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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, | |
6ed161e1 | 4 | @c 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 Free Software Foundation, Inc. |
b8d4c8d0 | 5 | @c See the file elisp.texi for copying conditions. |
6336d8c3 | 6 | @setfilename ../../info/variables |
b8d4c8d0 GM |
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. | |
b8d4c8d0 | 42 | * File Local Variables:: Handling local variable lists in files. |
eb22b78c | 43 | * Directory Local Variables:: Local variables common to all files in a directory. |
0f7766a4 | 44 | * Frame-Local Variables:: Frame-local bindings for variables. |
eb22b78c | 45 | * Future Local Variables:: New kinds of local values we might add some day. |
b8d4c8d0 GM |
46 | * Variable Aliases:: Variables that are aliases for other variables. |
47 | * Variables with Restricted Values:: Non-constant variables whose value can | |
48 | @emph{not} be an arbitrary Lisp object. | |
49 | @end menu | |
50 | ||
51 | @node Global Variables | |
52 | @section Global Variables | |
53 | @cindex global variable | |
54 | ||
55 | The simplest way to use a variable is @dfn{globally}. This means that | |
56 | the variable has just one value at a time, and this value is in effect | |
57 | (at least for the moment) throughout the Lisp system. The value remains | |
58 | in effect until you specify a new one. When a new value replaces the | |
59 | old one, no trace of the old value remains in the variable. | |
60 | ||
61 | You specify a value for a symbol with @code{setq}. For example, | |
62 | ||
63 | @example | |
64 | (setq x '(a b)) | |
65 | @end example | |
66 | ||
67 | @noindent | |
68 | gives the variable @code{x} the value @code{(a b)}. Note that | |
69 | @code{setq} does not evaluate its first argument, the name of the | |
70 | variable, but it does evaluate the second argument, the new value. | |
71 | ||
72 | Once the variable has a value, you can refer to it by using the symbol | |
73 | by itself as an expression. Thus, | |
74 | ||
75 | @example | |
76 | @group | |
77 | x @result{} (a b) | |
78 | @end group | |
79 | @end example | |
80 | ||
81 | @noindent | |
82 | assuming the @code{setq} form shown above has already been executed. | |
83 | ||
84 | If you do set the same variable again, the new value replaces the old | |
85 | one: | |
86 | ||
87 | @example | |
88 | @group | |
89 | x | |
90 | @result{} (a b) | |
91 | @end group | |
92 | @group | |
93 | (setq x 4) | |
94 | @result{} 4 | |
95 | @end group | |
96 | @group | |
97 | x | |
98 | @result{} 4 | |
99 | @end group | |
100 | @end example | |
101 | ||
102 | @node Constant Variables | |
103 | @section Variables that Never Change | |
104 | @kindex setting-constant | |
105 | @cindex keyword symbol | |
106 | @cindex variable with constant value | |
107 | @cindex constant variables | |
108 | @cindex symbol that evaluates to itself | |
109 | @cindex symbol with constant value | |
110 | ||
111 | In Emacs Lisp, certain symbols normally evaluate to themselves. These | |
112 | include @code{nil} and @code{t}, as well as any symbol whose name starts | |
113 | with @samp{:} (these are called @dfn{keywords}). These symbols cannot | |
114 | be rebound, nor can their values be changed. Any attempt to set or bind | |
115 | @code{nil} or @code{t} signals a @code{setting-constant} error. The | |
116 | same is true for a keyword (a symbol whose name starts with @samp{:}), | |
117 | if it is interned in the standard obarray, except that setting such a | |
118 | symbol to itself is not an error. | |
119 | ||
120 | @example | |
121 | @group | |
122 | nil @equiv{} 'nil | |
123 | @result{} nil | |
124 | @end group | |
125 | @group | |
126 | (setq nil 500) | |
127 | @error{} Attempt to set constant symbol: nil | |
128 | @end group | |
129 | @end example | |
130 | ||
131 | @defun keywordp object | |
132 | function returns @code{t} if @var{object} is a symbol whose name | |
133 | starts with @samp{:}, interned in the standard obarray, and returns | |
134 | @code{nil} otherwise. | |
135 | @end defun | |
136 | ||
137 | @node Local Variables | |
138 | @section Local Variables | |
139 | @cindex binding local variables | |
140 | @cindex local variables | |
141 | @cindex local binding | |
142 | @cindex global binding | |
143 | ||
144 | Global variables have values that last until explicitly superseded | |
145 | with new values. Sometimes it is useful to create variable values that | |
146 | exist temporarily---only until a certain part of the program finishes. | |
147 | These values are called @dfn{local}, and the variables so used are | |
148 | called @dfn{local variables}. | |
149 | ||
150 | For example, when a function is called, its argument variables receive | |
151 | new local values that last until the function exits. The @code{let} | |
152 | special form explicitly establishes new local values for specified | |
153 | variables; these last until exit from the @code{let} form. | |
154 | ||
155 | @cindex shadowing of variables | |
156 | Establishing a local value saves away the previous value (or lack of | |
157 | one) of the variable. When the life span of the local value is over, | |
158 | the previous value is restored. In the mean time, we say that the | |
159 | previous value is @dfn{shadowed} and @dfn{not visible}. Both global and | |
160 | local values may be shadowed (@pxref{Scope}). | |
161 | ||
162 | If you set a variable (such as with @code{setq}) while it is local, | |
163 | this replaces the local value; it does not alter the global value, or | |
164 | previous local values, that are shadowed. To model this behavior, we | |
165 | speak of a @dfn{local binding} of the variable as well as a local value. | |
166 | ||
167 | The local binding is a conceptual place that holds a local value. | |
168 | Entry to a function, or a special form such as @code{let}, creates the | |
169 | local binding; exit from the function or from the @code{let} removes the | |
170 | local binding. As long as the local binding lasts, the variable's value | |
171 | is stored within it. Use of @code{setq} or @code{set} while there is a | |
172 | local binding stores a different value into the local binding; it does | |
173 | not create a new binding. | |
174 | ||
175 | We also speak of the @dfn{global binding}, which is where | |
176 | (conceptually) the global value is kept. | |
177 | ||
178 | @cindex current binding | |
179 | A variable can have more than one local binding at a time (for | |
180 | example, if there are nested @code{let} forms that bind it). In such a | |
181 | case, the most recently created local binding that still exists is the | |
182 | @dfn{current binding} of the variable. (This rule is called | |
183 | @dfn{dynamic scoping}; see @ref{Variable Scoping}.) If there are no | |
184 | local bindings, the variable's global binding is its current binding. | |
185 | We sometimes call the current binding the @dfn{most-local existing | |
186 | binding}, for emphasis. Ordinary evaluation of a symbol always returns | |
187 | the value of its current binding. | |
188 | ||
189 | The special forms @code{let} and @code{let*} exist to create | |
190 | local bindings. | |
191 | ||
192 | @defspec let (bindings@dots{}) forms@dots{} | |
193 | This special form binds variables according to @var{bindings} and then | |
194 | evaluates all of the @var{forms} in textual order. The @code{let}-form | |
195 | returns the value of the last form in @var{forms}. | |
196 | ||
197 | Each of the @var{bindings} is either @w{(i) a} symbol, in which case | |
198 | that symbol is bound to @code{nil}; or @w{(ii) a} list of the form | |
199 | @code{(@var{symbol} @var{value-form})}, in which case @var{symbol} is | |
200 | bound to the result of evaluating @var{value-form}. If @var{value-form} | |
201 | is omitted, @code{nil} is used. | |
202 | ||
203 | All of the @var{value-form}s in @var{bindings} are evaluated in the | |
204 | order they appear and @emph{before} binding any of the symbols to them. | |
205 | Here is an example of this: @code{z} is bound to the old value of | |
206 | @code{y}, which is 2, not the new value of @code{y}, which is 1. | |
207 | ||
208 | @example | |
209 | @group | |
210 | (setq y 2) | |
211 | @result{} 2 | |
212 | @end group | |
213 | @group | |
214 | (let ((y 1) | |
215 | (z y)) | |
216 | (list y z)) | |
217 | @result{} (1 2) | |
218 | @end group | |
219 | @end example | |
220 | @end defspec | |
221 | ||
222 | @defspec let* (bindings@dots{}) forms@dots{} | |
223 | This special form is like @code{let}, but it binds each variable right | |
224 | after computing its local value, before computing the local value for | |
225 | the next variable. Therefore, an expression in @var{bindings} can | |
226 | reasonably refer to the preceding symbols bound in this @code{let*} | |
227 | form. Compare the following example with the example above for | |
228 | @code{let}. | |
229 | ||
230 | @example | |
231 | @group | |
232 | (setq y 2) | |
233 | @result{} 2 | |
234 | @end group | |
235 | @group | |
236 | (let* ((y 1) | |
237 | (z y)) ; @r{Use the just-established value of @code{y}.} | |
238 | (list y z)) | |
239 | @result{} (1 1) | |
240 | @end group | |
241 | @end example | |
242 | @end defspec | |
243 | ||
244 | Here is a complete list of the other facilities that create local | |
245 | bindings: | |
246 | ||
247 | @itemize @bullet | |
248 | @item | |
249 | Function calls (@pxref{Functions}). | |
250 | ||
251 | @item | |
252 | Macro calls (@pxref{Macros}). | |
253 | ||
254 | @item | |
255 | @code{condition-case} (@pxref{Errors}). | |
256 | @end itemize | |
257 | ||
258 | Variables can also have buffer-local bindings (@pxref{Buffer-Local | |
e388c68f RS |
259 | Variables}); a few variables have terminal-local bindings |
260 | (@pxref{Multiple Displays}). These kinds of bindings work somewhat | |
261 | like ordinary local bindings, but they are localized depending on | |
262 | ``where'' you are in Emacs, rather than localized in time. | |
b8d4c8d0 GM |
263 | |
264 | @defvar max-specpdl-size | |
265 | @anchor{Definition of max-specpdl-size} | |
266 | @cindex variable limit error | |
267 | @cindex evaluation error | |
268 | @cindex infinite recursion | |
269 | This variable defines the limit on the total number of local variable | |
270 | bindings and @code{unwind-protect} cleanups (@pxref{Cleanups,, | |
271 | Cleaning Up from Nonlocal Exits}) that are allowed before signaling an | |
272 | error (with data @code{"Variable binding depth exceeds | |
273 | max-specpdl-size"}). | |
274 | ||
275 | This limit, with the associated error when it is exceeded, is one way | |
276 | that Lisp avoids infinite recursion on an ill-defined function. | |
277 | @code{max-lisp-eval-depth} provides another limit on depth of nesting. | |
278 | @xref{Definition of max-lisp-eval-depth,, Eval}. | |
279 | ||
280 | The default value is 1000. Entry to the Lisp debugger increases the | |
281 | value, if there is little room left, to make sure the debugger itself | |
282 | has room to execute. | |
283 | @end defvar | |
284 | ||
285 | @node Void Variables | |
286 | @section When a Variable is ``Void'' | |
