2 @c This is part of the GNU Emacs Lisp Reference Manual.
3 @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998 Free Software Foundation, Inc.
4 @c See the file elisp.texi for copying conditions.
5 @setfilename ../info/variables
6 @node Variables, Functions, Control Structures, Top
10 A @dfn{variable} is a name used in a program to stand for a value.
11 Nearly all programming languages have variables of some sort. In the
12 text of a Lisp program, variables are written using the syntax for
15 In Lisp, unlike most programming languages, programs are represented
16 primarily as Lisp objects and only secondarily as text. The Lisp
17 objects used for variables are symbols: the symbol name is the variable
18 name, and the variable's value is stored in the value cell of the
19 symbol. The use of a symbol as a variable is independent of its use as
20 a function name. @xref{Symbol Components}.
22 The Lisp objects that constitute a Lisp program determine the textual
23 form of the program---it is simply the read syntax for those Lisp
24 objects. This is why, for example, a variable in a textual Lisp program
25 is written using the read syntax for the symbol that represents the
29 * Global Variables:: Variable values that exist permanently, everywhere.
30 * Constant Variables:: Certain "variables" have values that never change.
31 * Local Variables:: Variable values that exist only temporarily.
32 * Void Variables:: Symbols that lack values.
33 * Defining Variables:: A definition says a symbol is used as a variable.
34 * Tips for Defining:: How to avoid bad results from quitting
35 within the code to initialize a variable.
36 * Accessing Variables:: Examining values of variables whose names
37 are known only at run time.
38 * Setting Variables:: Storing new values in variables.
39 * Variable Scoping:: How Lisp chooses among local and global values.
40 * Buffer-Local Variables:: Variable values in effect only in one buffer.
41 * Frame-Local Variables:: Variable values in effect only in one frame.
42 * Future Local Variables:: New kinds of local values we might add some day.
45 @node Global Variables
46 @section Global Variables
47 @cindex global variable
49 The simplest way to use a variable is @dfn{globally}. This means that
50 the variable has just one value at a time, and this value is in effect
51 (at least for the moment) throughout the Lisp system. The value remains
52 in effect until you specify a new one. When a new value replaces the
53 old one, no trace of the old value remains in the variable.
55 You specify a value for a symbol with @code{setq}. For example,
62 gives the variable @code{x} the value @code{(a b)}. Note that
63 @code{setq} does not evaluate its first argument, the name of the
64 variable, but it does evaluate the second argument, the new value.
66 Once the variable has a value, you can refer to it by using the symbol
67 by itself as an expression. Thus,
76 assuming the @code{setq} form shown above has already been executed.
78 If you do set the same variable again, the new value replaces the old
96 @node Constant Variables
97 @section Variables that Never Change
100 @kindex setting-constant
101 @cindex keyword symbol
103 In Emacs Lisp, certain symbols normally evaluate to themselves. These
104 include @code{nil} and @code{t}, as well as any symbol whose name starts
105 with @samp{:} (these are called @dfn{keywords}). These symbols cannot
106 be rebound, nor can their values be changed. Any attempt to set or bind
107 @code{nil} or @code{t} signals a @code{setting-constant} error. The
108 same is true for a keyword (a symbol whose name starts with @samp{:}),
109 if it is interned in the standard obarray, except that setting such a
110 symbol to itself is not an error.
119 @error{} Attempt to set constant symbol: nil
123 @defvar keyword-symbols-constant-flag
124 @tindex keyword-symbols-constant-flag
125 If this variable is @code{nil}, you are allowed to set and bind symbols
126 whose names start with @samp{:} however you wish. This is to make it
127 possible to run old Lisp programs which do that.
130 @node Local Variables
131 @section Local Variables
132 @cindex binding local variables
133 @cindex local variables
134 @cindex local binding
135 @cindex global binding
137 Global variables have values that last until explicitly superseded
138 with new values. Sometimes it is useful to create variable values that
139 exist temporarily---only until a certain part of the program finishes.
140 These values are called @dfn{local}, and the variables so used are
141 called @dfn{local variables}.
143 For example, when a function is called, its argument variables receive
144 new local values that last until the function exits. The @code{let}
145 special form explicitly establishes new local values for specified
146 variables; these last until exit from the @code{let} form.
148 @cindex shadowing of variables
149 Establishing a local value saves away the previous value (or lack of
150 one) of the variable. When the life span of the local value is over,
151 the previous value is restored. In the mean time, we say that the
152 previous value is @dfn{shadowed} and @dfn{not visible}. Both global and
153 local values may be shadowed (@pxref{Scope}).
155 If you set a variable (such as with @code{setq}) while it is local,
156 this replaces the local value; it does not alter the global value, or
157 previous local values, that are shadowed. To model this behavior, we
158 speak of a @dfn{local binding} of the variable as well as a local value.
160 The local binding is a conceptual place that holds a local value.
161 Entry to a function, or a special form such as @code{let}, creates the
162 local binding; exit from the function or from the @code{let} removes the
163 local binding. As long as the local binding lasts, the variable's value
164 is stored within it. Use of @code{setq} or @code{set} while there is a
165 local binding stores a different value into the local binding; it does
166 not create a new binding.
168 We also speak of the @dfn{global binding}, which is where
169 (conceptually) the global value is kept.
171 @cindex current binding
172 A variable can have more than one local binding at a time (for
173 example, if there are nested @code{let} forms that bind it). In such a
174 case, the most recently created local binding that still exists is the
175 @dfn{current binding} of the variable. (This rule is called
176 @dfn{dynamic scoping}; see @ref{Variable Scoping}.) If there are no
177 local bindings, the variable's global binding is its current binding.
178 We sometimes call the current binding the @dfn{most-local existing
179 binding}, for emphasis. Ordinary evaluation of a symbol always returns
180 the value of its current binding.
182 The special forms @code{let} and @code{let*} exist to create
185 @defspec let (bindings@dots{}) forms@dots{}
186 This special form binds variables according to @var{bindings} and then
187 evaluates all of the @var{forms} in textual order. The @code{let}-form
188 returns the value of the last form in @var{forms}.
