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
102 In Emacs Lisp, certain symbols normally evaluate to themselves. These
103 include @code{nil} and @code{t}, as well as any symbol whose name starts
104 with @samp{:}. These symbols cannot be rebound, nor can their values be
105 changed. Any attempt to set or bind @code{nil} or @code{t} signals a
106 @code{setting-constant} error. The same is true for a symbol whose name
107 starts with @samp{:}, except that you are allowed to set such a symbol to
117 @error{} Attempt to set constant symbol: nil
121 @defvar keyword-symbols-constant-flag
122 @tindex keyword-symbols-constant-flag
123 If this variable is @code{nil}, you are allowed to set and bind symbols
124 whose names start with @samp{:} as you wish. This is to make it
125 possible to run old Lisp programs which do that.
128 @node Local Variables
129 @section Local Variables
130 @cindex binding local variables
131 @cindex local variables
132 @cindex local binding
133 @cindex global binding
135 Global variables have values that last until explicitly superseded
136 with new values. Sometimes it is useful to create variable values that
137 exist temporarily---only until a certain part of the program finishes.
138 These values are called @dfn{local}, and the variables so used are
139 called @dfn{local variables}.
141 For example, when a function is called, its argument variables receive
142 new local values that last until the function exits. The @code{let}
143 special form explicitly establishes new local values for specified
144 variables; these last until exit from the @code{let} form.
146 @cindex shadowing of variables
147 Establishing a local value saves away the previous value (or lack of
148 one) of the variable. When the life span of the local value is over,
149 the previous value is restored. In the mean time, we say that the
150 previous value is @dfn{shadowed} and @dfn{not visible}. Both global and
151 local values may be shadowed (@pxref{Scope}).
153 If you set a variable (such as with @code{setq}) while it is local,
154 this replaces the local value; it does not alter the global value, or
155 previous local values, that are shadowed. To model this behavior, we
156 speak of a @dfn{local binding} of the variable as well as a local value.
158 The local binding is a conceptual place that holds a local value.
159 Entry to a function, or a special form such as @code{let}, creates the
160 local binding; exit from the function or from the @code{let} removes the
161 local binding. As long as the local binding lasts, the variable's value
162 is stored within it. Use of @code{setq} or @code{set} while there is a
163 local binding stores a different value into the local binding; it does
164 not create a new binding.
166 We also speak of the @dfn{global binding}, which is where
167 (conceptually) the global value is kept.
169 @cindex current binding
170 A variable can have more than one local binding at a time (for
171 example, if there are nested @code{let} forms that bind it). In such a
172 case, the most recently created local binding that still exists is the
173 @dfn{current binding} of the variable. (This rule is called
174 @dfn{dynamic scoping}; see @ref{Variable Scoping}.) If there are no
175 local bindings, the variable's global binding is its current binding.
176 We sometimes call the current binding the @dfn{most-local existing
177 binding}, for emphasis. Ordinary evaluation of a symbol always returns
178 the value of its current binding.
180 The special forms @code{let} and @code{let*} exist to create
183 @defspec let (bindings@dots{}) forms@dots{}
184 This special form binds variables according to @var{bindings} and then
185 evaluates all of the @var{forms} in textual order. The @code{let}-form
186 returns the value of the last form in @var{forms}.
188 Each of the @var{bindings} is either @w{(i) a} symbol, in which case
189 that symbol is bound to @code{nil}; or @w{(ii) a} list of the form
190 @code{(@var{symbol} @var{value-form})}, in which case @var{symbol} is
191 bound to the result of evaluating @var{value-form}. If @var{value-form}
192 is omitted, @code{nil} is used.
194 All of the @var{value-form}s in @var{bindings} are evaluated in the
195 order they appear and @emph{before} binding any of the symbols to them.
196 Here is an example of this: @code{Z} is bound to the old value of
197 @code{Y}, which is 2, not the new value of @code{Y}, which is 1.
213 @defspec let* (bindings@dots{}) forms@dots{}
214 This special form is like @code{let}, but it binds each variable right
215 after computing its local value, before computing the local value for
216 the next variable. Therefore, an expression in @var{bindings} can
217 reasonably refer to the preceding symbols bound in this @code{let*}
218 form. Compare the following example with the example above for
228 (Z Y)) ; @r{Use the just-established value of @code{Y}.}
235 Here is a complete list of the other facilities that create local
240 Function calls (@pxref{Functions}).
243 Macro calls (@pxref{Macros}).
246 @code{condition-case} (@pxref{Errors}).
249 Variables can also have buffer-local bindings (@pxref{Buffer-Local
250 Variables}) and frame-local bindings (@pxref{Frame-Local Variables}); a
251 few variables have terminal-local bindings (@pxref{Multiple Displays}).
252 These kinds of bindings work somewhat like ordinary local bindings, but
253 they are localized depending on ``where'' you are in Emacs, rather than
256 @defvar max-specpdl-size
257 @cindex variable limit error
258 @cindex evaluation error
259 @cindex infinite recursion
260 This variable defines the limit on the total number of local variable
261 bindings and @code{unwind-protect} cleanups (@pxref{Nonlocal Exits})
262 that are allowed before signaling an error (with data @code{"Variable
263 binding depth exceeds max-specpdl-size"}).
265 This limit, with the associated error when it is exceeded, is one way
266 that Lisp avoids infinite recursion on an ill-defined function.
267 @code{max-lisp-eval-depth} provides another limit on depth of nesting.
270 The default value is 600. Entry to the Lisp debugger increases the
271 value, if there is little room left, to make sure the debugger itself
276 @section When a Variable is ``Void''
277 @kindex void-variable
278 @cindex void variable
280 If you have never given a symbol any value as a global variable, we
281 say that that symbol's global value is @dfn{void}. In other words, the
282 symbol's value cell does not have any Lisp object in it. If you try to
283 evaluate the symbol, you get a @code{void-variable} error rather than
286 Note that a value of @code{nil} is not the same as void. The symbol
287 @code{nil} is a Lisp object and can be the value of a variable just as any
288 other object can be; but it is @emph{a value}. A void variable does not
291 After you have given a variable a value, you can make it void once more
292 using @code{makunbound}.
