2 @c This is part of the GNU Emacs Lisp Reference Manual.
3 @c Copyright (C) 1990-1995, 1998-2011 Free Software Foundation, Inc.
4 @c See the file elisp.texi for copying conditions.
5 @setfilename ../../info/variables
6 @node Variables, Functions, Control Structures, Top
10 A @dfn{variable} is a name used in a program to stand for a value.
11 Nearly all programming languages have variables of some sort. In the
12 text of a Lisp program, variables are written using the syntax for
15 In Lisp, unlike most programming languages, programs are represented
16 primarily as Lisp objects and only secondarily as text. The Lisp
17 objects used for variables are symbols: the symbol name is the
18 variable name, and the variable's value is stored in the value cell of
19 the symbol. The use of a symbol as a variable is independent of its
20 use as a function name. @xref{Symbol Components}.
22 The textual form of a Lisp program is given by the read syntax of
23 the Lisp objects that constitute the program. Hence, a variable in a
24 textual Lisp program is written using the read syntax for the symbol
25 representing the variable.
28 * Global Variables:: Variable values that exist permanently, everywhere.
29 * Constant Variables:: Certain "variables" have values that never change.
30 * Local Variables:: Variable values that exist only temporarily.
31 * Void Variables:: Symbols that lack values.
32 * Defining Variables:: A definition says a symbol is used as a variable.
33 * Tips for Defining:: Things you should think about when you
35 * Accessing Variables:: Examining values of variables whose names
36 are known only at run time.
37 * Setting Variables:: Storing new values in variables.
38 * Variable Scoping:: How Lisp chooses among local and global values.
39 * Buffer-Local Variables:: Variable values in effect only in one buffer.
40 * File Local Variables:: Handling local variable lists in files.
41 * Directory Local Variables:: Local variables common to all files in a directory.
42 * Variable Aliases:: Variables that are aliases for other variables.
43 * Variables with Restricted Values:: Non-constant variables whose value can
44 @emph{not} be an arbitrary Lisp object.
47 @node Global Variables
48 @section Global Variables
49 @cindex global variable
51 The simplest way to use a variable is @dfn{globally}. This means that
52 the variable has just one value at a time, and this value is in effect
53 (at least for the moment) throughout the Lisp system. The value remains
54 in effect until you specify a new one. When a new value replaces the
55 old one, no trace of the old value remains in the variable.
57 You specify a value for a symbol with @code{setq}. For example,
64 gives the variable @code{x} the value @code{(a b)}. Note that
65 @code{setq} is a special form (@pxref{Special Forms}); it does not
66 evaluate its first argument, the name of the variable, but it does
67 evaluate the second argument, the new value.
69 Once the variable has a value, you can refer to it by using the
70 symbol itself as an expression. Thus,
79 assuming the @code{setq} form shown above has already been executed.
81 If you do set the same variable again, the new value replaces the old
99 @node Constant Variables
100 @section Variables that Never Change
101 @cindex @code{setting-constant} error
102 @cindex keyword symbol
103 @cindex variable with constant value
104 @cindex constant variables
105 @cindex symbol that evaluates to itself
106 @cindex symbol with constant value
108 In Emacs Lisp, certain symbols normally evaluate to themselves. These
109 include @code{nil} and @code{t}, as well as any symbol whose name starts
110 with @samp{:} (these are called @dfn{keywords}). These symbols cannot
111 be rebound, nor can their values be changed. Any attempt to set or bind
112 @code{nil} or @code{t} signals a @code{setting-constant} error. The
113 same is true for a keyword (a symbol whose name starts with @samp{:}),
114 if it is interned in the standard obarray, except that setting such a
115 symbol to itself is not an error.
124 @error{} Attempt to set constant symbol: nil
128 @defun keywordp object
129 function returns @code{t} if @var{object} is a symbol whose name
130 starts with @samp{:}, interned in the standard obarray, and returns
131 @code{nil} otherwise.
134 These constants are fundamentally different from the ``constants''
135 defined using the @code{defconst} special form (@pxref{Defining
136 Variables}). A @code{defconst} form serves to inform human readers
137 that you do not intend to change the value of a variable, but Emacs
138 does not raise an error if you actually change it.
140 @node Local Variables
141 @section Local Variables
142 @cindex binding local variables
143 @cindex local variables
144 @cindex local binding
145 @cindex global binding
147 Global variables have values that last until explicitly superseded
148 with new values. Sometimes it is useful to create variable values that
149 exist temporarily---only until a certain part of the program finishes.
150 These values are called @dfn{local}, and the variables so used are
151 called @dfn{local variables}.
153 For example, when a function is called, its argument variables receive
154 new local values that last until the function exits. The @code{let}
155 special form explicitly establishes new local values for specified
156 variables; these last until exit from the @code{let} form.
158 @cindex shadowing of variables
159 Establishing a local value saves away the variable's previous value
160 (or lack of one). We say that the previous value is @dfn{shadowed}
161 and @dfn{not visible}. Both global and local values may be shadowed
162 (@pxref{Scope}). After the life span of the local value is over, the
163 previous value (or lack of one) is restored.
165 If you set a variable (such as with @code{setq}) while it is local,
166 this replaces the local value; it does not alter the global value, or
167 previous local values, that are shadowed. To model this behavior, we
168 speak of a @dfn{local binding} of the variable as well as a local value.
170 The local binding is a conceptual place that holds a local value.
171 Entering a function, or a special form such as @code{let}, creates the
172 local binding; exiting the function or the @code{let} removes the
173 local binding. While the local binding lasts, the variable's value is
174 stored within it. Using @code{setq} or @code{set} while there is a
175 local binding stores a different value into the local binding; it does
176 not create a new binding.
178 We also speak of the @dfn{global binding}, which is where
179 (conceptually) the global value is kept.
181 @cindex current binding
182 A variable can have more than one local binding at a time (for
183 example, if there are nested @code{let} forms that bind it). In such a
184 case, the most recently created local binding that still exists is the
185 @dfn{current binding} of the variable. (This rule is called
186 @dfn{dynamic scoping}; see @ref{Variable Scoping}.) If there are no
187 local bindings, the variable's global binding is its current binding.
188 We sometimes call the current binding the @dfn{most-local existing
189 binding}, for emphasis. Ordinary evaluation of a symbol always returns
190 the value of its current binding.
192 The special forms @code{let} and @code{let*} exist to create
195 @defspec let (bindings@dots{}) forms@dots{}
196 This special form binds variables according to @var{bindings} and then
197 evaluates all of the @var{forms} in textual order. The @code{let}-form
198 returns the value of the last form in @var{forms}.
200 Each of the @var{bindings} is either @w{(i) a} symbol, in which case
201 that symbol is bound to @code{nil}; or @w{(ii) a} list of the form
202 @code{(@var{symbol} @var{value-form})}, in which case @var{symbol} is
203 bound to the result of evaluating @var{value-form}. If @var{value-form}
204 is omitted, @code{nil} is used.
206 All of the @var{value-form}s in @var{bindings} are evaluated in the
207 order they appear and @emph{before} binding any of the symbols to them.
208 Here is an example of this: @code{z} is bound to the old value of
209 @code{y}, which is 2, not the new value of @code{y}, which is 1.
225 @defspec let* (bindings@dots{}) forms@dots{}
226 This special form is like @code{let}, but it binds each variable right
227 after computing its local value, before computing the local value for
228 the next variable. Therefore, an expression in @var{bindings} can
229 reasonably refer to the preceding symbols bound in this @code{let*}
230 form. Compare the following example with the example above for
240 (z y)) ; @r{Use the just-established value of @code{y}.}
247 Here is a complete list of the other facilities that create local
252 Function calls (@pxref{Functions}).
255 Macro calls (@pxref{Macros}).
258 @code{condition-case} (@pxref{Errors}).
261 Variables can also have buffer-local bindings (@pxref{Buffer-Local
262 Variables}); a few variables have terminal-local bindings
263 (@pxref{Multiple Terminals}). These kinds of bindings work somewhat
264 like ordinary local bindings, but they are localized depending on
265 ``where'' you are in Emacs, rather than localized in time.
267 @defopt max-specpdl-size
268 @anchor{Definition of max-specpdl-size}
269 @cindex variable limit error
270 @cindex evaluation error
271 @cindex infinite recursion
272 This variable defines the limit on the total number of local variable
273 bindings and @code{unwind-protect} cleanups (see @ref{Cleanups,,
274 Cleaning Up from Nonlocal Exits}) that are allowed before Emacs
275 signals an error (with data @code{"Variable binding depth exceeds
278 This limit, with the associated error when it is exceeded, is one way
279 that Lisp avoids infinite recursion on an ill-defined function.
