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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 |
177c0ea7 | 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 |
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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. | |
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37 | * Accessing Variables:: Examining values of variables whose names |
38 | are known only at run time. | |
39 | * Setting Variables:: Storing new values in variables. | |
40 | * Variable Scoping:: How Lisp chooses among local and global values. | |
41 | * Buffer-Local Variables:: Variable values in effect only in one buffer. | |
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42 | * 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. | |
26236f6d | 44 | * Variable Aliases:: Variables that are aliases for other variables. |
b50c9214 | 45 | * File Local Variables:: Handling local variable lists in files. |
74ab3aa3 LT |
46 | * Variables with Restricted Values:: Non-constant variables whose value can |
47 | @emph{not} be an arbitrary Lisp object. | |
e6512bcf RS |
48 | @end menu |
49 | ||
50 | @node Global Variables | |
51 | @section Global Variables | |
52 | @cindex global variable | |
53 | ||
54 | The simplest way to use a variable is @dfn{globally}. This means that | |
55 | the variable has just one value at a time, and this value is in effect | |
56 | (at least for the moment) throughout the Lisp system. The value remains | |
57 | in effect until you specify a new one. When a new value replaces the | |
58 | old one, no trace of the old value remains in the variable. | |
59 | ||
60 | You specify a value for a symbol with @code{setq}. For example, | |
61 | ||
62 | @example | |
63 | (setq x '(a b)) | |
64 | @end example | |
65 | ||
66 | @noindent | |
67 | gives the variable @code{x} the value @code{(a b)}. Note that | |
68 | @code{setq} does not evaluate its first argument, the name of the | |
69 | variable, but it does evaluate the second argument, the new value. | |
70 | ||
71 | Once the variable has a value, you can refer to it by using the symbol | |
72 | by itself as an expression. Thus, | |
73 | ||
74 | @example | |
75 | @group | |
76 | x @result{} (a b) | |
77 | @end group | |
78 | @end example | |
79 | ||
80 | @noindent | |
81 | assuming the @code{setq} form shown above has already been executed. | |
82 | ||
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83 | If you do set the same variable again, the new value replaces the old |
84 | one: | |
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85 | |
86 | @example | |
87 | @group | |
88 | x | |
89 | @result{} (a b) | |
90 | @end group | |
91 | @group | |
92 | (setq x 4) | |
93 | @result{} 4 | |
94 | @end group | |
95 | @group | |
96 | x | |
97 | @result{} 4 | |
98 | @end group | |
99 | @end example | |
100 | ||
101 | @node Constant Variables | |
8241495d | 102 | @section Variables that Never Change |
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103 | @vindex nil |
104 | @vindex t | |
105 | @kindex setting-constant | |
95ed62c7 | 106 | @cindex keyword symbol |
e6512bcf | 107 | |
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108 | In Emacs Lisp, certain symbols normally evaluate to themselves. These |
109 | include @code{nil} and @code{t}, as well as any symbol whose name starts | |
95ed62c7 GM |
110 | with @samp{:} (these are called @dfn{keywords}). These symbols cannot |
111 | be rebound, nor can their values be changed. Any attempt to set or bind | |
112 | @code{nil} or @code{t} signals a @code{setting-constant} error. The | |
113 | same is true for a keyword (a symbol whose name starts with @samp{:}), | |
114 | if it is interned in the standard obarray, except that setting such a | |
115 | symbol to itself is not an error. | |
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116 | |
117 | @example | |
118 | @group | |
119 | nil @equiv{} 'nil | |
120 | @result{} nil | |
121 | @end group | |
122 | @group | |
123 | (setq nil 500) | |
124 | @error{} Attempt to set constant symbol: nil | |
125 | @end group | |
126 | @end example | |
127 | ||
e88399c8 DL |
128 | @defun keywordp object |
129 | @tindex keywordp | |
130 | function returns @code{t} if @var{object} is a symbol whose name | |
131 | starts with @samp{:}, interned in the standard obarray, and returns | |
132 | @code{nil} otherwise. | |
133 | @end defun | |
134 | ||
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135 | @node Local Variables |
136 | @section Local Variables | |
137 | @cindex binding local variables | |
138 | @cindex local variables | |
139 | @cindex local binding | |
140 | @cindex global binding | |
141 | ||
142 | Global variables have values that last until explicitly superseded | |
143 | with new values. Sometimes it is useful to create variable values that | |
969fe9b5 | 144 | exist temporarily---only until a certain part of the program finishes. |
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145 | These values are called @dfn{local}, and the variables so used are |
146 | called @dfn{local variables}. | |
147 | ||
148 | For example, when a function is called, its argument variables receive | |
149 | new local values that last until the function exits. The @code{let} | |
150 | special form explicitly establishes new local values for specified | |
151 | variables; these last until exit from the @code{let} form. | |
152 | ||
153 | @cindex shadowing of variables | |
154 | Establishing a local value saves away the previous value (or lack of | |
155 | one) of the variable. When the life span of the local value is over, | |
156 | the previous value is restored. In the mean time, we say that the | |
157 | previous value is @dfn{shadowed} and @dfn{not visible}. Both global and | |
158 | local values may be shadowed (@pxref{Scope}). | |
159 | ||
160 | If you set a variable (such as with @code{setq}) while it is local, | |
161 | this replaces the local value; it does not alter the global value, or | |
969fe9b5 | 162 | previous local values, that are shadowed. To model this behavior, we |
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163 | speak of a @dfn{local binding} of the variable as well as a local value. |
164 | ||
165 | The local binding is a conceptual place that holds a local value. | |
166 | Entry to a function, or a special form such as @code{let}, creates the | |
167 | local binding; exit from the function or from the @code{let} removes the | |
168 | local binding. As long as the local binding lasts, the variable's value | |
169 | is stored within it. Use of @code{setq} or @code{set} while there is a | |
170 | local binding stores a different value into the local binding; it does | |
171 | not create a new binding. | |
172 | ||
173 | We also speak of the @dfn{global binding}, which is where | |
174 | (conceptually) the global value is kept. | |
175 | ||
176 | @cindex current binding | |
177 | A variable can have more than one local binding at a time (for | |
178 | example, if there are nested @code{let} forms that bind it). In such a | |
179 | case, the most recently created local binding that still exists is the | |
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180 | @dfn{current binding} of the variable. (This rule is called |
181 | @dfn{dynamic scoping}; see @ref{Variable Scoping}.) If there are no | |
182 | local bindings, the variable's global binding is its current binding. | |
183 | We sometimes call the current binding the @dfn{most-local existing | |
184 | binding}, for emphasis. Ordinary evaluation of a symbol always returns | |
185 | the value of its current binding. | |
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186 | |
187 | The special forms @code{let} and @code{let*} exist to create | |
188 | local bindings. | |
189 | ||
190 | @defspec let (bindings@dots{}) forms@dots{} | |
f57ddf67 | 191 | This special form binds variables according to @var{bindings} and then |
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192 | evaluates all of the @var{forms} in textual order. The @code{let}-form |
193 | returns the value of the last form in @var{forms}. | |
194 | ||
195 | Each of the @var{bindings} is either @w{(i) a} symbol, in which case | |
196 | that symbol is bound to @code{nil}; or @w{(ii) a} list of the form | |
197 | @code{(@var{symbol} @var{value-form})}, in which case @var{symbol} is | |
198 | bound to the result of evaluating @var{value-form}. If @var{value-form} | |
199 | is omitted, @code{nil} is used. | |
200 | ||
201 | All of the @var{value-form}s in @var{bindings} are evaluated in the | |
f9f59935 | 202 | order they appear and @emph{before} binding any of the symbols to them. |
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203 | Here is an example of this: @code{z} is bound to the old value of |
204 | @code{y}, which is 2, not the new value of @code{y}, which is 1. | |
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205 | |
206 | @example | |
207 | @group | |
5c646a2d | 208 | (setq y 2) |
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209 | @result{} 2 |
210 | @end group | |
211 | @group | |
5c646a2d RS |
212 | (let ((y 1) |
213 | (z y)) | |
214 | (list y z)) | |
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215 | @result{} (1 2) |
216 | @end group | |
217 | @end example | |
218 | @end defspec | |
219 | ||
220 | @defspec let* (bindings@dots{}) forms@dots{} | |
221 | This special form is like @code{let}, but it binds each variable right | |
222 | after computing its local value, before computing the local value for | |
223 | the next variable. Therefore, an expression in @var{bindings} can | |
224 | reasonably refer to the preceding symbols bound in this @code{let*} | |
225 | form. Compare the following example with the example above for | |
226 | @code{let}. | |
227 | ||
228 | @example | |
229 | @group | |
5c646a2d | 230 | (setq y 2) |
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231 | @result{} 2 |
232 | @end group | |
233 | @group | |
5c646a2d RS |
234 | (let* ((y 1) |
235 | (z y)) ; @r{Use the just-established value of @code{y}.} | |
236 | (list y z)) | |
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237 | @result{} (1 1) |
238 | @end group | |
239 | @end example | |
240 | @end defspec | |
241 | ||
f57ddf67 | 242 | Here is a complete list of the other facilities that create local |
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243 | bindings: |
244 | ||
245 | @itemize @bullet | |
246 | @item | |
247 | Function calls (@pxref{Functions}). | |
248 | ||
249 | @item | |
250 | Macro calls (@pxref{Macros}). | |
251 | ||
252 | @item | |
253 | @code{condition-case} (@pxref{Errors}). | |
254 | @end itemize | |
255 | ||
bfe721d1 | 256 | Variables can also have buffer-local bindings (@pxref{Buffer-Local |
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257 | Variables}) and frame-local bindings (@pxref{Frame-Local Variables}); a |
258 | few variables have terminal-local bindings (@pxref{Multiple Displays}). | |
259 | These kinds of bindings work somewhat like ordinary local bindings, but | |
