2 @c This is part of the GNU Emacs Lisp Reference Manual.
3 @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2000,
4 @c 2001, 2002, 2003, 2004, 2005, 2006, 2007 Free Software Foundation, Inc.
5 @c See the file elisp.texi for copying conditions.
6 @setfilename ../info/variables
7 @node Variables, Functions, Control Structures, Top
11 A @dfn{variable} is a name used in a program to stand for a value.
12 Nearly all programming languages have variables of some sort. In the
13 text of a Lisp program, variables are written using the syntax for
16 In Lisp, unlike most programming languages, programs are represented
17 primarily as Lisp objects and only secondarily as text. The Lisp
18 objects used for variables are symbols: the symbol name is the variable
19 name, and the variable's value is stored in the value cell of the
20 symbol. The use of a symbol as a variable is independent of its use as
21 a function name. @xref{Symbol Components}.
23 The Lisp objects that constitute a Lisp program determine the textual
24 form of the program---it is simply the read syntax for those Lisp
25 objects. This is why, for example, a variable in a textual Lisp program
26 is written using the read syntax for the symbol that represents the
30 * Global Variables:: Variable values that exist permanently, everywhere.
31 * Constant Variables:: Certain "variables" have values that never change.
32 * Local Variables:: Variable values that exist only temporarily.
33 * Void Variables:: Symbols that lack values.
34 * Defining Variables:: A definition says a symbol is used as a variable.
35 * Tips for Defining:: Things you should think about when you
37 * Accessing Variables:: Examining values of variables whose names
38 are known only at run time.
39 * Setting Variables:: Storing new values in variables.
40 * Variable Scoping:: How Lisp chooses among local and global values.
41 * Buffer-Local Variables:: Variable values in effect only in one buffer.
42 * Frame-Local Variables:: Variable values in effect only in one frame.
43 * Future Local Variables:: New kinds of local values we might add some day.
44 * File Local Variables:: Handling local variable lists in files.
45 * Variable Aliases:: Variables that are aliases for other variables.
46 * Variables with Restricted Values:: Non-constant variables whose value can
47 @emph{not} be an arbitrary Lisp object.
50 @node Global Variables
51 @section Global Variables
52 @cindex global variable
54 The simplest way to use a variable is @dfn{globally}. This means that
55 the variable has just one value at a time, and this value is in effect
56 (at least for the moment) throughout the Lisp system. The value remains
57 in effect until you specify a new one. When a new value replaces the
58 old one, no trace of the old value remains in the variable.
60 You specify a value for a symbol with @code{setq}. For example,
67 gives the variable @code{x} the value @code{(a b)}. Note that
68 @code{setq} does not evaluate its first argument, the name of the
69 variable, but it does evaluate the second argument, the new value.
71 Once the variable has a value, you can refer to it by using the symbol
72 by itself as an expression. Thus,
81 assuming the @code{setq} form shown above has already been executed.
83 If you do set the same variable again, the new value replaces the old
101 @node Constant Variables
102 @section Variables that Never Change
103 @kindex setting-constant
104 @cindex keyword symbol
105 @cindex constant variables
106 @cindex symbols that evaluate to themselves
107 @cindex symbols with constant values
109 In Emacs Lisp, certain symbols normally evaluate to themselves. These
110 include @code{nil} and @code{t}, as well as any symbol whose name starts
111 with @samp{:} (these are called @dfn{keywords}). These symbols cannot
112 be rebound, nor can their values be changed. Any attempt to set or bind
113 @code{nil} or @code{t} signals a @code{setting-constant} error. The
114 same is true for a keyword (a symbol whose name starts with @samp{:}),
115 if it is interned in the standard obarray, except that setting such a
116 symbol to itself is not an error.
125 @error{} Attempt to set constant symbol: nil
129 @defun keywordp object
130 function returns @code{t} if @var{object} is a symbol whose name
131 starts with @samp{:}, interned in the standard obarray, and returns
132 @code{nil} otherwise.
135 @node Local Variables
136 @section Local Variables
137 @cindex binding local variables
138 @cindex local variables
139 @cindex local binding
140 @cindex global binding
142 Global variables have values that last until explicitly superseded
143 with new values. Sometimes it is useful to create variable values that
144 exist temporarily---only until a certain part of the program finishes.
145 These values are called @dfn{local}, and the variables so used are
146 called @dfn{local variables}.
148 For example, when a function is called, its argument variables receive
149 new local values that last until the function exits. The @code{let}
150 special form explicitly establishes new local values for specified
151 variables; these last until exit from the @code{let} form.
153 @cindex shadowing of variables
154 Establishing a local value saves away the previous value (or lack of
155 one) of the variable. When the life span of the local value is over,
156 the previous value is restored. In the mean time, we say that the
157 previous value is @dfn{shadowed} and @dfn{not visible}. Both global and
158 local values may be shadowed (@pxref{Scope}).
160 If you set a variable (such as with @code{setq}) while it is local,
161 this replaces the local value; it does not alter the global value, or
162 previous local values, that are shadowed. To model this behavior, we
163 speak of a @dfn{local binding} of the variable as well as a local value.
165 The local binding is a conceptual place that holds a local value.
166 Entry to a function, or a special form such as @code{let}, creates the
167 local binding; exit from the function or from the @code{let} removes the
168 local binding. As long as the local binding lasts, the variable's value
169 is stored within it. Use of @code{setq} or @code{set} while there is a
170 local binding stores a different value into the local binding; it does
171 not create a new binding.
173 We also speak of the @dfn{global binding}, which is where
174 (conceptually) the global value is kept.
176 @cindex current binding
177 A variable can have more than one local binding at a time (for
178 example, if there are nested @code{let} forms that bind it). In such a
179 case, the most recently created local binding that still exists is the
180 @dfn{current binding} of the variable. (This rule is called
181 @dfn{dynamic scoping}; see @ref{Variable Scoping}.) If there are no
182 local bindings, the variable's global binding is its current binding.
183 We sometimes call the current binding the @dfn{most-local existing
184 binding}, for emphasis. Ordinary evaluation of a symbol always returns
185 the value of its current binding.
187 The special forms @code{let} and @code{let*} exist to create
190 @defspec let (bindings@dots{}) forms@dots{}
191 This special form binds variables according to @var{bindings} and then
192 evaluates all of the @var{forms} in textual order. The @code{let}-form
193 returns the value of the last form in @var{forms}.
195 Each of the @var{bindings} is either @w{(i) a} symbol, in which case
196 that symbol is bound to @code{nil}; or @w{(ii) a} list of the form
197 @code{(@var{symbol} @var{value-form})}, in which case @var{symbol} is
198 bound to the result of evaluating @var{value-form}. If @var{value-form}
199 is omitted, @code{nil} is used.
201 All of the @var{value-form}s in @var{bindings} are evaluated in the
202 order they appear and @emph{before} binding any of the symbols to them.
203 Here is an example of this: @code{z} is bound to the old value of
204 @code{y}, which is 2, not the new value of @code{y}, which is 1.
220 @defspec let* (bindings@dots{}) forms@dots{}
221 This special form is like @code{let}, but it binds each variable right
222 after computing its local value, before computing the local value for
223 the next variable. Therefore, an expression in @var{bindings} can
224 reasonably refer to the preceding symbols bound in this @code{let*}
225 form. Compare the following example with the example above for
235 (z y)) ; @r{Use the just-established value of @code{y}.}
242 Here is a complete list of the other facilities that create local
247 Function calls (@pxref{Functions}).
250 Macro calls (@pxref{Macros}).
253 @code{condition-case} (@pxref{Errors}).