190 Each of the @var{bindings} is either @w{(i) a} symbol, in which case
191 that symbol is bound to @code{nil}; or @w{(ii) a} list of the form
192 @code{(@var{symbol} @var{value-form})}, in which case @var{symbol} is
193 bound to the result of evaluating @var{value-form}. If @var{value-form}
194 is omitted, @code{nil} is used.
196 All of the @var{value-form}s in @var{bindings} are evaluated in the
197 order they appear and @emph{before} binding any of the symbols to them.
198 Here is an example of this: @code{Z} is bound to the old value of
199 @code{Y}, which is 2, not the new value of @code{Y}, which is 1.
215 @defspec let* (bindings@dots{}) forms@dots{}
216 This special form is like @code{let}, but it binds each variable right
217 after computing its local value, before computing the local value for
218 the next variable. Therefore, an expression in @var{bindings} can
219 reasonably refer to the preceding symbols bound in this @code{let*}
220 form. Compare the following example with the example above for
230 (Z Y)) ; @r{Use the just-established value of @code{Y}.}
237 Here is a complete list of the other facilities that create local
242 Function calls (@pxref{Functions}).
245 Macro calls (@pxref{Macros}).
248 @code{condition-case} (@pxref{Errors}).
251 Variables can also have buffer-local bindings (@pxref{Buffer-Local
252 Variables}) and frame-local bindings (@pxref{Frame-Local Variables}); a
253 few variables have terminal-local bindings (@pxref{Multiple Displays}).
254 These kinds of bindings work somewhat like ordinary local bindings, but
255 they are localized depending on ``where'' you are in Emacs, rather than
258 @defvar max-specpdl-size
259 @cindex variable limit error
260 @cindex evaluation error
261 @cindex infinite recursion
262 This variable defines the limit on the total number of local variable
263 bindings and @code{unwind-protect} cleanups (@pxref{Nonlocal Exits})
264 that are allowed before signaling an error (with data @code{"Variable
265 binding depth exceeds max-specpdl-size"}).
267 This limit, with the associated error when it is exceeded, is one way
268 that Lisp avoids infinite recursion on an ill-defined function.
269 @code{max-lisp-eval-depth} provides another limit on depth of nesting.
272 The default value is 600. Entry to the Lisp debugger increases the
273 value, if there is little room left, to make sure the debugger itself
278 @section When a Variable is ``Void''
279 @kindex void-variable
280 @cindex void variable
282 If you have never given a symbol any value as a global variable, we
283 say that that symbol's global value is @dfn{void}. In other words, the
284 symbol's value cell does not have any Lisp object in it. If you try to
285 evaluate the symbol, you get a @code{void-variable} error rather than
288 Note that a value of @code{nil} is not the same as void. The symbol
289 @code{nil} is a Lisp object and can be the value of a variable just as any
290 other object can be; but it is @emph{a value}. A void variable does not
293 After you have given a variable a value, you can make it void once more
294 using @code{makunbound}.
296 @defun makunbound symbol
297 This function makes the current variable binding of @var{symbol} void.
298 Subsequent attempts to use this symbol's value as a variable will signal
299 the error @code{void-variable}, unless and until you set it again.
301 @code{makunbound} returns @var{symbol}.
305 (makunbound 'x) ; @r{Make the global value of @code{x} void.}
310 @error{} Symbol's value as variable is void: x
314 If @var{symbol} is locally bound, @code{makunbound} affects the most
315 local existing binding. This is the only way a symbol can have a void
316 local binding, since all the constructs that create local bindings
317 create them with values. In this case, the voidness lasts at most as
318 long as the binding does; when the binding is removed due to exit from
319 the construct that made it, the previous local or global binding is
320 reexposed as usual, and the variable is no longer void unless the newly
321 reexposed binding was void all along.
325 (setq x 1) ; @r{Put a value in the global binding.}
327 (let ((x 2)) ; @r{Locally bind it.}
328 (makunbound 'x) ; @r{Void the local binding.}
330 @error{} Symbol's value as variable is void: x
333 x ; @r{The global binding is unchanged.}
336 (let ((x 2)) ; @r{Locally bind it.}
337 (let ((x 3)) ; @r{And again.}
338 (makunbound 'x) ; @r{Void the innermost-local binding.}
339 x)) ; @r{And refer: it's void.}
340 @error{} Symbol's value as variable is void: x
346 (makunbound 'x)) ; @r{Void inner binding, then remove it.}
347 x) ; @r{Now outer @code{let} binding is visible.}
353 A variable that has been made void with @code{makunbound} is
354 indistinguishable from one that has never received a value and has
357 You can use the function @code{boundp} to test whether a variable is
360 @defun boundp variable
361 @code{boundp} returns @code{t} if @var{variable} (a symbol) is not void;
362 more precisely, if its current binding is not void. It returns
363 @code{nil} otherwise.
367 (boundp 'abracadabra) ; @r{Starts out void.}
371 (let ((abracadabra 5)) ; @r{Locally bind it.}
372 (boundp 'abracadabra))
376 (boundp 'abracadabra) ; @r{Still globally void.}
380 (setq abracadabra 5) ; @r{Make it globally nonvoid.}
384 (boundp 'abracadabra)
390 @node Defining Variables
391 @section Defining Global Variables
392 @cindex variable definition
394 You may announce your intention to use a symbol as a global variable
395 with a @dfn{variable definition}: a special form, either @code{defconst}
398 In Emacs Lisp, definitions serve three purposes. First, they inform
399 people who read the code that certain symbols are @emph{intended} to be
400 used a certain way (as variables). Second, they inform the Lisp system
401 of these things, supplying a value and documentation. Third, they
402 provide information to utilities such as @code{etags} and
403 @code{make-docfile}, which create data bases of the functions and
404 variables in a program.
406 The difference between @code{defconst} and @code{defvar} is primarily
407 a matter of intent, serving to inform human readers of whether the value
408 should ever change. Emacs Lisp does not restrict the ways in which a
409 variable can be used based on @code{defconst} or @code{defvar}
410 declarations. However, it does make a difference for initialization:
411 @code{defconst} unconditionally initializes the variable, while
412 @code{defvar} initializes it only if it is void.