260 | they are localized depending on ``where'' you are in Emacs, rather than | |
261 | localized in time. | |
bfe721d1 | 262 | |
74ab3aa3 | 263 | @anchor{Definition of max-specpdl-size} |
e6512bcf RS |
264 | @defvar max-specpdl-size |
265 | @cindex variable limit error | |
266 | @cindex evaluation error | |
267 | @cindex infinite recursion | |
a9f0a989 | 268 | This variable defines the limit on the total number of local variable |
74ab3aa3 LT |
269 | bindings and @code{unwind-protect} cleanups (@pxref{Cleanups,, |
270 | Cleaning Up from Nonlocal Exits}) that are allowed before signaling an | |
271 | error (with data @code{"Variable binding depth exceeds | |
272 | max-specpdl-size"}). | |
e6512bcf | 273 | |
a9f0a989 | 274 | This limit, with the associated error when it is exceeded, is one way |
e6512bcf | 275 | that Lisp avoids infinite recursion on an ill-defined function. |
a9f0a989 | 276 | @code{max-lisp-eval-depth} provides another limit on depth of nesting. |
74ab3aa3 | 277 | @xref{Definition of max-lisp-eval-depth,, Eval}. |
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278 | |
279 | The default value is 600. Entry to the Lisp debugger increases the | |
280 | value, if there is little room left, to make sure the debugger itself | |
281 | has room to execute. | |
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282 | @end defvar |
283 | ||
284 | @node Void Variables | |
285 | @section When a Variable is ``Void'' | |
286 | @kindex void-variable | |
287 | @cindex void variable | |
288 | ||
289 | If you have never given a symbol any value as a global variable, we | |
290 | say that that symbol's global value is @dfn{void}. In other words, the | |
291 | symbol's value cell does not have any Lisp object in it. If you try to | |
292 | evaluate the symbol, you get a @code{void-variable} error rather than | |
293 | a value. | |
294 | ||
295 | Note that a value of @code{nil} is not the same as void. The symbol | |
296 | @code{nil} is a Lisp object and can be the value of a variable just as any | |
297 | other object can be; but it is @emph{a value}. A void variable does not | |
298 | have any value. | |
299 | ||
300 | After you have given a variable a value, you can make it void once more | |
301 | using @code{makunbound}. | |
302 | ||
303 | @defun makunbound symbol | |
f9f59935 | 304 | This function makes the current variable binding of @var{symbol} void. |
e6512bcf | 305 | Subsequent attempts to use this symbol's value as a variable will signal |
f9f59935 | 306 | the error @code{void-variable}, unless and until you set it again. |
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307 | |
308 | @code{makunbound} returns @var{symbol}. | |
309 | ||
310 | @example | |
311 | @group | |
969fe9b5 | 312 | (makunbound 'x) ; @r{Make the global value of @code{x} void.} |
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313 | @result{} x |
314 | @end group | |
315 | @group | |
316 | x | |
317 | @error{} Symbol's value as variable is void: x | |
318 | @end group | |
319 | @end example | |
320 | ||
321 | If @var{symbol} is locally bound, @code{makunbound} affects the most | |
322 | local existing binding. This is the only way a symbol can have a void | |
323 | local binding, since all the constructs that create local bindings | |
324 | create them with values. In this case, the voidness lasts at most as | |
325 | long as the binding does; when the binding is removed due to exit from | |
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326 | the construct that made it, the previous local or global binding is |
327 | reexposed as usual, and the variable is no longer void unless the newly | |
328 | reexposed binding was void all along. | |
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329 | |
330 | @smallexample | |
331 | @group | |
332 | (setq x 1) ; @r{Put a value in the global binding.} | |
333 | @result{} 1 | |
334 | (let ((x 2)) ; @r{Locally bind it.} | |
335 | (makunbound 'x) ; @r{Void the local binding.} | |
336 | x) | |
337 | @error{} Symbol's value as variable is void: x | |
338 | @end group | |
339 | @group | |
340 | x ; @r{The global binding is unchanged.} | |
341 | @result{} 1 | |
342 | ||
343 | (let ((x 2)) ; @r{Locally bind it.} | |
344 | (let ((x 3)) ; @r{And again.} | |
345 | (makunbound 'x) ; @r{Void the innermost-local binding.} | |
346 | x)) ; @r{And refer: it's void.} | |
347 | @error{} Symbol's value as variable is void: x | |
348 | @end group | |
349 | ||
350 | @group | |
351 | (let ((x 2)) | |
352 | (let ((x 3)) | |
353 | (makunbound 'x)) ; @r{Void inner binding, then remove it.} | |
354 | x) ; @r{Now outer @code{let} binding is visible.} | |
355 | @result{} 2 | |
356 | @end group | |
357 | @end smallexample | |
358 | @end defun | |
359 | ||
360 | A variable that has been made void with @code{makunbound} is | |
361 | indistinguishable from one that has never received a value and has | |
362 | always been void. | |
363 | ||
364 | You can use the function @code{boundp} to test whether a variable is | |
365 | currently void. | |
366 | ||
367 | @defun boundp variable | |
368 | @code{boundp} returns @code{t} if @var{variable} (a symbol) is not void; | |
369 | more precisely, if its current binding is not void. It returns | |
370 | @code{nil} otherwise. | |
371 | ||
372 | @smallexample | |
373 | @group | |
374 | (boundp 'abracadabra) ; @r{Starts out void.} | |
375 | @result{} nil | |
376 | @end group | |
377 | @group | |
378 | (let ((abracadabra 5)) ; @r{Locally bind it.} | |
379 | (boundp 'abracadabra)) | |
380 | @result{} t | |
381 | @end group | |
382 | @group | |
383 | (boundp 'abracadabra) ; @r{Still globally void.} | |
384 | @result{} nil | |
385 | @end group | |
386 | @group | |
387 | (setq abracadabra 5) ; @r{Make it globally nonvoid.} | |
388 | @result{} 5 | |
389 | @end group | |
390 | @group | |
391 | (boundp 'abracadabra) | |
392 | @result{} t | |
393 | @end group | |
394 | @end smallexample | |
395 | @end defun | |
396 | ||
397 | @node Defining Variables | |
398 | @section Defining Global Variables | |
f57ddf67 | 399 | @cindex variable definition |
e6512bcf RS |
400 | |
401 | You may announce your intention to use a symbol as a global variable | |
f57ddf67 RS |
402 | with a @dfn{variable definition}: a special form, either @code{defconst} |
403 | or @code{defvar}. | |
e6512bcf RS |
404 | |
405 | In Emacs Lisp, definitions serve three purposes. First, they inform | |
406 | people who read the code that certain symbols are @emph{intended} to be | |
407 | used a certain way (as variables). Second, they inform the Lisp system | |
408 | of these things, supplying a value and documentation. Third, they | |
409 | provide information to utilities such as @code{etags} and | |
410 | @code{make-docfile}, which create data bases of the functions and | |
411 | variables in a program. | |
412 | ||
413 | The difference between @code{defconst} and @code{defvar} is primarily | |
f9f59935 RS |
414 | a matter of intent, serving to inform human readers of whether the value |
415 | should ever change. Emacs Lisp does not restrict the ways in which a | |
416 | variable can be used based on @code{defconst} or @code{defvar} | |
f57ddf67 | 417 | declarations. However, it does make a difference for initialization: |
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418 | @code{defconst} unconditionally initializes the variable, while |
419 | @code{defvar} initializes it only if it is void. | |
420 | ||
f9f59935 | 421 | @ignore |
e6512bcf RS |
422 | One would expect user option variables to be defined with |
423 | @code{defconst}, since programs do not change them. Unfortunately, this | |
424 | has bad results if the definition is in a library that is not preloaded: | |
425 | @code{defconst} would override any prior value when the library is | |
426 | loaded. Users would like to be able to set user options in their init | |
427 | files, and override the default values given in the definitions. For | |
428 | this reason, user options must be defined with @code{defvar}. | |
f9f59935 | 429 | @end ignore |
e6512bcf RS |
430 | |
431 | @defspec defvar symbol [value [doc-string]] | |
f9f59935 RS |
432 | This special form defines @var{symbol} as a variable and can also |
433 | initialize and document it. The definition informs a person reading | |
434 | your code that @var{symbol} is used as a variable that might be set or | |
435 | changed. Note that @var{symbol} is not evaluated; the symbol to be | |
e6512bcf RS |
436 | defined must appear explicitly in the @code{defvar}. |
437 | ||
f9f59935 RS |
438 | If @var{symbol} is void and @var{value} is specified, @code{defvar} |
439 | evaluates it and sets @var{symbol} to the result. But if @var{symbol} | |
440 | already has a value (i.e., it is not void), @var{value} is not even | |
441 | evaluated, and @var{symbol}'s value remains unchanged. If @var{value} | |
442 | is omitted, the value of @var{symbol} is not changed in any case. | |
e6512bcf | 443 | |
a9f0a989 RS |
444 | If @var{symbol} has a buffer-local binding in the current buffer, |
445 | @code{defvar} operates on the default value, which is buffer-independent, | |
446 | not the current (buffer-local) binding. It sets the default value if | |
447 | the default value is void. @xref{Buffer-Local Variables}. | |
448 | ||
bfe721d1 KH |
449 | When you evaluate a top-level @code{defvar} form with @kbd{C-M-x} in |
450 | Emacs Lisp mode (@code{eval-defun}), a special feature of | |
a9f0a989 RS |
451 | @code{eval-defun} arranges to set the variable unconditionally, without |
452 | testing whether its value is void. | |
e6512bcf RS |
453 | |
454 | If the @var{doc-string} argument appears, it specifies the documentation | |
455 | for the variable. (This opportunity to specify documentation is one of | |
456 | the main benefits of defining the variable.) The documentation is | |
457 | stored in the symbol's @code{variable-documentation} property. The | |
458 | Emacs help functions (@pxref{Documentation}) look for this property. | |
459 | ||
2a2048f2 RS |
460 | If the variable is a user option that users would want to set |
461 | interactively, you should use @samp{*} as the first character of | |
462 | @var{doc-string}. This lets users set the variable conveniently using | |
463 | the @code{set-variable} command. Note that you should nearly always | |
464 | use @code{defcustom} instead of @code{defvar} to define these | |
465 | variables, so that users can use @kbd{M-x customize} and related | |
466 | commands to set them. @xref{Customization}. | |
e6512bcf | 467 | |
f9f59935 RS |
468 | Here are some examples. This form defines @code{foo} but does not |
469 | initialize it: | |
e6512bcf RS |
470 | |
471 | @example | |
472 | @group | |
473 | (defvar foo) | |
474 | @result{} foo | |
475 | @end group | |
476 | @end example | |
477 | ||
f9f59935 RS |
478 | This example initializes the value of @code{bar} to @code{23}, and gives |
479 | it a documentation string: | |
e6512bcf RS |
480 | |
481 | @example | |
482 | @group | |