287 | @kindex void-variable | |
288 | @cindex void variable | |
289 | ||
290 | If you have never given a symbol any value as a global variable, we | |
291 | say that that symbol's global value is @dfn{void}. In other words, the | |
292 | symbol's value cell does not have any Lisp object in it. If you try to | |
293 | evaluate the symbol, you get a @code{void-variable} error rather than | |
294 | a value. | |
295 | ||
296 | Note that a value of @code{nil} is not the same as void. The symbol | |
297 | @code{nil} is a Lisp object and can be the value of a variable just as any | |
298 | other object can be; but it is @emph{a value}. A void variable does not | |
299 | have any value. | |
300 | ||
301 | After you have given a variable a value, you can make it void once more | |
302 | using @code{makunbound}. | |
303 | ||
304 | @defun makunbound symbol | |
305 | This function makes the current variable binding of @var{symbol} void. | |
306 | Subsequent attempts to use this symbol's value as a variable will signal | |
307 | the error @code{void-variable}, unless and until you set it again. | |
308 | ||
309 | @code{makunbound} returns @var{symbol}. | |
310 | ||
311 | @example | |
312 | @group | |
313 | (makunbound 'x) ; @r{Make the global value of @code{x} void.} | |
314 | @result{} x | |
315 | @end group | |
316 | @group | |
317 | x | |
318 | @error{} Symbol's value as variable is void: x | |
319 | @end group | |
320 | @end example | |
321 | ||
322 | If @var{symbol} is locally bound, @code{makunbound} affects the most | |
323 | local existing binding. This is the only way a symbol can have a void | |
324 | local binding, since all the constructs that create local bindings | |
325 | create them with values. In this case, the voidness lasts at most as | |
326 | long as the binding does; when the binding is removed due to exit from | |
327 | the construct that made it, the previous local or global binding is | |
328 | reexposed as usual, and the variable is no longer void unless the newly | |
329 | reexposed binding was void all along. | |
330 | ||
331 | @smallexample | |
332 | @group | |
333 | (setq x 1) ; @r{Put a value in the global binding.} | |
334 | @result{} 1 | |
335 | (let ((x 2)) ; @r{Locally bind it.} | |
336 | (makunbound 'x) ; @r{Void the local binding.} | |
337 | x) | |
338 | @error{} Symbol's value as variable is void: x | |
339 | @end group | |
340 | @group | |
341 | x ; @r{The global binding is unchanged.} | |
342 | @result{} 1 | |
343 | ||
344 | (let ((x 2)) ; @r{Locally bind it.} | |
345 | (let ((x 3)) ; @r{And again.} | |
346 | (makunbound 'x) ; @r{Void the innermost-local binding.} | |
347 | x)) ; @r{And refer: it's void.} | |
348 | @error{} Symbol's value as variable is void: x | |
349 | @end group | |
350 | ||
351 | @group | |
352 | (let ((x 2)) | |
353 | (let ((x 3)) | |
354 | (makunbound 'x)) ; @r{Void inner binding, then remove it.} | |
355 | x) ; @r{Now outer @code{let} binding is visible.} | |
356 | @result{} 2 | |
357 | @end group | |
358 | @end smallexample | |
359 | @end defun | |
360 | ||
361 | A variable that has been made void with @code{makunbound} is | |
362 | indistinguishable from one that has never received a value and has | |
363 | always been void. | |
364 | ||
365 | You can use the function @code{boundp} to test whether a variable is | |
366 | currently void. | |
367 | ||
368 | @defun boundp variable | |
369 | @code{boundp} returns @code{t} if @var{variable} (a symbol) is not void; | |
370 | more precisely, if its current binding is not void. It returns | |
371 | @code{nil} otherwise. | |
372 | ||
373 | @smallexample | |
374 | @group | |
375 | (boundp 'abracadabra) ; @r{Starts out void.} | |
376 | @result{} nil | |
377 | @end group | |
378 | @group | |
379 | (let ((abracadabra 5)) ; @r{Locally bind it.} | |
380 | (boundp 'abracadabra)) | |
381 | @result{} t | |
382 | @end group | |
383 | @group | |
384 | (boundp 'abracadabra) ; @r{Still globally void.} | |
385 | @result{} nil | |
386 | @end group | |
387 | @group | |
388 | (setq abracadabra 5) ; @r{Make it globally nonvoid.} | |
389 | @result{} 5 | |
390 | @end group | |
391 | @group | |
392 | (boundp 'abracadabra) | |
393 | @result{} t | |
394 | @end group | |
395 | @end smallexample | |
396 | @end defun | |
397 | ||
398 | @node Defining Variables | |
399 | @section Defining Global Variables | |
400 | @cindex variable definition | |
401 | ||
402 | You may announce your intention to use a symbol as a global variable | |
403 | with a @dfn{variable definition}: a special form, either @code{defconst} | |
404 | or @code{defvar}. | |
405 | ||
406 | In Emacs Lisp, definitions serve three purposes. First, they inform | |
407 | people who read the code that certain symbols are @emph{intended} to be | |
408 | used a certain way (as variables). Second, they inform the Lisp system | |
409 | of these things, supplying a value and documentation. Third, they | |
410 | provide information to utilities such as @code{etags} and | |
411 | @code{make-docfile}, which create data bases of the functions and | |
412 | variables in a program. | |
413 | ||
414 | The difference between @code{defconst} and @code{defvar} is primarily | |
415 | a matter of intent, serving to inform human readers of whether the value | |
416 | should ever change. Emacs Lisp does not restrict the ways in which a | |
417 | variable can be used based on @code{defconst} or @code{defvar} | |
418 | declarations. However, it does make a difference for initialization: | |
419 | @code{defconst} unconditionally initializes the variable, while | |
420 | @code{defvar} initializes it only if it is void. | |
421 | ||
422 | @ignore | |
423 | One would expect user option variables to be defined with | |
424 | @code{defconst}, since programs do not change them. Unfortunately, this | |
425 | has bad results if the definition is in a library that is not preloaded: | |
426 | @code{defconst} would override any prior value when the library is | |
427 | loaded. Users would like to be able to set user options in their init | |
428 | files, and override the default values given in the definitions. For | |
429 | this reason, user options must be defined with @code{defvar}. | |
430 | @end ignore | |
431 | ||
432 | @defspec defvar symbol [value [doc-string]] | |
433 | This special form defines @var{symbol} as a variable and can also | |
434 | initialize and document it. The definition informs a person reading | |
435 | your code that @var{symbol} is used as a variable that might be set or | |
436 | changed. Note that @var{symbol} is not evaluated; the symbol to be | |
437 | defined must appear explicitly in the @code{defvar}. | |
438 | ||
439 | If @var{symbol} is void and @var{value} is specified, @code{defvar} | |
440 | evaluates it and sets @var{symbol} to the result. But if @var{symbol} | |
441 | already has a value (i.e., it is not void), @var{value} is not even | |
442 | evaluated, and @var{symbol}'s value remains unchanged. If @var{value} | |
443 | is omitted, the value of @var{symbol} is not changed in any case. | |
444 | ||
445 | If @var{symbol} has a buffer-local binding in the current buffer, | |
446 | @code{defvar} operates on the default value, which is buffer-independent, | |
447 | not the current (buffer-local) binding. It sets the default value if | |
448 | the default value is void. @xref{Buffer-Local Variables}. | |
449 | ||
450 | When you evaluate a top-level @code{defvar} form with @kbd{C-M-x} in | |
451 | Emacs Lisp mode (@code{eval-defun}), a special feature of | |
452 | @code{eval-defun} arranges to set the variable unconditionally, without | |
453 | testing whether its value is void. | |
454 | ||
455 | If the @var{doc-string} argument appears, it specifies the documentation | |
456 | for the variable. (This opportunity to specify documentation is one of | |
457 | the main benefits of defining the variable.) The documentation is | |
458 | stored in the symbol's @code{variable-documentation} property. The | |
459 | Emacs help functions (@pxref{Documentation}) look for this property. | |
460 | ||
923dfbaa CY |
461 | If the documentation string begins with the character @samp{*}, Emacs |
462 | allows users to set it interactively using the @code{set-variable} | |
463 | command. However, you should nearly always use @code{defcustom} | |
464 | instead of @code{defvar} to define such variables, so that users can | |
465 | use @kbd{M-x customize} and related commands to set them. In that | |
466 | case, it is not necessary to begin the documentation string with | |
467 | @samp{*}. @xref{Customization}. | |
b8d4c8d0 GM |
468 | |
469 | Here are some examples. This form defines @code{foo} but does not | |
470 | initialize it: | |
471 | ||
472 | @example | |
473 | @group | |
474 | (defvar foo) | |
475 | @result{} foo | |
476 | @end group | |
477 | @end example | |
478 | ||
479 | This example initializes the value of @code{bar} to @code{23}, and gives | |
480 | it a documentation string: | |
481 | ||
482 | @example | |
483 | @group | |
484 | (defvar bar 23 | |
485 | "The normal weight of a bar.") | |
486 | @result{} bar | |
487 | @end group | |
488 | @end example | |
489 | ||
490 | The following form changes the documentation string for @code{bar}, | |
491 | making it a user option, but does not change the value, since @code{bar} | |
492 | already has a value. (The addition @code{(1+ nil)} would get an error | |
493 | if it were evaluated, but since it is not evaluated, there is no error.) | |
494 | ||
495 | @example | |
496 | @group | |
497 | (defvar bar (1+ nil) | |
498 | "*The normal weight of a bar.") | |
499 | @result{} bar | |
500 | @end group | |
501 | @group | |
502 | bar | |
503 | @result{} 23 | |
504 | @end group | |
505 | @end example | |
506 | ||
507 | Here is an equivalent expression for the @code{defvar} special form: | |
508 | ||
509 | @example | |
510 | @group | |
511 | (defvar @var{symbol} @var{value} @var{doc-string}) | |
512 | @equiv{} | |
513 | (progn | |
514 | (if (not (boundp '@var{symbol})) | |
515 | (setq @var{symbol} @var{value})) | |
516 | (if '@var{doc-string} | |
517 | (put '@var{symbol} 'variable-documentation '@var{doc-string})) | |
518 | '@var{symbol}) | |
519 | @end group | |
520 | @end example | |
521 | ||
522 | The @code{defvar} form returns @var{symbol}, but it is normally used | |
523 | at top level in a file where its value does not matter. | |
524 | @end defspec | |
525 | ||
526 | @defspec defconst symbol value [doc-string] | |
527 | This special form defines @var{symbol} as a value and initializes it. | |
528 | It informs a person reading your code that @var{symbol} has a standard | |
529 | global value, established here, that should not be changed by the user | |
530 | or by other programs. Note that @var{symbol} is not evaluated; the | |
531 | symbol to be defined must appear explicitly in the @code{defconst}. | |
532 | ||
533 | @code{defconst} always evaluates @var{value}, and sets the value of | |
534 | @var{symbol} to the result. If @var{symbol} does have a buffer-local | |
535 | binding in the current buffer, @code{defconst} sets the default value, | |
