Commit | Line | Data |
---|---|---|
e6512bcf RS |
1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
b50c9214 | 3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2000 |
fd897522 | 4 | @c Free Software Foundation, Inc. |
e6512bcf RS |
5 | @c See the file elisp.texi for copying conditions. |
6 | @setfilename ../info/variables | |
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 | |
f57ddf67 | 24 | form of the program---it is simply the read syntax for those Lisp |
e6512bcf RS |
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. | |
b50c9214 RS |
35 | * Tips for Defining:: Things you should think about when you |
36 | define a variable. | |
e6512bcf RS |
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. | |
969fe9b5 RS |
42 | * Frame-Local Variables:: Variable values in effect only in one frame. |
43 | * Future Local Variables:: New kinds of local values we might add some day. | |
b50c9214 | 44 | * File Local Variables:: Handling local variable lists in files. |
e6512bcf RS |
45 | @end menu |
46 | ||
47 | @node Global Variables | |
48 | @section Global Variables | |
49 | @cindex global variable | |
50 | ||
51 | The simplest way to use a variable is @dfn{globally}. This means that | |
52 | the variable has just one value at a time, and this value is in effect | |
53 | (at least for the moment) throughout the Lisp system. The value remains | |
54 | in effect until you specify a new one. When a new value replaces the | |
55 | old one, no trace of the old value remains in the variable. | |
56 | ||
57 | You specify a value for a symbol with @code{setq}. For example, | |
58 | ||
59 | @example | |
60 | (setq x '(a b)) | |
61 | @end example | |
62 | ||
63 | @noindent | |
64 | gives the variable @code{x} the value @code{(a b)}. Note that | |
65 | @code{setq} does not evaluate its first argument, the name of the | |
66 | variable, but it does evaluate the second argument, the new value. | |
67 | ||
68 | Once the variable has a value, you can refer to it by using the symbol | |
69 | by itself as an expression. Thus, | |
70 | ||
71 | @example | |
72 | @group | |
73 | x @result{} (a b) | |
74 | @end group | |
75 | @end example | |
76 | ||
77 | @noindent | |
78 | assuming the @code{setq} form shown above has already been executed. | |
79 | ||
969fe9b5 RS |
80 | If you do set the same variable again, the new value replaces the old |
81 | one: | |
e6512bcf RS |
82 | |
83 | @example | |
84 | @group | |
85 | x | |
86 | @result{} (a b) | |
87 | @end group | |
88 | @group | |
89 | (setq x 4) | |
90 | @result{} 4 | |
91 | @end group | |
92 | @group | |
93 | x | |
94 | @result{} 4 | |
95 | @end group | |
96 | @end example | |
97 | ||
98 | @node Constant Variables | |
8241495d | 99 | @section Variables that Never Change |
e6512bcf RS |
100 | @vindex nil |
101 | @vindex t | |
102 | @kindex setting-constant | |
95ed62c7 | 103 | @cindex keyword symbol |
e6512bcf | 104 | |
969fe9b5 RS |
105 | In Emacs Lisp, certain symbols normally evaluate to themselves. These |
106 | include @code{nil} and @code{t}, as well as any symbol whose name starts | |
95ed62c7 GM |
107 | with @samp{:} (these are called @dfn{keywords}). These symbols cannot |
108 | be rebound, nor can their values be changed. Any attempt to set or bind | |
109 | @code{nil} or @code{t} signals a @code{setting-constant} error. The | |
110 | same is true for a keyword (a symbol whose name starts with @samp{:}), | |
111 | if it is interned in the standard obarray, except that setting such a | |
112 | symbol to itself is not an error. | |
e6512bcf RS |
113 | |
114 | @example | |
115 | @group | |
116 | nil @equiv{} 'nil | |
117 | @result{} nil | |
118 | @end group | |
119 | @group | |
120 | (setq nil 500) | |
121 | @error{} Attempt to set constant symbol: nil | |
122 | @end group | |
123 | @end example | |
124 | ||
e88399c8 DL |
125 | @defun keywordp object |
126 | @tindex keywordp | |
127 | function returns @code{t} if @var{object} is a symbol whose name | |
128 | starts with @samp{:}, interned in the standard obarray, and returns | |
129 | @code{nil} otherwise. | |
130 | @end defun | |
131 | ||
e6512bcf RS |
132 | @node Local Variables |
133 | @section Local Variables | |
134 | @cindex binding local variables | |
135 | @cindex local variables | |
136 | @cindex local binding | |
137 | @cindex global binding | |
138 | ||
139 | Global variables have values that last until explicitly superseded | |
140 | with new values. Sometimes it is useful to create variable values that | |
969fe9b5 | 141 | exist temporarily---only until a certain part of the program finishes. |
e6512bcf RS |
142 | These values are called @dfn{local}, and the variables so used are |
143 | called @dfn{local variables}. | |
144 | ||
145 | For example, when a function is called, its argument variables receive | |
146 | new local values that last until the function exits. The @code{let} | |
147 | special form explicitly establishes new local values for specified | |
148 | variables; these last until exit from the @code{let} form. | |
149 | ||
150 | @cindex shadowing of variables | |
151 | Establishing a local value saves away the previous value (or lack of | |
152 | one) of the variable. When the life span of the local value is over, | |
153 | the previous value is restored. In the mean time, we say that the | |
154 | previous value is @dfn{shadowed} and @dfn{not visible}. Both global and | |
155 | local values may be shadowed (@pxref{Scope}). | |
156 | ||
157 | If you set a variable (such as with @code{setq}) while it is local, | |
158 | this replaces the local value; it does not alter the global value, or | |
969fe9b5 | 159 | previous local values, that are shadowed. To model this behavior, we |
e6512bcf RS |
160 | speak of a @dfn{local binding} of the variable as well as a local value. |
161 | ||
162 | The local binding is a conceptual place that holds a local value. | |
163 | Entry to a function, or a special form such as @code{let}, creates the | |
164 | local binding; exit from the function or from the @code{let} removes the | |
165 | local binding. As long as the local binding lasts, the variable's value | |
166 | is stored within it. Use of @code{setq} or @code{set} while there is a | |
167 | local binding stores a different value into the local binding; it does | |
168 | not create a new binding. | |
169 | ||
170 | We also speak of the @dfn{global binding}, which is where | |
171 | (conceptually) the global value is kept. | |
172 | ||
173 | @cindex current binding | |
174 | A variable can have more than one local binding at a time (for | |
175 | example, if there are nested @code{let} forms that bind it). In such a | |
176 | case, the most recently created local binding that still exists is the | |
969fe9b5 RS |
177 | @dfn{current binding} of the variable. (This rule is called |
178 | @dfn{dynamic scoping}; see @ref{Variable Scoping}.) If there are no | |
179 | local bindings, the variable's global binding is its current binding. | |
180 | We sometimes call the current binding the @dfn{most-local existing | |
181 | binding}, for emphasis. Ordinary evaluation of a symbol always returns | |
182 | the value of its current binding. | |
e6512bcf RS |
183 | |
184 | The special forms @code{let} and @code{let*} exist to create | |
185 | local bindings. | |
186 | ||
187 | @defspec let (bindings@dots{}) forms@dots{} | |
f57ddf67 | 188 | This special form binds variables according to @var{bindings} and then |
e6512bcf RS |
189 | evaluates all of the @var{forms} in textual order. The @code{let}-form |
190 | returns the value of the last form in @var{forms}. | |
191 | ||
192 | Each of the @var{bindings} is either @w{(i) a} symbol, in which case | |
193 | that symbol is bound to @code{nil}; or @w{(ii) a} list of the form | |
194 | @code{(@var{symbol} @var{value-form})}, in which case @var{symbol} is | |
195 | bound to the result of evaluating @var{value-form}. If @var{value-form} | |
196 | is omitted, @code{nil} is used. | |
197 | ||
198 | All of the @var{value-form}s in @var{bindings} are evaluated in the | |
f9f59935 RS |
199 | order they appear and @emph{before} binding any of the symbols to them. |
200 | Here is an example of this: @code{Z} is bound to the old value of | |
201 | @code{Y}, which is 2, not the new value of @code{Y}, which is 1. | |
e6512bcf RS |
202 | |
203 | @example | |
204 | @group | |
205 | (setq Y 2) | |
206 | @result{} 2 | |
207 | @end group | |
208 | @group | |
209 | (let ((Y 1) | |
210 | (Z Y)) | |
211 | (list Y Z)) | |
212 | @result{} (1 2) | |
213 | @end group | |
214 | @end example | |
215 | @end defspec | |
216 | ||
217 | @defspec let* (bindings@dots{}) forms@dots{} | |
218 | This special form is like @code{let}, but it binds each variable right | |
219 | after computing its local value, before computing the local value for | |
220 | the next variable. Therefore, an expression in @var{bindings} can | |
221 | reasonably refer to the preceding symbols bound in this @code{let*} | |
222 | form. Compare the following example with the example above for | |
223 | @code{let}. | |
224 | ||
225 | @example | |
226 | @group | |
227 | (setq Y 2) | |
228 | @result{} 2 | |
229 | @end group | |
230 | @group | |
231 | (let* ((Y 1) | |
232 | (Z Y)) ; @r{Use the just-established value of @code{Y}.} | |
233 | (list Y Z)) | |
234 | @result{} (1 1) | |
235 | @end group | |
236 | @end example | |
237 | @end defspec | |
238 | ||
f57ddf67 | 239 | Here is a complete list of the other facilities that create local |
e6512bcf RS |
240 | bindings: |
241 | ||
242 | @itemize @bullet | |
243 | @item | |
244 | Function calls (@pxref{Functions}). | |
245 | ||
246 | @item | |
247 | Macro calls (@pxref{Macros}). | |
248 | ||
249 | @item | |
250 | @code{condition-case} (@pxref{Errors}). | |
