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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, 2000, | |
4 | @c 2001, 2002, 2003, 2004, 2005, 2006, 2007 Free Software Foundation, Inc. | |
5 | @c See the file elisp.texi for copying conditions. | |
6336d8c3 | 6 | @setfilename ../../info/variables |
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7 | @node Variables, Functions, Control Structures, Top |
8 | @chapter Variables | |
9 | @cindex variable | |
10 | ||
11 | A @dfn{variable} is a name used in a program to stand for a value. | |
12 | Nearly all programming languages have variables of some sort. In the | |
13 | text of a Lisp program, variables are written using the syntax for | |
14 | symbols. | |
15 | ||
16 | In Lisp, unlike most programming languages, programs are represented | |
17 | primarily as Lisp objects and only secondarily as text. The Lisp | |
18 | objects used for variables are symbols: the symbol name is the variable | |
19 | name, and the variable's value is stored in the value cell of the | |
20 | symbol. The use of a symbol as a variable is independent of its use as | |
21 | a function name. @xref{Symbol Components}. | |
22 | ||
23 | The Lisp objects that constitute a Lisp program determine the textual | |
24 | form of the program---it is simply the read syntax for those Lisp | |
25 | objects. This is why, for example, a variable in a textual Lisp program | |
26 | is written using the read syntax for the symbol that represents the | |
27 | variable. | |
28 | ||
29 | @menu | |
30 | * Global Variables:: Variable values that exist permanently, everywhere. | |
31 | * Constant Variables:: Certain "variables" have values that never change. | |
32 | * Local Variables:: Variable values that exist only temporarily. | |
33 | * Void Variables:: Symbols that lack values. | |
34 | * Defining Variables:: A definition says a symbol is used as a variable. | |
35 | * Tips for Defining:: Things you should think about when you | |
36 | define a variable. | |
37 | * Accessing Variables:: Examining values of variables whose names | |
38 | are known only at run time. | |
39 | * Setting Variables:: Storing new values in variables. | |
40 | * Variable Scoping:: How Lisp chooses among local and global values. | |
41 | * Buffer-Local Variables:: Variable values in effect only in one buffer. | |
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42 | * Future Local Variables:: New kinds of local values we might add some day. |
43 | * File Local Variables:: Handling local variable lists in files. | |
44 | * Variable Aliases:: Variables that are aliases for other variables. | |
45 | * Variables with Restricted Values:: Non-constant variables whose value can | |
46 | @emph{not} be an arbitrary Lisp object. | |
47 | @end menu | |
48 | ||
49 | @node Global Variables | |
50 | @section Global Variables | |
51 | @cindex global variable | |
52 | ||
53 | The simplest way to use a variable is @dfn{globally}. This means that | |
54 | the variable has just one value at a time, and this value is in effect | |
55 | (at least for the moment) throughout the Lisp system. The value remains | |
56 | in effect until you specify a new one. When a new value replaces the | |
57 | old one, no trace of the old value remains in the variable. | |
58 | ||
59 | You specify a value for a symbol with @code{setq}. For example, | |
60 | ||
61 | @example | |
62 | (setq x '(a b)) | |
63 | @end example | |
64 | ||
65 | @noindent | |
66 | gives the variable @code{x} the value @code{(a b)}. Note that | |
67 | @code{setq} does not evaluate its first argument, the name of the | |
68 | variable, but it does evaluate the second argument, the new value. | |
69 | ||
70 | Once the variable has a value, you can refer to it by using the symbol | |
71 | by itself as an expression. Thus, | |
72 | ||
73 | @example | |
74 | @group | |
75 | x @result{} (a b) | |
76 | @end group | |
77 | @end example | |
78 | ||
79 | @noindent | |
80 | assuming the @code{setq} form shown above has already been executed. | |
81 | ||
82 | If you do set the same variable again, the new value replaces the old | |
83 | one: | |
84 | ||
85 | @example | |
86 | @group | |
87 | x | |
88 | @result{} (a b) | |
89 | @end group | |
90 | @group | |
91 | (setq x 4) | |
92 | @result{} 4 | |
93 | @end group | |
94 | @group | |
95 | x | |
96 | @result{} 4 | |
97 | @end group | |
98 | @end example | |
99 | ||
100 | @node Constant Variables | |
101 | @section Variables that Never Change | |
102 | @kindex setting-constant | |
103 | @cindex keyword symbol | |
104 | @cindex variable with constant value | |
105 | @cindex constant variables | |
106 | @cindex symbol that evaluates to itself | |
107 | @cindex symbol with constant value | |
108 | ||
109 | In Emacs Lisp, certain symbols normally evaluate to themselves. These | |
110 | include @code{nil} and @code{t}, as well as any symbol whose name starts | |
111 | with @samp{:} (these are called @dfn{keywords}). These symbols cannot | |
112 | be rebound, nor can their values be changed. Any attempt to set or bind | |
113 | @code{nil} or @code{t} signals a @code{setting-constant} error. The | |
114 | same is true for a keyword (a symbol whose name starts with @samp{:}), | |
115 | if it is interned in the standard obarray, except that setting such a | |
116 | symbol to itself is not an error. | |
117 | ||
118 | @example | |
119 | @group | |
120 | nil @equiv{} 'nil | |
121 | @result{} nil | |
122 | @end group | |
123 | @group | |
124 | (setq nil 500) | |
125 | @error{} Attempt to set constant symbol: nil | |
126 | @end group | |
127 | @end example | |
128 | ||
129 | @defun keywordp object | |
130 | function returns @code{t} if @var{object} is a symbol whose name | |
131 | starts with @samp{:}, interned in the standard obarray, and returns | |
132 | @code{nil} otherwise. | |
133 | @end defun | |
134 | ||
135 | @node Local Variables | |
136 | @section Local Variables | |
137 | @cindex binding local variables | |
138 | @cindex local variables | |
139 | @cindex local binding | |
140 | @cindex global binding | |
141 | ||
142 | Global variables have values that last until explicitly superseded | |
143 | with new values. Sometimes it is useful to create variable values that | |
144 | exist temporarily---only until a certain part of the program finishes. | |
145 | These values are called @dfn{local}, and the variables so used are | |
146 | called @dfn{local variables}. | |
147 | ||
148 | For example, when a function is called, its argument variables receive | |
149 | new local values that last until the function exits. The @code{let} | |
150 | special form explicitly establishes new local values for specified | |
151 | variables; these last until exit from the @code{let} form. | |
152 | ||
153 | @cindex shadowing of variables | |
154 | Establishing a local value saves away the previous value (or lack of | |
155 | one) of the variable. When the life span of the local value is over, | |
156 | the previous value is restored. In the mean time, we say that the | |
157 | previous value is @dfn{shadowed} and @dfn{not visible}. Both global and | |
158 | local values may be shadowed (@pxref{Scope}). | |
159 | ||
160 | If you set a variable (such as with @code{setq}) while it is local, | |
161 | this replaces the local value; it does not alter the global value, or | |
162 | previous local values, that are shadowed. To model this behavior, we | |
163 | speak of a @dfn{local binding} of the variable as well as a local value. | |
164 | ||
165 | The local binding is a conceptual place that holds a local value. | |
166 | Entry to a function, or a special form such as @code{let}, creates the | |
167 | local binding; exit from the function or from the @code{let} removes the | |
168 | local binding. As long as the local binding lasts, the variable's value | |
169 | is stored within it. Use of @code{setq} or @code{set} while there is a | |
170 | local binding stores a different value into the local binding; it does | |
171 | not create a new binding. | |
172 | ||
173 | We also speak of the @dfn{global binding}, which is where | |
174 | (conceptually) the global value is kept. | |
175 | ||
176 | @cindex current binding | |
177 | A variable can have more than one local binding at a time (for | |
178 | example, if there are nested @code{let} forms that bind it). In such a | |
179 | case, the most recently created local binding that still exists is the | |
180 | @dfn{current binding} of the variable. (This rule is called | |
181 | @dfn{dynamic scoping}; see @ref{Variable Scoping}.) If there are no | |
182 | local bindings, the variable's global binding is its current binding. | |
183 | We sometimes call the current binding the @dfn{most-local existing | |
184 | binding}, for emphasis. Ordinary evaluation of a symbol always returns | |
185 | the value of its current binding. | |
186 | ||
187 | The special forms @code{let} and @code{let*} exist to create | |
188 | local bindings. | |
189 | ||
190 | @defspec let (bindings@dots{}) forms@dots{} | |
191 | This special form binds variables according to @var{bindings} and then | |
192 | evaluates all of the @var{forms} in textual order. The @code{let}-form | |
193 | returns the value of the last form in @var{forms}. | |
194 | ||
195 | Each of the @var{bindings} is either @w{(i) a} symbol, in which case | |
196 | that symbol is bound to @code{nil}; or @w{(ii) a} list of the form | |
197 | @code{(@var{symbol} @var{value-form})}, in which case @var{symbol} is | |
198 | bound to the result of evaluating @var{value-form}. If @var{value-form} | |
199 | is omitted, @code{nil} is used. | |
200 | ||
201 | All of the @var{value-form}s in @var{bindings} are evaluated in the | |
202 | order they appear and @emph{before} binding any of the symbols to them. | |
203 | Here is an example of this: @code{z} is bound to the old value of | |
204 | @code{y}, which is 2, not the new value of @code{y}, which is 1. | |
205 | ||
206 | @example | |
207 | @group | |
208 | (setq y 2) | |
209 | @result{} 2 | |
210 | @end group | |
211 | @group | |
212 | (let ((y 1) | |
213 | (z y)) | |
214 | (list y z)) | |
215 | @result{} (1 2) | |
216 | @end group | |
217 | @end example | |
218 | @end defspec | |
219 | ||
220 | @defspec let* (bindings@dots{}) forms@dots{} | |
221 | This special form is like @code{let}, but it binds each variable right | |