536 | not the buffer-local value. (But you should not be making | |
537 | buffer-local bindings for a symbol that is defined with | |
538 | @code{defconst}.) | |
539 | ||
540 | Here, @code{pi} is a constant that presumably ought not to be changed | |
541 | by anyone (attempts by the Indiana State Legislature notwithstanding). | |
542 | As the second form illustrates, however, this is only advisory. | |
543 | ||
544 | @example | |
545 | @group | |
546 | (defconst pi 3.1415 "Pi to five places.") | |
547 | @result{} pi | |
548 | @end group | |
549 | @group | |
550 | (setq pi 3) | |
551 | @result{} pi | |
552 | @end group | |
553 | @group | |
554 | pi | |
555 | @result{} 3 | |
556 | @end group | |
557 | @end example | |
558 | @end defspec | |
559 | ||
560 | @defun user-variable-p variable | |
561 | @cindex user option | |
562 | This function returns @code{t} if @var{variable} is a user option---a | |
563 | variable intended to be set by the user for customization---and | |
564 | @code{nil} otherwise. (Variables other than user options exist for the | |
565 | internal purposes of Lisp programs, and users need not know about them.) | |
566 | ||
567 | User option variables are distinguished from other variables either | |
568 | though being declared using @code{defcustom}@footnote{They may also be | |
569 | declared equivalently in @file{cus-start.el}.} or by the first character | |
570 | of their @code{variable-documentation} property. If the property exists | |
571 | and is a string, and its first character is @samp{*}, then the variable | |
572 | is a user option. Aliases of user options are also user options. | |
573 | @end defun | |
574 | ||
575 | @kindex variable-interactive | |
576 | If a user option variable has a @code{variable-interactive} property, | |
577 | the @code{set-variable} command uses that value to control reading the | |
578 | new value for the variable. The property's value is used as if it were | |
579 | specified in @code{interactive} (@pxref{Using Interactive}). However, | |
580 | this feature is largely obsoleted by @code{defcustom} | |
581 | (@pxref{Customization}). | |
582 | ||
583 | @strong{Warning:} If the @code{defconst} and @code{defvar} special | |
584 | forms are used while the variable has a local binding (made with | |
585 | @code{let}, or a function argument), they set the local-binding's | |
586 | value; the top-level binding is not changed. This is not what you | |
587 | usually want. To prevent it, use these special forms at top level in | |
588 | a file, where normally no local binding is in effect, and make sure to | |
589 | load the file before making a local binding for the variable. | |
590 | ||
591 | @node Tips for Defining | |
592 | @section Tips for Defining Variables Robustly | |
593 | ||
594 | When you define a variable whose value is a function, or a list of | |
595 | functions, use a name that ends in @samp{-function} or | |
596 | @samp{-functions}, respectively. | |
597 | ||
598 | There are several other variable name conventions; | |
599 | here is a complete list: | |
600 | ||
601 | @table @samp | |
602 | @item @dots{}-hook | |
603 | The variable is a normal hook (@pxref{Hooks}). | |
604 | ||
605 | @item @dots{}-function | |
606 | The value is a function. | |
607 | ||
608 | @item @dots{}-functions | |
609 | The value is a list of functions. | |
610 | ||
611 | @item @dots{}-form | |
612 | The value is a form (an expression). | |
613 | ||
614 | @item @dots{}-forms | |
615 | The value is a list of forms (expressions). | |
616 | ||
617 | @item @dots{}-predicate | |
618 | The value is a predicate---a function of one argument that returns | |
619 | non-@code{nil} for ``good'' arguments and @code{nil} for ``bad'' | |
620 | arguments. | |
621 | ||
622 | @item @dots{}-flag | |
623 | The value is significant only as to whether it is @code{nil} or not. | |
cc5a5e2d SM |
624 | Since such variables often end up acquiring more values over time, |
625 | this convetion is not strongly recommended. | |
b8d4c8d0 GM |
626 | |
627 | @item @dots{}-program | |
628 | The value is a program name. | |
629 | ||
630 | @item @dots{}-command | |
631 | The value is a whole shell command. | |
632 | ||
633 | @item @dots{}-switches | |
634 | The value specifies options for a command. | |
635 | @end table | |
636 | ||
637 | When you define a variable, always consider whether you should mark | |
638 | it as ``risky''; see @ref{File Local Variables}. | |
639 | ||
640 | When defining and initializing a variable that holds a complicated | |
641 | value (such as a keymap with bindings in it), it's best to put the | |
642 | entire computation of the value into the @code{defvar}, like this: | |
643 | ||
644 | @example | |
645 | (defvar my-mode-map | |
646 | (let ((map (make-sparse-keymap))) | |
647 | (define-key map "\C-c\C-a" 'my-command) | |
648 | @dots{} | |
649 | map) | |
650 | @var{docstring}) | |
651 | @end example | |
652 | ||
653 | @noindent | |
654 | This method has several benefits. First, if the user quits while | |
655 | loading the file, the variable is either still uninitialized or | |
656 | initialized properly, never in-between. If it is still uninitialized, | |
657 | reloading the file will initialize it properly. Second, reloading the | |
658 | file once the variable is initialized will not alter it; that is | |
659 | important if the user has run hooks to alter part of the contents (such | |
660 | as, to rebind keys). Third, evaluating the @code{defvar} form with | |
661 | @kbd{C-M-x} @emph{will} reinitialize the map completely. | |
662 | ||
663 | Putting so much code in the @code{defvar} form has one disadvantage: | |
664 | it puts the documentation string far away from the line which names the | |
665 | variable. Here's a safe way to avoid that: | |
666 | ||
667 | @example | |
668 | (defvar my-mode-map nil | |
669 | @var{docstring}) | |
670 | (unless my-mode-map | |
671 | (let ((map (make-sparse-keymap))) | |
672 | (define-key map "\C-c\C-a" 'my-command) | |
673 | @dots{} | |
674 | (setq my-mode-map map))) | |
675 | @end example | |
676 | ||
677 | @noindent | |
678 | This has all the same advantages as putting the initialization inside | |
679 | the @code{defvar}, except that you must type @kbd{C-M-x} twice, once on | |
680 | each form, if you do want to reinitialize the variable. | |
681 | ||
682 | But be careful not to write the code like this: | |
683 | ||
684 | @example | |
685 | (defvar my-mode-map nil | |
686 | @var{docstring}) | |
687 | (unless my-mode-map | |
688 | (setq my-mode-map (make-sparse-keymap)) | |
689 | (define-key my-mode-map "\C-c\C-a" 'my-command) | |
690 | @dots{}) | |
691 | @end example | |
692 | ||
693 | @noindent | |
694 | This code sets the variable, then alters it, but it does so in more than | |
695 | one step. If the user quits just after the @code{setq}, that leaves the | |
696 | variable neither correctly initialized nor void nor @code{nil}. Once | |
697 | that happens, reloading the file will not initialize the variable; it | |
698 | will remain incomplete. | |
699 | ||
700 | @node Accessing Variables | |
701 | @section Accessing Variable Values | |
702 | ||
703 | The usual way to reference a variable is to write the symbol which | |
704 | names it (@pxref{Symbol Forms}). This requires you to specify the | |
705 | variable name when you write the program. Usually that is exactly what | |
706 | you want to do. Occasionally you need to choose at run time which | |
707 | variable to reference; then you can use @code{symbol-value}. | |
708 | ||
709 | @defun symbol-value symbol | |
710 | This function returns the value of @var{symbol}. This is the value in | |
711 | the innermost local binding of the symbol, or its global value if it | |
712 | has no local bindings. | |
713 | ||
714 | @example | |
715 | @group | |
716 | (setq abracadabra 5) | |
717 | @result{} 5 | |
718 | @end group | |
719 | @group | |
720 | (setq foo 9) | |
721 | @result{} 9 | |
722 | @end group | |
723 | ||
724 | @group | |
725 | ;; @r{Here the symbol @code{abracadabra}} | |
726 | ;; @r{is the symbol whose value is examined.} | |
727 | (let ((abracadabra 'foo)) | |
728 | (symbol-value 'abracadabra)) | |
729 | @result{} foo | |
730 | @end group | |
731 | ||
732 | @group | |
733 | ;; @r{Here, the value of @code{abracadabra},} | |
734 | ;; @r{which is @code{foo},} | |
735 | ;; @r{is the symbol whose value is examined.} | |
736 | (let ((abracadabra 'foo)) | |
737 | (symbol-value abracadabra)) | |
738 | @result{} 9 | |
739 | @end group | |
740 | ||
741 | @group | |
742 | (symbol-value 'abracadabra) | |
743 | @result{} 5 | |
744 | @end group | |
745 | @end example | |
746 | ||
747 | A @code{void-variable} error is signaled if the current binding of | |
748 | @var{symbol} is void. | |
749 | @end defun | |
750 | ||
751 | @node Setting Variables | |
752 | @section How to Alter a Variable Value | |
753 | ||
754 | The usual way to change the value of a variable is with the special | |
755 | form @code{setq}. When you need to compute the choice of variable at | |
756 | run time, use the function @code{set}. | |
757 | ||
758 | @defspec setq [symbol form]@dots{} | |
759 | This special form is the most common method of changing a variable's | |
760 | value. Each @var{symbol} is given a new value, which is the result of | |
761 | evaluating the corresponding @var{form}. The most-local existing | |
762 | binding of the symbol is changed. | |
763 | ||
764 | @code{setq} does not evaluate @var{symbol}; it sets the symbol that you | |
765 | write. We say that this argument is @dfn{automatically quoted}. The | |
766 | @samp{q} in @code{setq} stands for ``quoted.'' | |
767 | ||
768 | The value of the @code{setq} form is the value of the last @var{form}. | |
769 | ||
770 | @example | |
771 | @group | |
772 | (setq x (1+ 2)) | |
773 | @result{} 3 | |
774 | @end group | |
775 | x ; @r{@code{x} now has a global value.} | |
776 | @result{} 3 | |
777 | @group | |
778 | (let ((x 5)) | |
779 | (setq x 6) ; @r{The local binding of @code{x} is set.} | |
780 | x) | |
781 | @result{} 6 | |
782 | @end group | |
783 | x ; @r{The global value is unchanged.} | |
784 | @result{} 3 | |
785 | @end example | |
786 | ||
787 | Note that the first @var{form} is evaluated, then the first | |
788 | @var{symbol} is set, then the second @var{form} is evaluated, then the | |
789 | second @var{symbol} is set, and so on: | |
790 | ||
791 | @example | |
792 | @group | |
793 | (setq x 10 ; @r{Notice that @code{x} is set before} | |
794 | y (1+ x)) ; @r{the value of @code{y} is computed.} | |
795 | @result{} 11 | |
796 | @end group | |
797 | @end example | |
798 | @end defspec | |
799 | ||
800 | @defun set symbol value | |
801 | This function sets @var{symbol}'s value to @var{value}, then returns | |
802 | @var{value}. Since @code{set} is a function, the expression written for | |
803 | @var{symbol} is evaluated to obtain the symbol to set. | |
804 | ||
805 | The most-local existing binding of the variable is the binding that is | |
806 | set; shadowed bindings are not affected. | |
807 | ||
808 | @example | |
809 | @group | |
810 | (set one 1) | |
811 | @error{} Symbol's value as variable is void: one | |