415 One would expect user option variables to be defined with
416 @code{defconst}, since programs do not change them. Unfortunately, this
417 has bad results if the definition is in a library that is not preloaded:
418 @code{defconst} would override any prior value when the library is
419 loaded. Users would like to be able to set user options in their init
420 files, and override the default values given in the definitions. For
421 this reason, user options must be defined with @code{defvar}.
424 @defspec defvar symbol [value [doc-string]]
425 This special form defines @var{symbol} as a variable and can also
426 initialize and document it. The definition informs a person reading
427 your code that @var{symbol} is used as a variable that might be set or
428 changed. Note that @var{symbol} is not evaluated; the symbol to be
429 defined must appear explicitly in the @code{defvar}.
431 If @var{symbol} is void and @var{value} is specified, @code{defvar}
432 evaluates it and sets @var{symbol} to the result. But if @var{symbol}
433 already has a value (i.e., it is not void), @var{value} is not even
434 evaluated, and @var{symbol}'s value remains unchanged. If @var{value}
435 is omitted, the value of @var{symbol} is not changed in any case.
437 If @var{symbol} has a buffer-local binding in the current buffer,
438 @code{defvar} operates on the default value, which is buffer-independent,
439 not the current (buffer-local) binding. It sets the default value if
440 the default value is void. @xref{Buffer-Local Variables}.
442 When you evaluate a top-level @code{defvar} form with @kbd{C-M-x} in
443 Emacs Lisp mode (@code{eval-defun}), a special feature of
444 @code{eval-defun} arranges to set the variable unconditionally, without
445 testing whether its value is void.
447 If the @var{doc-string} argument appears, it specifies the documentation
448 for the variable. (This opportunity to specify documentation is one of
449 the main benefits of defining the variable.) The documentation is
450 stored in the symbol's @code{variable-documentation} property. The
451 Emacs help functions (@pxref{Documentation}) look for this property.
453 If the first character of @var{doc-string} is @samp{*}, it means that
454 this variable is considered a user option. This lets users set the
455 variable conveniently using the commands @code{set-variable} and
456 @code{edit-options}. However, it is better to use @code{defcustom}
457 instead of @code{defvar} for user option variables, so you can specify
458 customization information. @xref{Customization}.
460 Here are some examples. This form defines @code{foo} but does not
470 This example initializes the value of @code{bar} to @code{23}, and gives
471 it a documentation string:
476 "The normal weight of a bar.")
481 The following form changes the documentation string for @code{bar},
482 making it a user option, but does not change the value, since @code{bar}
483 already has a value. (The addition @code{(1+ nil)} would get an error
484 if it were evaluated, but since it is not evaluated, there is no error.)
489 "*The normal weight of a bar.")
498 Here is an equivalent expression for the @code{defvar} special form:
502 (defvar @var{symbol} @var{value} @var{doc-string})
505 (if (not (boundp '@var{symbol}))
506 (setq @var{symbol} @var{value}))
507 (if '@var{doc-string}
508 (put '@var{symbol} 'variable-documentation '@var{doc-string}))
513 The @code{defvar} form returns @var{symbol}, but it is normally used
514 at top level in a file where its value does not matter.
517 @defspec defconst symbol [value [doc-string]]
518 This special form defines @var{symbol} as a value and initializes it.
519 It informs a person reading your code that @var{symbol} has a standard
520 global value, established here, that should not be changed by the user
521 or by other programs. Note that @var{symbol} is not evaluated; the
522 symbol to be defined must appear explicitly in the @code{defconst}.
524 @code{defconst} always evaluates @var{value}, and sets the value of
525 @var{symbol} to the result if @var{value} is given. If @var{symbol}
526 does have a buffer-local binding in the current buffer, @code{defconst}
527 sets the default value, not the buffer-local value. (But you should not
528 be making buffer-local bindings for a symbol that is defined with
531 Here, @code{pi} is a constant that presumably ought not to be changed
532 by anyone (attempts by the Indiana State Legislature notwithstanding).
533 As the second form illustrates, however, this is only advisory.
537 (defconst pi 3.1415 "Pi to five places.")
551 @defun user-variable-p variable
553 This function returns @code{t} if @var{variable} is a user option---a
554 variable intended to be set by the user for customization---and
555 @code{nil} otherwise. (Variables other than user options exist for the
556 internal purposes of Lisp programs, and users need not know about them.)
558 User option variables are distinguished from other variables by the
559 first character of the @code{variable-documentation} property. If the
560 property exists and is a string, and its first character is @samp{*},
561 then the variable is a user option.
564 @kindex variable-interactive
565 If a user option variable has a @code{variable-interactive} property,
566 the @code{set-variable} command uses that value to control reading the
567 new value for the variable. The property's value is used as if it were
568 specified in @code{interactive} (@pxref{Using Interactive}). However,
569 this feature is largely obsoleted by @code{defcustom}
570 (@pxref{Customization}).
572 @strong{Warning:} If the @code{defconst} and @code{defvar} special
573 forms are used while the variable has a local binding, they set the
574 local binding's value; the global binding is not changed. This is not
575 what we really want. To prevent it, use these special forms at top
576 level in a file, where normally no local binding is in effect, and make
577 sure to load the file before making a local binding for the variable.
579 @node Tips for Defining
580 @section Tips for Defining Variables Robustly
582 When defining and initializing a variable that holds a complicated
583 value (such as a keymap with bindings in it), it's best to put the
584 entire computation of the value into the @code{defvar}, like this:
588 (let ((map (make-sparse-keymap)))
589 (define-key map "\C-c\C-a" 'my-command)
596 This method has several benefits. First, if the user quits while
597 loading the file, the variable is either still uninitialized or
598 initialized properly, never in-between. If it is still uninitialized,
599 reloading the file will initialize it properly. Second, reloading the
600 file once the variable is initialized will not alter it; that is
601 important if the user has run hooks to alter part of the contents (such
602 as, to rebind keys). Third, evaluating the @code{defvar} form with
603 @kbd{C-M-x} @emph{will} reinitialize the map completely.