280 @code{max-lisp-eval-depth} provides another limit on depth of nesting.
281 @xref{Definition of max-lisp-eval-depth,, Eval}.
283 The default value is 1000. Entry to the Lisp debugger increases the
284 value, if there is little room left, to make sure the debugger itself
289 @section When a Variable is ``Void''
290 @cindex @code{void-variable} error
291 @cindex void variable
293 If you have never given a symbol any value as a global variable, we
294 say that that symbol's global value is @dfn{void}. In other words, the
295 symbol's value cell does not have any Lisp object in it. If you try to
296 evaluate the symbol, you get a @code{void-variable} error rather than
299 Note that a value of @code{nil} is not the same as void. The symbol
300 @code{nil} is a Lisp object and can be the value of a variable just as any
301 other object can be; but it is @emph{a value}. A void variable does not
304 After you have given a variable a value, you can make it void once more
305 using @code{makunbound}.
307 @defun makunbound symbol
308 This function makes the current variable binding of @var{symbol} void.
309 Subsequent attempts to use this symbol's value as a variable will signal
310 the error @code{void-variable}, unless and until you set it again.
312 @code{makunbound} returns @var{symbol}.
316 (makunbound 'x) ; @r{Make the global value of @code{x} void.}
321 @error{} Symbol's value as variable is void: x
325 If @var{symbol} is locally bound, @code{makunbound} affects the most
326 local existing binding. This is the only way a symbol can have a void
327 local binding, since all the constructs that create local bindings
328 create them with values. In this case, the voidness lasts at most as
329 long as the binding does; when the binding is removed due to exit from
330 the construct that made it, the previous local or global binding is
331 reexposed as usual, and the variable is no longer void unless the newly
332 reexposed binding was void all along.
336 (setq x 1) ; @r{Put a value in the global binding.}
338 (let ((x 2)) ; @r{Locally bind it.}
339 (makunbound 'x) ; @r{Void the local binding.}
341 @error{} Symbol's value as variable is void: x
344 x ; @r{The global binding is unchanged.}
347 (let ((x 2)) ; @r{Locally bind it.}
348 (let ((x 3)) ; @r{And again.}
349 (makunbound 'x) ; @r{Void the innermost-local binding.}
350 x)) ; @r{And refer: it's void.}
351 @error{} Symbol's value as variable is void: x
357 (makunbound 'x)) ; @r{Void inner binding, then remove it.}
358 x) ; @r{Now outer @code{let} binding is visible.}
364 A variable that has been made void with @code{makunbound} is
365 indistinguishable from one that has never received a value and has
368 You can use the function @code{boundp} to test whether a variable is
371 @defun boundp variable
372 @code{boundp} returns @code{t} if @var{variable} (a symbol) is not void;
373 more precisely, if its current binding is not void. It returns
374 @code{nil} otherwise.
378 (boundp 'abracadabra) ; @r{Starts out void.}
382 (let ((abracadabra 5)) ; @r{Locally bind it.}
383 (boundp 'abracadabra))
387 (boundp 'abracadabra) ; @r{Still globally void.}
391 (setq abracadabra 5) ; @r{Make it globally nonvoid.}
395 (boundp 'abracadabra)
401 @node Defining Variables
402 @section Defining Global Variables
403 @cindex variable definition
405 You may announce your intention to use a symbol as a global variable
406 with a @dfn{variable definition}: a special form, either @code{defconst}
409 In Emacs Lisp, definitions serve three purposes. First, they inform
410 people who read the code that certain symbols are @emph{intended} to be
411 used a certain way (as variables). Second, they inform the Lisp system
412 of these things, supplying a value and documentation. Third, they
413 provide information to utilities such as @code{etags} and
414 @code{make-docfile}, which create data bases of the functions and
415 variables in a program.
417 The difference between @code{defconst} and @code{defvar} is primarily
418 a matter of intent, serving to inform human readers of whether the value
419 should ever change. Emacs Lisp does not restrict the ways in which a
420 variable can be used based on @code{defconst} or @code{defvar}
421 declarations. However, it does make a difference for initialization:
422 @code{defconst} unconditionally initializes the variable, while
423 @code{defvar} initializes it only if it is void.
426 One would expect user option variables to be defined with
427 @code{defconst}, since programs do not change them. Unfortunately, this
428 has bad results if the definition is in a library that is not preloaded:
429 @code{defconst} would override any prior value when the library is
430 loaded. Users would like to be able to set user options in their init
431 files, and override the default values given in the definitions. For
432 this reason, user options must be defined with @code{defvar}.
435 @defspec defvar symbol [value [doc-string]]
436 This special form defines @var{symbol} as a variable and can also
437 initialize and document it. The definition informs a person reading
438 your code that @var{symbol} is used as a variable that might be set or
439 changed. It also declares this variable as @dfn{special}, meaning that it
440 should always use dynamic scoping rules. Note that @var{symbol} is not
441 evaluated; the symbol to be defined must appear explicitly in the
444 If @var{symbol} is void and @var{value} is specified, @code{defvar}
445 evaluates it and sets @var{symbol} to the result. But if @var{symbol}
446 already has a value (i.e., it is not void), @var{value} is not even
447 evaluated, and @var{symbol}'s value remains unchanged.
448 If @var{value} is omitted, the value of @var{symbol} is not changed in any
449 case; instead, the only effect of @code{defvar} is to declare locally that this
450 variable exists elsewhere and should hence always use dynamic scoping rules.
452 If @var{symbol} has a buffer-local binding in the current buffer,
453 @code{defvar} operates on the default value, which is buffer-independent,
454 not the current (buffer-local) binding. It sets the default value if
455 the default value is void. @xref{Buffer-Local Variables}.
457 When you evaluate a top-level @code{defvar} form with @kbd{C-M-x} in
458 Emacs Lisp mode (@code{eval-defun}), a special feature of
459 @code{eval-defun} arranges to set the variable unconditionally, without
460 testing whether its value is void.
462 If the @var{doc-string} argument appears, it specifies the documentation
463 for the variable. (This opportunity to specify documentation is one of
464 the main benefits of defining the variable.) The documentation is
465 stored in the symbol's @code{variable-documentation} property. The
466 Emacs help functions (@pxref{Documentation}) look for this property.
468 If the documentation string begins with the character @samp{*}, Emacs
469 allows users to set it interactively using the @code{set-variable}
470 command. However, you should nearly always use @code{defcustom}
471 instead of @code{defvar} to define such variables, so that users can
472 use @kbd{M-x customize} and related commands to set them. In that
473 case, it is not necessary to begin the documentation string with
474 @samp{*}. @xref{Customization}.
476 Here are some examples. This form defines @code{foo} but does not
486 This example initializes the value of @code{bar} to @code{23}, and gives
487 it a documentation string:
492 "The normal weight of a bar.")
497 The following form changes the documentation string for @code{bar},
498 making it a user option, but does not change the value, since @code{bar}
499 already has a value. (The addition @code{(1+ nil)} would get an error
500 if it were evaluated, but since it is not evaluated, there is no error.)
505 "*The normal weight of a bar.")
514 Here is an equivalent expression for the @code{defvar} special form:
518 (defvar @var{symbol} @var{value} @var{doc-string})
521 (if (not (boundp '@var{symbol}))
522 (setq @var{symbol} @var{value}))
523 (if '@var{doc-string}
524 (put '@var{symbol} 'variable-documentation '@var{doc-string}))
529 The @code{defvar} form returns @var{symbol}, but it is normally used
530 at top level in a file where its value does not matter.
533 @cindex constant variables
534 @defspec defconst symbol value [doc-string]
535 This special form defines @var{symbol} as a value and initializes it.
536 It informs a person reading your code that @var{symbol} has a standard
537 global value, established here, that should not be changed by the user
538 or by other programs. Note that @var{symbol} is not evaluated; the
539 symbol to be defined must appear explicitly in the @code{defconst}.