483 | (defvar bar 23 | |
484 | "The normal weight of a bar.") | |
485 | @result{} bar | |
486 | @end group | |
487 | @end example | |
488 | ||
489 | The following form changes the documentation string for @code{bar}, | |
490 | making it a user option, but does not change the value, since @code{bar} | |
f9f59935 RS |
491 | already has a value. (The addition @code{(1+ nil)} would get an error |
492 | if it were evaluated, but since it is not evaluated, there is no error.) | |
e6512bcf RS |
493 | |
494 | @example | |
495 | @group | |
f9f59935 | 496 | (defvar bar (1+ nil) |
e6512bcf RS |
497 | "*The normal weight of a bar.") |
498 | @result{} bar | |
499 | @end group | |
500 | @group | |
501 | bar | |
502 | @result{} 23 | |
503 | @end group | |
504 | @end example | |
505 | ||
506 | Here is an equivalent expression for the @code{defvar} special form: | |
507 | ||
508 | @example | |
509 | @group | |
510 | (defvar @var{symbol} @var{value} @var{doc-string}) | |
511 | @equiv{} | |
512 | (progn | |
513 | (if (not (boundp '@var{symbol})) | |
514 | (setq @var{symbol} @var{value})) | |
25ef19c5 RS |
515 | (if '@var{doc-string} |
516 | (put '@var{symbol} 'variable-documentation '@var{doc-string})) | |
e6512bcf RS |
517 | '@var{symbol}) |
518 | @end group | |
519 | @end example | |
520 | ||
521 | The @code{defvar} form returns @var{symbol}, but it is normally used | |
522 | at top level in a file where its value does not matter. | |
523 | @end defspec | |
524 | ||
f36e1798 | 525 | @defspec defconst symbol value [doc-string] |
e6512bcf | 526 | This special form defines @var{symbol} as a value and initializes it. |
f9f59935 RS |
527 | It informs a person reading your code that @var{symbol} has a standard |
528 | global value, established here, that should not be changed by the user | |
529 | or by other programs. Note that @var{symbol} is not evaluated; the | |
e6512bcf RS |
530 | symbol to be defined must appear explicitly in the @code{defconst}. |
531 | ||
f9f59935 | 532 | @code{defconst} always evaluates @var{value}, and sets the value of |
f36e1798 LT |
533 | @var{symbol} to the result. If @var{symbol} does have a buffer-local |
534 | binding in the current buffer, @code{defconst} sets the default value, | |
535 | not the buffer-local value. (But you should not be making | |
536 | buffer-local bindings for a symbol that is defined with | |
a9f0a989 | 537 | @code{defconst}.) |
e6512bcf RS |
538 | |
539 | Here, @code{pi} is a constant that presumably ought not to be changed | |
540 | by anyone (attempts by the Indiana State Legislature notwithstanding). | |
541 | As the second form illustrates, however, this is only advisory. | |
542 | ||
543 | @example | |
544 | @group | |
545 | (defconst pi 3.1415 "Pi to five places.") | |
546 | @result{} pi | |
547 | @end group | |
548 | @group | |
549 | (setq pi 3) | |
550 | @result{} pi | |
551 | @end group | |
552 | @group | |
553 | pi | |
554 | @result{} 3 | |
555 | @end group | |
556 | @end example | |
557 | @end defspec | |
558 | ||
559 | @defun user-variable-p variable | |
560 | @cindex user option | |
f57ddf67 | 561 | This function returns @code{t} if @var{variable} is a user option---a |
e6512bcf RS |
562 | variable intended to be set by the user for customization---and |
563 | @code{nil} otherwise. (Variables other than user options exist for the | |
564 | internal purposes of Lisp programs, and users need not know about them.) | |
565 | ||
e88399c8 DL |
566 | User option variables are distinguished from other variables either |
567 | though being declared using @code{defcustom}@footnote{They may also be | |
568 | declared equivalently in @file{cus-start.el}.} or by the first character | |
569 | of their @code{variable-documentation} property. If the property exists | |
570 | and is a string, and its first character is @samp{*}, then the variable | |
571 | is a user option. | |
e6512bcf RS |
572 | @end defun |
573 | ||
113613ea | 574 | @kindex variable-interactive |
e6512bcf | 575 | If a user option variable has a @code{variable-interactive} property, |
bfe721d1 KH |
576 | the @code{set-variable} command uses that value to control reading the |
577 | new value for the variable. The property's value is used as if it were | |
8241495d RS |
578 | specified in @code{interactive} (@pxref{Using Interactive}). However, |
579 | this feature is largely obsoleted by @code{defcustom} | |
580 | (@pxref{Customization}). | |
e6512bcf | 581 | |
b22f3a19 | 582 | @strong{Warning:} If the @code{defconst} and @code{defvar} special |
6fe50867 RS |
583 | forms are used while the variable has a local binding (made with |
584 | @code{let}, or a function argument), they set the local-binding's | |
585 | value; the top-level binding is not changed. This is not what you | |
586 | usually want. To prevent it, use these special forms at top level in | |
587 | a file, where normally no local binding is in effect, and make sure to | |
588 | load the file before making a local binding for the variable. | |
e6512bcf | 589 | |
aa9b77f6 RS |
590 | @node Tips for Defining |
591 | @section Tips for Defining Variables Robustly | |
592 | ||
b50c9214 RS |
593 | When you define a variable whose value is a function, or a list of |
594 | functions, use a name that ends in @samp{-function} or | |
595 | @samp{-functions}, respectively. | |
596 | ||
597 | There are several other variable name conventions; | |
598 | here is a complete list: | |
599 | ||
600 | @table @samp | |
601 | @item @dots{}-hook | |
602 | The variable is a normal hook (@pxref{Hooks}). | |
603 | ||
604 | @item @dots{}-function | |
605 | The value is a function. | |
606 | ||
607 | @item @dots{}-functions | |
608 | The value is a list of functions. | |
609 | ||
610 | @item @dots{}-form | |
611 | The value is a form (an expression). | |
612 | ||
80561aaa | 613 | @item @dots{}-forms |
b50c9214 RS |
614 | The value is a list of forms (expressions). |
615 | ||
616 | @item @dots{}-predicate | |
617 | The value is a predicate---a function of one argument that returns | |
618 | non-@code{nil} for ``good'' arguments and @code{nil} for ``bad'' | |
619 | arguments. | |
620 | ||
621 | @item @dots{}-flag | |
622 | The value is significant only as to whether it is @code{nil} or not. | |
623 | ||
624 | @item @dots{}-program | |
625 | The value is a program name. | |
626 | ||
627 | @item @dots{}-command | |
628 | The value is a whole shell command. | |
629 | ||
630 | @item @samp{}-switches | |
631 | The value specifies options for a command. | |
632 | @end table | |
633 | ||
80561aaa | 634 | When you define a variable, always consider whether you should mark |
b50c9214 RS |
635 | it as ``risky''; see @ref{File Local Variables}. |
636 | ||
aa9b77f6 RS |
637 | When defining and initializing a variable that holds a complicated |
638 | value (such as a keymap with bindings in it), it's best to put the | |
639 | entire computation of the value into the @code{defvar}, like this: | |
640 | ||
641 | @example | |
642 | (defvar my-mode-map | |
643 | (let ((map (make-sparse-keymap))) | |
f9f59935 | 644 | (define-key map "\C-c\C-a" 'my-command) |
aa9b77f6 RS |
645 | @dots{} |
646 | map) | |
647 | @var{docstring}) | |
648 | @end example | |
649 | ||
650 | @noindent | |
651 | This method has several benefits. First, if the user quits while | |
652 | loading the file, the variable is either still uninitialized or | |
969fe9b5 | 653 | initialized properly, never in-between. If it is still uninitialized, |
aa9b77f6 RS |
654 | reloading the file will initialize it properly. Second, reloading the |
655 | file once the variable is initialized will not alter it; that is | |
656 | important if the user has run hooks to alter part of the contents (such | |
657 | as, to rebind keys). Third, evaluating the @code{defvar} form with | |
658 | @kbd{C-M-x} @emph{will} reinitialize the map completely. | |
659 | ||
660 | Putting so much code in the @code{defvar} form has one disadvantage: | |
661 | it puts the documentation string far away from the line which names the | |
662 | variable. Here's a safe way to avoid that: | |
663 | ||
664 | @example | |
665 | (defvar my-mode-map nil | |
666 | @var{docstring}) | |
8241495d | 667 | (unless my-mode-map |
aa9b77f6 | 668 | (let ((map (make-sparse-keymap))) |
b548072f | 669 | (define-key map "\C-c\C-a" 'my-command) |
aa9b77f6 RS |
670 | @dots{} |
671 | (setq my-mode-map map))) | |
672 | @end example | |
673 | ||
674 | @noindent | |
675 | This has all the same advantages as putting the initialization inside | |
676 | the @code{defvar}, except that you must type @kbd{C-M-x} twice, once on | |
677 | each form, if you do want to reinitialize the variable. | |
678 | ||
679 | But be careful not to write the code like this: | |
680 | ||
681 | @example | |
682 | (defvar my-mode-map nil | |
683 | @var{docstring}) | |
8241495d | 684 | (unless my-mode-map |
aa9b77f6 RS |
685 | (setq my-mode-map (make-sparse-keymap)) |
686 | (define-key my-mode-map "\C-c\C-a" 'my-command) | |
687 | @dots{}) | |
688 | @end example | |
689 | ||
690 | @noindent | |
969fe9b5 RS |
691 | This code sets the variable, then alters it, but it does so in more than |
692 | one step. If the user quits just after the @code{setq}, that leaves the | |
693 | variable neither correctly initialized nor void nor @code{nil}. Once | |
694 | that happens, reloading the file will not initialize the variable; it | |
695 | will remain incomplete. | |
aa9b77f6 | 696 | |
e6512bcf RS |
697 | @node Accessing Variables |
698 | @section Accessing Variable Values | |
699 | ||
700 | The usual way to reference a variable is to write the symbol which | |
701 | names it (@pxref{Symbol Forms}). This requires you to specify the | |
702 | variable name when you write the program. Usually that is exactly what | |
703 | you want to do. Occasionally you need to choose at run time which | |
704 | variable to reference; then you can use @code{symbol-value}. | |
705 | ||
706 | @defun symbol-value symbol | |
707 | This function returns the value of @var{symbol}. This is the value in | |
708 | the innermost local binding of the symbol, or its global value if it | |
709 | has no local bindings. | |
710 | ||
711 | @example | |
712 | @group | |
713 | (setq abracadabra 5) | |
714 | @result{} 5 | |
715 | @end group | |
716 | @group | |
717 | (setq foo 9) | |
718 | @result{} 9 | |
719 | @end group | |
720 | ||
721 | @group | |
722 | ;; @r{Here the symbol @code{abracadabra}} | |
723 | ;; @r{is the symbol whose value is examined.} | |
724 | (let ((abracadabra 'foo)) | |
725 | (symbol-value 'abracadabra)) | |
726 | @result{} foo | |
727 | @end group | |
728 | ||
729 | @group | |
730 | ;; @r{Here the value of @code{abracadabra},} | |
731 | ;; @r{which is @code{foo},} | |
732 | ;; @r{is the symbol whose value is examined.} | |
733 | (let ((abracadabra 'foo)) | |
734 | (symbol-value abracadabra)) | |
735 | @result{} 9 | |
736 | @end group | |
737 | ||
738 | @group | |
739 | (symbol-value 'abracadabra) | |
740 | @result{} 5 | |
741 | @end group | |
742 | @end example | |
743 | ||
969fe9b5 RS |