294 @defun makunbound symbol
295 This function makes the current variable binding of @var{symbol} void.
296 Subsequent attempts to use this symbol's value as a variable will signal
297 the error @code{void-variable}, unless and until you set it again.
299 @code{makunbound} returns @var{symbol}.
303 (makunbound 'x) ; @r{Make the global value of @code{x} void.}
308 @error{} Symbol's value as variable is void: x
312 If @var{symbol} is locally bound, @code{makunbound} affects the most
313 local existing binding. This is the only way a symbol can have a void
314 local binding, since all the constructs that create local bindings
315 create them with values. In this case, the voidness lasts at most as
316 long as the binding does; when the binding is removed due to exit from
317 the construct that made it, the previous local or global binding is
318 reexposed as usual, and the variable is no longer void unless the newly
319 reexposed binding was void all along.
323 (setq x 1) ; @r{Put a value in the global binding.}
325 (let ((x 2)) ; @r{Locally bind it.}
326 (makunbound 'x) ; @r{Void the local binding.}
328 @error{} Symbol's value as variable is void: x
331 x ; @r{The global binding is unchanged.}
334 (let ((x 2)) ; @r{Locally bind it.}
335 (let ((x 3)) ; @r{And again.}
336 (makunbound 'x) ; @r{Void the innermost-local binding.}
337 x)) ; @r{And refer: it's void.}
338 @error{} Symbol's value as variable is void: x
344 (makunbound 'x)) ; @r{Void inner binding, then remove it.}
345 x) ; @r{Now outer @code{let} binding is visible.}
351 A variable that has been made void with @code{makunbound} is
352 indistinguishable from one that has never received a value and has
355 You can use the function @code{boundp} to test whether a variable is
358 @defun boundp variable
359 @code{boundp} returns @code{t} if @var{variable} (a symbol) is not void;
360 more precisely, if its current binding is not void. It returns
361 @code{nil} otherwise.
365 (boundp 'abracadabra) ; @r{Starts out void.}
369 (let ((abracadabra 5)) ; @r{Locally bind it.}
370 (boundp 'abracadabra))
374 (boundp 'abracadabra) ; @r{Still globally void.}
378 (setq abracadabra 5) ; @r{Make it globally nonvoid.}
382 (boundp 'abracadabra)
388 @node Defining Variables
389 @section Defining Global Variables
390 @cindex variable definition
392 You may announce your intention to use a symbol as a global variable
393 with a @dfn{variable definition}: a special form, either @code{defconst}
396 In Emacs Lisp, definitions serve three purposes. First, they inform
397 people who read the code that certain symbols are @emph{intended} to be
398 used a certain way (as variables). Second, they inform the Lisp system
399 of these things, supplying a value and documentation. Third, they
400 provide information to utilities such as @code{etags} and
401 @code{make-docfile}, which create data bases of the functions and
402 variables in a program.
404 The difference between @code{defconst} and @code{defvar} is primarily
405 a matter of intent, serving to inform human readers of whether the value
406 should ever change. Emacs Lisp does not restrict the ways in which a
407 variable can be used based on @code{defconst} or @code{defvar}
408 declarations. However, it does make a difference for initialization:
409 @code{defconst} unconditionally initializes the variable, while
410 @code{defvar} initializes it only if it is void.
413 One would expect user option variables to be defined with
414 @code{defconst}, since programs do not change them. Unfortunately, this
415 has bad results if the definition is in a library that is not preloaded:
416 @code{defconst} would override any prior value when the library is
417 loaded. Users would like to be able to set user options in their init
418 files, and override the default values given in the definitions. For
419 this reason, user options must be defined with @code{defvar}.
422 @defspec defvar symbol [value [doc-string]]
423 This special form defines @var{symbol} as a variable and can also
424 initialize and document it. The definition informs a person reading
425 your code that @var{symbol} is used as a variable that might be set or
426 changed. Note that @var{symbol} is not evaluated; the symbol to be
427 defined must appear explicitly in the @code{defvar}.
429 If @var{symbol} is void and @var{value} is specified, @code{defvar}
430 evaluates it and sets @var{symbol} to the result. But if @var{symbol}
431 already has a value (i.e., it is not void), @var{value} is not even
432 evaluated, and @var{symbol}'s value remains unchanged. If @var{value}
433 is omitted, the value of @var{symbol} is not changed in any case.
435 If @var{symbol} has a buffer-local binding in the current buffer,
436 @code{defvar} operates on the default value, which is buffer-independent,
437 not the current (buffer-local) binding. It sets the default value if
438 the default value is void. @xref{Buffer-Local Variables}.
440 When you evaluate a top-level @code{defvar} form with @kbd{C-M-x} in
441 Emacs Lisp mode (@code{eval-defun}), a special feature of
442 @code{eval-defun} arranges to set the variable unconditionally, without
443 testing whether its value is void.
445 If the @var{doc-string} argument appears, it specifies the documentation
446 for the variable. (This opportunity to specify documentation is one of
447 the main benefits of defining the variable.) The documentation is
448 stored in the symbol's @code{variable-documentation} property. The
449 Emacs help functions (@pxref{Documentation}) look for this property.
451 If the first character of @var{doc-string} is @samp{*}, it means that
452 this variable is considered a user option. This lets users set the
453 variable conveniently using the commands @code{set-variable} and
454 @code{edit-options}. However, it is better to use @code{defcustom}
455 instead of @code{defvar} for user option variables, so you can specify
456 customization information. @xref{Customization}.