541 @code{defconst} always evaluates @var{value}, and sets the value of
542 @var{symbol} to the result. If @var{symbol} does have a buffer-local
543 binding in the current buffer, @code{defconst} sets the default value,
544 not the buffer-local value. (But you should not be making
545 buffer-local bindings for a symbol that is defined with
548 An example of the use of @code{defconst} is Emacs' definition of
549 @code{float-pi}---the mathematical constant @math{pi}, which ought not
550 to be changed by anyone (attempts by the Indiana State Legislature
551 notwithstanding). As the second form illustrates, however,
552 @code{defconst} is only advisory.
556 (defconst float-pi 3.141592653589793 "The value of Pi.")
570 @defun user-variable-p variable
572 This function returns @code{t} if @var{variable} is a user option---a
573 variable intended to be set by the user for customization---and
574 @code{nil} otherwise. (Variables other than user options exist for the
575 internal purposes of Lisp programs, and users need not know about them.)
577 User option variables are distinguished from other variables either
578 though being declared using @code{defcustom}@footnote{They may also be
579 declared equivalently in @file{cus-start.el}.} or by the first character
580 of their @code{variable-documentation} property. If the property exists
581 and is a string, and its first character is @samp{*}, then the variable
582 is a user option. Aliases of user options are also user options.
585 @cindex @code{variable-interactive} property
587 If a user option variable has a @code{variable-interactive} property,
588 the @code{set-variable} command uses that value to control reading the
589 new value for the variable. The property's value is used as if it were
590 specified in @code{interactive} (@pxref{Using Interactive}). However,
591 this feature is largely obsoleted by @code{defcustom}
592 (@pxref{Customization}).
594 @strong{Warning:} If the @code{defconst} and @code{defvar} special
595 forms are used while the variable has a local binding (made with
596 @code{let}, or a function argument), they set the local-binding's
597 value; the top-level binding is not changed. This is not what you
598 usually want. To prevent it, use these special forms at top level in
599 a file, where normally no local binding is in effect, and make sure to
600 load the file before making a local binding for the variable.
602 @node Tips for Defining
603 @section Tips for Defining Variables Robustly
605 When you define a variable whose value is a function, or a list of
606 functions, use a name that ends in @samp{-function} or
607 @samp{-functions}, respectively.
609 There are several other variable name conventions;
610 here is a complete list:
614 The variable is a normal hook (@pxref{Hooks}).
616 @item @dots{}-function
617 The value is a function.
619 @item @dots{}-functions
620 The value is a list of functions.
623 The value is a form (an expression).
626 The value is a list of forms (expressions).
628 @item @dots{}-predicate
629 The value is a predicate---a function of one argument that returns
630 non-@code{nil} for ``good'' arguments and @code{nil} for ``bad''
634 The value is significant only as to whether it is @code{nil} or not.
635 Since such variables often end up acquiring more values over time,
636 this convention is not strongly recommended.
638 @item @dots{}-program
639 The value is a program name.
641 @item @dots{}-command
642 The value is a whole shell command.
644 @item @dots{}-switches
645 The value specifies options for a command.
648 When you define a variable, always consider whether you should mark
649 it as ``safe'' or ``risky''; see @ref{File Local Variables}.
651 When defining and initializing a variable that holds a complicated
652 value (such as a keymap with bindings in it), it's best to put the
653 entire computation of the value into the @code{defvar}, like this:
657 (let ((map (make-sparse-keymap)))
658 (define-key map "\C-c\C-a" 'my-command)
665 This method has several benefits. First, if the user quits while
666 loading the file, the variable is either still uninitialized or
667 initialized properly, never in-between. If it is still uninitialized,
668 reloading the file will initialize it properly. Second, reloading the
669 file once the variable is initialized will not alter it; that is
670 important if the user has run hooks to alter part of the contents (such
671 as, to rebind keys). Third, evaluating the @code{defvar} form with
672 @kbd{C-M-x} @emph{will} reinitialize the map completely.
674 Putting so much code in the @code{defvar} form has one disadvantage:
675 it puts the documentation string far away from the line which names the
676 variable. Here's a safe way to avoid that:
679 (defvar my-mode-map nil
682 (let ((map (make-sparse-keymap)))
683 (define-key map "\C-c\C-a" 'my-command)
685 (setq my-mode-map map)))
689 This has all the same advantages as putting the initialization inside
690 the @code{defvar}, except that you must type @kbd{C-M-x} twice, once on
691 each form, if you do want to reinitialize the variable.
693 But be careful not to write the code like this:
696 (defvar my-mode-map nil
699 (setq my-mode-map (make-sparse-keymap))
700 (define-key my-mode-map "\C-c\C-a" 'my-command)
705 This code sets the variable, then alters it, but it does so in more than
706 one step. If the user quits just after the @code{setq}, that leaves the
707 variable neither correctly initialized nor void nor @code{nil}. Once
708 that happens, reloading the file will not initialize the variable; it
709 will remain incomplete.
711 @node Accessing Variables
712 @section Accessing Variable Values
714 The usual way to reference a variable is to write the symbol which
715 names it (@pxref{Symbol Forms}). This requires you to specify the
716 variable name when you write the program. Usually that is exactly what
717 you want to do. Occasionally you need to choose at run time which
718 variable to reference; then you can use @code{symbol-value}.
720 @defun symbol-value symbol
721 This function returns the value of @var{symbol}. This is the value in
722 the innermost local binding of the symbol, or its global value if it
723 has no local bindings.
736 ;; @r{Here the symbol @code{abracadabra}}
737 ;; @r{is the symbol whose value is examined.}
738 (let ((abracadabra 'foo))
739 (symbol-value 'abracadabra))
744 ;; @r{Here, the value of @code{abracadabra},}
745 ;; @r{which is @code{foo},}
746 ;; @r{is the symbol whose value is examined.}
747 (let ((abracadabra 'foo))
748 (symbol-value abracadabra))
753 (symbol-value 'abracadabra)
758 A @code{void-variable} error is signaled if the current binding of
759 @var{symbol} is void.
762 @node Setting Variables
763 @section How to Alter a Variable Value
765 The usual way to change the value of a variable is with the special
766 form @code{setq}. When you need to compute the choice of variable at
767 run time, use the function @code{set}.
769 @defspec setq [symbol form]@dots{}
770 This special form is the most common method of changing a variable's
771 value. Each @var{symbol} is given a new value, which is the result of
772 evaluating the corresponding @var{form}. The most-local existing
773 binding of the symbol is changed.
775 @code{setq} does not evaluate @var{symbol}; it sets the symbol that you
776 write. We say that this argument is @dfn{automatically quoted}. The
777 @samp{q} in @code{setq} stands for ``quoted.''
779 The value of the @code{setq} form is the value of the last @var{form}.
786 x ; @r{@code{x} now has a global value.}
790 (setq x 6) ; @r{The local binding of @code{x} is set.}
794 x ; @r{The global value is unchanged.}
798 Note that the first @var{form} is evaluated, then the first
799 @var{symbol} is set, then the second @var{form} is evaluated, then the
800 second @var{symbol} is set, and so on:
804 (setq x 10 ; @r{Notice that @code{x} is set before}
805 y (1+ x)) ; @r{the value of @code{y} is computed.}
811 @defun set symbol value
812 This function sets @var{symbol}'s value to @var{value}, then returns
813 @var{value}. Since @code{set} is a function, the expression written for
814 @var{symbol} is evaluated to obtain the symbol to set.
816 The most-local existing binding of the variable is the binding that is
817 set; shadowed bindings are not affected.
822 @error{} Symbol's value as variable is void: one
833 (set two 2) ; @r{@code{two} evaluates to symbol @code{one}.}
837 one ; @r{So it is @code{one} that was set.}
839 (let ((one 1)) ; @r{This binding of @code{one} is set,}
840 (set 'one 3) ; @r{not the global value.}
850 If @var{symbol} is not actually a symbol, a @code{wrong-type-argument}
855 @error{} Wrong type argument: symbolp, (x y)
858 Logically speaking, @code{set} is a more fundamental primitive than
859 @code{setq}. Any use of @code{setq} can be trivially rewritten to use
860 @code{set}; @code{setq} could even be defined as a macro, given the
861 availability of @code{set}. However, @code{set} itself is rarely used;
862 beginners hardly need to know about it. It is useful only for choosing
863 at run time which variable to set. For example, the command
864 @code{set-variable}, which reads a variable name from the user and then
865 sets the variable, needs to use @code{set}.
867 @cindex CL note---@code{set} local
869 @b{Common Lisp note:} In Common Lisp, @code{set} always changes the
870 symbol's ``special'' or dynamic value, ignoring any lexical bindings.