222 | after computing its local value, before computing the local value for | |
223 | the next variable. Therefore, an expression in @var{bindings} can | |
224 | reasonably refer to the preceding symbols bound in this @code{let*} | |
225 | form. Compare the following example with the example above for | |
226 | @code{let}. | |
227 | ||
228 | @example | |
229 | @group | |
230 | (setq y 2) | |
231 | @result{} 2 | |
232 | @end group | |
233 | @group | |
234 | (let* ((y 1) | |
235 | (z y)) ; @r{Use the just-established value of @code{y}.} | |
236 | (list y z)) | |
237 | @result{} (1 1) | |
238 | @end group | |
239 | @end example | |
240 | @end defspec | |
241 | ||
242 | Here is a complete list of the other facilities that create local | |
243 | bindings: | |
244 | ||
245 | @itemize @bullet | |
246 | @item | |
247 | Function calls (@pxref{Functions}). | |
248 | ||
249 | @item | |
250 | Macro calls (@pxref{Macros}). | |
251 | ||
252 | @item | |
253 | @code{condition-case} (@pxref{Errors}). | |
254 | @end itemize | |
255 | ||
256 | Variables can also have buffer-local bindings (@pxref{Buffer-Local | |
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257 | Variables}); a few variables have terminal-local bindings |
258 | (@pxref{Multiple Displays}). These kinds of bindings work somewhat | |
259 | like ordinary local bindings, but they are localized depending on | |
260 | ``where'' you are in Emacs, rather than localized in time. | |
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261 | |
262 | @defvar max-specpdl-size | |
263 | @anchor{Definition of max-specpdl-size} | |
264 | @cindex variable limit error | |
265 | @cindex evaluation error | |
266 | @cindex infinite recursion | |
267 | This variable defines the limit on the total number of local variable | |
268 | bindings and @code{unwind-protect} cleanups (@pxref{Cleanups,, | |
269 | Cleaning Up from Nonlocal Exits}) that are allowed before signaling an | |
270 | error (with data @code{"Variable binding depth exceeds | |
271 | max-specpdl-size"}). | |
272 | ||
273 | This limit, with the associated error when it is exceeded, is one way | |
274 | that Lisp avoids infinite recursion on an ill-defined function. | |
275 | @code{max-lisp-eval-depth} provides another limit on depth of nesting. | |
276 | @xref{Definition of max-lisp-eval-depth,, Eval}. | |
277 | ||
278 | The default value is 1000. Entry to the Lisp debugger increases the | |
279 | value, if there is little room left, to make sure the debugger itself | |
280 | has room to execute. | |
281 | @end defvar | |
282 | ||
283 | @node Void Variables | |
284 | @section When a Variable is ``Void'' | |
285 | @kindex void-variable | |
286 | @cindex void variable | |
287 | ||
288 | If you have never given a symbol any value as a global variable, we | |
289 | say that that symbol's global value is @dfn{void}. In other words, the | |
290 | symbol's value cell does not have any Lisp object in it. If you try to | |
291 | evaluate the symbol, you get a @code{void-variable} error rather than | |
292 | a value. | |
293 | ||
294 | Note that a value of @code{nil} is not the same as void. The symbol | |
295 | @code{nil} is a Lisp object and can be the value of a variable just as any | |
296 | other object can be; but it is @emph{a value}. A void variable does not | |
297 | have any value. | |
298 | ||
299 | After you have given a variable a value, you can make it void once more | |
300 | using @code{makunbound}. | |
301 | ||
302 | @defun makunbound symbol | |
303 | This function makes the current variable binding of @var{symbol} void. | |
304 | Subsequent attempts to use this symbol's value as a variable will signal | |
305 | the error @code{void-variable}, unless and until you set it again. | |
306 | ||
307 | @code{makunbound} returns @var{symbol}. | |
308 | ||
309 | @example | |
310 | @group | |
311 | (makunbound 'x) ; @r{Make the global value of @code{x} void.} | |
312 | @result{} x | |
313 | @end group | |
314 | @group | |
315 | x | |
316 | @error{} Symbol's value as variable is void: x | |
317 | @end group | |
318 | @end example | |
319 | ||
320 | If @var{symbol} is locally bound, @code{makunbound} affects the most | |
321 | local existing binding. This is the only way a symbol can have a void | |
322 | local binding, since all the constructs that create local bindings | |
323 | create them with values. In this case, the voidness lasts at most as | |
324 | long as the binding does; when the binding is removed due to exit from | |
325 | the construct that made it, the previous local or global binding is | |
326 | reexposed as usual, and the variable is no longer void unless the newly | |
327 | reexposed binding was void all along. | |
328 | ||
329 | @smallexample | |
330 | @group | |
331 | (setq x 1) ; @r{Put a value in the global binding.} | |
332 | @result{} 1 | |
333 | (let ((x 2)) ; @r{Locally bind it.} | |
334 | (makunbound 'x) ; @r{Void the local binding.} | |
335 | x) | |
336 | @error{} Symbol's value as variable is void: x | |
337 | @end group | |
338 | @group | |
339 | x ; @r{The global binding is unchanged.} | |
340 | @result{} 1 | |
341 | ||
342 | (let ((x 2)) ; @r{Locally bind it.} | |
343 | (let ((x 3)) ; @r{And again.} | |
344 | (makunbound 'x) ; @r{Void the innermost-local binding.} | |
345 | x)) ; @r{And refer: it's void.} | |
346 | @error{} Symbol's value as variable is void: x | |
347 | @end group | |
348 | ||
349 | @group | |
350 | (let ((x 2)) | |
351 | (let ((x 3)) | |
352 | (makunbound 'x)) ; @r{Void inner binding, then remove it.} | |
353 | x) ; @r{Now outer @code{let} binding is visible.} | |
354 | @result{} 2 | |
355 | @end group | |
356 | @end smallexample | |
357 | @end defun | |
358 | ||
359 | A variable that has been made void with @code{makunbound} is | |
360 | indistinguishable from one that has never received a value and has | |
361 | always been void. | |
362 | ||
363 | You can use the function @code{boundp} to test whether a variable is | |
364 | currently void. | |
365 | ||
366 | @defun boundp variable | |
367 | @code{boundp} returns @code{t} if @var{variable} (a symbol) is not void; | |
368 | more precisely, if its current binding is not void. It returns | |
369 | @code{nil} otherwise. | |
370 | ||
371 | @smallexample | |
372 | @group | |
373 | (boundp 'abracadabra) ; @r{Starts out void.} | |
374 | @result{} nil | |
375 | @end group | |
376 | @group | |
377 | (let ((abracadabra 5)) ; @r{Locally bind it.} | |
378 | (boundp 'abracadabra)) | |
379 | @result{} t | |
380 | @end group | |
381 | @group | |
382 | (boundp 'abracadabra) ; @r{Still globally void.} | |
383 | @result{} nil | |
384 | @end group | |
385 | @group | |
386 | (setq abracadabra 5) ; @r{Make it globally nonvoid.} | |
387 | @result{} 5 | |
388 | @end group | |
389 | @group | |
390 | (boundp 'abracadabra) | |
391 | @result{} t | |
392 | @end group | |
393 | @end smallexample | |
394 | @end defun | |
395 | ||
396 | @node Defining Variables | |
397 | @section Defining Global Variables | |
398 | @cindex variable definition | |
399 | ||
400 | You may announce your intention to use a symbol as a global variable | |
401 | with a @dfn{variable definition}: a special form, either @code{defconst} | |
402 | or @code{defvar}. | |
403 | ||
404 | In Emacs Lisp, definitions serve three purposes. First, they inform | |
405 | people who read the code that certain symbols are @emph{intended} to be | |
406 | used a certain way (as variables). Second, they inform the Lisp system | |
407 | of these things, supplying a value and documentation. Third, they | |
408 | provide information to utilities such as @code{etags} and | |
409 | @code{make-docfile}, which create data bases of the functions and | |
410 | variables in a program. | |
411 | ||
412 | The difference between @code{defconst} and @code{defvar} is primarily | |
413 | a matter of intent, serving to inform human readers of whether the value | |
414 | should ever change. Emacs Lisp does not restrict the ways in which a | |
415 | variable can be used based on @code{defconst} or @code{defvar} | |
416 | declarations. However, it does make a difference for initialization: | |
417 | @code{defconst} unconditionally initializes the variable, while | |
418 | @code{defvar} initializes it only if it is void. | |
419 | ||
420 | @ignore | |
421 | One would expect user option variables to be defined with | |
422 | @code{defconst}, since programs do not change them. Unfortunately, this | |
423 | has bad results if the definition is in a library that is not preloaded: | |
424 | @code{defconst} would override any prior value when the library is | |
425 | loaded. Users would like to be able to set user options in their init | |
426 | files, and override the default values given in the definitions. For | |
427 | this reason, user options must be defined with @code{defvar}. | |
428 | @end ignore | |
429 | ||
430 | @defspec defvar symbol [value [doc-string]] | |
431 | This special form defines @var{symbol} as a variable and can also | |
432 | initialize and document it. The definition informs a person reading | |
433 | your code that @var{symbol} is used as a variable that might be set or | |
434 | changed. Note that @var{symbol} is not evaluated; the symbol to be | |
435 | defined must appear explicitly in the @code{defvar}. | |
436 | ||
437 | If @var{symbol} is void and @var{value} is specified, @code{defvar} | |
438 | evaluates it and sets @var{symbol} to the result. But if @var{symbol} | |
439 | already has a value (i.e., it is not void), @var{value} is not even | |
440 | evaluated, and @var{symbol}'s value remains unchanged. If @var{value} | |
441 | is omitted, the value of @var{symbol} is not changed in any case. | |
442 | ||
443 | If @var{symbol} has a buffer-local binding in the current buffer, | |
444 | @code{defvar} operates on the default value, which is buffer-independent, | |
445 | not the current (buffer-local) binding. It sets the default value if | |
446 | the default value is void. @xref{Buffer-Local Variables}. | |
447 | ||
448 | When you evaluate a top-level @code{defvar} form with @kbd{C-M-x} in | |
449 | Emacs Lisp mode (@code{eval-defun}), a special feature of | |
450 | @code{eval-defun} arranges to set the variable unconditionally, without | |
451 | testing whether its value is void. | |
452 | ||
453 | If the @var{doc-string} argument appears, it specifies the documentation | |