458 Here are some examples. This form defines @code{foo} but does not
468 This example initializes the value of @code{bar} to @code{23}, and gives
469 it a documentation string:
474 "The normal weight of a bar.")
479 The following form changes the documentation string for @code{bar},
480 making it a user option, but does not change the value, since @code{bar}
481 already has a value. (The addition @code{(1+ nil)} would get an error
482 if it were evaluated, but since it is not evaluated, there is no error.)
487 "*The normal weight of a bar.")
496 Here is an equivalent expression for the @code{defvar} special form:
500 (defvar @var{symbol} @var{value} @var{doc-string})
503 (if (not (boundp '@var{symbol}))
504 (setq @var{symbol} @var{value}))
505 (if '@var{doc-string}
506 (put '@var{symbol} 'variable-documentation '@var{doc-string}))
511 The @code{defvar} form returns @var{symbol}, but it is normally used
512 at top level in a file where its value does not matter.
515 @defspec defconst symbol [value [doc-string]]
516 This special form defines @var{symbol} as a value and initializes it.
517 It informs a person reading your code that @var{symbol} has a standard
518 global value, established here, that should not be changed by the user
519 or by other programs. Note that @var{symbol} is not evaluated; the
520 symbol to be defined must appear explicitly in the @code{defconst}.
522 @code{defconst} always evaluates @var{value}, and sets the value of
523 @var{symbol} to the result if @var{value} is given. If @var{symbol}
524 does have a buffer-local binding in the current buffer, @code{defconst}
525 sets the default value, not the buffer-local value. (But you should not
526 be making buffer-local bindings for a symbol that is defined with
529 Here, @code{pi} is a constant that presumably ought not to be changed
530 by anyone (attempts by the Indiana State Legislature notwithstanding).
531 As the second form illustrates, however, this is only advisory.
535 (defconst pi 3.1415 "Pi to five places.")
549 @defun user-variable-p variable
551 This function returns @code{t} if @var{variable} is a user option---a
552 variable intended to be set by the user for customization---and
553 @code{nil} otherwise. (Variables other than user options exist for the
554 internal purposes of Lisp programs, and users need not know about them.)
556 User option variables are distinguished from other variables by the
557 first character of the @code{variable-documentation} property. If the
558 property exists and is a string, and its first character is @samp{*},
559 then the variable is a user option.
562 @kindex variable-interactive
563 If a user option variable has a @code{variable-interactive} property,
564 the @code{set-variable} command uses that value to control reading the
565 new value for the variable. The property's value is used as if it were
566 to @code{interactive} (@pxref{Using Interactive}). However, this feature
567 is largely obsoleted by @code{defcustom} (@pxref{Customization}).
569 @strong{Warning:} If the @code{defconst} and @code{defvar} special
570 forms are used while the variable has a local binding, they set the
571 local binding's value; the global binding is not changed. This is not
572 what we really want. To prevent it, use these special forms at top
573 level in a file, where normally no local binding is in effect, and make
574 sure to load the file before making a local binding for the variable.
576 @node Tips for Defining
577 @section Tips for Defining Variables Robustly
579 When defining and initializing a variable that holds a complicated
580 value (such as a keymap with bindings in it), it's best to put the
581 entire computation of the value into the @code{defvar}, like this:
585 (let ((map (make-sparse-keymap)))
586 (define-key map "\C-c\C-a" 'my-command)
593 This method has several benefits. First, if the user quits while
594 loading the file, the variable is either still uninitialized or
595 initialized properly, never in-between. If it is still uninitialized,
596 reloading the file will initialize it properly. Second, reloading the
597 file once the variable is initialized will not alter it; that is
598 important if the user has run hooks to alter part of the contents (such
599 as, to rebind keys). Third, evaluating the @code{defvar} form with
600 @kbd{C-M-x} @emph{will} reinitialize the map completely.
602 Putting so much code in the @code{defvar} form has one disadvantage:
603 it puts the documentation string far away from the line which names the
604 variable. Here's a safe way to avoid that:
607 (defvar my-mode-map nil
611 (let ((map (make-sparse-keymap)))
612 (define-key my-mode-map "\C-c\C-a" 'my-command)
614 (setq my-mode-map map)))
618 This has all the same advantages as putting the initialization inside
619 the @code{defvar}, except that you must type @kbd{C-M-x} twice, once on
620 each form, if you do want to reinitialize the variable.
622 But be careful not to write the code like this:
625 (defvar my-mode-map nil
629 (setq my-mode-map (make-sparse-keymap))
630 (define-key my-mode-map "\C-c\C-a" 'my-command)
635 This code sets the variable, then alters it, but it does so in more than
636 one step. If the user quits just after the @code{setq}, that leaves the
637 variable neither correctly initialized nor void nor @code{nil}. Once
638 that happens, reloading the file will not initialize the variable; it
639 will remain incomplete.
641 @node Accessing Variables
642 @section Accessing Variable Values
644 The usual way to reference a variable is to write the symbol which
645 names it (@pxref{Symbol Forms}). This requires you to specify the
646 variable name when you write the program. Usually that is exactly what
647 you want to do. Occasionally you need to choose at run time which
648 variable to reference; then you can use @code{symbol-value}.
650 @defun symbol-value symbol
651 This function returns the value of @var{symbol}. This is the value in
652 the innermost local binding of the symbol, or its global value if it
653 has no local bindings.
666 ;; @r{Here the symbol @code{abracadabra}}
667 ;; @r{is the symbol whose value is examined.}
668 (let ((abracadabra 'foo))
669 (symbol-value 'abracadabra))
674 ;; @r{Here the value of @code{abracadabra},}
675 ;; @r{which is @code{foo},}
676 ;; @r{is the symbol whose value is examined.}
677 (let ((abracadabra 'foo))
678 (symbol-value abracadabra))
683 (symbol-value 'abracadabra)
688 A @code{void-variable} error is signaled if the current binding of
689 @var{symbol} is void.
