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, 2008, 2009, 2010, 2011, 2012
5 @c Free Software Foundation, Inc.
6 @c See the file elisp.texi for copying conditions.
7 @setfilename ../../info/variables
8 @node Variables, Functions, Control Structures, Top
12 A @dfn{variable} is a name used in a program to stand for a value.
13 Nearly all programming languages have variables of some sort. In the
14 text of a Lisp program, variables are written using the syntax for
17 In Lisp, unlike most programming languages, programs are represented
18 primarily as Lisp objects and only secondarily as text. The Lisp
19 objects used for variables are symbols: the symbol name is the
20 variable name, and the variable's value is stored in the value cell of
21 the symbol. The use of a symbol as a variable is independent of its
22 use as a function name. @xref{Symbol Components}.
24 The textual form of a Lisp program is given by the read syntax of
25 the Lisp objects that constitute the program. Hence, a variable in a
26 textual Lisp program is written using the read syntax for the symbol
27 representing the variable.
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 * File Local Variables:: Handling local variable lists in files.
43 * Directory Local Variables:: Local variables common to all files in a directory.
44 * Frame-Local Variables:: Frame-local bindings for variables.
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} is a special form (@pxref{Special Forms}); it does not
69 evaluate its first argument, the name of the variable, but it does
70 evaluate the second argument, the new value.
72 Once the variable has a value, you can refer to it by using the
73 symbol itself as an expression. Thus,
82 assuming the @code{setq} form shown above has already been executed.
84 If you do set the same variable again, the new value replaces the old
102 @node Constant Variables
103 @section Variables that Never Change
104 @kindex setting-constant
105 @cindex keyword symbol
106 @cindex variable with constant value
107 @cindex constant variables
108 @cindex symbol that evaluates to itself
109 @cindex symbol with constant value
111 In Emacs Lisp, certain symbols normally evaluate to themselves. These
112 include @code{nil} and @code{t}, as well as any symbol whose name starts
113 with @samp{:} (these are called @dfn{keywords}). These symbols cannot
114 be rebound, nor can their values be changed. Any attempt to set or bind
115 @code{nil} or @code{t} signals a @code{setting-constant} error. The
116 same is true for a keyword (a symbol whose name starts with @samp{:}),
117 if it is interned in the standard obarray, except that setting such a
118 symbol to itself is not an error.
127 @error{} Attempt to set constant symbol: nil
131 @defun keywordp object
132 function returns @code{t} if @var{object} is a symbol whose name
133 starts with @samp{:}, interned in the standard obarray, and returns
134 @code{nil} otherwise.
137 These constants are fundamentally different from the ``constants''
138 defined using the @code{defconst} special form (@pxref{Defining
139 Variables}). A @code{defconst} form serves to inform human readers
140 that you do not intend to change the value of a variable, but Emacs
141 does not raise an error if you actually change it.
143 @node Local Variables
144 @section Local Variables
145 @cindex binding local variables
146 @cindex local variables
147 @cindex local binding
148 @cindex global binding
150 Global variables have values that last until explicitly superseded
151 with new values. Sometimes it is useful to create variable values that
152 exist temporarily---only until a certain part of the program finishes.
153 These values are called @dfn{local}, and the variables so used are
154 called @dfn{local variables}.
156 For example, when a function is called, its argument variables receive
157 new local values that last until the function exits. The @code{let}
158 special form explicitly establishes new local values for specified
159 variables; these last until exit from the @code{let} form.
161 @cindex shadowing of variables
162 Establishing a local value saves away the variable's previous value
163 (or lack of one). We say that the previous value is @dfn{shadowed}
164 and @dfn{not visible}. Both global and local values may be shadowed
165 (@pxref{Scope}). After the life span of the local value is over, the
166 previous value (or lack of one) is restored.
168 If you set a variable (such as with @code{setq}) while it is local,
169 this replaces the local value; it does not alter the global value, or
170 previous local values, that are shadowed. To model this behavior, we
171 speak of a @dfn{local binding} of the variable as well as a local value.
173 The local binding is a conceptual place that holds a local value.
174 Entering a function, or a special form such as @code{let}, creates the
175 local binding; exiting the function or the @code{let} removes the
176 local binding. While the local binding lasts, the variable's value is
177 stored within it. Using @code{setq} or @code{set} while there is a
178 local binding stores a different value into the local binding; it does
179 not create a new binding.
181 We also speak of the @dfn{global binding}, which is where
182 (conceptually) the global value is kept.
184 @cindex current binding
185 A variable can have more than one local binding at a time (for
186 example, if there are nested @code{let} forms that bind it). In such a
187 case, the most recently created local binding that still exists is the
188 @dfn{current binding} of the variable. (This rule is called
189 @dfn{dynamic scoping}; see @ref{Variable Scoping}.) If there are no
190 local bindings, the variable's global binding is its current binding.
191 We sometimes call the current binding the @dfn{most-local existing
192 binding}, for emphasis. Ordinary evaluation of a symbol always returns
193 the value of its current binding.
195 The special forms @code{let} and @code{let*} exist to create
198 @defspec let (bindings@dots{}) forms@dots{}
199 This special form binds variables according to @var{bindings} and then
200 evaluates all of the @var{forms} in textual order. The @code{let}-form
201 returns the value of the last form in @var{forms}.
203 Each of the @var{bindings} is either @w{(i) a} symbol, in which case
204 that symbol is bound to @code{nil}; or @w{(ii) a} list of the form
205 @code{(@var{symbol} @var{value-form})}, in which case @var{symbol} is
206 bound to the result of evaluating @var{value-form}. If @var{value-form}
207 is omitted, @code{nil} is used.
209 All of the @var{value-form}s in @var{bindings} are evaluated in the
210 order they appear and @emph{before} binding any of the symbols to them.
211 Here is an example of this: @code{z} is bound to the old value of
212 @code{y}, which is 2, not the new value of @code{y}, which is 1.
228 @defspec let* (bindings@dots{}) forms@dots{}
229 This special form is like @code{let}, but it binds each variable right
230 after computing its local value, before computing the local value for
231 the next variable. Therefore, an expression in @var{bindings} can
232 reasonably refer to the preceding symbols bound in this @code{let*}
233 form. Compare the following example with the example above for
243 (z y)) ; @r{Use the just-established value of @code{y}.}
250 Here is a complete list of the other facilities that create local
255 Function calls (@pxref{Functions}).
258 Macro calls (@pxref{Macros}).
261 @code{condition-case} (@pxref{Errors}).
264 Variables can also have buffer-local bindings (@pxref{Buffer-Local
265 Variables}); a few variables have terminal-local bindings
266 (@pxref{Multiple Terminals}). These kinds of bindings work somewhat
267 like ordinary local bindings, but they are localized depending on
268 ``where'' you are in Emacs, rather than localized in time.
270 @defopt max-specpdl-size
271 @anchor{Definition of max-specpdl-size}
272 @cindex variable limit error
273 @cindex evaluation error
274 @cindex infinite recursion
275 This variable defines the limit on the total number of local variable
276 bindings and @code{unwind-protect} cleanups (see @ref{Cleanups,,
277 Cleaning Up from Nonlocal Exits}) that are allowed before Emacs
278 signals an error (with data @code{"Variable binding depth exceeds
281 This limit, with the associated error when it is exceeded, is one way
282 that Lisp avoids infinite recursion on an ill-defined function.
283 @code{max-lisp-eval-depth} provides another limit on depth of nesting.
284 @xref{Definition of max-lisp-eval-depth,, Eval}.