871 In Emacs Lisp, all variables and all bindings are dynamic, so @code{set}
872 always affects the most local existing binding.
876 @node Variable Scoping
877 @section Scoping Rules for Variable Bindings
879 A given symbol @code{foo} can have several local variable bindings,
880 established at different places in the Lisp program, as well as a global
881 binding. The most recently established binding takes precedence over
886 @cindex dynamic scoping
887 @cindex lexical scoping
888 By default, local bindings in Emacs Lisp have @dfn{indefinite scope} and
889 @dfn{dynamic extent}. @dfn{Scope} refers to @emph{where} textually in
890 the source code the binding can be accessed. ``Indefinite scope'' means
891 that any part of the program can potentially access the variable
892 binding. @dfn{Extent} refers to @emph{when}, as the program is
893 executing, the binding exists. ``Dynamic extent'' means that the binding
894 lasts as long as the activation of the construct that established it.
896 The combination of dynamic extent and indefinite scope is called
897 @dfn{dynamic scoping}. By contrast, most programming languages use
898 @dfn{lexical scoping}, in which references to a local variable must be
899 located textually within the function or block that binds the variable.
900 Emacs can also support lexical scoping, upon request (@pxref{Lexical
903 @cindex CL note---special variables
905 @b{Common Lisp note:} Variables declared ``special'' in Common Lisp are
906 dynamically scoped, like all variables in Emacs Lisp.
910 * Scope:: Scope means where in the program a value is visible.
911 Comparison with other languages.
912 * Extent:: Extent means how long in time a value exists.
913 * Impl of Scope:: Two ways to implement dynamic scoping.
914 * Using Scoping:: How to use dynamic scoping carefully and avoid problems.
915 * Lexical Binding:: Use of lexical scoping.
921 Emacs Lisp uses @dfn{indefinite scope} for local variable bindings.
922 This means that any function anywhere in the program text might access a
923 given binding of a variable. Consider the following function
928 (defun binder (x) ; @r{@code{x} is bound in @code{binder}.}
929 (foo 5)) ; @r{@code{foo} is some other function.}
933 (defun user () ; @r{@code{x} is used ``free'' in @code{user}.}
938 In a lexically scoped language, the binding of @code{x} in
939 @code{binder} would never be accessible in @code{user}, because
940 @code{user} is not textually contained within the function
941 @code{binder}. However, in dynamically-scoped Emacs Lisp, @code{user}
942 may or may not refer to the binding of @code{x} established in
943 @code{binder}, depending on the circumstances:
947 If we call @code{user} directly without calling @code{binder} at all,
948 then whatever binding of @code{x} is found, it cannot come from
952 If we define @code{foo} as follows and then call @code{binder}, then the
953 binding made in @code{binder} will be seen in @code{user}:
963 However, if we define @code{foo} as follows and then call @code{binder},
964 then the binding made in @code{binder} @emph{will not} be seen in
973 Here, when @code{foo} is called by @code{binder}, it binds @code{x}.
974 (The binding in @code{foo} is said to @dfn{shadow} the one made in
975 @code{binder}.) Therefore, @code{user} will access the @code{x} bound
976 by @code{foo} instead of the one bound by @code{binder}.
979 Emacs Lisp used dynamic scoping by default because simple implementations of
980 lexical scoping are slow. In addition, every Lisp system needs to offer
981 dynamic scoping at least as an option; if lexical scoping is the norm, there
982 must be a way to specify dynamic scoping instead for a particular variable.
983 Nowadays, Emacs offers both, but the default is still to use exclusively
989 @dfn{Extent} refers to the time during program execution that a
990 variable name is valid. In Emacs Lisp, a variable is valid only while
991 the form that bound it is executing. This is called @dfn{dynamic
992 extent}. ``Local'' or ``automatic'' variables in most languages,
993 including C and Pascal, have dynamic extent.
995 One alternative to dynamic extent is @dfn{indefinite extent}. This
996 means that a variable binding can live on past the exit from the form
997 that made the binding. Common Lisp and Scheme, for example, support
998 this, but Emacs Lisp does not.
1000 To illustrate this, the function below, @code{make-add}, returns a
1001 function that purports to add @var{n} to its own argument @var{m}. This
1002 would work in Common Lisp, but it does not do the job in Emacs Lisp,
1003 because after the call to @code{make-add} exits, the variable @code{n}
1004 is no longer bound to the actual argument 2.
1008 (function (lambda (m) (+ n m)))) ; @r{Return a function.}
1010 (fset 'add2 (make-add 2)) ; @r{Define function @code{add2}}
1011 ; @r{with @code{(make-add 2)}.}
1012 @result{} (lambda (m) (+ n m))
1013 (add2 4) ; @r{Try to add 2 to 4.}
1014 @error{} Symbol's value as variable is void: n
1017 @cindex closures not available
1018 Some Lisp dialects have ``closures,'' objects that are like functions
1019 but record additional variable bindings. Emacs Lisp does not have
1023 @subsection Implementation of Dynamic Scoping
1024 @cindex deep binding
1026 A simple sample implementation (which is not how Emacs Lisp actually
1027 works) may help you understand dynamic binding. This technique is
1028 called @dfn{deep binding} and was used in early Lisp systems.
1030 Suppose there is a stack of bindings, which are variable-value pairs.
1031 At entry to a function or to a @code{let} form, we can push bindings
1032 onto the stack for the arguments or local variables created there. We
1033 can pop those bindings from the stack at exit from the binding
1036 We can find the value of a variable by searching the stack from top to
1037 bottom for a binding for that variable; the value from that binding is
1038 the value of the variable. To set the variable, we search for the
1039 current binding, then store the new value into that binding.
1041 As you can see, a function's bindings remain in effect as long as it
1042 continues execution, even during its calls to other functions. That is
1043 why we say the extent of the binding is dynamic. And any other function
1044 can refer to the bindings, if it uses the same variables while the
1045 bindings are in effect. That is why we say the scope is indefinite.
1047 @cindex shallow binding
1048 The actual implementation of variable scoping in GNU Emacs Lisp uses a
1049 technique called @dfn{shallow binding}. Each variable has a standard
1050 place in which its current value is always found---the value cell of the
1053 In shallow binding, setting the variable works by storing a value in
1054 the value cell. Creating a new binding works by pushing the old value
1055 (belonging to a previous binding) onto a stack, and storing the new
1056 local value in the value cell. Eliminating a binding works by popping
1057 the old value off the stack, into the value cell.
1059 We use shallow binding because it has the same results as deep
1060 binding, but runs faster, since there is never a need to search for a
1064 @subsection Proper Use of Dynamic Scoping
1066 Binding a variable in one function and using it in another is a
1067 powerful technique, but if used without restraint, it can make programs
1068 hard to understand. There are two clean ways to use this technique:
1072 Use or bind the variable only in a few related functions, written close
1073 together in one file. Such a variable is used for communication within
1076 You should write comments to inform other programmers that they can see
1077 all uses of the variable before them, and to advise them not to add uses
1081 Give the variable a well-defined, documented meaning, and make all
1082 appropriate functions refer to it (but not bind it or set it) wherever
1083 that meaning is relevant. For example, the variable
1084 @code{case-fold-search} is defined as ``non-@code{nil} means ignore case
1085 when searching''; various search and replace functions refer to it
1086 directly or through their subroutines, but do not bind or set it.
1088 Then you can bind the variable in other programs, knowing reliably what
1092 In either case, you should define the variable with @code{defvar}.
1093 This helps other people understand your program by telling them to look
1094 for inter-function usage. It also avoids a warning from the byte
1095 compiler. Choose the variable's name to avoid name conflicts---don't
1096 use short names like @code{x}.
1099 @node Lexical Binding
1100 @subsection Use of Lexical Scoping
1102 Emacs Lisp can be evaluated in two different modes: in dynamic binding mode or
1103 lexical binding mode. In dynamic binding mode, all local variables use dynamic
1104 scoping, whereas in lexical binding mode variables that have been declared
1105 @dfn{special} (i.e., declared with @code{defvar} or @code{defconst}) use
1106 dynamic scoping and all others use lexical scoping.
1108 @defvar lexical-binding
1109 When non-nil, evaluation of Lisp code uses lexical scoping for non-special
1110 local variables instead of dynamic scoping. If nil, dynamic scoping is used
1111 for all local variables. This variable is typically set for a whole Elisp file
1112 via file local variables (@pxref{File Local Variables}).