251 | @end itemize | |
252 | ||
bfe721d1 | 253 | Variables can also have buffer-local bindings (@pxref{Buffer-Local |
a9f0a989 RS |
254 | Variables}) and frame-local bindings (@pxref{Frame-Local Variables}); a |
255 | few variables have terminal-local bindings (@pxref{Multiple Displays}). | |
256 | These kinds of bindings work somewhat like ordinary local bindings, but | |
257 | they are localized depending on ``where'' you are in Emacs, rather than | |
258 | localized in time. | |
bfe721d1 | 259 | |
e6512bcf RS |
260 | @defvar max-specpdl-size |
261 | @cindex variable limit error | |
262 | @cindex evaluation error | |
263 | @cindex infinite recursion | |
a9f0a989 | 264 | This variable defines the limit on the total number of local variable |
e6512bcf RS |
265 | bindings and @code{unwind-protect} cleanups (@pxref{Nonlocal Exits}) |
266 | that are allowed before signaling an error (with data @code{"Variable | |
267 | binding depth exceeds max-specpdl-size"}). | |
268 | ||
a9f0a989 | 269 | This limit, with the associated error when it is exceeded, is one way |
e6512bcf | 270 | that Lisp avoids infinite recursion on an ill-defined function. |
a9f0a989 | 271 | @code{max-lisp-eval-depth} provides another limit on depth of nesting. |
e6512bcf | 272 | @xref{Eval}. |
a9f0a989 RS |
273 | |
274 | The default value is 600. Entry to the Lisp debugger increases the | |
275 | value, if there is little room left, to make sure the debugger itself | |
276 | has room to execute. | |
e6512bcf RS |
277 | @end defvar |
278 | ||
279 | @node Void Variables | |
280 | @section When a Variable is ``Void'' | |
281 | @kindex void-variable | |
282 | @cindex void variable | |
283 | ||
284 | If you have never given a symbol any value as a global variable, we | |
285 | say that that symbol's global value is @dfn{void}. In other words, the | |
286 | symbol's value cell does not have any Lisp object in it. If you try to | |
287 | evaluate the symbol, you get a @code{void-variable} error rather than | |
288 | a value. | |
289 | ||
290 | Note that a value of @code{nil} is not the same as void. The symbol | |
291 | @code{nil} is a Lisp object and can be the value of a variable just as any | |
292 | other object can be; but it is @emph{a value}. A void variable does not | |
293 | have any value. | |
294 | ||
295 | After you have given a variable a value, you can make it void once more | |
296 | using @code{makunbound}. | |
297 | ||
298 | @defun makunbound symbol | |
f9f59935 | 299 | This function makes the current variable binding of @var{symbol} void. |
e6512bcf | 300 | Subsequent attempts to use this symbol's value as a variable will signal |
f9f59935 | 301 | the error @code{void-variable}, unless and until you set it again. |
e6512bcf RS |
302 | |
303 | @code{makunbound} returns @var{symbol}. | |
304 | ||
305 | @example | |
306 | @group | |
969fe9b5 | 307 | (makunbound 'x) ; @r{Make the global value of @code{x} void.} |
e6512bcf RS |
308 | @result{} x |
309 | @end group | |
310 | @group | |
311 | x | |
312 | @error{} Symbol's value as variable is void: x | |
313 | @end group | |
314 | @end example | |
315 | ||
316 | If @var{symbol} is locally bound, @code{makunbound} affects the most | |
317 | local existing binding. This is the only way a symbol can have a void | |
318 | local binding, since all the constructs that create local bindings | |
319 | create them with values. In this case, the voidness lasts at most as | |
320 | long as the binding does; when the binding is removed due to exit from | |
969fe9b5 RS |
321 | the construct that made it, the previous local or global binding is |
322 | reexposed as usual, and the variable is no longer void unless the newly | |
323 | reexposed binding was void all along. | |
e6512bcf RS |
324 | |
325 | @smallexample | |
326 | @group | |
327 | (setq x 1) ; @r{Put a value in the global binding.} | |
328 | @result{} 1 | |
329 | (let ((x 2)) ; @r{Locally bind it.} | |
330 | (makunbound 'x) ; @r{Void the local binding.} | |
331 | x) | |
332 | @error{} Symbol's value as variable is void: x | |
333 | @end group | |
334 | @group | |
335 | x ; @r{The global binding is unchanged.} | |
336 | @result{} 1 | |
337 | ||
338 | (let ((x 2)) ; @r{Locally bind it.} | |
339 | (let ((x 3)) ; @r{And again.} | |
340 | (makunbound 'x) ; @r{Void the innermost-local binding.} | |
341 | x)) ; @r{And refer: it's void.} | |
342 | @error{} Symbol's value as variable is void: x | |
343 | @end group | |
344 | ||
345 | @group | |
346 | (let ((x 2)) | |
347 | (let ((x 3)) | |
348 | (makunbound 'x)) ; @r{Void inner binding, then remove it.} | |
349 | x) ; @r{Now outer @code{let} binding is visible.} | |
350 | @result{} 2 | |
351 | @end group | |
352 | @end smallexample | |
353 | @end defun | |
354 | ||
355 | A variable that has been made void with @code{makunbound} is | |
356 | indistinguishable from one that has never received a value and has | |
357 | always been void. | |
358 | ||
359 | You can use the function @code{boundp} to test whether a variable is | |
360 | currently void. | |
361 | ||
362 | @defun boundp variable | |
363 | @code{boundp} returns @code{t} if @var{variable} (a symbol) is not void; | |
364 | more precisely, if its current binding is not void. It returns | |
365 | @code{nil} otherwise. | |
366 | ||
367 | @smallexample | |
368 | @group | |
369 | (boundp 'abracadabra) ; @r{Starts out void.} | |
370 | @result{} nil | |
371 | @end group | |
372 | @group | |
373 | (let ((abracadabra 5)) ; @r{Locally bind it.} | |
374 | (boundp 'abracadabra)) | |
375 | @result{} t | |
376 | @end group | |
377 | @group | |
378 | (boundp 'abracadabra) ; @r{Still globally void.} | |
379 | @result{} nil | |
380 | @end group | |
381 | @group | |
382 | (setq abracadabra 5) ; @r{Make it globally nonvoid.} | |
383 | @result{} 5 | |
384 | @end group | |
385 | @group | |
386 | (boundp 'abracadabra) | |
387 | @result{} t | |
388 | @end group | |
389 | @end smallexample | |
390 | @end defun | |
391 | ||
392 | @node Defining Variables | |
393 | @section Defining Global Variables | |
f57ddf67 | 394 | @cindex variable definition |
e6512bcf RS |
395 | |
396 | You may announce your intention to use a symbol as a global variable | |
f57ddf67 RS |
397 | with a @dfn{variable definition}: a special form, either @code{defconst} |
398 | or @code{defvar}. | |
e6512bcf RS |
399 | |
400 | In Emacs Lisp, definitions serve three purposes. First, they inform | |
401 | people who read the code that certain symbols are @emph{intended} to be | |
402 | used a certain way (as variables). Second, they inform the Lisp system | |
403 | of these things, supplying a value and documentation. Third, they | |
404 | provide information to utilities such as @code{etags} and | |
405 | @code{make-docfile}, which create data bases of the functions and | |
406 | variables in a program. | |
407 | ||
408 | The difference between @code{defconst} and @code{defvar} is primarily | |
f9f59935 RS |
409 | a matter of intent, serving to inform human readers of whether the value |
410 | should ever change. Emacs Lisp does not restrict the ways in which a | |
411 | variable can be used based on @code{defconst} or @code{defvar} | |
f57ddf67 | 412 | declarations. However, it does make a difference for initialization: |
e6512bcf RS |
413 | @code{defconst} unconditionally initializes the variable, while |
414 | @code{defvar} initializes it only if it is void. | |
415 | ||
f9f59935 | 416 | @ignore |
e6512bcf RS |
417 | One would expect user option variables to be defined with |
418 | @code{defconst}, since programs do not change them. Unfortunately, this | |
419 | has bad results if the definition is in a library that is not preloaded: | |
420 | @code{defconst} would override any prior value when the library is | |
421 | loaded. Users would like to be able to set user options in their init | |
422 | files, and override the default values given in the definitions. For | |
423 | this reason, user options must be defined with @code{defvar}. | |
f9f59935 | 424 | @end ignore |
e6512bcf RS |
425 | |
426 | @defspec defvar symbol [value [doc-string]] | |
f9f59935 RS |
427 | This special form defines @var{symbol} as a variable and can also |
428 | initialize and document it. The definition informs a person reading | |
429 | your code that @var{symbol} is used as a variable that might be set or | |
430 | changed. Note that @var{symbol} is not evaluated; the symbol to be | |
e6512bcf RS |
431 | defined must appear explicitly in the @code{defvar}. |
432 | ||
f9f59935 RS |
433 | If @var{symbol} is void and @var{value} is specified, @code{defvar} |
434 | evaluates it and sets @var{symbol} to the result. But if @var{symbol} | |
435 | already has a value (i.e., it is not void), @var{value} is not even | |
436 | evaluated, and @var{symbol}'s value remains unchanged. If @var{value} | |
437 | is omitted, the value of @var{symbol} is not changed in any case. | |
e6512bcf | 438 | |
a9f0a989 RS |
439 | If @var{symbol} has a buffer-local binding in the current buffer, |
440 | @code{defvar} operates on the default value, which is buffer-independent, | |
441 | not the current (buffer-local) binding. It sets the default value if | |
442 | the default value is void. @xref{Buffer-Local Variables}. | |
443 | ||
bfe721d1 KH |
444 | When you evaluate a top-level @code{defvar} form with @kbd{C-M-x} in |
445 | Emacs Lisp mode (@code{eval-defun}), a special feature of | |
a9f0a989 RS |
446 | @code{eval-defun} arranges to set the variable unconditionally, without |
447 | testing whether its value is void. | |
e6512bcf RS |
448 | |
449 | If the @var{doc-string} argument appears, it specifies the documentation | |
450 | for the variable. (This opportunity to specify documentation is one of | |