692 @node Setting Variables
693 @section How to Alter a Variable Value
695 The usual way to change the value of a variable is with the special
696 form @code{setq}. When you need to compute the choice of variable at
697 run time, use the function @code{set}.
699 @defspec setq [symbol form]@dots{}
700 This special form is the most common method of changing a variable's
701 value. Each @var{symbol} is given a new value, which is the result of
702 evaluating the corresponding @var{form}. The most-local existing
703 binding of the symbol is changed.
705 @code{setq} does not evaluate @var{symbol}; it sets the symbol that you
706 write. We say that this argument is @dfn{automatically quoted}. The
707 @samp{q} in @code{setq} stands for ``quoted.''
709 The value of the @code{setq} form is the value of the last @var{form}.
716 x ; @r{@code{x} now has a global value.}
720 (setq x 6) ; @r{The local binding of @code{x} is set.}
724 x ; @r{The global value is unchanged.}
728 Note that the first @var{form} is evaluated, then the first
729 @var{symbol} is set, then the second @var{form} is evaluated, then the
730 second @var{symbol} is set, and so on:
734 (setq x 10 ; @r{Notice that @code{x} is set before}
735 y (1+ x)) ; @r{the value of @code{y} is computed.}
741 @defun set symbol value
742 This function sets @var{symbol}'s value to @var{value}, then returns
743 @var{value}. Since @code{set} is a function, the expression written for
744 @var{symbol} is evaluated to obtain the symbol to set.
746 The most-local existing binding of the variable is the binding that is
747 set; shadowed bindings are not affected.
752 @error{} Symbol's value as variable is void: one
763 (set two 2) ; @r{@code{two} evaluates to symbol @code{one}.}
767 one ; @r{So it is @code{one} that was set.}
769 (let ((one 1)) ; @r{This binding of @code{one} is set,}
770 (set 'one 3) ; @r{not the global value.}
780 If @var{symbol} is not actually a symbol, a @code{wrong-type-argument}
785 @error{} Wrong type argument: symbolp, (x y)
788 Logically speaking, @code{set} is a more fundamental primitive than
789 @code{setq}. Any use of @code{setq} can be trivially rewritten to use
790 @code{set}; @code{setq} could even be defined as a macro, given the
791 availability of @code{set}. However, @code{set} itself is rarely used;
792 beginners hardly need to know about it. It is useful only for choosing
793 at run time which variable to set. For example, the command
794 @code{set-variable}, which reads a variable name from the user and then
795 sets the variable, needs to use @code{set}.
797 @cindex CL note---@code{set} local
799 @b{Common Lisp note:} In Common Lisp, @code{set} always changes the
800 symbol's ``special'' or dynamic value, ignoring any lexical bindings.
801 In Emacs Lisp, all variables and all bindings are dynamic, so @code{set}
802 always affects the most local existing binding.
806 One other function for setting a variable is designed to add
807 an element to a list if it is not already present in the list.
809 @defun add-to-list symbol element
810 This function sets the variable @var{symbol} by consing @var{element}
811 onto the old value, if @var{element} is not already a member of that
812 value. It returns the resulting list, whether updated or not. The
813 value of @var{symbol} had better be a list already before the call.
815 The argument @var{symbol} is not implicitly quoted; @code{add-to-list}
816 is an ordinary function, like @code{set} and unlike @code{setq}. Quote
817 the argument yourself if that is what you want.
820 Here's a scenario showing how to use @code{add-to-list}:
826 (add-to-list 'foo 'c) ;; @r{Add @code{c}.}
829 (add-to-list 'foo 'b) ;; @r{No effect.}
832 foo ;; @r{@code{foo} was changed.}
836 An equivalent expression for @code{(add-to-list '@var{var}
837 @var{value})} is this:
840 (or (member @var{value} @var{var})
841 (setq @var{var} (cons @var{value} @var{var})))
844 @node Variable Scoping
845 @section Scoping Rules for Variable Bindings
847 A given symbol @code{foo} can have several local variable bindings,
848 established at different places in the Lisp program, as well as a global
849 binding. The most recently established binding takes precedence over
854 @cindex dynamic scoping
855 Local bindings in Emacs Lisp have @dfn{indefinite scope} and
856 @dfn{dynamic extent}. @dfn{Scope} refers to @emph{where} textually in
857 the source code the binding can be accessed. Indefinite scope means
858 that any part of the program can potentially access the variable
859 binding. @dfn{Extent} refers to @emph{when}, as the program is
860 executing, the binding exists. Dynamic extent means that the binding
861 lasts as long as the activation of the construct that established it.
863 The combination of dynamic extent and indefinite scope is called
864 @dfn{dynamic scoping}. By contrast, most programming languages use
865 @dfn{lexical scoping}, in which references to a local variable must be
866 located textually within the function or block that binds the variable.
868 @cindex CL note---special variables
870 @b{Common Lisp note:} Variables declared ``special'' in Common Lisp are
871 dynamically scoped, like all variables in Emacs Lisp.
875 * Scope:: Scope means where in the program a value is visible.
876 Comparison with other languages.
877 * Extent:: Extent means how long in time a value exists.
878 * Impl of Scope:: Two ways to implement dynamic scoping.
879 * Using Scoping:: How to use dynamic scoping carefully and avoid problems.