744 | A @code{void-variable} error is signaled if the current binding of |
745 | @var{symbol} is void. | |
e6512bcf RS |
746 | @end defun |
747 | ||
748 | @node Setting Variables | |
749 | @section How to Alter a Variable Value | |
750 | ||
751 | The usual way to change the value of a variable is with the special | |
752 | form @code{setq}. When you need to compute the choice of variable at | |
753 | run time, use the function @code{set}. | |
754 | ||
755 | @defspec setq [symbol form]@dots{} | |
756 | This special form is the most common method of changing a variable's | |
757 | value. Each @var{symbol} is given a new value, which is the result of | |
758 | evaluating the corresponding @var{form}. The most-local existing | |
759 | binding of the symbol is changed. | |
760 | ||
761 | @code{setq} does not evaluate @var{symbol}; it sets the symbol that you | |
762 | write. We say that this argument is @dfn{automatically quoted}. The | |
763 | @samp{q} in @code{setq} stands for ``quoted.'' | |
764 | ||
765 | The value of the @code{setq} form is the value of the last @var{form}. | |
766 | ||
767 | @example | |
768 | @group | |
769 | (setq x (1+ 2)) | |
770 | @result{} 3 | |
771 | @end group | |
772 | x ; @r{@code{x} now has a global value.} | |
773 | @result{} 3 | |
774 | @group | |
177c0ea7 | 775 | (let ((x 5)) |
e6512bcf RS |
776 | (setq x 6) ; @r{The local binding of @code{x} is set.} |
777 | x) | |
778 | @result{} 6 | |
779 | @end group | |
780 | x ; @r{The global value is unchanged.} | |
781 | @result{} 3 | |
782 | @end example | |
783 | ||
784 | Note that the first @var{form} is evaluated, then the first | |
785 | @var{symbol} is set, then the second @var{form} is evaluated, then the | |
786 | second @var{symbol} is set, and so on: | |
787 | ||
788 | @example | |
789 | @group | |
790 | (setq x 10 ; @r{Notice that @code{x} is set before} | |
791 | y (1+ x)) ; @r{the value of @code{y} is computed.} | |
177c0ea7 | 792 | @result{} 11 |
e6512bcf RS |
793 | @end group |
794 | @end example | |
795 | @end defspec | |
796 | ||
797 | @defun set symbol value | |
798 | This function sets @var{symbol}'s value to @var{value}, then returns | |
799 | @var{value}. Since @code{set} is a function, the expression written for | |
800 | @var{symbol} is evaluated to obtain the symbol to set. | |
801 | ||
802 | The most-local existing binding of the variable is the binding that is | |
f57ddf67 | 803 | set; shadowed bindings are not affected. |
e6512bcf RS |
804 | |
805 | @example | |
806 | @group | |
807 | (set one 1) | |
808 | @error{} Symbol's value as variable is void: one | |
809 | @end group | |
810 | @group | |
811 | (set 'one 1) | |
812 | @result{} 1 | |
813 | @end group | |
814 | @group | |
815 | (set 'two 'one) | |
816 | @result{} one | |
817 | @end group | |
818 | @group | |
819 | (set two 2) ; @r{@code{two} evaluates to symbol @code{one}.} | |
820 | @result{} 2 | |
821 | @end group | |
822 | @group | |
823 | one ; @r{So it is @code{one} that was set.} | |
824 | @result{} 2 | |
825 | (let ((one 1)) ; @r{This binding of @code{one} is set,} | |
826 | (set 'one 3) ; @r{not the global value.} | |
827 | one) | |
828 | @result{} 3 | |
829 | @end group | |
830 | @group | |
831 | one | |
832 | @result{} 2 | |
833 | @end group | |
834 | @end example | |
835 | ||
f57ddf67 RS |
836 | If @var{symbol} is not actually a symbol, a @code{wrong-type-argument} |
837 | error is signaled. | |
838 | ||
839 | @example | |
840 | (set '(x y) 'z) | |
841 | @error{} Wrong type argument: symbolp, (x y) | |
842 | @end example | |
843 | ||
e6512bcf RS |
844 | Logically speaking, @code{set} is a more fundamental primitive than |
845 | @code{setq}. Any use of @code{setq} can be trivially rewritten to use | |
846 | @code{set}; @code{setq} could even be defined as a macro, given the | |
847 | availability of @code{set}. However, @code{set} itself is rarely used; | |
f57ddf67 RS |
848 | beginners hardly need to know about it. It is useful only for choosing |
849 | at run time which variable to set. For example, the command | |
e6512bcf RS |
850 | @code{set-variable}, which reads a variable name from the user and then |
851 | sets the variable, needs to use @code{set}. | |
852 | ||
853 | @cindex CL note---@code{set} local | |
854 | @quotation | |
f57ddf67 | 855 | @b{Common Lisp note:} In Common Lisp, @code{set} always changes the |
f9f59935 RS |
856 | symbol's ``special'' or dynamic value, ignoring any lexical bindings. |
857 | In Emacs Lisp, all variables and all bindings are dynamic, so @code{set} | |
e6512bcf RS |
858 | always affects the most local existing binding. |
859 | @end quotation | |
860 | @end defun | |
861 | ||
22697dac KH |
862 | One other function for setting a variable is designed to add |
863 | an element to a list if it is not already present in the list. | |
864 | ||
f36e1798 | 865 | @defun add-to-list symbol element &optional append |
22697dac KH |
866 | This function sets the variable @var{symbol} by consing @var{element} |
867 | onto the old value, if @var{element} is not already a member of that | |
bfe721d1 KH |
868 | value. It returns the resulting list, whether updated or not. The |
869 | value of @var{symbol} had better be a list already before the call. | |
f36e1798 LT |
870 | Membership is tested using @code{equal}. |
871 | ||
872 | Normally, if @var{element} is added, it is added to the front of | |
873 | @var{symbol}, but if the optional argument @var{append} is | |
874 | non-@code{nil}, it is added at the end. | |
bfe721d1 KH |
875 | |
876 | The argument @var{symbol} is not implicitly quoted; @code{add-to-list} | |
877 | is an ordinary function, like @code{set} and unlike @code{setq}. Quote | |
878 | the argument yourself if that is what you want. | |
969fe9b5 | 879 | @end defun |
22697dac KH |
880 | |
881 | Here's a scenario showing how to use @code{add-to-list}: | |
882 | ||
883 | @example | |
884 | (setq foo '(a b)) | |
885 | @result{} (a b) | |
886 | ||
887 | (add-to-list 'foo 'c) ;; @r{Add @code{c}.} | |
888 | @result{} (c a b) | |
889 | ||
890 | (add-to-list 'foo 'b) ;; @r{No effect.} | |
891 | @result{} (c a b) | |
892 | ||
893 | foo ;; @r{@code{foo} was changed.} | |
894 | @result{} (c a b) | |
895 | @end example | |
22697dac KH |
896 | |
897 | An equivalent expression for @code{(add-to-list '@var{var} | |
898 | @var{value})} is this: | |
899 | ||
900 | @example | |
901 | (or (member @var{value} @var{var}) | |
902 | (setq @var{var} (cons @var{value} @var{var}))) | |
903 | @end example | |
904 | ||
e6512bcf RS |
905 | @node Variable Scoping |
906 | @section Scoping Rules for Variable Bindings | |
907 | ||
f9f59935 | 908 | A given symbol @code{foo} can have several local variable bindings, |
e6512bcf RS |
909 | established at different places in the Lisp program, as well as a global |
910 | binding. The most recently established binding takes precedence over | |
911 | the others. | |
912 | ||
913 | @cindex scope | |
914 | @cindex extent | |
915 | @cindex dynamic scoping | |
04b6b342 | 916 | @cindex lexical scoping |
e6512bcf RS |
917 | Local bindings in Emacs Lisp have @dfn{indefinite scope} and |
918 | @dfn{dynamic extent}. @dfn{Scope} refers to @emph{where} textually in | |
8241495d | 919 | the source code the binding can be accessed. ``Indefinite scope'' means |
e6512bcf RS |
920 | that any part of the program can potentially access the variable |
921 | binding. @dfn{Extent} refers to @emph{when}, as the program is | |
8241495d | 922 | executing, the binding exists. ``Dynamic extent'' means that the binding |
e6512bcf RS |
923 | lasts as long as the activation of the construct that established it. |
924 | ||
925 | The combination of dynamic extent and indefinite scope is called | |
926 | @dfn{dynamic scoping}. By contrast, most programming languages use | |
927 | @dfn{lexical scoping}, in which references to a local variable must be | |
928 | located textually within the function or block that binds the variable. | |
929 | ||
930 | @cindex CL note---special variables | |
931 | @quotation | |
f9f59935 RS |
932 | @b{Common Lisp note:} Variables declared ``special'' in Common Lisp are |
933 | dynamically scoped, like all variables in Emacs Lisp. | |
e6512bcf RS |
934 | @end quotation |
935 | ||
936 | @menu | |
937 | * Scope:: Scope means where in the program a value is visible. | |
938 | Comparison with other languages. | |
939 | * Extent:: Extent means how long in time a value exists. | |
940 | * Impl of Scope:: Two ways to implement dynamic scoping. | |
941 | * Using Scoping:: How to use dynamic scoping carefully and avoid problems. | |
942 | @end menu | |
943 | ||
944 | @node Scope | |
945 | @subsection Scope | |
946 | ||
947 | Emacs Lisp uses @dfn{indefinite scope} for local variable bindings. | |
948 | This means that any function anywhere in the program text might access a | |
949 | given binding of a variable. Consider the following function | |
950 | definitions: | |
951 | ||
952 | @example | |
953 | @group | |
954 | (defun binder (x) ; @r{@code{x} is bound in @code{binder}.} | |
955 | (foo 5)) ; @r{@code{foo} is some other function.} | |
956 | @end group | |
957 | ||
958 | @group | |
f9f59935 | 959 | (defun user () ; @r{@code{x} is used ``free'' in @code{user}.} |
e6512bcf RS |
960 | (list x)) |
961 | @end group | |
962 | @end example | |
963 | ||
964 | In a lexically scoped language, the binding of @code{x} in | |
965 | @code{binder} would never be accessible in @code{user}, because | |
966 | @code{user} is not textually contained within the function | |
8241495d | 967 | @code{binder}. However, in dynamically-scoped Emacs Lisp, @code{user} |
e6512bcf | 968 | may or may not refer to the binding of @code{x} established in |
8241495d | 969 | @code{binder}, depending on the circumstances: |
e6512bcf RS |
970 | |
971 | @itemize @bullet | |
972 | @item | |
973 | If we call @code{user} directly without calling @code{binder} at all, | |
974 | then whatever binding of @code{x} is found, it cannot come from | |
975 | @code{binder}. | |
976 | ||
977 | @item | |
f9f59935 | 978 | If we define @code{foo} as follows and then call @code{binder}, then the |
e6512bcf RS |
979 | binding made in @code{binder} will be seen in @code{user}: |
980 | ||
981 | @example | |
982 | @group | |
983 | (defun foo (lose) | |
984 | (user)) | |
985 | @end group | |
986 | @end example | |
987 | ||
988 | @item | |
f9f59935 RS |
989 | However, if we define @code{foo} as follows and then call @code{binder}, |
990 | then the binding made in @code{binder} @emph{will not} be seen in | |
991 | @code{user}: | |
e6512bcf RS |
992 | |
993 | @example | |
994 | (defun foo (x) | |
995 | (user)) | |
996 | @end example | |
997 | ||
998 | @noindent | |
999 | Here, when @code{foo} is called by @code{binder}, it binds @code{x}. | |
1000 | (The binding in @code{foo} is said to @dfn{shadow} the one made in | |
1001 | @code{binder}.) Therefore, @code{user} will access the @code{x} bound | |
1002 | by @code{foo} instead of the one bound by @code{binder}. | |
1003 | @end itemize | |
1004 | ||
f9f59935 RS |