451 | the main benefits of defining the variable.) The documentation is | |
452 | stored in the symbol's @code{variable-documentation} property. The | |
453 | Emacs help functions (@pxref{Documentation}) look for this property. | |
454 | ||
455 | If the first character of @var{doc-string} is @samp{*}, it means that | |
456 | this variable is considered a user option. This lets users set the | |
969fe9b5 | 457 | variable conveniently using the commands @code{set-variable} and |
f9f59935 | 458 | @code{edit-options}. However, it is better to use @code{defcustom} |
969fe9b5 | 459 | instead of @code{defvar} for user option variables, so you can specify |
f9f59935 | 460 | customization information. @xref{Customization}. |
e6512bcf | 461 | |
f9f59935 RS |
462 | Here are some examples. This form defines @code{foo} but does not |
463 | initialize it: | |
e6512bcf RS |
464 | |
465 | @example | |
466 | @group | |
467 | (defvar foo) | |
468 | @result{} foo | |
469 | @end group | |
470 | @end example | |
471 | ||
f9f59935 RS |
472 | This example initializes the value of @code{bar} to @code{23}, and gives |
473 | it a documentation string: | |
e6512bcf RS |
474 | |
475 | @example | |
476 | @group | |
477 | (defvar bar 23 | |
478 | "The normal weight of a bar.") | |
479 | @result{} bar | |
480 | @end group | |
481 | @end example | |
482 | ||
483 | The following form changes the documentation string for @code{bar}, | |
484 | making it a user option, but does not change the value, since @code{bar} | |
f9f59935 RS |
485 | already has a value. (The addition @code{(1+ nil)} would get an error |
486 | if it were evaluated, but since it is not evaluated, there is no error.) | |
e6512bcf RS |
487 | |
488 | @example | |
489 | @group | |
f9f59935 | 490 | (defvar bar (1+ nil) |
e6512bcf RS |
491 | "*The normal weight of a bar.") |
492 | @result{} bar | |
493 | @end group | |
494 | @group | |
495 | bar | |
496 | @result{} 23 | |
497 | @end group | |
498 | @end example | |
499 | ||
500 | Here is an equivalent expression for the @code{defvar} special form: | |
501 | ||
502 | @example | |
503 | @group | |
504 | (defvar @var{symbol} @var{value} @var{doc-string}) | |
505 | @equiv{} | |
506 | (progn | |
507 | (if (not (boundp '@var{symbol})) | |
508 | (setq @var{symbol} @var{value})) | |
25ef19c5 RS |
509 | (if '@var{doc-string} |
510 | (put '@var{symbol} 'variable-documentation '@var{doc-string})) | |
e6512bcf RS |
511 | '@var{symbol}) |
512 | @end group | |
513 | @end example | |
514 | ||
515 | The @code{defvar} form returns @var{symbol}, but it is normally used | |
516 | at top level in a file where its value does not matter. | |
517 | @end defspec | |
518 | ||
519 | @defspec defconst symbol [value [doc-string]] | |
520 | This special form defines @var{symbol} as a value and initializes it. | |
f9f59935 RS |
521 | It informs a person reading your code that @var{symbol} has a standard |
522 | global value, established here, that should not be changed by the user | |
523 | or by other programs. Note that @var{symbol} is not evaluated; the | |
e6512bcf RS |
524 | symbol to be defined must appear explicitly in the @code{defconst}. |
525 | ||
f9f59935 RS |
526 | @code{defconst} always evaluates @var{value}, and sets the value of |
527 | @var{symbol} to the result if @var{value} is given. If @var{symbol} | |
a9f0a989 RS |
528 | does have a buffer-local binding in the current buffer, @code{defconst} |
529 | sets the default value, not the buffer-local value. (But you should not | |
530 | be making buffer-local bindings for a symbol that is defined with | |
531 | @code{defconst}.) | |
e6512bcf RS |
532 | |
533 | Here, @code{pi} is a constant that presumably ought not to be changed | |
534 | by anyone (attempts by the Indiana State Legislature notwithstanding). | |
535 | As the second form illustrates, however, this is only advisory. | |
536 | ||
537 | @example | |
538 | @group | |
539 | (defconst pi 3.1415 "Pi to five places.") | |
540 | @result{} pi | |
541 | @end group | |
542 | @group | |
543 | (setq pi 3) | |
544 | @result{} pi | |
545 | @end group | |
546 | @group | |
547 | pi | |
548 | @result{} 3 | |
549 | @end group | |
550 | @end example | |
551 | @end defspec | |
552 | ||
553 | @defun user-variable-p variable | |
554 | @cindex user option | |
f57ddf67 | 555 | This function returns @code{t} if @var{variable} is a user option---a |
e6512bcf RS |
556 | variable intended to be set by the user for customization---and |
557 | @code{nil} otherwise. (Variables other than user options exist for the | |
558 | internal purposes of Lisp programs, and users need not know about them.) | |
559 | ||
e88399c8 DL |
560 | User option variables are distinguished from other variables either |
561 | though being declared using @code{defcustom}@footnote{They may also be | |
562 | declared equivalently in @file{cus-start.el}.} or by the first character | |
563 | of their @code{variable-documentation} property. If the property exists | |
564 | and is a string, and its first character is @samp{*}, then the variable | |
565 | is a user option. | |
e6512bcf RS |
566 | @end defun |
567 | ||
113613ea | 568 | @kindex variable-interactive |
e6512bcf | 569 | If a user option variable has a @code{variable-interactive} property, |
bfe721d1 KH |
570 | the @code{set-variable} command uses that value to control reading the |
571 | new value for the variable. The property's value is used as if it were | |
8241495d RS |
572 | specified in @code{interactive} (@pxref{Using Interactive}). However, |
573 | this feature is largely obsoleted by @code{defcustom} | |
574 | (@pxref{Customization}). | |
e6512bcf | 575 | |
b22f3a19 | 576 | @strong{Warning:} If the @code{defconst} and @code{defvar} special |
e6512bcf RS |
577 | forms are used while the variable has a local binding, they set the |
578 | local binding's value; the global binding is not changed. This is not | |
b50c9214 | 579 | what you usually want. To prevent it, use these special forms at top |
e6512bcf RS |
580 | level in a file, where normally no local binding is in effect, and make |
581 | sure to load the file before making a local binding for the variable. | |
582 | ||
aa9b77f6 RS |
583 | @node Tips for Defining |
584 | @section Tips for Defining Variables Robustly | |
585 | ||
b50c9214 RS |
586 | When you define a variable whose value is a function, or a list of |
587 | functions, use a name that ends in @samp{-function} or | |
588 | @samp{-functions}, respectively. | |
589 | ||
590 | There are several other variable name conventions; | |
591 | here is a complete list: | |
592 | ||
593 | @table @samp | |
594 | @item @dots{}-hook | |
595 | The variable is a normal hook (@pxref{Hooks}). | |
596 | ||
597 | @item @dots{}-function | |
598 | The value is a function. | |
599 | ||
600 | @item @dots{}-functions | |
601 | The value is a list of functions. | |
602 | ||
603 | @item @dots{}-form | |
604 | The value is a form (an expression). | |
605 | ||
606 | @item @dots{}-functions | |
607 | The value is a list of forms (expressions). | |
608 | ||
609 | @item @dots{}-predicate | |
610 | The value is a predicate---a function of one argument that returns | |
611 | non-@code{nil} for ``good'' arguments and @code{nil} for ``bad'' | |
612 | arguments. | |
613 | ||
614 | @item @dots{}-flag | |
615 | The value is significant only as to whether it is @code{nil} or not. | |
616 | ||
617 | @item @dots{}-program | |
618 | The value is a program name. | |
619 | ||
620 | @item @dots{}-command | |
621 | The value is a whole shell command. | |
622 | ||
623 | @item @samp{}-switches | |
624 | The value specifies options for a command. | |
625 | @end table | |
626 | ||
627 | When you define a variable, always cvonsider whether you should mark | |
628 | it as ``risky''; see @ref{File Local Variables}. | |
629 | ||
aa9b77f6 RS |
630 | When defining and initializing a variable that holds a complicated |
631 | value (such as a keymap with bindings in it), it's best to put the | |
632 | entire computation of the value into the @code{defvar}, like this: | |
633 | ||
634 | @example | |
635 | (defvar my-mode-map | |
636 | (let ((map (make-sparse-keymap))) | |
f9f59935 | 637 | (define-key map "\C-c\C-a" 'my-command) |
aa9b77f6 RS |
638 | @dots{} |
639 | map) | |
640 | @var{docstring}) | |
641 | @end example | |
642 | ||
643 | @noindent | |
644 | This method has several benefits. First, if the user quits while | |
645 | loading the file, the variable is either still uninitialized or | |
969fe9b5 | 646 | initialized properly, never in-between. If it is still uninitialized, |
aa9b77f6 RS |
647 | reloading the file will initialize it properly. Second, reloading the |
648 | file once the variable is initialized will not alter it; that is | |
649 | important if the user has run hooks to alter part of the contents (such | |
650 | as, to rebind keys). Third, evaluating the @code{defvar} form with | |
651 | @kbd{C-M-x} @emph{will} reinitialize the map completely. | |
652 | ||
653 | Putting so much code in the @code{defvar} form has one disadvantage: | |
654 | it puts the documentation string far away from the line which names the | |
655 | variable. Here's a safe way to avoid that: | |
656 | ||
657 | @example | |
658 | (defvar my-mode-map nil | |
659 | @var{docstring}) | |
8241495d | 660 | (unless my-mode-map |
aa9b77f6 | 661 | (let ((map (make-sparse-keymap))) |
b548072f | 662 | (define-key map "\C-c\C-a" 'my-command) |
aa9b77f6 RS |
663 | @dots{} |
664 | (setq my-mode-map map))) | |
665 | @end example | |
666 | ||
667 | @noindent | |
668 | This has all the same advantages as putting the initialization inside | |
669 | the @code{defvar}, except that you must type @kbd{C-M-x} twice, once on | |
670 | each form, if you do want to reinitialize the variable. | |
671 | ||