812 | @end group | |
813 | @group | |
814 | (set 'one 1) | |
815 | @result{} 1 | |
816 | @end group | |
817 | @group | |
818 | (set 'two 'one) | |
819 | @result{} one | |
820 | @end group | |
821 | @group | |
822 | (set two 2) ; @r{@code{two} evaluates to symbol @code{one}.} | |
823 | @result{} 2 | |
824 | @end group | |
825 | @group | |
826 | one ; @r{So it is @code{one} that was set.} | |
827 | @result{} 2 | |
828 | (let ((one 1)) ; @r{This binding of @code{one} is set,} | |
829 | (set 'one 3) ; @r{not the global value.} | |
830 | one) | |
831 | @result{} 3 | |
832 | @end group | |
833 | @group | |
834 | one | |
835 | @result{} 2 | |
836 | @end group | |
837 | @end example | |
838 | ||
839 | If @var{symbol} is not actually a symbol, a @code{wrong-type-argument} | |
840 | error is signaled. | |
841 | ||
842 | @example | |
843 | (set '(x y) 'z) | |
844 | @error{} Wrong type argument: symbolp, (x y) | |
845 | @end example | |
846 | ||
847 | Logically speaking, @code{set} is a more fundamental primitive than | |
848 | @code{setq}. Any use of @code{setq} can be trivially rewritten to use | |
849 | @code{set}; @code{setq} could even be defined as a macro, given the | |
850 | availability of @code{set}. However, @code{set} itself is rarely used; | |
851 | beginners hardly need to know about it. It is useful only for choosing | |
852 | at run time which variable to set. For example, the command | |
853 | @code{set-variable}, which reads a variable name from the user and then | |
854 | sets the variable, needs to use @code{set}. | |
855 | ||
856 | @cindex CL note---@code{set} local | |
857 | @quotation | |
858 | @b{Common Lisp note:} In Common Lisp, @code{set} always changes the | |
859 | symbol's ``special'' or dynamic value, ignoring any lexical bindings. | |
860 | In Emacs Lisp, all variables and all bindings are dynamic, so @code{set} | |
861 | always affects the most local existing binding. | |
862 | @end quotation | |
863 | @end defun | |
864 | ||
865 | @node Variable Scoping | |
866 | @section Scoping Rules for Variable Bindings | |
867 | ||
868 | A given symbol @code{foo} can have several local variable bindings, | |
869 | established at different places in the Lisp program, as well as a global | |
870 | binding. The most recently established binding takes precedence over | |
871 | the others. | |
872 | ||
873 | @cindex scope | |
874 | @cindex extent | |
875 | @cindex dynamic scoping | |
876 | @cindex lexical scoping | |
877 | Local bindings in Emacs Lisp have @dfn{indefinite scope} and | |
878 | @dfn{dynamic extent}. @dfn{Scope} refers to @emph{where} textually in | |
879 | the source code the binding can be accessed. ``Indefinite scope'' means | |
880 | that any part of the program can potentially access the variable | |
881 | binding. @dfn{Extent} refers to @emph{when}, as the program is | |
882 | executing, the binding exists. ``Dynamic extent'' means that the binding | |
883 | lasts as long as the activation of the construct that established it. | |
884 | ||
885 | The combination of dynamic extent and indefinite scope is called | |
886 | @dfn{dynamic scoping}. By contrast, most programming languages use | |
887 | @dfn{lexical scoping}, in which references to a local variable must be | |
888 | located textually within the function or block that binds the variable. | |
889 | ||
890 | @cindex CL note---special variables | |
891 | @quotation | |
892 | @b{Common Lisp note:} Variables declared ``special'' in Common Lisp are | |
893 | dynamically scoped, like all variables in Emacs Lisp. | |
894 | @end quotation | |
895 | ||
896 | @menu | |
897 | * Scope:: Scope means where in the program a value is visible. | |
898 | Comparison with other languages. | |
899 | * Extent:: Extent means how long in time a value exists. | |
900 | * Impl of Scope:: Two ways to implement dynamic scoping. | |
901 | * Using Scoping:: How to use dynamic scoping carefully and avoid problems. | |
902 | @end menu | |
903 | ||
904 | @node Scope | |
905 | @subsection Scope | |
906 | ||
907 | Emacs Lisp uses @dfn{indefinite scope} for local variable bindings. | |
908 | This means that any function anywhere in the program text might access a | |
909 | given binding of a variable. Consider the following function | |
910 | definitions: | |
911 | ||
912 | @example | |
913 | @group | |
914 | (defun binder (x) ; @r{@code{x} is bound in @code{binder}.} | |
915 | (foo 5)) ; @r{@code{foo} is some other function.} | |
916 | @end group | |
917 | ||
918 | @group | |
919 | (defun user () ; @r{@code{x} is used ``free'' in @code{user}.} | |
920 | (list x)) | |
921 | @end group | |
922 | @end example | |
923 | ||
924 | In a lexically scoped language, the binding of @code{x} in | |
925 | @code{binder} would never be accessible in @code{user}, because | |
926 | @code{user} is not textually contained within the function | |
927 | @code{binder}. However, in dynamically-scoped Emacs Lisp, @code{user} | |
928 | may or may not refer to the binding of @code{x} established in | |
929 | @code{binder}, depending on the circumstances: | |
930 | ||
931 | @itemize @bullet | |
932 | @item | |
933 | If we call @code{user} directly without calling @code{binder} at all, | |
934 | then whatever binding of @code{x} is found, it cannot come from | |
935 | @code{binder}. | |
936 | ||
937 | @item | |
938 | If we define @code{foo} as follows and then call @code{binder}, then the | |
939 | binding made in @code{binder} will be seen in @code{user}: | |
940 | ||
941 | @example | |
942 | @group | |
943 | (defun foo (lose) | |
944 | (user)) | |
945 | @end group | |
946 | @end example | |
947 | ||
948 | @item | |
949 | However, if we define @code{foo} as follows and then call @code{binder}, | |
950 | then the binding made in @code{binder} @emph{will not} be seen in | |
951 | @code{user}: | |
952 | ||
953 | @example | |
954 | (defun foo (x) | |
955 | (user)) | |
956 | @end example | |
957 | ||
958 | @noindent | |
959 | Here, when @code{foo} is called by @code{binder}, it binds @code{x}. | |
960 | (The binding in @code{foo} is said to @dfn{shadow} the one made in | |
961 | @code{binder}.) Therefore, @code{user} will access the @code{x} bound | |
962 | by @code{foo} instead of the one bound by @code{binder}. | |
963 | @end itemize | |
964 | ||
965 | Emacs Lisp uses dynamic scoping because simple implementations of | |
966 | lexical scoping are slow. In addition, every Lisp system needs to offer | |
967 | dynamic scoping at least as an option; if lexical scoping is the norm, | |
968 | there must be a way to specify dynamic scoping instead for a particular | |
969 | variable. It might not be a bad thing for Emacs to offer both, but | |
970 | implementing it with dynamic scoping only was much easier. | |
971 | ||
972 | @node Extent | |
973 | @subsection Extent | |
974 | ||
975 | @dfn{Extent} refers to the time during program execution that a | |
976 | variable name is valid. In Emacs Lisp, a variable is valid only while | |
977 | the form that bound it is executing. This is called @dfn{dynamic | |
978 | extent}. ``Local'' or ``automatic'' variables in most languages, | |
979 | including C and Pascal, have dynamic extent. | |
980 | ||
981 | One alternative to dynamic extent is @dfn{indefinite extent}. This | |
982 | means that a variable binding can live on past the exit from the form | |
983 | that made the binding. Common Lisp and Scheme, for example, support | |
984 | this, but Emacs Lisp does not. | |
985 | ||
986 | To illustrate this, the function below, @code{make-add}, returns a | |
987 | function that purports to add @var{n} to its own argument @var{m}. This | |
988 | would work in Common Lisp, but it does not do the job in Emacs Lisp, | |
989 | because after the call to @code{make-add} exits, the variable @code{n} | |
990 | is no longer bound to the actual argument 2. | |
991 | ||
992 | @example | |
993 | (defun make-add (n) | |
994 | (function (lambda (m) (+ n m)))) ; @r{Return a function.} | |
995 | @result{} make-add | |
996 | (fset 'add2 (make-add 2)) ; @r{Define function @code{add2}} | |
997 | ; @r{with @code{(make-add 2)}.} | |
998 | @result{} (lambda (m) (+ n m)) | |
999 | (add2 4) ; @r{Try to add 2 to 4.} | |
1000 | @error{} Symbol's value as variable is void: n | |
1001 | @end example | |
1002 | ||
1003 | @cindex closures not available | |
1004 | Some Lisp dialects have ``closures,'' objects that are like functions | |
1005 | but record additional variable bindings. Emacs Lisp does not have | |
1006 | closures. | |
1007 | ||
1008 | @node Impl of Scope | |
1009 | @subsection Implementation of Dynamic Scoping | |
1010 | @cindex deep binding | |
1011 | ||
1012 | A simple sample implementation (which is not how Emacs Lisp actually | |
1013 | works) may help you understand dynamic binding. This technique is | |
1014 | called @dfn{deep binding} and was used in early Lisp systems. | |
1015 | ||
1016 | Suppose there is a stack of bindings, which are variable-value pairs. | |
1017 | At entry to a function or to a @code{let} form, we can push bindings | |
1018 | onto the stack for the arguments or local variables created there. We | |
1019 | can pop those bindings from the stack at exit from the binding | |
1020 | construct. | |
1021 | ||
1022 | We can find the value of a variable by searching the stack from top to | |
1023 | bottom for a binding for that variable; the value from that binding is | |
1024 | the value of the variable. To set the variable, we search for the | |
1025 | current binding, then store the new value into that binding. | |
1026 | ||
1027 | As you can see, a function's bindings remain in effect as long as it | |
1028 | continues execution, even during its calls to other functions. That is | |
1029 | why we say the extent of the binding is dynamic. And any other function | |
1030 | can refer to the bindings, if it uses the same variables while the | |
1031 | bindings are in effect. That is why we say the scope is indefinite. | |
1032 | ||
1033 | @cindex shallow binding | |
1034 | The actual implementation of variable scoping in GNU Emacs Lisp uses a | |
1035 | technique called @dfn{shallow binding}. Each variable has a standard | |
1036 | place in which its current value is always found---the value cell of the | |
1037 | symbol. | |
1038 | ||
1039 | In shallow binding, setting the variable works by storing a value in | |
1040 | the value cell. Creating a new binding works by pushing the old value | |
1041 | (belonging to a previous binding) onto a stack, and storing the new | |
1042 | local value in the value cell. Eliminating a binding works by popping | |
1043 | the old value off the stack, into the value cell. | |
1044 | ||
1045 | We use shallow binding because it has the same results as deep | |
1046 | binding, but runs faster, since there is never a need to search for a | |
1047 | binding. | |
1048 | ||
1049 | @node Using Scoping | |
1050 | @subsection Proper Use of Dynamic Scoping | |
1051 | ||
1052 | Binding a variable in one function and using it in another is a | |
1053 | powerful technique, but if used without restraint, it can make programs | |
1054 | hard to understand. There are two clean ways to use this technique: | |
