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 variable with constant value
106 @cindex constant variables
107 @cindex symbol that evaluates to itself
108 @cindex symbol with constant value
110 In Emacs Lisp, certain symbols normally evaluate to themselves. These
111 include @code{nil} and @code{t}, as well as any symbol whose name starts
112 with @samp{:} (these are called @dfn{keywords}). These symbols cannot
113 be rebound, nor can their values be changed. Any attempt to set or bind
114 @code{nil} or @code{t} signals a @code{setting-constant} error. The
115 same is true for a keyword (a symbol whose name starts with @samp{:}),
116 if it is interned in the standard obarray, except that setting such a
117 symbol to itself is not an error.
126 @error{} Attempt to set constant symbol: nil
130 @defun keywordp object
131 function returns @code{t} if @var{object} is a symbol whose name
132 starts with @samp{:}, interned in the standard obarray, and returns
133 @code{nil} otherwise.
136 @node Local Variables
137 @section Local Variables
138 @cindex binding local variables
139 @cindex local variables
140 @cindex local binding
141 @cindex global binding
143 Global variables have values that last until explicitly superseded
144 with new values. Sometimes it is useful to create variable values that
145 exist temporarily---only until a certain part of the program finishes.
146 These values are called @dfn{local}, and the variables so used are
147 called @dfn{local variables}.
149 For example, when a function is called, its argument variables receive
150 new local values that last until the function exits. The @code{let}
151 special form explicitly establishes new local values for specified
152 variables; these last until exit from the @code{let} form.
154 @cindex shadowing of variables
155 Establishing a local value saves away the previous value (or lack of
156 one) of the variable. When the life span of the local value is over,
157 the previous value is restored. In the mean time, we say that the
158 previous value is @dfn{shadowed} and @dfn{not visible}. Both global and
159 local values may be shadowed (@pxref{Scope}).
161 If you set a variable (such as with @code{setq}) while it is local,
162 this replaces the local value; it does not alter the global value, or
163 previous local values, that are shadowed. To model this behavior, we
164 speak of a @dfn{local binding} of the variable as well as a local value.
166 The local binding is a conceptual place that holds a local value.
167 Entry to a function, or a special form such as @code{let}, creates the
168 local binding; exit from the function or from the @code{let} removes the
169 local binding. As long as the local binding lasts, the variable's value
170 is stored within it. Use of @code{setq} or @code{set} while there is a
171 local binding stores a different value into the local binding; it does
172 not create a new binding.
174 We also speak of the @dfn{global binding}, which is where
175 (conceptually) the global value is kept.
177 @cindex current binding
178 A variable can have more than one local binding at a time (for
179 example, if there are nested @code{let} forms that bind it). In such a
180 case, the most recently created local binding that still exists is the
181 @dfn{current binding} of the variable. (This rule is called
182 @dfn{dynamic scoping}; see @ref{Variable Scoping}.) If there are no
183 local bindings, the variable's global binding is its current binding.
184 We sometimes call the current binding the @dfn{most-local existing
185 binding}, for emphasis. Ordinary evaluation of a symbol always returns
186 the value of its current binding.
188 The special forms @code{let} and @code{let*} exist to create
191 @defspec let (bindings@dots{}) forms@dots{}
192 This special form binds variables according to @var{bindings} and then
193 evaluates all of the @var{forms} in textual order. The @code{let}-form
194 returns the value of the last form in @var{forms}.
196 Each of the @var{bindings} is either @w{(i) a} symbol, in which case
197 that symbol is bound to @code{nil}; or @w{(ii) a} list of the form
198 @code{(@var{symbol} @var{value-form})}, in which case @var{symbol} is
199 bound to the result of evaluating @var{value-form}. If @var{value-form}
200 is omitted, @code{nil} is used.
202 All of the @var{value-form}s in @var{bindings} are evaluated in the
203 order they appear and @emph{before} binding any of the symbols to them.
204 Here is an example of this: @code{z} is bound to the old value of
205 @code{y}, which is 2, not the new value of @code{y}, which is 1.
221 @defspec let* (bindings@dots{}) forms@dots{}
222 This special form is like @code{let}, but it binds each variable right
223 after computing its local value, before computing the local value for
224 the next variable. Therefore, an expression in @var{bindings} can
225 reasonably refer to the preceding symbols bound in this @code{let*}
226 form. Compare the following example with the example above for
236 (z y)) ; @r{Use the just-established value of @code{y}.}
243 Here is a complete list of the other facilities that create local
248 Function calls (@pxref{Functions}).
251 Macro calls (@pxref{Macros}).
254 @code{condition-case} (@pxref{Errors}).
257 Variables can also have buffer-local bindings (@pxref{Buffer-Local
258 Variables}) and frame-local bindings (@pxref{Frame-Local Variables}); a
259 few variables have terminal-local bindings (@pxref{Multiple Displays}).
260 These kinds of bindings work somewhat like ordinary local bindings, but
261 they are localized depending on ``where'' you are in Emacs, rather than
264 @defvar max-specpdl-size
265 @anchor{Definition of max-specpdl-size}
266 @cindex variable limit error
267 @cindex evaluation error
268 @cindex infinite recursion
269 This variable defines the limit on the total number of local variable
270 bindings and @code{unwind-protect} cleanups (@pxref{Cleanups,,
271 Cleaning Up from Nonlocal Exits}) that are allowed before signaling an
272 error (with data @code{"Variable binding depth exceeds
275 This limit, with the associated error when it is exceeded, is one way
276 that Lisp avoids infinite recursion on an ill-defined function.
277 @code{max-lisp-eval-depth} provides another limit on depth of nesting.
278 @xref{Definition of max-lisp-eval-depth,, Eval}.
280 The default value is 1000. Entry to the Lisp debugger increases the
281 value, if there is little room left, to make sure the debugger itself
286 @section When a Variable is ``Void''
287 @kindex void-variable
288 @cindex void variable
290 If you have never given a symbol any value as a global variable, we
291 say that that symbol's global value is @dfn{void}. In other words, the
292 symbol's value cell does not have any Lisp object in it. If you try to
293 evaluate the symbol, you get a @code{void-variable} error rather than
296 Note that a value of @code{nil} is not the same as void. The symbol
297 @code{nil} is a Lisp object and can be the value of a variable just as any
298 other object can be; but it is @emph{a value}. A void variable does not
301 After you have given a variable a value, you can make it void once more
302 using @code{makunbound}.
304 @defun makunbound symbol
305 This function makes the current variable binding of @var{symbol} void.
306 Subsequent attempts to use this symbol's value as a variable will signal
307 the error @code{void-variable}, unless and until you set it again.
309 @code{makunbound} returns @var{symbol}.
313 (makunbound 'x) ; @r{Make the global value of @code{x} void.}
318 @error{} Symbol's value as variable is void: x
322 If @var{symbol} is locally bound, @code{makunbound} affects the most
323 local existing binding. This is the only way a symbol can have a void
324 local binding, since all the constructs that create local bindings
325 create them with values. In this case, the voidness lasts at most as
326 long as the binding does; when the binding is removed due to exit from
327 the construct that made it, the previous local or global binding is
328 reexposed as usual, and the variable is no longer void unless the newly
329 reexposed binding was void all along.