672 | But be careful not to write the code like this: | |
673 | ||
674 | @example | |
675 | (defvar my-mode-map nil | |
676 | @var{docstring}) | |
8241495d | 677 | (unless my-mode-map |
aa9b77f6 RS |
678 | (setq my-mode-map (make-sparse-keymap)) |
679 | (define-key my-mode-map "\C-c\C-a" 'my-command) | |
680 | @dots{}) | |
681 | @end example | |
682 | ||
683 | @noindent | |
969fe9b5 RS |
684 | This code sets the variable, then alters it, but it does so in more than |
685 | one step. If the user quits just after the @code{setq}, that leaves the | |
686 | variable neither correctly initialized nor void nor @code{nil}. Once | |
687 | that happens, reloading the file will not initialize the variable; it | |
688 | will remain incomplete. | |
aa9b77f6 | 689 | |
e6512bcf RS |
690 | @node Accessing Variables |
691 | @section Accessing Variable Values | |
692 | ||
693 | The usual way to reference a variable is to write the symbol which | |
694 | names it (@pxref{Symbol Forms}). This requires you to specify the | |
695 | variable name when you write the program. Usually that is exactly what | |
696 | you want to do. Occasionally you need to choose at run time which | |
697 | variable to reference; then you can use @code{symbol-value}. | |
698 | ||
699 | @defun symbol-value symbol | |
700 | This function returns the value of @var{symbol}. This is the value in | |
701 | the innermost local binding of the symbol, or its global value if it | |
702 | has no local bindings. | |
703 | ||
704 | @example | |
705 | @group | |
706 | (setq abracadabra 5) | |
707 | @result{} 5 | |
708 | @end group | |
709 | @group | |
710 | (setq foo 9) | |
711 | @result{} 9 | |
712 | @end group | |
713 | ||
714 | @group | |
715 | ;; @r{Here the symbol @code{abracadabra}} | |
716 | ;; @r{is the symbol whose value is examined.} | |
717 | (let ((abracadabra 'foo)) | |
718 | (symbol-value 'abracadabra)) | |
719 | @result{} foo | |
720 | @end group | |
721 | ||
722 | @group | |
723 | ;; @r{Here the value of @code{abracadabra},} | |
724 | ;; @r{which is @code{foo},} | |
725 | ;; @r{is the symbol whose value is examined.} | |
726 | (let ((abracadabra 'foo)) | |
727 | (symbol-value abracadabra)) | |
728 | @result{} 9 | |
729 | @end group | |
730 | ||
731 | @group | |
732 | (symbol-value 'abracadabra) | |
733 | @result{} 5 | |
734 | @end group | |
735 | @end example | |
736 | ||
969fe9b5 RS |
737 | A @code{void-variable} error is signaled if the current binding of |
738 | @var{symbol} is void. | |
e6512bcf RS |
739 | @end defun |
740 | ||
741 | @node Setting Variables | |
742 | @section How to Alter a Variable Value | |
743 | ||
744 | The usual way to change the value of a variable is with the special | |
745 | form @code{setq}. When you need to compute the choice of variable at | |
746 | run time, use the function @code{set}. | |
747 | ||
748 | @defspec setq [symbol form]@dots{} | |
749 | This special form is the most common method of changing a variable's | |
750 | value. Each @var{symbol} is given a new value, which is the result of | |
751 | evaluating the corresponding @var{form}. The most-local existing | |
752 | binding of the symbol is changed. | |
753 | ||
754 | @code{setq} does not evaluate @var{symbol}; it sets the symbol that you | |
755 | write. We say that this argument is @dfn{automatically quoted}. The | |
756 | @samp{q} in @code{setq} stands for ``quoted.'' | |
757 | ||
758 | The value of the @code{setq} form is the value of the last @var{form}. | |
759 | ||
760 | @example | |
761 | @group | |
762 | (setq x (1+ 2)) | |
763 | @result{} 3 | |
764 | @end group | |
765 | x ; @r{@code{x} now has a global value.} | |
766 | @result{} 3 | |
767 | @group | |
768 | (let ((x 5)) | |
769 | (setq x 6) ; @r{The local binding of @code{x} is set.} | |
770 | x) | |
771 | @result{} 6 | |
772 | @end group | |
773 | x ; @r{The global value is unchanged.} | |
774 | @result{} 3 | |
775 | @end example | |
776 | ||
777 | Note that the first @var{form} is evaluated, then the first | |
778 | @var{symbol} is set, then the second @var{form} is evaluated, then the | |
779 | second @var{symbol} is set, and so on: | |
780 | ||
781 | @example | |
782 | @group | |
783 | (setq x 10 ; @r{Notice that @code{x} is set before} | |
784 | y (1+ x)) ; @r{the value of @code{y} is computed.} | |
785 | @result{} 11 | |
786 | @end group | |
787 | @end example | |
788 | @end defspec | |
789 | ||
790 | @defun set symbol value | |
791 | This function sets @var{symbol}'s value to @var{value}, then returns | |
792 | @var{value}. Since @code{set} is a function, the expression written for | |
793 | @var{symbol} is evaluated to obtain the symbol to set. | |
794 | ||
795 | The most-local existing binding of the variable is the binding that is | |
f57ddf67 | 796 | set; shadowed bindings are not affected. |
e6512bcf RS |
797 | |
798 | @example | |
799 | @group | |
800 | (set one 1) | |
801 | @error{} Symbol's value as variable is void: one | |
802 | @end group | |
803 | @group | |
804 | (set 'one 1) | |
805 | @result{} 1 | |
806 | @end group | |
807 | @group | |
808 | (set 'two 'one) | |
809 | @result{} one | |
810 | @end group | |
811 | @group | |
812 | (set two 2) ; @r{@code{two} evaluates to symbol @code{one}.} | |
813 | @result{} 2 | |
814 | @end group | |
815 | @group | |
816 | one ; @r{So it is @code{one} that was set.} | |
817 | @result{} 2 | |
818 | (let ((one 1)) ; @r{This binding of @code{one} is set,} | |
819 | (set 'one 3) ; @r{not the global value.} | |
820 | one) | |
821 | @result{} 3 | |
822 | @end group | |
823 | @group | |
824 | one | |
825 | @result{} 2 | |
826 | @end group | |
827 | @end example | |
828 | ||
f57ddf67 RS |
829 | If @var{symbol} is not actually a symbol, a @code{wrong-type-argument} |
830 | error is signaled. | |
831 | ||
832 | @example | |
833 | (set '(x y) 'z) | |
834 | @error{} Wrong type argument: symbolp, (x y) | |
835 | @end example | |
836 | ||
e6512bcf RS |
837 | Logically speaking, @code{set} is a more fundamental primitive than |
838 | @code{setq}. Any use of @code{setq} can be trivially rewritten to use | |
839 | @code{set}; @code{setq} could even be defined as a macro, given the | |
840 | availability of @code{set}. However, @code{set} itself is rarely used; | |
f57ddf67 RS |
841 | beginners hardly need to know about it. It is useful only for choosing |
842 | at run time which variable to set. For example, the command | |
e6512bcf RS |
843 | @code{set-variable}, which reads a variable name from the user and then |
844 | sets the variable, needs to use @code{set}. | |
845 | ||
846 | @cindex CL note---@code{set} local | |
847 | @quotation | |
f57ddf67 | 848 | @b{Common Lisp note:} In Common Lisp, @code{set} always changes the |
f9f59935 RS |
849 | symbol's ``special'' or dynamic value, ignoring any lexical bindings. |
850 | In Emacs Lisp, all variables and all bindings are dynamic, so @code{set} | |
e6512bcf RS |
851 | always affects the most local existing binding. |
852 | @end quotation | |
853 | @end defun | |
854 | ||
22697dac KH |
855 | One other function for setting a variable is designed to add |
856 | an element to a list if it is not already present in the list. | |
857 | ||
858 | @defun add-to-list symbol element | |
859 | This function sets the variable @var{symbol} by consing @var{element} | |
860 | onto the old value, if @var{element} is not already a member of that | |
bfe721d1 KH |
861 | value. It returns the resulting list, whether updated or not. The |
862 | value of @var{symbol} had better be a list already before the call. | |
863 | ||
864 | The argument @var{symbol} is not implicitly quoted; @code{add-to-list} | |
865 | is an ordinary function, like @code{set} and unlike @code{setq}. Quote | |
866 | the argument yourself if that is what you want. | |
969fe9b5 | 867 | @end defun |
22697dac KH |
868 | |
869 | Here's a scenario showing how to use @code{add-to-list}: | |
870 | ||
871 | @example | |
872 | (setq foo '(a b)) | |
873 | @result{} (a b) | |
874 | ||
875 | (add-to-list 'foo 'c) ;; @r{Add @code{c}.} | |
876 | @result{} (c a b) | |
877 | ||
878 | (add-to-list 'foo 'b) ;; @r{No effect.} | |
879 | @result{} (c a b) | |
880 | ||
881 | foo ;; @r{@code{foo} was changed.} | |
882 | @result{} (c a b) | |
883 | @end example | |
22697dac KH |
884 | |
885 | An equivalent expression for @code{(add-to-list '@var{var} | |
886 | @var{value})} is this: | |
887 | ||
888 | @example | |
889 | (or (member @var{value} @var{var}) | |
890 | (setq @var{var} (cons @var{value} @var{var}))) | |
891 | @end example | |
892 | ||
e6512bcf RS |
893 | @node Variable Scoping |
894 | @section Scoping Rules for Variable Bindings | |
895 | ||
f9f59935 | 896 | A given symbol @code{foo} can have several local variable bindings, |
e6512bcf RS |
897 | established at different places in the Lisp program, as well as a global |
898 | binding. The most recently established binding takes precedence over | |
899 | the others. | |
900 | ||
901 | @cindex scope | |
902 | @cindex extent | |
903 | @cindex dynamic scoping | |
904 | Local bindings in Emacs Lisp have @dfn{indefinite scope} and | |
905 | @dfn{dynamic extent}. @dfn{Scope} refers to @emph{where} textually in | |
8241495d | 906 | the source code the binding can be accessed. ``Indefinite scope'' means |
e6512bcf RS |
907 | that any part of the program can potentially access the variable |
908 | binding. @dfn{Extent} refers to @emph{when}, as the program is | |
8241495d | 909 | executing, the binding exists. ``Dynamic extent'' means that the binding |
e6512bcf RS |
910 | lasts as long as the activation of the construct that established it. |
911 | ||
912 | The combination of dynamic extent and indefinite scope is called | |
913 | @dfn{dynamic scoping}. By contrast, most programming languages use | |