885 Emacs Lisp uses @dfn{indefinite scope} for local variable bindings.
886 This means that any function anywhere in the program text might access a
887 given binding of a variable. Consider the following function
892 (defun binder (x) ; @r{@code{x} is bound in @code{binder}.}
893 (foo 5)) ; @r{@code{foo} is some other function.}
897 (defun user () ; @r{@code{x} is used ``free'' in @code{user}.}
902 In a lexically scoped language, the binding of @code{x} in
903 @code{binder} would never be accessible in @code{user}, because
904 @code{user} is not textually contained within the function
905 @code{binder}. However, in dynamically scoped Emacs Lisp, @code{user}
906 may or may not refer to the binding of @code{x} established in
907 @code{binder}, depending on circumstances:
911 If we call @code{user} directly without calling @code{binder} at all,
912 then whatever binding of @code{x} is found, it cannot come from
916 If we define @code{foo} as follows and then call @code{binder}, then the
917 binding made in @code{binder} will be seen in @code{user}:
927 However, if we define @code{foo} as follows and then call @code{binder},
928 then the binding made in @code{binder} @emph{will not} be seen in
937 Here, when @code{foo} is called by @code{binder}, it binds @code{x}.
938 (The binding in @code{foo} is said to @dfn{shadow} the one made in
939 @code{binder}.) Therefore, @code{user} will access the @code{x} bound
940 by @code{foo} instead of the one bound by @code{binder}.
943 Emacs Lisp uses dynamic scoping because simple implementations of
944 lexical scoping are slow. In addition, every Lisp system needs to offer
945 dynamic scoping at least as an option; if lexical scoping is the norm,
946 there must be a way to specify dynamic scoping instead for a particular
947 variable. It might not be a bad thing for Emacs to offer both, but
948 implementing it with dynamic scoping only was much easier.
953 @dfn{Extent} refers to the time during program execution that a
954 variable name is valid. In Emacs Lisp, a variable is valid only while
955 the form that bound it is executing. This is called @dfn{dynamic
956 extent}. ``Local'' or ``automatic'' variables in most languages,
957 including C and Pascal, have dynamic extent.
959 One alternative to dynamic extent is @dfn{indefinite extent}. This
960 means that a variable binding can live on past the exit from the form
961 that made the binding. Common Lisp and Scheme, for example, support
962 this, but Emacs Lisp does not.
964 To illustrate this, the function below, @code{make-add}, returns a
965 function that purports to add @var{n} to its own argument @var{m}. This
966 would work in Common Lisp, but it does not do the job in Emacs Lisp,
967 because after the call to @code{make-add} exits, the variable @code{n}
968 is no longer bound to the actual argument 2.
972 (function (lambda (m) (+ n m)))) ; @r{Return a function.}
974 (fset 'add2 (make-add 2)) ; @r{Define function @code{add2}}
975 ; @r{with @code{(make-add 2)}.}
976 @result{} (lambda (m) (+ n m))
977 (add2 4) ; @r{Try to add 2 to 4.}
978 @error{} Symbol's value as variable is void: n
981 @cindex closures not available
982 Some Lisp dialects have ``closures'', objects that are like functions
983 but record additional variable bindings. Emacs Lisp does not have
987 @subsection Implementation of Dynamic Scoping
990 A simple sample implementation (which is not how Emacs Lisp actually
991 works) may help you understand dynamic binding. This technique is
992 called @dfn{deep binding} and was used in early Lisp systems.
994 Suppose there is a stack of bindings, which are variable-value pairs.
995 At entry to a function or to a @code{let} form, we can push bindings
996 onto the stack for the arguments or local variables created there. We
997 can pop those bindings from the stack at exit from the binding
1000 We can find the value of a variable by searching the stack from top to
1001 bottom for a binding for that variable; the value from that binding is
1002 the value of the variable. To set the variable, we search for the
1003 current binding, then store the new value into that binding.
1005 As you can see, a function's bindings remain in effect as long as it
1006 continues execution, even during its calls to other functions. That is
1007 why we say the extent of the binding is dynamic. And any other function
1008 can refer to the bindings, if it uses the same variables while the
1009 bindings are in effect. That is why we say the scope is indefinite.
1011 @cindex shallow binding
1012 The actual implementation of variable scoping in GNU Emacs Lisp uses a
1013 technique called @dfn{shallow binding}. Each variable has a standard
1014 place in which its current value is always found---the value cell of the
1017 In shallow binding, setting the variable works by storing a value in
1018 the value cell. Creating a new binding works by pushing the old value
1019 (belonging to a previous binding) onto a stack, and storing the new
1020 local value in the value cell. Eliminating a binding works by popping
1021 the old value off the stack, into the value cell.
1023 We use shallow binding because it has the same results as deep
1024 binding, but runs faster, since there is never a need to search for a
1028 @subsection Proper Use of Dynamic Scoping
1030 Binding a variable in one function and using it in another is a
1031 powerful technique, but if used without restraint, it can make programs
1032 hard to understand. There are two clean ways to use this technique:
1036 Use or bind the variable only in a few related functions, written close
1037 together in one file. Such a variable is used for communication within
1040 You should write comments to inform other programmers that they can see
1041 all uses of the variable before them, and to advise them not to add uses
1045 Give the variable a well-defined, documented meaning, and make all
1046 appropriate functions refer to it (but not bind it or set it) wherever
1047 that meaning is relevant. For example, the variable
1048 @code{case-fold-search} is defined as ``non-@code{nil} means ignore case
1049 when searching''; various search and replace functions refer to it
1050 directly or through their subroutines, but do not bind or set it.
1052 Then you can bind the variable in other programs, knowing reliably what
1056 In either case, you should define the variable with @code{defvar}.
1057 This helps other people understand your program by telling them to look
1058 for inter-function usage. It also avoids a warning from the byte
1059 compiler. Choose the variable's name to avoid name conflicts---don't
1060 use short names like @code{x}.
1062 @node Buffer-Local Variables
1063 @section Buffer-Local Variables
1064 @cindex variables, buffer-local
1065 @cindex buffer-local variables
1067 Global and local variable bindings are found in most programming
1068 languages in one form or another. Emacs also supports additional,
1069 unusual kinds of variable binding: @dfn{buffer-local} bindings, which
1070 apply only in one buffer, and frame-local bindings, which apply only in
1071 one frame. Having different values for a variable in different buffers
1072 and/or frames is an important customization method.