286 The default value is 1000. Entry to the Lisp debugger increases the
287 value, if there is little room left, to make sure the debugger itself
292 @section When a Variable is ``Void''
293 @kindex void-variable
294 @cindex void variable
296 If you have never given a symbol any value as a global variable, we
297 say that that symbol's global value is @dfn{void}. In other words, the
298 symbol's value cell does not have any Lisp object in it. If you try to
299 evaluate the symbol, you get a @code{void-variable} error rather than
302 Note that a value of @code{nil} is not the same as void. The symbol
303 @code{nil} is a Lisp object and can be the value of a variable just as any
304 other object can be; but it is @emph{a value}. A void variable does not
307 After you have given a variable a value, you can make it void once more
308 using @code{makunbound}.
310 @defun makunbound symbol
311 This function makes the current variable binding of @var{symbol} void.
312 Subsequent attempts to use this symbol's value as a variable will signal
313 the error @code{void-variable}, unless and until you set it again.
315 @code{makunbound} returns @var{symbol}.
319 (makunbound 'x) ; @r{Make the global value of @code{x} void.}
324 @error{} Symbol's value as variable is void: x
328 If @var{symbol} is locally bound, @code{makunbound} affects the most
329 local existing binding. This is the only way a symbol can have a void
330 local binding, since all the constructs that create local bindings
331 create them with values. In this case, the voidness lasts at most as
332 long as the binding does; when the binding is removed due to exit from
333 the construct that made it, the previous local or global binding is
334 reexposed as usual, and the variable is no longer void unless the newly
335 reexposed binding was void all along.
339 (setq x 1) ; @r{Put a value in the global binding.}
341 (let ((x 2)) ; @r{Locally bind it.}
342 (makunbound 'x) ; @r{Void the local binding.}
344 @error{} Symbol's value as variable is void: x
347 x ; @r{The global binding is unchanged.}
350 (let ((x 2)) ; @r{Locally bind it.}
351 (let ((x 3)) ; @r{And again.}
352 (makunbound 'x) ; @r{Void the innermost-local binding.}
353 x)) ; @r{And refer: it's void.}
354 @error{} Symbol's value as variable is void: x
360 (makunbound 'x)) ; @r{Void inner binding, then remove it.}
361 x) ; @r{Now outer @code{let} binding is visible.}
367 A variable that has been made void with @code{makunbound} is
368 indistinguishable from one that has never received a value and has
371 You can use the function @code{boundp} to test whether a variable is
374 @defun boundp variable
375 @code{boundp} returns @code{t} if @var{variable} (a symbol) is not void;
376 more precisely, if its current binding is not void. It returns
377 @code{nil} otherwise.
381 (boundp 'abracadabra) ; @r{Starts out void.}
385 (let ((abracadabra 5)) ; @r{Locally bind it.}
386 (boundp 'abracadabra))
390 (boundp 'abracadabra) ; @r{Still globally void.}
394 (setq abracadabra 5) ; @r{Make it globally nonvoid.}
398 (boundp 'abracadabra)
404 @node Defining Variables
405 @section Defining Global Variables
406 @cindex variable definition
408 You may announce your intention to use a symbol as a global variable
409 with a @dfn{variable definition}: a special form, either @code{defconst}
412 In Emacs Lisp, definitions serve three purposes. First, they inform
413 people who read the code that certain symbols are @emph{intended} to be
414 used a certain way (as variables). Second, they inform the Lisp system
415 of these things, supplying a value and documentation. Third, they
416 provide information to utilities such as @code{etags} and
417 @code{make-docfile}, which create data bases of the functions and
418 variables in a program.
420 The difference between @code{defconst} and @code{defvar} is primarily
421 a matter of intent, serving to inform human readers of whether the value
422 should ever change. Emacs Lisp does not restrict the ways in which a
423 variable can be used based on @code{defconst} or @code{defvar}
424 declarations. However, it does make a difference for initialization:
425 @code{defconst} unconditionally initializes the variable, while
426 @code{defvar} initializes it only if it is void.
429 One would expect user option variables to be defined with
430 @code{defconst}, since programs do not change them. Unfortunately, this
431 has bad results if the definition is in a library that is not preloaded:
432 @code{defconst} would override any prior value when the library is
433 loaded. Users would like to be able to set user options in their init
434 files, and override the default values given in the definitions. For
435 this reason, user options must be defined with @code{defvar}.
438 @defspec defvar symbol [value [doc-string]]
439 This special form defines @var{symbol} as a variable and can also
440 initialize and document it. The definition informs a person reading
441 your code that @var{symbol} is used as a variable that might be set or
442 changed. Note that @var{symbol} is not evaluated; the symbol to be
443 defined must appear explicitly in the @code{defvar}.
445 If @var{symbol} is void and @var{value} is specified, @code{defvar}
446 evaluates it and sets @var{symbol} to the result. But if @var{symbol}
447 already has a value (i.e., it is not void), @var{value} is not even
448 evaluated, and @var{symbol}'s value remains unchanged. If @var{value}
449 is omitted, the value of @var{symbol} is not changed in any case.
451 If @var{symbol} has a buffer-local binding in the current buffer,
452 @code{defvar} operates on the default value, which is buffer-independent,
453 not the current (buffer-local) binding. It sets the default value if
454 the default value is void. @xref{Buffer-Local Variables}.
456 When you evaluate a top-level @code{defvar} form with @kbd{C-M-x} in
457 Emacs Lisp mode (@code{eval-defun}), a special feature of
458 @code{eval-defun} arranges to set the variable unconditionally, without
459 testing whether its value is void.
461 If the @var{doc-string} argument appears, it specifies the documentation
462 for the variable. (This opportunity to specify documentation is one of
463 the main benefits of defining the variable.) The documentation is
464 stored in the symbol's @code{variable-documentation} property. The
465 Emacs help functions (@pxref{Documentation}) look for this property.
467 If the documentation string begins with the character @samp{*}, Emacs
468 allows users to set it interactively using the @code{set-variable}
469 command. However, you should nearly always use @code{defcustom}
470 instead of @code{defvar} to define such variables, so that users can
471 use @kbd{M-x customize} and related commands to set them. In that
472 case, it is not necessary to begin the documentation string with
473 @samp{*}. @xref{Customization}.
475 Here are some examples. This form defines @code{foo} but does not
485 This example initializes the value of @code{bar} to @code{23}, and gives
486 it a documentation string:
491 "The normal weight of a bar.")
496 The following form changes the documentation string for @code{bar},
497 making it a user option, but does not change the value, since @code{bar}
498 already has a value. (The addition @code{(1+ nil)} would get an error
499 if it were evaluated, but since it is not evaluated, there is no error.)
504 "*The normal weight of a bar.")
513 Here is an equivalent expression for the @code{defvar} special form:
517 (defvar @var{symbol} @var{value} @var{doc-string})
520 (if (not (boundp '@var{symbol}))
521 (setq @var{symbol} @var{value}))
522 (if '@var{doc-string}
523 (put '@var{symbol} 'variable-documentation '@var{doc-string}))
528 The @code{defvar} form returns @var{symbol}, but it is normally used
529 at top level in a file where its value does not matter.
532 @cindex constant variables
533 @defspec defconst symbol value [doc-string]
534 This special form defines @var{symbol} as a value and initializes it.
535 It informs a person reading your code that @var{symbol} has a standard
536 global value, established here, that should not be changed by the user
537 or by other programs. Note that @var{symbol} is not evaluated; the
538 symbol to be defined must appear explicitly in the @code{defconst}.
540 @code{defconst} always evaluates @var{value}, and sets the value of
541 @var{symbol} to the result. If @var{symbol} does have a buffer-local
542 binding in the current buffer, @code{defconst} sets the default value,
543 not the buffer-local value. (But you should not be making
544 buffer-local bindings for a symbol that is defined with
547 An example of the use of @code{defconst} is Emacs' definition of
548 @code{float-pi}---the mathematical constant @math{pi}, which ought not
549 to be changed by anyone (attempts by the Indiana State Legislature
550 notwithstanding). As the second form illustrates, however,
551 @code{defconst} is only advisory.
555 (defconst float-pi 3.141592653589793 "The value of Pi.")