333 (setq x 1) ; @r{Put a value in the global binding.}
335 (let ((x 2)) ; @r{Locally bind it.}
336 (makunbound 'x) ; @r{Void the local binding.}
338 @error{} Symbol's value as variable is void: x
341 x ; @r{The global binding is unchanged.}
344 (let ((x 2)) ; @r{Locally bind it.}
345 (let ((x 3)) ; @r{And again.}
346 (makunbound 'x) ; @r{Void the innermost-local binding.}
347 x)) ; @r{And refer: it's void.}
348 @error{} Symbol's value as variable is void: x
354 (makunbound 'x)) ; @r{Void inner binding, then remove it.}
355 x) ; @r{Now outer @code{let} binding is visible.}
361 A variable that has been made void with @code{makunbound} is
362 indistinguishable from one that has never received a value and has
365 You can use the function @code{boundp} to test whether a variable is
368 @defun boundp variable
369 @code{boundp} returns @code{t} if @var{variable} (a symbol) is not void;
370 more precisely, if its current binding is not void. It returns
371 @code{nil} otherwise.
375 (boundp 'abracadabra) ; @r{Starts out void.}
379 (let ((abracadabra 5)) ; @r{Locally bind it.}
380 (boundp 'abracadabra))
384 (boundp 'abracadabra) ; @r{Still globally void.}
388 (setq abracadabra 5) ; @r{Make it globally nonvoid.}
392 (boundp 'abracadabra)
398 @node Defining Variables
399 @section Defining Global Variables
400 @cindex variable definition
402 You may announce your intention to use a symbol as a global variable
403 with a @dfn{variable definition}: a special form, either @code{defconst}
406 In Emacs Lisp, definitions serve three purposes. First, they inform
407 people who read the code that certain symbols are @emph{intended} to be
408 used a certain way (as variables). Second, they inform the Lisp system
409 of these things, supplying a value and documentation. Third, they
410 provide information to utilities such as @code{etags} and
411 @code{make-docfile}, which create data bases of the functions and
412 variables in a program.
414 The difference between @code{defconst} and @code{defvar} is primarily
415 a matter of intent, serving to inform human readers of whether the value
416 should ever change. Emacs Lisp does not restrict the ways in which a
417 variable can be used based on @code{defconst} or @code{defvar}
418 declarations. However, it does make a difference for initialization:
419 @code{defconst} unconditionally initializes the variable, while
420 @code{defvar} initializes it only if it is void.
423 One would expect user option variables to be defined with
424 @code{defconst}, since programs do not change them. Unfortunately, this
425 has bad results if the definition is in a library that is not preloaded:
426 @code{defconst} would override any prior value when the library is
427 loaded. Users would like to be able to set user options in their init
428 files, and override the default values given in the definitions. For
429 this reason, user options must be defined with @code{defvar}.
432 @defspec defvar symbol [value [doc-string]]
433 This special form defines @var{symbol} as a variable and can also
434 initialize and document it. The definition informs a person reading
435 your code that @var{symbol} is used as a variable that might be set or
436 changed. Note that @var{symbol} is not evaluated; the symbol to be
437 defined must appear explicitly in the @code{defvar}.
439 If @var{symbol} is void and @var{value} is specified, @code{defvar}
440 evaluates it and sets @var{symbol} to the result. But if @var{symbol}
441 already has a value (i.e., it is not void), @var{value} is not even
442 evaluated, and @var{symbol}'s value remains unchanged. If @var{value}
443 is omitted, the value of @var{symbol} is not changed in any case.
445 If @var{symbol} has a buffer-local binding in the current buffer,
446 @code{defvar} operates on the default value, which is buffer-independent,
447 not the current (buffer-local) binding. It sets the default value if
448 the default value is void. @xref{Buffer-Local Variables}.
450 When you evaluate a top-level @code{defvar} form with @kbd{C-M-x} in
451 Emacs Lisp mode (@code{eval-defun}), a special feature of
452 @code{eval-defun} arranges to set the variable unconditionally, without
453 testing whether its value is void.
455 If the @var{doc-string} argument appears, it specifies the documentation
456 for the variable. (This opportunity to specify documentation is one of
457 the main benefits of defining the variable.) The documentation is
458 stored in the symbol's @code{variable-documentation} property. The
459 Emacs help functions (@pxref{Documentation}) look for this property.
461 If the variable is a user option that users would want to set
462 interactively, you should use @samp{*} as the first character of
463 @var{doc-string}. This lets users set the variable conveniently using
464 the @code{set-variable} command. Note that you should nearly always
465 use @code{defcustom} instead of @code{defvar} to define these
466 variables, so that users can use @kbd{M-x customize} and related
467 commands to set them. @xref{Customization}.
469 Here are some examples. This form defines @code{foo} but does not
479 This example initializes the value of @code{bar} to @code{23}, and gives
480 it a documentation string:
485 "The normal weight of a bar.")
490 The following form changes the documentation string for @code{bar},
491 making it a user option, but does not change the value, since @code{bar}
492 already has a value. (The addition @code{(1+ nil)} would get an error
493 if it were evaluated, but since it is not evaluated, there is no error.)
498 "*The normal weight of a bar.")
507 Here is an equivalent expression for the @code{defvar} special form:
511 (defvar @var{symbol} @var{value} @var{doc-string})
514 (if (not (boundp '@var{symbol}))
515 (setq @var{symbol} @var{value}))
516 (if '@var{doc-string}
517 (put '@var{symbol} 'variable-documentation '@var{doc-string}))
522 The @code{defvar} form returns @var{symbol}, but it is normally used
523 at top level in a file where its value does not matter.
526 @defspec defconst symbol value [doc-string]
527 This special form defines @var{symbol} as a value and initializes it.
528 It informs a person reading your code that @var{symbol} has a standard
529 global value, established here, that should not be changed by the user
530 or by other programs. Note that @var{symbol} is not evaluated; the
531 symbol to be defined must appear explicitly in the @code{defconst}.
533 @code{defconst} always evaluates @var{value}, and sets the value of
534 @var{symbol} to the result. If @var{symbol} does have a buffer-local
535 binding in the current buffer, @code{defconst} sets the default value,
536 not the buffer-local value. (But you should not be making
537 buffer-local bindings for a symbol that is defined with
540 Here, @code{pi} is a constant that presumably ought not to be changed
541 by anyone (attempts by the Indiana State Legislature notwithstanding).
542 As the second form illustrates, however, this is only advisory.
546 (defconst pi 3.1415 "Pi to five places.")
560 @defun user-variable-p variable
562 This function returns @code{t} if @var{variable} is a user option---a
563 variable intended to be set by the user for customization---and
564 @code{nil} otherwise. (Variables other than user options exist for the
565 internal purposes of Lisp programs, and users need not know about them.)
567 User option variables are distinguished from other variables either
568 though being declared using @code{defcustom}@footnote{They may also be
569 declared equivalently in @file{cus-start.el}.} or by the first character
570 of their @code{variable-documentation} property. If the property exists
571 and is a string, and its first character is @samp{*}, then the variable
572 is a user option. Aliases of user options are also user options.
575 @kindex variable-interactive
576 If a user option variable has a @code{variable-interactive} property,
577 the @code{set-variable} command uses that value to control reading the
578 new value for the variable. The property's value is used as if it were
579 specified in @code{interactive} (@pxref{Using Interactive}). However,
580 this feature is largely obsoleted by @code{defcustom}
581 (@pxref{Customization}).