605 Putting so much code in the @code{defvar} form has one disadvantage:
606 it puts the documentation string far away from the line which names the
607 variable. Here's a safe way to avoid that:
610 (defvar my-mode-map nil
613 (let ((map (make-sparse-keymap)))
614 (define-key my-mode-map "\C-c\C-a" 'my-command)
616 (setq my-mode-map map)))
620 This has all the same advantages as putting the initialization inside
621 the @code{defvar}, except that you must type @kbd{C-M-x} twice, once on
622 each form, if you do want to reinitialize the variable.
624 But be careful not to write the code like this:
627 (defvar my-mode-map nil
630 (setq my-mode-map (make-sparse-keymap))
631 (define-key my-mode-map "\C-c\C-a" 'my-command)
636 This code sets the variable, then alters it, but it does so in more than
637 one step. If the user quits just after the @code{setq}, that leaves the
638 variable neither correctly initialized nor void nor @code{nil}. Once
639 that happens, reloading the file will not initialize the variable; it
640 will remain incomplete.
642 @node Accessing Variables
643 @section Accessing Variable Values
645 The usual way to reference a variable is to write the symbol which
646 names it (@pxref{Symbol Forms}). This requires you to specify the
647 variable name when you write the program. Usually that is exactly what
648 you want to do. Occasionally you need to choose at run time which
649 variable to reference; then you can use @code{symbol-value}.
651 @defun symbol-value symbol
652 This function returns the value of @var{symbol}. This is the value in
653 the innermost local binding of the symbol, or its global value if it
654 has no local bindings.
667 ;; @r{Here the symbol @code{abracadabra}}
668 ;; @r{is the symbol whose value is examined.}
669 (let ((abracadabra 'foo))
670 (symbol-value 'abracadabra))
675 ;; @r{Here the value of @code{abracadabra},}
676 ;; @r{which is @code{foo},}
677 ;; @r{is the symbol whose value is examined.}
678 (let ((abracadabra 'foo))
679 (symbol-value abracadabra))
684 (symbol-value 'abracadabra)
689 A @code{void-variable} error is signaled if the current binding of
690 @var{symbol} is void.
693 @node Setting Variables
694 @section How to Alter a Variable Value
696 The usual way to change the value of a variable is with the special
697 form @code{setq}. When you need to compute the choice of variable at
698 run time, use the function @code{set}.
700 @defspec setq [symbol form]@dots{}
701 This special form is the most common method of changing a variable's
702 value. Each @var{symbol} is given a new value, which is the result of
703 evaluating the corresponding @var{form}. The most-local existing
704 binding of the symbol is changed.
706 @code{setq} does not evaluate @var{symbol}; it sets the symbol that you
707 write. We say that this argument is @dfn{automatically quoted}. The
708 @samp{q} in @code{setq} stands for ``quoted.''
710 The value of the @code{setq} form is the value of the last @var{form}.
717 x ; @r{@code{x} now has a global value.}
721 (setq x 6) ; @r{The local binding of @code{x} is set.}
725 x ; @r{The global value is unchanged.}
729 Note that the first @var{form} is evaluated, then the first
730 @var{symbol} is set, then the second @var{form} is evaluated, then the
731 second @var{symbol} is set, and so on:
735 (setq x 10 ; @r{Notice that @code{x} is set before}
736 y (1+ x)) ; @r{the value of @code{y} is computed.}
742 @defun set symbol value
743 This function sets @var{symbol}'s value to @var{value}, then returns
744 @var{value}. Since @code{set} is a function, the expression written for
745 @var{symbol} is evaluated to obtain the symbol to set.
747 The most-local existing binding of the variable is the binding that is
748 set; shadowed bindings are not affected.
753 @error{} Symbol's value as variable is void: one
764 (set two 2) ; @r{@code{two} evaluates to symbol @code{one}.}
768 one ; @r{So it is @code{one} that was set.}
770 (let ((one 1)) ; @r{This binding of @code{one} is set,}
771 (set 'one 3) ; @r{not the global value.}
781 If @var{symbol} is not actually a symbol, a @code{wrong-type-argument}
786 @error{} Wrong type argument: symbolp, (x y)
789 Logically speaking, @code{set} is a more fundamental primitive than
790 @code{setq}. Any use of @code{setq} can be trivially rewritten to use
791 @code{set}; @code{setq} could even be defined as a macro, given the
792 availability of @code{set}. However, @code{set} itself is rarely used;
793 beginners hardly need to know about it. It is useful only for choosing
794 at run time which variable to set. For example, the command
795 @code{set-variable}, which reads a variable name from the user and then
796 sets the variable, needs to use @code{set}.
798 @cindex CL note---@code{set} local
800 @b{Common Lisp note:} In Common Lisp, @code{set} always changes the
801 symbol's ``special'' or dynamic value, ignoring any lexical bindings.
802 In Emacs Lisp, all variables and all bindings are dynamic, so @code{set}
803 always affects the most local existing binding.
807 One other function for setting a variable is designed to add
808 an element to a list if it is not already present in the list.
810 @defun add-to-list symbol element
811 This function sets the variable @var{symbol} by consing @var{element}
812 onto the old value, if @var{element} is not already a member of that
813 value. It returns the resulting list, whether updated or not. The
814 value of @var{symbol} had better be a list already before the call.
816 The argument @var{symbol} is not implicitly quoted; @code{add-to-list}
817 is an ordinary function, like @code{set} and unlike @code{setq}. Quote
818 the argument yourself if that is what you want.
821 Here's a scenario showing how to use @code{add-to-list}:
827 (add-to-list 'foo 'c) ;; @r{Add @code{c}.}
830 (add-to-list 'foo 'b) ;; @r{No effect.}
833 foo ;; @r{@code{foo} was changed.}
837 An equivalent expression for @code{(add-to-list '@var{var}
838 @var{value})} is this:
841 (or (member @var{value} @var{var})
842 (setq @var{var} (cons @var{value} @var{var})))
845 @node Variable Scoping
846 @section Scoping Rules for Variable Bindings
848 A given symbol @code{foo} can have several local variable bindings,
849 established at different places in the Lisp program, as well as a global
850 binding. The most recently established binding takes precedence over
855 @cindex dynamic scoping
856 Local bindings in Emacs Lisp have @dfn{indefinite scope} and
857 @dfn{dynamic extent}. @dfn{Scope} refers to @emph{where} textually in
858 the source code the binding can be accessed. ``Indefinite scope'' means
859 that any part of the program can potentially access the variable
860 binding. @dfn{Extent} refers to @emph{when}, as the program is
861 executing, the binding exists. ``Dynamic extent'' means that the binding
862 lasts as long as the activation of the construct that established it.
