group | |
624 | (let ((x 5)) | |
625 | (setq x 6) ; @r{The local binding of @code{x} is set.} | |
626 | x) | |
627 | @result{} 6 | |
628 | @end group | |
629 | x ; @r{The global value is unchanged.} | |
630 | @result{} 3 | |
631 | @end example | |
632 | ||
633 | Note that the first @var{form} is evaluated, then the first | |
634 | @var{symbol} is set, then the second @var{form} is evaluated, then the | |
635 | second @var{symbol} is set, and so on: | |
636 | ||
637 | @example | |
638 | @group | |
639 | (setq x 10 ; @r{Notice that @code{x} is set before} | |
640 | y (1+ x)) ; @r{the value of @code{y} is computed.} | |
641 | @result{} 11 | |
642 | @end group | |
643 | @end example | |
644 | @end defspec | |
645 | ||
646 | @defun set symbol value | |
647 | This function sets @var{symbol}'s value to @var{value}, then returns | |
648 | @var{value}. Since @code{set} is a function, the expression written for | |
649 | @var{symbol} is evaluated to obtain the symbol to set. | |
650 | ||
651 | The most-local existing binding of the variable is the binding that is | |
f57ddf67 | 652 | set; shadowed bindings are not affected. |
e6512bcf RS |
653 | |
654 | @example | |
655 | @group | |
656 | (set one 1) | |
657 | @error{} Symbol's value as variable is void: one | |
658 | @end group | |
659 | @group | |
660 | (set 'one 1) | |
661 | @result{} 1 | |
662 | @end group | |
663 | @group | |
664 | (set 'two 'one) | |
665 | @result{} one | |
666 | @end group | |
667 | @group | |
668 | (set two 2) ; @r{@code{two} evaluates to symbol @code{one}.} | |
669 | @result{} 2 | |
670 | @end group | |
671 | @group | |
672 | one ; @r{So it is @code{one} that was set.} | |
673 | @result{} 2 | |
674 | (let ((one 1)) ; @r{This binding of @code{one} is set,} | |
675 | (set 'one 3) ; @r{not the global value.} | |
676 | one) | |
677 | @result{} 3 | |
678 | @end group | |
679 | @group | |
680 | one | |
681 | @result{} 2 | |
682 | @end group | |
683 | @end example | |
684 | ||
f57ddf67 RS |
685 | If @var{symbol} is not actually a symbol, a @code{wrong-type-argument} |
686 | error is signaled. | |
687 | ||
688 | @example | |
689 | (set '(x y) 'z) | |
690 | @error{} Wrong type argument: symbolp, (x y) | |
691 | @end example | |
692 | ||
e6512bcf RS |
693 | Logically speaking, @code{set} is a more fundamental primitive than |
694 | @code{setq}. Any use of @code{setq} can be trivially rewritten to use | |
695 | @code{set}; @code{setq} could even be defined as a macro, given the | |
696 | availability of @code{set}. However, @code{set} itself is rarely used; | |
f57ddf67 RS |
697 | beginners hardly need to know about it. It is useful only for choosing |
698 | at run time which variable to set. For example, the command | |
e6512bcf RS |
699 | @code{set-variable}, which reads a variable name from the user and then |
700 | sets the variable, needs to use @code{set}. | |
701 | ||
702 | @cindex CL note---@code{set} local | |
703 | @quotation | |
f57ddf67 | 704 | @b{Common Lisp note:} In Common Lisp, @code{set} always changes the |
e6512bcf RS |
705 | symbol's special value, ignoring any lexical bindings. In Emacs Lisp, |
706 | all variables and all bindings are (in effect) special, so @code{set} | |
707 | always affects the most local existing binding. | |
708 | @end quotation | |
709 | @end defun | |
710 | ||
711 | @node Variable Scoping | |
712 | @section Scoping Rules for Variable Bindings | |
713 | ||
714 | A given symbol @code{foo} may have several local variable bindings, | |
715 | established at different places in the Lisp program, as well as a global | |
716 | binding. The most recently established binding takes precedence over | |
717 | the others. | |
718 | ||
719 | @cindex scope | |
720 | @cindex extent | |
721 | @cindex dynamic scoping | |
722 | Local bindings in Emacs Lisp have @dfn{indefinite scope} and | |
723 | @dfn{dynamic extent}. @dfn{Scope} refers to @emph{where} textually in | |
724 | the source code the binding can be accessed. Indefinite scope means | |
725 | that any part of the program can potentially access the variable | |
726 | binding. @dfn{Extent} refers to @emph{when}, as the program is | |