1074 This section describes buffer-local bindings; for frame-local
1075 bindings, see the following section, @ref{Frame-Local Variables}. (A few
1076 variables have bindings that are local to each terminal; see
1077 @ref{Multiple Displays}.)
1080 * Intro to Buffer-Local:: Introduction and concepts.
1081 * Creating Buffer-Local:: Creating and destroying buffer-local bindings.
1082 * Default Value:: The default value is seen in buffers
1083 that don't have their own buffer-local values.
1086 @node Intro to Buffer-Local
1087 @subsection Introduction to Buffer-Local Variables
1089 A buffer-local variable has a buffer-local binding associated with a
1090 particular buffer. The binding is in effect when that buffer is
1091 current; otherwise, it is not in effect. If you set the variable while
1092 a buffer-local binding is in effect, the new value goes in that binding,
1093 so its other bindings are unchanged. This means that the change is
1094 visible only in the buffer where you made it.
1096 The variable's ordinary binding, which is not associated with any
1097 specific buffer, is called the @dfn{default binding}. In most cases,
1098 this is the global binding.
1100 A variable can have buffer-local bindings in some buffers but not in
1101 other buffers. The default binding is shared by all the buffers that
1102 don't have their own bindings for the variable. (This includes all
1103 newly created buffers.) If you set the variable in a buffer that does
1104 not have a buffer-local binding for it, this sets the default binding
1105 (assuming there are no frame-local bindings to complicate the matter),
1106 so the new value is visible in all the buffers that see the default
1109 The most common use of buffer-local bindings is for major modes to change
1110 variables that control the behavior of commands. For example, C mode and
1111 Lisp mode both set the variable @code{paragraph-start} to specify that only
1112 blank lines separate paragraphs. They do this by making the variable
1113 buffer-local in the buffer that is being put into C mode or Lisp mode, and
1114 then setting it to the new value for that mode. @xref{Major Modes}.
1116 The usual way to make a buffer-local binding is with
1117 @code{make-local-variable}, which is what major mode commands typically
1118 use. This affects just the current buffer; all other buffers (including
1119 those yet to be created) will continue to share the default value unless
1120 they are explicitly given their own buffer-local bindings.
1122 @cindex automatically buffer-local
1123 A more powerful operation is to mark the variable as
1124 @dfn{automatically buffer-local} by calling
1125 @code{make-variable-buffer-local}. You can think of this as making the
1126 variable local in all buffers, even those yet to be created. More
1127 precisely, the effect is that setting the variable automatically makes
1128 the variable local to the current buffer if it is not already so. All
1129 buffers start out by sharing the default value of the variable as usual,
1130 but setting the variable creates a buffer-local binding for the current
1131 buffer. The new value is stored in the buffer-local binding, leaving
1132 the default binding untouched. This means that the default value cannot
1133 be changed with @code{setq} in any buffer; the only way to change it is
1134 with @code{setq-default}.
1136 @strong{Warning:} When a variable has buffer-local values in one or
1137 more buffers, you can get Emacs very confused by binding the variable
1138 with @code{let}, changing to a different current buffer in which a
1139 different binding is in effect, and then exiting the @code{let}. This
1140 can scramble the values of the buffer-local and default bindings.
1142 To preserve your sanity, avoid using a variable in that way. If you
1143 use @code{save-excursion} around each piece of code that changes to a
1144 different current buffer, you will not have this problem
1145 (@pxref{Excursions}). Here is an example of what to avoid:
1151 (make-local-variable 'foo)
1158 foo @result{} 'a ; @r{The old buffer-local value from buffer @samp{a}}
1159 ; @r{is now the default value.}
1163 foo @result{} 'temp ; @r{The local @code{let} value that should be gone}
1164 ; @r{is now the buffer-local value in buffer @samp{a}.}
1169 But @code{save-excursion} as shown here avoids the problem:
1180 Note that references to @code{foo} in @var{body} access the
1181 buffer-local binding of buffer @samp{b}.
1183 When a file specifies local variable values, these become buffer-local
1184 values when you visit the file. @xref{File Variables,,, emacs, The
1187 @node Creating Buffer-Local
1188 @subsection Creating and Deleting Buffer-Local Bindings
1190 @deffn Command make-local-variable variable
1191 This function creates a buffer-local binding in the current buffer for
1192 @var{variable} (a symbol). Other buffers are not affected. The value
1193 returned is @var{variable}.
1196 The buffer-local value of @var{variable} starts out as the same value
1197 @var{variable} previously had. If @var{variable} was void, it remains
1202 ;; @r{In buffer @samp{b1}:}
1203 (setq foo 5) ; @r{Affects all buffers.}
1207 (make-local-variable 'foo) ; @r{Now it is local in @samp{b1}.}
1211 foo ; @r{That did not change}
1212 @result{} 5 ; @r{the value.}
1215 (setq foo 6) ; @r{Change the value}
1216 @result{} 6 ; @r{in @samp{b1}.}
1224 ;; @r{In buffer @samp{b2}, the value hasn't changed.}
1232 Making a variable buffer-local within a @code{let}-binding for that
1233 variable does not work reliably, unless the buffer in which you do this
1234 is not current either on entry to or exit from the @code{let}. This is
1235 because @code{let} does not distinguish between different kinds of
1236 bindings; it knows only which variable the binding was made for.
1238 If the variable is terminal-local, this function signals an error. Such
1239 variables cannot have buffer-local bindings as well. @xref{Multiple
1242 @strong{Note:} do not use @code{make-local-variable} for a hook
1243 variable. Instead, use @code{make-local-hook}. @xref{Hooks}.