256 Variables can also have buffer-local bindings (@pxref{Buffer-Local
257 Variables}) and frame-local bindings (@pxref{Frame-Local Variables}); a
258 few variables have terminal-local bindings (@pxref{Multiple Displays}).
259 These kinds of bindings work somewhat like ordinary local bindings, but
260 they are localized depending on ``where'' you are in Emacs, rather than
263 @defvar max-specpdl-size
264 @anchor{Definition of max-specpdl-size}
265 @cindex variable limit error
266 @cindex evaluation error
267 @cindex infinite recursion
268 This variable defines the limit on the total number of local variable
269 bindings and @code{unwind-protect} cleanups (@pxref{Cleanups,,
270 Cleaning Up from Nonlocal Exits}) that are allowed before signaling an
271 error (with data @code{"Variable binding depth exceeds
274 This limit, with the associated error when it is exceeded, is one way
275 that Lisp avoids infinite recursion on an ill-defined function.
276 @code{max-lisp-eval-depth} provides another limit on depth of nesting.
277 @xref{Definition of max-lisp-eval-depth,, Eval}.
279 The default value is 1000. Entry to the Lisp debugger increases the
280 value, if there is little room left, to make sure the debugger itself
285 @section When a Variable is ``Void''
286 @kindex void-variable
287 @cindex void variable
289 If you have never given a symbol any value as a global variable, we
290 say that that symbol's global value is @dfn{void}. In other words, the
291 symbol's value cell does not have any Lisp object in it. If you try to
292 evaluate the symbol, you get a @code{void-variable} error rather than
295 Note that a value of @code{nil} is not the same as void. The symbol
296 @code{nil} is a Lisp object and can be the value of a variable just as any
297 other object can be; but it is @emph{a value}. A void variable does not
300 After you have given a variable a value, you can make it void once more
301 using @code{makunbound}.
303 @defun makunbound symbol
304 This function makes the current variable binding of @var{symbol} void.
305 Subsequent attempts to use this symbol's value as a variable will signal
306 the error @code{void-variable}, unless and until you set it again.
308 @code{makunbound} returns @var{symbol}.
312 (makunbound 'x) ; @r{Make the global value of @code{x} void.}
317 @error{} Symbol's value as variable is void: x
321 If @var{symbol} is locally bound, @code{makunbound} affects the most
322 local existing binding. This is the only way a symbol can have a void
323 local binding, since all the constructs that create local bindings
324 create them with values. In this case, the voidness lasts at most as
325 long as the binding does; when the binding is removed due to exit from
326 the construct that made it, the previous local or global binding is
327 reexposed as usual, and the variable is no longer void unless the newly
328 reexposed binding was void all along.
332 (setq x 1) ; @r{Put a value in the global binding.}
334 (let ((x 2)) ; @r{Locally bind it.}
335 (makunbound 'x) ; @r{Void the local binding.}
337 @error{} Symbol's value as variable is void: x
340 x ; @r{The global binding is unchanged.}
343 (let ((x 2)) ; @r{Locally bind it.}
344 (let ((x 3)) ; @r{And again.}
345 (makunbound 'x) ; @r{Void the innermost-local binding.}
346 x)) ; @r{And refer: it's void.}
347 @error{} Symbol's value as variable is void: x
353 (makunbound 'x)) ; @r{Void inner binding, then remove it.}
354 x) ; @r{Now outer @code{let} binding is visible.}
360 A variable that has been made void with @code{makunbound} is
361 indistinguishable from one that has never received a value and has
364 You can use the function @code{boundp} to test whether a variable is
367 @defun boundp variable
368 @code{boundp} returns @code{t} if @var{variable} (a symbol) is not void;
369 more precisely, if its current binding is not void. It returns
370 @code{nil} otherwise.
374 (boundp 'abracadabra) ; @r{Starts out void.}
378 (let ((abracadabra 5)) ; @r{Locally bind it.}
379 (boundp 'abracadabra))
383 (boundp 'abracadabra) ; @r{Still globally void.}
387 (setq abracadabra 5) ; @r{Make it globally nonvoid.}
391 (boundp 'abracadabra)
397 @node Defining Variables
398 @section Defining Global Variables
399 @cindex variable definition
401 You may announce your intention to use a symbol as a global variable
402 with a @dfn{variable definition}: a special form, either @code{defconst}
405 In Emacs Lisp, definitions serve three purposes. First, they inform
406 people who read the code that certain symbols are @emph{intended} to be
407 used a certain way (as variables). Second, they inform the Lisp system
408 of these things, supplying a value and documentation. Third, they
409 provide information to utilities such as @code{etags} and
410 @code{make-docfile}, which create data bases of the functions and
411 variables in a program.
413 The difference between @code{defconst} and @code{defvar} is primarily
414 a matter of intent, serving to inform human readers of whether the value
415 should ever change. Emacs Lisp does not restrict the ways in which a
416 variable can be used based on @code{defconst} or @code{defvar}
417 declarations. However, it does make a difference for initialization:
418 @code{defconst} unconditionally initializes the variable, while
419 @code{defvar} initializes it only if it is void.
422 One would expect user option variables to be defined with
423 @code{defconst}, since programs do not change them. Unfortunately, this
424 has bad results if the definition is in a library that is not preloaded:
425 @code{defconst} would override any prior value when the library is
426 loaded. Users would like to be able to set user options in their init
427 files, and override the default values given in the definitions. For
428 this reason, user options must be defined with @code{defvar}.
431 @defspec defvar symbol [value [doc-string]]
432 This special form defines @var{symbol} as a variable and can also
433 initialize and document it. The definition informs a person reading
434 your code that @var{symbol} is used as a variable that might be set or
435 changed. Note that @var{symbol} is not evaluated; the symbol to be
436 defined must appear explicitly in the @code{defvar}.
438 If @var{symbol} is void and @var{value} is specified, @code{defvar}
439 evaluates it and sets @var{symbol} to the result. But if @var{symbol}
440 already has a value (i.e., it is not void), @var{value} is not even
441 evaluated, and @var{symbol}'s value remains unchanged. If @var{value}
442 is omitted, the value of @var{symbol} is not changed in any case.
444 If @var{symbol} has a buffer-local binding in the current buffer,
445 @code{defvar} operates on the default value, which is buffer-independent,
446 not the current (buffer-local) binding. It sets the default value if
447 the default value is void. @xref{Buffer-Local Variables}.
449 When you evaluate a top-level @code{defvar} form with @kbd{C-M-x} in
450 Emacs Lisp mode (@code{eval-defun}), a special feature of
451 @code{eval-defun} arranges to set the variable unconditionally, without
452 testing whether its value is void.
454 If the @var{doc-string} argument appears, it specifies the documentation
455 for the variable. (This opportunity to specify documentation is one of
456 the main benefits of defining the variable.) The documentation is
457 stored in the symbol's @code{variable-documentation} property. The
458 Emacs help functions (@pxref{Documentation}) look for this property.
460 If the variable is a user option that users would want to set
461 interactively, you should use @samp{*} as the first character of
462 @var{doc-string}. This lets users set the variable conveniently using
463 the @code{set-variable} command. Note that you should nearly always
464 use @code{defcustom} instead of @code{defvar} to define these
465 variables, so that users can use @kbd{M-x customize} and related
466 commands to set them. @xref{Customization}.
468 Here are some examples. This form defines @code{foo} but does not
478 This example initializes the value of @code{bar} to @code{23}, and gives
479 it a documentation string:
484 "The normal weight of a bar.")
489 The following form changes the documentation string for @code{bar},
490 making it a user option, but does not change the value, since @code{bar}
491 already has a value. (The addition @code{(1+ nil)} would get an error
492 if it were evaluated, but since it is not evaluated, there is no error.)