727 | executing, the binding exists. Dynamic extent means that the binding | |
728 | lasts as long as the activation of the construct that established it. | |
729 | ||
730 | The combination of dynamic extent and indefinite scope is called | |
731 | @dfn{dynamic scoping}. By contrast, most programming languages use | |
732 | @dfn{lexical scoping}, in which references to a local variable must be | |
733 | located textually within the function or block that binds the variable. | |
734 | ||
735 | @cindex CL note---special variables | |
736 | @quotation | |
b22f3a19 | 737 | @b{Common Lisp note:} Variables declared ``special'' in Common Lisp |
f57ddf67 | 738 | are dynamically scoped, like variables in Emacs Lisp. |
e6512bcf RS |
739 | @end quotation |
740 | ||
741 | @menu | |
742 | * Scope:: Scope means where in the program a value is visible. | |
743 | Comparison with other languages. | |
744 | * Extent:: Extent means how long in time a value exists. | |
745 | * Impl of Scope:: Two ways to implement dynamic scoping. | |
746 | * Using Scoping:: How to use dynamic scoping carefully and avoid problems. | |
747 | @end menu | |
748 | ||
749 | @node Scope | |
750 | @subsection Scope | |
751 | ||
752 | Emacs Lisp uses @dfn{indefinite scope} for local variable bindings. | |
753 | This means that any function anywhere in the program text might access a | |
754 | given binding of a variable. Consider the following function | |
755 | definitions: | |
756 | ||
757 | @example | |
758 | @group | |
759 | (defun binder (x) ; @r{@code{x} is bound in @code{binder}.} | |
760 | (foo 5)) ; @r{@code{foo} is some other function.} | |
761 | @end group | |
762 | ||
763 | @group | |
764 | (defun user () ; @r{@code{x} is used in @code{user}.} | |
765 | (list x)) | |
766 | @end group | |
767 | @end example | |
768 | ||
769 | In a lexically scoped language, the binding of @code{x} in | |
770 | @code{binder} would never be accessible in @code{user}, because | |
771 | @code{user} is not textually contained within the function | |
772 | @code{binder}. However, in dynamically scoped Emacs Lisp, @code{user} | |
773 | may or may not refer to the binding of @code{x} established in | |
774 | @code{binder}, depending on circumstances: | |
775 | ||
776 | @itemize @bullet | |
777 | @item | |
778 | If we call @code{user} directly without calling @code{binder} at all, | |
779 | then whatever binding of @code{x} is found, it cannot come from | |
780 | @code{binder}. | |
781 | ||
782 | @item | |
783 | If we define @code{foo} as follows and call @code{binder}, then the | |
784 | binding made in @code{binder} will be seen in @code{user}: | |
785 | ||
786 | @example | |
787 | @group | |
788 | (defun foo (lose) | |
789 | (user)) | |
790 | @end group | |
791 | @end example | |
792 | ||
793 | @item | |
794 | If we define @code{foo} as follows and call @code{binder}, then the | |
795 | binding made in @code{binder} @emph{will not} be seen in @code{user}: | |
796 | ||
797 | @example | |
798 | (defun foo (x) | |
799 | (user)) | |
800 | @end example | |
801 | ||
802 | @noindent | |
803 | Here, when @code{foo} is called by @code{binder}, it binds @code{x}. | |
804 | (The binding in @code{foo} is said to @dfn{shadow} the one made in | |
805 | @code{binder}.) Therefore, @code{user} will access the @code{x} bound | |
806 | by @code{foo} instead of the one bound by @code{binder}. | |
807 | @end itemize | |
808 | ||
809 | @node Extent | |
810 | @subsection Extent | |
811 | ||
812 | @dfn{Extent} refers to the time during program execution that a | |
813 | variable name is valid. In Emacs Lisp, a variable is valid only while | |
814 | the form that bound it is executing. This is called @dfn{dynamic | |
815 | extent}. ``Local'' or ``automatic'' variables in most languages, | |
816 | including C and Pascal, have dynamic extent. | |
817 | ||
818 | One alternative to dynamic extent is @dfn{indefinite extent}. This | |
819 | means that a variable binding can live on past the exit from the form | |
820 | that made the binding. Common Lisp and Scheme, for example, support | |
821 | this, but Emacs Lisp does not. | |
822 | ||
823 | To illustrate this, the function below, @code{make-add}, returns a | |