454 | for the variable. (This opportunity to specify documentation is one of | |
455 | the main benefits of defining the variable.) The documentation is | |
456 | stored in the symbol's @code{variable-documentation} property. The | |
457 | Emacs help functions (@pxref{Documentation}) look for this property. | |
458 | ||
459 | If the variable is a user option that users would want to set | |
460 | interactively, you should use @samp{*} as the first character of | |
461 | @var{doc-string}. This lets users set the variable conveniently using | |
462 | the @code{set-variable} command. Note that you should nearly always | |
463 | use @code{defcustom} instead of @code{defvar} to define these | |
464 | variables, so that users can use @kbd{M-x customize} and related | |
465 | commands to set them. @xref{Customization}. | |
466 | ||
467 | Here are some examples. This form defines @code{foo} but does not | |
468 | initialize it: | |
469 | ||
470 | @example | |
471 | @group | |
472 | (defvar foo) | |
473 | @result{} foo | |
474 | @end group | |
475 | @end example | |
476 | ||
477 | This example initializes the value of @code{bar} to @code{23}, and gives | |
478 | it a documentation string: | |
479 | ||
480 | @example | |
481 | @group | |
482 | (defvar bar 23 | |
483 | "The normal weight of a bar.") | |
484 | @result{} bar | |
485 | @end group | |
486 | @end example | |
487 | ||
488 | The following form changes the documentation string for @code{bar}, | |
489 | making it a user option, but does not change the value, since @code{bar} | |
490 | already has a value. (The addition @code{(1+ nil)} would get an error | |
491 | if it were evaluated, but since it is not evaluated, there is no error.) | |
492 | ||
493 | @example | |
494 | @group | |
495 | (defvar bar (1+ nil) | |
496 | "*The normal weight of a bar.") | |
497 | @result{} bar | |
498 | @end group | |
499 | @group | |
500 | bar | |
501 | @result{} 23 | |
502 | @end group | |
503 | @end example | |
504 | ||
505 | Here is an equivalent expression for the @code{defvar} special form: | |
506 | ||
507 | @example | |
508 | @group | |
509 | (defvar @var{symbol} @var{value} @var{doc-string}) | |
510 | @equiv{} | |
511 | (progn | |
512 | (if (not (boundp '@var{symbol})) | |
513 | (setq @var{symbol} @var{value})) | |
514 | (if '@var{doc-string} | |
515 | (put '@var{symbol} 'variable-documentation '@var{doc-string})) | |
516 | '@var{symbol}) | |
517 | @end group | |
518 | @end example | |
519 | ||
520 | The @code{defvar} form returns @var{symbol}, but it is normally used | |
521 | at top level in a file where its value does not matter. | |
522 | @end defspec | |
523 | ||
524 | @defspec defconst symbol value [doc-string] | |
525 | This special form defines @var{symbol} as a value and initializes it. | |
526 | It informs a person reading your code that @var{symbol} has a standard | |
527 | global value, established here, that should not be changed by the user | |
528 | or by other programs. Note that @var{symbol} is not evaluated; the | |
529 | symbol to be defined must appear explicitly in the @code{defconst}. | |
530 | ||
531 | @code{defconst} always evaluates @var{value}, and sets the value of | |
532 | @var{symbol} to the result. If @var{symbol} does have a buffer-local | |
533 | binding in the current buffer, @code{defconst} sets the default value, | |
534 | not the buffer-local value. (But you should not be making | |
535 | buffer-local bindings for a symbol that is defined with | |
536 | @code{defconst}.) | |
537 | ||
538 | Here, @code{pi} is a constant that presumably ought not to be changed | |
539 | by anyone (attempts by the Indiana State Legislature notwithstanding). | |
540 | As the second form illustrates, however, this is only advisory. | |
541 | ||
542 | @example | |
543 | @group | |
544 | (defconst pi 3.1415 "Pi to five places.") | |
545 | @result{} pi | |
546 | @end group | |
547 | @group | |
548 | (setq pi 3) | |
549 | @result{} pi | |
550 | @end group | |
551 | @group | |
552 | pi | |
553 | @result{} 3 | |
554 | @end group | |
555 | @end example | |
556 | @end defspec | |
557 | ||
558 | @defun user-variable-p variable | |
559 | @cindex user option | |
560 | This function returns @code{t} if @var{variable} is a user option---a | |
561 | variable intended to be set by the user for customization---and | |
562 | @code{nil} otherwise. (Variables other than user options exist for the | |
563 | internal purposes of Lisp programs, and users need not know about them.) | |
564 | ||
565 | User option variables are distinguished from other variables either | |
566 | though being declared using @code{defcustom}@footnote{They may also be | |
567 | declared equivalently in @file{cus-start.el}.} or by the first character | |
568 | of their @code{variable-documentation} property. If the property exists | |
569 | and is a string, and its first character is @samp{*}, then the variable | |
570 | is a user option. Aliases of user options are also user options. | |
571 | @end defun | |
572 | ||
573 | @kindex variable-interactive | |
574 | If a user option variable has a @code{variable-interactive} property, | |
575 | the @code{set-variable} command uses that value to control reading the | |
576 | new value for the variable. The property's value is used as if it were | |
577 | specified in @code{interactive} (@pxref{Using Interactive}). However, | |
578 | this feature is largely obsoleted by @code{defcustom} | |
579 | (@pxref{Customization}). | |
580 | ||
581 | @strong{Warning:} If the @code{defconst} and @code{defvar} special | |
582 | forms are used while the variable has a local binding (made with | |
583 | @code{let}, or a function argument), they set the local-binding's | |
584 | value; the top-level binding is not changed. This is not what you | |
585 | usually want. To prevent it, use these special forms at top level in | |
586 | a file, where normally no local binding is in effect, and make sure to | |
587 | load the file before making a local binding for the variable. | |
588 | ||
589 | @node Tips for Defining | |
590 | @section Tips for Defining Variables Robustly | |
591 | ||
592 | When you define a variable whose value is a function, or a list of | |
593 | functions, use a name that ends in @samp{-function} or | |
594 | @samp{-functions}, respectively. | |
595 | ||
596 | There are several other variable name conventions; | |
597 | here is a complete list: | |
598 | ||
599 | @table @samp | |
600 | @item @dots{}-hook | |
601 | The variable is a normal hook (@pxref{Hooks}). | |
602 | ||
603 | @item @dots{}-function | |
604 | The value is a function. | |
605 | ||
606 | @item @dots{}-functions | |
607 | The value is a list of functions. | |
608 | ||
609 | @item @dots{}-form | |
610 | The value is a form (an expression). | |
611 | ||
612 | @item @dots{}-forms | |
613 | The value is a list of forms (expressions). | |
614 | ||
615 | @item @dots{}-predicate | |
616 | The value is a predicate---a function of one argument that returns | |
617 | non-@code{nil} for ``good'' arguments and @code{nil} for ``bad'' | |
618 | arguments. | |
619 | ||
620 | @item @dots{}-flag | |
621 | The value is significant only as to whether it is @code{nil} or not. | |
622 | ||
623 | @item @dots{}-program | |
624 | The value is a program name. | |
625 | ||
626 | @item @dots{}-command | |
627 | The value is a whole shell command. | |
628 | ||
629 | @item @dots{}-switches | |
630 | The value specifies options for a command. | |
631 | @end table | |
632 | ||
633 | When you define a variable, always consider whether you should mark | |
634 | it as ``risky''; see @ref{File Local Variables}. | |
635 | ||
636 | When defining and initializing a variable that holds a complicated | |
637 | value (such as a keymap with bindings in it), it's best to put the | |
638 | entire computation of the value into the @code{defvar}, like this: | |
639 | ||
640 | @example | |
641 | (defvar my-mode-map | |
642 | (let ((map (make-sparse-keymap))) | |
643 | (define-key map "\C-c\C-a" 'my-command) | |
644 | @dots{} | |
645 | map) | |
646 | @var{docstring}) | |
647 | @end example | |
648 | ||
649 | @noindent | |
650 | This method has several benefits. First, if the user quits while | |
651 | loading the file, the variable is either still uninitialized or | |
652 | initialized properly, never in-between. If it is still uninitialized, | |
653 | reloading the file will initialize it properly. Second, reloading the | |
654 | file once the variable is initialized will not alter it; that is | |
655 | important if the user has run hooks to alter part of the contents (such | |
656 | as, to rebind keys). Third, evaluating the @code{defvar} form with | |
657 | @kbd{C-M-x} @emph{will} reinitialize the map completely. | |
658 | ||
659 | Putting so much code in the @code{defvar} form has one disadvantage: | |
660 | it puts the documentation string far away from the line which names the | |
661 | variable. Here's a safe way to avoid that: | |
662 | ||
663 | @example | |
664 | (defvar my-mode-map nil | |
665 | @var{docstring}) | |
666 | (unless my-mode-map | |
667 | (let ((map (make-sparse-keymap))) | |
668 | (define-key map "\C-c\C-a" 'my-command) | |
669 | @dots{} | |
670 | (setq my-mode-map map))) | |
671 | @end example | |
672 | ||
673 | @noindent | |
674 | This has all the same advantages as putting the initialization inside | |
675 | the @code{defvar}, except that you must type @kbd{C-M-x} twice, once on | |
676 | each form, if you do want to reinitialize the variable. | |
677 | ||
678 | But be careful not to write the code like this: | |
679 | ||
680 | @example | |
681 | (defvar my-mode-map nil | |
682 | @var{docstring}) | |
683 | (unless my-mode-map | |
684 | (setq my-mode-map (make-sparse-keymap)) | |
685 | (define-key my-mode-map "\C-c\C-a" 'my-command) | |
686 | @dots{}) | |
687 | @end example | |
688 | ||
689 | @noindent | |
690 | This code sets the variable, then alters it, but it does so in more than | |
691 | one step. If the user quits just after the @code{setq}, that leaves the | |
692 | variable neither correctly initialized nor void nor @code{nil}. Once | |
693 | that happens, reloading the file will not initialize the variable; it | |
694 | will remain incomplete. | |
695 | ||
696 | @node Accessing Variables | |
697 | @section Accessing Variable Values | |
698 | ||