1055 | ||
1056 | @itemize @bullet | |
1057 | @item | |
1058 | Use or bind the variable only in a few related functions, written close | |
1059 | together in one file. Such a variable is used for communication within | |
1060 | one program. | |
1061 | ||
1062 | You should write comments to inform other programmers that they can see | |
1063 | all uses of the variable before them, and to advise them not to add uses | |
1064 | elsewhere. | |
1065 | ||
1066 | @item | |
1067 | Give the variable a well-defined, documented meaning, and make all | |
1068 | appropriate functions refer to it (but not bind it or set it) wherever | |
1069 | that meaning is relevant. For example, the variable | |
1070 | @code{case-fold-search} is defined as ``non-@code{nil} means ignore case | |
1071 | when searching''; various search and replace functions refer to it | |
1072 | directly or through their subroutines, but do not bind or set it. | |
1073 | ||
1074 | Then you can bind the variable in other programs, knowing reliably what | |
1075 | the effect will be. | |
1076 | @end itemize | |
1077 | ||
1078 | In either case, you should define the variable with @code{defvar}. | |
1079 | This helps other people understand your program by telling them to look | |
1080 | for inter-function usage. It also avoids a warning from the byte | |
1081 | compiler. Choose the variable's name to avoid name conflicts---don't | |
1082 | use short names like @code{x}. | |
1083 | ||
1084 | @node Buffer-Local Variables | |
1085 | @section Buffer-Local Variables | |
1086 | @cindex variable, buffer-local | |
1087 | @cindex buffer-local variables | |
1088 | ||
1089 | Global and local variable bindings are found in most programming | |
e388c68f RS |
1090 | languages in one form or another. Emacs, however, also supports |
1091 | additional, unusual kinds of variable binding, such as | |
1092 | @dfn{buffer-local} bindings, which apply only in one buffer. Having | |
1093 | different values for a variable in different buffers is an important | |
1094 | customization method. (A few variables have bindings that are local | |
0f7766a4 EZ |
1095 | to each terminal; see @ref{Multiple Displays}. You can also have |
1096 | frame-local bindings for variables; see @ref{Frame-Local Variables}.) | |
b8d4c8d0 GM |
1097 | |
1098 | @menu | |
1099 | * Intro to Buffer-Local:: Introduction and concepts. | |
1100 | * Creating Buffer-Local:: Creating and destroying buffer-local bindings. | |
1101 | * Default Value:: The default value is seen in buffers | |
1102 | that don't have their own buffer-local values. | |
1103 | @end menu | |
1104 | ||
1105 | @node Intro to Buffer-Local | |
1106 | @subsection Introduction to Buffer-Local Variables | |
1107 | ||
1108 | A buffer-local variable has a buffer-local binding associated with a | |
1109 | particular buffer. The binding is in effect when that buffer is | |
1110 | current; otherwise, it is not in effect. If you set the variable while | |
1111 | a buffer-local binding is in effect, the new value goes in that binding, | |
1112 | so its other bindings are unchanged. This means that the change is | |
1113 | visible only in the buffer where you made it. | |
1114 | ||
1115 | The variable's ordinary binding, which is not associated with any | |
1116 | specific buffer, is called the @dfn{default binding}. In most cases, | |
1117 | this is the global binding. | |
1118 | ||
1119 | A variable can have buffer-local bindings in some buffers but not in | |
1120 | other buffers. The default binding is shared by all the buffers that | |
1121 | don't have their own bindings for the variable. (This includes all | |
1122 | newly-created buffers.) If you set the variable in a buffer that does | |
e388c68f | 1123 | not have a buffer-local binding for it, this sets the default binding, |
b8d4c8d0 GM |
1124 | so the new value is visible in all the buffers that see the default |
1125 | binding. | |
1126 | ||
1127 | The most common use of buffer-local bindings is for major modes to change | |
1128 | variables that control the behavior of commands. For example, C mode and | |
1129 | Lisp mode both set the variable @code{paragraph-start} to specify that only | |
1130 | blank lines separate paragraphs. They do this by making the variable | |
1131 | buffer-local in the buffer that is being put into C mode or Lisp mode, and | |
1132 | then setting it to the new value for that mode. @xref{Major Modes}. | |
1133 | ||
1134 | The usual way to make a buffer-local binding is with | |
1135 | @code{make-local-variable}, which is what major mode commands typically | |
1136 | use. This affects just the current buffer; all other buffers (including | |
1137 | those yet to be created) will continue to share the default value unless | |
1138 | they are explicitly given their own buffer-local bindings. | |
1139 | ||
1140 | @cindex automatically buffer-local | |
1141 | A more powerful operation is to mark the variable as | |
1142 | @dfn{automatically buffer-local} by calling | |
1143 | @code{make-variable-buffer-local}. You can think of this as making the | |
1144 | variable local in all buffers, even those yet to be created. More | |
1145 | precisely, the effect is that setting the variable automatically makes | |
1146 | the variable local to the current buffer if it is not already so. All | |
1147 | buffers start out by sharing the default value of the variable as usual, | |
1148 | but setting the variable creates a buffer-local binding for the current | |
1149 | buffer. The new value is stored in the buffer-local binding, leaving | |
1150 | the default binding untouched. This means that the default value cannot | |
1151 | be changed with @code{setq} in any buffer; the only way to change it is | |
1152 | with @code{setq-default}. | |
1153 | ||
e388c68f | 1154 | @strong{Warning:} When a variable has buffer-local |
b8d4c8d0 GM |
1155 | bindings in one or more buffers, @code{let} rebinds the binding that's |
1156 | currently in effect. For instance, if the current buffer has a | |
1157 | buffer-local value, @code{let} temporarily rebinds that. If no | |
e388c68f | 1158 | buffer-local bindings are in effect, @code{let} rebinds |
b8d4c8d0 GM |
1159 | the default value. If inside the @code{let} you then change to a |
1160 | different current buffer in which a different binding is in effect, | |
1161 | you won't see the @code{let} binding any more. And if you exit the | |
1162 | @code{let} while still in the other buffer, you won't see the | |
1163 | unbinding occur (though it will occur properly). Here is an example | |
1164 | to illustrate: | |
1165 | ||
1166 | @example | |
1167 | @group | |
1168 | (setq foo 'g) | |
1169 | (set-buffer "a") | |
1170 | (make-local-variable 'foo) | |
1171 | @end group | |
1172 | (setq foo 'a) | |
1173 | (let ((foo 'temp)) | |
1174 | ;; foo @result{} 'temp ; @r{let binding in buffer @samp{a}} | |
1175 | (set-buffer "b") | |
1176 | ;; foo @result{} 'g ; @r{the global value since foo is not local in @samp{b}} | |
1177 | @var{body}@dots{}) | |
1178 | @group | |
1179 | foo @result{} 'g ; @r{exiting restored the local value in buffer @samp{a},} | |
1180 | ; @r{but we don't see that in buffer @samp{b}} | |
1181 | @end group | |
1182 | @group | |
1183 | (set-buffer "a") ; @r{verify the local value was restored} | |
1184 | foo @result{} 'a | |
1185 | @end group | |
1186 | @end example | |
1187 | ||
1188 | Note that references to @code{foo} in @var{body} access the | |
1189 | buffer-local binding of buffer @samp{b}. | |
1190 | ||
1191 | When a file specifies local variable values, these become buffer-local | |
1192 | values when you visit the file. @xref{File Variables,,, emacs, The | |
1193 | GNU Emacs Manual}. | |
1194 | ||
0f7766a4 EZ |
1195 | A variable cannot be both buffer-local and frame-local |
1196 | (@pxref{Frame-Local Variables}) or terminal-local (@pxref{Multiple | |
1197 | Displays}). | |
1198 | ||
b8d4c8d0 GM |
1199 | @node Creating Buffer-Local |
1200 | @subsection Creating and Deleting Buffer-Local Bindings | |
1201 | ||
1202 | @deffn Command make-local-variable variable | |
1203 | This function creates a buffer-local binding in the current buffer for | |
1204 | @var{variable} (a symbol). Other buffers are not affected. The value | |
1205 | returned is @var{variable}. | |
1206 | ||
1207 | @c Emacs 19 feature | |
1208 | The buffer-local value of @var{variable} starts out as the same value | |
1209 | @var{variable} previously had. If @var{variable} was void, it remains | |
1210 | void. | |
1211 | ||
1212 | @example | |
1213 | @group | |
1214 | ;; @r{In buffer @samp{b1}:} | |
1215 | (setq foo 5) ; @r{Affects all buffers.} | |
1216 | @result{} 5 | |
1217 | @end group | |
1218 | @group | |
1219 | (make-local-variable 'foo) ; @r{Now it is local in @samp{b1}.} | |
1220 | @result{} foo | |
1221 | @end group | |
1222 | @group | |
1223 | foo ; @r{That did not change} | |
1224 | @result{} 5 ; @r{the value.} | |
1225 | @end group | |
1226 | @group | |
1227 | (setq foo 6) ; @r{Change the value} | |
1228 | @result{} 6 ; @r{in @samp{b1}.} | |
1229 | @end group | |
1230 | @group | |
1231 | foo | |
1232 | @result{} 6 | |
1233 | @end group | |
1234 | ||
1235 | @group | |
1236 | ;; @r{In buffer @samp{b2}, the value hasn't changed.} | |
1237 | (save-excursion | |
1238 | (set-buffer "b2") | |
1239 | foo) | |
1240 | @result{} 5 | |
1241 | @end group | |
1242 | @end example | |
1243 | ||
1244 | Making a variable buffer-local within a @code{let}-binding for that | |
1245 | variable does not work reliably, unless the buffer in which you do this | |
1246 | is not current either on entry to or exit from the @code{let}. This is | |
1247 | because @code{let} does not distinguish between different kinds of | |
1248 | bindings; it knows only which variable the binding was made for. | |
1249 | ||
0f7766a4 EZ |
1250 | If the variable is terminal-local (@pxref{Multiple Displays}), or |
1251 | frame-local (@pxref{Frame-Local Variables}), this function signals an | |
1252 | error. Such variables cannot have buffer-local bindings as well. | |
b8d4c8d0 GM |
1253 | |
1254 | @strong{Warning:} do not use @code{make-local-variable} for a hook | |
1255 | variable. The hook variables are automatically made buffer-local as | |
1256 | needed if you use the @var{local} argument to @code{add-hook} or | |
1257 | @code{remove-hook}. | |
1258 | @end deffn | |
1259 | ||
1260 | @deffn Command make-variable-buffer-local variable | |
1261 | This function marks @var{variable} (a symbol) automatically | |
1262 | buffer-local, so that any subsequent attempt to set it will make it | |
1263 | local to the current buffer at the time. | |
1264 | ||
1265 | A peculiar wrinkle of this feature is that binding the variable (with | |
1266 | @code{let} or other binding constructs) does not create a buffer-local | |
1267 | binding for it. Only setting the variable (with @code{set} or | |
1268 | @code{setq}), while the variable does not have a @code{let}-style | |
1269 | binding that was made in the current buffer, does so. | |
1270 | ||
1271 | If @var{variable} does not have a default value, then calling this | |