824 | function that purports to add @var{n} to its own argument @var{m}. | |
825 | This would work in Common Lisp, but it does not work as intended in | |
826 | Emacs Lisp, because after the call to @code{make-add} exits, the | |
827 | variable @code{n} is no longer bound to the actual argument 2. | |
828 | ||
829 | @example | |
830 | (defun make-add (n) | |
831 | (function (lambda (m) (+ n m)))) ; @r{Return a function.} | |
832 | @result{} make-add | |
833 | (fset 'add2 (make-add 2)) ; @r{Define function @code{add2}} | |
834 | ; @r{with @code{(make-add 2)}.} | |
835 | @result{} (lambda (m) (+ n m)) | |
836 | (add2 4) ; @r{Try to add 2 to 4.} | |
837 | @error{} Symbol's value as variable is void: n | |
838 | @end example | |
839 | ||
840 | @cindex closures not available | |
841 | Some Lisp dialects have ``closures'', objects that are like functions | |
842 | but record additional variable bindings. Emacs Lisp does not have | |
843 | closures. | |
844 | ||
845 | @node Impl of Scope | |
846 | @subsection Implementation of Dynamic Scoping | |
847 | @cindex deep binding | |
848 | ||
849 | A simple sample implementation (which is not how Emacs Lisp actually | |
850 | works) may help you understand dynamic binding. This technique is | |
851 | called @dfn{deep binding} and was used in early Lisp systems. | |
852 | ||
853 | Suppose there is a stack of bindings: variable-value pairs. At entry | |
854 | to a function or to a @code{let} form, we can push bindings on the stack | |
855 | for the arguments or local variables created there. We can pop those | |
856 | bindings from the stack at exit from the binding construct. | |
857 | ||
858 | We can find the value of a variable by searching the stack from top to | |
859 | bottom for a binding for that variable; the value from that binding is | |
860 | the value of the variable. To set the variable, we search for the | |
861 | current binding, then store the new value into that binding. | |
862 | ||
863 | As you can see, a function's bindings remain in effect as long as it | |
864 | continues execution, even during its calls to other functions. That is | |
865 | why we say the extent of the binding is dynamic. And any other function | |
866 | can refer to the bindings, if it uses the same variables while the | |
867 | bindings are in effect. That is why we say the scope is indefinite. | |
868 | ||
869 | @cindex shallow binding | |
870 | The actual implementation of variable scoping in GNU Emacs Lisp uses a | |
871 | technique called @dfn{shallow binding}. Each variable has a standard | |
872 | place in which its current value is always found---the value cell of the | |
873 | symbol. | |
874 | ||
875 | In shallow binding, setting the variable works by storing a value in | |
876 | the value cell. Creating a new binding works by pushing the old value | |
877 | (belonging to a previous binding) on a stack, and storing the local value | |
878 | in the value cell. Eliminating a binding works by popping the old value | |
879 | off the stack, into the value cell. | |
880 | ||
881 | We use shallow binding because it has the same results as deep | |
882 | binding, but runs faster, since there is never a need to search for a | |
883 | binding. | |
884 | ||
885 | @node Using Scoping | |
886 | @subsection Proper Use of Dynamic Scoping | |
887 | ||
888 | Binding a variable in one function and using it in another is a | |
889 | powerful technique, but if used without restraint, it can make programs | |
890 | hard to understand. There are two clean ways to use this technique: | |
891 | ||
892 | @itemize @bullet | |
893 | @item | |
894 | Use or bind the variable only in a few related functions, written close | |
895 | together in one file. Such a variable is used for communication within | |
896 | one program. | |
897 | ||
898 | You should write comments to inform other programmers that they can see | |
899 | all uses of the variable before them, and to advise them not to add uses | |
900 | elsewhere. | |
901 | ||
902 | @item | |
903 | Give the variable a well-defined, documented meaning, and make all | |
904 | appropriate functions refer to it (but not bind it or set it) wherever | |