1005 | Emacs Lisp uses dynamic scoping because simple implementations of |
1006 | lexical scoping are slow. In addition, every Lisp system needs to offer | |
1007 | dynamic scoping at least as an option; if lexical scoping is the norm, | |
1008 | there must be a way to specify dynamic scoping instead for a particular | |
1009 | variable. It might not be a bad thing for Emacs to offer both, but | |
1010 | implementing it with dynamic scoping only was much easier. | |
1011 | ||
e6512bcf RS |
1012 | @node Extent |
1013 | @subsection Extent | |
1014 | ||
1015 | @dfn{Extent} refers to the time during program execution that a | |
1016 | variable name is valid. In Emacs Lisp, a variable is valid only while | |
1017 | the form that bound it is executing. This is called @dfn{dynamic | |
1018 | extent}. ``Local'' or ``automatic'' variables in most languages, | |
1019 | including C and Pascal, have dynamic extent. | |
1020 | ||
1021 | One alternative to dynamic extent is @dfn{indefinite extent}. This | |
1022 | means that a variable binding can live on past the exit from the form | |
1023 | that made the binding. Common Lisp and Scheme, for example, support | |
1024 | this, but Emacs Lisp does not. | |
1025 | ||
1026 | To illustrate this, the function below, @code{make-add}, returns a | |
f9f59935 RS |
1027 | function that purports to add @var{n} to its own argument @var{m}. This |
1028 | would work in Common Lisp, but it does not do the job in Emacs Lisp, | |
1029 | because after the call to @code{make-add} exits, the variable @code{n} | |
1030 | is no longer bound to the actual argument 2. | |
e6512bcf RS |
1031 | |
1032 | @example | |
1033 | (defun make-add (n) | |
1034 | (function (lambda (m) (+ n m)))) ; @r{Return a function.} | |
1035 | @result{} make-add | |
1036 | (fset 'add2 (make-add 2)) ; @r{Define function @code{add2}} | |
1037 | ; @r{with @code{(make-add 2)}.} | |
1038 | @result{} (lambda (m) (+ n m)) | |
1039 | (add2 4) ; @r{Try to add 2 to 4.} | |
1040 | @error{} Symbol's value as variable is void: n | |
1041 | @end example | |
1042 | ||
1043 | @cindex closures not available | |
1044 | Some Lisp dialects have ``closures'', objects that are like functions | |
1045 | but record additional variable bindings. Emacs Lisp does not have | |
1046 | closures. | |
1047 | ||
1048 | @node Impl of Scope | |
1049 | @subsection Implementation of Dynamic Scoping | |
1050 | @cindex deep binding | |
1051 | ||
1052 | A simple sample implementation (which is not how Emacs Lisp actually | |
1053 | works) may help you understand dynamic binding. This technique is | |
1054 | called @dfn{deep binding} and was used in early Lisp systems. | |
1055 | ||
969fe9b5 RS |
1056 | Suppose there is a stack of bindings, which are variable-value pairs. |
1057 | At entry to a function or to a @code{let} form, we can push bindings | |
1058 | onto the stack for the arguments or local variables created there. We | |
1059 | can pop those bindings from the stack at exit from the binding | |
1060 | construct. | |
e6512bcf RS |
1061 | |
1062 | We can find the value of a variable by searching the stack from top to | |
1063 | bottom for a binding for that variable; the value from that binding is | |
1064 | the value of the variable. To set the variable, we search for the | |
1065 | current binding, then store the new value into that binding. | |
1066 | ||
1067 | As you can see, a function's bindings remain in effect as long as it | |
1068 | continues execution, even during its calls to other functions. That is | |
1069 | why we say the extent of the binding is dynamic. And any other function | |
1070 | can refer to the bindings, if it uses the same variables while the | |
1071 | bindings are in effect. That is why we say the scope is indefinite. | |
1072 | ||
1073 | @cindex shallow binding | |
1074 | The actual implementation of variable scoping in GNU Emacs Lisp uses a | |
1075 | technique called @dfn{shallow binding}. Each variable has a standard | |
1076 | place in which its current value is always found---the value cell of the | |
1077 | symbol. | |
1078 | ||
1079 | In shallow binding, setting the variable works by storing a value in | |
1080 | the value cell. Creating a new binding works by pushing the old value | |
f9f59935 RS |
1081 | (belonging to a previous binding) onto a stack, and storing the new |
1082 | local value in the value cell. Eliminating a binding works by popping | |
1083 | the old value off the stack, into the value cell. | |
e6512bcf RS |
1084 | |
1085 | We use shallow binding because it has the same results as deep | |
1086 | binding, but runs faster, since there is never a need to search for a | |
1087 | binding. | |
1088 | ||
1089 | @node Using Scoping | |
1090 | @subsection Proper Use of Dynamic Scoping | |
1091 | ||
1092 | Binding a variable in one function and using it in another is a | |
1093 | powerful technique, but if used without restraint, it can make programs | |
1094 | hard to understand. There are two clean ways to use this technique: | |
1095 | ||
1096 | @itemize @bullet | |
1097 | @item | |
1098 | Use or bind the variable only in a few related functions, written close | |
1099 | together in one file. Such a variable is used for communication within | |
1100 | one program. | |
1101 | ||
1102 | You should write comments to inform other programmers that they can see | |
1103 | all uses of the variable before them, and to advise them not to add uses | |
1104 | elsewhere. | |
1105 | ||
1106 | @item | |
1107 | Give the variable a well-defined, documented meaning, and make all | |
1108 | appropriate functions refer to it (but not bind it or set it) wherever | |
1109 | that meaning is relevant. For example, the variable | |
1110 | @code{case-fold-search} is defined as ``non-@code{nil} means ignore case | |
1111 | when searching''; various search and replace functions refer to it | |
1112 | directly or through their subroutines, but do not bind or set it. | |
1113 | ||
1114 | Then you can bind the variable in other programs, knowing reliably what | |
1115 | the effect will be. | |
1116 | @end itemize | |
1117 | ||
bfe721d1 KH |
1118 | In either case, you should define the variable with @code{defvar}. |
1119 | This helps other people understand your program by telling them to look | |
1120 | for inter-function usage. It also avoids a warning from the byte | |
1121 | compiler. Choose the variable's name to avoid name conflicts---don't | |
1122 | use short names like @code{x}. | |
1123 | ||
e6512bcf RS |
1124 | @node Buffer-Local Variables |
1125 | @section Buffer-Local Variables | |
1126 | @cindex variables, buffer-local | |
1127 | @cindex buffer-local variables | |
1128 | ||
1129 | Global and local variable bindings are found in most programming | |
8241495d | 1130 | languages in one form or another. Emacs, however, also supports additional, |
969fe9b5 | 1131 | unusual kinds of variable binding: @dfn{buffer-local} bindings, which |
8241495d | 1132 | apply only in one buffer, and @dfn{frame-local} bindings, which apply only in |
969fe9b5 RS |
1133 | one frame. Having different values for a variable in different buffers |
1134 | and/or frames is an important customization method. | |
1135 | ||
1136 | This section describes buffer-local bindings; for frame-local | |
1137 | bindings, see the following section, @ref{Frame-Local Variables}. (A few | |
1911e6e5 | 1138 | variables have bindings that are local to each terminal; see |
969fe9b5 | 1139 | @ref{Multiple Displays}.) |
e6512bcf RS |
1140 | |
1141 | @menu | |
1142 | * Intro to Buffer-Local:: Introduction and concepts. | |
1143 | * Creating Buffer-Local:: Creating and destroying buffer-local bindings. | |
1144 | * Default Value:: The default value is seen in buffers | |
969fe9b5 | 1145 | that don't have their own buffer-local values. |
e6512bcf RS |
1146 | @end menu |
1147 | ||
1148 | @node Intro to Buffer-Local | |
1149 | @subsection Introduction to Buffer-Local Variables | |
1150 | ||
1151 | A buffer-local variable has a buffer-local binding associated with a | |
1152 | particular buffer. The binding is in effect when that buffer is | |
1153 | current; otherwise, it is not in effect. If you set the variable while | |
1154 | a buffer-local binding is in effect, the new value goes in that binding, | |
f9f59935 RS |
1155 | so its other bindings are unchanged. This means that the change is |
1156 | visible only in the buffer where you made it. | |
1157 | ||
1158 | The variable's ordinary binding, which is not associated with any | |
1159 | specific buffer, is called the @dfn{default binding}. In most cases, | |
1160 | this is the global binding. | |
e6512bcf | 1161 | |
f9f59935 RS |
1162 | A variable can have buffer-local bindings in some buffers but not in |
1163 | other buffers. The default binding is shared by all the buffers that | |
969fe9b5 | 1164 | don't have their own bindings for the variable. (This includes all |
8241495d | 1165 | newly-created buffers.) If you set the variable in a buffer that does |
969fe9b5 RS |
1166 | not have a buffer-local binding for it, this sets the default binding |
1167 | (assuming there are no frame-local bindings to complicate the matter), | |
1168 | so the new value is visible in all the buffers that see the default | |
1169 | binding. | |
e6512bcf RS |
1170 | |
1171 | The most common use of buffer-local bindings is for major modes to change | |
1172 | variables that control the behavior of commands. For example, C mode and | |
1173 | Lisp mode both set the variable @code{paragraph-start} to specify that only | |
1174 | blank lines separate paragraphs. They do this by making the variable | |
1175 | buffer-local in the buffer that is being put into C mode or Lisp mode, and | |
969fe9b5 | 1176 | then setting it to the new value for that mode. @xref{Major Modes}. |
e6512bcf RS |
1177 | |
1178 | The usual way to make a buffer-local binding is with | |
969fe9b5 RS |
1179 | @code{make-local-variable}, which is what major mode commands typically |
1180 | use. This affects just the current buffer; all other buffers (including | |
1181 | those yet to be created) will continue to share the default value unless | |
1182 | they are explicitly given their own buffer-local bindings. | |
e6512bcf RS |
1183 | |
1184 | @cindex automatically buffer-local | |
1185 | A more powerful operation is to mark the variable as | |
1186 | @dfn{automatically buffer-local} by calling | |
1187 | @code{make-variable-buffer-local}. You can think of this as making the | |
1188 | variable local in all buffers, even those yet to be created. More | |
1189 | precisely, the effect is that setting the variable automatically makes | |
1190 | the variable local to the current buffer if it is not already so. All | |
f9f59935 | 1191 | buffers start out by sharing the default value of the variable as usual, |
969fe9b5 | 1192 | but setting the variable creates a buffer-local binding for the current |
e6512bcf | 1193 | buffer. The new value is stored in the buffer-local binding, leaving |
969fe9b5 RS |