1272 | command will give it a default value of @code{nil}. If @var{variable} | |
1273 | already has a default value, that value remains unchanged. | |
1274 | Subsequently calling @code{makunbound} on @var{variable} will result | |
1275 | in a void buffer-local value and leave the default value unaffected. | |
1276 | ||
1277 | The value returned is @var{variable}. | |
1278 | ||
1279 | @strong{Warning:} Don't assume that you should use | |
1280 | @code{make-variable-buffer-local} for user-option variables, simply | |
1281 | because users @emph{might} want to customize them differently in | |
1282 | different buffers. Users can make any variable local, when they wish | |
1283 | to. It is better to leave the choice to them. | |
1284 | ||
1285 | The time to use @code{make-variable-buffer-local} is when it is crucial | |
1286 | that no two buffers ever share the same binding. For example, when a | |
1287 | variable is used for internal purposes in a Lisp program which depends | |
1288 | on having separate values in separate buffers, then using | |
1289 | @code{make-variable-buffer-local} can be the best solution. | |
1290 | @end deffn | |
1291 | ||
1292 | @defun local-variable-p variable &optional buffer | |
1293 | This returns @code{t} if @var{variable} is buffer-local in buffer | |
1294 | @var{buffer} (which defaults to the current buffer); otherwise, | |
1295 | @code{nil}. | |
1296 | @end defun | |
1297 | ||
1298 | @defun local-variable-if-set-p variable &optional buffer | |
1299 | This returns @code{t} if @var{variable} will become buffer-local in | |
1300 | buffer @var{buffer} (which defaults to the current buffer) if it is | |
1301 | set there. | |
1302 | @end defun | |
1303 | ||
1304 | @defun buffer-local-value variable buffer | |
1305 | This function returns the buffer-local binding of @var{variable} (a | |
1306 | symbol) in buffer @var{buffer}. If @var{variable} does not have a | |
1307 | buffer-local binding in buffer @var{buffer}, it returns the default | |
1308 | value (@pxref{Default Value}) of @var{variable} instead. | |
1309 | @end defun | |
1310 | ||
1311 | @defun buffer-local-variables &optional buffer | |
1312 | This function returns a list describing the buffer-local variables in | |
1313 | buffer @var{buffer}. (If @var{buffer} is omitted, the current buffer is | |
1314 | used.) It returns an association list (@pxref{Association Lists}) in | |
1315 | which each element contains one buffer-local variable and its value. | |
1316 | However, when a variable's buffer-local binding in @var{buffer} is void, | |
1317 | then the variable appears directly in the resulting list. | |
1318 | ||
1319 | @example | |
1320 | @group | |
1321 | (make-local-variable 'foobar) | |
1322 | (makunbound 'foobar) | |
1323 | (make-local-variable 'bind-me) | |
1324 | (setq bind-me 69) | |
1325 | @end group | |
1326 | (setq lcl (buffer-local-variables)) | |
1327 | ;; @r{First, built-in variables local in all buffers:} | |
1328 | @result{} ((mark-active . nil) | |
1329 | (buffer-undo-list . nil) | |
1330 | (mode-name . "Fundamental") | |
1331 | @dots{} | |
1332 | @group | |
1333 | ;; @r{Next, non-built-in buffer-local variables.} | |
1334 | ;; @r{This one is buffer-local and void:} | |
1335 | foobar | |
1336 | ;; @r{This one is buffer-local and nonvoid:} | |
1337 | (bind-me . 69)) | |
1338 | @end group | |
1339 | @end example | |
1340 | ||
1341 | Note that storing new values into the @sc{cdr}s of cons cells in this | |
1342 | list does @emph{not} change the buffer-local values of the variables. | |
1343 | @end defun | |
1344 | ||
1345 | @deffn Command kill-local-variable variable | |
1346 | This function deletes the buffer-local binding (if any) for | |
1347 | @var{variable} (a symbol) in the current buffer. As a result, the | |
1348 | default binding of @var{variable} becomes visible in this buffer. This | |
1349 | typically results in a change in the value of @var{variable}, since the | |
1350 | default value is usually different from the buffer-local value just | |
1351 | eliminated. | |
1352 | ||
1353 | If you kill the buffer-local binding of a variable that automatically | |
1354 | becomes buffer-local when set, this makes the default value visible in | |
1355 | the current buffer. However, if you set the variable again, that will | |
1356 | once again create a buffer-local binding for it. | |
1357 | ||
1358 | @code{kill-local-variable} returns @var{variable}. | |
1359 | ||
1360 | This function is a command because it is sometimes useful to kill one | |
1361 | buffer-local variable interactively, just as it is useful to create | |
1362 | buffer-local variables interactively. | |
1363 | @end deffn | |
1364 | ||
1365 | @defun kill-all-local-variables | |
1366 | This function eliminates all the buffer-local variable bindings of the | |
ee666f84 EZ |
1367 | current buffer except for variables marked as ``permanent'' and local |
1368 | hook functions that have a non-@code{nil} @code{permanent-local-hook} | |
1369 | property (@pxref{Setting Hooks}). As a result, the buffer will see | |
1370 | the default values of most variables. | |
b8d4c8d0 GM |
1371 | |
1372 | This function also resets certain other information pertaining to the | |
1373 | buffer: it sets the local keymap to @code{nil}, the syntax table to the | |
1374 | value of @code{(standard-syntax-table)}, the case table to | |
1375 | @code{(standard-case-table)}, and the abbrev table to the value of | |
1376 | @code{fundamental-mode-abbrev-table}. | |
1377 | ||
1378 | The very first thing this function does is run the normal hook | |
1379 | @code{change-major-mode-hook} (see below). | |
1380 | ||
1381 | Every major mode command begins by calling this function, which has the | |
1382 | effect of switching to Fundamental mode and erasing most of the effects | |
1383 | of the previous major mode. To ensure that this does its job, the | |
1384 | variables that major modes set should not be marked permanent. | |
1385 | ||
1386 | @code{kill-all-local-variables} returns @code{nil}. | |
1387 | @end defun | |
1388 | ||
1389 | @defvar change-major-mode-hook | |
1390 | The function @code{kill-all-local-variables} runs this normal hook | |
1391 | before it does anything else. This gives major modes a way to arrange | |
1392 | for something special to be done if the user switches to a different | |
1393 | major mode. It is also useful for buffer-specific minor modes | |
1394 | that should be forgotten if the user changes the major mode. | |
1395 | ||
1396 | For best results, make this variable buffer-local, so that it will | |
1397 | disappear after doing its job and will not interfere with the | |
1398 | subsequent major mode. @xref{Hooks}. | |
1399 | @end defvar | |
1400 | ||
1401 | @c Emacs 19 feature | |
1402 | @cindex permanent local variable | |
1403 | A buffer-local variable is @dfn{permanent} if the variable name (a | |
1404 | symbol) has a @code{permanent-local} property that is non-@code{nil}. | |
1405 | Permanent locals are appropriate for data pertaining to where the file | |
1406 | came from or how to save it, rather than with how to edit the contents. | |
1407 | ||
1408 | @node Default Value | |
1409 | @subsection The Default Value of a Buffer-Local Variable | |
1410 | @cindex default value | |
1411 | ||
1412 | The global value of a variable with buffer-local bindings is also | |
1413 | called the @dfn{default} value, because it is the value that is in | |
1414 | effect whenever neither the current buffer nor the selected frame has | |
1415 | its own binding for the variable. | |
1416 | ||
1417 | The functions @code{default-value} and @code{setq-default} access and | |
1418 | change a variable's default value regardless of whether the current | |
1419 | buffer has a buffer-local binding. For example, you could use | |
1420 | @code{setq-default} to change the default setting of | |
1421 | @code{paragraph-start} for most buffers; and this would work even when | |
1422 | you are in a C or Lisp mode buffer that has a buffer-local value for | |
1423 | this variable. | |
1424 | ||
1425 | @c Emacs 19 feature | |
1426 | The special forms @code{defvar} and @code{defconst} also set the | |
1427 | default value (if they set the variable at all), rather than any | |
e388c68f | 1428 | buffer-local value. |
b8d4c8d0 GM |
1429 | |
1430 | @defun default-value symbol | |
1431 | This function returns @var{symbol}'s default value. This is the value | |
1432 | that is seen in buffers and frames that do not have their own values for | |
1433 | this variable. If @var{symbol} is not buffer-local, this is equivalent | |
1434 | to @code{symbol-value} (@pxref{Accessing Variables}). | |
1435 | @end defun | |
1436 | ||
1437 | @c Emacs 19 feature | |
1438 | @defun default-boundp symbol | |
1439 | The function @code{default-boundp} tells you whether @var{symbol}'s | |
1440 | default value is nonvoid. If @code{(default-boundp 'foo)} returns | |
1441 | @code{nil}, then @code{(default-value 'foo)} would get an error. | |
1442 | ||
1443 | @code{default-boundp} is to @code{default-value} as @code{boundp} is to | |
1444 | @code{symbol-value}. | |
1445 | @end defun | |
1446 | ||
1447 | @defspec setq-default [symbol form]@dots{} | |
1448 | This special form gives each @var{symbol} a new default value, which is | |
1449 | the result of evaluating the corresponding @var{form}. It does not | |
1450 | evaluate @var{symbol}, but does evaluate @var{form}. The value of the | |
1451 | @code{setq-default} form is the value of the last @var{form}. | |
1452 | ||
1453 | If a @var{symbol} is not buffer-local for the current buffer, and is not | |
1454 | marked automatically buffer-local, @code{setq-default} has the same | |
1455 | effect as @code{setq}. If @var{symbol} is buffer-local for the current | |
1456 | buffer, then this changes the value that other buffers will see (as long | |
1457 | as they don't have a buffer-local value), but not the value that the | |
1458 | current buffer sees. | |
1459 | ||
1460 | @example | |
1461 | @group | |
1462 | ;; @r{In buffer @samp{foo}:} | |
1463 | (make-local-variable 'buffer-local) | |
1464 | @result{} buffer-local | |
1465 | @end group | |
1466 | @group | |
1467 | (setq buffer-local 'value-in-foo) | |
1468 | @result{} value-in-foo | |
1469 | @end group | |
1470 | @group | |
1471 | (setq-default buffer-local 'new-default) | |
1472 | @result{} new-default | |
1473 | @end group | |
1474 | @group | |
1475 | buffer-local | |
1476 | @result{} value-in-foo | |
1477 | @end group | |
1478 | @group | |
1479 | (default-value 'buffer-local) | |
1480 | @result{} new-default | |
1481 | @end group | |
1482 | ||
1483 | @group | |
1484 | ;; @r{In (the new) buffer @samp{bar}:} | |
1485 | buffer-local | |
1486 | @result{} new-default | |
1487 | @end group | |
1488 | @group | |
1489 | (default-value 'buffer-local) | |
1490 | @result{} new-default | |
1491 | @end group | |
1492 | @group | |
1493 | (setq buffer-local 'another-default) | |
1494 | @result{} another-default | |
1495 | @end group | |
1496 | @group | |
1497 | (default-value 'buffer-local) | |
1498 | @result{} another-default | |
1499 | @end group | |
1500 | ||
1501 | @group | |
1502 | ;; @r{Back in buffer @samp{foo}:} | |