905 | that meaning is relevant. For example, the variable | |
906 | @code{case-fold-search} is defined as ``non-@code{nil} means ignore case | |
907 | when searching''; various search and replace functions refer to it | |
908 | directly or through their subroutines, but do not bind or set it. | |
909 | ||
910 | Then you can bind the variable in other programs, knowing reliably what | |
911 | the effect will be. | |
912 | @end itemize | |
913 | ||
914 | @node Buffer-Local Variables | |
915 | @section Buffer-Local Variables | |
916 | @cindex variables, buffer-local | |
917 | @cindex buffer-local variables | |
918 | ||
919 | Global and local variable bindings are found in most programming | |
920 | languages in one form or another. Emacs also supports another, unusual | |
921 | kind of variable binding: @dfn{buffer-local} bindings, which apply only | |
922 | to one buffer. Emacs Lisp is meant for programming editing commands, | |
923 | and having different values for a variable in different buffers is an | |
924 | important customization method. | |
925 | ||
926 | @menu | |
927 | * Intro to Buffer-Local:: Introduction and concepts. | |
928 | * Creating Buffer-Local:: Creating and destroying buffer-local bindings. | |
929 | * Default Value:: The default value is seen in buffers | |
930 | that don't have their own local values. | |
931 | @end menu | |
932 | ||
933 | @node Intro to Buffer-Local | |
934 | @subsection Introduction to Buffer-Local Variables | |
935 | ||
936 | A buffer-local variable has a buffer-local binding associated with a | |
937 | particular buffer. The binding is in effect when that buffer is | |
938 | current; otherwise, it is not in effect. If you set the variable while | |
939 | a buffer-local binding is in effect, the new value goes in that binding, | |
940 | so the global binding is unchanged; this means that the change is | |
941 | visible in that buffer alone. | |
942 | ||
943 | A variable may have buffer-local bindings in some buffers but not in | |
944 | others. The global binding is shared by all the buffers that don't have | |
945 | their own bindings. Thus, if you set the variable in a buffer that does | |
946 | not have a buffer-local binding for it, the new value is visible in all | |
947 | buffers except those with buffer-local bindings. (Here we are assuming | |
948 | that there are no @code{let}-style local bindings to complicate the issue.) | |
949 | ||
950 | The most common use of buffer-local bindings is for major modes to change | |
951 | variables that control the behavior of commands. For example, C mode and | |
952 | Lisp mode both set the variable @code{paragraph-start} to specify that only | |
953 | blank lines separate paragraphs. They do this by making the variable | |
954 | buffer-local in the buffer that is being put into C mode or Lisp mode, and | |
955 | then setting it to the new value for that mode. | |
956 | ||
957 | The usual way to make a buffer-local binding is with | |
958 | @code{make-local-variable}, which is what major mode commands use. This | |
959 | affects just the current buffer; all other buffers (including those yet to | |
960 | be created) continue to share the global value. | |
961 | ||
962 | @cindex automatically buffer-local | |
963 | A more powerful operation is to mark the variable as | |
964 | @dfn{automatically buffer-local} by calling | |
965 | @code{make-variable-buffer-local}. You can think of this as making the | |
966 | variable local in all buffers, even those yet to be created. More | |
967 | precisely, the effect is that setting the variable automatically makes | |
968 | the variable local to the current buffer if it is not already so. All | |
969 | buffers start out by sharing the global value of the variable as usual, | |
970 | but any @code{setq} creates a buffer-local binding for the current | |
971 | buffer. The new value is stored in the buffer-local binding, leaving | |
972 | the (default) global binding untouched. The global value can no longer | |
973 | be changed with @code{setq}; you need to use @code{setq-default} to do | |
974 | that. | |
975 | ||