569 @defun user-variable-p variable
571 This function returns @code{t} if @var{variable} is a user option---a
572 variable intended to be set by the user for customization---and
573 @code{nil} otherwise. (Variables other than user options exist for the
574 internal purposes of Lisp programs, and users need not know about them.)
576 User option variables are distinguished from other variables either
577 though being declared using @code{defcustom}@footnote{They may also be
578 declared equivalently in @file{cus-start.el}.} or by the first character
579 of their @code{variable-documentation} property. If the property exists
580 and is a string, and its first character is @samp{*}, then the variable
581 is a user option. Aliases of user options are also user options.
584 @kindex variable-interactive
585 If a user option variable has a @code{variable-interactive} property,
586 the @code{set-variable} command uses that value to control reading the
587 new value for the variable. The property's value is used as if it were
588 specified in @code{interactive} (@pxref{Using Interactive}). However,
589 this feature is largely obsoleted by @code{defcustom}
590 (@pxref{Customization}).
592 @strong{Warning:} If the @code{defconst} and @code{defvar} special
593 forms are used while the variable has a local binding (made with
594 @code{let}, or a function argument), they set the local-binding's
595 value; the top-level binding is not changed. This is not what you
596 usually want. To prevent it, use these special forms at top level in
597 a file, where normally no local binding is in effect, and make sure to
598 load the file before making a local binding for the variable.
600 @node Tips for Defining
601 @section Tips for Defining Variables Robustly
603 When you define a variable whose value is a function, or a list of
604 functions, use a name that ends in @samp{-function} or
605 @samp{-functions}, respectively.
607 There are several other variable name conventions;
608 here is a complete list:
612 The variable is a normal hook (@pxref{Hooks}).
614 @item @dots{}-function
615 The value is a function.
617 @item @dots{}-functions
618 The value is a list of functions.
621 The value is a form (an expression).
624 The value is a list of forms (expressions).
626 @item @dots{}-predicate
627 The value is a predicate---a function of one argument that returns
628 non-@code{nil} for ``good'' arguments and @code{nil} for ``bad''
632 The value is significant only as to whether it is @code{nil} or not.
633 Since such variables often end up acquiring more values over time,
634 this convention is not strongly recommended.
636 @item @dots{}-program
637 The value is a program name.
639 @item @dots{}-command
640 The value is a whole shell command.
642 @item @dots{}-switches
643 The value specifies options for a command.
646 When you define a variable, always consider whether you should mark
647 it as ``safe'' or ``risky''; see @ref{File Local Variables}.
649 When defining and initializing a variable that holds a complicated
650 value (such as a keymap with bindings in it), it's best to put the
651 entire computation of the value into the @code{defvar}, like this:
655 (let ((map (make-sparse-keymap)))
656 (define-key map "\C-c\C-a" 'my-command)
663 This method has several benefits. First, if the user quits while
664 loading the file, the variable is either still uninitialized or
665 initialized properly, never in-between. If it is still uninitialized,
666 reloading the file will initialize it properly. Second, reloading the
667 file once the variable is initialized will not alter it; that is
668 important if the user has run hooks to alter part of the contents (such
669 as, to rebind keys). Third, evaluating the @code{defvar} form with
670 @kbd{C-M-x} @emph{will} reinitialize the map completely.
672 Putting so much code in the @code{defvar} form has one disadvantage:
673 it puts the documentation string far away from the line which names the
674 variable. Here's a safe way to avoid that:
677 (defvar my-mode-map nil
680 (let ((map (make-sparse-keymap)))
681 (define-key map "\C-c\C-a" 'my-command)
683 (setq my-mode-map map)))
687 This has all the same advantages as putting the initialization inside
688 the @code{defvar}, except that you must type @kbd{C-M-x} twice, once on
689 each form, if you do want to reinitialize the variable.
691 But be careful not to write the code like this:
694 (defvar my-mode-map nil
697 (setq my-mode-map (make-sparse-keymap))
698 (define-key my-mode-map "\C-c\C-a" 'my-command)
703 This code sets the variable, then alters it, but it does so in more than
704 one step. If the user quits just after the @code{setq}, that leaves the
705 variable neither correctly initialized nor void nor @code{nil}. Once
706 that happens, reloading the file will not initialize the variable; it
707 will remain incomplete.
709 @node Accessing Variables
710 @section Accessing Variable Values
712 The usual way to reference a variable is to write the symbol which
713 names it (@pxref{Symbol Forms}). This requires you to specify the
714 variable name when you write the program. Usually that is exactly what
715 you want to do. Occasionally you need to choose at run time which
716 variable to reference; then you can use @code{symbol-value}.
718 @defun symbol-value symbol
719 This function returns the value of @var{symbol}. This is the value in
720 the innermost local binding of the symbol, or its global value if it
721 has no local bindings.
734 ;; @r{Here the symbol @code{abracadabra}}
735 ;; @r{is the symbol whose value is examined.}
736 (let ((abracadabra 'foo))
737 (symbol-value 'abracadabra))
742 ;; @r{Here, the value of @code{abracadabra},}
743 ;; @r{which is @code{foo},}
744 ;; @r{is the symbol whose value is examined.}
745 (let ((abracadabra 'foo))
746 (symbol-value abracadabra))
751 (symbol-value 'abracadabra)
756 A @code{void-variable} error is signaled if the current binding of
757 @var{symbol} is void.
760 @node Setting Variables
761 @section How to Alter a Variable Value
763 The usual way to change the value of a variable is with the special
764 form @code{setq}. When you need to compute the choice of variable at
765 run time, use the function @code{set}.
767 @defspec setq [symbol form]@dots{}
768 This special form is the most common method of changing a variable's
769 value. Each @var{symbol} is given a new value, which is the result of
770 evaluating the corresponding @var{form}. The most-local existing
771 binding of the symbol is changed.
773 @code{setq} does not evaluate @var{symbol}; it sets the symbol that you
774 write. We say that this argument is @dfn{automatically quoted}. The
775 @samp{q} in @code{setq} stands for ``quoted.''
777 The value of the @code{setq} form is the value of the last @var{form}.
784 x ; @r{@code{x} now has a global value.}
788 (setq x 6) ; @r{The local binding of @code{x} is set.}
792 x ; @r{The global value is unchanged.}
796 Note that the first @var{form} is evaluated, then the first
797 @var{symbol} is set, then the second @var{form} is evaluated, then the
798 second @var{symbol} is set, and so on:
802 (setq x 10 ; @r{Notice that @code{x} is set before}
803 y (1+ x)) ; @r{the value of @code{y} is computed.}
809 @defun set symbol value
810 This function sets @var{symbol}'s value to @var{value}, then returns
811 @var{value}. Since @code{set} is a function, the expression written for
812 @var{symbol} is evaluated to obtain the symbol to set.
814 The most-local existing binding of the variable is the binding that is
815 set; shadowed bindings are not affected.
820 @error{} Symbol's value as variable is void: one
831 (set two 2) ; @r{@code{two} evaluates to symbol @code{one}.}
835 one ; @r{So it is @code{one} that was set.}
837 (let ((one 1)) ; @r{This binding of @code{one} is set,}
838 (set 'one 3) ; @r{not the global value.}
848 If @var{symbol} is not actually a symbol, a @code{wrong-type-argument}
853 @error{} Wrong type argument: symbolp, (x y)
856 Logically speaking, @code{set} is a more fundamental primitive than
857 @code{setq}. Any use of @code{setq} can be trivially rewritten to use
858 @code{set}; @code{setq} could even be defined as a macro, given the
859 availability of @code{set}. However, @code{set} itself is rarely used;
860 beginners hardly need to know about it. It is useful only for choosing
861 at run time which variable to set. For example, the command
862 @code{set-variable}, which reads a variable name from the user and then
863 sets the variable, needs to use @code{set}.
865 @cindex CL note---@code{set} local
867 @b{Common Lisp note:} In Common Lisp, @code{set} always changes the
868 symbol's ``special'' or dynamic value, ignoring any lexical bindings.