1503 | buffer-local | |
1504 | @result{} value-in-foo | |
1505 | (default-value 'buffer-local) | |
1506 | @result{} another-default | |
1507 | @end group | |
1508 | @end example | |
1509 | @end defspec | |
1510 | ||
1511 | @defun set-default symbol value | |
1512 | This function is like @code{setq-default}, except that @var{symbol} is | |
1513 | an ordinary evaluated argument. | |
1514 | ||
1515 | @example | |
1516 | @group | |
1517 | (set-default (car '(a b c)) 23) | |
1518 | @result{} 23 | |
1519 | @end group | |
1520 | @group | |
1521 | (default-value 'a) | |
1522 | @result{} 23 | |
1523 | @end group | |
1524 | @end example | |
1525 | @end defun | |
1526 | ||
b8d4c8d0 GM |
1527 | @node File Local Variables |
1528 | @section File Local Variables | |
1529 | @cindex file local variables | |
1530 | ||
1531 | A file can specify local variable values; Emacs uses these to create | |
1532 | buffer-local bindings for those variables in the buffer visiting that | |
1533 | file. @xref{File variables, , Local Variables in Files, emacs, The | |
1534 | GNU Emacs Manual}, for basic information about file local variables. | |
1535 | This section describes the functions and variables that affect | |
1536 | processing of file local variables. | |
1537 | ||
1538 | @defopt enable-local-variables | |
1539 | This variable controls whether to process file local variables. | |
1540 | The possible values are: | |
1541 | ||
1542 | @table @asis | |
1543 | @item @code{t} (the default) | |
1544 | Set the safe variables, and query (once) about any unsafe variables. | |
1545 | @item @code{:safe} | |
1546 | Set only the safe variables and do not query. | |
1547 | @item @code{:all} | |
1548 | Set all the variables and do not query. | |
1549 | @item @code{nil} | |
1550 | Don't set any variables. | |
1551 | @item anything else | |
1552 | Query (once) about all the variables. | |
1553 | @end table | |
1554 | @end defopt | |
1555 | ||
1556 | @defun hack-local-variables &optional mode-only | |
1557 | This function parses, and binds or evaluates as appropriate, any local | |
1558 | variables specified by the contents of the current buffer. The variable | |
1559 | @code{enable-local-variables} has its effect here. However, this | |
1560 | function does not look for the @samp{mode:} local variable in the | |
1561 | @w{@samp{-*-}} line. @code{set-auto-mode} does that, also taking | |
1562 | @code{enable-local-variables} into account (@pxref{Auto Major Mode}). | |
1563 | ||
291703b5 | 1564 | This function works by walking the alist stored in |
3a57591a | 1565 | @code{file-local-variables-alist} and applying each local variable in |
291703b5 EZ |
1566 | turn. It calls @code{before-hack-local-variables-hook} and |
1567 | @code{hack-local-variables-hook} before and after applying the | |
1568 | variables, respectively. | |
1569 | ||
b8d4c8d0 GM |
1570 | If the optional argument @var{mode-only} is non-@code{nil}, then all |
1571 | this function does is return @code{t} if the @w{@samp{-*-}} line or | |
1572 | the local variables list specifies a mode and @code{nil} otherwise. | |
1573 | It does not set the mode nor any other file local variable. | |
1574 | @end defun | |
1575 | ||
291703b5 EZ |
1576 | @defvar file-local-variables-alist |
1577 | This buffer-local variable holds the alist of file-local variable | |
1578 | settings. Each element of the alist is of the form | |
1579 | @w{@code{(@var{var} . @var{value})}}, where @var{var} is a symbol of | |
1580 | the local variable and @var{value} is its value. When Emacs visits a | |
1581 | file, it first collects all the file-local variables into this alist, | |
1582 | and then the @code{hack-local-variables} function applies them one by | |
1583 | one. | |
1584 | @end defvar | |
1585 | ||
1586 | @defvar before-hack-local-variables-hook | |
1587 | Emacs calls this hook immediately before applying file-local variables | |
1588 | stored in @code{file-local-variables-alist}. | |
1589 | @end defvar | |
1590 | ||
1591 | @defvar hack-local-variables-hook | |
1592 | Emacs calls this hook immediately after it finishes applying | |
1593 | file-local variables stored in @code{file-local-variables-alist}. | |
1594 | @end defvar | |
1595 | ||
b8d4c8d0 GM |
1596 | If a file local variable could specify a function that would |
1597 | be called later, or an expression that would be executed later, simply | |
1598 | visiting a file could take over your Emacs. Emacs takes several | |
1599 | measures to prevent this. | |
1600 | ||
1601 | @cindex safe local variable | |
1602 | You can specify safe values for a variable with a | |
1603 | @code{safe-local-variable} property. The property has to be | |
1604 | a function of one argument; any value is safe if the function | |
1605 | returns non-@code{nil} given that value. Many commonly encountered | |
1606 | file variables standardly have @code{safe-local-variable} properties, | |
1607 | including @code{fill-column}, @code{fill-prefix}, and | |
1608 | @code{indent-tabs-mode}. For boolean-valued variables that are safe, | |
1609 | use @code{booleanp} as the property value. Lambda expressions should | |
1610 | be quoted so that @code{describe-variable} can display the predicate. | |
1611 | ||
1612 | @defopt safe-local-variable-values | |
1613 | This variable provides another way to mark some variable values as | |
1614 | safe. It is a list of cons cells @code{(@var{var} . @var{val})}, | |
1615 | where @var{var} is a variable name and @var{val} is a value which is | |
1616 | safe for that variable. | |
1617 | ||
1618 | When Emacs asks the user whether or not to obey a set of file local | |
1619 | variable specifications, the user can choose to mark them as safe. | |
1620 | Doing so adds those variable/value pairs to | |
1621 | @code{safe-local-variable-values}, and saves it to the user's custom | |
1622 | file. | |
1623 | @end defopt | |
1624 | ||
1625 | @defun safe-local-variable-p sym val | |
1626 | This function returns non-@code{nil} if it is safe to give @var{sym} | |
1627 | the value @var{val}, based on the above criteria. | |
1628 | @end defun | |
1629 | ||
1630 | @c @cindex risky local variable Duplicates risky-local-variable | |
1631 | Some variables are considered @dfn{risky}. A variable whose name | |
1632 | ends in any of @samp{-command}, @samp{-frame-alist}, @samp{-function}, | |
1633 | @samp{-functions}, @samp{-hook}, @samp{-hooks}, @samp{-form}, | |
1634 | @samp{-forms}, @samp{-map}, @samp{-map-alist}, @samp{-mode-alist}, | |
1635 | @samp{-program}, or @samp{-predicate} is considered risky. The | |
1636 | variables @samp{font-lock-keywords}, @samp{font-lock-keywords} | |
1637 | followed by a digit, and @samp{font-lock-syntactic-keywords} are also | |
1638 | considered risky. Finally, any variable whose name has a | |
1639 | non-@code{nil} @code{risky-local-variable} property is considered | |
1640 | risky. | |
1641 | ||
1642 | @defun risky-local-variable-p sym | |
1643 | This function returns non-@code{nil} if @var{sym} is a risky variable, | |
1644 | based on the above criteria. | |
1645 | @end defun | |
1646 | ||
1647 | If a variable is risky, it will not be entered automatically into | |
1648 | @code{safe-local-variable-values} as described above. Therefore, | |
1649 | Emacs will always query before setting a risky variable, unless the | |
1650 | user explicitly allows the setting by customizing | |
1651 | @code{safe-local-variable-values} directly. | |
1652 | ||
1653 | @defvar ignored-local-variables | |
1654 | This variable holds a list of variables that should not be given local | |
1655 | values by files. Any value specified for one of these variables is | |
1656 | completely ignored. | |
1657 | @end defvar | |
1658 | ||
1659 | The @samp{Eval:} ``variable'' is also a potential loophole, so Emacs | |
1660 | normally asks for confirmation before handling it. | |
1661 | ||
1662 | @defopt enable-local-eval | |
1663 | This variable controls processing of @samp{Eval:} in @samp{-*-} lines | |
1664 | or local variables | |
1665 | lists in files being visited. A value of @code{t} means process them | |
1666 | unconditionally; @code{nil} means ignore them; anything else means ask | |
1667 | the user what to do for each file. The default value is @code{maybe}. | |
1668 | @end defopt | |
1669 | ||
1670 | @defopt safe-local-eval-forms | |
1671 | This variable holds a list of expressions that are safe to | |
1672 | evaluate when found in the @samp{Eval:} ``variable'' in a file | |
1673 | local variables list. | |
1674 | @end defopt | |
1675 | ||
1676 | If the expression is a function call and the function has a | |
1677 | @code{safe-local-eval-function} property, the property value | |
1678 | determines whether the expression is safe to evaluate. The property | |
1679 | value can be a predicate to call to test the expression, a list of | |
1680 | such predicates (it's safe if any predicate succeeds), or @code{t} | |
1681 | (always safe provided the arguments are constant). | |
1682 | ||
1683 | Text properties are also potential loopholes, since their values | |
1684 | could include functions to call. So Emacs discards all text | |
1685 | properties from string values specified for file local variables. | |
1686 | ||
eb22b78c EZ |
1687 | @node Directory Local Variables |
1688 | @section Directory Local Variables | |
1689 | @cindex directory local variables | |
1690 | ||
1691 | A directory can specify local variable values common to all files in | |
1692 | that directory; Emacs uses these to create buffer-local bindings for | |
1693 | those variables in buffers visiting any file in that directory. This | |
1694 | is useful when the files in the directory belong to some @dfn{project} | |
1695 | and therefore share the same local variables. | |
1696 | ||
1697 | There are two different methods for specifying directory local | |
1698 | variables: by putting them in a special file, or by defining a | |
1699 | @dfn{project class} for that directory. | |
1700 | ||
1701 | @defvr Constant dir-locals-file | |
1702 | This constant is the name of the file where Emacs expects to find the | |
1703 | directory-local variables. The name of the file is | |
1704 | @file{.dir-locals.el}@footnote{ | |
1705 | The MS-DOS version of Emacs uses @file{_dir-locals.el} instead, due to | |
1706 | limitations of the DOS filesystems. | |
1707 | }. A file by that name in a directory causes Emacs to apply its | |
1708 | settings to any file in that directory or any of its subdirectories. | |
1709 | If some of the subdirectories have their own @file{.dir-locals.el} | |
1710 | files, Emacs uses the settings from the deepest file it finds starting | |
1711 | from the file's directory and moving up the directory tree. The file | |
1712 | specifies local variables as a specially formatted list; see | |
1713 | @ref{Directory Variables, , Per-directory Local Variables, emacs, The | |
1714 | GNU Emacs Manual}, for more details. | |
1715 | @end defvr | |
1716 | ||
1717 | @defun hack-dir-local-variables | |
1718 | This function reads the @code{.dir-locals.el} file and stores the | |
1719 | directory-local variables in @code{file-local-variables-alist} that is | |