b22f3a19 | 976 | @strong{Warning:} When a variable has local values in one or more |
e6512bcf RS |
977 | buffers, you can get Emacs very confused by binding the variable with |
978 | @code{let}, changing to a different current buffer in which a different | |
979 | binding is in effect, and then exiting the @code{let}. This can | |
980 | scramble the values of the global and local bindings. | |
981 | ||
982 | To preserve your sanity, avoid that series of actions. If you use | |
983 | @code{save-excursion} around each piece of code that changes to a | |
984 | different current buffer, you will not have this problem. Here is an | |
985 | example of what to avoid: | |
986 | ||
987 | @example | |
988 | @group | |
989 | (setq foo 'b) | |
990 | (set-buffer "a") | |
991 | (make-local-variable 'foo) | |
992 | @end group | |
993 | (setq foo 'a) | |
994 | (let ((foo 'temp)) | |
995 | (set-buffer "b") | |
996 | @dots{}) | |
997 | @group | |
998 | foo @result{} 'a ; @r{The old buffer-local value from buffer @samp{a}} | |
999 | ; @r{is now the default value.} | |
1000 | @end group | |
1001 | @group | |
1002 | (set-buffer "a") | |
1003 | foo @result{} 'temp ; @r{The local value that should be gone} | |
1004 | ; @r{is now the buffer-local value in buffer @samp{a}.} | |
1005 | @end group | |
1006 | @end example | |
1007 | ||
1008 | @noindent | |
1009 | But @code{save-excursion} as shown here avoids the problem: | |
1010 | ||
1011 | @example | |
1012 | @group | |
1013 | (let ((foo 'temp)) | |
1014 | (save-excursion | |
1015 | (set-buffer "b") | |
1016 | @var{body}@dots{})) | |
1017 | @end group | |
1018 | @end example | |
1019 | ||
1020 | Note that references to @code{foo} in @var{body} access the | |
1021 | buffer-local binding of buffer @samp{b}. | |
1022 | ||
1023 | When a file specifies local variable values, these become buffer-local | |
f57ddf67 | 1024 | values when you visit the file. @xref{Auto Major Mode}. |
e6512bcf RS |
1025 | |
1026 | @node Creating Buffer-Local | |
1027 | @subsection Creating and Deleting Buffer-Local Bindings | |
1028 | ||
1029 | @deffn Command make-local-variable variable | |
1030 | This function creates a buffer-local binding in the current buffer for | |
1031 | @var{variable} (a symbol). Other buffers are not affected. The value | |
1032 | returned is @var{variable}. | |
1033 | ||
1034 | @c Emacs 19 feature | |
1035 | The buffer-local value of @var{variable} starts out as the same value | |
1036 | @var{variable} previously had. If @var{variable} was void, it remains | |
1037 | void. | |
1038 | ||
1039 | @example | |
1040 | @group | |
1041 | ;; @r{In buffer @samp{b1}:} | |
1042 | (setq foo 5) ; @r{Affects all buffers.} | |
1043 | @result{} 5 | |
1044 | @end group | |
1045 | @group | |
1046 | (make-local-variable 'foo) ; @r{Now it is local in @samp{b1}.} | |
1047 | @result{} foo | |
1048 | @end group | |
1049 | @group | |
1050 | foo ; @r{That did not change} | |
1051 | @result{} 5 ; @r{the value.} | |
1052 | @end group | |
1053 | @group | |
1054 | (setq foo 6) ; @r{Change the value} | |
1055 | @result{} 6 ; @r{in @samp{b1}.} | |
1056 | @end group | |
1057 | @group | |
1058 | foo | |
1059 | @result{} 6 | |
1060 | @end group | |
1061 | ||
1062 | @group | |
1063 | ;; @r{In buffer @samp{b2}, the value hasn't changed.} | |
1064 | (save-excursion | |
1065 | (set-buffer "b2") | |
1066 | foo) | |
1067 | @result{} 5 | |
1068 | @end group | |
1069 | @end example | |
e8505179 RS |
1070 | |
1071 | Making a variable buffer-local within a @code{let}-binding for that | |
1072 | variable does not work. This is because @code{let} does not distinguish | |
1073 | between different kinds of bindings; it knows only which variable the | |
1074 | binding was made for. | |
e6512bcf RS |
1075 | @end deffn |
1076 | ||
1077 | @deffn Command make-variable-buffer-local variable | |
1078 | This function marks @var{variable} (a symbol) automatically | |
1079 | buffer-local, so that any subsequent attempt to set it will make it | |
1080 | local to the current buffer at the time. | |
1081 | ||
1082 | The value returned is @var{variable}. | |
1083 | @end deffn | |
1084 | ||
1085 | @defun buffer-local-variables &optional buffer | |