869 In Emacs Lisp, all variables and all bindings are dynamic, so @code{set}
870 always affects the most local existing binding.
874 @node Variable Scoping
875 @section Scoping Rules for Variable Bindings
877 A given symbol @code{foo} can have several local variable bindings,
878 established at different places in the Lisp program, as well as a global
879 binding. The most recently established binding takes precedence over
884 @cindex dynamic scoping
885 @cindex lexical scoping
886 Local bindings in Emacs Lisp have @dfn{indefinite scope} and
887 @dfn{dynamic extent}. @dfn{Scope} refers to @emph{where} textually in
888 the source code the binding can be accessed. ``Indefinite scope'' means
889 that any part of the program can potentially access the variable
890 binding. @dfn{Extent} refers to @emph{when}, as the program is
891 executing, the binding exists. ``Dynamic extent'' means that the binding
892 lasts as long as the activation of the construct that established it.
894 The combination of dynamic extent and indefinite scope is called
895 @dfn{dynamic scoping}. By contrast, most programming languages use
896 @dfn{lexical scoping}, in which references to a local variable must be
897 located textually within the function or block that binds the variable.
899 @cindex CL note---special variables
901 @b{Common Lisp note:} Variables declared ``special'' in Common Lisp are
902 dynamically scoped, like all variables in Emacs Lisp.
906 * Scope:: Scope means where in the program a value is visible.
907 Comparison with other languages.
908 * Extent:: Extent means how long in time a value exists.
909 * Impl of Scope:: Two ways to implement dynamic scoping.
910 * Using Scoping:: How to use dynamic scoping carefully and avoid problems.
916 Emacs Lisp uses @dfn{indefinite scope} for local variable bindings.
917 This means that any function anywhere in the program text might access a
918 given binding of a variable. Consider the following function
923 (defun binder (x) ; @r{@code{x} is bound in @code{binder}.}
924 (foo 5)) ; @r{@code{foo} is some other function.}
928 (defun user () ; @r{@code{x} is used ``free'' in @code{user}.}
933 In a lexically scoped language, the binding of @code{x} in
934 @code{binder} would never be accessible in @code{user}, because
935 @code{user} is not textually contained within the function
936 @code{binder}. However, in dynamically-scoped Emacs Lisp, @code{user}
937 may or may not refer to the binding of @code{x} established in
938 @code{binder}, depending on the circumstances:
942 If we call @code{user} directly without calling @code{binder} at all,
943 then whatever binding of @code{x} is found, it cannot come from
947 If we define @code{foo} as follows and then call @code{binder}, then the
948 binding made in @code{binder} will be seen in @code{user}:
958 However, if we define @code{foo} as follows and then call @code{binder},
959 then the binding made in @code{binder} @emph{will not} be seen in
968 Here, when @code{foo} is called by @code{binder}, it binds @code{x}.
969 (The binding in @code{foo} is said to @dfn{shadow} the one made in
970 @code{binder}.) Therefore, @code{user} will access the @code{x} bound
971 by @code{foo} instead of the one bound by @code{binder}.
974 Emacs Lisp uses dynamic scoping because simple implementations of
975 lexical scoping are slow. In addition, every Lisp system needs to offer
976 dynamic scoping at least as an option; if lexical scoping is the norm,
977 there must be a way to specify dynamic scoping instead for a particular
978 variable. It might not be a bad thing for Emacs to offer both, but
979 implementing it with dynamic scoping only was much easier.
984 @dfn{Extent} refers to the time during program execution that a
985 variable name is valid. In Emacs Lisp, a variable is valid only while
986 the form that bound it is executing. This is called @dfn{dynamic
987 extent}. ``Local'' or ``automatic'' variables in most languages,
988 including C and Pascal, have dynamic extent.
990 One alternative to dynamic extent is @dfn{indefinite extent}. This
991 means that a variable binding can live on past the exit from the form
992 that made the binding. Common Lisp and Scheme, for example, support
993 this, but Emacs Lisp does not.
995 To illustrate this, the function below, @code{make-add}, returns a
996 function that purports to add @var{n} to its own argument @var{m}. This
997 would work in Common Lisp, but it does not do the job in Emacs Lisp,
998 because after the call to @code{make-add} exits, the variable @code{n}
999 is no longer bound to the actual argument 2.
1003 (function (lambda (m) (+ n m)))) ; @r{Return a function.}
1005 (fset 'add2 (make-add 2)) ; @r{Define function @code{add2}}
1006 ; @r{with @code{(make-add 2)}.}
1007 @result{} (lambda (m) (+ n m))
1008 (add2 4) ; @r{Try to add 2 to 4.}
1009 @error{} Symbol's value as variable is void: n
1012 @cindex closures not available
1013 Some Lisp dialects have ``closures,'' objects that are like functions
1014 but record additional variable bindings. Emacs Lisp does not have
1018 @subsection Implementation of Dynamic Scoping
1019 @cindex deep binding
1021 A simple sample implementation (which is not how Emacs Lisp actually
1022 works) may help you understand dynamic binding. This technique is
1023 called @dfn{deep binding} and was used in early Lisp systems.
1025 Suppose there is a stack of bindings, which are variable-value pairs.
1026 At entry to a function or to a @code{let} form, we can push bindings
1027 onto the stack for the arguments or local variables created there. We
1028 can pop those bindings from the stack at exit from the binding
1031 We can find the value of a variable by searching the stack from top to
1032 bottom for a binding for that variable; the value from that binding is
1033 the value of the variable. To set the variable, we search for the
1034 current binding, then store the new value into that binding.
1036 As you can see, a function's bindings remain in effect as long as it
1037 continues execution, even during its calls to other functions. That is
1038 why we say the extent of the binding is dynamic. And any other function
1039 can refer to the bindings, if it uses the same variables while the
1040 bindings are in effect. That is why we say the scope is indefinite.
1042 @cindex shallow binding
1043 The actual implementation of variable scoping in GNU Emacs Lisp uses a
1044 technique called @dfn{shallow binding}. Each variable has a standard
1045 place in which its current value is always found---the value cell of the
1048 In shallow binding, setting the variable works by storing a value in
1049 the value cell. Creating a new binding works by pushing the old value
1050 (belonging to a previous binding) onto a stack, and storing the new
1051 local value in the value cell. Eliminating a binding works by popping
1052 the old value off the stack, into the value cell.
1054 We use shallow binding because it has the same results as deep
1055 binding, but runs faster, since there is never a need to search for a
1059 @subsection Proper Use of Dynamic Scoping
1061 Binding a variable in one function and using it in another is a
1062 powerful technique, but if used without restraint, it can make programs
1063 hard to understand. There are two clean ways to use this technique:
1067 Use or bind the variable only in a few related functions, written close
1068 together in one file. Such a variable is used for communication within
1071 You should write comments to inform other programmers that they can see
1072 all uses of the variable before them, and to advise them not to add uses
1076 Give the variable a well-defined, documented meaning, and make all
1077 appropriate functions refer to it (but not bind it or set it) wherever
1078 that meaning is relevant. For example, the variable
1079 @code{case-fold-search} is defined as ``non-@code{nil} means ignore case
1080 when searching''; various search and replace functions refer to it
1081 directly or through their subroutines, but do not bind or set it.
1083 Then you can bind the variable in other programs, knowing reliably what
1087 In either case, you should define the variable with @code{defvar}.
1088 This helps other people understand your program by telling them to look
1089 for inter-function usage. It also avoids a warning from the byte
1090 compiler. Choose the variable's name to avoid name conflicts---don't
1091 use short names like @code{x}.