864 The combination of dynamic extent and indefinite scope is called
865 @dfn{dynamic scoping}. By contrast, most programming languages use
866 @dfn{lexical scoping}, in which references to a local variable must be
867 located textually within the function or block that binds the variable.
869 @cindex CL note---special variables
871 @b{Common Lisp note:} Variables declared ``special'' in Common Lisp are
872 dynamically scoped, like all variables in Emacs Lisp.
876 * Scope:: Scope means where in the program a value is visible.
877 Comparison with other languages.
878 * Extent:: Extent means how long in time a value exists.
879 * Impl of Scope:: Two ways to implement dynamic scoping.
880 * Using Scoping:: How to use dynamic scoping carefully and avoid problems.
886 Emacs Lisp uses @dfn{indefinite scope} for local variable bindings.
887 This means that any function anywhere in the program text might access a
888 given binding of a variable. Consider the following function
893 (defun binder (x) ; @r{@code{x} is bound in @code{binder}.}
894 (foo 5)) ; @r{@code{foo} is some other function.}
898 (defun user () ; @r{@code{x} is used ``free'' in @code{user}.}
903 In a lexically scoped language, the binding of @code{x} in
904 @code{binder} would never be accessible in @code{user}, because
905 @code{user} is not textually contained within the function
906 @code{binder}. However, in dynamically-scoped Emacs Lisp, @code{user}
907 may or may not refer to the binding of @code{x} established in
908 @code{binder}, depending on the circumstances:
912 If we call @code{user} directly without calling @code{binder} at all,
913 then whatever binding of @code{x} is found, it cannot come from
917 If we define @code{foo} as follows and then call @code{binder}, then the
918 binding made in @code{binder} will be seen in @code{user}:
928 However, if we define @code{foo} as follows and then call @code{binder},
929 then the binding made in @code{binder} @emph{will not} be seen in
938 Here, when @code{foo} is called by @code{binder}, it binds @code{x}.
939 (The binding in @code{foo} is said to @dfn{shadow} the one made in
940 @code{binder}.) Therefore, @code{user} will access the @code{x} bound
941 by @code{foo} instead of the one bound by @code{binder}.
944 Emacs Lisp uses dynamic scoping because simple implementations of
945 lexical scoping are slow. In addition, every Lisp system needs to offer
946 dynamic scoping at least as an option; if lexical scoping is the norm,
947 there must be a way to specify dynamic scoping instead for a particular
948 variable. It might not be a bad thing for Emacs to offer both, but
949 implementing it with dynamic scoping only was much easier.
954 @dfn{Extent} refers to the time during program execution that a
955 variable name is valid. In Emacs Lisp, a variable is valid only while
956 the form that bound it is executing. This is called @dfn{dynamic
957 extent}. ``Local'' or ``automatic'' variables in most languages,
958 including C and Pascal, have dynamic extent.
960 One alternative to dynamic extent is @dfn{indefinite extent}. This
961 means that a variable binding can live on past the exit from the form
962 that made the binding. Common Lisp and Scheme, for example, support
963 this, but Emacs Lisp does not.
965 To illustrate this, the function below, @code{make-add}, returns a
966 function that purports to add @var{n} to its own argument @var{m}. This
967 would work in Common Lisp, but it does not do the job in Emacs Lisp,
968 because after the call to @code{make-add} exits, the variable @code{n}
969 is no longer bound to the actual argument 2.
973 (function (lambda (m) (+ n m)))) ; @r{Return a function.}
975 (fset 'add2 (make-add 2)) ; @r{Define function @code{add2}}
976 ; @r{with @code{(make-add 2)}.}
977 @result{} (lambda (m) (+ n m))
978 (add2 4) ; @r{Try to add 2 to 4.}
979 @error{} Symbol's value as variable is void: n
982 @cindex closures not available
983 Some Lisp dialects have ``closures'', objects that are like functions
984 but record additional variable bindings. Emacs Lisp does not have
988 @subsection Implementation of Dynamic Scoping
991 A simple sample implementation (which is not how Emacs Lisp actually
992 works) may help you understand dynamic binding. This technique is
993 called @dfn{deep binding} and was used in early Lisp systems.
995 Suppose there is a stack of bindings, which are variable-value pairs.
996 At entry to a function or to a @code{let} form, we can push bindings
997 onto the stack for the arguments or local variables created there. We
998 can pop those bindings from the stack at exit from the binding
1001 We can find the value of a variable by searching the stack from top to
1002 bottom for a binding for that variable; the value from that binding is
1003 the value of the variable. To set the variable, we search for the
1004 current binding, then store the new value into that binding.
1006 As you can see, a function's bindings remain in effect as long as it
1007 continues execution, even during its calls to other functions. That is
1008 why we say the extent of the binding is dynamic. And any other function
1009 can refer to the bindings, if it uses the same variables while the
1010 bindings are in effect. That is why we say the scope is indefinite.
1012 @cindex shallow binding
1013 The actual implementation of variable scoping in GNU Emacs Lisp uses a
1014 technique called @dfn{shallow binding}. Each variable has a standard
1015 place in which its current value is always found---the value cell of the
1018 In shallow binding, setting the variable works by storing a value in
1019 the value cell. Creating a new binding works by pushing the old value
1020 (belonging to a previous binding) onto a stack, and storing the new
1021 local value in the value cell. Eliminating a binding works by popping
1022 the old value off the stack, into the value cell.
1024 We use shallow binding because it has the same results as deep
1025 binding, but runs faster, since there is never a need to search for a
1029 @subsection Proper Use of Dynamic Scoping
1031 Binding a variable in one function and using it in another is a
1032 powerful technique, but if used without restraint, it can make programs
1033 hard to understand. There are two clean ways to use this technique:
1037 Use or bind the variable only in a few related functions, written close
1038 together in one file. Such a variable is used for communication within
1041 You should write comments to inform other programmers that they can see
1042 all uses of the variable before them, and to advise them not to add uses
1046 Give the variable a well-defined, documented meaning, and make all
1047 appropriate functions refer to it (but not bind it or set it) wherever
1048 that meaning is relevant. For example, the variable
1049 @code{case-fold-search} is defined as ``non-@code{nil} means ignore case
1050 when searching''; various search and replace functions refer to it
1051 directly or through their subroutines, but do not bind or set it.