497 "*The normal weight of a bar.")
506 Here is an equivalent expression for the @code{defvar} special form:
510 (defvar @var{symbol} @var{value} @var{doc-string})
513 (if (not (boundp '@var{symbol}))
514 (setq @var{symbol} @var{value}))
515 (if '@var{doc-string}
516 (put '@var{symbol} 'variable-documentation '@var{doc-string}))
521 The @code{defvar} form returns @var{symbol}, but it is normally used
522 at top level in a file where its value does not matter.
525 @defspec defconst symbol value [doc-string]
526 This special form defines @var{symbol} as a value and initializes it.
527 It informs a person reading your code that @var{symbol} has a standard
528 global value, established here, that should not be changed by the user
529 or by other programs. Note that @var{symbol} is not evaluated; the
530 symbol to be defined must appear explicitly in the @code{defconst}.
532 @code{defconst} always evaluates @var{value}, and sets the value of
533 @var{symbol} to the result. If @var{symbol} does have a buffer-local
534 binding in the current buffer, @code{defconst} sets the default value,
535 not the buffer-local value. (But you should not be making
536 buffer-local bindings for a symbol that is defined with
539 Here, @code{pi} is a constant that presumably ought not to be changed
540 by anyone (attempts by the Indiana State Legislature notwithstanding).
541 As the second form illustrates, however, this is only advisory.
545 (defconst pi 3.1415 "Pi to five places.")
559 @defun user-variable-p variable
561 This function returns @code{t} if @var{variable} is a user option---a
562 variable intended to be set by the user for customization---and
563 @code{nil} otherwise. (Variables other than user options exist for the
564 internal purposes of Lisp programs, and users need not know about them.)
566 User option variables are distinguished from other variables either
567 though being declared using @code{defcustom}@footnote{They may also be
568 declared equivalently in @file{cus-start.el}.} or by the first character
569 of their @code{variable-documentation} property. If the property exists
570 and is a string, and its first character is @samp{*}, then the variable
571 is a user option. Aliases of user options are also user options.
574 @kindex variable-interactive
575 If a user option variable has a @code{variable-interactive} property,
576 the @code{set-variable} command uses that value to control reading the
577 new value for the variable. The property's value is used as if it were
578 specified in @code{interactive} (@pxref{Using Interactive}). However,
579 this feature is largely obsoleted by @code{defcustom}
580 (@pxref{Customization}).
582 @strong{Warning:} If the @code{defconst} and @code{defvar} special
583 forms are used while the variable has a local binding (made with
584 @code{let}, or a function argument), they set the local-binding's
585 value; the top-level binding is not changed. This is not what you
586 usually want. To prevent it, use these special forms at top level in
587 a file, where normally no local binding is in effect, and make sure to
588 load the file before making a local binding for the variable.
590 @node Tips for Defining
591 @section Tips for Defining Variables Robustly
593 When you define a variable whose value is a function, or a list of
594 functions, use a name that ends in @samp{-function} or
595 @samp{-functions}, respectively.
597 There are several other variable name conventions;
598 here is a complete list:
602 The variable is a normal hook (@pxref{Hooks}).
604 @item @dots{}-function
605 The value is a function.
607 @item @dots{}-functions
608 The value is a list of functions.
611 The value is a form (an expression).
614 The value is a list of forms (expressions).
616 @item @dots{}-predicate
617 The value is a predicate---a function of one argument that returns
618 non-@code{nil} for ``good'' arguments and @code{nil} for ``bad''
622 The value is significant only as to whether it is @code{nil} or not.
624 @item @dots{}-program
625 The value is a program name.
627 @item @dots{}-command
628 The value is a whole shell command.
630 @item @dots{}-switches
631 The value specifies options for a command.
634 When you define a variable, always consider whether you should mark
635 it as ``risky''; see @ref{File Local Variables}.
637 When defining and initializing a variable that holds a complicated
638 value (such as a keymap with bindings in it), it's best to put the
639 entire computation of the value into the @code{defvar}, like this:
643 (let ((map (make-sparse-keymap)))
644 (define-key map "\C-c\C-a" 'my-command)
651 This method has several benefits. First, if the user quits while
652 loading the file, the variable is either still uninitialized or
653 initialized properly, never in-between. If it is still uninitialized,
654 reloading the file will initialize it properly. Second, reloading the
655 file once the variable is initialized will not alter it; that is
656 important if the user has run hooks to alter part of the contents (such
657 as, to rebind keys). Third, evaluating the @code{defvar} form with
658 @kbd{C-M-x} @emph{will} reinitialize the map completely.
660 Putting so much code in the @code{defvar} form has one disadvantage:
661 it puts the documentation string far away from the line which names the
662 variable. Here's a safe way to avoid that:
665 (defvar my-mode-map nil
668 (let ((map (make-sparse-keymap)))
669 (define-key map "\C-c\C-a" 'my-command)
671 (setq my-mode-map map)))
675 This has all the same advantages as putting the initialization inside
676 the @code{defvar}, except that you must type @kbd{C-M-x} twice, once on
677 each form, if you do want to reinitialize the variable.
679 But be careful not to write the code like this:
682 (defvar my-mode-map nil
685 (setq my-mode-map (make-sparse-keymap))
686 (define-key my-mode-map "\C-c\C-a" 'my-command)
691 This code sets the variable, then alters it, but it does so in more than
692 one step. If the user quits just after the @code{setq}, that leaves the
693 variable neither correctly initialized nor void nor @code{nil}. Once
694 that happens, reloading the file will not initialize the variable; it
695 will remain incomplete.
697 @node Accessing Variables
698 @section Accessing Variable Values
700 The usual way to reference a variable is to write the symbol which
701 names it (@pxref{Symbol Forms}). This requires you to specify the
702 variable name when you write the program. Usually that is exactly what
703 you want to do. Occasionally you need to choose at run time which
704 variable to reference; then you can use @code{symbol-value}.
706 @defun symbol-value symbol
707 This function returns the value of @var{symbol}. This is the value in
708 the innermost local binding of the symbol, or its global value if it
709 has no local bindings.
722 ;; @r{Here the symbol @code{abracadabra}}
723 ;; @r{is the symbol whose value is examined.}
724 (let ((abracadabra 'foo))
725 (symbol-value 'abracadabra))
730 ;; @r{Here, the value of @code{abracadabra},}
731 ;; @r{which is @code{foo},}
732 ;; @r{is the symbol whose value is examined.}
733 (let ((abracadabra 'foo))
734 (symbol-value abracadabra))
739 (symbol-value 'abracadabra)
744 A @code{void-variable} error is signaled if the current binding of
745 @var{symbol} is void.
748 @node Setting Variables
749 @section How to Alter a Variable Value
751 The usual way to change the value of a variable is with the special
752 form @code{setq}. When you need to compute the choice of variable at
753 run time, use the function @code{set}.
755 @defspec setq [symbol form]@dots{}
756 This special form is the most common method of changing a variable's
757 value. Each @var{symbol} is given a new value, which is the result of
758 evaluating the corresponding @var{form}. The most-local existing
759 binding of the symbol is changed.
761 @code{setq} does not evaluate @var{symbol}; it sets the symbol that you
762 write. We say that this argument is @dfn{automatically quoted}. The
763 @samp{q} in @code{setq} stands for ``quoted.''