699 | The usual way to reference a variable is to write the symbol which | |
700 | names it (@pxref{Symbol Forms}). This requires you to specify the | |
701 | variable name when you write the program. Usually that is exactly what | |
702 | you want to do. Occasionally you need to choose at run time which | |
703 | variable to reference; then you can use @code{symbol-value}. | |
704 | ||
705 | @defun symbol-value symbol | |
706 | This function returns the value of @var{symbol}. This is the value in | |
707 | the innermost local binding of the symbol, or its global value if it | |
708 | has no local bindings. | |
709 | ||
710 | @example | |
711 | @group | |
712 | (setq abracadabra 5) | |
713 | @result{} 5 | |
714 | @end group | |
715 | @group | |
716 | (setq foo 9) | |
717 | @result{} 9 | |
718 | @end group | |
719 | ||
720 | @group | |
721 | ;; @r{Here the symbol @code{abracadabra}} | |
722 | ;; @r{is the symbol whose value is examined.} | |
723 | (let ((abracadabra 'foo)) | |
724 | (symbol-value 'abracadabra)) | |
725 | @result{} foo | |
726 | @end group | |
727 | ||
728 | @group | |
729 | ;; @r{Here, the value of @code{abracadabra},} | |
730 | ;; @r{which is @code{foo},} | |
731 | ;; @r{is the symbol whose value is examined.} | |
732 | (let ((abracadabra 'foo)) | |
733 | (symbol-value abracadabra)) | |
734 | @result{} 9 | |
735 | @end group | |
736 | ||
737 | @group | |
738 | (symbol-value 'abracadabra) | |
739 | @result{} 5 | |
740 | @end group | |
741 | @end example | |
742 | ||
743 | A @code{void-variable} error is signaled if the current binding of | |
744 | @var{symbol} is void. | |
745 | @end defun | |
746 | ||
747 | @node Setting Variables | |
748 | @section How to Alter a Variable Value | |
749 | ||
750 | The usual way to change the value of a variable is with the special | |
751 | form @code{setq}. When you need to compute the choice of variable at | |
752 | run time, use the function @code{set}. | |
753 | ||
754 | @defspec setq [symbol form]@dots{} | |
755 | This special form is the most common method of changing a variable's | |
756 | value. Each @var{symbol} is given a new value, which is the result of | |
757 | evaluating the corresponding @var{form}. The most-local existing | |
758 | binding of the symbol is changed. | |
759 | ||
760 | @code{setq} does not evaluate @var{symbol}; it sets the symbol that you | |
761 | write. We say that this argument is @dfn{automatically quoted}. The | |
762 | @samp{q} in @code{setq} stands for ``quoted.'' | |
763 | ||
764 | The value of the @code{setq} form is the value of the last @var{form}. | |
765 | ||
766 | @example | |
767 | @group | |
768 | (setq x (1+ 2)) | |
769 | @result{} 3 | |
770 | @end group | |
771 | x ; @r{@code{x} now has a global value.} | |
772 | @result{} 3 | |
773 | @group | |
774 | (let ((x 5)) | |
775 | (setq x 6) ; @r{The local binding of @code{x} is set.} | |
776 | x) | |
777 | @result{} 6 | |
778 | @end group | |
779 | x ; @r{The global value is unchanged.} | |
780 | @result{} 3 | |
781 | @end example | |
782 | ||
783 | Note that the first @var{form} is evaluated, then the first | |
784 | @var{symbol} is set, then the second @var{form} is evaluated, then the | |
785 | second @var{symbol} is set, and so on: | |
786 | ||
787 | @example | |
788 | @group | |
789 | (setq x 10 ; @r{Notice that @code{x} is set before} | |
790 | y (1+ x)) ; @r{the value of @code{y} is computed.} | |
791 | @result{} 11 | |
792 | @end group | |
793 | @end example | |
794 | @end defspec | |
795 | ||
796 | @defun set symbol value | |
797 | This function sets @var{symbol}'s value to @var{value}, then returns | |
798 | @var{value}. Since @code{set} is a function, the expression written for | |
799 | @var{symbol} is evaluated to obtain the symbol to set. | |
800 | ||
801 | The most-local existing binding of the variable is the binding that is | |
802 | set; shadowed bindings are not affected. | |
803 | ||
804 | @example | |
805 | @group | |
806 | (set one 1) | |
807 | @error{} Symbol's value as variable is void: one | |
808 | @end group | |
809 | @group | |
810 | (set 'one 1) | |
811 | @result{} 1 | |
812 | @end group | |
813 | @group | |
814 | (set 'two 'one) | |
815 | @result{} one | |
816 | @end group | |
817 | @group | |
818 | (set two 2) ; @r{@code{two} evaluates to symbol @code{one}.} | |
819 | @result{} 2 | |
820 | @end group | |
821 | @group | |
822 | one ; @r{So it is @code{one} that was set.} | |
823 | @result{} 2 | |
824 | (let ((one 1)) ; @r{This binding of @code{one} is set,} | |
825 | (set 'one 3) ; @r{not the global value.} | |
826 | one) | |
827 | @result{} 3 | |
828 | @end group | |
829 | @group | |
830 | one | |
831 | @result{} 2 | |
832 | @end group | |
833 | @end example | |
834 | ||
835 | If @var{symbol} is not actually a symbol, a @code{wrong-type-argument} | |
836 | error is signaled. | |
837 | ||
838 | @example | |
839 | (set '(x y) 'z) | |
840 | @error{} Wrong type argument: symbolp, (x y) | |
841 | @end example | |
842 | ||
843 | Logically speaking, @code{set} is a more fundamental primitive than | |
844 | @code{setq}. Any use of @code{setq} can be trivially rewritten to use | |
845 | @code{set}; @code{setq} could even be defined as a macro, given the | |
846 | availability of @code{set}. However, @code{set} itself is rarely used; | |
847 | beginners hardly need to know about it. It is useful only for choosing | |
848 | at run time which variable to set. For example, the command | |
849 | @code{set-variable}, which reads a variable name from the user and then | |
850 | sets the variable, needs to use @code{set}. | |
851 | ||
852 | @cindex CL note---@code{set} local | |
853 | @quotation | |
854 | @b{Common Lisp note:} In Common Lisp, @code{set} always changes the | |
855 | symbol's ``special'' or dynamic value, ignoring any lexical bindings. | |
856 | In Emacs Lisp, all variables and all bindings are dynamic, so @code{set} | |
857 | always affects the most local existing binding. | |
858 | @end quotation | |
859 | @end defun | |
860 | ||
861 | @node Variable Scoping | |
862 | @section Scoping Rules for Variable Bindings | |
863 | ||
864 | A given symbol @code{foo} can have several local variable bindings, | |
865 | established at different places in the Lisp program, as well as a global | |
866 | binding. The most recently established binding takes precedence over | |
867 | the others. | |
868 | ||
869 | @cindex scope | |
870 | @cindex extent | |
871 | @cindex dynamic scoping | |
872 | @cindex lexical scoping | |
873 | Local bindings in Emacs Lisp have @dfn{indefinite scope} and | |
874 | @dfn{dynamic extent}. @dfn{Scope} refers to @emph{where} textually in | |
875 | the source code the binding can be accessed. ``Indefinite scope'' means | |
876 | that any part of the program can potentially access the variable | |
877 | binding. @dfn{Extent} refers to @emph{when}, as the program is | |
878 | executing, the binding exists. ``Dynamic extent'' means that the binding | |
879 | lasts as long as the activation of the construct that established it. | |
880 | ||
881 | The combination of dynamic extent and indefinite scope is called | |
882 | @dfn{dynamic scoping}. By contrast, most programming languages use | |
883 | @dfn{lexical scoping}, in which references to a local variable must be | |
884 | located textually within the function or block that binds the variable. | |
885 | ||
886 | @cindex CL note---special variables | |
887 | @quotation | |
888 | @b{Common Lisp note:} Variables declared ``special'' in Common Lisp are | |
889 | dynamically scoped, like all variables in Emacs Lisp. | |
890 | @end quotation | |
891 | ||
892 | @menu | |
893 | * Scope:: Scope means where in the program a value is visible. | |
894 | Comparison with other languages. | |
895 | * Extent:: Extent means how long in time a value exists. | |
896 | * Impl of Scope:: Two ways to implement dynamic scoping. | |
897 | * Using Scoping:: How to use dynamic scoping carefully and avoid problems. | |
898 | @end menu | |
899 | ||
900 | @node Scope | |
901 | @subsection Scope | |
902 | ||
903 | Emacs Lisp uses @dfn{indefinite scope} for local variable bindings. | |
904 | This means that any function anywhere in the program text might access a | |
905 | given binding of a variable. Consider the following function | |
906 | definitions: | |
907 | ||
908 | @example | |
909 | @group | |
910 | (defun binder (x) ; @r{@code{x} is bound in @code{binder}.} | |
911 | (foo 5)) ; @r{@code{foo} is some other function.} | |
912 | @end group | |
913 | ||
914 | @group | |
915 | (defun user () ; @r{@code{x} is used ``free'' in @code{user}.} | |
916 | (list x)) | |
917 | @end group | |
918 | @end example | |
919 | ||
920 | In a lexically scoped language, the binding of @code{x} in | |
921 | @code{binder} would never be accessible in @code{user}, because | |
922 | @code{user} is not textually contained within the function | |
923 | @code{binder}. However, in dynamically-scoped Emacs Lisp, @code{user} | |
924 | may or may not refer to the binding of @code{x} established in | |
925 | @code{binder}, depending on the circumstances: | |
926 | ||
927 | @itemize @bullet | |
928 | @item | |
929 | If we call @code{user} directly without calling @code{binder} at all, | |
930 | then whatever binding of @code{x} is found, it cannot come from | |
931 | @code{binder}. | |
932 | ||
933 | @item | |
934 | If we define @code{foo} as follows and then call @code{binder}, then the | |
935 | binding made in @code{binder} will be seen in @code{user}: | |
936 | ||
937 | @example | |
938 | @group | |
939 | (defun foo (lose) | |
940 | (user)) | |
941 | @end group | |
942 | @end example | |
943 | ||
944 | @item | |
945 | However, if we define @code{foo} as follows and then call @code{binder}, | |
946 | then the binding made in @code{binder} @emph{will not} be seen in | |
947 | @code{user}: | |
948 | ||
949 | @example | |
950 | (defun foo (x) | |
951 | (user)) | |
952 | @end example | |
953 | ||
954 | @noindent | |
955 | Here, when @code{foo} is called by @code{binder}, it binds @code{x}. | |
956 | (The binding in @code{foo} is said to @dfn{shadow} the one made in | |
957 | @code{binder}.) Therefore, @code{user} will access the @code{x} bound | |
958 | by @code{foo} instead of the one bound by @code{binder}. | |
959 | @end itemize | |
960 | ||
961 | Emacs Lisp uses dynamic scoping because simple implementations of | |