1115 @defun special-variable-p SYMBOL
1116 Return whether SYMBOL has been declared as a special variable, via
1117 @code{defvar} or @code{defconst}.
1120 The use of a special variable as a formal argument in a function is generally
1121 discouraged and its behavior in lexical binding mode is unspecified (it may use
1122 lexical scoping sometimes and dynamic scoping other times).
1124 Functions like @code{symbol-value}, @code{boundp}, or @code{set} only know
1125 about dynamically scoped variables, so you cannot get the value of a lexical
1126 variable via @code{symbol-value} and neither can you change it via @code{set}.
1127 Another particularity is that code in the body of a @code{defun} or
1128 @code{defmacro} cannot refer to surrounding lexical variables.
1130 Evaluation of a @code{lambda} expression in lexical binding mode will not just
1131 return that lambda expression unchanged, as in the dynamic binding case, but
1132 will instead construct a new object that remembers the current lexical
1133 environment in which that lambda expression was defined, so that the function
1134 body can later be evaluated in the proper context. Those objects are called
1135 @dfn{closures}. They are also functions, in the sense that they are accepted
1136 by @code{funcall}, and they are represented by a cons cell whose @code{car} is
1137 the symbol @code{closure}.
1140 * Converting to Lexical Binding:: How to start using lexical scoping
1143 @node Converting to Lexical Binding
1144 @subsubsection Converting a package to use lexical scoping
1146 Lexical scoping, as currently implemented, does not bring many significant
1147 benefits, unless you are a seasoned functional programmer addicted to
1148 higher-order functions. But its importance will increase in the future:
1149 lexical scoping opens up a lot more opportunities for optimization, so
1150 lexically scoped code is likely to run faster in future Emacs versions, and it
1151 is much more friendly to concurrency, which we want to add in the near future.
1153 Converting a package to lexical binding is usually pretty easy and should not
1154 break backward compatibility: just add a file-local variable setting
1155 @code{lexical-binding} to @code{t} and add declarations of the form
1156 @code{(defvar @var{VAR})} for every variable which still needs to use
1159 To find which variables need this declaration, the simplest solution is to
1160 check the byte-compiler's warnings. The byte-compiler will usually find those
1161 variables either because they are used outside of a let-binding (leading to
1162 warnings about reference or assignment to ``free variable @var{VAR}'') or
1163 because they are let-bound but not used within the let-binding (leading to
1164 warnings about ``unused lexical variable @var{VAR}'').
1166 In cases where a dynamically scoped variable was bound as a function argument,
1167 you will also need to move this binding to a @code{let}. These cases are also
1168 flagged by the byte-compiler.
1170 To silence byte-compiler warnings about unused variables, just use a variable
1171 name that start with an underscore, which the byte-compiler interpret as an
1172 indication that this is a variable known not to be used.
1174 In most cases, the resulting code will then work with either setting of
1175 @code{lexical-binding}, so it can still be used with older Emacsen (which will
1176 simply ignore the @code{lexical-binding} variable setting).
1178 @node Buffer-Local Variables
1179 @section Buffer-Local Variables
1180 @cindex variable, buffer-local
1181 @cindex buffer-local variables
1183 Global and local variable bindings are found in most programming
1184 languages in one form or another. Emacs, however, also supports
1185 additional, unusual kinds of variable binding, such as
1186 @dfn{buffer-local} bindings, which apply only in one buffer. Having
1187 different values for a variable in different buffers is an important
1188 customization method. (Variables can also have bindings that are
1189 local to each terminal. @xref{Multiple Terminals}.)
1192 * Intro to Buffer-Local:: Introduction and concepts.
1193 * Creating Buffer-Local:: Creating and destroying buffer-local bindings.
1194 * Default Value:: The default value is seen in buffers
1195 that don't have their own buffer-local values.
1198 @node Intro to Buffer-Local
1199 @subsection Introduction to Buffer-Local Variables
1201 A buffer-local variable has a buffer-local binding associated with a
1202 particular buffer. The binding is in effect when that buffer is
1203 current; otherwise, it is not in effect. If you set the variable while
1204 a buffer-local binding is in effect, the new value goes in that binding,
1205 so its other bindings are unchanged. This means that the change is
1206 visible only in the buffer where you made it.
1208 The variable's ordinary binding, which is not associated with any
1209 specific buffer, is called the @dfn{default binding}. In most cases,
1210 this is the global binding.
1212 A variable can have buffer-local bindings in some buffers but not in
1213 other buffers. The default binding is shared by all the buffers that
1214 don't have their own bindings for the variable. (This includes all
1215 newly-created buffers.) If you set the variable in a buffer that does
1216 not have a buffer-local binding for it, this sets the default binding,
1217 so the new value is visible in all the buffers that see the default
1220 The most common use of buffer-local bindings is for major modes to change
1221 variables that control the behavior of commands. For example, C mode and
1222 Lisp mode both set the variable @code{paragraph-start} to specify that only
1223 blank lines separate paragraphs. They do this by making the variable
1224 buffer-local in the buffer that is being put into C mode or Lisp mode, and
1225 then setting it to the new value for that mode. @xref{Major Modes}.
1227 The usual way to make a buffer-local binding is with
1228 @code{make-local-variable}, which is what major mode commands typically
1229 use. This affects just the current buffer; all other buffers (including
1230 those yet to be created) will continue to share the default value unless
1231 they are explicitly given their own buffer-local bindings.
1233 @cindex automatically buffer-local
1234 A more powerful operation is to mark the variable as
1235 @dfn{automatically buffer-local} by calling
1236 @code{make-variable-buffer-local}. You can think of this as making the
1237 variable local in all buffers, even those yet to be created. More
1238 precisely, the effect is that setting the variable automatically makes
1239 the variable local to the current buffer if it is not already so. All
1240 buffers start out by sharing the default value of the variable as usual,
1241 but setting the variable creates a buffer-local binding for the current
1242 buffer. The new value is stored in the buffer-local binding, leaving
1243 the default binding untouched. This means that the default value cannot
1244 be changed with @code{setq} in any buffer; the only way to change it is
1245 with @code{setq-default}.
1247 @strong{Warning:} When a variable has buffer-local
1248 bindings in one or more buffers, @code{let} rebinds the binding that's
1249 currently in effect. For instance, if the current buffer has a
1250 buffer-local value, @code{let} temporarily rebinds that. If no
1251 buffer-local bindings are in effect, @code{let} rebinds
1252 the default value. If inside the @code{let} you then change to a
1253 different current buffer in which a different binding is in effect,
1254 you won't see the @code{let} binding any more. And if you exit the
1255 @code{let} while still in the other buffer, you won't see the
1256 unbinding occur (though it will occur properly). Here is an example
1263 (make-local-variable 'foo)
1267 ;; foo @result{} 'temp ; @r{let binding in buffer @samp{a}}
1269 ;; foo @result{} 'g ; @r{the global value since foo is not local in @samp{b}}
1272 foo @result{} 'g ; @r{exiting restored the local value in buffer @samp{a},}
1273 ; @r{but we don't see that in buffer @samp{b}}
1276 (set-buffer "a") ; @r{verify the local value was restored}
1281 Note that references to @code{foo} in @var{body} access the
1282 buffer-local binding of buffer @samp{b}.
1284 When a file specifies local variable values, these become buffer-local
1285 values when you visit the file. @xref{File Variables,,, emacs, The
1288 A buffer-local variable cannot be made terminal-local
1289 (@pxref{Multiple Terminals}).
1291 @node Creating Buffer-Local
1292 @subsection Creating and Deleting Buffer-Local Bindings
1294 @deffn Command make-local-variable variable
1295 This function creates a buffer-local binding in the current buffer for
1296 @var{variable} (a symbol). Other buffers are not affected. The value
1297 returned is @var{variable}.
1299 The buffer-local value of @var{variable} starts out as the same value
1300 @var{variable} previously had. If @var{variable} was void, it remains
1305 ;; @r{In buffer @samp{b1}:}
1306 (setq foo 5) ; @r{Affects all buffers.}
1310 (make-local-variable 'foo) ; @r{Now it is local in @samp{b1}.}
1314 foo ; @r{That did not change}
1315 @result{} 5 ; @r{the value.}
1318 (setq foo 6) ; @r{Change the value}
1319 @result{} 6 ; @r{in @samp{b1}.}
1327 ;; @r{In buffer @samp{b2}, the value hasn't changed.}
1328 (with-current-buffer "b2"
1334 Making a variable buffer-local within a @code{let}-binding for that
1335 variable does not work reliably, unless the buffer in which you do this
1336 is not current either on entry to or exit from the @code{let}. This is
1337 because @code{let} does not distinguish between different kinds of
1338 bindings; it knows only which variable the binding was made for.