765 The value of the @code{setq} form is the value of the last @var{form}.
772 x ; @r{@code{x} now has a global value.}
776 (setq x 6) ; @r{The local binding of @code{x} is set.}
780 x ; @r{The global value is unchanged.}
784 Note that the first @var{form} is evaluated, then the first
785 @var{symbol} is set, then the second @var{form} is evaluated, then the
786 second @var{symbol} is set, and so on:
790 (setq x 10 ; @r{Notice that @code{x} is set before}
791 y (1+ x)) ; @r{the value of @code{y} is computed.}
797 @defun set symbol value
798 This function sets @var{symbol}'s value to @var{value}, then returns
799 @var{value}. Since @code{set} is a function, the expression written for
800 @var{symbol} is evaluated to obtain the symbol to set.
802 The most-local existing binding of the variable is the binding that is
803 set; shadowed bindings are not affected.
808 @error{} Symbol's value as variable is void: one
819 (set two 2) ; @r{@code{two} evaluates to symbol @code{one}.}
823 one ; @r{So it is @code{one} that was set.}
825 (let ((one 1)) ; @r{This binding of @code{one} is set,}
826 (set 'one 3) ; @r{not the global value.}
836 If @var{symbol} is not actually a symbol, a @code{wrong-type-argument}
841 @error{} Wrong type argument: symbolp, (x y)
844 Logically speaking, @code{set} is a more fundamental primitive than
845 @code{setq}. Any use of @code{setq} can be trivially rewritten to use
846 @code{set}; @code{setq} could even be defined as a macro, given the
847 availability of @code{set}. However, @code{set} itself is rarely used;
848 beginners hardly need to know about it. It is useful only for choosing
849 at run time which variable to set. For example, the command
850 @code{set-variable}, which reads a variable name from the user and then
851 sets the variable, needs to use @code{set}.
853 @cindex CL note---@code{set} local
855 @b{Common Lisp note:} In Common Lisp, @code{set} always changes the
856 symbol's ``special'' or dynamic value, ignoring any lexical bindings.
857 In Emacs Lisp, all variables and all bindings are dynamic, so @code{set}
858 always affects the most local existing binding.
862 @node Variable Scoping
863 @section Scoping Rules for Variable Bindings
865 A given symbol @code{foo} can have several local variable bindings,
866 established at different places in the Lisp program, as well as a global
867 binding. The most recently established binding takes precedence over
872 @cindex dynamic scoping
873 @cindex lexical scoping
874 Local bindings in Emacs Lisp have @dfn{indefinite scope} and
875 @dfn{dynamic extent}. @dfn{Scope} refers to @emph{where} textually in
876 the source code the binding can be accessed. ``Indefinite scope'' means
877 that any part of the program can potentially access the variable
878 binding. @dfn{Extent} refers to @emph{when}, as the program is
879 executing, the binding exists. ``Dynamic extent'' means that the binding
880 lasts as long as the activation of the construct that established it.
882 The combination of dynamic extent and indefinite scope is called
883 @dfn{dynamic scoping}. By contrast, most programming languages use
884 @dfn{lexical scoping}, in which references to a local variable must be
885 located textually within the function or block that binds the variable.
887 @cindex CL note---special variables
889 @b{Common Lisp note:} Variables declared ``special'' in Common Lisp are
890 dynamically scoped, like all variables in Emacs Lisp.
894 * Scope:: Scope means where in the program a value is visible.
895 Comparison with other languages.
896 * Extent:: Extent means how long in time a value exists.
897 * Impl of Scope:: Two ways to implement dynamic scoping.
898 * Using Scoping:: How to use dynamic scoping carefully and avoid problems.
904 Emacs Lisp uses @dfn{indefinite scope} for local variable bindings.
905 This means that any function anywhere in the program text might access a
906 given binding of a variable. Consider the following function
911 (defun binder (x) ; @r{@code{x} is bound in @code{binder}.}
912 (foo 5)) ; @r{@code{foo} is some other function.}
916 (defun user () ; @r{@code{x} is used ``free'' in @code{user}.}
921 In a lexically scoped language, the binding of @code{x} in
922 @code{binder} would never be accessible in @code{user}, because
923 @code{user} is not textually contained within the function
924 @code{binder}. However, in dynamically-scoped Emacs Lisp, @code{user}
925 may or may not refer to the binding of @code{x} established in
926 @code{binder}, depending on the circumstances:
930 If we call @code{user} directly without calling @code{binder} at all,
931 then whatever binding of @code{x} is found, it cannot come from
935 If we define @code{foo} as follows and then call @code{binder}, then the
936 binding made in @code{binder} will be seen in @code{user}:
946 However, if we define @code{foo} as follows and then call @code{binder},
947 then the binding made in @code{binder} @emph{will not} be seen in
956 Here, when @code{foo} is called by @code{binder}, it binds @code{x}.
957 (The binding in @code{foo} is said to @dfn{shadow} the one made in
958 @code{binder}.) Therefore, @code{user} will access the @code{x} bound
959 by @code{foo} instead of the one bound by @code{binder}.
962 Emacs Lisp uses dynamic scoping because simple implementations of
963 lexical scoping are slow. In addition, every Lisp system needs to offer
964 dynamic scoping at least as an option; if lexical scoping is the norm,
965 there must be a way to specify dynamic scoping instead for a particular
966 variable. It might not be a bad thing for Emacs to offer both, but
967 implementing it with dynamic scoping only was much easier.
972 @dfn{Extent} refers to the time during program execution that a
973 variable name is valid. In Emacs Lisp, a variable is valid only while
974 the form that bound it is executing. This is called @dfn{dynamic
975 extent}. ``Local'' or ``automatic'' variables in most languages,
976 including C and Pascal, have dynamic extent.
978 One alternative to dynamic extent is @dfn{indefinite extent}. This
979 means that a variable binding can live on past the exit from the form
980 that made the binding. Common Lisp and Scheme, for example, support
981 this, but Emacs Lisp does not.
983 To illustrate this, the function below, @code{make-add}, returns a
984 function that purports to add @var{n} to its own argument @var{m}. This
985 would work in Common Lisp, but it does not do the job in Emacs Lisp,
986 because after the call to @code{make-add} exits, the variable @code{n}
987 is no longer bound to the actual argument 2.
991 (function (lambda (m) (+ n m)))) ; @r{Return a function.}
993 (fset 'add2 (make-add 2)) ; @r{Define function @code{add2}}
994 ; @r{with @code{(make-add 2)}.}
995 @result{} (lambda (m) (+ n m))
996 (add2 4) ; @r{Try to add 2 to 4.}
997 @error{} Symbol's value as variable is void: n
1000 @cindex closures not available
1001 Some Lisp dialects have ``closures,'' objects that are like functions
1002 but record additional variable bindings. Emacs Lisp does not have
1006 @subsection Implementation of Dynamic Scoping
1007 @cindex deep binding
1009 A simple sample implementation (which is not how Emacs Lisp actually
1010 works) may help you understand dynamic binding. This technique is
1011 called @dfn{deep binding} and was used in early Lisp systems.
1013 Suppose there is a stack of bindings, which are variable-value pairs.
1014 At entry to a function or to a @code{let} form, we can push bindings
1015 onto the stack for the arguments or local variables created there. We
1016 can pop those bindings from the stack at exit from the binding
1019 We can find the value of a variable by searching the stack from top to
1020 bottom for a binding for that variable; the value from that binding is
1021 the value of the variable. To set the variable, we search for the
1022 current binding, then store the new value into that binding.
1024 As you can see, a function's bindings remain in effect as long as it
1025 continues execution, even during its calls to other functions. That is
1026 why we say the extent of the binding is dynamic. And any other function
1027 can refer to the bindings, if it uses the same variables while the
1028 bindings are in effect. That is why we say the scope is indefinite.