962 | lexical scoping are slow. In addition, every Lisp system needs to offer | |
963 | dynamic scoping at least as an option; if lexical scoping is the norm, | |
964 | there must be a way to specify dynamic scoping instead for a particular | |
965 | variable. It might not be a bad thing for Emacs to offer both, but | |
966 | implementing it with dynamic scoping only was much easier. | |
967 | ||
968 | @node Extent | |
969 | @subsection Extent | |
970 | ||
971 | @dfn{Extent} refers to the time during program execution that a | |
972 | variable name is valid. In Emacs Lisp, a variable is valid only while | |
973 | the form that bound it is executing. This is called @dfn{dynamic | |
974 | extent}. ``Local'' or ``automatic'' variables in most languages, | |
975 | including C and Pascal, have dynamic extent. | |
976 | ||
977 | One alternative to dynamic extent is @dfn{indefinite extent}. This | |
978 | means that a variable binding can live on past the exit from the form | |
979 | that made the binding. Common Lisp and Scheme, for example, support | |
980 | this, but Emacs Lisp does not. | |
981 | ||
982 | To illustrate this, the function below, @code{make-add}, returns a | |
983 | function that purports to add @var{n} to its own argument @var{m}. This | |
984 | would work in Common Lisp, but it does not do the job in Emacs Lisp, | |
985 | because after the call to @code{make-add} exits, the variable @code{n} | |
986 | is no longer bound to the actual argument 2. | |
987 | ||
988 | @example | |
989 | (defun make-add (n) | |
990 | (function (lambda (m) (+ n m)))) ; @r{Return a function.} | |
991 | @result{} make-add | |
992 | (fset 'add2 (make-add 2)) ; @r{Define function @code{add2}} | |
993 | ; @r{with @code{(make-add 2)}.} | |
994 | @result{} (lambda (m) (+ n m)) | |
995 | (add2 4) ; @r{Try to add 2 to 4.} | |
996 | @error{} Symbol's value as variable is void: n | |
997 | @end example | |
998 | ||
999 | @cindex closures not available | |
1000 | Some Lisp dialects have ``closures,'' objects that are like functions | |
1001 | but record additional variable bindings. Emacs Lisp does not have | |
1002 | closures. | |
1003 | ||
1004 | @node Impl of Scope | |
1005 | @subsection Implementation of Dynamic Scoping | |
1006 | @cindex deep binding | |
1007 | ||
1008 | A simple sample implementation (which is not how Emacs Lisp actually | |
1009 | works) may help you understand dynamic binding. This technique is | |
1010 | called @dfn{deep binding} and was used in early Lisp systems. | |
1011 | ||
1012 | Suppose there is a stack of bindings, which are variable-value pairs. | |
1013 | At entry to a function or to a @code{let} form, we can push bindings | |
1014 | onto the stack for the arguments or local variables created there. We | |
1015 | can pop those bindings from the stack at exit from the binding | |
1016 | construct. | |
1017 | ||
1018 | We can find the value of a variable by searching the stack from top to | |
1019 | bottom for a binding for that variable; the value from that binding is | |
1020 | the value of the variable. To set the variable, we search for the | |
1021 | current binding, then store the new value into that binding. | |
1022 | ||
1023 | As you can see, a function's bindings remain in effect as long as it | |
1024 | continues execution, even during its calls to other functions. That is | |
1025 | why we say the extent of the binding is dynamic. And any other function | |
1026 | can refer to the bindings, if it uses the same variables while the | |
1027 | bindings are in effect. That is why we say the scope is indefinite. | |
1028 | ||
1029 | @cindex shallow binding | |
1030 | The actual implementation of variable scoping in GNU Emacs Lisp uses a | |
1031 | technique called @dfn{shallow binding}. Each variable has a standard | |
1032 | place in which its current value is always found---the value cell of the | |
1033 | symbol. | |
1034 | ||
1035 | In shallow binding, setting the variable works by storing a value in | |
1036 | the value cell. Creating a new binding works by pushing the old value | |
1037 | (belonging to a previous binding) onto a stack, and storing the new | |
1038 | local value in the value cell. Eliminating a binding works by popping | |
1039 | the old value off the stack, into the value cell. | |
1040 | ||
1041 | We use shallow binding because it has the same results as deep | |
1042 | binding, but runs faster, since there is never a need to search for a | |
1043 | binding. | |
1044 | ||
1045 | @node Using Scoping | |
1046 | @subsection Proper Use of Dynamic Scoping | |
1047 | ||
1048 | Binding a variable in one function and using it in another is a | |
1049 | powerful technique, but if used without restraint, it can make programs | |
1050 | hard to understand. There are two clean ways to use this technique: | |
1051 | ||
1052 | @itemize @bullet | |
1053 | @item | |
1054 | Use or bind the variable only in a few related functions, written close | |
1055 | together in one file. Such a variable is used for communication within | |
1056 | one program. | |
1057 | ||
1058 | You should write comments to inform other programmers that they can see | |
1059 | all uses of the variable before them, and to advise them not to add uses | |
1060 | elsewhere. | |
1061 | ||
1062 | @item | |
1063 | Give the variable a well-defined, documented meaning, and make all | |
1064 | appropriate functions refer to it (but not bind it or set it) wherever | |
1065 | that meaning is relevant. For example, the variable | |
1066 | @code{case-fold-search} is defined as ``non-@code{nil} means ignore case | |
1067 | when searching''; various search and replace functions refer to it | |
1068 | directly or through their subroutines, but do not bind or set it. | |
1069 | ||
1070 | Then you can bind the variable in other programs, knowing reliably what | |
1071 | the effect will be. | |
1072 | @end itemize | |
1073 | ||
1074 | In either case, you should define the variable with @code{defvar}. | |
1075 | This helps other people understand your program by telling them to look | |
1076 | for inter-function usage. It also avoids a warning from the byte | |
1077 | compiler. Choose the variable's name to avoid name conflicts---don't | |
1078 | use short names like @code{x}. | |
1079 | ||
1080 | @node Buffer-Local Variables | |
1081 | @section Buffer-Local Variables | |
1082 | @cindex variable, buffer-local | |
1083 | @cindex buffer-local variables | |
1084 | ||
1085 | Global and local variable bindings are found in most programming | |
e388c68f RS |
1086 | languages in one form or another. Emacs, however, also supports |
1087 | additional, unusual kinds of variable binding, such as | |
1088 | @dfn{buffer-local} bindings, which apply only in one buffer. Having | |
1089 | different values for a variable in different buffers is an important | |
1090 | customization method. (A few variables have bindings that are local | |
1091 | to each terminal; see @ref{Multiple Displays}.) | |
b8d4c8d0 GM |
1092 | |
1093 | @menu | |
1094 | * Intro to Buffer-Local:: Introduction and concepts. | |
1095 | * Creating Buffer-Local:: Creating and destroying buffer-local bindings. | |
1096 | * Default Value:: The default value is seen in buffers | |
1097 | that don't have their own buffer-local values. | |
1098 | @end menu | |
1099 | ||
1100 | @node Intro to Buffer-Local | |
1101 | @subsection Introduction to Buffer-Local Variables | |
1102 | ||
1103 | A buffer-local variable has a buffer-local binding associated with a | |
1104 | particular buffer. The binding is in effect when that buffer is | |
1105 | current; otherwise, it is not in effect. If you set the variable while | |
1106 | a buffer-local binding is in effect, the new value goes in that binding, | |
1107 | so its other bindings are unchanged. This means that the change is | |
1108 | visible only in the buffer where you made it. | |
1109 | ||
1110 | The variable's ordinary binding, which is not associated with any | |
1111 | specific buffer, is called the @dfn{default binding}. In most cases, | |
1112 | this is the global binding. | |
1113 | ||
1114 | A variable can have buffer-local bindings in some buffers but not in | |
1115 | other buffers. The default binding is shared by all the buffers that | |
1116 | don't have their own bindings for the variable. (This includes all | |
1117 | newly-created buffers.) If you set the variable in a buffer that does | |
e388c68f | 1118 | not have a buffer-local binding for it, this sets the default binding, |
b8d4c8d0 GM |
1119 | so the new value is visible in all the buffers that see the default |
1120 | binding. | |
1121 | ||
1122 | The most common use of buffer-local bindings is for major modes to change | |
1123 | variables that control the behavior of commands. For example, C mode and | |
1124 | Lisp mode both set the variable @code{paragraph-start} to specify that only | |
1125 | blank lines separate paragraphs. They do this by making the variable | |
1126 | buffer-local in the buffer that is being put into C mode or Lisp mode, and | |
1127 | then setting it to the new value for that mode. @xref{Major Modes}. | |
1128 | ||
1129 | The usual way to make a buffer-local binding is with | |
1130 | @code{make-local-variable}, which is what major mode commands typically | |
1131 | use. This affects just the current buffer; all other buffers (including | |
1132 | those yet to be created) will continue to share the default value unless | |
1133 | they are explicitly given their own buffer-local bindings. | |
1134 | ||
1135 | @cindex automatically buffer-local | |
1136 | A more powerful operation is to mark the variable as | |
1137 | @dfn{automatically buffer-local} by calling | |
1138 | @code{make-variable-buffer-local}. You can think of this as making the | |
1139 | variable local in all buffers, even those yet to be created. More | |
1140 | precisely, the effect is that setting the variable automatically makes | |
1141 | the variable local to the current buffer if it is not already so. All | |
1142 | buffers start out by sharing the default value of the variable as usual, | |
1143 | but setting the variable creates a buffer-local binding for the current | |
1144 | buffer. The new value is stored in the buffer-local binding, leaving | |
1145 | the default binding untouched. This means that the default value cannot | |