583 @strong{Warning:} If the @code{defconst} and @code{defvar} special
584 forms are used while the variable has a local binding (made with
585 @code{let}, or a function argument), they set the local-binding's
586 value; the top-level binding is not changed. This is not what you
587 usually want. To prevent it, use these special forms at top level in
588 a file, where normally no local binding is in effect, and make sure to
589 load the file before making a local binding for the variable.
591 @node Tips for Defining
592 @section Tips for Defining Variables Robustly
594 When you define a variable whose value is a function, or a list of
595 functions, use a name that ends in @samp{-function} or
596 @samp{-functions}, respectively.
598 There are several other variable name conventions;
599 here is a complete list:
603 The variable is a normal hook (@pxref{Hooks}).
605 @item @dots{}-function
606 The value is a function.
608 @item @dots{}-functions
609 The value is a list of functions.
612 The value is a form (an expression).
615 The value is a list of forms (expressions).
617 @item @dots{}-predicate
618 The value is a predicate---a function of one argument that returns
619 non-@code{nil} for ``good'' arguments and @code{nil} for ``bad''
623 The value is significant only as to whether it is @code{nil} or not.
625 @item @dots{}-program
626 The value is a program name.
628 @item @dots{}-command
629 The value is a whole shell command.
631 @item @dots{}-switches
632 The value specifies options for a command.
635 When you define a variable, always consider whether you should mark
636 it as ``risky''; see @ref{File Local Variables}.
638 When defining and initializing a variable that holds a complicated
639 value (such as a keymap with bindings in it), it's best to put the
640 entire computation of the value into the @code{defvar}, like this:
644 (let ((map (make-sparse-keymap)))
645 (define-key map "\C-c\C-a" 'my-command)
652 This method has several benefits. First, if the user quits while
653 loading the file, the variable is either still uninitialized or
654 initialized properly, never in-between. If it is still uninitialized,
655 reloading the file will initialize it properly. Second, reloading the
656 file once the variable is initialized will not alter it; that is
657 important if the user has run hooks to alter part of the contents (such
658 as, to rebind keys). Third, evaluating the @code{defvar} form with
659 @kbd{C-M-x} @emph{will} reinitialize the map completely.
661 Putting so much code in the @code{defvar} form has one disadvantage:
662 it puts the documentation string far away from the line which names the
663 variable. Here's a safe way to avoid that:
666 (defvar my-mode-map nil
669 (let ((map (make-sparse-keymap)))
670 (define-key map "\C-c\C-a" 'my-command)
672 (setq my-mode-map map)))
676 This has all the same advantages as putting the initialization inside
677 the @code{defvar}, except that you must type @kbd{C-M-x} twice, once on
678 each form, if you do want to reinitialize the variable.
680 But be careful not to write the code like this:
683 (defvar my-mode-map nil
686 (setq my-mode-map (make-sparse-keymap))
687 (define-key my-mode-map "\C-c\C-a" 'my-command)
692 This code sets the variable, then alters it, but it does so in more than
693 one step. If the user quits just after the @code{setq}, that leaves the
694 variable neither correctly initialized nor void nor @code{nil}. Once
695 that happens, reloading the file will not initialize the variable; it
696 will remain incomplete.
698 @node Accessing Variables
699 @section Accessing Variable Values
701 The usual way to reference a variable is to write the symbol which
702 names it (@pxref{Symbol Forms}). This requires you to specify the
703 variable name when you write the program. Usually that is exactly what
704 you want to do. Occasionally you need to choose at run time which
705 variable to reference; then you can use @code{symbol-value}.
707 @defun symbol-value symbol
708 This function returns the value of @var{symbol}. This is the value in
709 the innermost local binding of the symbol, or its global value if it
710 has no local bindings.
723 ;; @r{Here the symbol @code{abracadabra}}
724 ;; @r{is the symbol whose value is examined.}
725 (let ((abracadabra 'foo))
726 (symbol-value 'abracadabra))
731 ;; @r{Here, the value of @code{abracadabra},}
732 ;; @r{which is @code{foo},}
733 ;; @r{is the symbol whose value is examined.}
734 (let ((abracadabra 'foo))
735 (symbol-value abracadabra))
740 (symbol-value 'abracadabra)
745 A @code{void-variable} error is signaled if the current binding of
746 @var{symbol} is void.
749 @node Setting Variables
750 @section How to Alter a Variable Value
752 The usual way to change the value of a variable is with the special
753 form @code{setq}. When you need to compute the choice of variable at
754 run time, use the function @code{set}.
756 @defspec setq [symbol form]@dots{}
757 This special form is the most common method of changing a variable's
758 value. Each @var{symbol} is given a new value, which is the result of
759 evaluating the corresponding @var{form}. The most-local existing
760 binding of the symbol is changed.
762 @code{setq} does not evaluate @var{symbol}; it sets the symbol that you
763 write. We say that this argument is @dfn{automatically quoted}. The
764 @samp{q} in @code{setq} stands for ``quoted.''
766 The value of the @code{setq} form is the value of the last @var{form}.
773 x ; @r{@code{x} now has a global value.}
777 (setq x 6) ; @r{The local binding of @code{x} is set.}
781 x ; @r{The global value is unchanged.}
785 Note that the first @var{form} is evaluated, then the first
786 @var{symbol} is set, then the second @var{form} is evaluated, then the
787 second @var{symbol} is set, and so on:
791 (setq x 10 ; @r{Notice that @code{x} is set before}
792 y (1+ x)) ; @r{the value of @code{y} is computed.}
798 @defun set symbol value
799 This function sets @var{symbol}'s value to @var{value}, then returns
800 @var{value}. Since @code{set} is a function, the expression written for
801 @var{symbol} is evaluated to obtain the symbol to set.
803 The most-local existing binding of the variable is the binding that is
804 set; shadowed bindings are not affected.
809 @error{} Symbol's value as variable is void: one
820 (set two 2) ; @r{@code{two} evaluates to symbol @code{one}.}
824 one ; @r{So it is @code{one} that was set.}
826 (let ((one 1)) ; @r{This binding of @code{one} is set,}
827 (set 'one 3) ; @r{not the global value.}
837 If @var{symbol} is not actually a symbol, a @code{wrong-type-argument}
842 @error{} Wrong type argument: symbolp, (x y)
845 Logically speaking, @code{set} is a more fundamental primitive than
846 @code{setq}. Any use of @code{setq} can be trivially rewritten to use
847 @code{set}; @code{setq} could even be defined as a macro, given the
848 availability of @code{set}. However, @code{set} itself is rarely used;
849 beginners hardly need to know about it. It is useful only for choosing
850 at run time which variable to set. For example, the command
851 @code{set-variable}, which reads a variable name from the user and then
852 sets the variable, needs to use @code{set}.
854 @cindex CL note---@code{set} local
856 @b{Common Lisp note:} In Common Lisp, @code{set} always changes the
857 symbol's ``special'' or dynamic value, ignoring any lexical bindings.
858 In Emacs Lisp, all variables and all bindings are dynamic, so @code{set}
859 always affects the most local existing binding.