1194 | the default binding untouched. This means that the default value cannot |
1195 | be changed with @code{setq} in any buffer; the only way to change it is | |
1196 | with @code{setq-default}. | |
f9f59935 | 1197 | |
beab64dc RS |
1198 | @strong{Warning:} When a variable has buffer-local or frame-local |
1199 | bindings in one or more buffers, @code{let} rebinds the binding that's | |
1200 | currently in effect. For instance, if the current buffer has a | |
1201 | buffer-local value, @code{let} temporarily rebinds that. If no | |
1202 | buffer-local or frame-local bindings are in effect, @code{let} rebinds | |
1203 | the default value. If inside the @code{let} you then change to a | |
1204 | different current buffer in which a different binding is in effect, | |
1205 | you won't see the @code{let} binding any more. And if you exit the | |
1206 | @code{let} while still in the other buffer, you won't see the | |
1207 | unbinding occur (though it will occur properly). Here is an example | |
1208 | to illustrate: | |
e6512bcf RS |
1209 | |
1210 | @example | |
1211 | @group | |
c152cd2a | 1212 | (setq foo 'g) |
e6512bcf RS |
1213 | (set-buffer "a") |
1214 | (make-local-variable 'foo) | |
1215 | @end group | |
1216 | (setq foo 'a) | |
1217 | (let ((foo 'temp)) | |
c152cd2a | 1218 | ;; foo @result{} 'temp ; @r{let binding in buffer @samp{a}} |
e6512bcf | 1219 | (set-buffer "b") |
c152cd2a | 1220 | ;; foo @result{} 'g ; @r{the global value since foo is not local in @samp{b}} |
bfe721d1 | 1221 | @var{body}@dots{}) |
e6512bcf | 1222 | @group |
beab64dc RS |
1223 | foo @result{} 'g ; @r{exiting restored the local value in buffer @samp{a},} |
1224 | ; @r{but we don't see that in buffer @samp{b}} | |
e6512bcf RS |
1225 | @end group |
1226 | @group | |
beab64dc RS |
1227 | (set-buffer "a") ; @r{verify the local value was restored} |
1228 | foo @result{} 'a | |
e6512bcf RS |
1229 | @end group |
1230 | @end example | |
1231 | ||
1232 | Note that references to @code{foo} in @var{body} access the | |
1233 | buffer-local binding of buffer @samp{b}. | |
1234 | ||
1235 | When a file specifies local variable values, these become buffer-local | |
969fe9b5 RS |
1236 | values when you visit the file. @xref{File Variables,,, emacs, The |
1237 | GNU Emacs Manual}. | |
e6512bcf RS |
1238 | |
1239 | @node Creating Buffer-Local | |
1240 | @subsection Creating and Deleting Buffer-Local Bindings | |
1241 | ||
1242 | @deffn Command make-local-variable variable | |
1243 | This function creates a buffer-local binding in the current buffer for | |
1244 | @var{variable} (a symbol). Other buffers are not affected. The value | |
1245 | returned is @var{variable}. | |
1246 | ||
1247 | @c Emacs 19 feature | |
1248 | The buffer-local value of @var{variable} starts out as the same value | |
1249 | @var{variable} previously had. If @var{variable} was void, it remains | |
1250 | void. | |
1251 | ||
1252 | @example | |
1253 | @group | |
1254 | ;; @r{In buffer @samp{b1}:} | |
1255 | (setq foo 5) ; @r{Affects all buffers.} | |
1256 | @result{} 5 | |
1257 | @end group | |
1258 | @group | |
1259 | (make-local-variable 'foo) ; @r{Now it is local in @samp{b1}.} | |
1260 | @result{} foo | |
1261 | @end group | |
1262 | @group | |
1263 | foo ; @r{That did not change} | |
1264 | @result{} 5 ; @r{the value.} | |
1265 | @end group | |
1266 | @group | |
1267 | (setq foo 6) ; @r{Change the value} | |
1268 | @result{} 6 ; @r{in @samp{b1}.} | |
1269 | @end group | |
1270 | @group | |
1271 | foo | |
1272 | @result{} 6 | |
1273 | @end group | |
1274 | ||
1275 | @group | |
1276 | ;; @r{In buffer @samp{b2}, the value hasn't changed.} | |
1277 | (save-excursion | |
1278 | (set-buffer "b2") | |
1279 | foo) | |
1280 | @result{} 5 | |
1281 | @end group | |
1282 | @end example | |
e8505179 RS |
1283 | |
1284 | Making a variable buffer-local within a @code{let}-binding for that | |
969fe9b5 RS |
1285 | variable does not work reliably, unless the buffer in which you do this |
1286 | is not current either on entry to or exit from the @code{let}. This is | |
1287 | because @code{let} does not distinguish between different kinds of | |
1288 | bindings; it knows only which variable the binding was made for. | |
22697dac | 1289 | |
bfe721d1 KH |
1290 | If the variable is terminal-local, this function signals an error. Such |
1291 | variables cannot have buffer-local bindings as well. @xref{Multiple | |
1292 | Displays}. | |
1293 | ||
6142d1d0 RS |
1294 | @strong{Warning:} do not use @code{make-local-variable} for a hook |
1295 | variable. The hook variables are automatically made buffer-local as | |
1296 | needed if you use the @var{local} argument to @code{add-hook} or | |
89cda0c5 | 1297 | @code{remove-hook}. |
e6512bcf RS |
1298 | @end deffn |
1299 | ||
1300 | @deffn Command make-variable-buffer-local variable | |
1301 | This function marks @var{variable} (a symbol) automatically | |
1302 | buffer-local, so that any subsequent attempt to set it will make it | |
1303 | local to the current buffer at the time. | |
1304 | ||
969fe9b5 RS |
1305 | A peculiar wrinkle of this feature is that binding the variable (with |
1306 | @code{let} or other binding constructs) does not create a buffer-local | |
1307 | binding for it. Only setting the variable (with @code{set} or | |
f36e1798 LT |
1308 | @code{setq}), while the variable does not have a @code{let}-style |
1309 | binding that was made in the current buffer, does so. | |
1310 | ||
1311 | If @var{variable} does not have a default value, then calling this | |
1312 | command will give it a default value of @code{nil}. If @var{variable} | |
1313 | already has a default value, that value remains unchanged. | |
1314 | Subsequently calling @code{makunbound} on @var{variable} will result | |
1315 | in a void buffer-local value and leave the default value unaffected. | |
ab4b1835 | 1316 | |
969fe9b5 | 1317 | The value returned is @var{variable}. |
ab4b1835 | 1318 | |
969fe9b5 RS |
1319 | @strong{Warning:} Don't assume that you should use |
1320 | @code{make-variable-buffer-local} for user-option variables, simply | |
1321 | because users @emph{might} want to customize them differently in | |
1322 | different buffers. Users can make any variable local, when they wish | |
1323 | to. It is better to leave the choice to them. | |
1324 | ||
1325 | The time to use @code{make-variable-buffer-local} is when it is crucial | |
1326 | that no two buffers ever share the same binding. For example, when a | |
1327 | variable is used for internal purposes in a Lisp program which depends | |
1328 | on having separate values in separate buffers, then using | |
1329 | @code{make-variable-buffer-local} can be the best solution. | |
e6512bcf RS |
1330 | @end deffn |
1331 | ||
bfe721d1 KH |
1332 | @defun local-variable-p variable &optional buffer |
1333 | This returns @code{t} if @var{variable} is buffer-local in buffer | |
1334 | @var{buffer} (which defaults to the current buffer); otherwise, | |
1335 | @code{nil}. | |
1336 | @end defun | |
1337 | ||
dbcd9cd7 RS |
1338 | @defun local-variable-if-set-p variable &optional buffer |
1339 | This returns @code{t} if @var{variable} will become buffer-local in | |
1340 | buffer @var{buffer} (which defaults to the current buffer) if it is | |
1341 | set there. | |
1342 | @end defun | |
1343 | ||
a1c07461 | 1344 | @defun buffer-local-value variable buffer |
2b99c69a RS |
1345 | This function returns the buffer-local binding of @var{variable} (a |
1346 | symbol) in buffer @var{buffer}. If @var{variable} does not have a | |
1347 | buffer-local binding in buffer @var{buffer}, it returns the default | |
1348 | value (@pxref{Default Value}) of @var{variable} instead. | |
a1c07461 RS |
1349 | @end defun |
1350 | ||
e6512bcf RS |
1351 | @defun buffer-local-variables &optional buffer |
1352 | This function returns a list describing the buffer-local variables in | |
969fe9b5 RS |
1353 | buffer @var{buffer}. (If @var{buffer} is omitted, the current buffer is |
1354 | used.) It returns an association list (@pxref{Association Lists}) in | |
1355 | which each element contains one buffer-local variable and its value. | |
1356 | However, when a variable's buffer-local binding in @var{buffer} is void, | |
1357 | then the variable appears directly in the resulting list. | |
e6512bcf RS |
1358 | |
1359 | @example | |
1360 | @group | |
1361 | (make-local-variable 'foobar) | |
1362 | (makunbound 'foobar) | |
1363 | (make-local-variable 'bind-me) | |
1364 | (setq bind-me 69) | |
1365 | @end group | |
1366 | (setq lcl (buffer-local-variables)) | |
1367 | ;; @r{First, built-in variables local in all buffers:} | |
1368 | @result{} ((mark-active . nil) | |
a9f0a989 | 1369 | (buffer-undo-list . nil) |
e6512bcf RS |
1370 | (mode-name . "Fundamental") |
1371 | @dots{} | |
1372 | @group | |
177c0ea7 | 1373 | ;; @r{Next, non-built-in buffer-local variables.} |
f9f59935 | 1374 | ;; @r{This one is buffer-local and void:} |
e6512bcf | 1375 | foobar |
f9f59935 | 1376 | ;; @r{This one is buffer-local and nonvoid:} |
e6512bcf RS |
1377 | (bind-me . 69)) |
1378 | @end group | |
1379 | @end example | |
1380 | ||
1381 | Note that storing new values into the @sc{cdr}s of cons cells in this | |
f9f59935 | 1382 | list does @emph{not} change the buffer-local values of the variables. |
e6512bcf RS |
1383 | @end defun |
1384 | ||
1385 | @deffn Command kill-local-variable variable | |
1386 | This function deletes the buffer-local binding (if any) for | |
1387 | @var{variable} (a symbol) in the current buffer. As a result, the | |
f9f59935 RS |
1388 | default binding of @var{variable} becomes visible in this buffer. This |
1389 | typically results in a change in the value of @var{variable}, since the | |
1390 | default value is usually different from the buffer-local value just | |
1391 | eliminated. | |
e6512bcf | 1392 | |
f9f59935 RS |
1393 | If you kill the buffer-local binding of a variable that automatically |
1394 | becomes buffer-local when set, this makes the default value visible in | |
1395 | the current buffer. However, if you set the variable again, that will | |
1396 | once again create a buffer-local binding for it. | |
e6512bcf RS |
1397 | |
1398 | @code{kill-local-variable} returns @var{variable}. | |
f57ddf67 RS |
1399 | |
1400 | This function is a command because it is sometimes useful to kill one | |
1401 | buffer-local variable interactively, just as it is useful to create | |
1402 | buffer-local variables interactively. | |
e6512bcf RS |
1403 | @end deffn |
1404 | ||
1405 | @defun kill-all-local-variables | |
1406 | This function eliminates all the buffer-local variable bindings of the | |
1407 | current buffer except for variables marked as ``permanent''. As a | |
1408 | result, the buffer will see the default values of most variables. | |
1409 | ||
1410 | This function also resets certain other information pertaining to the | |
1411 | buffer: it sets the local keymap to @code{nil}, the syntax table to the | |
a9f0a989 RS |
1412 | value of @code{(standard-syntax-table)}, the case table to |