914 | @dfn{lexical scoping}, in which references to a local variable must be | |
915 | located textually within the function or block that binds the variable. | |
916 | ||
917 | @cindex CL note---special variables | |
918 | @quotation | |
f9f59935 RS |
919 | @b{Common Lisp note:} Variables declared ``special'' in Common Lisp are |
920 | dynamically scoped, like all variables in Emacs Lisp. | |
e6512bcf RS |
921 | @end quotation |
922 | ||
923 | @menu | |
924 | * Scope:: Scope means where in the program a value is visible. | |
925 | Comparison with other languages. | |
926 | * Extent:: Extent means how long in time a value exists. | |
927 | * Impl of Scope:: Two ways to implement dynamic scoping. | |
928 | * Using Scoping:: How to use dynamic scoping carefully and avoid problems. | |
929 | @end menu | |
930 | ||
931 | @node Scope | |
932 | @subsection Scope | |
933 | ||
934 | Emacs Lisp uses @dfn{indefinite scope} for local variable bindings. | |
935 | This means that any function anywhere in the program text might access a | |
936 | given binding of a variable. Consider the following function | |
937 | definitions: | |
938 | ||
939 | @example | |
940 | @group | |
941 | (defun binder (x) ; @r{@code{x} is bound in @code{binder}.} | |
942 | (foo 5)) ; @r{@code{foo} is some other function.} | |
943 | @end group | |
944 | ||
945 | @group | |
f9f59935 | 946 | (defun user () ; @r{@code{x} is used ``free'' in @code{user}.} |
e6512bcf RS |
947 | (list x)) |
948 | @end group | |
949 | @end example | |
950 | ||
951 | In a lexically scoped language, the binding of @code{x} in | |
952 | @code{binder} would never be accessible in @code{user}, because | |
953 | @code{user} is not textually contained within the function | |
8241495d | 954 | @code{binder}. However, in dynamically-scoped Emacs Lisp, @code{user} |
e6512bcf | 955 | may or may not refer to the binding of @code{x} established in |
8241495d | 956 | @code{binder}, depending on the circumstances: |
e6512bcf RS |
957 | |
958 | @itemize @bullet | |
959 | @item | |
960 | If we call @code{user} directly without calling @code{binder} at all, | |
961 | then whatever binding of @code{x} is found, it cannot come from | |
962 | @code{binder}. | |
963 | ||
964 | @item | |
f9f59935 | 965 | If we define @code{foo} as follows and then call @code{binder}, then the |
e6512bcf RS |
966 | binding made in @code{binder} will be seen in @code{user}: |
967 | ||
968 | @example | |
969 | @group | |
970 | (defun foo (lose) | |
971 | (user)) | |
972 | @end group | |
973 | @end example | |
974 | ||
975 | @item | |
f9f59935 RS |
976 | However, if we define @code{foo} as follows and then call @code{binder}, |
977 | then the binding made in @code{binder} @emph{will not} be seen in | |
978 | @code{user}: | |
e6512bcf RS |
979 | |
980 | @example | |
981 | (defun foo (x) | |
982 | (user)) | |
983 | @end example | |
984 | ||
985 | @noindent | |
986 | Here, when @code{foo} is called by @code{binder}, it binds @code{x}. | |
987 | (The binding in @code{foo} is said to @dfn{shadow} the one made in | |
988 | @code{binder}.) Therefore, @code{user} will access the @code{x} bound | |
989 | by @code{foo} instead of the one bound by @code{binder}. | |
990 | @end itemize | |
991 | ||
f9f59935 RS |
992 | Emacs Lisp uses dynamic scoping because simple implementations of |
993 | lexical scoping are slow. In addition, every Lisp system needs to offer | |
994 | dynamic scoping at least as an option; if lexical scoping is the norm, | |
995 | there must be a way to specify dynamic scoping instead for a particular | |
996 | variable. It might not be a bad thing for Emacs to offer both, but | |
997 | implementing it with dynamic scoping only was much easier. | |
998 | ||
e6512bcf RS |
999 | @node Extent |
1000 | @subsection Extent | |
1001 | ||
1002 | @dfn{Extent} refers to the time during program execution that a | |
1003 | variable name is valid. In Emacs Lisp, a variable is valid only while | |
1004 | the form that bound it is executing. This is called @dfn{dynamic | |
1005 | extent}. ``Local'' or ``automatic'' variables in most languages, | |
1006 | including C and Pascal, have dynamic extent. | |
1007 | ||
1008 | One alternative to dynamic extent is @dfn{indefinite extent}. This | |
1009 | means that a variable binding can live on past the exit from the form | |
1010 | that made the binding. Common Lisp and Scheme, for example, support | |
1011 | this, but Emacs Lisp does not. | |
1012 | ||
1013 | To illustrate this, the function below, @code{make-add}, returns a | |
f9f59935 RS |
1014 | function that purports to add @var{n} to its own argument @var{m}. This |
1015 | would work in Common Lisp, but it does not do the job in Emacs Lisp, | |
1016 | because after the call to @code{make-add} exits, the variable @code{n} | |
1017 | is no longer bound to the actual argument 2. | |
e6512bcf RS |
1018 | |
1019 | @example | |
1020 | (defun make-add (n) | |
1021 | (function (lambda (m) (+ n m)))) ; @r{Return a function.} | |
1022 | @result{} make-add | |
1023 | (fset 'add2 (make-add 2)) ; @r{Define function @code{add2}} | |
1024 | ; @r{with @code{(make-add 2)}.} | |
1025 | @result{} (lambda (m) (+ n m)) | |
1026 | (add2 4) ; @r{Try to add 2 to 4.} | |
1027 | @error{} Symbol's value as variable is void: n | |
1028 | @end example | |
1029 | ||
1030 | @cindex closures not available | |
1031 | Some Lisp dialects have ``closures'', objects that are like functions | |
1032 | but record additional variable bindings. Emacs Lisp does not have | |
1033 | closures. | |
1034 | ||
1035 | @node Impl of Scope | |
1036 | @subsection Implementation of Dynamic Scoping | |
1037 | @cindex deep binding | |
1038 | ||
1039 | A simple sample implementation (which is not how Emacs Lisp actually | |
1040 | works) may help you understand dynamic binding. This technique is | |
1041 | called @dfn{deep binding} and was used in early Lisp systems. | |
1042 | ||
969fe9b5 RS |
1043 | Suppose there is a stack of bindings, which are variable-value pairs. |
1044 | At entry to a function or to a @code{let} form, we can push bindings | |
1045 | onto the stack for the arguments or local variables created there. We | |
1046 | can pop those bindings from the stack at exit from the binding | |
1047 | construct. | |
e6512bcf RS |
1048 | |
1049 | We can find the value of a variable by searching the stack from top to | |
1050 | bottom for a binding for that variable; the value from that binding is | |
1051 | the value of the variable. To set the variable, we search for the | |
1052 | current binding, then store the new value into that binding. | |
1053 | ||
1054 | As you can see, a function's bindings remain in effect as long as it | |
1055 | continues execution, even during its calls to other functions. That is | |
1056 | why we say the extent of the binding is dynamic. And any other function | |
1057 | can refer to the bindings, if it uses the same variables while the | |
1058 | bindings are in effect. That is why we say the scope is indefinite. | |
1059 | ||
1060 | @cindex shallow binding | |
1061 | The actual implementation of variable scoping in GNU Emacs Lisp uses a | |
1062 | technique called @dfn{shallow binding}. Each variable has a standard | |
1063 | place in which its current value is always found---the value cell of the | |
1064 | symbol. | |
1065 | ||
1066 | In shallow binding, setting the variable works by storing a value in | |
1067 | the value cell. Creating a new binding works by pushing the old value | |
f9f59935 RS |
1068 | (belonging to a previous binding) onto a stack, and storing the new |
1069 | local value in the value cell. Eliminating a binding works by popping | |
1070 | the old value off the stack, into the value cell. | |
e6512bcf RS |
1071 | |
1072 | We use shallow binding because it has the same results as deep | |
1073 | binding, but runs faster, since there is never a need to search for a | |
1074 | binding. | |
1075 | ||
1076 | @node Using Scoping | |
1077 | @subsection Proper Use of Dynamic Scoping | |
1078 | ||
1079 | Binding a variable in one function and using it in another is a | |
1080 | powerful technique, but if used without restraint, it can make programs | |
1081 | hard to understand. There are two clean ways to use this technique: | |
1082 | ||
1083 | @itemize @bullet | |
1084 | @item | |
1085 | Use or bind the variable only in a few related functions, written close | |
1086 | together in one file. Such a variable is used for communication within | |
1087 | one program. | |
1088 | ||
1089 | You should write comments to inform other programmers that they can see | |
1090 | all uses of the variable before them, and to advise them not to add uses | |
1091 | elsewhere. | |
1092 | ||
1093 | @item | |
1094 | Give the variable a well-defined, documented meaning, and make all | |
1095 | appropriate functions refer to it (but not bind it or set it) wherever | |
1096 | that meaning is relevant. For example, the variable | |
1097 | @code{case-fold-search} is defined as ``non-@code{nil} means ignore case | |
1098 | when searching''; various search and replace functions refer to it | |
1099 | directly or through their subroutines, but do not bind or set it. | |
1100 | ||
1101 | Then you can bind the variable in other programs, knowing reliably what | |
1102 | the effect will be. | |
1103 | @end itemize | |
1104 | ||
bfe721d1 KH |
1105 | In either case, you should define the variable with @code{defvar}. |
1106 | This helps other people understand your program by telling them to look | |
1107 | for inter-function usage. It also avoids a warning from the byte | |
1108 | compiler. Choose the variable's name to avoid name conflicts---don't | |
1109 | use short names like @code{x}. | |
1110 | ||
e6512bcf RS |
1111 | @node Buffer-Local Variables |