1053 Then you can bind the variable in other programs, knowing reliably what
1057 In either case, you should define the variable with @code{defvar}.
1058 This helps other people understand your program by telling them to look
1059 for inter-function usage. It also avoids a warning from the byte
1060 compiler. Choose the variable's name to avoid name conflicts---don't
1061 use short names like @code{x}.
1063 @node Buffer-Local Variables
1064 @section Buffer-Local Variables
1065 @cindex variables, buffer-local
1066 @cindex buffer-local variables
1068 Global and local variable bindings are found in most programming
1069 languages in one form or another. Emacs, however, also supports additional,
1070 unusual kinds of variable binding: @dfn{buffer-local} bindings, which
1071 apply only in one buffer, and @dfn{frame-local} bindings, which apply only in
1072 one frame. Having different values for a variable in different buffers
1073 and/or frames is an important customization method.
1075 This section describes buffer-local bindings; for frame-local
1076 bindings, see the following section, @ref{Frame-Local Variables}. (A few
1077 variables have bindings that are local to each terminal; see
1078 @ref{Multiple Displays}.)
1081 * Intro to Buffer-Local:: Introduction and concepts.
1082 * Creating Buffer-Local:: Creating and destroying buffer-local bindings.
1083 * Default Value:: The default value is seen in buffers
1084 that don't have their own buffer-local values.
1087 @node Intro to Buffer-Local
1088 @subsection Introduction to Buffer-Local Variables
1090 A buffer-local variable has a buffer-local binding associated with a
1091 particular buffer. The binding is in effect when that buffer is
1092 current; otherwise, it is not in effect. If you set the variable while
1093 a buffer-local binding is in effect, the new value goes in that binding,
1094 so its other bindings are unchanged. This means that the change is
1095 visible only in the buffer where you made it.
1097 The variable's ordinary binding, which is not associated with any
1098 specific buffer, is called the @dfn{default binding}. In most cases,
1099 this is the global binding.
1101 A variable can have buffer-local bindings in some buffers but not in
1102 other buffers. The default binding is shared by all the buffers that
1103 don't have their own bindings for the variable. (This includes all
1104 newly-created buffers.) If you set the variable in a buffer that does
1105 not have a buffer-local binding for it, this sets the default binding
1106 (assuming there are no frame-local bindings to complicate the matter),
1107 so the new value is visible in all the buffers that see the default
1110 The most common use of buffer-local bindings is for major modes to change
1111 variables that control the behavior of commands. For example, C mode and
1112 Lisp mode both set the variable @code{paragraph-start} to specify that only
1113 blank lines separate paragraphs. They do this by making the variable
1114 buffer-local in the buffer that is being put into C mode or Lisp mode, and
1115 then setting it to the new value for that mode. @xref{Major Modes}.
1117 The usual way to make a buffer-local binding is with
1118 @code{make-local-variable}, which is what major mode commands typically
1119 use. This affects just the current buffer; all other buffers (including
1120 those yet to be created) will continue to share the default value unless
1121 they are explicitly given their own buffer-local bindings.
1123 @cindex automatically buffer-local
1124 A more powerful operation is to mark the variable as
1125 @dfn{automatically buffer-local} by calling
1126 @code{make-variable-buffer-local}. You can think of this as making the
1127 variable local in all buffers, even those yet to be created. More
1128 precisely, the effect is that setting the variable automatically makes
1129 the variable local to the current buffer if it is not already so. All
1130 buffers start out by sharing the default value of the variable as usual,
1131 but setting the variable creates a buffer-local binding for the current
1132 buffer. The new value is stored in the buffer-local binding, leaving
1133 the default binding untouched. This means that the default value cannot
1134 be changed with @code{setq} in any buffer; the only way to change it is
1135 with @code{setq-default}.
1137 @strong{Warning:} When a variable has buffer-local values in one or
1138 more buffers, you can get Emacs very confused by binding the variable
1139 with @code{let}, changing to a different current buffer in which a
1140 different binding is in effect, and then exiting the @code{let}. This
1141 can scramble the values of the buffer-local and default bindings.
1143 To preserve your sanity, avoid using a variable in that way. If you
1144 use @code{save-excursion} around each piece of code that changes to a
1145 different current buffer, you will not have this problem
1146 (@pxref{Excursions}). Here is an example of what to avoid:
1152 (make-local-variable 'foo)
1159 foo @result{} 'a ; @r{The old buffer-local value from buffer @samp{a}}
1160 ; @r{is now the default value.}
1164 foo @result{} 'temp ; @r{The local @code{let} value that should be gone}
1165 ; @r{is now the buffer-local value in buffer @samp{a}.}
1170 But @code{save-excursion} as shown here avoids the problem:
1181 Note that references to @code{foo} in @var{body} access the
1182 buffer-local binding of buffer @samp{b}.
1184 When a file specifies local variable values, these become buffer-local
1185 values when you visit the file. @xref{File Variables,,, emacs, The
1188 @node Creating Buffer-Local
1189 @subsection Creating and Deleting Buffer-Local Bindings
1191 @deffn Command make-local-variable variable
1192 This function creates a buffer-local binding in the current buffer for
1193 @var{variable} (a symbol). Other buffers are not affected. The value
1194 returned is @var{variable}.
1197 The buffer-local value of @var{variable} starts out as the same value
1198 @var{variable} previously had. If @var{variable} was void, it remains
1203 ;; @r{In buffer @samp{b1}:}
1204 (setq foo 5) ; @r{Affects all buffers.}
1208 (make-local-variable 'foo) ; @r{Now it is local in @samp{b1}.}
1212 foo ; @r{That did not change}
1213 @result{} 5 ; @r{the value.}
1216 (setq foo 6) ; @r{Change the value}
1217 @result{} 6 ; @r{in @samp{b1}.}
1225 ;; @r{In buffer @samp{b2}, the value hasn't changed.}
1233 Making a variable buffer-local within a @code{let}-binding for that
1234 variable does not work reliably, unless the buffer in which you do this
1235 is not current either on entry to or exit from the @code{let}. This is
1236 because @code{let} does not distinguish between different kinds of
1237 bindings; it knows only which variable the binding was made for.