1093 @node Buffer-Local Variables
1094 @section Buffer-Local Variables
1095 @cindex variable, buffer-local
1096 @cindex buffer-local variables
1098 Global and local variable bindings are found in most programming
1099 languages in one form or another. Emacs, however, also supports
1100 additional, unusual kinds of variable binding, such as
1101 @dfn{buffer-local} bindings, which apply only in one buffer. Having
1102 different values for a variable in different buffers is an important
1103 customization method. (Variables can also have bindings that are
1104 local to each terminal, or to each frame. @xref{Multiple Terminals},
1105 and @xref{Frame-Local Variables}.)
1108 * Intro to Buffer-Local:: Introduction and concepts.
1109 * Creating Buffer-Local:: Creating and destroying buffer-local bindings.
1110 * Default Value:: The default value is seen in buffers
1111 that don't have their own buffer-local values.
1114 @node Intro to Buffer-Local
1115 @subsection Introduction to Buffer-Local Variables
1117 A buffer-local variable has a buffer-local binding associated with a
1118 particular buffer. The binding is in effect when that buffer is
1119 current; otherwise, it is not in effect. If you set the variable while
1120 a buffer-local binding is in effect, the new value goes in that binding,
1121 so its other bindings are unchanged. This means that the change is
1122 visible only in the buffer where you made it.
1124 The variable's ordinary binding, which is not associated with any
1125 specific buffer, is called the @dfn{default binding}. In most cases,
1126 this is the global binding.
1128 A variable can have buffer-local bindings in some buffers but not in
1129 other buffers. The default binding is shared by all the buffers that
1130 don't have their own bindings for the variable. (This includes all
1131 newly-created buffers.) If you set the variable in a buffer that does
1132 not have a buffer-local binding for it, this sets the default binding,
1133 so the new value is visible in all the buffers that see the default
1136 The most common use of buffer-local bindings is for major modes to change
1137 variables that control the behavior of commands. For example, C mode and
1138 Lisp mode both set the variable @code{paragraph-start} to specify that only
1139 blank lines separate paragraphs. They do this by making the variable
1140 buffer-local in the buffer that is being put into C mode or Lisp mode, and
1141 then setting it to the new value for that mode. @xref{Major Modes}.
1143 The usual way to make a buffer-local binding is with
1144 @code{make-local-variable}, which is what major mode commands typically
1145 use. This affects just the current buffer; all other buffers (including
1146 those yet to be created) will continue to share the default value unless
1147 they are explicitly given their own buffer-local bindings.
1149 @cindex automatically buffer-local
1150 A more powerful operation is to mark the variable as
1151 @dfn{automatically buffer-local} by calling
1152 @code{make-variable-buffer-local}. You can think of this as making the
1153 variable local in all buffers, even those yet to be created. More
1154 precisely, the effect is that setting the variable automatically makes
1155 the variable local to the current buffer if it is not already so. All
1156 buffers start out by sharing the default value of the variable as usual,
1157 but setting the variable creates a buffer-local binding for the current
1158 buffer. The new value is stored in the buffer-local binding, leaving
1159 the default binding untouched. This means that the default value cannot
1160 be changed with @code{setq} in any buffer; the only way to change it is
1161 with @code{setq-default}.
1163 @strong{Warning:} When a variable has buffer-local
1164 bindings in one or more buffers, @code{let} rebinds the binding that's
1165 currently in effect. For instance, if the current buffer has a
1166 buffer-local value, @code{let} temporarily rebinds that. If no
1167 buffer-local bindings are in effect, @code{let} rebinds
1168 the default value. If inside the @code{let} you then change to a
1169 different current buffer in which a different binding is in effect,
1170 you won't see the @code{let} binding any more. And if you exit the
1171 @code{let} while still in the other buffer, you won't see the
1172 unbinding occur (though it will occur properly). Here is an example
1179 (make-local-variable 'foo)
1183 ;; foo @result{} 'temp ; @r{let binding in buffer @samp{a}}
1185 ;; foo @result{} 'g ; @r{the global value since foo is not local in @samp{b}}
1188 foo @result{} 'g ; @r{exiting restored the local value in buffer @samp{a},}
1189 ; @r{but we don't see that in buffer @samp{b}}
1192 (set-buffer "a") ; @r{verify the local value was restored}
1197 Note that references to @code{foo} in @var{body} access the
1198 buffer-local binding of buffer @samp{b}.
1200 When a file specifies local variable values, these become buffer-local
1201 values when you visit the file. @xref{File Variables,,, emacs, The
1204 A buffer-local variable cannot be made frame-local
1205 (@pxref{Frame-Local Variables}) or terminal-local (@pxref{Multiple
1208 @node Creating Buffer-Local
1209 @subsection Creating and Deleting Buffer-Local Bindings
1211 @deffn Command make-local-variable variable
1212 This function creates a buffer-local binding in the current buffer for
1213 @var{variable} (a symbol). Other buffers are not affected. The value
1214 returned is @var{variable}.
1216 The buffer-local value of @var{variable} starts out as the same value
1217 @var{variable} previously had. If @var{variable} was void, it remains
1222 ;; @r{In buffer @samp{b1}:}
1223 (setq foo 5) ; @r{Affects all buffers.}
1227 (make-local-variable 'foo) ; @r{Now it is local in @samp{b1}.}
1231 foo ; @r{That did not change}
1232 @result{} 5 ; @r{the value.}
1235 (setq foo 6) ; @r{Change the value}
1236 @result{} 6 ; @r{in @samp{b1}.}
1244 ;; @r{In buffer @samp{b2}, the value hasn't changed.}
1245 (with-current-buffer "b2"
1251 Making a variable buffer-local within a @code{let}-binding for that
1252 variable does not work reliably, unless the buffer in which you do this
1253 is not current either on entry to or exit from the @code{let}. This is
1254 because @code{let} does not distinguish between different kinds of
1255 bindings; it knows only which variable the binding was made for.
1257 If the variable is terminal-local (@pxref{Multiple Terminals}), or
1258 frame-local (@pxref{Frame-Local Variables}), this function signals an
1259 error. Such variables cannot have buffer-local bindings as well.
1261 @strong{Warning:} do not use @code{make-local-variable} for a hook
1262 variable. The hook variables are automatically made buffer-local as
1263 needed if you use the @var{local} argument to @code{add-hook} or
1267 @deffn Command make-variable-buffer-local variable
1268 This function marks @var{variable} (a symbol) automatically
1269 buffer-local, so that any subsequent attempt to set it will make it
1270 local to the current buffer at the time.
1271 Contrary to @code{make-local-variable} with which it is often confused, this
1272 cannot be undone, and affects the behavior of the variable in all buffers.
1274 A peculiar wrinkle of this feature is that binding the variable (with
1275 @code{let} or other binding constructs) does not create a buffer-local
1276 binding for it. Only setting the variable (with @code{set} or
1277 @code{setq}), while the variable does not have a @code{let}-style
1278 binding that was made in the current buffer, does so.
1280 If @var{variable} does not have a default value, then calling this
1281 command will give it a default value of @code{nil}. If @var{variable}
1282 already has a default value, that value remains unchanged.
1283 Subsequently calling @code{makunbound} on @var{variable} will result
1284 in a void buffer-local value and leave the default value unaffected.
1286 The value returned is @var{variable}.
1288 @strong{Warning:} Don't assume that you should use
1289 @code{make-variable-buffer-local} for user-option variables, simply
1290 because users @emph{might} want to customize them differently in
1291 different buffers. Users can make any variable local, when they wish
1292 to. It is better to leave the choice to them.
1294 The time to use @code{make-variable-buffer-local} is when it is crucial
1295 that no two buffers ever share the same binding. For example, when a
1296 variable is used for internal purposes in a Lisp program which depends
1297 on having separate values in separate buffers, then using
1298 @code{make-variable-buffer-local} can be the best solution.