1030 @cindex shallow binding
1031 The actual implementation of variable scoping in GNU Emacs Lisp uses a
1032 technique called @dfn{shallow binding}. Each variable has a standard
1033 place in which its current value is always found---the value cell of the
1036 In shallow binding, setting the variable works by storing a value in
1037 the value cell. Creating a new binding works by pushing the old value
1038 (belonging to a previous binding) onto a stack, and storing the new
1039 local value in the value cell. Eliminating a binding works by popping
1040 the old value off the stack, into the value cell.
1042 We use shallow binding because it has the same results as deep
1043 binding, but runs faster, since there is never a need to search for a
1047 @subsection Proper Use of Dynamic Scoping
1049 Binding a variable in one function and using it in another is a
1050 powerful technique, but if used without restraint, it can make programs
1051 hard to understand. There are two clean ways to use this technique:
1055 Use or bind the variable only in a few related functions, written close
1056 together in one file. Such a variable is used for communication within
1059 You should write comments to inform other programmers that they can see
1060 all uses of the variable before them, and to advise them not to add uses
1064 Give the variable a well-defined, documented meaning, and make all
1065 appropriate functions refer to it (but not bind it or set it) wherever
1066 that meaning is relevant. For example, the variable
1067 @code{case-fold-search} is defined as ``non-@code{nil} means ignore case
1068 when searching''; various search and replace functions refer to it
1069 directly or through their subroutines, but do not bind or set it.
1071 Then you can bind the variable in other programs, knowing reliably what
1075 In either case, you should define the variable with @code{defvar}.
1076 This helps other people understand your program by telling them to look
1077 for inter-function usage. It also avoids a warning from the byte
1078 compiler. Choose the variable's name to avoid name conflicts---don't
1079 use short names like @code{x}.
1081 @node Buffer-Local Variables
1082 @section Buffer-Local Variables
1083 @cindex variables, buffer-local
1084 @cindex buffer-local variables
1086 Global and local variable bindings are found in most programming
1087 languages in one form or another. Emacs, however, also supports additional,
1088 unusual kinds of variable binding: @dfn{buffer-local} bindings, which
1089 apply only in one buffer, and @dfn{frame-local} bindings, which apply only in
1090 one frame. Having different values for a variable in different buffers
1091 and/or frames is an important customization method.
1093 This section describes buffer-local bindings; for frame-local
1094 bindings, see the following section, @ref{Frame-Local Variables}. (A few
1095 variables have bindings that are local to each terminal; see
1096 @ref{Multiple Displays}.)
1099 * Intro to Buffer-Local:: Introduction and concepts.
1100 * Creating Buffer-Local:: Creating and destroying buffer-local bindings.
1101 * Default Value:: The default value is seen in buffers
1102 that don't have their own buffer-local values.
1105 @node Intro to Buffer-Local
1106 @subsection Introduction to Buffer-Local Variables
1108 A buffer-local variable has a buffer-local binding associated with a
1109 particular buffer. The binding is in effect when that buffer is
1110 current; otherwise, it is not in effect. If you set the variable while
1111 a buffer-local binding is in effect, the new value goes in that binding,
1112 so its other bindings are unchanged. This means that the change is
1113 visible only in the buffer where you made it.
1115 The variable's ordinary binding, which is not associated with any
1116 specific buffer, is called the @dfn{default binding}. In most cases,
1117 this is the global binding.
1119 A variable can have buffer-local bindings in some buffers but not in
1120 other buffers. The default binding is shared by all the buffers that
1121 don't have their own bindings for the variable. (This includes all
1122 newly-created buffers.) If you set the variable in a buffer that does
1123 not have a buffer-local binding for it, this sets the default binding
1124 (assuming there are no frame-local bindings to complicate the matter),
1125 so the new value is visible in all the buffers that see the default
1128 The most common use of buffer-local bindings is for major modes to change
1129 variables that control the behavior of commands. For example, C mode and
1130 Lisp mode both set the variable @code{paragraph-start} to specify that only
1131 blank lines separate paragraphs. They do this by making the variable
1132 buffer-local in the buffer that is being put into C mode or Lisp mode, and
1133 then setting it to the new value for that mode. @xref{Major Modes}.
1135 The usual way to make a buffer-local binding is with
1136 @code{make-local-variable}, which is what major mode commands typically
1137 use. This affects just the current buffer; all other buffers (including
1138 those yet to be created) will continue to share the default value unless
1139 they are explicitly given their own buffer-local bindings.
1141 @cindex automatically buffer-local
1142 A more powerful operation is to mark the variable as
1143 @dfn{automatically buffer-local} by calling
1144 @code{make-variable-buffer-local}. You can think of this as making the
1145 variable local in all buffers, even those yet to be created. More
1146 precisely, the effect is that setting the variable automatically makes
1147 the variable local to the current buffer if it is not already so. All
1148 buffers start out by sharing the default value of the variable as usual,
1149 but setting the variable creates a buffer-local binding for the current
1150 buffer. The new value is stored in the buffer-local binding, leaving
1151 the default binding untouched. This means that the default value cannot
1152 be changed with @code{setq} in any buffer; the only way to change it is
1153 with @code{setq-default}.
1155 @strong{Warning:} When a variable has buffer-local or frame-local
1156 bindings in one or more buffers, @code{let} rebinds the binding that's
1157 currently in effect. For instance, if the current buffer has a
1158 buffer-local value, @code{let} temporarily rebinds that. If no
1159 buffer-local or frame-local bindings are in effect, @code{let} rebinds
1160 the default value. If inside the @code{let} you then change to a
1161 different current buffer in which a different binding is in effect,
1162 you won't see the @code{let} binding any more. And if you exit the
1163 @code{let} while still in the other buffer, you won't see the
1164 unbinding occur (though it will occur properly). Here is an example
1171 (make-local-variable 'foo)
1175 ;; foo @result{} 'temp ; @r{let binding in buffer @samp{a}}
1177 ;; foo @result{} 'g ; @r{the global value since foo is not local in @samp{b}}
1180 foo @result{} 'g ; @r{exiting restored the local value in buffer @samp{a},}
1181 ; @r{but we don't see that in buffer @samp{b}}
1184 (set-buffer "a") ; @r{verify the local value was restored}
1189 Note that references to @code{foo} in @var{body} access the
1190 buffer-local binding of buffer @samp{b}.
1192 When a file specifies local variable values, these become buffer-local
1193 values when you visit the file. @xref{File Variables,,, emacs, The
1196 @node Creating Buffer-Local
1197 @subsection Creating and Deleting Buffer-Local Bindings
1199 @deffn Command make-local-variable variable
1200 This function creates a buffer-local binding in the current buffer for
1201 @var{variable} (a symbol). Other buffers are not affected. The value
1202 returned is @var{variable}.
1205 The buffer-local value of @var{variable} starts out as the same value
1206 @var{variable} previously had. If @var{variable} was void, it remains
1211 ;; @r{In buffer @samp{b1}:}
1212 (setq foo 5) ; @r{Affects all buffers.}
1216 (make-local-variable 'foo) ; @r{Now it is local in @samp{b1}.}
1220 foo ; @r{That did not change}
1221 @result{} 5 ; @r{the value.}
1224 (setq foo 6) ; @r{Change the value}
1225 @result{} 6 ; @r{in @samp{b1}.}
1233 ;; @r{In buffer @samp{b2}, the value hasn't changed.}
1241 Making a variable buffer-local within a @code{let}-binding for that
1242 variable does not work reliably, unless the buffer in which you do this
1243 is not current either on entry to or exit from the @code{let}. This is
1244 because @code{let} does not distinguish between different kinds of
1245 bindings; it knows only which variable the binding was made for.