1112 | @section Buffer-Local Variables | |
1113 | @cindex variables, buffer-local | |
1114 | @cindex buffer-local variables | |
1115 | ||
1116 | Global and local variable bindings are found in most programming | |
8241495d | 1117 | languages in one form or another. Emacs, however, also supports additional, |
969fe9b5 | 1118 | unusual kinds of variable binding: @dfn{buffer-local} bindings, which |
8241495d | 1119 | apply only in one buffer, and @dfn{frame-local} bindings, which apply only in |
969fe9b5 RS |
1120 | one frame. Having different values for a variable in different buffers |
1121 | and/or frames is an important customization method. | |
1122 | ||
1123 | This section describes buffer-local bindings; for frame-local | |
1124 | bindings, see the following section, @ref{Frame-Local Variables}. (A few | |
1911e6e5 | 1125 | variables have bindings that are local to each terminal; see |
969fe9b5 | 1126 | @ref{Multiple Displays}.) |
e6512bcf RS |
1127 | |
1128 | @menu | |
1129 | * Intro to Buffer-Local:: Introduction and concepts. | |
1130 | * Creating Buffer-Local:: Creating and destroying buffer-local bindings. | |
1131 | * Default Value:: The default value is seen in buffers | |
969fe9b5 | 1132 | that don't have their own buffer-local values. |
e6512bcf RS |
1133 | @end menu |
1134 | ||
1135 | @node Intro to Buffer-Local | |
1136 | @subsection Introduction to Buffer-Local Variables | |
1137 | ||
1138 | A buffer-local variable has a buffer-local binding associated with a | |
1139 | particular buffer. The binding is in effect when that buffer is | |
1140 | current; otherwise, it is not in effect. If you set the variable while | |
1141 | a buffer-local binding is in effect, the new value goes in that binding, | |
f9f59935 RS |
1142 | so its other bindings are unchanged. This means that the change is |
1143 | visible only in the buffer where you made it. | |
1144 | ||
1145 | The variable's ordinary binding, which is not associated with any | |
1146 | specific buffer, is called the @dfn{default binding}. In most cases, | |
1147 | this is the global binding. | |
e6512bcf | 1148 | |
f9f59935 RS |
1149 | A variable can have buffer-local bindings in some buffers but not in |
1150 | other buffers. The default binding is shared by all the buffers that | |
969fe9b5 | 1151 | don't have their own bindings for the variable. (This includes all |
8241495d | 1152 | newly-created buffers.) If you set the variable in a buffer that does |
969fe9b5 RS |
1153 | not have a buffer-local binding for it, this sets the default binding |
1154 | (assuming there are no frame-local bindings to complicate the matter), | |
1155 | so the new value is visible in all the buffers that see the default | |
1156 | binding. | |
e6512bcf RS |
1157 | |
1158 | The most common use of buffer-local bindings is for major modes to change | |
1159 | variables that control the behavior of commands. For example, C mode and | |
1160 | Lisp mode both set the variable @code{paragraph-start} to specify that only | |
1161 | blank lines separate paragraphs. They do this by making the variable | |
1162 | buffer-local in the buffer that is being put into C mode or Lisp mode, and | |
969fe9b5 | 1163 | then setting it to the new value for that mode. @xref{Major Modes}. |
e6512bcf RS |
1164 | |
1165 | The usual way to make a buffer-local binding is with | |
969fe9b5 RS |
1166 | @code{make-local-variable}, which is what major mode commands typically |
1167 | use. This affects just the current buffer; all other buffers (including | |
1168 | those yet to be created) will continue to share the default value unless | |
1169 | they are explicitly given their own buffer-local bindings. | |
e6512bcf RS |
1170 | |
1171 | @cindex automatically buffer-local | |
1172 | A more powerful operation is to mark the variable as | |
1173 | @dfn{automatically buffer-local} by calling | |
1174 | @code{make-variable-buffer-local}. You can think of this as making the | |
1175 | variable local in all buffers, even those yet to be created. More | |
1176 | precisely, the effect is that setting the variable automatically makes | |
1177 | the variable local to the current buffer if it is not already so. All | |
f9f59935 | 1178 | buffers start out by sharing the default value of the variable as usual, |
969fe9b5 | 1179 | but setting the variable creates a buffer-local binding for the current |
e6512bcf | 1180 | buffer. The new value is stored in the buffer-local binding, leaving |
969fe9b5 RS |
1181 | the default binding untouched. This means that the default value cannot |
1182 | be changed with @code{setq} in any buffer; the only way to change it is | |
1183 | with @code{setq-default}. | |
f9f59935 RS |
1184 | |
1185 | @strong{Warning:} When a variable has buffer-local values in one or | |
1186 | more buffers, you can get Emacs very confused by binding the variable | |
1187 | with @code{let}, changing to a different current buffer in which a | |
1188 | different binding is in effect, and then exiting the @code{let}. This | |
1189 | can scramble the values of the buffer-local and default bindings. | |
1190 | ||
1191 | To preserve your sanity, avoid using a variable in that way. If you | |
1192 | use @code{save-excursion} around each piece of code that changes to a | |
a9f0a989 RS |
1193 | different current buffer, you will not have this problem |
1194 | (@pxref{Excursions}). Here is an example of what to avoid: | |
e6512bcf RS |
1195 | |
1196 | @example | |
1197 | @group | |
1198 | (setq foo 'b) | |
1199 | (set-buffer "a") | |
1200 | (make-local-variable 'foo) | |
1201 | @end group | |
1202 | (setq foo 'a) | |
1203 | (let ((foo 'temp)) | |
1204 | (set-buffer "b") | |
bfe721d1 | 1205 | @var{body}@dots{}) |
e6512bcf RS |
1206 | @group |
1207 | foo @result{} 'a ; @r{The old buffer-local value from buffer @samp{a}} | |
1208 | ; @r{is now the default value.} | |
1209 | @end group | |
1210 | @group | |
1211 | (set-buffer "a") | |
f9f59935 | 1212 | foo @result{} 'temp ; @r{The local @code{let} value that should be gone} |
e6512bcf RS |
1213 | ; @r{is now the buffer-local value in buffer @samp{a}.} |
1214 | @end group | |
1215 | @end example | |
1216 | ||
1217 | @noindent | |
1218 | But @code{save-excursion} as shown here avoids the problem: | |
1219 | ||
1220 | @example | |
1221 | @group | |
1222 | (let ((foo 'temp)) | |
1223 | (save-excursion | |
1224 | (set-buffer "b") | |
1225 | @var{body}@dots{})) | |
1226 | @end group | |
1227 | @end example | |
1228 | ||
1229 | Note that references to @code{foo} in @var{body} access the | |
1230 | buffer-local binding of buffer @samp{b}. | |
1231 | ||
1232 | When a file specifies local variable values, these become buffer-local | |
969fe9b5 RS |
1233 | values when you visit the file. @xref{File Variables,,, emacs, The |
1234 | GNU Emacs Manual}. | |
e6512bcf RS |
1235 | |
1236 | @node Creating Buffer-Local | |
1237 | @subsection Creating and Deleting Buffer-Local Bindings | |
1238 | ||
1239 | @deffn Command make-local-variable variable | |
1240 | This function creates a buffer-local binding in the current buffer for | |
1241 | @var{variable} (a symbol). Other buffers are not affected. The value | |
1242 | returned is @var{variable}. | |
1243 | ||
1244 | @c Emacs 19 feature | |
1245 | The buffer-local value of @var{variable} starts out as the same value | |
1246 | @var{variable} previously had. If @var{variable} was void, it remains | |
1247 | void. | |
1248 | ||
1249 | @example | |
1250 | @group | |
1251 | ;; @r{In buffer @samp{b1}:} | |
1252 | (setq foo 5) ; @r{Affects all buffers.} | |
1253 | @result{} 5 | |
1254 | @end group | |
1255 | @group | |
1256 | (make-local-variable 'foo) ; @r{Now it is local in @samp{b1}.} | |
1257 | @result{} foo | |
1258 | @end group | |
1259 | @group | |
1260 | foo ; @r{That did not change} | |
1261 | @result{} 5 ; @r{the value.} | |
1262 | @end group | |
1263 | @group | |
1264 | (setq foo 6) ; @r{Change the value} | |
1265 | @result{} 6 ; @r{in @samp{b1}.} | |
1266 | @end group | |
1267 | @group | |
1268 | foo | |
1269 | @result{} 6 | |
1270 | @end group | |
1271 | ||
1272 | @group | |
1273 | ;; @r{In buffer @samp{b2}, the value hasn't changed.} | |
1274 | (save-excursion | |
1275 | (set-buffer "b2") | |
1276 | foo) | |
1277 | @result{} 5 | |
1278 | @end group | |
1279 | @end example | |
e8505179 RS |
1280 | |
1281 | Making a variable buffer-local within a @code{let}-binding for that | |
969fe9b5 RS |
1282 | variable does not work reliably, unless the buffer in which you do this |
1283 | is not current either on entry to or exit from the @code{let}. This is | |
1284 | because @code{let} does not distinguish between different kinds of | |
1285 | bindings; it knows only which variable the binding was made for. | |
22697dac | 1286 | |
bfe721d1 KH |
1287 | If the variable is terminal-local, this function signals an error. Such |
1288 | variables cannot have buffer-local bindings as well. @xref{Multiple | |
1289 | Displays}. | |
1290 | ||
8241495d | 1291 | @strong{Note:} Do not use @code{make-local-variable} for a hook |
22697dac | 1292 | variable. Instead, use @code{make-local-hook}. @xref{Hooks}. |
e6512bcf RS |
1293 | @end deffn |
1294 | ||
1295 | @deffn Command make-variable-buffer-local variable | |
1296 | This function marks @var{variable} (a symbol) automatically | |
1297 | buffer-local, so that any subsequent attempt to set it will make it | |
1298 | local to the current buffer at the time. | |
1299 | ||
969fe9b5 RS |
1300 | A peculiar wrinkle of this feature is that binding the variable (with |
1301 | @code{let} or other binding constructs) does not create a buffer-local | |
1302 | binding for it. Only setting the variable (with @code{set} or | |
1303 | @code{setq}) does so. | |
ab4b1835 | 1304 | |