1239 If the variable is terminal-local, this function signals an error. Such
1240 variables cannot have buffer-local bindings as well. @xref{Multiple
1243 @strong{Note:} Do not use @code{make-local-variable} for a hook
1244 variable. Instead, use @code{make-local-hook}. @xref{Hooks}.
1247 @deffn Command make-variable-buffer-local variable
1248 This function marks @var{variable} (a symbol) automatically
1249 buffer-local, so that any subsequent attempt to set it will make it
1250 local to the current buffer at the time.
1252 A peculiar wrinkle of this feature is that binding the variable (with
1253 @code{let} or other binding constructs) does not create a buffer-local
1254 binding for it. Only setting the variable (with @code{set} or
1255 @code{setq}) does so.
1257 The value returned is @var{variable}.
1259 @strong{Warning:} Don't assume that you should use
1260 @code{make-variable-buffer-local} for user-option variables, simply
1261 because users @emph{might} want to customize them differently in
1262 different buffers. Users can make any variable local, when they wish
1263 to. It is better to leave the choice to them.
1265 The time to use @code{make-variable-buffer-local} is when it is crucial
1266 that no two buffers ever share the same binding. For example, when a
1267 variable is used for internal purposes in a Lisp program which depends
1268 on having separate values in separate buffers, then using
1269 @code{make-variable-buffer-local} can be the best solution.
1272 @defun local-variable-p variable &optional buffer
1273 This returns @code{t} if @var{variable} is buffer-local in buffer
1274 @var{buffer} (which defaults to the current buffer); otherwise,
1278 @defun buffer-local-variables &optional buffer
1279 This function returns a list describing the buffer-local variables in
1280 buffer @var{buffer}. (If @var{buffer} is omitted, the current buffer is
1281 used.) It returns an association list (@pxref{Association Lists}) in
1282 which each element contains one buffer-local variable and its value.
1283 However, when a variable's buffer-local binding in @var{buffer} is void,
1284 then the variable appears directly in the resulting list.
1288 (make-local-variable 'foobar)
1289 (makunbound 'foobar)
1290 (make-local-variable 'bind-me)
1293 (setq lcl (buffer-local-variables))
1294 ;; @r{First, built-in variables local in all buffers:}
1295 @result{} ((mark-active . nil)
1296 (buffer-undo-list . nil)
1297 (mode-name . "Fundamental")
1300 ;; @r{Next, non-built-in buffer-local variables.}
1301 ;; @r{This one is buffer-local and void:}
1303 ;; @r{This one is buffer-local and nonvoid:}
1308 Note that storing new values into the @sc{cdr}s of cons cells in this
1309 list does @emph{not} change the buffer-local values of the variables.
1312 @deffn Command kill-local-variable variable
1313 This function deletes the buffer-local binding (if any) for
1314 @var{variable} (a symbol) in the current buffer. As a result, the
1315 default binding of @var{variable} becomes visible in this buffer. This
1316 typically results in a change in the value of @var{variable}, since the
1317 default value is usually different from the buffer-local value just
1320 If you kill the buffer-local binding of a variable that automatically
1321 becomes buffer-local when set, this makes the default value visible in
1322 the current buffer. However, if you set the variable again, that will
1323 once again create a buffer-local binding for it.
1325 @code{kill-local-variable} returns @var{variable}.
1327 This function is a command because it is sometimes useful to kill one
1328 buffer-local variable interactively, just as it is useful to create
1329 buffer-local variables interactively.
1332 @defun kill-all-local-variables
1333 This function eliminates all the buffer-local variable bindings of the
1334 current buffer except for variables marked as ``permanent''. As a
1335 result, the buffer will see the default values of most variables.
1337 This function also resets certain other information pertaining to the
1338 buffer: it sets the local keymap to @code{nil}, the syntax table to the
1339 value of @code{(standard-syntax-table)}, the case table to
1340 @code{(standard-case-table)}, and the abbrev table to the value of
1341 @code{fundamental-mode-abbrev-table}.
1343 The very first thing this function does is run the normal hook
1344 @code{change-major-mode-hook} (see below).
1346 Every major mode command begins by calling this function, which has the
1347 effect of switching to Fundamental mode and erasing most of the effects
1348 of the previous major mode. To ensure that this does its job, the
1349 variables that major modes set should not be marked permanent.
1351 @code{kill-all-local-variables} returns @code{nil}.
1354 @defvar change-major-mode-hook
1355 The function @code{kill-all-local-variables} runs this normal hook
1356 before it does anything else. This gives major modes a way to arrange
1357 for something special to be done if the user switches to a different
1358 major mode. For best results, make this variable buffer-local, so that
1359 it will disappear after doing its job and will not interfere with the
1360 subsequent major mode. @xref{Hooks}.
1364 @cindex permanent local variable
1365 A buffer-local variable is @dfn{permanent} if the variable name (a
1366 symbol) has a @code{permanent-local} property that is non-@code{nil}.
1367 Permanent locals are appropriate for data pertaining to where the file
1368 came from or how to save it, rather than with how to edit the contents.
1371 @subsection The Default Value of a Buffer-Local Variable
1372 @cindex default value
1374 The global value of a variable with buffer-local bindings is also
1375 called the @dfn{default} value, because it is the value that is in
1376 effect whenever neither the current buffer nor the selected frame has
1377 its own binding for the variable.
1379 The functions @code{default-value} and @code{setq-default} access and
1380 change a variable's default value regardless of whether the current
1381 buffer has a buffer-local binding. For example, you could use
1382 @code{setq-default} to change the default setting of
1383 @code{paragraph-start} for most buffers; and this would work even when
1384 you are in a C or Lisp mode buffer that has a buffer-local value for
1388 The special forms @code{defvar} and @code{defconst} also set the
1389 default value (if they set the variable at all), rather than any
1390 buffer-local or frame-local value.