863 @node Variable Scoping
864 @section Scoping Rules for Variable Bindings
866 A given symbol @code{foo} can have several local variable bindings,
867 established at different places in the Lisp program, as well as a global
868 binding. The most recently established binding takes precedence over
873 @cindex dynamic scoping
874 @cindex lexical scoping
875 Local bindings in Emacs Lisp have @dfn{indefinite scope} and
876 @dfn{dynamic extent}. @dfn{Scope} refers to @emph{where} textually in
877 the source code the binding can be accessed. ``Indefinite scope'' means
878 that any part of the program can potentially access the variable
879 binding. @dfn{Extent} refers to @emph{when}, as the program is
880 executing, the binding exists. ``Dynamic extent'' means that the binding
881 lasts as long as the activation of the construct that established it.
883 The combination of dynamic extent and indefinite scope is called
884 @dfn{dynamic scoping}. By contrast, most programming languages use
885 @dfn{lexical scoping}, in which references to a local variable must be
886 located textually within the function or block that binds the variable.
888 @cindex CL note---special variables
890 @b{Common Lisp note:} Variables declared ``special'' in Common Lisp are
891 dynamically scoped, like all variables in Emacs Lisp.
895 * Scope:: Scope means where in the program a value is visible.
896 Comparison with other languages.
897 * Extent:: Extent means how long in time a value exists.
898 * Impl of Scope:: Two ways to implement dynamic scoping.
899 * Using Scoping:: How to use dynamic scoping carefully and avoid problems.
905 Emacs Lisp uses @dfn{indefinite scope} for local variable bindings.
906 This means that any function anywhere in the program text might access a
907 given binding of a variable. Consider the following function
912 (defun binder (x) ; @r{@code{x} is bound in @code{binder}.}
913 (foo 5)) ; @r{@code{foo} is some other function.}
917 (defun user () ; @r{@code{x} is used ``free'' in @code{user}.}
922 In a lexically scoped language, the binding of @code{x} in
923 @code{binder} would never be accessible in @code{user}, because
924 @code{user} is not textually contained within the function
925 @code{binder}. However, in dynamically-scoped Emacs Lisp, @code{user}
926 may or may not refer to the binding of @code{x} established in
927 @code{binder}, depending on the circumstances:
931 If we call @code{user} directly without calling @code{binder} at all,
932 then whatever binding of @code{x} is found, it cannot come from
936 If we define @code{foo} as follows and then call @code{binder}, then the
937 binding made in @code{binder} will be seen in @code{user}:
947 However, if we define @code{foo} as follows and then call @code{binder},
948 then the binding made in @code{binder} @emph{will not} be seen in
957 Here, when @code{foo} is called by @code{binder}, it binds @code{x}.
958 (The binding in @code{foo} is said to @dfn{shadow} the one made in
959 @code{binder}.) Therefore, @code{user} will access the @code{x} bound
960 by @code{foo} instead of the one bound by @code{binder}.
963 Emacs Lisp uses dynamic scoping because simple implementations of
964 lexical scoping are slow. In addition, every Lisp system needs to offer
965 dynamic scoping at least as an option; if lexical scoping is the norm,
966 there must be a way to specify dynamic scoping instead for a particular
967 variable. It might not be a bad thing for Emacs to offer both, but
968 implementing it with dynamic scoping only was much easier.
973 @dfn{Extent} refers to the time during program execution that a
974 variable name is valid. In Emacs Lisp, a variable is valid only while
975 the form that bound it is executing. This is called @dfn{dynamic
976 extent}. ``Local'' or ``automatic'' variables in most languages,
977 including C and Pascal, have dynamic extent.
979 One alternative to dynamic extent is @dfn{indefinite extent}. This
980 means that a variable binding can live on past the exit from the form
981 that made the binding. Common Lisp and Scheme, for example, support
982 this, but Emacs Lisp does not.
984 To illustrate this, the function below, @code{make-add}, returns a
985 function that purports to add @var{n} to its own argument @var{m}. This
986 would work in Common Lisp, but it does not do the job in Emacs Lisp,
987 because after the call to @code{make-add} exits, the variable @code{n}
988 is no longer bound to the actual argument 2.
992 (function (lambda (m) (+ n m)))) ; @r{Return a function.}
994 (fset 'add2 (make-add 2)) ; @r{Define function @code{add2}}
995 ; @r{with @code{(make-add 2)}.}
996 @result{} (lambda (m) (+ n m))
997 (add2 4) ; @r{Try to add 2 to 4.}
998 @error{} Symbol's value as variable is void: n
1001 @cindex closures not available
1002 Some Lisp dialects have ``closures,'' objects that are like functions
1003 but record additional variable bindings. Emacs Lisp does not have
1007 @subsection Implementation of Dynamic Scoping
1008 @cindex deep binding
1010 A simple sample implementation (which is not how Emacs Lisp actually
1011 works) may help you understand dynamic binding. This technique is
1012 called @dfn{deep binding} and was used in early Lisp systems.
1014 Suppose there is a stack of bindings, which are variable-value pairs.
1015 At entry to a function or to a @code{let} form, we can push bindings
1016 onto the stack for the arguments or local variables created there. We
1017 can pop those bindings from the stack at exit from the binding
1020 We can find the value of a variable by searching the stack from top to
1021 bottom for a binding for that variable; the value from that binding is
1022 the value of the variable. To set the variable, we search for the
1023 current binding, then store the new value into that binding.
1025 As you can see, a function's bindings remain in effect as long as it
1026 continues execution, even during its calls to other functions. That is
1027 why we say the extent of the binding is dynamic. And any other function
1028 can refer to the bindings, if it uses the same variables while the
1029 bindings are in effect. That is why we say the scope is indefinite.
1031 @cindex shallow binding
1032 The actual implementation of variable scoping in GNU Emacs Lisp uses a
1033 technique called @dfn{shallow binding}. Each variable has a standard
1034 place in which its current value is always found---the value cell of the
1037 In shallow binding, setting the variable works by storing a value in
1038 the value cell. Creating a new binding works by pushing the old value
1039 (belonging to a previous binding) onto a stack, and storing the new
1040 local value in the value cell. Eliminating a binding works by popping
1041 the old value off the stack, into the value cell.
1043 We use shallow binding because it has the same results as deep
1044 binding, but runs faster, since there is never a need to search for a
1048 @subsection Proper Use of Dynamic Scoping
1050 Binding a variable in one function and using it in another is a
1051 powerful technique, but if used without restraint, it can make programs
1052 hard to understand. There are two clean ways to use this technique:
1056 Use or bind the variable only in a few related functions, written close
1057 together in one file. Such a variable is used for communication within
1060 You should write comments to inform other programmers that they can see
1061 all uses of the variable before them, and to advise them not to add uses
1065 Give the variable a well-defined, documented meaning, and make all
1066 appropriate functions refer to it (but not bind it or set it) wherever
1067 that meaning is relevant. For example, the variable
1068 @code{case-fold-search} is defined as ``non-@code{nil} means ignore case
1069 when searching''; various search and replace functions refer to it
1070 directly or through their subroutines, but do not bind or set it.
1072 Then you can bind the variable in other programs, knowing reliably what
1076 In either case, you should define the variable with @code{defvar}.
1077 This helps other people understand your program by telling them to look
1078 for inter-function usage. It also avoids a warning from the byte
1079 compiler. Choose the variable's name to avoid name conflicts---don't
1080 use short names like @code{x}.