969fe9b5 | 1305 | The value returned is @var{variable}. |
ab4b1835 | 1306 | |
969fe9b5 RS |
1307 | @strong{Warning:} Don't assume that you should use |
1308 | @code{make-variable-buffer-local} for user-option variables, simply | |
1309 | because users @emph{might} want to customize them differently in | |
1310 | different buffers. Users can make any variable local, when they wish | |
1311 | to. It is better to leave the choice to them. | |
1312 | ||
1313 | The time to use @code{make-variable-buffer-local} is when it is crucial | |
1314 | that no two buffers ever share the same binding. For example, when a | |
1315 | variable is used for internal purposes in a Lisp program which depends | |
1316 | on having separate values in separate buffers, then using | |
1317 | @code{make-variable-buffer-local} can be the best solution. | |
e6512bcf RS |
1318 | @end deffn |
1319 | ||
bfe721d1 KH |
1320 | @defun local-variable-p variable &optional buffer |
1321 | This returns @code{t} if @var{variable} is buffer-local in buffer | |
1322 | @var{buffer} (which defaults to the current buffer); otherwise, | |
1323 | @code{nil}. | |
1324 | @end defun | |
1325 | ||
e6512bcf RS |
1326 | @defun buffer-local-variables &optional buffer |
1327 | This function returns a list describing the buffer-local variables in | |
969fe9b5 RS |
1328 | buffer @var{buffer}. (If @var{buffer} is omitted, the current buffer is |
1329 | used.) It returns an association list (@pxref{Association Lists}) in | |
1330 | which each element contains one buffer-local variable and its value. | |
1331 | However, when a variable's buffer-local binding in @var{buffer} is void, | |
1332 | then the variable appears directly in the resulting list. | |
e6512bcf RS |
1333 | |
1334 | @example | |
1335 | @group | |
1336 | (make-local-variable 'foobar) | |
1337 | (makunbound 'foobar) | |
1338 | (make-local-variable 'bind-me) | |
1339 | (setq bind-me 69) | |
1340 | @end group | |
1341 | (setq lcl (buffer-local-variables)) | |
1342 | ;; @r{First, built-in variables local in all buffers:} | |
1343 | @result{} ((mark-active . nil) | |
a9f0a989 | 1344 | (buffer-undo-list . nil) |
e6512bcf RS |
1345 | (mode-name . "Fundamental") |
1346 | @dots{} | |
1347 | @group | |
f9f59935 RS |
1348 | ;; @r{Next, non-built-in buffer-local variables.} |
1349 | ;; @r{This one is buffer-local and void:} | |
e6512bcf | 1350 | foobar |
f9f59935 | 1351 | ;; @r{This one is buffer-local and nonvoid:} |
e6512bcf RS |
1352 | (bind-me . 69)) |
1353 | @end group | |
1354 | @end example | |
1355 | ||
1356 | Note that storing new values into the @sc{cdr}s of cons cells in this | |
f9f59935 | 1357 | list does @emph{not} change the buffer-local values of the variables. |
e6512bcf RS |
1358 | @end defun |
1359 | ||
1360 | @deffn Command kill-local-variable variable | |
1361 | This function deletes the buffer-local binding (if any) for | |
1362 | @var{variable} (a symbol) in the current buffer. As a result, the | |
f9f59935 RS |
1363 | default binding of @var{variable} becomes visible in this buffer. This |
1364 | typically results in a change in the value of @var{variable}, since the | |
1365 | default value is usually different from the buffer-local value just | |
1366 | eliminated. | |
e6512bcf | 1367 | |
f9f59935 RS |
1368 | If you kill the buffer-local binding of a variable that automatically |
1369 | becomes buffer-local when set, this makes the default value visible in | |
1370 | the current buffer. However, if you set the variable again, that will | |
1371 | once again create a buffer-local binding for it. | |
e6512bcf RS |
1372 | |
1373 | @code{kill-local-variable} returns @var{variable}. | |
f57ddf67 RS |
1374 | |
1375 | This function is a command because it is sometimes useful to kill one | |
1376 | buffer-local variable interactively, just as it is useful to create | |
1377 | buffer-local variables interactively. | |
e6512bcf RS |
1378 | @end deffn |
1379 | ||
1380 | @defun kill-all-local-variables | |
1381 | This function eliminates all the buffer-local variable bindings of the | |
1382 | current buffer except for variables marked as ``permanent''. As a | |
1383 | result, the buffer will see the default values of most variables. | |
1384 | ||
1385 | This function also resets certain other information pertaining to the | |
1386 | buffer: it sets the local keymap to @code{nil}, the syntax table to the | |
a9f0a989 RS |
1387 | value of @code{(standard-syntax-table)}, the case table to |
1388 | @code{(standard-case-table)}, and the abbrev table to the value of | |
1389 | @code{fundamental-mode-abbrev-table}. | |
e6512bcf | 1390 | |
f9f59935 | 1391 | The very first thing this function does is run the normal hook |
969fe9b5 | 1392 | @code{change-major-mode-hook} (see below). |
f9f59935 | 1393 | |
e6512bcf RS |
1394 | Every major mode command begins by calling this function, which has the |
1395 | effect of switching to Fundamental mode and erasing most of the effects | |
1396 | of the previous major mode. To ensure that this does its job, the | |
1397 | variables that major modes set should not be marked permanent. | |
1398 | ||
1399 | @code{kill-all-local-variables} returns @code{nil}. | |
1400 | @end defun | |
1401 | ||
969fe9b5 RS |
1402 | @defvar change-major-mode-hook |
1403 | The function @code{kill-all-local-variables} runs this normal hook | |
1404 | before it does anything else. This gives major modes a way to arrange | |
1405 | for something special to be done if the user switches to a different | |
1406 | major mode. For best results, make this variable buffer-local, so that | |
1407 | it will disappear after doing its job and will not interfere with the | |
1408 | subsequent major mode. @xref{Hooks}. | |
1409 | @end defvar | |
1410 | ||
e6512bcf RS |
1411 | @c Emacs 19 feature |
1412 | @cindex permanent local variable | |
f9f59935 RS |
1413 | A buffer-local variable is @dfn{permanent} if the variable name (a |
1414 | symbol) has a @code{permanent-local} property that is non-@code{nil}. | |
1415 | Permanent locals are appropriate for data pertaining to where the file | |
1416 | came from or how to save it, rather than with how to edit the contents. | |
e6512bcf RS |
1417 | |
1418 | @node Default Value | |
1419 | @subsection The Default Value of a Buffer-Local Variable | |
1420 | @cindex default value | |
1421 | ||
1422 | The global value of a variable with buffer-local bindings is also | |
1423 | called the @dfn{default} value, because it is the value that is in | |
969fe9b5 RS |
1424 | effect whenever neither the current buffer nor the selected frame has |
1425 | its own binding for the variable. | |
e6512bcf RS |
1426 | |
1427 | The functions @code{default-value} and @code{setq-default} access and | |
1428 | change a variable's default value regardless of whether the current | |
1429 | buffer has a buffer-local binding. For example, you could use | |
1430 | @code{setq-default} to change the default setting of | |
1431 | @code{paragraph-start} for most buffers; and this would work even when | |
f57ddf67 | 1432 | you are in a C or Lisp mode buffer that has a buffer-local value for |
e6512bcf RS |
1433 | this variable. |
1434 | ||
1435 | @c Emacs 19 feature | |
1436 | The special forms @code{defvar} and @code{defconst} also set the | |
f9f59935 | 1437 | default value (if they set the variable at all), rather than any |
969fe9b5 | 1438 | buffer-local or frame-local value. |
e6512bcf RS |
1439 | |
1440 | @defun default-value symbol | |
1441 | This function returns @var{symbol}'s default value. This is the value | |
969fe9b5 RS |
1442 | that is seen in buffers and frames that do not have their own values for |
1443 | this variable. If @var{symbol} is not buffer-local, this is equivalent | |
1444 | to @code{symbol-value} (@pxref{Accessing Variables}). | |
e6512bcf RS |
1445 | @end defun |
1446 | ||
1447 | @c Emacs 19 feature | |
f57ddf67 RS |
1448 | @defun default-boundp symbol |
1449 | The function @code{default-boundp} tells you whether @var{symbol}'s | |
e6512bcf RS |
1450 | default value is nonvoid. If @code{(default-boundp 'foo)} returns |
1451 | @code{nil}, then @code{(default-value 'foo)} would get an error. | |
1452 | ||
1453 | @code{default-boundp} is to @code{default-value} as @code{boundp} is to | |
1454 | @code{symbol-value}. | |
1455 | @end defun | |
1456 | ||
a9f0a989 RS |
1457 | @defspec setq-default [symbol form]@dots{} |
1458 | This special form gives each @var{symbol} a new default value, which is | |
1459 | the result of evaluating the corresponding @var{form}. It does not | |
1460 | evaluate @var{symbol}, but does evaluate @var{form}. The value of the | |
1461 | @code{setq-default} form is the value of the last @var{form}. | |
e6512bcf RS |
1462 | |
1463 | If a @var{symbol} is not buffer-local for the current buffer, and is not | |
1464 | marked automatically buffer-local, @code{setq-default} has the same | |
1465 | effect as @code{setq}. If @var{symbol} is buffer-local for the current | |
1466 | buffer, then this changes the value that other buffers will see (as long | |
1467 | as they don't have a buffer-local value), but not the value that the | |
1468 | current buffer sees. | |
1469 | ||
1470 | @example | |
1471 | @group | |
1472 | ;; @r{In buffer @samp{foo}:} | |
969fe9b5 RS |
1473 | (make-local-variable 'buffer-local) |
1474 | @result{} buffer-local | |
e6512bcf RS |
1475 | @end group |
1476 | @group | |
969fe9b5 | 1477 | (setq buffer-local 'value-in-foo) |
e6512bcf RS |
1478 | @result{} value-in-foo |
1479 | @end group | |
1480 | @group | |
969fe9b5 | 1481 | (setq-default buffer-local 'new-default) |
e6512bcf RS |
1482 | @result{} new-default |
1483 | @end group | |
1484 | @group | |
969fe9b5 | 1485 | buffer-local |
e6512bcf RS |
1486 | @result{} value-in-foo |