1146 | be changed with @code{setq} in any buffer; the only way to change it is | |
1147 | with @code{setq-default}. | |
1148 | ||
e388c68f | 1149 | @strong{Warning:} When a variable has buffer-local |
b8d4c8d0 GM |
1150 | bindings in one or more buffers, @code{let} rebinds the binding that's |
1151 | currently in effect. For instance, if the current buffer has a | |
1152 | buffer-local value, @code{let} temporarily rebinds that. If no | |
e388c68f | 1153 | buffer-local bindings are in effect, @code{let} rebinds |
b8d4c8d0 GM |
1154 | the default value. If inside the @code{let} you then change to a |
1155 | different current buffer in which a different binding is in effect, | |
1156 | you won't see the @code{let} binding any more. And if you exit the | |
1157 | @code{let} while still in the other buffer, you won't see the | |
1158 | unbinding occur (though it will occur properly). Here is an example | |
1159 | to illustrate: | |
1160 | ||
1161 | @example | |
1162 | @group | |
1163 | (setq foo 'g) | |
1164 | (set-buffer "a") | |
1165 | (make-local-variable 'foo) | |
1166 | @end group | |
1167 | (setq foo 'a) | |
1168 | (let ((foo 'temp)) | |
1169 | ;; foo @result{} 'temp ; @r{let binding in buffer @samp{a}} | |
1170 | (set-buffer "b") | |
1171 | ;; foo @result{} 'g ; @r{the global value since foo is not local in @samp{b}} | |
1172 | @var{body}@dots{}) | |
1173 | @group | |
1174 | foo @result{} 'g ; @r{exiting restored the local value in buffer @samp{a},} | |
1175 | ; @r{but we don't see that in buffer @samp{b}} | |
1176 | @end group | |
1177 | @group | |
1178 | (set-buffer "a") ; @r{verify the local value was restored} | |
1179 | foo @result{} 'a | |
1180 | @end group | |
1181 | @end example | |
1182 | ||
1183 | Note that references to @code{foo} in @var{body} access the | |
1184 | buffer-local binding of buffer @samp{b}. | |
1185 | ||
1186 | When a file specifies local variable values, these become buffer-local | |
1187 | values when you visit the file. @xref{File Variables,,, emacs, The | |
1188 | GNU Emacs Manual}. | |
1189 | ||
1190 | @node Creating Buffer-Local | |
1191 | @subsection Creating and Deleting Buffer-Local Bindings | |
1192 | ||
1193 | @deffn Command make-local-variable variable | |
1194 | This function creates a buffer-local binding in the current buffer for | |
1195 | @var{variable} (a symbol). Other buffers are not affected. The value | |
1196 | returned is @var{variable}. | |
1197 | ||
1198 | @c Emacs 19 feature | |
1199 | The buffer-local value of @var{variable} starts out as the same value | |
1200 | @var{variable} previously had. If @var{variable} was void, it remains | |
1201 | void. | |
1202 | ||
1203 | @example | |
1204 | @group | |
1205 | ;; @r{In buffer @samp{b1}:} | |
1206 | (setq foo 5) ; @r{Affects all buffers.} | |
1207 | @result{} 5 | |
1208 | @end group | |
1209 | @group | |
1210 | (make-local-variable 'foo) ; @r{Now it is local in @samp{b1}.} | |
1211 | @result{} foo | |
1212 | @end group | |
1213 | @group | |
1214 | foo ; @r{That did not change} | |
1215 | @result{} 5 ; @r{the value.} | |
1216 | @end group | |
1217 | @group | |
1218 | (setq foo 6) ; @r{Change the value} | |
1219 | @result{} 6 ; @r{in @samp{b1}.} | |
1220 | @end group | |
1221 | @group | |
1222 | foo | |
1223 | @result{} 6 | |
1224 | @end group | |
1225 | ||
1226 | @group | |
1227 | ;; @r{In buffer @samp{b2}, the value hasn't changed.} | |
1228 | (save-excursion | |
1229 | (set-buffer "b2") | |
1230 | foo) | |
1231 | @result{} 5 | |
1232 | @end group | |
1233 | @end example | |
1234 | ||
1235 | Making a variable buffer-local within a @code{let}-binding for that | |
1236 | variable does not work reliably, unless the buffer in which you do this | |
1237 | is not current either on entry to or exit from the @code{let}. This is | |
1238 | because @code{let} does not distinguish between different kinds of | |
1239 | bindings; it knows only which variable the binding was made for. | |
1240 | ||
1241 | If the variable is terminal-local, this function signals an error. Such | |
1242 | variables cannot have buffer-local bindings as well. @xref{Multiple | |
1243 | Displays}. | |
1244 | ||
1245 | @strong{Warning:} do not use @code{make-local-variable} for a hook | |
1246 | variable. The hook variables are automatically made buffer-local as | |
1247 | needed if you use the @var{local} argument to @code{add-hook} or | |
1248 | @code{remove-hook}. | |
1249 | @end deffn | |
1250 | ||
1251 | @deffn Command make-variable-buffer-local variable | |
1252 | This function marks @var{variable} (a symbol) automatically | |
1253 | buffer-local, so that any subsequent attempt to set it will make it | |
1254 | local to the current buffer at the time. | |
1255 | ||
1256 | A peculiar wrinkle of this feature is that binding the variable (with | |
1257 | @code{let} or other binding constructs) does not create a buffer-local | |
1258 | binding for it. Only setting the variable (with @code{set} or | |
1259 | @code{setq}), while the variable does not have a @code{let}-style | |
1260 | binding that was made in the current buffer, does so. | |
1261 | ||
1262 | If @var{variable} does not have a default value, then calling this | |
1263 | command will give it a default value of @code{nil}. If @var{variable} | |
1264 | already has a default value, that value remains unchanged. | |
1265 | Subsequently calling @code{makunbound} on @var{variable} will result | |
1266 | in a void buffer-local value and leave the default value unaffected. | |
1267 | ||
1268 | The value returned is @var{variable}. | |
1269 | ||
1270 | @strong{Warning:} Don't assume that you should use | |
1271 | @code{make-variable-buffer-local} for user-option variables, simply | |
1272 | because users @emph{might} want to customize them differently in | |
1273 | different buffers. Users can make any variable local, when they wish | |
1274 | to. It is better to leave the choice to them. | |
1275 | ||
1276 | The time to use @code{make-variable-buffer-local} is when it is crucial | |
1277 | that no two buffers ever share the same binding. For example, when a | |
1278 | variable is used for internal purposes in a Lisp program which depends | |
1279 | on having separate values in separate buffers, then using | |
1280 | @code{make-variable-buffer-local} can be the best solution. | |
1281 | @end deffn | |
1282 | ||
1283 | @defun local-variable-p variable &optional buffer | |
1284 | This returns @code{t} if @var{variable} is buffer-local in buffer | |
1285 | @var{buffer} (which defaults to the current buffer); otherwise, | |
1286 | @code{nil}. | |
1287 | @end defun | |
1288 | ||
1289 | @defun local-variable-if-set-p variable &optional buffer | |
1290 | This returns @code{t} if @var{variable} will become buffer-local in | |
1291 | buffer @var{buffer} (which defaults to the current buffer) if it is | |
1292 | set there. | |
1293 | @end defun | |
1294 | ||
1295 | @defun buffer-local-value variable buffer | |
1296 | This function returns the buffer-local binding of @var{variable} (a | |
1297 | symbol) in buffer @var{buffer}. If @var{variable} does not have a | |
1298 | buffer-local binding in buffer @var{buffer}, it returns the default | |
1299 | value (@pxref{Default Value}) of @var{variable} instead. | |
1300 | @end defun | |
1301 | ||
1302 | @defun buffer-local-variables &optional buffer | |
1303 | This function returns a list describing the buffer-local variables in | |
1304 | buffer @var{buffer}. (If @var{buffer} is omitted, the current buffer is | |
1305 | used.) It returns an association list (@pxref{Association Lists}) in | |
1306 | which each element contains one buffer-local variable and its value. | |
1307 | However, when a variable's buffer-local binding in @var{buffer} is void, | |
1308 | then the variable appears directly in the resulting list. | |
1309 | ||
1310 | @example | |
1311 | @group | |
1312 | (make-local-variable 'foobar) | |
1313 | (makunbound 'foobar) | |
1314 | (make-local-variable 'bind-me) | |
1315 | (setq bind-me 69) | |
1316 | @end group | |
1317 | (setq lcl (buffer-local-variables)) | |
1318 | ;; @r{First, built-in variables local in all buffers:} | |
1319 | @result{} ((mark-active . nil) | |
1320 | (buffer-undo-list . nil) | |
1321 | (mode-name . "Fundamental") | |
1322 | @dots{} | |
1323 | @group | |
1324 | ;; @r{Next, non-built-in buffer-local variables.} | |
1325 | ;; @r{This one is buffer-local and void:} | |
1326 | foobar | |
1327 | ;; @r{This one is buffer-local and nonvoid:} | |
1328 | (bind-me . 69)) | |
1329 | @end group | |
1330 | @end example | |
1331 | ||
1332 | Note that storing new values into the @sc{cdr}s of cons cells in this | |
1333 | list does @emph{not} change the buffer-local values of the variables. | |
1334 | @end defun | |
1335 | ||
1336 | @deffn Command kill-local-variable variable | |
1337 | This function deletes the buffer-local binding (if any) for | |
1338 | @var{variable} (a symbol) in the current buffer. As a result, the | |
1339 | default binding of @var{variable} becomes visible in this buffer. This | |
1340 | typically results in a change in the value of @var{variable}, since the | |
1341 | default value is usually different from the buffer-local value just | |
1342 | eliminated. | |
1343 | ||
1344 | If you kill the buffer-local binding of a variable that automatically | |
1345 | becomes buffer-local when set, this makes the default value visible in | |
1346 | the current buffer. However, if you set the variable again, that will | |
1347 | once again create a buffer-local binding for it. | |
1348 | ||
1349 | @code{kill-local-variable} returns @var{variable}. | |
1350 | ||
1351 | This function is a command because it is sometimes useful to kill one | |
1352 | buffer-local variable interactively, just as it is useful to create | |
1353 | buffer-local variables interactively. | |
1354 | @end deffn | |
1355 | ||
1356 | @defun kill-all-local-variables | |
1357 | This function eliminates all the buffer-local variable bindings of the | |
1358 | current buffer except for variables marked as ``permanent.'' As a | |
1359 | result, the buffer will see the default values of most variables. | |
1360 | ||
1361 | This function also resets certain other information pertaining to the | |
1362 | buffer: it sets the local keymap to @code{nil}, the syntax table to the | |
1363 | value of @code{(standard-syntax-table)}, the case table to | |