1082 @node Buffer-Local Variables
1083 @section Buffer-Local Variables
1084 @cindex variable, buffer-local
1085 @cindex buffer-local variables
1087 Global and local variable bindings are found in most programming
1088 languages in one form or another. Emacs, however, also supports additional,
1089 unusual kinds of variable binding: @dfn{buffer-local} bindings, which
1090 apply only in one buffer, and @dfn{frame-local} bindings, which apply only in
1091 one frame. Having different values for a variable in different buffers
1092 and/or frames is an important customization method.
1094 This section describes buffer-local bindings; for frame-local
1095 bindings, see the following section, @ref{Frame-Local Variables}. (A few
1096 variables have bindings that are local to each terminal; see
1097 @ref{Multiple Displays}.)
1100 * Intro to Buffer-Local:: Introduction and concepts.
1101 * Creating Buffer-Local:: Creating and destroying buffer-local bindings.
1102 * Default Value:: The default value is seen in buffers
1103 that don't have their own buffer-local values.
1106 @node Intro to Buffer-Local
1107 @subsection Introduction to Buffer-Local Variables
1109 A buffer-local variable has a buffer-local binding associated with a
1110 particular buffer. The binding is in effect when that buffer is
1111 current; otherwise, it is not in effect. If you set the variable while
1112 a buffer-local binding is in effect, the new value goes in that binding,
1113 so its other bindings are unchanged. This means that the change is
1114 visible only in the buffer where you made it.
1116 The variable's ordinary binding, which is not associated with any
1117 specific buffer, is called the @dfn{default binding}. In most cases,
1118 this is the global binding.
1120 A variable can have buffer-local bindings in some buffers but not in
1121 other buffers. The default binding is shared by all the buffers that
1122 don't have their own bindings for the variable. (This includes all
1123 newly-created buffers.) If you set the variable in a buffer that does
1124 not have a buffer-local binding for it, this sets the default binding
1125 (assuming there are no frame-local bindings to complicate the matter),
1126 so the new value is visible in all the buffers that see the default
1129 The most common use of buffer-local bindings is for major modes to change
1130 variables that control the behavior of commands. For example, C mode and
1131 Lisp mode both set the variable @code{paragraph-start} to specify that only
1132 blank lines separate paragraphs. They do this by making the variable
1133 buffer-local in the buffer that is being put into C mode or Lisp mode, and
1134 then setting it to the new value for that mode. @xref{Major Modes}.
1136 The usual way to make a buffer-local binding is with
1137 @code{make-local-variable}, which is what major mode commands typically
1138 use. This affects just the current buffer; all other buffers (including
1139 those yet to be created) will continue to share the default value unless
1140 they are explicitly given their own buffer-local bindings.
1142 @cindex automatically buffer-local
1143 A more powerful operation is to mark the variable as
1144 @dfn{automatically buffer-local} by calling
1145 @code{make-variable-buffer-local}. You can think of this as making the
1146 variable local in all buffers, even those yet to be created. More
1147 precisely, the effect is that setting the variable automatically makes
1148 the variable local to the current buffer if it is not already so. All
1149 buffers start out by sharing the default value of the variable as usual,
1150 but setting the variable creates a buffer-local binding for the current
1151 buffer. The new value is stored in the buffer-local binding, leaving
1152 the default binding untouched. This means that the default value cannot
1153 be changed with @code{setq} in any buffer; the only way to change it is
1154 with @code{setq-default}.
1156 @strong{Warning:} When a variable has buffer-local or frame-local
1157 bindings in one or more buffers, @code{let} rebinds the binding that's
1158 currently in effect. For instance, if the current buffer has a
1159 buffer-local value, @code{let} temporarily rebinds that. If no
1160 buffer-local or frame-local bindings are in effect, @code{let} rebinds
1161 the default value. If inside the @code{let} you then change to a
1162 different current buffer in which a different binding is in effect,
1163 you won't see the @code{let} binding any more. And if you exit the
1164 @code{let} while still in the other buffer, you won't see the
1165 unbinding occur (though it will occur properly). Here is an example
1172 (make-local-variable 'foo)
1176 ;; foo @result{} 'temp ; @r{let binding in buffer @samp{a}}
1178 ;; foo @result{} 'g ; @r{the global value since foo is not local in @samp{b}}
1181 foo @result{} 'g ; @r{exiting restored the local value in buffer @samp{a},}
1182 ; @r{but we don't see that in buffer @samp{b}}
1185 (set-buffer "a") ; @r{verify the local value was restored}
1190 Note that references to @code{foo} in @var{body} access the
1191 buffer-local binding of buffer @samp{b}.
1193 When a file specifies local variable values, these become buffer-local
1194 values when you visit the file. @xref{File Variables,,, emacs, The
1197 @node Creating Buffer-Local
1198 @subsection Creating and Deleting Buffer-Local Bindings
1200 @deffn Command make-local-variable variable
1201 This function creates a buffer-local binding in the current buffer for
1202 @var{variable} (a symbol). Other buffers are not affected. The value
1203 returned is @var{variable}.
1206 The buffer-local value of @var{variable} starts out as the same value
1207 @var{variable} previously had. If @var{variable} was void, it remains
1212 ;; @r{In buffer @samp{b1}:}
1213 (setq foo 5) ; @r{Affects all buffers.}
1217 (make-local-variable 'foo) ; @r{Now it is local in @samp{b1}.}
1221 foo ; @r{That did not change}
1222 @result{} 5 ; @r{the value.}
1225 (setq foo 6) ; @r{Change the value}
1226 @result{} 6 ; @r{in @samp{b1}.}
1234 ;; @r{In buffer @samp{b2}, the value hasn't changed.}
1242 Making a variable buffer-local within a @code{let}-binding for that
1243 variable does not work reliably, unless the buffer in which you do this
1244 is not current either on entry to or exit from the @code{let}. This is
1245 because @code{let} does not distinguish between different kinds of
1246 bindings; it knows only which variable the binding was made for.
1248 If the variable is terminal-local, this function signals an error. Such
1249 variables cannot have buffer-local bindings as well. @xref{Multiple
1252 @strong{Warning:} do not use @code{make-local-variable} for a hook
1253 variable. The hook variables are automatically made buffer-local as
1254 needed if you use the @var{local} argument to @code{add-hook} or
1258 @deffn Command make-variable-buffer-local variable
1259 This function marks @var{variable} (a symbol) automatically
1260 buffer-local, so that any subsequent attempt to set it will make it
1261 local to the current buffer at the time.
1263 A peculiar wrinkle of this feature is that binding the variable (with
1264 @code{let} or other binding constructs) does not create a buffer-local
1265 binding for it. Only setting the variable (with @code{set} or
1266 @code{setq}), while the variable does not have a @code{let}-style
1267 binding that was made in the current buffer, does so.
1269 If @var{variable} does not have a default value, then calling this
1270 command will give it a default value of @code{nil}. If @var{variable}
1271 already has a default value, that value remains unchanged.
1272 Subsequently calling @code{makunbound} on @var{variable} will result
1273 in a void buffer-local value and leave the default value unaffected.