1720 | local to the buffer visiting any file in the directory, without | |
1721 | applying them. It also stores the directory-local settings in | |
1722 | @code{dir-locals-class-alist}, where it defines a special class for | |
1723 | the directory in which @file{.dir-locals.el} file was found. This | |
1724 | function works by calling @code{dir-locals-set-class-variables} and | |
1725 | @code{dir-locals-set-directory-class}, described below. | |
1726 | @end defun | |
1727 | ||
1728 | @defun dir-locals-set-class-variables class variables | |
1729 | This function defines a set of variable settings for the named | |
1730 | @var{class}, which is a symbol. You can later assign the class to one | |
1731 | or more directories, and Emacs will apply those variable settings to | |
1732 | all files in those directories. The list in @var{variables} can be of | |
1733 | one of the two forms: @code{(@var{major-mode} . @var{alist})} or | |
1734 | @code{(@var{directory} . @var{list})}. With the first form, if the | |
1735 | file's buffer turns on a mode that is derived from @var{major-mode}, | |
1736 | then the all the variables in the associated @var{alist} are applied; | |
1737 | @var{alist} should be of the form @code{(@var{name} . @var{value})}. | |
1738 | A special value @code{nil} for @var{major-mode} means the settings are | |
1739 | applicable to any mode. | |
1740 | ||
1741 | With the second form of @var{variables}, if @var{directory} is the | |
1742 | initial substring of the file's directory, then @var{list} is applied | |
1743 | recursively by following the above rules; @var{list} should be of one | |
1744 | of the two forms accepted by this function in @var{variables}. | |
1745 | @end defun | |
1746 | ||
1747 | @defun dir-locals-set-directory-class directory class | |
1748 | This function assigns @var{class} to all the files in @code{directory} | |
1749 | and its subdirectories. Thereafter, all the variable settings | |
1750 | specified for @var{class} will be applied to any visited file in | |
1751 | @var{directory} and its children. @var{class} must have been already | |
1752 | defined by @code{dir-locals-set-class-variables} | |
1753 | @end defun | |
1754 | ||
1755 | @defvar dir-locals-class-alist | |
1756 | This alist holds the class symbols and the associated variable | |
1757 | settings. It is updated by @code{dir-locals-set-class-variables}. | |
1758 | @end defvar | |
1759 | ||
1760 | @defvar dir-locals-directory-alist | |
1761 | This alist holds directory names and their assigned class names. It | |
1762 | is updated by @code{dir-locals-set-directory-class}. | |
1763 | @end defvar | |
1764 | ||
0f7766a4 EZ |
1765 | @node Frame-Local Variables |
1766 | @section Frame-Local Values for Variables | |
1767 | @cindex frame-local variables | |
1768 | ||
1769 | In addition to buffer-local variable bindings (@pxref{Buffer-Local | |
1770 | Variables}), Emacs supports @dfn{frame-local} bindings. A frame-local | |
1771 | binding for a variable is in effect in a frame for which it was | |
1772 | defined. Note that you cannot have frame-local bindings for a | |
1773 | variable that has a buffer-local binding. | |
1774 | ||
1775 | To define a frame-specific binding for a variable @var{var} in frame | |
1776 | @var{frame}, set the @var{var} frame parameter for that frame: | |
1777 | ||
1778 | @lisp | |
1779 | (modify-frame-parameters @var{frame} '((@var{var} . @var{value}))) | |
1780 | @end lisp | |
1781 | ||
1782 | @noindent | |
1783 | This causes the variable @var{var} to be bound to the specified | |
1784 | @var{value} in the named @var{frame}. @xref{Parameter Access, | |
1785 | modify-frame-parameters}. | |
1786 | ||
1787 | @findex make-variable-frame-local@r{, deprecated} | |
1788 | Use of @code{make-variable-frame-local} for defining frame-local | |
1789 | bindings is deprecated since Emacs 22.2. | |
1790 | ||
1791 | To check the frame-specific values of such variables, use | |
1792 | @code{frame-parameter} (@pxref{Parameter Access, frame-parameter}). | |
1793 | ||
eb22b78c EZ |
1794 | @node Future Local Variables |
1795 | @section Possible Future Local Variables | |
1796 | ||
1797 | We have considered the idea of bindings that are local to a category | |
1798 | of frames---for example, all color frames, or all frames with dark | |
1799 | backgrounds. We have not implemented them because it is not clear that | |
1800 | this feature is really useful. You can get more or less the same | |
1801 | results by adding a function to @code{after-make-frame-functions}, set up to | |
1802 | define a particular frame parameter according to the appropriate | |
1803 | conditions for each frame. | |
1804 | ||
1805 | It would also be possible to implement window-local bindings. We | |
1806 | don't know of many situations where they would be useful, and it seems | |
1807 | that indirect buffers (@pxref{Indirect Buffers}) with buffer-local | |
1808 | bindings offer a way to handle these situations more robustly. | |
1809 | ||
1810 | If sufficient application is found for either of these two kinds of | |
1811 | local bindings, we will provide it in a subsequent Emacs version. | |
1812 | ||
b8d4c8d0 GM |
1813 | @node Variable Aliases |
1814 | @section Variable Aliases | |
1815 | @cindex variable aliases | |
1816 | ||
1817 | It is sometimes useful to make two variables synonyms, so that both | |
1818 | variables always have the same value, and changing either one also | |
1819 | changes the other. Whenever you change the name of a | |
1820 | variable---either because you realize its old name was not well | |
1821 | chosen, or because its meaning has partly changed---it can be useful | |
1822 | to keep the old name as an @emph{alias} of the new one for | |
1823 | compatibility. You can do this with @code{defvaralias}. | |
1824 | ||
1825 | @defun defvaralias new-alias base-variable &optional docstring | |
1826 | This function defines the symbol @var{new-alias} as a variable alias | |
1827 | for symbol @var{base-variable}. This means that retrieving the value | |
1828 | of @var{new-alias} returns the value of @var{base-variable}, and | |
1829 | changing the value of @var{new-alias} changes the value of | |
1830 | @var{base-variable}. The two aliased variable names always share the | |
1831 | same value and the same bindings. | |
1832 | ||
1833 | If the @var{docstring} argument is non-@code{nil}, it specifies the | |
1834 | documentation for @var{new-alias}; otherwise, the alias gets the same | |
1835 | documentation as @var{base-variable} has, if any, unless | |
1836 | @var{base-variable} is itself an alias, in which case @var{new-alias} gets | |
1837 | the documentation of the variable at the end of the chain of aliases. | |
1838 | ||
1839 | This function returns @var{base-variable}. | |
1840 | @end defun | |
1841 | ||
1842 | Variable aliases are convenient for replacing an old name for a | |
1843 | variable with a new name. @code{make-obsolete-variable} declares that | |
1844 | the old name is obsolete and therefore that it may be removed at some | |
1845 | stage in the future. | |
1846 | ||
1847 | @defun make-obsolete-variable obsolete-name current-name &optional when | |
fc997332 | 1848 | This function makes the byte compiler warn that the variable |
b8d4c8d0 GM |
1849 | @var{obsolete-name} is obsolete. If @var{current-name} is a symbol, it is |
1850 | the variable's new name; then the warning message says to use | |
1851 | @var{current-name} instead of @var{obsolete-name}. If @var{current-name} | |
1852 | is a string, this is the message and there is no replacement variable. | |
1853 | ||
1854 | If provided, @var{when} should be a string indicating when the | |
1855 | variable was first made obsolete---for example, a date or a release | |
1856 | number. | |
1857 | @end defun | |
1858 | ||
1859 | You can make two variables synonyms and declare one obsolete at the | |
1860 | same time using the macro @code{define-obsolete-variable-alias}. | |
1861 | ||
1862 | @defmac define-obsolete-variable-alias obsolete-name current-name &optional when docstring | |
1863 | This macro marks the variable @var{obsolete-name} as obsolete and also | |
1864 | makes it an alias for the variable @var{current-name}. It is | |
1865 | equivalent to the following: | |
1866 | ||
1867 | @example | |
1868 | (defvaralias @var{obsolete-name} @var{current-name} @var{docstring}) | |
1869 | (make-obsolete-variable @var{obsolete-name} @var{current-name} @var{when}) | |
1870 | @end example | |
1871 | @end defmac | |
1872 | ||
1873 | @defun indirect-variable variable | |
1874 | This function returns the variable at the end of the chain of aliases | |
1875 | of @var{variable}. If @var{variable} is not a symbol, or if @var{variable} is | |
1876 | not defined as an alias, the function returns @var{variable}. | |
1877 | ||
1878 | This function signals a @code{cyclic-variable-indirection} error if | |
1879 | there is a loop in the chain of symbols. | |
1880 | @end defun | |
1881 | ||
1882 | @example | |
1883 | (defvaralias 'foo 'bar) | |
1884 | (indirect-variable 'foo) | |
1885 | @result{} bar | |
1886 | (indirect-variable 'bar) | |
1887 | @result{} bar | |
1888 | (setq bar 2) | |
1889 | bar | |
1890 | @result{} 2 | |
1891 | @group | |
1892 | foo | |
1893 | @result{} 2 | |
1894 | @end group | |
1895 | (setq foo 0) | |
1896 | bar | |
1897 | @result{} 0 | |
1898 | foo | |
1899 | @result{} 0 | |
1900 | @end example | |
1901 | ||
1902 | @node Variables with Restricted Values | |
1903 | @section Variables with Restricted Values | |
1904 | ||
1905 | Ordinary Lisp variables can be assigned any value that is a valid | |
1906 | Lisp object. However, certain Lisp variables are not defined in Lisp, | |
1907 | but in C. Most of these variables are defined in the C code using | |
1908 | @code{DEFVAR_LISP}. Like variables defined in Lisp, these can take on | |
1909 | any value. However, some variables are defined using | |
1910 | @code{DEFVAR_INT} or @code{DEFVAR_BOOL}. @xref{Defining Lisp | |
1911 | variables in C,, Writing Emacs Primitives}, in particular the | |
1912 | description of functions of the type @code{syms_of_@var{filename}}, | |
1913 | for a brief discussion of the C implementation. | |
1914 | ||
1915 | Variables of type @code{DEFVAR_BOOL} can only take on the values | |
1916 | @code{nil} or @code{t}. Attempting to assign them any other value | |
1917 | will set them to @code{t}: | |
1918 | ||
1919 | @example | |
1920 | (let ((display-hourglass 5)) | |
1921 | display-hourglass) | |
1922 | @result{} t | |
1923 | @end example | |
1924 | ||
1925 | @defvar byte-boolean-vars | |
1926 | This variable holds a list of all variables of type @code{DEFVAR_BOOL}. | |
1927 | @end defvar | |
1928 | ||
1929 | Variables of type @code{DEFVAR_INT} can only take on integer values. | |
1930 | Attempting to assign them any other value will result in an error: | |
1931 | ||
1932 | @example | |
1933 | (setq window-min-height 5.0) | |
1934 | @error{} Wrong type argument: integerp, 5.0 | |
1935 | @end example | |
1936 | ||
1937 | @ignore | |
1938 | arch-tag: 5ff62c44-2b51-47bb-99d4-fea5aeec5d3e | |
1939 | @end ignore |