1364 | @code{(standard-case-table)}, and the abbrev table to the value of | |
1365 | @code{fundamental-mode-abbrev-table}. | |
1366 | ||
1367 | The very first thing this function does is run the normal hook | |
1368 | @code{change-major-mode-hook} (see below). | |
1369 | ||
1370 | Every major mode command begins by calling this function, which has the | |
1371 | effect of switching to Fundamental mode and erasing most of the effects | |
1372 | of the previous major mode. To ensure that this does its job, the | |
1373 | variables that major modes set should not be marked permanent. | |
1374 | ||
1375 | @code{kill-all-local-variables} returns @code{nil}. | |
1376 | @end defun | |
1377 | ||
1378 | @defvar change-major-mode-hook | |
1379 | The function @code{kill-all-local-variables} runs this normal hook | |
1380 | before it does anything else. This gives major modes a way to arrange | |
1381 | for something special to be done if the user switches to a different | |
1382 | major mode. It is also useful for buffer-specific minor modes | |
1383 | that should be forgotten if the user changes the major mode. | |
1384 | ||
1385 | For best results, make this variable buffer-local, so that it will | |
1386 | disappear after doing its job and will not interfere with the | |
1387 | subsequent major mode. @xref{Hooks}. | |
1388 | @end defvar | |
1389 | ||
1390 | @c Emacs 19 feature | |
1391 | @cindex permanent local variable | |
1392 | A buffer-local variable is @dfn{permanent} if the variable name (a | |
1393 | symbol) has a @code{permanent-local} property that is non-@code{nil}. | |
1394 | Permanent locals are appropriate for data pertaining to where the file | |
1395 | came from or how to save it, rather than with how to edit the contents. | |
1396 | ||
1397 | @node Default Value | |
1398 | @subsection The Default Value of a Buffer-Local Variable | |
1399 | @cindex default value | |
1400 | ||
1401 | The global value of a variable with buffer-local bindings is also | |
1402 | called the @dfn{default} value, because it is the value that is in | |
1403 | effect whenever neither the current buffer nor the selected frame has | |
1404 | its own binding for the variable. | |
1405 | ||
1406 | The functions @code{default-value} and @code{setq-default} access and | |
1407 | change a variable's default value regardless of whether the current | |
1408 | buffer has a buffer-local binding. For example, you could use | |
1409 | @code{setq-default} to change the default setting of | |
1410 | @code{paragraph-start} for most buffers; and this would work even when | |
1411 | you are in a C or Lisp mode buffer that has a buffer-local value for | |
1412 | this variable. | |
1413 | ||
1414 | @c Emacs 19 feature | |
1415 | The special forms @code{defvar} and @code{defconst} also set the | |
1416 | default value (if they set the variable at all), rather than any | |
e388c68f | 1417 | buffer-local value. |
b8d4c8d0 GM |
1418 | |
1419 | @defun default-value symbol | |
1420 | This function returns @var{symbol}'s default value. This is the value | |
1421 | that is seen in buffers and frames that do not have their own values for | |
1422 | this variable. If @var{symbol} is not buffer-local, this is equivalent | |
1423 | to @code{symbol-value} (@pxref{Accessing Variables}). | |
1424 | @end defun | |
1425 | ||
1426 | @c Emacs 19 feature | |
1427 | @defun default-boundp symbol | |
1428 | The function @code{default-boundp} tells you whether @var{symbol}'s | |
1429 | default value is nonvoid. If @code{(default-boundp 'foo)} returns | |
1430 | @code{nil}, then @code{(default-value 'foo)} would get an error. | |
1431 | ||
1432 | @code{default-boundp} is to @code{default-value} as @code{boundp} is to | |
1433 | @code{symbol-value}. | |
1434 | @end defun | |
1435 | ||
1436 | @defspec setq-default [symbol form]@dots{} | |
1437 | This special form gives each @var{symbol} a new default value, which is | |
1438 | the result of evaluating the corresponding @var{form}. It does not | |
1439 | evaluate @var{symbol}, but does evaluate @var{form}. The value of the | |
1440 | @code{setq-default} form is the value of the last @var{form}. | |
1441 | ||
1442 | If a @var{symbol} is not buffer-local for the current buffer, and is not | |
1443 | marked automatically buffer-local, @code{setq-default} has the same | |
1444 | effect as @code{setq}. If @var{symbol} is buffer-local for the current | |
1445 | buffer, then this changes the value that other buffers will see (as long | |
1446 | as they don't have a buffer-local value), but not the value that the | |
1447 | current buffer sees. | |
1448 | ||
1449 | @example | |
1450 | @group | |
1451 | ;; @r{In buffer @samp{foo}:} | |
1452 | (make-local-variable 'buffer-local) | |
1453 | @result{} buffer-local | |
1454 | @end group | |
1455 | @group | |
1456 | (setq buffer-local 'value-in-foo) | |
1457 | @result{} value-in-foo | |
1458 | @end group | |
1459 | @group | |
1460 | (setq-default buffer-local 'new-default) | |
1461 | @result{} new-default | |
1462 | @end group | |
1463 | @group | |
1464 | buffer-local | |
1465 | @result{} value-in-foo | |
1466 | @end group | |
1467 | @group | |
1468 | (default-value 'buffer-local) | |
1469 | @result{} new-default | |
1470 | @end group | |
1471 | ||
1472 | @group | |
1473 | ;; @r{In (the new) buffer @samp{bar}:} | |
1474 | buffer-local | |
1475 | @result{} new-default | |
1476 | @end group | |
1477 | @group | |
1478 | (default-value 'buffer-local) | |
1479 | @result{} new-default | |
1480 | @end group | |
1481 | @group | |
1482 | (setq buffer-local 'another-default) | |
1483 | @result{} another-default | |
1484 | @end group | |
1485 | @group | |
1486 | (default-value 'buffer-local) | |
1487 | @result{} another-default | |
1488 | @end group | |
1489 | ||
1490 | @group | |
1491 | ;; @r{Back in buffer @samp{foo}:} | |
1492 | buffer-local | |
1493 | @result{} value-in-foo | |
1494 | (default-value 'buffer-local) | |
1495 | @result{} another-default | |
1496 | @end group | |
1497 | @end example | |
1498 | @end defspec | |
1499 | ||
1500 | @defun set-default symbol value | |
1501 | This function is like @code{setq-default}, except that @var{symbol} is | |
1502 | an ordinary evaluated argument. | |
1503 | ||
1504 | @example | |
1505 | @group | |
1506 | (set-default (car '(a b c)) 23) | |
1507 | @result{} 23 | |
1508 | @end group | |
1509 | @group | |
1510 | (default-value 'a) | |
1511 | @result{} 23 | |
1512 | @end group | |
1513 | @end example | |
1514 | @end defun | |
1515 | ||
b8d4c8d0 GM |
1516 | @node Future Local Variables |
1517 | @section Possible Future Local Variables | |
1518 | ||
1519 | We have considered the idea of bindings that are local to a category | |
1520 | of frames---for example, all color frames, or all frames with dark | |
1521 | backgrounds. We have not implemented them because it is not clear that | |
1522 | this feature is really useful. You can get more or less the same | |
1523 | results by adding a function to @code{after-make-frame-functions}, set up to | |
1524 | define a particular frame parameter according to the appropriate | |
1525 | conditions for each frame. | |
1526 | ||
1527 | It would also be possible to implement window-local bindings. We | |
1528 | don't know of many situations where they would be useful, and it seems | |
1529 | that indirect buffers (@pxref{Indirect Buffers}) with buffer-local | |
1530 | bindings offer a way to handle these situations more robustly. | |
1531 | ||
1532 | If sufficient application is found for either of these two kinds of | |
1533 | local bindings, we will provide it in a subsequent Emacs version. | |
1534 | ||
1535 | @node File Local Variables | |
1536 | @section File Local Variables | |
1537 | @cindex file local variables | |
1538 | ||
1539 | A file can specify local variable values; Emacs uses these to create | |
1540 | buffer-local bindings for those variables in the buffer visiting that | |
1541 | file. @xref{File variables, , Local Variables in Files, emacs, The | |
1542 | GNU Emacs Manual}, for basic information about file local variables. | |
1543 | This section describes the functions and variables that affect | |
1544 | processing of file local variables. | |
1545 | ||
1546 | @defopt enable-local-variables | |
1547 | This variable controls whether to process file local variables. | |
1548 | The possible values are: | |
1549 | ||
1550 | @table @asis | |
1551 | @item @code{t} (the default) | |
1552 | Set the safe variables, and query (once) about any unsafe variables. | |
1553 | @item @code{:safe} | |
1554 | Set only the safe variables and do not query. | |
1555 | @item @code{:all} | |
1556 | Set all the variables and do not query. | |
1557 | @item @code{nil} | |
1558 | Don't set any variables. | |
1559 | @item anything else | |
1560 | Query (once) about all the variables. | |
1561 | @end table | |
1562 | @end defopt | |
1563 | ||
1564 | @defun hack-local-variables &optional mode-only | |
1565 | This function parses, and binds or evaluates as appropriate, any local | |
1566 | variables specified by the contents of the current buffer. The variable | |
1567 | @code{enable-local-variables} has its effect here. However, this | |
1568 | function does not look for the @samp{mode:} local variable in the | |
1569 | @w{@samp{-*-}} line. @code{set-auto-mode} does that, also taking | |
1570 | @code{enable-local-variables} into account (@pxref{Auto Major Mode}). | |
1571 | ||
1572 | If the optional argument @var{mode-only} is non-@code{nil}, then all | |
1573 | this function does is return @code{t} if the @w{@samp{-*-}} line or | |
1574 | the local variables list specifies a mode and @code{nil} otherwise. | |
1575 | It does not set the mode nor any other file local variable. | |
1576 | @end defun | |
1577 | ||
1578 | If a file local variable could specify a function that would | |
1579 | be called later, or an expression that would be executed later, simply | |
1580 | visiting a file could take over your Emacs. Emacs takes several | |
1581 | measures to prevent this. | |
1582 | ||
1583 | @cindex safe local variable | |
1584 | You can specify safe values for a variable with a | |
1585 | @code{safe-local-variable} property. The property has to be | |
1586 | a function of one argument; any value is safe if the function | |
1587 | returns non-@code{nil} given that value. Many commonly encountered | |