1301 @defun local-variable-p variable &optional buffer
1302 This returns @code{t} if @var{variable} is buffer-local in buffer
1303 @var{buffer} (which defaults to the current buffer); otherwise,
1307 @defun local-variable-if-set-p variable &optional buffer
1308 This returns @code{t} if @var{variable} will become buffer-local in
1309 buffer @var{buffer} (which defaults to the current buffer) if it is
1313 @defun buffer-local-value variable buffer
1314 This function returns the buffer-local binding of @var{variable} (a
1315 symbol) in buffer @var{buffer}. If @var{variable} does not have a
1316 buffer-local binding in buffer @var{buffer}, it returns the default
1317 value (@pxref{Default Value}) of @var{variable} instead.
1320 @defun buffer-local-variables &optional buffer
1321 This function returns a list describing the buffer-local variables in
1322 buffer @var{buffer}. (If @var{buffer} is omitted, the current buffer is
1323 used.) It returns an association list (@pxref{Association Lists}) in
1324 which each element contains one buffer-local variable and its value.
1325 However, when a variable's buffer-local binding in @var{buffer} is void,
1326 then the variable appears directly in the resulting list.
1330 (make-local-variable 'foobar)
1331 (makunbound 'foobar)
1332 (make-local-variable 'bind-me)
1335 (setq lcl (buffer-local-variables))
1336 ;; @r{First, built-in variables local in all buffers:}
1337 @result{} ((mark-active . nil)
1338 (buffer-undo-list . nil)
1339 (mode-name . "Fundamental")
1342 ;; @r{Next, non-built-in buffer-local variables.}
1343 ;; @r{This one is buffer-local and void:}
1345 ;; @r{This one is buffer-local and nonvoid:}
1350 Note that storing new values into the @sc{cdr}s of cons cells in this
1351 list does @emph{not} change the buffer-local values of the variables.
1354 @deffn Command kill-local-variable variable
1355 This function deletes the buffer-local binding (if any) for
1356 @var{variable} (a symbol) in the current buffer. As a result, the
1357 default binding of @var{variable} becomes visible in this buffer. This
1358 typically results in a change in the value of @var{variable}, since the
1359 default value is usually different from the buffer-local value just
1362 If you kill the buffer-local binding of a variable that automatically
1363 becomes buffer-local when set, this makes the default value visible in
1364 the current buffer. However, if you set the variable again, that will
1365 once again create a buffer-local binding for it.
1367 @code{kill-local-variable} returns @var{variable}.
1369 This function is a command because it is sometimes useful to kill one
1370 buffer-local variable interactively, just as it is useful to create
1371 buffer-local variables interactively.
1374 @defun kill-all-local-variables
1375 This function eliminates all the buffer-local variable bindings of the
1376 current buffer except for variables marked as ``permanent'' and local
1377 hook functions that have a non-@code{nil} @code{permanent-local-hook}
1378 property (@pxref{Setting Hooks}). As a result, the buffer will see
1379 the default values of most variables.
1381 This function also resets certain other information pertaining to the
1382 buffer: it sets the local keymap to @code{nil}, the syntax table to the
1383 value of @code{(standard-syntax-table)}, the case table to
1384 @code{(standard-case-table)}, and the abbrev table to the value of
1385 @code{fundamental-mode-abbrev-table}.
1387 The very first thing this function does is run the normal hook
1388 @code{change-major-mode-hook} (see below).
1390 Every major mode command begins by calling this function, which has the
1391 effect of switching to Fundamental mode and erasing most of the effects
1392 of the previous major mode. To ensure that this does its job, the
1393 variables that major modes set should not be marked permanent.
1395 @code{kill-all-local-variables} returns @code{nil}.
1398 @defvar change-major-mode-hook
1399 The function @code{kill-all-local-variables} runs this normal hook
1400 before it does anything else. This gives major modes a way to arrange
1401 for something special to be done if the user switches to a different
1402 major mode. It is also useful for buffer-specific minor modes
1403 that should be forgotten if the user changes the major mode.
1405 For best results, make this variable buffer-local, so that it will
1406 disappear after doing its job and will not interfere with the
1407 subsequent major mode. @xref{Hooks}.
1411 @cindex permanent local variable
1412 A buffer-local variable is @dfn{permanent} if the variable name (a
1413 symbol) has a @code{permanent-local} property that is non-@code{nil}.
1414 Such variables are unaffected by @code{kill-all-local-variables}, and
1415 their local bindings are therefore not cleared by changing major modes.
1416 Permanent locals are appropriate for data pertaining to where the file
1417 came from or how to save it, rather than with how to edit the contents.
1420 @subsection The Default Value of a Buffer-Local Variable
1421 @cindex default value
1423 The global value of a variable with buffer-local bindings is also
1424 called the @dfn{default} value, because it is the value that is in
1425 effect whenever neither the current buffer nor the selected frame has
1426 its own binding for the variable.
1428 The functions @code{default-value} and @code{setq-default} access and
1429 change a variable's default value regardless of whether the current
1430 buffer has a buffer-local binding. For example, you could use
1431 @code{setq-default} to change the default setting of
1432 @code{paragraph-start} for most buffers; and this would work even when
1433 you are in a C or Lisp mode buffer that has a buffer-local value for
1437 The special forms @code{defvar} and @code{defconst} also set the
1438 default value (if they set the variable at all), rather than any
1441 @defun default-value symbol
1442 This function returns @var{symbol}'s default value. This is the value
1443 that is seen in buffers and frames that do not have their own values for
1444 this variable. If @var{symbol} is not buffer-local, this is equivalent
1445 to @code{symbol-value} (@pxref{Accessing Variables}).
1449 @defun default-boundp symbol
1450 The function @code{default-boundp} tells you whether @var{symbol}'s
1451 default value is nonvoid. If @code{(default-boundp 'foo)} returns
1452 @code{nil}, then @code{(default-value 'foo)} would get an error.
1454 @code{default-boundp} is to @code{default-value} as @code{boundp} is to
1455 @code{symbol-value}.
1458 @defspec setq-default [symbol form]@dots{}
1459 This special form gives each @var{symbol} a new default value, which is
1460 the result of evaluating the corresponding @var{form}. It does not
1461 evaluate @var{symbol}, but does evaluate @var{form}. The value of the
1462 @code{setq-default} form is the value of the last @var{form}.
1464 If a @var{symbol} is not buffer-local for the current buffer, and is not
1465 marked automatically buffer-local, @code{setq-default} has the same
1466 effect as @code{setq}. If @var{symbol} is buffer-local for the current
1467 buffer, then this changes the value that other buffers will see (as long
1468 as they don't have a buffer-local value), but not the value that the
1469 current buffer sees.
1473 ;; @r{In buffer @samp{foo}:}
1474 (make-local-variable 'buffer-local)
1475 @result{} buffer-local
1478 (setq buffer-local 'value-in-foo)
1479 @result{} value-in-foo
1482 (setq-default buffer-local 'new-default)
1483 @result{} new-default
1487 @result{} value-in-foo
1490 (default-value 'buffer-local)
1491 @result{} new-default
1495 ;; @r{In (the new) buffer @samp{bar}:}
1497 @result{} new-default
1500 (default-value 'buffer-local)
1501 @result{} new-default
1504 (setq buffer-local 'another-default)
1505 @result{} another-default
1508 (default-value 'buffer-local)
1509 @result{} another-default
1513 ;; @r{Back in buffer @samp{foo}:}
1515 @result{} value-in-foo
1516 (default-value 'buffer-local)
1517 @result{} another-default
1522 @defun set-default symbol value
1523 This function is like @code{setq-default}, except that @var{symbol} is
1524 an ordinary evaluated argument.
1528 (set-default (car '(a b c)) 23)
1538 @node File Local Variables
1539 @section File Local Variables
1540 @cindex file local variables
1542 A file can specify local variable values; Emacs uses these to create
1543 buffer-local bindings for those variables in the buffer visiting that
1544 file. @xref{File variables, , Local Variables in Files, emacs, The
1545 GNU Emacs Manual}, for basic information about file-local variables.
1546 This section describes the functions and variables that affect how
1547 file-local variables are processed.