1247 If the variable is terminal-local, this function signals an error. Such
1248 variables cannot have buffer-local bindings as well. @xref{Multiple
1251 @strong{Warning:} do not use @code{make-local-variable} for a hook
1252 variable. The hook variables are automatically made buffer-local as
1253 needed if you use the @var{local} argument to @code{add-hook} or
1257 @deffn Command make-variable-buffer-local variable
1258 This function marks @var{variable} (a symbol) automatically
1259 buffer-local, so that any subsequent attempt to set it will make it
1260 local to the current buffer at the time.
1262 A peculiar wrinkle of this feature is that binding the variable (with
1263 @code{let} or other binding constructs) does not create a buffer-local
1264 binding for it. Only setting the variable (with @code{set} or
1265 @code{setq}), while the variable does not have a @code{let}-style
1266 binding that was made in the current buffer, does so.
1268 If @var{variable} does not have a default value, then calling this
1269 command will give it a default value of @code{nil}. If @var{variable}
1270 already has a default value, that value remains unchanged.
1271 Subsequently calling @code{makunbound} on @var{variable} will result
1272 in a void buffer-local value and leave the default value unaffected.
1274 The value returned is @var{variable}.
1276 @strong{Warning:} Don't assume that you should use
1277 @code{make-variable-buffer-local} for user-option variables, simply
1278 because users @emph{might} want to customize them differently in
1279 different buffers. Users can make any variable local, when they wish
1280 to. It is better to leave the choice to them.
1282 The time to use @code{make-variable-buffer-local} is when it is crucial
1283 that no two buffers ever share the same binding. For example, when a
1284 variable is used for internal purposes in a Lisp program which depends
1285 on having separate values in separate buffers, then using
1286 @code{make-variable-buffer-local} can be the best solution.
1289 @defun local-variable-p variable &optional buffer
1290 This returns @code{t} if @var{variable} is buffer-local in buffer
1291 @var{buffer} (which defaults to the current buffer); otherwise,
1295 @defun local-variable-if-set-p variable &optional buffer
1296 This returns @code{t} if @var{variable} will become buffer-local in
1297 buffer @var{buffer} (which defaults to the current buffer) if it is
1301 @defun buffer-local-value variable buffer
1302 This function returns the buffer-local binding of @var{variable} (a
1303 symbol) in buffer @var{buffer}. If @var{variable} does not have a
1304 buffer-local binding in buffer @var{buffer}, it returns the default
1305 value (@pxref{Default Value}) of @var{variable} instead.
1308 @defun buffer-local-variables &optional buffer
1309 This function returns a list describing the buffer-local variables in
1310 buffer @var{buffer}. (If @var{buffer} is omitted, the current buffer is
1311 used.) It returns an association list (@pxref{Association Lists}) in
1312 which each element contains one buffer-local variable and its value.
1313 However, when a variable's buffer-local binding in @var{buffer} is void,
1314 then the variable appears directly in the resulting list.
1318 (make-local-variable 'foobar)
1319 (makunbound 'foobar)
1320 (make-local-variable 'bind-me)
1323 (setq lcl (buffer-local-variables))
1324 ;; @r{First, built-in variables local in all buffers:}
1325 @result{} ((mark-active . nil)
1326 (buffer-undo-list . nil)
1327 (mode-name . "Fundamental")
1330 ;; @r{Next, non-built-in buffer-local variables.}
1331 ;; @r{This one is buffer-local and void:}
1333 ;; @r{This one is buffer-local and nonvoid:}
1338 Note that storing new values into the @sc{cdr}s of cons cells in this
1339 list does @emph{not} change the buffer-local values of the variables.
1342 @deffn Command kill-local-variable variable
1343 This function deletes the buffer-local binding (if any) for
1344 @var{variable} (a symbol) in the current buffer. As a result, the
1345 default binding of @var{variable} becomes visible in this buffer. This
1346 typically results in a change in the value of @var{variable}, since the
1347 default value is usually different from the buffer-local value just
1350 If you kill the buffer-local binding of a variable that automatically
1351 becomes buffer-local when set, this makes the default value visible in
1352 the current buffer. However, if you set the variable again, that will
1353 once again create a buffer-local binding for it.
1355 @code{kill-local-variable} returns @var{variable}.
1357 This function is a command because it is sometimes useful to kill one
1358 buffer-local variable interactively, just as it is useful to create
1359 buffer-local variables interactively.
1362 @defun kill-all-local-variables
1363 This function eliminates all the buffer-local variable bindings of the
1364 current buffer except for variables marked as ``permanent.'' As a
1365 result, the buffer will see the default values of most variables.
1367 This function also resets certain other information pertaining to the
1368 buffer: it sets the local keymap to @code{nil}, the syntax table to the
1369 value of @code{(standard-syntax-table)}, the case table to
1370 @code{(standard-case-table)}, and the abbrev table to the value of
1371 @code{fundamental-mode-abbrev-table}.
1373 The very first thing this function does is run the normal hook
1374 @code{change-major-mode-hook} (see below).
1376 Every major mode command begins by calling this function, which has the
1377 effect of switching to Fundamental mode and erasing most of the effects
1378 of the previous major mode. To ensure that this does its job, the
1379 variables that major modes set should not be marked permanent.
1381 @code{kill-all-local-variables} returns @code{nil}.
1384 @defvar change-major-mode-hook
1385 The function @code{kill-all-local-variables} runs this normal hook
1386 before it does anything else. This gives major modes a way to arrange
1387 for something special to be done if the user switches to a different
1388 major mode. It is also useful for buffer-specific minor modes
1389 that should be forgotten if the user changes the major mode.
1391 For best results, make this variable buffer-local, so that it will
1392 disappear after doing its job and will not interfere with the
1393 subsequent major mode. @xref{Hooks}.
1397 @cindex permanent local variable
1398 A buffer-local variable is @dfn{permanent} if the variable name (a
1399 symbol) has a @code{permanent-local} property that is non-@code{nil}.
1400 Permanent locals are appropriate for data pertaining to where the file
1401 came from or how to save it, rather than with how to edit the contents.
1404 @subsection The Default Value of a Buffer-Local Variable
1405 @cindex default value
1407 The global value of a variable with buffer-local bindings is also
1408 called the @dfn{default} value, because it is the value that is in
1409 effect whenever neither the current buffer nor the selected frame has
1410 its own binding for the variable.
1412 The functions @code{default-value} and @code{setq-default} access and
1413 change a variable's default value regardless of whether the current
1414 buffer has a buffer-local binding. For example, you could use
1415 @code{setq-default} to change the default setting of
1416 @code{paragraph-start} for most buffers; and this would work even when
1417 you are in a C or Lisp mode buffer that has a buffer-local value for
1421 The special forms @code{defvar} and @code{defconst} also set the
1422 default value (if they set the variable at all), rather than any
1423 buffer-local or frame-local value.
1425 @defun default-value symbol
1426 This function returns @var{symbol}'s default value. This is the value
1427 that is seen in buffers and frames that do not have their own values for
1428 this variable. If @var{symbol} is not buffer-local, this is equivalent
1429 to @code{symbol-value} (@pxref{Accessing Variables}).
1433 @defun default-boundp symbol
1434 The function @code{default-boundp} tells you whether @var{symbol}'s
1435 default value is nonvoid. If @code{(default-boundp 'foo)} returns
1436 @code{nil}, then @code{(default-value 'foo)} would get an error.
1438 @code{default-boundp} is to @code{default-value} as @code{boundp} is to
1439 @code{symbol-value}.