1275 The value returned is @var{variable}.
1277 @strong{Warning:} Don't assume that you should use
1278 @code{make-variable-buffer-local} for user-option variables, simply
1279 because users @emph{might} want to customize them differently in
1280 different buffers. Users can make any variable local, when they wish
1281 to. It is better to leave the choice to them.
1283 The time to use @code{make-variable-buffer-local} is when it is crucial
1284 that no two buffers ever share the same binding. For example, when a
1285 variable is used for internal purposes in a Lisp program which depends
1286 on having separate values in separate buffers, then using
1287 @code{make-variable-buffer-local} can be the best solution.
1290 @defun local-variable-p variable &optional buffer
1291 This returns @code{t} if @var{variable} is buffer-local in buffer
1292 @var{buffer} (which defaults to the current buffer); otherwise,
1296 @defun local-variable-if-set-p variable &optional buffer
1297 This returns @code{t} if @var{variable} will become buffer-local in
1298 buffer @var{buffer} (which defaults to the current buffer) if it is
1302 @defun buffer-local-value variable buffer
1303 This function returns the buffer-local binding of @var{variable} (a
1304 symbol) in buffer @var{buffer}. If @var{variable} does not have a
1305 buffer-local binding in buffer @var{buffer}, it returns the default
1306 value (@pxref{Default Value}) of @var{variable} instead.
1309 @defun buffer-local-variables &optional buffer
1310 This function returns a list describing the buffer-local variables in
1311 buffer @var{buffer}. (If @var{buffer} is omitted, the current buffer is
1312 used.) It returns an association list (@pxref{Association Lists}) in
1313 which each element contains one buffer-local variable and its value.
1314 However, when a variable's buffer-local binding in @var{buffer} is void,
1315 then the variable appears directly in the resulting list.
1319 (make-local-variable 'foobar)
1320 (makunbound 'foobar)
1321 (make-local-variable 'bind-me)
1324 (setq lcl (buffer-local-variables))
1325 ;; @r{First, built-in variables local in all buffers:}
1326 @result{} ((mark-active . nil)
1327 (buffer-undo-list . nil)
1328 (mode-name . "Fundamental")
1331 ;; @r{Next, non-built-in buffer-local variables.}
1332 ;; @r{This one is buffer-local and void:}
1334 ;; @r{This one is buffer-local and nonvoid:}
1339 Note that storing new values into the @sc{cdr}s of cons cells in this
1340 list does @emph{not} change the buffer-local values of the variables.
1343 @deffn Command kill-local-variable variable
1344 This function deletes the buffer-local binding (if any) for
1345 @var{variable} (a symbol) in the current buffer. As a result, the
1346 default binding of @var{variable} becomes visible in this buffer. This
1347 typically results in a change in the value of @var{variable}, since the
1348 default value is usually different from the buffer-local value just
1351 If you kill the buffer-local binding of a variable that automatically
1352 becomes buffer-local when set, this makes the default value visible in
1353 the current buffer. However, if you set the variable again, that will
1354 once again create a buffer-local binding for it.
1356 @code{kill-local-variable} returns @var{variable}.
1358 This function is a command because it is sometimes useful to kill one
1359 buffer-local variable interactively, just as it is useful to create
1360 buffer-local variables interactively.
1363 @defun kill-all-local-variables
1364 This function eliminates all the buffer-local variable bindings of the
1365 current buffer except for variables marked as ``permanent.'' As a
1366 result, the buffer will see the default values of most variables.
1368 This function also resets certain other information pertaining to the
1369 buffer: it sets the local keymap to @code{nil}, the syntax table to the
1370 value of @code{(standard-syntax-table)}, the case table to
1371 @code{(standard-case-table)}, and the abbrev table to the value of
1372 @code{fundamental-mode-abbrev-table}.
1374 The very first thing this function does is run the normal hook
1375 @code{change-major-mode-hook} (see below).
1377 Every major mode command begins by calling this function, which has the
1378 effect of switching to Fundamental mode and erasing most of the effects
1379 of the previous major mode. To ensure that this does its job, the
1380 variables that major modes set should not be marked permanent.
1382 @code{kill-all-local-variables} returns @code{nil}.
1385 @defvar change-major-mode-hook
1386 The function @code{kill-all-local-variables} runs this normal hook
1387 before it does anything else. This gives major modes a way to arrange
1388 for something special to be done if the user switches to a different
1389 major mode. It is also useful for buffer-specific minor modes
1390 that should be forgotten if the user changes the major mode.
1392 For best results, make this variable buffer-local, so that it will
1393 disappear after doing its job and will not interfere with the
1394 subsequent major mode. @xref{Hooks}.
1398 @cindex permanent local variable
1399 A buffer-local variable is @dfn{permanent} if the variable name (a
1400 symbol) has a @code{permanent-local} property that is non-@code{nil}.
1401 Permanent locals are appropriate for data pertaining to where the file
1402 came from or how to save it, rather than with how to edit the contents.
1405 @subsection The Default Value of a Buffer-Local Variable
1406 @cindex default value
1408 The global value of a variable with buffer-local bindings is also
1409 called the @dfn{default} value, because it is the value that is in
1410 effect whenever neither the current buffer nor the selected frame has
1411 its own binding for the variable.
1413 The functions @code{default-value} and @code{setq-default} access and
1414 change a variable's default value regardless of whether the current
1415 buffer has a buffer-local binding. For example, you could use
1416 @code{setq-default} to change the default setting of
1417 @code{paragraph-start} for most buffers; and this would work even when
1418 you are in a C or Lisp mode buffer that has a buffer-local value for
1422 The special forms @code{defvar} and @code{defconst} also set the
1423 default value (if they set the variable at all), rather than any
1424 buffer-local or frame-local value.
1426 @defun default-value symbol
1427 This function returns @var{symbol}'s default value. This is the value
1428 that is seen in buffers and frames that do not have their own values for
1429 this variable. If @var{symbol} is not buffer-local, this is equivalent
1430 to @code{symbol-value} (@pxref{Accessing Variables}).
1434 @defun default-boundp symbol
1435 The function @code{default-boundp} tells you whether @var{symbol}'s
1436 default value is nonvoid. If @code{(default-boundp 'foo)} returns
1437 @code{nil}, then @code{(default-value 'foo)} would get an error.
1439 @code{default-boundp} is to @code{default-value} as @code{boundp} is to
1440 @code{symbol-value}.
1443 @defspec setq-default [symbol form]@dots{}
1444 This special form gives each @var{symbol} a new default value, which is
1445 the result of evaluating the corresponding @var{form}. It does not
1446 evaluate @var{symbol}, but does evaluate @var{form}. The value of the
1447 @code{setq-default} form is the value of the last @var{form}.
1449 If a @var{symbol} is not buffer-local for the current buffer, and is not
1450 marked automatically buffer-local, @code{setq-default} has the same
1451 effect as @code{setq}. If @var{symbol} is buffer-local for the current
1452 buffer, then this changes the value that other buffers will see (as long
1453 as they don't have a buffer-local value), but not the value that the
1454 current buffer sees.