1340 If the variable is terminal-local (@pxref{Multiple Terminals}), this
1341 function signals an error. Such variables cannot have buffer-local
1344 @strong{Warning:} do not use @code{make-local-variable} for a hook
1345 variable. The hook variables are automatically made buffer-local as
1346 needed if you use the @var{local} argument to @code{add-hook} or
1350 @deffn Command make-variable-buffer-local variable
1351 This function marks @var{variable} (a symbol) automatically
1352 buffer-local, so that any subsequent attempt to set it will make it
1353 local to the current buffer at the time.
1355 A peculiar wrinkle of this feature is that binding the variable (with
1356 @code{let} or other binding constructs) does not create a buffer-local
1357 binding for it. Only setting the variable (with @code{set} or
1358 @code{setq}), while the variable does not have a @code{let}-style
1359 binding that was made in the current buffer, does so.
1361 If @var{variable} does not have a default value, then calling this
1362 command will give it a default value of @code{nil}. If @var{variable}
1363 already has a default value, that value remains unchanged.
1364 Subsequently calling @code{makunbound} on @var{variable} will result
1365 in a void buffer-local value and leave the default value unaffected.
1367 The value returned is @var{variable}.
1369 @strong{Warning:} Don't assume that you should use
1370 @code{make-variable-buffer-local} for user-option variables, simply
1371 because users @emph{might} want to customize them differently in
1372 different buffers. Users can make any variable local, when they wish
1373 to. It is better to leave the choice to them.
1375 The time to use @code{make-variable-buffer-local} is when it is crucial
1376 that no two buffers ever share the same binding. For example, when a
1377 variable is used for internal purposes in a Lisp program which depends
1378 on having separate values in separate buffers, then using
1379 @code{make-variable-buffer-local} can be the best solution.
1382 @defun local-variable-p variable &optional buffer
1383 This returns @code{t} if @var{variable} is buffer-local in buffer
1384 @var{buffer} (which defaults to the current buffer); otherwise,
1388 @defun local-variable-if-set-p variable &optional buffer
1389 This returns @code{t} if @var{variable} will become buffer-local in
1390 buffer @var{buffer} (which defaults to the current buffer) if it is
1394 @defun buffer-local-value variable buffer
1395 This function returns the buffer-local binding of @var{variable} (a
1396 symbol) in buffer @var{buffer}. If @var{variable} does not have a
1397 buffer-local binding in buffer @var{buffer}, it returns the default
1398 value (@pxref{Default Value}) of @var{variable} instead.
1401 @defun buffer-local-variables &optional buffer
1402 This function returns a list describing the buffer-local variables in
1403 buffer @var{buffer}. (If @var{buffer} is omitted, the current buffer is
1404 used.) It returns an association list (@pxref{Association Lists}) in
1405 which each element contains one buffer-local variable and its value.
1406 However, when a variable's buffer-local binding in @var{buffer} is void,
1407 then the variable appears directly in the resulting list.
1411 (make-local-variable 'foobar)
1412 (makunbound 'foobar)
1413 (make-local-variable 'bind-me)
1416 (setq lcl (buffer-local-variables))
1417 ;; @r{First, built-in variables local in all buffers:}
1418 @result{} ((mark-active . nil)
1419 (buffer-undo-list . nil)
1420 (mode-name . "Fundamental")
1423 ;; @r{Next, non-built-in buffer-local variables.}
1424 ;; @r{This one is buffer-local and void:}
1426 ;; @r{This one is buffer-local and nonvoid:}
1431 Note that storing new values into the @sc{cdr}s of cons cells in this
1432 list does @emph{not} change the buffer-local values of the variables.
1435 @deffn Command kill-local-variable variable
1436 This function deletes the buffer-local binding (if any) for
1437 @var{variable} (a symbol) in the current buffer. As a result, the
1438 default binding of @var{variable} becomes visible in this buffer. This
1439 typically results in a change in the value of @var{variable}, since the
1440 default value is usually different from the buffer-local value just
1443 If you kill the buffer-local binding of a variable that automatically
1444 becomes buffer-local when set, this makes the default value visible in
1445 the current buffer. However, if you set the variable again, that will
1446 once again create a buffer-local binding for it.
1448 @code{kill-local-variable} returns @var{variable}.
1450 This function is a command because it is sometimes useful to kill one
1451 buffer-local variable interactively, just as it is useful to create
1452 buffer-local variables interactively.
1455 @defun kill-all-local-variables
1456 This function eliminates all the buffer-local variable bindings of the
1457 current buffer except for variables marked as ``permanent'' and local
1458 hook functions that have a non-@code{nil} @code{permanent-local-hook}
1459 property (@pxref{Setting Hooks}). As a result, the buffer will see
1460 the default values of most variables.
1462 This function also resets certain other information pertaining to the
1463 buffer: it sets the local keymap to @code{nil}, the syntax table to the
1464 value of @code{(standard-syntax-table)}, the case table to
1465 @code{(standard-case-table)}, and the abbrev table to the value of
1466 @code{fundamental-mode-abbrev-table}.
1468 The very first thing this function does is run the normal hook
1469 @code{change-major-mode-hook} (see below).
1471 Every major mode command begins by calling this function, which has the
1472 effect of switching to Fundamental mode and erasing most of the effects
1473 of the previous major mode. To ensure that this does its job, the
1474 variables that major modes set should not be marked permanent.
1476 @code{kill-all-local-variables} returns @code{nil}.
1479 @defvar change-major-mode-hook
1480 The function @code{kill-all-local-variables} runs this normal hook
1481 before it does anything else. This gives major modes a way to arrange
1482 for something special to be done if the user switches to a different
1483 major mode. It is also useful for buffer-specific minor modes
1484 that should be forgotten if the user changes the major mode.
1486 For best results, make this variable buffer-local, so that it will
1487 disappear after doing its job and will not interfere with the
1488 subsequent major mode. @xref{Hooks}.
1492 @cindex permanent local variable
1493 A buffer-local variable is @dfn{permanent} if the variable name (a
1494 symbol) has a @code{permanent-local} property that is non-@code{nil}.
1495 Such variables are unaffected by @code{kill-all-local-variables}, and
1496 their local bindings are therefore not cleared by changing major modes.
1497 Permanent locals are appropriate for data pertaining to where the file
1498 came from or how to save it, rather than with how to edit the contents.
1501 @subsection The Default Value of a Buffer-Local Variable
1502 @cindex default value
1504 The global value of a variable with buffer-local bindings is also
1505 called the @dfn{default} value, because it is the value that is in
1506 effect whenever neither the current buffer nor the selected frame has
1507 its own binding for the variable.
1509 The functions @code{default-value} and @code{setq-default} access and
1510 change a variable's default value regardless of whether the current
1511 buffer has a buffer-local binding. For example, you could use
1512 @code{setq-default} to change the default setting of
1513 @code{paragraph-start} for most buffers; and this would work even when
1514 you are in a C or Lisp mode buffer that has a buffer-local value for
1518 The special forms @code{defvar} and @code{defconst} also set the
1519 default value (if they set the variable at all), rather than any
1522 @defun default-value symbol
1523 This function returns @var{symbol}'s default value. This is the value
1524 that is seen in buffers and frames that do not have their own values for
1525 this variable. If @var{symbol} is not buffer-local, this is equivalent
1526 to @code{symbol-value} (@pxref{Accessing Variables}).
1530 @defun default-boundp symbol
1531 The function @code{default-boundp} tells you whether @var{symbol}'s
1532 default value is nonvoid. If @code{(default-boundp 'foo)} returns
1533 @code{nil}, then @code{(default-value 'foo)} would get an error.
1535 @code{default-boundp} is to @code{default-value} as @code{boundp} is to
1536 @code{symbol-value}.
1539 @defspec setq-default [symbol form]@dots{}
1540 This special form gives each @var{symbol} a new default value, which is
1541 the result of evaluating the corresponding @var{form}. It does not
1542 evaluate @var{symbol}, but does evaluate @var{form}. The value of the
1543 @code{setq-default} form is the value of the last @var{form}.