1588 | file variables standardly have @code{safe-local-variable} properties, | |
1589 | including @code{fill-column}, @code{fill-prefix}, and | |
1590 | @code{indent-tabs-mode}. For boolean-valued variables that are safe, | |
1591 | use @code{booleanp} as the property value. Lambda expressions should | |
1592 | be quoted so that @code{describe-variable} can display the predicate. | |
1593 | ||
1594 | @defopt safe-local-variable-values | |
1595 | This variable provides another way to mark some variable values as | |
1596 | safe. It is a list of cons cells @code{(@var{var} . @var{val})}, | |
1597 | where @var{var} is a variable name and @var{val} is a value which is | |
1598 | safe for that variable. | |
1599 | ||
1600 | When Emacs asks the user whether or not to obey a set of file local | |
1601 | variable specifications, the user can choose to mark them as safe. | |
1602 | Doing so adds those variable/value pairs to | |
1603 | @code{safe-local-variable-values}, and saves it to the user's custom | |
1604 | file. | |
1605 | @end defopt | |
1606 | ||
1607 | @defun safe-local-variable-p sym val | |
1608 | This function returns non-@code{nil} if it is safe to give @var{sym} | |
1609 | the value @var{val}, based on the above criteria. | |
1610 | @end defun | |
1611 | ||
1612 | @c @cindex risky local variable Duplicates risky-local-variable | |
1613 | Some variables are considered @dfn{risky}. A variable whose name | |
1614 | ends in any of @samp{-command}, @samp{-frame-alist}, @samp{-function}, | |
1615 | @samp{-functions}, @samp{-hook}, @samp{-hooks}, @samp{-form}, | |
1616 | @samp{-forms}, @samp{-map}, @samp{-map-alist}, @samp{-mode-alist}, | |
1617 | @samp{-program}, or @samp{-predicate} is considered risky. The | |
1618 | variables @samp{font-lock-keywords}, @samp{font-lock-keywords} | |
1619 | followed by a digit, and @samp{font-lock-syntactic-keywords} are also | |
1620 | considered risky. Finally, any variable whose name has a | |
1621 | non-@code{nil} @code{risky-local-variable} property is considered | |
1622 | risky. | |
1623 | ||
1624 | @defun risky-local-variable-p sym | |
1625 | This function returns non-@code{nil} if @var{sym} is a risky variable, | |
1626 | based on the above criteria. | |
1627 | @end defun | |
1628 | ||
1629 | If a variable is risky, it will not be entered automatically into | |
1630 | @code{safe-local-variable-values} as described above. Therefore, | |
1631 | Emacs will always query before setting a risky variable, unless the | |
1632 | user explicitly allows the setting by customizing | |
1633 | @code{safe-local-variable-values} directly. | |
1634 | ||
1635 | @defvar ignored-local-variables | |
1636 | This variable holds a list of variables that should not be given local | |
1637 | values by files. Any value specified for one of these variables is | |
1638 | completely ignored. | |
1639 | @end defvar | |
1640 | ||
1641 | The @samp{Eval:} ``variable'' is also a potential loophole, so Emacs | |
1642 | normally asks for confirmation before handling it. | |
1643 | ||
1644 | @defopt enable-local-eval | |
1645 | This variable controls processing of @samp{Eval:} in @samp{-*-} lines | |
1646 | or local variables | |
1647 | lists in files being visited. A value of @code{t} means process them | |
1648 | unconditionally; @code{nil} means ignore them; anything else means ask | |
1649 | the user what to do for each file. The default value is @code{maybe}. | |
1650 | @end defopt | |
1651 | ||
1652 | @defopt safe-local-eval-forms | |
1653 | This variable holds a list of expressions that are safe to | |
1654 | evaluate when found in the @samp{Eval:} ``variable'' in a file | |
1655 | local variables list. | |
1656 | @end defopt | |
1657 | ||
1658 | If the expression is a function call and the function has a | |
1659 | @code{safe-local-eval-function} property, the property value | |
1660 | determines whether the expression is safe to evaluate. The property | |
1661 | value can be a predicate to call to test the expression, a list of | |
1662 | such predicates (it's safe if any predicate succeeds), or @code{t} | |
1663 | (always safe provided the arguments are constant). | |
1664 | ||
1665 | Text properties are also potential loopholes, since their values | |
1666 | could include functions to call. So Emacs discards all text | |
1667 | properties from string values specified for file local variables. | |
1668 | ||
1669 | @node Variable Aliases | |
1670 | @section Variable Aliases | |
1671 | @cindex variable aliases | |
1672 | ||
1673 | It is sometimes useful to make two variables synonyms, so that both | |
1674 | variables always have the same value, and changing either one also | |
1675 | changes the other. Whenever you change the name of a | |
1676 | variable---either because you realize its old name was not well | |
1677 | chosen, or because its meaning has partly changed---it can be useful | |
1678 | to keep the old name as an @emph{alias} of the new one for | |
1679 | compatibility. You can do this with @code{defvaralias}. | |
1680 | ||
1681 | @defun defvaralias new-alias base-variable &optional docstring | |
1682 | This function defines the symbol @var{new-alias} as a variable alias | |
1683 | for symbol @var{base-variable}. This means that retrieving the value | |
1684 | of @var{new-alias} returns the value of @var{base-variable}, and | |
1685 | changing the value of @var{new-alias} changes the value of | |
1686 | @var{base-variable}. The two aliased variable names always share the | |
1687 | same value and the same bindings. | |
1688 | ||
1689 | If the @var{docstring} argument is non-@code{nil}, it specifies the | |
1690 | documentation for @var{new-alias}; otherwise, the alias gets the same | |
1691 | documentation as @var{base-variable} has, if any, unless | |
1692 | @var{base-variable} is itself an alias, in which case @var{new-alias} gets | |
1693 | the documentation of the variable at the end of the chain of aliases. | |
1694 | ||
1695 | This function returns @var{base-variable}. | |
1696 | @end defun | |
1697 | ||
1698 | Variable aliases are convenient for replacing an old name for a | |
1699 | variable with a new name. @code{make-obsolete-variable} declares that | |
1700 | the old name is obsolete and therefore that it may be removed at some | |
1701 | stage in the future. | |
1702 | ||
1703 | @defun make-obsolete-variable obsolete-name current-name &optional when | |
1704 | This function makes the byte-compiler warn that the variable | |
1705 | @var{obsolete-name} is obsolete. If @var{current-name} is a symbol, it is | |
1706 | the variable's new name; then the warning message says to use | |
1707 | @var{current-name} instead of @var{obsolete-name}. If @var{current-name} | |
1708 | is a string, this is the message and there is no replacement variable. | |
1709 | ||
1710 | If provided, @var{when} should be a string indicating when the | |
1711 | variable was first made obsolete---for example, a date or a release | |
1712 | number. | |
1713 | @end defun | |
1714 | ||
1715 | You can make two variables synonyms and declare one obsolete at the | |
1716 | same time using the macro @code{define-obsolete-variable-alias}. | |
1717 | ||
1718 | @defmac define-obsolete-variable-alias obsolete-name current-name &optional when docstring | |
1719 | This macro marks the variable @var{obsolete-name} as obsolete and also | |
1720 | makes it an alias for the variable @var{current-name}. It is | |
1721 | equivalent to the following: | |
1722 | ||
1723 | @example | |
1724 | (defvaralias @var{obsolete-name} @var{current-name} @var{docstring}) | |
1725 | (make-obsolete-variable @var{obsolete-name} @var{current-name} @var{when}) | |
1726 | @end example | |
1727 | @end defmac | |
1728 | ||
1729 | @defun indirect-variable variable | |
1730 | This function returns the variable at the end of the chain of aliases | |
1731 | of @var{variable}. If @var{variable} is not a symbol, or if @var{variable} is | |
1732 | not defined as an alias, the function returns @var{variable}. | |
1733 | ||
1734 | This function signals a @code{cyclic-variable-indirection} error if | |
1735 | there is a loop in the chain of symbols. | |
1736 | @end defun | |
1737 | ||
1738 | @example | |
1739 | (defvaralias 'foo 'bar) | |
1740 | (indirect-variable 'foo) | |
1741 | @result{} bar | |
1742 | (indirect-variable 'bar) | |
1743 | @result{} bar | |
1744 | (setq bar 2) | |
1745 | bar | |
1746 | @result{} 2 | |
1747 | @group | |
1748 | foo | |
1749 | @result{} 2 | |
1750 | @end group | |
1751 | (setq foo 0) | |
1752 | bar | |
1753 | @result{} 0 | |
1754 | foo | |
1755 | @result{} 0 | |
1756 | @end example | |
1757 | ||
1758 | @node Variables with Restricted Values | |
1759 | @section Variables with Restricted Values | |
1760 | ||
1761 | Ordinary Lisp variables can be assigned any value that is a valid | |
1762 | Lisp object. However, certain Lisp variables are not defined in Lisp, | |
1763 | but in C. Most of these variables are defined in the C code using | |
1764 | @code{DEFVAR_LISP}. Like variables defined in Lisp, these can take on | |
1765 | any value. However, some variables are defined using | |
1766 | @code{DEFVAR_INT} or @code{DEFVAR_BOOL}. @xref{Defining Lisp | |
1767 | variables in C,, Writing Emacs Primitives}, in particular the | |
1768 | description of functions of the type @code{syms_of_@var{filename}}, | |
1769 | for a brief discussion of the C implementation. | |
1770 | ||
1771 | Variables of type @code{DEFVAR_BOOL} can only take on the values | |
1772 | @code{nil} or @code{t}. Attempting to assign them any other value | |
1773 | will set them to @code{t}: | |
1774 | ||
1775 | @example | |
1776 | (let ((display-hourglass 5)) | |
1777 | display-hourglass) | |
1778 | @result{} t | |
1779 | @end example | |
1780 | ||
1781 | @defvar byte-boolean-vars | |
1782 | This variable holds a list of all variables of type @code{DEFVAR_BOOL}. | |
1783 | @end defvar | |
1784 | ||
1785 | Variables of type @code{DEFVAR_INT} can only take on integer values. | |
1786 | Attempting to assign them any other value will result in an error: | |
1787 | ||
1788 | @example | |
1789 | (setq window-min-height 5.0) | |
1790 | @error{} Wrong type argument: integerp, 5.0 | |
1791 | @end example | |
1792 | ||
1793 | @ignore | |
1794 | arch-tag: 5ff62c44-2b51-47bb-99d4-fea5aeec5d3e | |
1795 | @end ignore |