1086 | This function returns a list describing the buffer-local variables in | |
1087 | buffer @var{buffer}. It returns an association list (@pxref{Association | |
1088 | Lists}) in which each association contains one buffer-local variable and | |
1089 | its value. When a buffer-local variable is void in @var{buffer}, then | |
1090 | it appears directly in the resulting list. If @var{buffer} is omitted, | |
1091 | the current buffer is used. | |
1092 | ||
1093 | @example | |
1094 | @group | |
1095 | (make-local-variable 'foobar) | |
1096 | (makunbound 'foobar) | |
1097 | (make-local-variable 'bind-me) | |
1098 | (setq bind-me 69) | |
1099 | @end group | |
1100 | (setq lcl (buffer-local-variables)) | |
1101 | ;; @r{First, built-in variables local in all buffers:} | |
1102 | @result{} ((mark-active . nil) | |
1103 | (buffer-undo-list nil) | |
1104 | (mode-name . "Fundamental") | |
1105 | @dots{} | |
1106 | @group | |
1107 | ;; @r{Next, non-built-in local variables.} | |
1108 | ;; @r{This one is local and void:} | |
1109 | foobar | |
1110 | ;; @r{This one is local and nonvoid:} | |
1111 | (bind-me . 69)) | |
1112 | @end group | |
1113 | @end example | |
1114 | ||
1115 | Note that storing new values into the @sc{cdr}s of cons cells in this | |
1116 | list does @emph{not} change the local values of the variables. | |
1117 | @end defun | |
1118 | ||
1119 | @deffn Command kill-local-variable variable | |
1120 | This function deletes the buffer-local binding (if any) for | |
1121 | @var{variable} (a symbol) in the current buffer. As a result, the | |
1122 | global (default) binding of @var{variable} becomes visible in this | |
1123 | buffer. Usually this results in a change in the value of | |
1124 | @var{variable}, since the global value is usually different from the | |
1125 | buffer-local value just eliminated. | |
1126 | ||
1127 | If you kill the local binding of a variable that automatically becomes | |
1128 | local when set, this makes the global value visible in the current | |
1129 | buffer. However, if you set the variable again, that will once again | |
1130 | create a local binding for it. | |
1131 | ||
1132 | @code{kill-local-variable} returns @var{variable}. | |
f57ddf67 RS |
1133 | |
1134 | This function is a command because it is sometimes useful to kill one | |
1135 | buffer-local variable interactively, just as it is useful to create | |
1136 | buffer-local variables interactively. | |
e6512bcf RS |
1137 | @end deffn |
1138 | ||
1139 | @defun kill-all-local-variables | |
1140 | This function eliminates all the buffer-local variable bindings of the | |
1141 | current buffer except for variables marked as ``permanent''. As a | |
1142 | result, the buffer will see the default values of most variables. | |
1143 | ||
1144 | This function also resets certain other information pertaining to the | |
1145 | buffer: it sets the local keymap to @code{nil}, the syntax table to the | |
1146 | value of @code{standard-syntax-table}, and the abbrev table to the value | |
1147 | of @code{fundamental-mode-abbrev-table}. | |
1148 | ||
1149 | Every major mode command begins by calling this function, which has the | |
1150 | effect of switching to Fundamental mode and erasing most of the effects | |
1151 | of the previous major mode. To ensure that this does its job, the | |
1152 | variables that major modes set should not be marked permanent. | |
1153 | ||
1154 | @code{kill-all-local-variables} returns @code{nil}. | |
1155 | @end defun | |
1156 | ||
1157 | @c Emacs 19 feature | |
1158 | @cindex permanent local variable | |
1159 | A local variable is @dfn{permanent} if the variable name (a symbol) has a | |
1160 | @code{permanent-local} property that is non-@code{nil}. Permanent | |
1161 | locals are appropriate for data pertaining to where the file came from | |
1162 | or how to save it, rather than with how to edit the contents. | |
1163 | ||
1164 | @node Default Value | |
1165 | @subsection The Default Value of a Buffer-Local Variable | |
1166 | @cindex default value | |
1167 | ||
1168 | The global value of a variable with buffer-local bindings is also | |
1169 | called the @dfn{default} value, because it is the value that is in | |