1458 ;; @r{In buffer @samp{foo}:}
1459 (make-local-variable 'buffer-local)
1460 @result{} buffer-local
1463 (setq buffer-local 'value-in-foo)
1464 @result{} value-in-foo
1467 (setq-default buffer-local 'new-default)
1468 @result{} new-default
1472 @result{} value-in-foo
1475 (default-value 'buffer-local)
1476 @result{} new-default
1480 ;; @r{In (the new) buffer @samp{bar}:}
1482 @result{} new-default
1485 (default-value 'buffer-local)
1486 @result{} new-default
1489 (setq buffer-local 'another-default)
1490 @result{} another-default
1493 (default-value 'buffer-local)
1494 @result{} another-default
1498 ;; @r{Back in buffer @samp{foo}:}
1500 @result{} value-in-foo
1501 (default-value 'buffer-local)
1502 @result{} another-default
1507 @defun set-default symbol value
1508 This function is like @code{setq-default}, except that @var{symbol} is
1509 an ordinary evaluated argument.
1513 (set-default (car '(a b c)) 23)
1523 @node Frame-Local Variables
1524 @section Frame-Local Variables
1525 @cindex frame-local variables
1527 Just as variables can have buffer-local bindings, they can also have
1528 frame-local bindings. These bindings belong to one frame, and are in
1529 effect when that frame is selected. Frame-local bindings are actually
1530 frame parameters: you create a frame-local binding in a specific frame
1531 by calling @code{modify-frame-parameters} and specifying the variable
1532 name as the parameter name.
1534 To enable frame-local bindings for a certain variable, call the function
1535 @code{make-variable-frame-local}.
1537 @deffn Command make-variable-frame-local variable
1538 Enable the use of frame-local bindings for @var{variable}. This does
1539 not in itself create any frame-local bindings for the variable; however,
1540 if some frame already has a value for @var{variable} as a frame
1541 parameter, that value automatically becomes a frame-local binding.
1543 If @var{variable} does not have a default value, then calling this
1544 command will give it a default value of @code{nil}. If @var{variable}
1545 already has a default value, that value remains unchanged.
1547 If the variable is terminal-local, this function signals an error,
1548 because such variables cannot have frame-local bindings as well.
1549 @xref{Multiple Displays}. A few variables that are implemented
1550 specially in Emacs can be buffer-local, but can never be frame-local.
1552 This command returns @var{variable}.
1555 Buffer-local bindings take precedence over frame-local bindings. Thus,
1556 consider a variable @code{foo}: if the current buffer has a buffer-local
1557 binding for @code{foo}, that binding is active; otherwise, if the
1558 selected frame has a frame-local binding for @code{foo}, that binding is
1559 active; otherwise, the default binding of @code{foo} is active.
1561 Here is an example. First we prepare a few bindings for @code{foo}:
1564 (setq f1 (selected-frame))
1565 (make-variable-frame-local 'foo)
1567 ;; @r{Make a buffer-local binding for @code{foo} in @samp{b1}.}
1568 (set-buffer (get-buffer-create "b1"))
1569 (make-local-variable 'foo)
1572 ;; @r{Make a frame-local binding for @code{foo} in a new frame.}
1573 ;; @r{Store that frame in @code{f2}.}
1574 (setq f2 (make-frame))
1575 (modify-frame-parameters f2 '((foo . (f 2))))
1578 Now we examine @code{foo} in various contexts. Whenever the
1579 buffer @samp{b1} is current, its buffer-local binding is in effect,
1580 regardless of the selected frame:
1584 (set-buffer (get-buffer-create "b1"))
1589 (set-buffer (get-buffer-create "b1"))
1595 Otherwise, the frame gets a chance to provide the binding; when frame
1596 @code{f2} is selected, its frame-local binding is in effect:
1600 (set-buffer (get-buffer "*scratch*"))
1606 When neither the current buffer nor the selected frame provides
1607 a binding, the default binding is used:
1611 (set-buffer (get-buffer "*scratch*"))
1617 When the active binding of a variable is a frame-local binding, setting
1618 the variable changes that binding. You can observe the result with
1619 @code{frame-parameters}:
1623 (set-buffer (get-buffer "*scratch*"))
1625 (assq 'foo (frame-parameters f2))
1626 @result{} (foo . nobody)
1629 @node Future Local Variables
1630 @section Possible Future Local Variables
1632 We have considered the idea of bindings that are local to a category
1633 of frames---for example, all color frames, or all frames with dark
1634 backgrounds. We have not implemented them because it is not clear that
1635 this feature is really useful. You can get more or less the same
1636 results by adding a function to @code{after-make-frame-functions}, set up to
1637 define a particular frame parameter according to the appropriate
1638 conditions for each frame.
1640 It would also be possible to implement window-local bindings. We
1641 don't know of many situations where they would be useful, and it seems
1642 that indirect buffers (@pxref{Indirect Buffers}) with buffer-local
1643 bindings offer a way to handle these situations more robustly.
1645 If sufficient application is found for either of these two kinds of
1646 local bindings, we will provide it in a subsequent Emacs version.
1648 @node File Local Variables
1649 @section File Local Variables
1650 @cindex file local variables
1652 A file can specify local variable values; Emacs uses these to create
1653 buffer-local bindings for those variables in the buffer visiting that
1654 file. @xref{File variables, , Local Variables in Files, emacs, The
1655 GNU Emacs Manual}, for basic information about file local variables.
1656 This section describes the functions and variables that affect
1657 processing of file local variables.
1659 @defopt enable-local-variables
1660 This variable controls whether to process file local variables.
1661 The possible values are:
1664 @item @code{t} (the default)
1665 Set the safe variables, and query (once) about any unsafe variables.
1667 Set only the safe variables and do not query.
1669 Set all the variables and do not query.
1671 Don't set any variables.
1673 Query (once) about all the variables.
1677 @defun hack-local-variables &optional mode-only
1678 This function parses, and binds or evaluates as appropriate, any local
1679 variables specified by the contents of the current buffer. The variable
1680 @code{enable-local-variables} has its effect here. However, this
1681 function does not look for the @samp{mode:} local variable in the
1682 @w{@samp{-*-}} line. @code{set-auto-mode} does that, also taking
1683 @code{enable-local-variables} into account (@pxref{Auto Major Mode}).
1685 If the optional argument @var{mode-only} is non-@code{nil}, then all
1686 this function does is return @code{t} if the @w{@samp{-*-}} line or
1687 the local variables list specifies a mode and @code{nil} otherwise.
1688 It does not set the mode nor any other file local variable.
1691 If a file local variable could specify a function that would
1692 be called later, or an expression that would be executed later, simply
1693 visiting a file could take over your Emacs. Emacs takes several
1694 measures to prevent this.
1696 @cindex safe local variable
1697 You can specify safe values for a variable with a
1698 @code{safe-local-variable} property. The property has to be
1699 a function of one argument; any value is safe if the function
1700 returns non-@code{nil} given that value. Many commonly encountered
1701 file variables standardly have @code{safe-local-variable} properties,
1702 including @code{fill-column}, @code{fill-prefix}, and
1703 @code{indent-tabs-mode}. For boolean-valued variables that are safe,
1704 use @code{booleanp} as the property value. Lambda expressions should
1705 be quoted so that @code{describe-variable} can display the predicate.
1707 @defopt safe-local-variable-values
1708 This variable provides another way to mark some variable values as
1709 safe. It is a list of cons cells @code{(@var{var} . @var{val})},
1710 where @var{var} is a variable name and @var{val} is a value which is
1711 safe for that variable.