1549 If a file-local variable could specify an arbitrary function or Lisp
1550 expression that would be called later, visiting a file could take over
1551 your Emacs. Emacs protects against this by automatically setting only
1552 those file-local variables whose specified values are known to be
1553 safe. Other file-local variables are set only if the user agrees.
1555 For additional safety, @code{read-circle} is temporarily bound to
1556 @code{nil} when Emacs reads file-local variables (@pxref{Input
1557 Functions}). This prevents the Lisp reader from recognizing circular
1558 and shared Lisp structures (@pxref{Circular Objects}).
1560 @defopt enable-local-variables
1561 This variable controls whether to process file-local variables.
1562 The possible values are:
1565 @item @code{t} (the default)
1566 Set the safe variables, and query (once) about any unsafe variables.
1568 Set only the safe variables and do not query.
1570 Set all the variables and do not query.
1572 Don't set any variables.
1574 Query (once) about all the variables.
1578 @defun hack-local-variables &optional mode-only
1579 This function parses, and binds or evaluates as appropriate, any local
1580 variables specified by the contents of the current buffer. The variable
1581 @code{enable-local-variables} has its effect here. However, this
1582 function does not look for the @samp{mode:} local variable in the
1583 @w{@samp{-*-}} line. @code{set-auto-mode} does that, also taking
1584 @code{enable-local-variables} into account (@pxref{Auto Major Mode}).
1586 This function works by walking the alist stored in
1587 @code{file-local-variables-alist} and applying each local variable in
1588 turn. It calls @code{before-hack-local-variables-hook} and
1589 @code{hack-local-variables-hook} before and after applying the
1590 variables, respectively.
1592 If the optional argument @var{mode-only} is non-@code{nil}, then all
1593 this function does is return @code{t} if the @w{@samp{-*-}} line or
1594 the local variables list specifies a mode and @code{nil} otherwise.
1595 It does not set the mode nor any other file-local variable.
1598 @defvar file-local-variables-alist
1599 This buffer-local variable holds the alist of file-local variable
1600 settings. Each element of the alist is of the form
1601 @w{@code{(@var{var} . @var{value})}}, where @var{var} is a symbol of
1602 the local variable and @var{value} is its value. When Emacs visits a
1603 file, it first collects all the file-local variables into this alist,
1604 and then the @code{hack-local-variables} function applies them one by
1608 @defvar before-hack-local-variables-hook
1609 Emacs calls this hook immediately before applying file-local variables
1610 stored in @code{file-local-variables-alist}.
1613 @defvar hack-local-variables-hook
1614 Emacs calls this hook immediately after it finishes applying
1615 file-local variables stored in @code{file-local-variables-alist}.
1618 @cindex safe local variable
1619 You can specify safe values for a variable with a
1620 @code{safe-local-variable} property. The property has to be a
1621 function of one argument; any value is safe if the function returns
1622 non-@code{nil} given that value. Many commonly-encountered file
1623 variables have @code{safe-local-variable} properties; these include
1624 @code{fill-column}, @code{fill-prefix}, and @code{indent-tabs-mode}.
1625 For boolean-valued variables that are safe, use @code{booleanp} as the
1626 property value. Lambda expressions should be quoted so that
1627 @code{describe-variable} can display the predicate.
1629 @defopt safe-local-variable-values
1630 This variable provides another way to mark some variable values as
1631 safe. It is a list of cons cells @code{(@var{var} . @var{val})},
1632 where @var{var} is a variable name and @var{val} is a value which is
1633 safe for that variable.
1635 When Emacs asks the user whether or not to obey a set of file-local
1636 variable specifications, the user can choose to mark them as safe.
1637 Doing so adds those variable/value pairs to
1638 @code{safe-local-variable-values}, and saves it to the user's custom
1642 @defun safe-local-variable-p sym val
1643 This function returns non-@code{nil} if it is safe to give @var{sym}
1644 the value @var{val}, based on the above criteria.
1647 @c @cindex risky local variable Duplicates risky-local-variable
1648 Some variables are considered @dfn{risky}. A variable whose name
1649 ends in any of @samp{-command}, @samp{-frame-alist}, @samp{-function},
1650 @samp{-functions}, @samp{-hook}, @samp{-hooks}, @samp{-form},
1651 @samp{-forms}, @samp{-map}, @samp{-map-alist}, @samp{-mode-alist},
1652 @samp{-program}, or @samp{-predicate} is considered risky. The
1653 variables @samp{font-lock-keywords}, @samp{font-lock-keywords}
1654 followed by a digit, and @samp{font-lock-syntactic-keywords} are also
1655 considered risky. Finally, any variable whose name has a
1656 non-@code{nil} @code{risky-local-variable} property is considered
1659 @defun risky-local-variable-p sym
1660 This function returns non-@code{nil} if @var{sym} is a risky variable,
1661 based on the above criteria.
1664 If a variable is risky, it will not be entered automatically into
1665 @code{safe-local-variable-values} as described above. Therefore,
1666 Emacs will always query before setting a risky variable, unless the
1667 user explicitly allows the setting by customizing
1668 @code{safe-local-variable-values} directly.
1670 @defvar ignored-local-variables
1671 This variable holds a list of variables that should not be given local
1672 values by files. Any value specified for one of these variables is
1676 The @samp{Eval:} ``variable'' is also a potential loophole, so Emacs
1677 normally asks for confirmation before handling it.
1679 @defopt enable-local-eval
1680 This variable controls processing of @samp{Eval:} in @samp{-*-} lines
1682 lists in files being visited. A value of @code{t} means process them
1683 unconditionally; @code{nil} means ignore them; anything else means ask
1684 the user what to do for each file. The default value is @code{maybe}.
1687 @defopt safe-local-eval-forms
1688 This variable holds a list of expressions that are safe to
1689 evaluate when found in the @samp{Eval:} ``variable'' in a file
1690 local variables list.
1693 If the expression is a function call and the function has a
1694 @code{safe-local-eval-function} property, the property value
1695 determines whether the expression is safe to evaluate. The property
1696 value can be a predicate to call to test the expression, a list of
1697 such predicates (it's safe if any predicate succeeds), or @code{t}
1698 (always safe provided the arguments are constant).
1700 Text properties are also potential loopholes, since their values
1701 could include functions to call. So Emacs discards all text
1702 properties from string values specified for file-local variables.
1704 @node Directory Local Variables
1705 @section Directory Local Variables
1706 @cindex directory local variables
1708 A directory can specify local variable values common to all files in
1709 that directory; Emacs uses these to create buffer-local bindings for
1710 those variables in buffers visiting any file in that directory. This
1711 is useful when the files in the directory belong to some @dfn{project}
1712 and therefore share the same local variables.
1714 There are two different methods for specifying directory local
1715 variables: by putting them in a special file, or by defining a
1716 @dfn{project class} for that directory.
1718 @defvr Constant dir-locals-file
1719 This constant is the name of the file where Emacs expects to find the
1720 directory-local variables. The name of the file is
1721 @file{.dir-locals.el}@footnote{
1722 The MS-DOS version of Emacs uses @file{_dir-locals.el} instead, due to
1723 limitations of the DOS filesystems.
1724 }. A file by that name in a directory causes Emacs to apply its
1725 settings to any file in that directory or any of its subdirectories.
1726 If some of the subdirectories have their own @file{.dir-locals.el}
1727 files, Emacs uses the settings from the deepest file it finds starting
1728 from the file's directory and moving up the directory tree. The file
1729 specifies local variables as a specially formatted list; see
1730 @ref{Directory Variables, , Per-directory Local Variables, emacs, The
1731 GNU Emacs Manual}, for more details.
1734 @defun hack-dir-local-variables
1735 This function reads the @code{.dir-locals.el} file and stores the
1736 directory-local variables in @code{file-local-variables-alist} that is
1737 local to the buffer visiting any file in the directory, without
1738 applying them. It also stores the directory-local settings in
1739 @code{dir-locals-class-alist}, where it defines a special class for
1740 the directory in which @file{.dir-locals.el} file was found. This
1741 function works by calling @code{dir-locals-set-class-variables} and
1742 @code{dir-locals-set-directory-class}, described below.