1487 | @end group | |
1488 | @group | |
969fe9b5 | 1489 | (default-value 'buffer-local) |
e6512bcf RS |
1490 | @result{} new-default |
1491 | @end group | |
1492 | ||
1493 | @group | |
1494 | ;; @r{In (the new) buffer @samp{bar}:} | |
969fe9b5 | 1495 | buffer-local |
e6512bcf RS |
1496 | @result{} new-default |
1497 | @end group | |
1498 | @group | |
969fe9b5 | 1499 | (default-value 'buffer-local) |
e6512bcf RS |
1500 | @result{} new-default |
1501 | @end group | |
1502 | @group | |
969fe9b5 | 1503 | (setq buffer-local 'another-default) |
e6512bcf RS |
1504 | @result{} another-default |
1505 | @end group | |
1506 | @group | |
969fe9b5 | 1507 | (default-value 'buffer-local) |
e6512bcf RS |
1508 | @result{} another-default |
1509 | @end group | |
1510 | ||
1511 | @group | |
1512 | ;; @r{Back in buffer @samp{foo}:} | |
969fe9b5 | 1513 | buffer-local |
e6512bcf | 1514 | @result{} value-in-foo |
969fe9b5 | 1515 | (default-value 'buffer-local) |
e6512bcf RS |
1516 | @result{} another-default |
1517 | @end group | |
1518 | @end example | |
1519 | @end defspec | |
1520 | ||
1521 | @defun set-default symbol value | |
1522 | This function is like @code{setq-default}, except that @var{symbol} is | |
f9f59935 | 1523 | an ordinary evaluated argument. |
e6512bcf RS |
1524 | |
1525 | @example | |
1526 | @group | |
1527 | (set-default (car '(a b c)) 23) | |
1528 | @result{} 23 | |
1529 | @end group | |
1530 | @group | |
1531 | (default-value 'a) | |
1532 | @result{} 23 | |
1533 | @end group | |
1534 | @end example | |
1535 | @end defun | |
969fe9b5 RS |
1536 | |
1537 | @node Frame-Local Variables | |
1538 | @section Frame-Local Variables | |
1539 | ||
1540 | Just as variables can have buffer-local bindings, they can also have | |
1541 | frame-local bindings. These bindings belong to one frame, and are in | |
1542 | effect when that frame is selected. Frame-local bindings are actually | |
1543 | frame parameters: you create a frame-local binding in a specific frame | |
1544 | by calling @code{modify-frame-parameters} and specifying the variable | |
1545 | name as the parameter name. | |
1546 | ||
1547 | To enable frame-local bindings for a certain variable, call the function | |
1548 | @code{make-variable-frame-local}. | |
1549 | ||
a9f0a989 | 1550 | @deffn Command make-variable-frame-local variable |
969fe9b5 RS |
1551 | Enable the use of frame-local bindings for @var{variable}. This does |
1552 | not in itself create any frame-local bindings for the variable; however, | |
1553 | if some frame already has a value for @var{variable} as a frame | |
1554 | parameter, that value automatically becomes a frame-local binding. | |
1555 | ||
a9f0a989 RS |
1556 | If the variable is terminal-local, this function signals an error, |
1557 | because such variables cannot have frame-local bindings as well. | |
1558 | @xref{Multiple Displays}. A few variables that are implemented | |
1559 | specially in Emacs can be (and usually are) buffer-local, but can never | |
1560 | be frame-local. | |
1561 | @end deffn | |
969fe9b5 RS |
1562 | |
1563 | Buffer-local bindings take precedence over frame-local bindings. Thus, | |
1564 | consider a variable @code{foo}: if the current buffer has a buffer-local | |
1565 | binding for @code{foo}, that binding is active; otherwise, if the | |
1566 | selected frame has a frame-local binding for @code{foo}, that binding is | |
1567 | active; otherwise, the default binding of @code{foo} is active. | |
1568 | ||
1569 | Here is an example. First we prepare a few bindings for @code{foo}: | |
1570 | ||
1571 | @example | |
1572 | (setq f1 (selected-frame)) | |
1573 | (make-variable-frame-local 'foo) | |
1574 | ||
1575 | ;; @r{Make a buffer-local binding for @code{foo} in @samp{b1}.} | |
1576 | (set-buffer (get-buffer-create "b1")) | |
1577 | (make-local-variable 'foo) | |
1578 | (setq foo '(b 1)) | |
1579 | ||
1580 | ;; @r{Make a frame-local binding for @code{foo} in a new frame.} | |
1581 | ;; @r{Store that frame in @code{f2}.} | |
1582 | (setq f2 (make-frame)) | |
1583 | (modify-frame-parameters f2 '((foo . (f 2)))) | |
1584 | @end example | |
1585 | ||
1586 | Now we examine @code{foo} in various contexts. Whenever the | |
1587 | buffer @samp{b1} is current, its buffer-local binding is in effect, | |
1588 | regardless of the selected frame: | |
1589 | ||
1590 | @example | |
1591 | (select-frame f1) | |
1592 | (set-buffer (get-buffer-create "b1")) | |
1593 | foo | |
1594 | @result{} (b 1) | |
1595 | ||
1596 | (select-frame f2) | |
1597 | (set-buffer (get-buffer-create "b1")) | |
1598 | foo | |
1599 | @result{} (b 1) | |
1600 | @end example | |
1601 | ||
1602 | @noindent | |
1603 | Otherwise, the frame gets a chance to provide the binding; when frame | |
1604 | @code{f2} is selected, its frame-local binding is in effect: | |
1605 | ||
1606 | @example | |
1607 | (select-frame f2) | |
1608 | (set-buffer (get-buffer "*scratch*")) | |
1609 | foo | |
1610 | @result{} (f 2) | |
1611 | @end example | |
1612 | ||
1613 | @noindent | |
1614 | When neither the current buffer nor the selected frame provides | |
1615 | a binding, the default binding is used: | |
1616 | ||
1617 | @example | |
1618 | (select-frame f1) | |
1619 | (set-buffer (get-buffer "*scratch*")) | |
1620 | foo | |
1621 | @result{} nil | |
1622 | @end example | |
1623 | ||
1624 | @noindent | |
1625 | When the active binding of a variable is a frame-local binding, setting | |
1626 | the variable changes that binding. You can observe the result with | |
1627 | @code{frame-parameters}: | |
1628 | ||
1629 | @example | |
1630 | (select-frame f2) | |
1631 | (set-buffer (get-buffer "*scratch*")) | |
1632 | (setq foo 'nobody) | |
1633 | (assq 'foo (frame-parameters f2)) | |
1634 | @result{} (foo . nobody) | |
1635 | @end example | |
1636 | ||
1637 | @node Future Local Variables | |
1638 | @section Possible Future Local Variables | |
1639 | ||
1640 | We have considered the idea of bindings that are local to a category | |
1641 | of frames---for example, all color frames, or all frames with dark | |
1642 | backgrounds. We have not implemented them because it is not clear that | |
1643 | this feature is really useful. You can get more or less the same | |
916a89db | 1644 | results by adding a function to @code{after-make-frame-functions}, set up to |
969fe9b5 RS |
1645 | define a particular frame parameter according to the appropriate |
1646 | conditions for each frame. | |
1647 | ||
1648 | It would also be possible to implement window-local bindings. We | |
1649 | don't know of many situations where they would be useful, and it seems | |
1650 | that indirect buffers (@pxref{Indirect Buffers}) with buffer-local | |
1651 | bindings offer a way to handle these situations more robustly. | |
1652 | ||
1653 | If sufficient application is found for either of these two kinds of | |
1654 | local bindings, we will provide it in a subsequent Emacs version. | |
1655 | ||
b50c9214 RS |
1656 | @node File Local Variables |
1657 | @section File Local Variables | |
1658 | ||
1659 | This section describes the functions and variables that affect | |
1660 | processing of local variables lists in files. | |
1661 | ||
1662 | @defopt enable-local-variables | |
1663 | This variable controls whether to process file local variables lists. A | |
1664 | value of @code{t} means process the local variables lists | |
1665 | unconditionally; @code{nil} means ignore them; anything else means ask | |
1666 | the user what to do for each file. The default value is @code{t}. | |
1667 | @end defopt | |
1668 | ||
1669 | @defun hack-local-variables &optional force | |
1670 | This function parses, and binds or evaluates as appropriate, any local | |
1671 | variables specified by the contents of the current buffer. The variable | |
1672 | @code{enable-local-variables} has its effect here. | |
1673 | ||
1674 | The argument @var{force} usually comes from the argument @var{find-file} | |
1675 | given to @code{normal-mode}. | |
1676 | @end defun | |
1677 | ||
1678 | If a file local variable list could specify the a function that will | |
1679 | be called later, or an expression that will be executed later, simply | |
1680 | visiting a file could take over your Emacs. To prevent this, Emacs | |
1681 | takes care not to allow local variable lists to set such variables. | |
1682 | ||
1683 | For one thing, any variable whose name ends in @samp{-function}, | |
1684 | @samp{-functions}, @samp{-hook}, @samp{-hooks}, @samp{-form}, | |
1685 | @samp{-forms}, @samp{-program}, @samp{-command} or @samp{-predicate} | |
1686 | cannot be set in a local variable list. In general, you should use such | |
1687 | a name whenever it is appropriate for the variable's meaning. | |
1688 | ||
1689 | In addition, any variable whose name has a non-@code{nil} | |
1690 | @code{risky-local-variable} property is also ignored. So are | |
1691 | all variables listed in @code{ignored-local-variables}: | |
1692 | ||
1693 | @defvar ignored-local-variables | |
1694 | This variable holds a list of variables that should not be | |
1695 | set by a file's local variables list. Any value specified | |
1696 | for one of these variables is ignored. | |
1697 | @end defvar | |
1698 | ||
1699 | The @samp{Eval:} ``variable'' is also a potential loophole, so Emacs | |
1700 | normally asks for confirmation before handling it. | |
969fe9b5 | 1701 | |
b50c9214 RS |
1702 | @defopt enable-local-eval |
1703 | This variable controls processing of @samp{Eval:} in local variables | |
1704 | lists in files being visited. A value of @code{t} means process them | |
1705 | unconditionally; @code{nil} means ignore them; anything else means ask | |
1706 | the user what to do for each file. The default value is @code{maybe}. | |
1707 | @end defopt |