1246 @deffn Command make-variable-buffer-local variable
1247 This function marks @var{variable} (a symbol) automatically
1248 buffer-local, so that any subsequent attempt to set it will make it
1249 local to the current buffer at the time.
1251 A peculiar wrinkle of this feature is that binding the variable (with
1252 @code{let} or other binding constructs) does not create a buffer-local
1253 binding for it. Only setting the variable (with @code{set} or
1254 @code{setq}) does so.
1256 The value returned is @var{variable}.
1258 @strong{Warning:} Don't assume that you should use
1259 @code{make-variable-buffer-local} for user-option variables, simply
1260 because users @emph{might} want to customize them differently in
1261 different buffers. Users can make any variable local, when they wish
1262 to. It is better to leave the choice to them.
1264 The time to use @code{make-variable-buffer-local} is when it is crucial
1265 that no two buffers ever share the same binding. For example, when a
1266 variable is used for internal purposes in a Lisp program which depends
1267 on having separate values in separate buffers, then using
1268 @code{make-variable-buffer-local} can be the best solution.
1271 @defun local-variable-p variable &optional buffer
1272 This returns @code{t} if @var{variable} is buffer-local in buffer
1273 @var{buffer} (which defaults to the current buffer); otherwise,
1277 @defun buffer-local-variables &optional buffer
1278 This function returns a list describing the buffer-local variables in
1279 buffer @var{buffer}. (If @var{buffer} is omitted, the current buffer is
1280 used.) It returns an association list (@pxref{Association Lists}) in
1281 which each element contains one buffer-local variable and its value.
1282 However, when a variable's buffer-local binding in @var{buffer} is void,
1283 then the variable appears directly in the resulting list.
1287 (make-local-variable 'foobar)
1288 (makunbound 'foobar)
1289 (make-local-variable 'bind-me)
1292 (setq lcl (buffer-local-variables))
1293 ;; @r{First, built-in variables local in all buffers:}
1294 @result{} ((mark-active . nil)
1295 (buffer-undo-list . nil)
1296 (mode-name . "Fundamental")
1299 ;; @r{Next, non-built-in buffer-local variables.}
1300 ;; @r{This one is buffer-local and void:}
1302 ;; @r{This one is buffer-local and nonvoid:}
1307 Note that storing new values into the @sc{cdr}s of cons cells in this
1308 list does @emph{not} change the buffer-local values of the variables.
1311 @deffn Command kill-local-variable variable
1312 This function deletes the buffer-local binding (if any) for
1313 @var{variable} (a symbol) in the current buffer. As a result, the
1314 default binding of @var{variable} becomes visible in this buffer. This
1315 typically results in a change in the value of @var{variable}, since the
1316 default value is usually different from the buffer-local value just
1319 If you kill the buffer-local binding of a variable that automatically
1320 becomes buffer-local when set, this makes the default value visible in
1321 the current buffer. However, if you set the variable again, that will
1322 once again create a buffer-local binding for it.
1324 @code{kill-local-variable} returns @var{variable}.
1326 This function is a command because it is sometimes useful to kill one
1327 buffer-local variable interactively, just as it is useful to create
1328 buffer-local variables interactively.
1331 @defun kill-all-local-variables
1332 This function eliminates all the buffer-local variable bindings of the
1333 current buffer except for variables marked as ``permanent''. As a
1334 result, the buffer will see the default values of most variables.
1336 This function also resets certain other information pertaining to the
1337 buffer: it sets the local keymap to @code{nil}, the syntax table to the
1338 value of @code{(standard-syntax-table)}, the case table to
1339 @code{(standard-case-table)}, and the abbrev table to the value of
1340 @code{fundamental-mode-abbrev-table}.
1342 The very first thing this function does is run the normal hook
1343 @code{change-major-mode-hook} (see below).
1345 Every major mode command begins by calling this function, which has the
1346 effect of switching to Fundamental mode and erasing most of the effects
1347 of the previous major mode. To ensure that this does its job, the
1348 variables that major modes set should not be marked permanent.
1350 @code{kill-all-local-variables} returns @code{nil}.
1353 @defvar change-major-mode-hook
1354 The function @code{kill-all-local-variables} runs this normal hook
1355 before it does anything else. This gives major modes a way to arrange
1356 for something special to be done if the user switches to a different
1357 major mode. For best results, make this variable buffer-local, so that
1358 it will disappear after doing its job and will not interfere with the
1359 subsequent major mode. @xref{Hooks}.
1363 @cindex permanent local variable
1364 A buffer-local variable is @dfn{permanent} if the variable name (a
1365 symbol) has a @code{permanent-local} property that is non-@code{nil}.
1366 Permanent locals are appropriate for data pertaining to where the file
1367 came from or how to save it, rather than with how to edit the contents.
1370 @subsection The Default Value of a Buffer-Local Variable
1371 @cindex default value
1373 The global value of a variable with buffer-local bindings is also
1374 called the @dfn{default} value, because it is the value that is in
1375 effect whenever neither the current buffer nor the selected frame has
1376 its own binding for the variable.
1378 The functions @code{default-value} and @code{setq-default} access and
1379 change a variable's default value regardless of whether the current
1380 buffer has a buffer-local binding. For example, you could use
1381 @code{setq-default} to change the default setting of
1382 @code{paragraph-start} for most buffers; and this would work even when
1383 you are in a C or Lisp mode buffer that has a buffer-local value for
1387 The special forms @code{defvar} and @code{defconst} also set the
1388 default value (if they set the variable at all), rather than any
1389 buffer-local or frame-local value.
1391 @defun default-value symbol
1392 This function returns @var{symbol}'s default value. This is the value
1393 that is seen in buffers and frames that do not have their own values for
1394 this variable. If @var{symbol} is not buffer-local, this is equivalent
1395 to @code{symbol-value} (@pxref{Accessing Variables}).