1392 @defun default-value symbol
1393 This function returns @var{symbol}'s default value. This is the value
1394 that is seen in buffers and frames that do not have their own values for
1395 this variable. If @var{symbol} is not buffer-local, this is equivalent
1396 to @code{symbol-value} (@pxref{Accessing Variables}).
1400 @defun default-boundp symbol
1401 The function @code{default-boundp} tells you whether @var{symbol}'s
1402 default value is nonvoid. If @code{(default-boundp 'foo)} returns
1403 @code{nil}, then @code{(default-value 'foo)} would get an error.
1405 @code{default-boundp} is to @code{default-value} as @code{boundp} is to
1406 @code{symbol-value}.
1409 @defspec setq-default [symbol form]@dots{}
1410 This special form gives each @var{symbol} a new default value, which is
1411 the result of evaluating the corresponding @var{form}. It does not
1412 evaluate @var{symbol}, but does evaluate @var{form}. The value of the
1413 @code{setq-default} form is the value of the last @var{form}.
1415 If a @var{symbol} is not buffer-local for the current buffer, and is not
1416 marked automatically buffer-local, @code{setq-default} has the same
1417 effect as @code{setq}. If @var{symbol} is buffer-local for the current
1418 buffer, then this changes the value that other buffers will see (as long
1419 as they don't have a buffer-local value), but not the value that the
1420 current buffer sees.
1424 ;; @r{In buffer @samp{foo}:}
1425 (make-local-variable 'buffer-local)
1426 @result{} buffer-local
1429 (setq buffer-local 'value-in-foo)
1430 @result{} value-in-foo
1433 (setq-default buffer-local 'new-default)
1434 @result{} new-default
1438 @result{} value-in-foo
1441 (default-value 'buffer-local)
1442 @result{} new-default
1446 ;; @r{In (the new) buffer @samp{bar}:}
1448 @result{} new-default
1451 (default-value 'buffer-local)
1452 @result{} new-default
1455 (setq buffer-local 'another-default)
1456 @result{} another-default
1459 (default-value 'buffer-local)
1460 @result{} another-default
1464 ;; @r{Back in buffer @samp{foo}:}
1466 @result{} value-in-foo
1467 (default-value 'buffer-local)
1468 @result{} another-default
1473 @defun set-default symbol value
1474 This function is like @code{setq-default}, except that @var{symbol} is
1475 an ordinary evaluated argument.
1479 (set-default (car '(a b c)) 23)
1489 @node Frame-Local Variables
1490 @section Frame-Local Variables
1492 Just as variables can have buffer-local bindings, they can also have
1493 frame-local bindings. These bindings belong to one frame, and are in
1494 effect when that frame is selected. Frame-local bindings are actually
1495 frame parameters: you create a frame-local binding in a specific frame
1496 by calling @code{modify-frame-parameters} and specifying the variable
1497 name as the parameter name.
1499 To enable frame-local bindings for a certain variable, call the function
1500 @code{make-variable-frame-local}.
1502 @deffn Command make-variable-frame-local variable
1503 Enable the use of frame-local bindings for @var{variable}. This does
1504 not in itself create any frame-local bindings for the variable; however,
1505 if some frame already has a value for @var{variable} as a frame
1506 parameter, that value automatically becomes a frame-local binding.
1508 If the variable is terminal-local, this function signals an error,
1509 because such variables cannot have frame-local bindings as well.
1510 @xref{Multiple Displays}. A few variables that are implemented
1511 specially in Emacs can be (and usually are) buffer-local, but can never
1515 Buffer-local bindings take precedence over frame-local bindings. Thus,
1516 consider a variable @code{foo}: if the current buffer has a buffer-local
1517 binding for @code{foo}, that binding is active; otherwise, if the
1518 selected frame has a frame-local binding for @code{foo}, that binding is
1519 active; otherwise, the default binding of @code{foo} is active.
1521 Here is an example. First we prepare a few bindings for @code{foo}:
1524 (setq f1 (selected-frame))
1525 (make-variable-frame-local 'foo)
1527 ;; @r{Make a buffer-local binding for @code{foo} in @samp{b1}.}
1528 (set-buffer (get-buffer-create "b1"))
1529 (make-local-variable 'foo)
1532 ;; @r{Make a frame-local binding for @code{foo} in a new frame.}
1533 ;; @r{Store that frame in @code{f2}.}
1534 (setq f2 (make-frame))
1535 (modify-frame-parameters f2 '((foo . (f 2))))
1538 Now we examine @code{foo} in various contexts. Whenever the
1539 buffer @samp{b1} is current, its buffer-local binding is in effect,
1540 regardless of the selected frame:
1544 (set-buffer (get-buffer-create "b1"))
1549 (set-buffer (get-buffer-create "b1"))
1555 Otherwise, the frame gets a chance to provide the binding; when frame
1556 @code{f2} is selected, its frame-local binding is in effect:
1560 (set-buffer (get-buffer "*scratch*"))
1566 When neither the current buffer nor the selected frame provides
1567 a binding, the default binding is used:
1571 (set-buffer (get-buffer "*scratch*"))
1577 When the active binding of a variable is a frame-local binding, setting
1578 the variable changes that binding. You can observe the result with
1579 @code{frame-parameters}:
1583 (set-buffer (get-buffer "*scratch*"))
1585 (assq 'foo (frame-parameters f2))
1586 @result{} (foo . nobody)
1589 @node Future Local Variables
1590 @section Possible Future Local Variables
1592 We have considered the idea of bindings that are local to a category
1593 of frames---for example, all color frames, or all frames with dark
1594 backgrounds. We have not implemented them because it is not clear that
1595 this feature is really useful. You can get more or less the same
1596 results by adding a function to @code{after-make-frame-hook}, set up to
1597 define a particular frame parameter according to the appropriate
1598 conditions for each frame.
1600 It would also be possible to implement window-local bindings. We
1601 don't know of many situations where they would be useful, and it seems
1602 that indirect buffers (@pxref{Indirect Buffers}) with buffer-local
1603 bindings offer a way to handle these situations more robustly.
1605 If sufficient application is found for either of these two kinds of
1606 local bindings, we will provide it in a subsequent Emacs version.