1545 If a @var{symbol} is not buffer-local for the current buffer, and is not
1546 marked automatically buffer-local, @code{setq-default} has the same
1547 effect as @code{setq}. If @var{symbol} is buffer-local for the current
1548 buffer, then this changes the value that other buffers will see (as long
1549 as they don't have a buffer-local value), but not the value that the
1550 current buffer sees.
1554 ;; @r{In buffer @samp{foo}:}
1555 (make-local-variable 'buffer-local)
1556 @result{} buffer-local
1559 (setq buffer-local 'value-in-foo)
1560 @result{} value-in-foo
1563 (setq-default buffer-local 'new-default)
1564 @result{} new-default
1568 @result{} value-in-foo
1571 (default-value 'buffer-local)
1572 @result{} new-default
1576 ;; @r{In (the new) buffer @samp{bar}:}
1578 @result{} new-default
1581 (default-value 'buffer-local)
1582 @result{} new-default
1585 (setq buffer-local 'another-default)
1586 @result{} another-default
1589 (default-value 'buffer-local)
1590 @result{} another-default
1594 ;; @r{Back in buffer @samp{foo}:}
1596 @result{} value-in-foo
1597 (default-value 'buffer-local)
1598 @result{} another-default
1603 @defun set-default symbol value
1604 This function is like @code{setq-default}, except that @var{symbol} is
1605 an ordinary evaluated argument.
1609 (set-default (car '(a b c)) 23)
1619 @node File Local Variables
1620 @section File Local Variables
1621 @cindex file local variables
1623 A file can specify local variable values; Emacs uses these to create
1624 buffer-local bindings for those variables in the buffer visiting that
1625 file. @xref{File variables, , Local Variables in Files, emacs, The
1626 GNU Emacs Manual}, for basic information about file-local variables.
1627 This section describes the functions and variables that affect how
1628 file-local variables are processed.
1630 If a file-local variable could specify an arbitrary function or Lisp
1631 expression that would be called later, visiting a file could take over
1632 your Emacs. Emacs protects against this by automatically setting only
1633 those file-local variables whose specified values are known to be
1634 safe. Other file-local variables are set only if the user agrees.
1636 For additional safety, @code{read-circle} is temporarily bound to
1637 @code{nil} when Emacs reads file-local variables (@pxref{Input
1638 Functions}). This prevents the Lisp reader from recognizing circular
1639 and shared Lisp structures (@pxref{Circular Objects}).
1641 @defopt enable-local-variables
1642 This variable controls whether to process file-local variables.
1643 The possible values are:
1646 @item @code{t} (the default)
1647 Set the safe variables, and query (once) about any unsafe variables.
1649 Set only the safe variables and do not query.
1651 Set all the variables and do not query.
1653 Don't set any variables.
1655 Query (once) about all the variables.
1659 @defun hack-local-variables &optional mode-only
1660 This function parses, and binds or evaluates as appropriate, any local
1661 variables specified by the contents of the current buffer. The variable
1662 @code{enable-local-variables} has its effect here. However, this
1663 function does not look for the @samp{mode:} local variable in the
1664 @w{@samp{-*-}} line. @code{set-auto-mode} does that, also taking
1665 @code{enable-local-variables} into account (@pxref{Auto Major Mode}).
1667 This function works by walking the alist stored in
1668 @code{file-local-variables-alist} and applying each local variable in
1669 turn. It calls @code{before-hack-local-variables-hook} and
1670 @code{hack-local-variables-hook} before and after applying the
1671 variables, respectively. It only calls the before-hook if the alist
1672 is non-@code{nil}; it always calls the other hook. This
1673 function ignores a @samp{mode} element if it specifies the same major
1674 mode as the buffer already has.
1676 If the optional argument @var{mode-only} is non-@code{nil}, then all
1677 this function does is return a symbol specifying the major mode,
1678 if the @w{@samp{-*-}} line or the local variables list specifies one,
1679 and @code{nil} otherwise. It does not set the mode nor any other
1680 file-local variable.
1683 @defvar file-local-variables-alist
1684 This buffer-local variable holds the alist of file-local variable
1685 settings. Each element of the alist is of the form
1686 @w{@code{(@var{var} . @var{value})}}, where @var{var} is a symbol of
1687 the local variable and @var{value} is its value. When Emacs visits a
1688 file, it first collects all the file-local variables into this alist,
1689 and then the @code{hack-local-variables} function applies them one by
1693 @defvar before-hack-local-variables-hook
1694 Emacs calls this hook immediately before applying file-local variables
1695 stored in @code{file-local-variables-alist}.
1698 @defvar hack-local-variables-hook
1699 Emacs calls this hook immediately after it finishes applying
1700 file-local variables stored in @code{file-local-variables-alist}.
1703 @cindex safe local variable
1704 You can specify safe values for a variable with a
1705 @code{safe-local-variable} property. The property has to be a
1706 function of one argument; any value is safe if the function returns
1707 non-@code{nil} given that value. Many commonly-encountered file
1708 variables have @code{safe-local-variable} properties; these include
1709 @code{fill-column}, @code{fill-prefix}, and @code{indent-tabs-mode}.
1710 For boolean-valued variables that are safe, use @code{booleanp} as the
1711 property value. Lambda expressions should be quoted so that
1712 @code{describe-variable} can display the predicate.
1714 @defopt safe-local-variable-values
1715 This variable provides another way to mark some variable values as
1716 safe. It is a list of cons cells @code{(@var{var} . @var{val})},
1717 where @var{var} is a variable name and @var{val} is a value which is
1718 safe for that variable.
1720 When Emacs asks the user whether or not to obey a set of file-local
1721 variable specifications, the user can choose to mark them as safe.
1722 Doing so adds those variable/value pairs to
1723 @code{safe-local-variable-values}, and saves it to the user's custom
1727 @defun safe-local-variable-p sym val
1728 This function returns non-@code{nil} if it is safe to give @var{sym}
1729 the value @var{val}, based on the above criteria.
1732 @c @cindex risky local variable Duplicates risky-local-variable
1733 Some variables are considered @dfn{risky}. A variable whose name
1734 ends in any of @samp{-command}, @samp{-frame-alist}, @samp{-function},
1735 @samp{-functions}, @samp{-hook}, @samp{-hooks}, @samp{-form},
1736 @samp{-forms}, @samp{-map}, @samp{-map-alist}, @samp{-mode-alist},
1737 @samp{-program}, or @samp{-predicate} is considered risky. The
1738 variables @samp{font-lock-keywords}, @samp{font-lock-keywords}
1739 followed by a digit, and @samp{font-lock-syntactic-keywords} are also
1740 considered risky. Finally, any variable whose name has a
1741 non-@code{nil} @code{risky-local-variable} property is considered
1744 @defun risky-local-variable-p sym
1745 This function returns non-@code{nil} if @var{sym} is a risky variable,
1746 based on the above criteria.
1749 If a variable is risky, it will not be entered automatically into
1750 @code{safe-local-variable-values} as described above. Therefore,
1751 Emacs will always query before setting a risky variable, unless the
1752 user explicitly allows the setting by customizing
1753 @code{safe-local-variable-values} directly.
1755 @defvar ignored-local-variables
1756 This variable holds a list of variables that should not be given local
1757 values by files. Any value specified for one of these variables is
1761 The @samp{Eval:} ``variable'' is also a potential loophole, so Emacs
1762 normally asks for confirmation before handling it.
1764 @defopt enable-local-eval
1765 This variable controls processing of @samp{Eval:} in @samp{-*-} lines
1767 lists in files being visited. A value of @code{t} means process them
1768 unconditionally; @code{nil} means ignore them; anything else means ask
1769 the user what to do for each file. The default value is @code{maybe}.
1772 @defopt safe-local-eval-forms
1773 This variable holds a list of expressions that are safe to
1774 evaluate when found in the @samp{Eval:} ``variable'' in a file
1775 local variables list.
1778 If the expression is a function call and the function has a
1779 @code{safe-local-eval-function} property, the property value
1780 determines whether the expression is safe to evaluate. The property
1781 value can be a predicate to call to test the expression, a list of
1782 such predicates (it's safe if any predicate succeeds), or @code{t}
1783 (always safe provided the arguments are constant).
1785 Text properties are also potential loopholes, since their values
1786 could include functions to call. So Emacs discards all text
1787 properties from string values specified for file-local variables.