1413 | @code{(standard-case-table)}, and the abbrev table to the value of | |
1414 | @code{fundamental-mode-abbrev-table}. | |
e6512bcf | 1415 | |
f9f59935 | 1416 | The very first thing this function does is run the normal hook |
969fe9b5 | 1417 | @code{change-major-mode-hook} (see below). |
f9f59935 | 1418 | |
e6512bcf RS |
1419 | Every major mode command begins by calling this function, which has the |
1420 | effect of switching to Fundamental mode and erasing most of the effects | |
1421 | of the previous major mode. To ensure that this does its job, the | |
1422 | variables that major modes set should not be marked permanent. | |
1423 | ||
1424 | @code{kill-all-local-variables} returns @code{nil}. | |
1425 | @end defun | |
1426 | ||
969fe9b5 RS |
1427 | @defvar change-major-mode-hook |
1428 | The function @code{kill-all-local-variables} runs this normal hook | |
1429 | before it does anything else. This gives major modes a way to arrange | |
1430 | for something special to be done if the user switches to a different | |
1431 | major mode. For best results, make this variable buffer-local, so that | |
1432 | it will disappear after doing its job and will not interfere with the | |
1433 | subsequent major mode. @xref{Hooks}. | |
1434 | @end defvar | |
1435 | ||
e6512bcf RS |
1436 | @c Emacs 19 feature |
1437 | @cindex permanent local variable | |
f9f59935 RS |
1438 | A buffer-local variable is @dfn{permanent} if the variable name (a |
1439 | symbol) has a @code{permanent-local} property that is non-@code{nil}. | |
1440 | Permanent locals are appropriate for data pertaining to where the file | |
1441 | came from or how to save it, rather than with how to edit the contents. | |
e6512bcf RS |
1442 | |
1443 | @node Default Value | |
1444 | @subsection The Default Value of a Buffer-Local Variable | |
1445 | @cindex default value | |
1446 | ||
1447 | The global value of a variable with buffer-local bindings is also | |
1448 | called the @dfn{default} value, because it is the value that is in | |
969fe9b5 RS |
1449 | effect whenever neither the current buffer nor the selected frame has |
1450 | its own binding for the variable. | |
e6512bcf RS |
1451 | |
1452 | The functions @code{default-value} and @code{setq-default} access and | |
1453 | change a variable's default value regardless of whether the current | |
1454 | buffer has a buffer-local binding. For example, you could use | |
1455 | @code{setq-default} to change the default setting of | |
1456 | @code{paragraph-start} for most buffers; and this would work even when | |
f57ddf67 | 1457 | you are in a C or Lisp mode buffer that has a buffer-local value for |
e6512bcf RS |
1458 | this variable. |
1459 | ||
1460 | @c Emacs 19 feature | |
1461 | The special forms @code{defvar} and @code{defconst} also set the | |
f9f59935 | 1462 | default value (if they set the variable at all), rather than any |
969fe9b5 | 1463 | buffer-local or frame-local value. |
e6512bcf RS |
1464 | |
1465 | @defun default-value symbol | |
1466 | This function returns @var{symbol}'s default value. This is the value | |
969fe9b5 RS |
1467 | that is seen in buffers and frames that do not have their own values for |
1468 | this variable. If @var{symbol} is not buffer-local, this is equivalent | |
1469 | to @code{symbol-value} (@pxref{Accessing Variables}). | |
e6512bcf RS |
1470 | @end defun |
1471 | ||
1472 | @c Emacs 19 feature | |
f57ddf67 RS |
1473 | @defun default-boundp symbol |
1474 | The function @code{default-boundp} tells you whether @var{symbol}'s | |
e6512bcf RS |
1475 | default value is nonvoid. If @code{(default-boundp 'foo)} returns |
1476 | @code{nil}, then @code{(default-value 'foo)} would get an error. | |
1477 | ||
1478 | @code{default-boundp} is to @code{default-value} as @code{boundp} is to | |
1479 | @code{symbol-value}. | |
1480 | @end defun | |
1481 | ||
a9f0a989 RS |
1482 | @defspec setq-default [symbol form]@dots{} |
1483 | This special form gives each @var{symbol} a new default value, which is | |
1484 | the result of evaluating the corresponding @var{form}. It does not | |
1485 | evaluate @var{symbol}, but does evaluate @var{form}. The value of the | |
1486 | @code{setq-default} form is the value of the last @var{form}. | |
e6512bcf RS |
1487 | |
1488 | If a @var{symbol} is not buffer-local for the current buffer, and is not | |
1489 | marked automatically buffer-local, @code{setq-default} has the same | |
1490 | effect as @code{setq}. If @var{symbol} is buffer-local for the current | |
1491 | buffer, then this changes the value that other buffers will see (as long | |
1492 | as they don't have a buffer-local value), but not the value that the | |
1493 | current buffer sees. | |
1494 | ||
1495 | @example | |
1496 | @group | |
1497 | ;; @r{In buffer @samp{foo}:} | |
969fe9b5 RS |
1498 | (make-local-variable 'buffer-local) |
1499 | @result{} buffer-local | |
e6512bcf RS |
1500 | @end group |
1501 | @group | |
969fe9b5 | 1502 | (setq buffer-local 'value-in-foo) |
e6512bcf RS |
1503 | @result{} value-in-foo |
1504 | @end group | |
1505 | @group | |
969fe9b5 | 1506 | (setq-default buffer-local 'new-default) |
e6512bcf RS |
1507 | @result{} new-default |
1508 | @end group | |
1509 | @group | |
969fe9b5 | 1510 | buffer-local |
e6512bcf RS |
1511 | @result{} value-in-foo |
1512 | @end group | |
1513 | @group | |
969fe9b5 | 1514 | (default-value 'buffer-local) |
e6512bcf RS |
1515 | @result{} new-default |
1516 | @end group | |
1517 | ||
1518 | @group | |
1519 | ;; @r{In (the new) buffer @samp{bar}:} | |
969fe9b5 | 1520 | buffer-local |
e6512bcf RS |
1521 | @result{} new-default |
1522 | @end group | |
1523 | @group | |
969fe9b5 | 1524 | (default-value 'buffer-local) |
e6512bcf RS |
1525 | @result{} new-default |
1526 | @end group | |
1527 | @group | |
969fe9b5 | 1528 | (setq buffer-local 'another-default) |
e6512bcf RS |
1529 | @result{} another-default |
1530 | @end group | |
1531 | @group | |
969fe9b5 | 1532 | (default-value 'buffer-local) |
e6512bcf RS |
1533 | @result{} another-default |
1534 | @end group | |
1535 | ||
1536 | @group | |
1537 | ;; @r{Back in buffer @samp{foo}:} | |
969fe9b5 | 1538 | buffer-local |
e6512bcf | 1539 | @result{} value-in-foo |
969fe9b5 | 1540 | (default-value 'buffer-local) |
e6512bcf RS |
1541 | @result{} another-default |
1542 | @end group | |
1543 | @end example | |
1544 | @end defspec | |
1545 | ||
1546 | @defun set-default symbol value | |
1547 | This function is like @code{setq-default}, except that @var{symbol} is | |
f9f59935 | 1548 | an ordinary evaluated argument. |
e6512bcf RS |
1549 | |
1550 | @example | |
1551 | @group | |
1552 | (set-default (car '(a b c)) 23) | |
1553 | @result{} 23 | |
1554 | @end group | |
1555 | @group | |
1556 | (default-value 'a) | |
1557 | @result{} 23 | |
1558 | @end group | |
1559 | @end example | |
1560 | @end defun | |
969fe9b5 RS |
1561 | |
1562 | @node Frame-Local Variables | |
1563 | @section Frame-Local Variables | |
1564 | ||
1565 | Just as variables can have buffer-local bindings, they can also have | |
1566 | frame-local bindings. These bindings belong to one frame, and are in | |
1567 | effect when that frame is selected. Frame-local bindings are actually | |
1568 | frame parameters: you create a frame-local binding in a specific frame | |
1569 | by calling @code{modify-frame-parameters} and specifying the variable | |
1570 | name as the parameter name. | |
1571 | ||
1572 | To enable frame-local bindings for a certain variable, call the function | |
1573 | @code{make-variable-frame-local}. | |
1574 | ||
a9f0a989 | 1575 | @deffn Command make-variable-frame-local variable |
969fe9b5 RS |
1576 | Enable the use of frame-local bindings for @var{variable}. This does |
1577 | not in itself create any frame-local bindings for the variable; however, | |
1578 | if some frame already has a value for @var{variable} as a frame | |
1579 | parameter, that value automatically becomes a frame-local binding. | |
1580 | ||
f36e1798 LT |
1581 | If @var{variable} does not have a default value, then calling this |
1582 | command will give it a default value of @code{nil}. If @var{variable} | |
1583 | already has a default value, that value remains unchanged. | |
1584 | ||
a9f0a989 RS |
1585 | If the variable is terminal-local, this function signals an error, |
1586 | because such variables cannot have frame-local bindings as well. | |
1587 | @xref{Multiple Displays}. A few variables that are implemented | |
1588 | specially in Emacs can be (and usually are) buffer-local, but can never | |
1589 | be frame-local. | |
f36e1798 LT |
1590 | |
1591 | This command returns @var{variable}. | |
a9f0a989 | 1592 | @end deffn |
969fe9b5 RS |
1593 | |
1594 | Buffer-local bindings take precedence over frame-local bindings. Thus, | |
1595 | consider a variable @code{foo}: if the current buffer has a buffer-local | |
1596 | binding for @code{foo}, that binding is active; otherwise, if the | |
1597 | selected frame has a frame-local binding for @code{foo}, that binding is | |
1598 | active; otherwise, the default binding of @code{foo} is active. | |
1599 | ||
1600 | Here is an example. First we prepare a few bindings for @code{foo}: | |
1601 | ||
1602 | @example | |
1603 | (setq f1 (selected-frame)) | |
1604 | (make-variable-frame-local 'foo) | |
1605 | ||
1606 | ;; @r{Make a buffer-local binding for @code{foo} in @samp{b1}.} | |
1607 | (set-buffer (get-buffer-create "b1")) | |
1608 | (make-local-variable 'foo) | |
1609 | (setq foo '(b 1)) | |
1610 | ||
1611 | ;; @r{Make a frame-local binding for @code{foo} in a new frame.} | |
1612 | ;; @r{Store that frame in @code{f2}.} | |
1613 | (setq f2 (make-frame)) | |
1614 | (modify-frame-parameters f2 '((foo . (f 2)))) | |
1615 | @end example | |
1616 | ||
1617 | Now we examine @code{foo} in various contexts. Whenever the | |
1618 | buffer @samp{b1} is current, its buffer-local binding is in effect, | |
1619 | regardless of the selected frame: | |
1620 | ||
1621 | @example | |
1622 | (select-frame f1) | |
1623 | (set-buffer (get-buffer-create "b1")) | |
1624 | foo | |
1625 | @result{} (b 1) | |
1626 | ||
1627 | (select-frame f2) | |
1628 | (set-buffer (get-buffer-create "b1")) | |
1629 | foo | |
1630 | @result{} (b 1) | |
1631 | @end example | |
1632 | ||
1633 | @noindent | |
1634 | Otherwise, the frame gets a chance to provide the binding; when frame | |
1635 | @code{f2} is selected, its frame-local binding is in effect: | |
1636 | ||
1637 | @example | |
1638 | (select-frame f2) | |
1639 | (set-buffer (get-buffer "*scratch*")) | |
1640 | foo | |
1641 | @result{} (f 2) | |
1642 | @end example | |
1643 | ||
1644 | @noindent | |
1645 | When neither the current buffer nor the selected frame provides | |
1646 | a binding, the default binding is used: | |
1647 | ||
1648 | @example | |
1649 | (select-frame f1) | |
1650 | (set-buffer (get-buffer "*scratch*")) | |
1651 | foo | |
1652 | @result{} nil | |
1653 | @end example | |
1654 | ||
1655 | @noindent | |
1656 | When the active binding of a variable is a frame-local binding, setting | |