1442 @defspec setq-default [symbol form]@dots{}
1443 This special form gives each @var{symbol} a new default value, which is
1444 the result of evaluating the corresponding @var{form}. It does not
1445 evaluate @var{symbol}, but does evaluate @var{form}. The value of the
1446 @code{setq-default} form is the value of the last @var{form}.
1448 If a @var{symbol} is not buffer-local for the current buffer, and is not
1449 marked automatically buffer-local, @code{setq-default} has the same
1450 effect as @code{setq}. If @var{symbol} is buffer-local for the current
1451 buffer, then this changes the value that other buffers will see (as long
1452 as they don't have a buffer-local value), but not the value that the
1453 current buffer sees.
1457 ;; @r{In buffer @samp{foo}:}
1458 (make-local-variable 'buffer-local)
1459 @result{} buffer-local
1462 (setq buffer-local 'value-in-foo)
1463 @result{} value-in-foo
1466 (setq-default buffer-local 'new-default)
1467 @result{} new-default
1471 @result{} value-in-foo
1474 (default-value 'buffer-local)
1475 @result{} new-default
1479 ;; @r{In (the new) buffer @samp{bar}:}
1481 @result{} new-default
1484 (default-value 'buffer-local)
1485 @result{} new-default
1488 (setq buffer-local 'another-default)
1489 @result{} another-default
1492 (default-value 'buffer-local)
1493 @result{} another-default
1497 ;; @r{Back in buffer @samp{foo}:}
1499 @result{} value-in-foo
1500 (default-value 'buffer-local)
1501 @result{} another-default
1506 @defun set-default symbol value
1507 This function is like @code{setq-default}, except that @var{symbol} is
1508 an ordinary evaluated argument.
1512 (set-default (car '(a b c)) 23)
1522 @node Frame-Local Variables
1523 @section Frame-Local Variables
1524 @cindex frame-local variables
1526 Just as variables can have buffer-local bindings, they can also have
1527 frame-local bindings. These bindings belong to one frame, and are in
1528 effect when that frame is selected. Frame-local bindings are actually
1529 frame parameters: you create a frame-local binding in a specific frame
1530 by calling @code{modify-frame-parameters} and specifying the variable
1531 name as the parameter name.
1533 To enable frame-local bindings for a certain variable, call the function
1534 @code{make-variable-frame-local}.
1536 @deffn Command make-variable-frame-local variable
1537 Enable the use of frame-local bindings for @var{variable}. This does
1538 not in itself create any frame-local bindings for the variable; however,
1539 if some frame already has a value for @var{variable} as a frame
1540 parameter, that value automatically becomes a frame-local binding.
1542 If @var{variable} does not have a default value, then calling this
1543 command will give it a default value of @code{nil}. If @var{variable}
1544 already has a default value, that value remains unchanged.
1546 If the variable is terminal-local, this function signals an error,
1547 because such variables cannot have frame-local bindings as well.
1548 @xref{Multiple Displays}. A few variables that are implemented
1549 specially in Emacs can be buffer-local, but can never be frame-local.
1551 This command returns @var{variable}.
1554 Buffer-local bindings take precedence over frame-local bindings. Thus,
1555 consider a variable @code{foo}: if the current buffer has a buffer-local
1556 binding for @code{foo}, that binding is active; otherwise, if the
1557 selected frame has a frame-local binding for @code{foo}, that binding is
1558 active; otherwise, the default binding of @code{foo} is active.
1560 Here is an example. First we prepare a few bindings for @code{foo}:
1563 (setq f1 (selected-frame))
1564 (make-variable-frame-local 'foo)
1566 ;; @r{Make a buffer-local binding for @code{foo} in @samp{b1}.}
1567 (set-buffer (get-buffer-create "b1"))
1568 (make-local-variable 'foo)
1571 ;; @r{Make a frame-local binding for @code{foo} in a new frame.}
1572 ;; @r{Store that frame in @code{f2}.}
1573 (setq f2 (make-frame))
1574 (modify-frame-parameters f2 '((foo . (f 2))))
1577 Now we examine @code{foo} in various contexts. Whenever the
1578 buffer @samp{b1} is current, its buffer-local binding is in effect,
1579 regardless of the selected frame:
1583 (set-buffer (get-buffer-create "b1"))
1588 (set-buffer (get-buffer-create "b1"))
1594 Otherwise, the frame gets a chance to provide the binding; when frame
1595 @code{f2} is selected, its frame-local binding is in effect:
1599 (set-buffer (get-buffer "*scratch*"))
1605 When neither the current buffer nor the selected frame provides
1606 a binding, the default binding is used:
1610 (set-buffer (get-buffer "*scratch*"))
1616 When the active binding of a variable is a frame-local binding, setting
1617 the variable changes that binding. You can observe the result with
1618 @code{frame-parameters}:
1622 (set-buffer (get-buffer "*scratch*"))
1624 (assq 'foo (frame-parameters f2))
1625 @result{} (foo . nobody)
1628 @node Future Local Variables
1629 @section Possible Future Local Variables
1631 We have considered the idea of bindings that are local to a category
1632 of frames---for example, all color frames, or all frames with dark
1633 backgrounds. We have not implemented them because it is not clear that
1634 this feature is really useful. You can get more or less the same
1635 results by adding a function to @code{after-make-frame-functions}, set up to
1636 define a particular frame parameter according to the appropriate
1637 conditions for each frame.
1639 It would also be possible to implement window-local bindings. We
1640 don't know of many situations where they would be useful, and it seems
1641 that indirect buffers (@pxref{Indirect Buffers}) with buffer-local
1642 bindings offer a way to handle these situations more robustly.
1644 If sufficient application is found for either of these two kinds of
1645 local bindings, we will provide it in a subsequent Emacs version.
1647 @node File Local Variables
1648 @section File Local Variables
1649 @cindex file local variables
1651 A file can specify local variable values; Emacs uses these to create
1652 buffer-local bindings for those variables in the buffer visiting that
1653 file. @xref{File variables, , Local Variables in Files, emacs, The
1654 GNU Emacs Manual}, for basic information about file local variables.
1655 This section describes the functions and variables that affect
1656 processing of file local variables.
1658 @defopt enable-local-variables
1659 This variable controls whether to process file local variables.
1660 The possible values are:
1663 @item @code{t} (the default)
1664 Set the safe variables, and query (once) about any unsafe variables.
1666 Set only the safe variables and do not query.
1668 Set all the variables and do not query.
1670 Don't set any variables.
1672 Query (once) about all the variables.
1676 @defun hack-local-variables &optional mode-only
1677 This function parses, and binds or evaluates as appropriate, any local
1678 variables specified by the contents of the current buffer. The variable
1679 @code{enable-local-variables} has its effect here. However, this
1680 function does not look for the @samp{mode:} local variable in the
1681 @w{@samp{-*-}} line. @code{set-auto-mode} does that, also taking
1682 @code{enable-local-variables} into account (@pxref{Auto Major Mode}).
1684 If the optional argument @var{mode-only} is non-@code{nil}, then all
1685 this function does is return @code{t} if the @w{@samp{-*-}} line or
1686 the local variables list specifies a mode and @code{nil} otherwise.
1687 It does not set the mode nor any other file local variable.
1690 If a file local variable could specify a function that would
1691 be called later, or an expression that would be executed later, simply
1692 visiting a file could take over your Emacs. Emacs takes several
1693 measures to prevent this.
1695 @cindex safe local variable
1696 You can specify safe values for a variable with a
1697 @code{safe-local-variable} property. The property has to be
1698 a function of one argument; any value is safe if the function
1699 returns non-@code{nil} given that value. Many commonly encountered
1700 file variables standardly have @code{safe-local-variable} properties,
1701 including @code{fill-column}, @code{fill-prefix}, and
1702 @code{indent-tabs-mode}. For boolean-valued variables that are safe,
1703 use @code{booleanp} as the property value. Lambda expressions should
1704 be quoted so that @code{describe-variable} can display the predicate.