1399 @defun default-boundp symbol
1400 The function @code{default-boundp} tells you whether @var{symbol}'s
1401 default value is nonvoid. If @code{(default-boundp 'foo)} returns
1402 @code{nil}, then @code{(default-value 'foo)} would get an error.
1404 @code{default-boundp} is to @code{default-value} as @code{boundp} is to
1405 @code{symbol-value}.
1408 @defspec setq-default [symbol form]@dots{}
1409 This special form gives each @var{symbol} a new default value, which is
1410 the result of evaluating the corresponding @var{form}. It does not
1411 evaluate @var{symbol}, but does evaluate @var{form}. The value of the
1412 @code{setq-default} form is the value of the last @var{form}.
1414 If a @var{symbol} is not buffer-local for the current buffer, and is not
1415 marked automatically buffer-local, @code{setq-default} has the same
1416 effect as @code{setq}. If @var{symbol} is buffer-local for the current
1417 buffer, then this changes the value that other buffers will see (as long
1418 as they don't have a buffer-local value), but not the value that the
1419 current buffer sees.
1423 ;; @r{In buffer @samp{foo}:}
1424 (make-local-variable 'buffer-local)
1425 @result{} buffer-local
1428 (setq buffer-local 'value-in-foo)
1429 @result{} value-in-foo
1432 (setq-default buffer-local 'new-default)
1433 @result{} new-default
1437 @result{} value-in-foo
1440 (default-value 'buffer-local)
1441 @result{} new-default
1445 ;; @r{In (the new) buffer @samp{bar}:}
1447 @result{} new-default
1450 (default-value 'buffer-local)
1451 @result{} new-default
1454 (setq buffer-local 'another-default)
1455 @result{} another-default
1458 (default-value 'buffer-local)
1459 @result{} another-default
1463 ;; @r{Back in buffer @samp{foo}:}
1465 @result{} value-in-foo
1466 (default-value 'buffer-local)
1467 @result{} another-default
1472 @defun set-default symbol value
1473 This function is like @code{setq-default}, except that @var{symbol} is
1474 an ordinary evaluated argument.
1478 (set-default (car '(a b c)) 23)
1488 @node Frame-Local Variables
1489 @section Frame-Local Variables
1491 Just as variables can have buffer-local bindings, they can also have
1492 frame-local bindings. These bindings belong to one frame, and are in
1493 effect when that frame is selected. Frame-local bindings are actually
1494 frame parameters: you create a frame-local binding in a specific frame
1495 by calling @code{modify-frame-parameters} and specifying the variable
1496 name as the parameter name.
1498 To enable frame-local bindings for a certain variable, call the function
1499 @code{make-variable-frame-local}.
1501 @deffn Command make-variable-frame-local variable
1502 Enable the use of frame-local bindings for @var{variable}. This does
1503 not in itself create any frame-local bindings for the variable; however,
1504 if some frame already has a value for @var{variable} as a frame
1505 parameter, that value automatically becomes a frame-local binding.
1507 If the variable is terminal-local, this function signals an error,
1508 because such variables cannot have frame-local bindings as well.
1509 @xref{Multiple Displays}. A few variables that are implemented
1510 specially in Emacs can be (and usually are) buffer-local, but can never
1514 Buffer-local bindings take precedence over frame-local bindings. Thus,
1515 consider a variable @code{foo}: if the current buffer has a buffer-local
1516 binding for @code{foo}, that binding is active; otherwise, if the
1517 selected frame has a frame-local binding for @code{foo}, that binding is
1518 active; otherwise, the default binding of @code{foo} is active.
1520 Here is an example. First we prepare a few bindings for @code{foo}:
1523 (setq f1 (selected-frame))
1524 (make-variable-frame-local 'foo)
1526 ;; @r{Make a buffer-local binding for @code{foo} in @samp{b1}.}
1527 (set-buffer (get-buffer-create "b1"))
1528 (make-local-variable 'foo)
1531 ;; @r{Make a frame-local binding for @code{foo} in a new frame.}
1532 ;; @r{Store that frame in @code{f2}.}
1533 (setq f2 (make-frame))
1534 (modify-frame-parameters f2 '((foo . (f 2))))
1537 Now we examine @code{foo} in various contexts. Whenever the
1538 buffer @samp{b1} is current, its buffer-local binding is in effect,
1539 regardless of the selected frame:
1543 (set-buffer (get-buffer-create "b1"))
1548 (set-buffer (get-buffer-create "b1"))
1554 Otherwise, the frame gets a chance to provide the binding; when frame
1555 @code{f2} is selected, its frame-local binding is in effect:
1559 (set-buffer (get-buffer "*scratch*"))
1565 When neither the current buffer nor the selected frame provides
1566 a binding, the default binding is used:
1570 (set-buffer (get-buffer "*scratch*"))
1576 When the active binding of a variable is a frame-local binding, setting
1577 the variable changes that binding. You can observe the result with
1578 @code{frame-parameters}:
1582 (set-buffer (get-buffer "*scratch*"))
1584 (assq 'foo (frame-parameters f2))
1585 @result{} (foo . nobody)
1588 @node Future Local Variables
1589 @section Possible Future Local Variables
1591 We have considered the idea of bindings that are local to a category
1592 of frames---for example, all color frames, or all frames with dark
1593 backgrounds. We have not implemented them because it is not clear that
1594 this feature is really useful. You can get more or less the same
1595 results by adding a function to @code{after-make-frame-hook}, set up to
1596 define a particular frame parameter according to the appropriate
1597 conditions for each frame.
1599 It would also be possible to implement window-local bindings. We
1600 don't know of many situations where they would be useful, and it seems
1601 that indirect buffers (@pxref{Indirect Buffers}) with buffer-local
1602 bindings offer a way to handle these situations more robustly.
1604 If sufficient application is found for either of these two kinds of
1605 local bindings, we will provide it in a subsequent Emacs version.