1713 When Emacs asks the user whether or not to obey a set of file local
1714 variable specifications, the user can choose to mark them as safe.
1715 Doing so adds those variable/value pairs to
1716 @code{safe-local-variable-values}, and saves it to the user's custom
1720 @defun safe-local-variable-p sym val
1721 This function returns non-@code{nil} if it is safe to give @var{sym}
1722 the value @var{val}, based on the above criteria.
1725 @c @cindex risky local variable Duplicates risky-local-variable
1726 Some variables are considered @dfn{risky}. A variable whose name
1727 ends in any of @samp{-command}, @samp{-frame-alist}, @samp{-function},
1728 @samp{-functions}, @samp{-hook}, @samp{-hooks}, @samp{-form},
1729 @samp{-forms}, @samp{-map}, @samp{-map-alist}, @samp{-mode-alist},
1730 @samp{-program}, or @samp{-predicate} is considered risky. The
1731 variables @samp{font-lock-keywords}, @samp{font-lock-keywords}
1732 followed by a digit, and @samp{font-lock-syntactic-keywords} are also
1733 considered risky. Finally, any variable whose name has a
1734 non-@code{nil} @code{risky-local-variable} property is considered
1737 @defun risky-local-variable-p sym
1738 This function returns non-@code{nil} if @var{sym} is a risky variable,
1739 based on the above criteria.
1742 If a variable is risky, it will not be entered automatically into
1743 @code{safe-local-variable-values} as described above. Therefore,
1744 Emacs will always query before setting a risky variable, unless the
1745 user explicitly allows the setting by customizing
1746 @code{safe-local-variable-values} directly.
1748 @defvar ignored-local-variables
1749 This variable holds a list of variables that should not be given local
1750 values by files. Any value specified for one of these variables is
1754 The @samp{Eval:} ``variable'' is also a potential loophole, so Emacs
1755 normally asks for confirmation before handling it.
1757 @defopt enable-local-eval
1758 This variable controls processing of @samp{Eval:} in @samp{-*-} lines
1760 lists in files being visited. A value of @code{t} means process them
1761 unconditionally; @code{nil} means ignore them; anything else means ask
1762 the user what to do for each file. The default value is @code{maybe}.
1765 @defopt safe-local-eval-forms
1766 This variable holds a list of expressions that are safe to
1767 evaluate when found in the @samp{Eval:} ``variable'' in a file
1768 local variables list.
1771 If the expression is a function call and the function has a
1772 @code{safe-local-eval-function} property, the property value
1773 determines whether the expression is safe to evaluate. The property
1774 value can be a predicate to call to test the expression, a list of
1775 such predicates (it's safe if any predicate succeeds), or @code{t}
1776 (always safe provided the arguments are constant).
1778 Text properties are also potential loopholes, since their values
1779 could include functions to call. So Emacs discards all text
1780 properties from string values specified for file local variables.
1782 @node Variable Aliases
1783 @section Variable Aliases
1784 @cindex variable aliases
1786 It is sometimes useful to make two variables synonyms, so that both
1787 variables always have the same value, and changing either one also
1788 changes the other. Whenever you change the name of a
1789 variable---either because you realize its old name was not well
1790 chosen, or because its meaning has partly changed---it can be useful
1791 to keep the old name as an @emph{alias} of the new one for
1792 compatibility. You can do this with @code{defvaralias}.
1794 @defun defvaralias new-alias base-variable &optional docstring
1795 This function defines the symbol @var{new-alias} as a variable alias
1796 for symbol @var{base-variable}. This means that retrieving the value
1797 of @var{new-alias} returns the value of @var{base-variable}, and
1798 changing the value of @var{new-alias} changes the value of
1799 @var{base-variable}. The two aliased variable names always share the
1800 same value and the same bindings.
1802 If the @var{docstring} argument is non-@code{nil}, it specifies the
1803 documentation for @var{new-alias}; otherwise, the alias gets the same
1804 documentation as @var{base-variable} has, if any, unless
1805 @var{base-variable} is itself an alias, in which case @var{new-alias} gets
1806 the documentation of the variable at the end of the chain of aliases.
1808 This function returns @var{base-variable}.
1811 Variable aliases are convenient for replacing an old name for a
1812 variable with a new name. @code{make-obsolete-variable} declares that
1813 the old name is obsolete and therefore that it may be removed at some
1814 stage in the future.
1816 @defun make-obsolete-variable obsolete-name current-name &optional when
1817 This function makes the byte-compiler warn that the variable
1818 @var{obsolete-name} is obsolete. If @var{current-name} is a symbol, it is
1819 the variable's new name; then the warning message says to use
1820 @var{current-name} instead of @var{obsolete-name}. If @var{current-name}
1821 is a string, this is the message and there is no replacement variable.
1823 If provided, @var{when} should be a string indicating when the
1824 variable was first made obsolete---for example, a date or a release
1828 You can make two variables synonyms and declare one obsolete at the
1829 same time using the macro @code{define-obsolete-variable-alias}.
1831 @defmac define-obsolete-variable-alias obsolete-name current-name &optional when docstring
1832 This macro marks the variable @var{obsolete-name} as obsolete and also
1833 makes it an alias for the variable @var{current-name}. It is
1834 equivalent to the following:
1837 (defvaralias @var{obsolete-name} @var{current-name} @var{docstring})
1838 (make-obsolete-variable @var{obsolete-name} @var{current-name} @var{when})
1842 @defun indirect-variable variable
1843 This function returns the variable at the end of the chain of aliases
1844 of @var{variable}. If @var{variable} is not a symbol, or if @var{variable} is
1845 not defined as an alias, the function returns @var{variable}.
1847 This function signals a @code{cyclic-variable-indirection} error if
1848 there is a loop in the chain of symbols.
1852 (defvaralias 'foo 'bar)
1853 (indirect-variable 'foo)
1855 (indirect-variable 'bar)
1871 @node Variables with Restricted Values
1872 @section Variables with Restricted Values
1874 Ordinary Lisp variables can be assigned any value that is a valid
1875 Lisp object. However, certain Lisp variables are not defined in Lisp,
1876 but in C. Most of these variables are defined in the C code using
1877 @code{DEFVAR_LISP}. Like variables defined in Lisp, these can take on
1878 any value. However, some variables are defined using
1879 @code{DEFVAR_INT} or @code{DEFVAR_BOOL}. @xref{Defining Lisp
1880 variables in C,, Writing Emacs Primitives}, in particular the
1881 description of functions of the type @code{syms_of_@var{filename}},
1882 for a brief discussion of the C implementation.
1884 Variables of type @code{DEFVAR_BOOL} can only take on the values
1885 @code{nil} or @code{t}. Attempting to assign them any other value
1886 will set them to @code{t}:
1889 (let ((display-hourglass 5))
1894 @defvar byte-boolean-vars
1895 This variable holds a list of all variables of type @code{DEFVAR_BOOL}.
1898 Variables of type @code{DEFVAR_INT} can only take on integer values.
1899 Attempting to assign them any other value will result in an error:
1902 (setq window-min-height 5.0)
1903 @error{} Wrong type argument: integerp, 5.0
1907 arch-tag: 5ff62c44-2b51-47bb-99d4-fea5aeec5d3e