1706 @defopt safe-local-variable-values
1707 This variable provides another way to mark some variable values as
1708 safe. It is a list of cons cells @code{(@var{var} . @var{val})},
1709 where @var{var} is a variable name and @var{val} is a value which is
1710 safe for that variable.
1712 When Emacs asks the user whether or not to obey a set of file local
1713 variable specifications, the user can choose to mark them as safe.
1714 Doing so adds those variable/value pairs to
1715 @code{safe-local-variable-values}, and saves it to the user's custom
1719 @defun safe-local-variable-p sym val
1720 This function returns non-@code{nil} if it is safe to give @var{sym}
1721 the value @var{val}, based on the above criteria.
1724 @cindex risky local variable
1725 Some variables are considered @dfn{risky}. A variable whose name
1726 ends in any of @samp{-command}, @samp{-frame-alist}, @samp{-function},
1727 @samp{-functions}, @samp{-hook}, @samp{-hooks}, @samp{-form},
1728 @samp{-forms}, @samp{-map}, @samp{-map-alist}, @samp{-mode-alist},
1729 @samp{-program}, or @samp{-predicate} is considered risky. The
1730 variables @samp{font-lock-keywords}, @samp{font-lock-keywords}
1731 followed by a digit, and @samp{font-lock-syntactic-keywords} are also
1732 considered risky. Finally, any variable whose name has a
1733 non-@code{nil} @code{risky-local-variable} property is considered
1736 @defun risky-local-variable-p sym
1737 This function returns non-@code{nil} if @var{sym} is a risky variable,
1738 based on the above criteria.
1741 If a variable is risky, it will not be entered automatically into
1742 @code{safe-local-variable-values} as described above. Therefore,
1743 Emacs will always query before setting a risky variable, unless the
1744 user explicitly allows the setting by customizing
1745 @code{safe-local-variable-values} directly.
1747 @defvar ignored-local-variables
1748 This variable holds a list of variables that should not be given local
1749 values by files. Any value specified for one of these variables is
1753 The @samp{Eval:} ``variable'' is also a potential loophole, so Emacs
1754 normally asks for confirmation before handling it.
1756 @defopt enable-local-eval
1757 This variable controls processing of @samp{Eval:} in @samp{-*-} lines
1759 lists in files being visited. A value of @code{t} means process them
1760 unconditionally; @code{nil} means ignore them; anything else means ask
1761 the user what to do for each file. The default value is @code{maybe}.
1764 @defopt safe-local-eval-forms
1765 This variable holds a list of expressions that are safe to
1766 evaluate when found in the @samp{Eval:} ``variable'' in a file
1767 local variables list.
1770 If the expression is a function call and the function has a
1771 @code{safe-local-eval-function} property, the property value
1772 determines whether the expression is safe to evaluate. The property
1773 value can be a predicate to call to test the expression, a list of
1774 such predicates (it's safe if any predicate succeeds), or @code{t}
1775 (always safe provided the arguments are constant).
1777 Text properties are also potential loopholes, since their values
1778 could include functions to call. So Emacs discards all text
1779 properties from string values specified for file local variables.
1781 @node Variable Aliases
1782 @section Variable Aliases
1783 @cindex variable aliases
1785 It is sometimes useful to make two variables synonyms, so that both
1786 variables always have the same value, and changing either one also
1787 changes the other. Whenever you change the name of a
1788 variable---either because you realize its old name was not well
1789 chosen, or because its meaning has partly changed---it can be useful
1790 to keep the old name as an @emph{alias} of the new one for
1791 compatibility. You can do this with @code{defvaralias}.
1793 @defun defvaralias new-alias base-variable &optional docstring
1794 This function defines the symbol @var{new-alias} as a variable alias
1795 for symbol @var{base-variable}. This means that retrieving the value
1796 of @var{new-alias} returns the value of @var{base-variable}, and
1797 changing the value of @var{new-alias} changes the value of
1798 @var{base-variable}. The two aliased variable names always share the
1799 same value and the same bindings.
1801 If the @var{docstring} argument is non-@code{nil}, it specifies the
1802 documentation for @var{new-alias}; otherwise, the alias gets the same
1803 documentation as @var{base-variable} has, if any, unless
1804 @var{base-variable} is itself an alias, in which case @var{new-alias} gets
1805 the documentation of the variable at the end of the chain of aliases.
1807 This function returns @var{base-variable}.
1810 Variable aliases are convenient for replacing an old name for a
1811 variable with a new name. @code{make-obsolete-variable} declares that
1812 the old name is obsolete and therefore that it may be removed at some
1813 stage in the future.
1815 @defun make-obsolete-variable obsolete-name current-name &optional when
1816 This function makes the byte-compiler warn that the variable
1817 @var{obsolete-name} is obsolete. If @var{current-name} is a symbol, it is
1818 the variable's new name; then the warning message says to use
1819 @var{current-name} instead of @var{obsolete-name}. If @var{current-name}
1820 is a string, this is the message and there is no replacement variable.
1822 If provided, @var{when} should be a string indicating when the
1823 variable was first made obsolete---for example, a date or a release
1827 You can make two variables synonyms and declare one obsolete at the
1828 same time using the macro @code{define-obsolete-variable-alias}.
1830 @defmac define-obsolete-variable-alias obsolete-name current-name &optional when docstring
1831 This macro marks the variable @var{obsolete-name} as obsolete and also
1832 makes it an alias for the variable @var{current-name}. It is
1833 equivalent to the following:
1836 (defvaralias @var{obsolete-name} @var{current-name} @var{docstring})
1837 (make-obsolete-variable @var{obsolete-name} @var{current-name} @var{when})
1841 @defun indirect-variable variable
1842 This function returns the variable at the end of the chain of aliases
1843 of @var{variable}. If @var{variable} is not a symbol, or if @var{variable} is
1844 not defined as an alias, the function returns @var{variable}.
1846 This function signals a @code{cyclic-variable-indirection} error if
1847 there is a loop in the chain of symbols.
1851 (defvaralias 'foo 'bar)
1852 (indirect-variable 'foo)
1854 (indirect-variable 'bar)
1870 @node Variables with Restricted Values
1871 @section Variables with Restricted Values
1873 Ordinary Lisp variables can be assigned any value that is a valid
1874 Lisp object. However, certain Lisp variables are not defined in Lisp,
1875 but in C. Most of these variables are defined in the C code using
1876 @code{DEFVAR_LISP}. Like variables defined in Lisp, these can take on
1877 any value. However, some variables are defined using
1878 @code{DEFVAR_INT} or @code{DEFVAR_BOOL}. @xref{Defining Lisp
1879 variables in C,, Writing Emacs Primitives}, in particular the
1880 description of functions of the type @code{syms_of_@var{filename}},
1881 for a brief discussion of the C implementation.
1883 Variables of type @code{DEFVAR_BOOL} can only take on the values
1884 @code{nil} or @code{t}. Attempting to assign them any other value
1885 will set them to @code{t}:
1888 (let ((display-hourglass 5))
1893 @defvar byte-boolean-vars
1894 This variable holds a list of all variables of type @code{DEFVAR_BOOL}.
1897 Variables of type @code{DEFVAR_INT} can only take on integer values.
1898 Attempting to assign them any other value will result in an error:
1901 (setq window-min-height 5.0)
1902 @error{} Wrong type argument: integerp, 5.0
1906 arch-tag: 5ff62c44-2b51-47bb-99d4-fea5aeec5d3e