1657 | the variable changes that binding. You can observe the result with | |
1658 | @code{frame-parameters}: | |
1659 | ||
1660 | @example | |
1661 | (select-frame f2) | |
1662 | (set-buffer (get-buffer "*scratch*")) | |
1663 | (setq foo 'nobody) | |
1664 | (assq 'foo (frame-parameters f2)) | |
1665 | @result{} (foo . nobody) | |
1666 | @end example | |
1667 | ||
1668 | @node Future Local Variables | |
1669 | @section Possible Future Local Variables | |
1670 | ||
1671 | We have considered the idea of bindings that are local to a category | |
1672 | of frames---for example, all color frames, or all frames with dark | |
1673 | backgrounds. We have not implemented them because it is not clear that | |
1674 | this feature is really useful. You can get more or less the same | |
916a89db | 1675 | results by adding a function to @code{after-make-frame-functions}, set up to |
969fe9b5 RS |
1676 | define a particular frame parameter according to the appropriate |
1677 | conditions for each frame. | |
1678 | ||
1679 | It would also be possible to implement window-local bindings. We | |
1680 | don't know of many situations where they would be useful, and it seems | |
1681 | that indirect buffers (@pxref{Indirect Buffers}) with buffer-local | |
1682 | bindings offer a way to handle these situations more robustly. | |
1683 | ||
1684 | If sufficient application is found for either of these two kinds of | |
1685 | local bindings, we will provide it in a subsequent Emacs version. | |
1686 | ||
26236f6d GM |
1687 | @node Variable Aliases |
1688 | @section Variable Aliases | |
1689 | ||
1690 | It is sometimes useful to make two variables synonyms, so that both | |
1691 | variables always have the same value, and changing either one also | |
1692 | changes the other. Whenever you change the name of a | |
1693 | variable---either because you realize its old name was not well | |
1694 | chosen, or because its meaning has partly changed---it can be useful | |
1695 | to keep the old name as an @emph{alias} of the new one for | |
1696 | compatibility. You can do this with @code{defvaralias}. | |
1697 | ||
a1c07461 | 1698 | @defun defvaralias alias-var base-var &optional docstring |
26236f6d | 1699 | This function defines the symbol @var{alias-var} as a variable alias |
32ebbc3a | 1700 | for symbol @var{base-var}. This means that retrieving the value of |
26236f6d GM |
1701 | @var{alias-var} returns the value of @var{base-var}, and changing the |
1702 | value of @var{alias-var} changes the value of @var{base-var}. | |
32ebbc3a | 1703 | |
a1c07461 RS |
1704 | If the @var{docstring} argument is non-@code{nil}, it specifies the |
1705 | documentation for @var{alias-var}; otherwise, the alias gets the same | |
f36e1798 LT |
1706 | documentation as @var{base-var} has, if any, unless @var{base-var} is |
1707 | itself an alias, in which case @var{alias-var} gets the documentation | |
1708 | of the variable at the end of the chain of aliases. | |
1709 | ||
1710 | This function returns @var{base-var}. | |
3fdb4c50 | 1711 | @end defun |
26236f6d GM |
1712 | |
1713 | @defun indirect-variable variable | |
1714 | This function returns the variable at the end of the chain of aliases | |
1715 | of @var{variable}. If @var{variable} is not a symbol, or if @var{variable} is | |
1716 | not defined as an alias, the function returns @var{variable}. | |
571fd4d5 JH |
1717 | |
1718 | This function signals a @code{cyclic-variable-indirection} error if | |
1719 | there is a loop in the chain of symbols. | |
26236f6d GM |
1720 | @end defun |
1721 | ||
1722 | @example | |
1723 | (defvaralias 'foo 'bar) | |
1724 | (indirect-variable 'foo) | |
1725 | @result{} bar | |
1726 | (indirect-variable 'bar) | |
1727 | @result{} bar | |
1728 | (setq bar 2) | |
1729 | bar | |
1730 | @result{} 2 | |
f36e1798 | 1731 | @group |
26236f6d GM |
1732 | foo |
1733 | @result{} 2 | |
f36e1798 | 1734 | @end group |
26236f6d GM |
1735 | (setq foo 0) |
1736 | bar | |
1737 | @result{} 0 | |
1738 | foo | |
1739 | @result{} 0 | |
1740 | @end example | |
1741 | ||
b50c9214 RS |
1742 | @node File Local Variables |
1743 | @section File Local Variables | |
1744 | ||
1745 | This section describes the functions and variables that affect | |
f36e1798 LT |
1746 | processing of local variables lists in files. @xref{File variables, , |
1747 | Local Variables in Files, emacs, The GNU Emacs Manual}, for basic | |
1748 | information about file local variables. | |
b50c9214 RS |
1749 | |
1750 | @defopt enable-local-variables | |
1751 | This variable controls whether to process file local variables lists. A | |
1752 | value of @code{t} means process the local variables lists | |
1753 | unconditionally; @code{nil} means ignore them; anything else means ask | |
1754 | the user what to do for each file. The default value is @code{t}. | |
1755 | @end defopt | |
1756 | ||
f36e1798 | 1757 | @defun hack-local-variables &optional mode-only |
b50c9214 RS |
1758 | This function parses, and binds or evaluates as appropriate, any local |
1759 | variables specified by the contents of the current buffer. The variable | |
f36e1798 LT |
1760 | @code{enable-local-variables} has its effect here. However, this |
1761 | function does not look for the @samp{mode:} local variable in the | |
1762 | @w{@samp{-*-}} line. @code{set-auto-mode} does that, also taking | |
1763 | @code{enable-local-variables} into account. | |
1764 | ||
1765 | If the optional argument @var{mode-only} is non-@code{nil}, then all | |
1766 | this function does is return @code{t} if the @w{@samp{-*-}} line | |
1767 | specifies a mode and @code{nil} otherwise. It does not set the mode | |
1768 | nor any other file local variable. It does not check whether a mode | |
1769 | is specified in the local variables list at the end of the file. | |
b50c9214 RS |
1770 | @end defun |
1771 | ||
24ff45f3 | 1772 | If a file local variable list could specify a function that will |
b50c9214 RS |
1773 | be called later, or an expression that will be executed later, simply |
1774 | visiting a file could take over your Emacs. To prevent this, Emacs | |
1775 | takes care not to allow local variable lists to set such variables. | |
1776 | ||
7ed9159a | 1777 | For one thing, any variable whose name ends in @samp{-command}, |
f5d2538f | 1778 | @samp{-frame-alist}, @samp{-function}, @samp{-functions}, |
7ed9159a JY |
1779 | @samp{-hook}, @samp{-hooks}, @samp{-form}, @samp{-forms}, @samp{-map}, |
1780 | @samp{-map-alist}, @samp{-mode-alist}, @samp{-program}, or | |
1781 | @samp{-predicate} cannot be set in a local variable list. In general, | |
1782 | you should use such a name whenever it is appropriate for the | |
1783 | variable's meaning. The variables @samp{font-lock-keywords}, | |
1784 | @samp{font-lock-keywords-[0-9]}, and | |
1785 | @samp{font-lock-syntactic-keywords} cannot be set in a local variable | |
f36e1798 LT |
1786 | list, either. These rules can be overridden by giving the variable's |
1787 | name a non-@code{nil} @code{safe-local-variable} property. If one | |
1788 | gives it a @code{safe-local-variable} property of @code{t}, then one | |
1789 | can give the variable any file local value. One can also give any | |
1790 | symbol, including the above, a @code{safe-local-variable} property | |
1791 | that is a function taking exactly one argument. In that case, giving | |
1792 | a variable with that name a file local value is only allowed if the | |
1793 | function returns non-@code{nil} when called with that value as | |
1794 | argument. | |
b50c9214 RS |
1795 | |
1796 | In addition, any variable whose name has a non-@code{nil} | |
7ed9159a JY |
1797 | @code{risky-local-variable} property is also ignored. So are all |
1798 | variables listed in @code{ignored-local-variables}: | |
b50c9214 RS |
1799 | |
1800 | @defvar ignored-local-variables | |
1801 | This variable holds a list of variables that should not be | |
1802 | set by a file's local variables list. Any value specified | |
1803 | for one of these variables is ignored. | |
1804 | @end defvar | |
1805 | ||
f36e1798 LT |
1806 | @defun risky-local-variable-p sym &optional val |
1807 | If @var{val} is non-@code{nil}, returns non-@code{nil} if giving | |
1808 | @var{sym} a file local value of @var{val} would be risky, for any of | |
1809 | the reasons stated above. If @var{val} is @code{nil} or omitted, only | |
1810 | returns @code{nil} if @var{sym} can be safely assigned any file local | |
1811 | value whatsoever. | |
7ed9159a JY |
1812 | @end defun |
1813 | ||
b50c9214 RS |
1814 | The @samp{Eval:} ``variable'' is also a potential loophole, so Emacs |
1815 | normally asks for confirmation before handling it. | |
969fe9b5 | 1816 | |
b50c9214 RS |
1817 | @defopt enable-local-eval |
1818 | This variable controls processing of @samp{Eval:} in local variables | |
1819 | lists in files being visited. A value of @code{t} means process them | |
1820 | unconditionally; @code{nil} means ignore them; anything else means ask | |
1821 | the user what to do for each file. The default value is @code{maybe}. | |
1822 | @end defopt | |
ab5796a9 | 1823 | |
2b99c69a RS |
1824 | Text properties are also potential loopholes, since their values |
1825 | could include functions to call. So Emacs discards all text | |
1826 | properties from string values specified in a file's local variables | |
1827 | list. | |
1828 | ||
74ab3aa3 LT |
1829 | @node Variables with Restricted Values |
1830 | @section Variables with Restricted Values | |
1831 | ||
1832 | Ordinary Lisp variables can be assigned any value that is a valid | |
1833 | Lisp object. However, certain Lisp variables are not defined in Lisp, | |
1834 | but in C. Most of these variables are defined in the C code using | |
1835 | @code{DEFVAR_LISP}. Like variables defined in Lisp, these can take on | |
1836 | any value. However, some variables are defined using | |
1837 | @code{DEFVAR_INT} or @code{DEFVAR_BOOL}. @xref{Defining Lisp | |
1838 | variables in C,, Writing Emacs Primitives}, in particular the | |
1839 | description of functions of the type @code{syms_of_@var{filename}}, | |
1840 | for a brief discussion of the C implementation. | |
1841 | ||
1842 | Variables of type @code{DEFVAR_BOOL} can only take on the values | |
1843 | @code{nil} or @code{t}. Attempting to assign them any other value | |
1844 | will set them to @code{t}: | |
1845 | ||
1846 | @example | |
1847 | (let ((display-hourglass 5)) | |
1848 | display-hourglass) | |
1849 | @result{} t | |
1850 | @end example | |
1851 | ||
1852 | @defvar byte-boolean-vars | |
1853 | This variable holds a list of all variables of type @code{DEFVAR_BOOL}. | |
1854 | @end defvar | |
1855 | ||
1856 | Variables of type @code{DEFVAR_INT} can only take on integer values. | |
1857 | Attempting to assign them any other value will result in an error: | |
1858 | ||
1859 | @example | |
1860 | (setq window-min-height 5.0) | |
1861 | @error{} Wrong type argument: integerp, 5.0 | |
1862 | @end example | |
1863 | ||
ab5796a9 MB |
1864 | @ignore |
1865 | arch-tag: 5ff62c44-2b51-47bb-99d4-fea5aeec5d3e | |
1866 | @end ignore |