1170 | effect except when specifically overridden. | |
1171 | ||
1172 | The functions @code{default-value} and @code{setq-default} access and | |
1173 | change a variable's default value regardless of whether the current | |
1174 | buffer has a buffer-local binding. For example, you could use | |
1175 | @code{setq-default} to change the default setting of | |
1176 | @code{paragraph-start} for most buffers; and this would work even when | |
f57ddf67 | 1177 | you are in a C or Lisp mode buffer that has a buffer-local value for |
e6512bcf RS |
1178 | this variable. |
1179 | ||
1180 | @c Emacs 19 feature | |
1181 | The special forms @code{defvar} and @code{defconst} also set the | |
1182 | default value (if they set the variable at all), rather than any local | |
1183 | value. | |
1184 | ||
1185 | @defun default-value symbol | |
1186 | This function returns @var{symbol}'s default value. This is the value | |
1187 | that is seen in buffers that do not have their own values for this | |
1188 | variable. If @var{symbol} is not buffer-local, this is equivalent to | |
1189 | @code{symbol-value} (@pxref{Accessing Variables}). | |
1190 | @end defun | |
1191 | ||
1192 | @c Emacs 19 feature | |
f57ddf67 RS |
1193 | @defun default-boundp symbol |
1194 | The function @code{default-boundp} tells you whether @var{symbol}'s | |
e6512bcf RS |
1195 | default value is nonvoid. If @code{(default-boundp 'foo)} returns |
1196 | @code{nil}, then @code{(default-value 'foo)} would get an error. | |
1197 | ||
1198 | @code{default-boundp} is to @code{default-value} as @code{boundp} is to | |
1199 | @code{symbol-value}. | |
1200 | @end defun | |
1201 | ||
1202 | @defspec setq-default symbol value | |
1203 | This sets the default value of @var{symbol} to @var{value}. It does not | |
1204 | evaluate @var{symbol}, but does evaluate @var{value}. The value of the | |
1205 | @code{setq-default} form is @var{value}. | |
1206 | ||
1207 | If a @var{symbol} is not buffer-local for the current buffer, and is not | |
1208 | marked automatically buffer-local, @code{setq-default} has the same | |
1209 | effect as @code{setq}. If @var{symbol} is buffer-local for the current | |
1210 | buffer, then this changes the value that other buffers will see (as long | |
1211 | as they don't have a buffer-local value), but not the value that the | |
1212 | current buffer sees. | |
1213 | ||
1214 | @example | |
1215 | @group | |
1216 | ;; @r{In buffer @samp{foo}:} | |
1217 | (make-local-variable 'local) | |
1218 | @result{} local | |
1219 | @end group | |
1220 | @group | |
1221 | (setq local 'value-in-foo) | |
1222 | @result{} value-in-foo | |
1223 | @end group | |
1224 | @group | |
1225 | (setq-default local 'new-default) | |
1226 | @result{} new-default | |
1227 | @end group | |
1228 | @group | |
1229 | local | |
1230 | @result{} value-in-foo | |
1231 | @end group | |
1232 | @group | |
1233 | (default-value 'local) | |
1234 | @result{} new-default | |
1235 | @end group | |
1236 | ||
1237 | @group | |
1238 | ;; @r{In (the new) buffer @samp{bar}:} | |
1239 | local | |
1240 | @result{} new-default | |
1241 | @end group | |
1242 | @group | |
1243 | (default-value 'local) | |
1244 | @result{} new-default | |
1245 | @end group | |
1246 | @group | |
1247 | (setq local 'another-default) | |
1248 | @result{} another-default | |
1249 | @end group | |
1250 | @group | |
1251 | (default-value 'local) | |
1252 | @result{} another-default | |
1253 | @end group | |
1254 | ||
1255 | @group | |
1256 | ;; @r{Back in buffer @samp{foo}:} | |
1257 | local | |
1258 | @result{} value-in-foo | |
1259 | (default-value 'local) | |
1260 | @result{} another-default | |
1261 | @end group | |
1262 | @end example | |
1263 | @end defspec | |
1264 | ||
1265 | @defun set-default symbol value | |
1266 | This function is like @code{setq-default}, except that @var{symbol} is | |
1267 | evaluated. | |
1268 | ||
1269 | @example | |
1270 | @group | |
1271 | (set-default (car '(a b c)) 23) | |
1272 | @result{} 23 | |
1273 | @end group | |
1274 | @group | |
1275 | (default-value 'a) | |
1276 | @result{} 23 | |
1277 | @end group | |
1278 | @end example | |
1279 | @end defun | |
1280 |