1789 @node Directory Local Variables
1790 @section Directory Local Variables
1791 @cindex directory local variables
1793 A directory can specify local variable values common to all files in
1794 that directory; Emacs uses these to create buffer-local bindings for
1795 those variables in buffers visiting any file in that directory. This
1796 is useful when the files in the directory belong to some @dfn{project}
1797 and therefore share the same local variables.
1799 There are two different methods for specifying directory local
1800 variables: by putting them in a special file, or by defining a
1801 @dfn{project class} for that directory.
1803 @defvr Constant dir-locals-file
1804 This constant is the name of the file where Emacs expects to find the
1805 directory-local variables. The name of the file is
1806 @file{.dir-locals.el}@footnote{
1807 The MS-DOS version of Emacs uses @file{_dir-locals.el} instead, due to
1808 limitations of the DOS filesystems.
1809 }. A file by that name in a directory causes Emacs to apply its
1810 settings to any file in that directory or any of its subdirectories
1811 (optionally, you can exclude subdirectories; see below).
1812 If some of the subdirectories have their own @file{.dir-locals.el}
1813 files, Emacs uses the settings from the deepest file it finds starting
1814 from the file's directory and moving up the directory tree. The file
1815 specifies local variables as a specially formatted list; see
1816 @ref{Directory Variables, , Per-directory Local Variables, emacs, The
1817 GNU Emacs Manual}, for more details.
1820 @defun hack-dir-local-variables
1821 This function reads the @code{.dir-locals.el} file and stores the
1822 directory-local variables in @code{file-local-variables-alist} that is
1823 local to the buffer visiting any file in the directory, without
1824 applying them. It also stores the directory-local settings in
1825 @code{dir-locals-class-alist}, where it defines a special class for
1826 the directory in which @file{.dir-locals.el} file was found. This
1827 function works by calling @code{dir-locals-set-class-variables} and
1828 @code{dir-locals-set-directory-class}, described below.
1831 @defun dir-locals-set-class-variables class variables
1832 This function defines a set of variable settings for the named
1833 @var{class}, which is a symbol. You can later assign the class to one
1834 or more directories, and Emacs will apply those variable settings to
1835 all files in those directories. The list in @var{variables} can be of
1836 one of the two forms: @code{(@var{major-mode} . @var{alist})} or
1837 @code{(@var{directory} . @var{list})}. With the first form, if the
1838 file's buffer turns on a mode that is derived from @var{major-mode},
1839 then the all the variables in the associated @var{alist} are applied;
1840 @var{alist} should be of the form @code{(@var{name} . @var{value})}.
1841 A special value @code{nil} for @var{major-mode} means the settings are
1842 applicable to any mode. In @var{alist}, you can use a special
1843 @var{name}: @code{subdirs}. If the associated value is
1844 @code{nil}, the alist is only applied to files in the relevant
1845 directory, not to those in any subdirectories.
1847 With the second form of @var{variables}, if @var{directory} is the
1848 initial substring of the file's directory, then @var{list} is applied
1849 recursively by following the above rules; @var{list} should be of one
1850 of the two forms accepted by this function in @var{variables}.
1853 @defun dir-locals-set-directory-class directory class &optional mtime
1854 This function assigns @var{class} to all the files in @code{directory}
1855 and its subdirectories. Thereafter, all the variable settings
1856 specified for @var{class} will be applied to any visited file in
1857 @var{directory} and its children. @var{class} must have been already
1858 defined by @code{dir-locals-set-class-variables}.
1860 Emacs uses this function internally when it loads directory variables
1861 from a @code{.dir-locals.el} file. In that case, the optional
1862 argument @var{mtime} holds the file modification time (as returned by
1863 @code{file-attributes}). Emacs uses this time to check stored
1864 local variables are still valid. If you are assigning a class
1865 directly, not via a file, this argument should be @code{nil}.
1868 @defvar dir-locals-class-alist
1869 This alist holds the class symbols and the associated variable
1870 settings. It is updated by @code{dir-locals-set-class-variables}.
1873 @defvar dir-locals-directory-cache
1874 This alist holds directory names, their assigned class names, and
1875 modification times of the associated directory local variables file
1876 (if there is one). The function @code{dir-locals-set-directory-class}
1880 @node Variable Aliases
1881 @section Variable Aliases
1882 @cindex variable aliases
1884 It is sometimes useful to make two variables synonyms, so that both
1885 variables always have the same value, and changing either one also
1886 changes the other. Whenever you change the name of a
1887 variable---either because you realize its old name was not well
1888 chosen, or because its meaning has partly changed---it can be useful
1889 to keep the old name as an @emph{alias} of the new one for
1890 compatibility. You can do this with @code{defvaralias}.
1892 @defun defvaralias new-alias base-variable &optional docstring
1893 This function defines the symbol @var{new-alias} as a variable alias
1894 for symbol @var{base-variable}. This means that retrieving the value
1895 of @var{new-alias} returns the value of @var{base-variable}, and
1896 changing the value of @var{new-alias} changes the value of
1897 @var{base-variable}. The two aliased variable names always share the
1898 same value and the same bindings.
1900 If the @var{docstring} argument is non-@code{nil}, it specifies the
1901 documentation for @var{new-alias}; otherwise, the alias gets the same
1902 documentation as @var{base-variable} has, if any, unless
1903 @var{base-variable} is itself an alias, in which case @var{new-alias} gets
1904 the documentation of the variable at the end of the chain of aliases.
1906 This function returns @var{base-variable}.
1909 Variable aliases are convenient for replacing an old name for a
1910 variable with a new name. @code{make-obsolete-variable} declares that
1911 the old name is obsolete and therefore that it may be removed at some
1912 stage in the future.
1914 @defun make-obsolete-variable obsolete-name current-name &optional when
1915 This function makes the byte compiler warn that the variable
1916 @var{obsolete-name} is obsolete. If @var{current-name} is a symbol, it is
1917 the variable's new name; then the warning message says to use
1918 @var{current-name} instead of @var{obsolete-name}. If @var{current-name}
1919 is a string, this is the message and there is no replacement variable.
1921 If provided, @var{when} should be a string indicating when the
1922 variable was first made obsolete---for example, a date or a release
1926 You can make two variables synonyms and declare one obsolete at the
1927 same time using the macro @code{define-obsolete-variable-alias}.
1929 @defmac define-obsolete-variable-alias obsolete-name current-name &optional when docstring
1930 This macro marks the variable @var{obsolete-name} as obsolete and also
1931 makes it an alias for the variable @var{current-name}. It is
1932 equivalent to the following:
1935 (defvaralias @var{obsolete-name} @var{current-name} @var{docstring})
1936 (make-obsolete-variable @var{obsolete-name} @var{current-name} @var{when})
1940 @defun indirect-variable variable
1941 This function returns the variable at the end of the chain of aliases
1942 of @var{variable}. If @var{variable} is not a symbol, or if @var{variable} is
1943 not defined as an alias, the function returns @var{variable}.
1945 This function signals a @code{cyclic-variable-indirection} error if
1946 there is a loop in the chain of symbols.
1950 (defvaralias 'foo 'bar)
1951 (indirect-variable 'foo)
1953 (indirect-variable 'bar)
1969 @node Variables with Restricted Values
1970 @section Variables with Restricted Values
1972 Ordinary Lisp variables can be assigned any value that is a valid
1973 Lisp object. However, certain Lisp variables are not defined in Lisp,
1974 but in C. Most of these variables are defined in the C code using
1975 @code{DEFVAR_LISP}. Like variables defined in Lisp, these can take on
1976 any value. However, some variables are defined using
1977 @code{DEFVAR_INT} or @code{DEFVAR_BOOL}. @xref{Defining Lisp
1978 variables in C,, Writing Emacs Primitives}, in particular the
1979 description of functions of the type @code{syms_of_@var{filename}},
1980 for a brief discussion of the C implementation.
1982 Variables of type @code{DEFVAR_BOOL} can only take on the values
1983 @code{nil} or @code{t}. Attempting to assign them any other value
1984 will set them to @code{t}:
1987 (let ((display-hourglass 5))
1992 @defvar byte-boolean-vars
1993 This variable holds a list of all variables of type @code{DEFVAR_BOOL}.
1996 Variables of type @code{DEFVAR_INT} can only take on integer values.
1997 Attempting to assign them any other value will result in an error:
2000 (setq window-min-height 5.0)
2001 @error{} Wrong type argument: integerp, 5.0