1745 @defun dir-locals-set-class-variables class variables
1746 This function defines a set of variable settings for the named
1747 @var{class}, which is a symbol. You can later assign the class to one
1748 or more directories, and Emacs will apply those variable settings to
1749 all files in those directories. The list in @var{variables} can be of
1750 one of the two forms: @code{(@var{major-mode} . @var{alist})} or
1751 @code{(@var{directory} . @var{list})}. With the first form, if the
1752 file's buffer turns on a mode that is derived from @var{major-mode},
1753 then the all the variables in the associated @var{alist} are applied;
1754 @var{alist} should be of the form @code{(@var{name} . @var{value})}.
1755 A special value @code{nil} for @var{major-mode} means the settings are
1756 applicable to any mode.
1758 With the second form of @var{variables}, if @var{directory} is the
1759 initial substring of the file's directory, then @var{list} is applied
1760 recursively by following the above rules; @var{list} should be of one
1761 of the two forms accepted by this function in @var{variables}.
1764 @defun dir-locals-set-directory-class directory class &optional mtime
1765 This function assigns @var{class} to all the files in @code{directory}
1766 and its subdirectories. Thereafter, all the variable settings
1767 specified for @var{class} will be applied to any visited file in
1768 @var{directory} and its children. @var{class} must have been already
1769 defined by @code{dir-locals-set-class-variables}.
1771 Emacs uses this function internally when it loads directory variables
1772 from a @code{.dir-locals.el} file. In that case, the optional
1773 argument @var{mtime} holds the file modification time (as returned by
1774 @code{file-attributes}). Emacs uses this time to check stored
1775 local variables are still valid. If you are assigning a class
1776 directly, not via a file, this argument should be @code{nil}.
1779 @defvar dir-locals-class-alist
1780 This alist holds the class symbols and the associated variable
1781 settings. It is updated by @code{dir-locals-set-class-variables}.
1784 @defvar dir-locals-directory-cache
1785 This alist holds directory names, their assigned class names, and
1786 modification times of the associated directory local variables file
1787 (if there is one). The function @code{dir-locals-set-directory-class}
1791 @node Frame-Local Variables
1792 @section Frame-Local Values for Variables
1793 @cindex frame-local variables
1795 In addition to buffer-local variable bindings (@pxref{Buffer-Local
1796 Variables}), Emacs supports @dfn{frame-local} bindings. A frame-local
1797 binding for a variable is in effect in a frame for which it was
1800 In practice, frame-local variables have not proven very useful.
1801 Ordinary frame parameters are generally used instead (@pxref{Frame
1802 Parameters}). The function @code{make-variable-frame-local}, which
1803 was used to define frame-local variables, has been deprecated since
1804 Emacs 22.2. However, you can still define a frame-specific binding
1805 for a variable @var{var} in frame @var{frame}, by setting the
1806 @var{var} frame parameter for that frame:
1809 (modify-frame-parameters @var{frame} '((@var{var} . @var{value})))
1813 This causes the variable @var{var} to be bound to the specified
1814 @var{value} in the named @var{frame}. To check the frame-specific
1815 values of such variables, use @code{frame-parameter}. @xref{Parameter
1818 Note that you cannot have a frame-local binding for a variable that
1819 has a buffer-local binding.
1821 @node Variable Aliases
1822 @section Variable Aliases
1823 @cindex variable aliases
1825 It is sometimes useful to make two variables synonyms, so that both
1826 variables always have the same value, and changing either one also
1827 changes the other. Whenever you change the name of a
1828 variable---either because you realize its old name was not well
1829 chosen, or because its meaning has partly changed---it can be useful
1830 to keep the old name as an @emph{alias} of the new one for
1831 compatibility. You can do this with @code{defvaralias}.
1833 @defun defvaralias new-alias base-variable &optional docstring
1834 This function defines the symbol @var{new-alias} as a variable alias
1835 for symbol @var{base-variable}. This means that retrieving the value
1836 of @var{new-alias} returns the value of @var{base-variable}, and
1837 changing the value of @var{new-alias} changes the value of
1838 @var{base-variable}. The two aliased variable names always share the
1839 same value and the same bindings.
1841 If the @var{docstring} argument is non-@code{nil}, it specifies the
1842 documentation for @var{new-alias}; otherwise, the alias gets the same
1843 documentation as @var{base-variable} has, if any, unless
1844 @var{base-variable} is itself an alias, in which case @var{new-alias} gets
1845 the documentation of the variable at the end of the chain of aliases.
1847 This function returns @var{base-variable}.
1850 Variable aliases are convenient for replacing an old name for a
1851 variable with a new name. @code{make-obsolete-variable} declares that
1852 the old name is obsolete and therefore that it may be removed at some
1853 stage in the future.
1855 @defun make-obsolete-variable obsolete-name current-name &optional when
1856 This function makes the byte compiler warn that the variable
1857 @var{obsolete-name} is obsolete. If @var{current-name} is a symbol, it is
1858 the variable's new name; then the warning message says to use
1859 @var{current-name} instead of @var{obsolete-name}. If @var{current-name}
1860 is a string, this is the message and there is no replacement variable.
1862 If provided, @var{when} should be a string indicating when the
1863 variable was first made obsolete---for example, a date or a release
1867 You can make two variables synonyms and declare one obsolete at the
1868 same time using the macro @code{define-obsolete-variable-alias}.
1870 @defmac define-obsolete-variable-alias obsolete-name current-name &optional when docstring
1871 This macro marks the variable @var{obsolete-name} as obsolete and also
1872 makes it an alias for the variable @var{current-name}. It is
1873 equivalent to the following:
1876 (defvaralias @var{obsolete-name} @var{current-name} @var{docstring})
1877 (make-obsolete-variable @var{obsolete-name} @var{current-name} @var{when})
1881 @defun indirect-variable variable
1882 This function returns the variable at the end of the chain of aliases
1883 of @var{variable}. If @var{variable} is not a symbol, or if @var{variable} is
1884 not defined as an alias, the function returns @var{variable}.
1886 This function signals a @code{cyclic-variable-indirection} error if
1887 there is a loop in the chain of symbols.
1891 (defvaralias 'foo 'bar)
1892 (indirect-variable 'foo)
1894 (indirect-variable 'bar)
1910 @node Variables with Restricted Values
1911 @section Variables with Restricted Values
1913 Ordinary Lisp variables can be assigned any value that is a valid
1914 Lisp object. However, certain Lisp variables are not defined in Lisp,
1915 but in C. Most of these variables are defined in the C code using
1916 @code{DEFVAR_LISP}. Like variables defined in Lisp, these can take on
1917 any value. However, some variables are defined using
1918 @code{DEFVAR_INT} or @code{DEFVAR_BOOL}. @xref{Defining Lisp
1919 variables in C,, Writing Emacs Primitives}, in particular the
1920 description of functions of the type @code{syms_of_@var{filename}},
1921 for a brief discussion of the C implementation.
1923 Variables of type @code{DEFVAR_BOOL} can only take on the values
1924 @code{nil} or @code{t}. Attempting to assign them any other value
1925 will set them to @code{t}:
1928 (let ((display-hourglass 5))
1933 @defvar byte-boolean-vars
1934 This variable holds a list of all variables of type @code{DEFVAR_BOOL}.
1937 Variables of type @code{DEFVAR_INT} can only take on integer values.
1938 Attempting to assign them any other value will result in an error:
1941 (setq window-min-height 5.0)
1942 @error{} Wrong type argument: integerp, 5.0
1946 arch-tag: 5ff62c44-2b51-47bb-99d4-fea5aeec5d3e