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 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.
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} does not evaluate its first argument, the name of the
66 variable, but it does evaluate the second argument, the new value.
68 Once the variable has a value, you can refer to it by using the symbol
69 by itself as an expression. Thus,
78 assuming the @code{setq} form shown above has already been executed.
80 If you do set the same variable again, the new value replaces the old
98 @node Constant Variables
99 @section Variables that Never Change
102 @kindex setting-constant
103 @cindex keyword symbol
105 In Emacs Lisp, certain symbols normally evaluate to themselves. These
106 include @code{nil} and @code{t}, as well as any symbol whose name starts
107 with @samp{:} (these are called @dfn{keywords}). These symbols cannot
108 be rebound, nor can their values be changed. Any attempt to set or bind
109 @code{nil} or @code{t} signals a @code{setting-constant} error. The
110 same is true for a keyword (a symbol whose name starts with @samp{:}),
111 if it is interned in the standard obarray, except that setting such a
112 symbol to itself is not an error.
121 @error{} Attempt to set constant symbol: nil
125 @defun keywordp object
127 function returns @code{t} if @var{object} is a symbol whose name
128 starts with @samp{:}, interned in the standard obarray, and returns
129 @code{nil} otherwise.
132 @node Local Variables
133 @section Local Variables
134 @cindex binding local variables
135 @cindex local variables
136 @cindex local binding
137 @cindex global binding
139 Global variables have values that last until explicitly superseded
140 with new values. Sometimes it is useful to create variable values that
141 exist temporarily---only until a certain part of the program finishes.
142 These values are called @dfn{local}, and the variables so used are
143 called @dfn{local variables}.
145 For example, when a function is called, its argument variables receive
146 new local values that last until the function exits. The @code{let}
147 special form explicitly establishes new local values for specified
148 variables; these last until exit from the @code{let} form.
150 @cindex shadowing of variables
151 Establishing a local value saves away the previous value (or lack of
152 one) of the variable. When the life span of the local value is over,
153 the previous value is restored. In the mean time, we say that the
154 previous value is @dfn{shadowed} and @dfn{not visible}. Both global and
155 local values may be shadowed (@pxref{Scope}).
157 If you set a variable (such as with @code{setq}) while it is local,
158 this replaces the local value; it does not alter the global value, or
159 previous local values, that are shadowed. To model this behavior, we
160 speak of a @dfn{local binding} of the variable as well as a local value.
162 The local binding is a conceptual place that holds a local value.
163 Entry to a function, or a special form such as @code{let}, creates the
164 local binding; exit from the function or from the @code{let} removes the
165 local binding. As long as the local binding lasts, the variable's value
166 is stored within it. Use of @code{setq} or @code{set} while there is a
167 local binding stores a different value into the local binding; it does
168 not create a new binding.
170 We also speak of the @dfn{global binding}, which is where
171 (conceptually) the global value is kept.
173 @cindex current binding
174 A variable can have more than one local binding at a time (for
175 example, if there are nested @code{let} forms that bind it). In such a
176 case, the most recently created local binding that still exists is the
177 @dfn{current binding} of the variable. (This rule is called
178 @dfn{dynamic scoping}; see @ref{Variable Scoping}.) If there are no
179 local bindings, the variable's global binding is its current binding.
180 We sometimes call the current binding the @dfn{most-local existing
181 binding}, for emphasis. Ordinary evaluation of a symbol always returns
182 the value of its current binding.
184 The special forms @code{let} and @code{let*} exist to create
187 @defspec let (bindings@dots{}) forms@dots{}
188 This special form binds variables according to @var{bindings} and then
189 evaluates all of the @var{forms} in textual order. The @code{let}-form
190 returns the value of the last form in @var{forms}.
192 Each of the @var{bindings} is either @w{(i) a} symbol, in which case
193 that symbol is bound to @code{nil}; or @w{(ii) a} list of the form
194 @code{(@var{symbol} @var{value-form})}, in which case @var{symbol} is
195 bound to the result of evaluating @var{value-form}. If @var{value-form}
196 is omitted, @code{nil} is used.
198 All of the @var{value-form}s in @var{bindings} are evaluated in the
199 order they appear and @emph{before} binding any of the symbols to them.
200 Here is an example of this: @code{Z} is bound to the old value of
201 @code{Y}, which is 2, not the new value of @code{Y}, which is 1.
217 @defspec let* (bindings@dots{}) forms@dots{}
218 This special form is like @code{let}, but it binds each variable right
219 after computing its local value, before computing the local value for
220 the next variable. Therefore, an expression in @var{bindings} can
221 reasonably refer to the preceding symbols bound in this @code{let*}
222 form. Compare the following example with the example above for
232 (Z Y)) ; @r{Use the just-established value of @code{Y}.}
239 Here is a complete list of the other facilities that create local
244 Function calls (@pxref{Functions}).
247 Macro calls (@pxref{Macros}).
250 @code{condition-case} (@pxref{Errors}).
253 Variables can also have buffer-local bindings (@pxref{Buffer-Local
254 Variables}) and frame-local bindings (@pxref{Frame-Local Variables}); a
255 few variables have terminal-local bindings (@pxref{Multiple Displays}).
256 These kinds of bindings work somewhat like ordinary local bindings, but
257 they are localized depending on ``where'' you are in Emacs, rather than
260 @defvar max-specpdl-size
261 @cindex variable limit error
262 @cindex evaluation error
263 @cindex infinite recursion
264 This variable defines the limit on the total number of local variable
265 bindings and @code{unwind-protect} cleanups (@pxref{Nonlocal Exits})
266 that are allowed before signaling an error (with data @code{"Variable
267 binding depth exceeds max-specpdl-size"}).
269 This limit, with the associated error when it is exceeded, is one way
270 that Lisp avoids infinite recursion on an ill-defined function.
271 @code{max-lisp-eval-depth} provides another limit on depth of nesting.
274 The default value is 600. Entry to the Lisp debugger increases the
275 value, if there is little room left, to make sure the debugger itself
280 @section When a Variable is ``Void''
281 @kindex void-variable
282 @cindex void variable
284 If you have never given a symbol any value as a global variable, we
285 say that that symbol's global value is @dfn{void}. In other words, the
286 symbol's value cell does not have any Lisp object in it. If you try to
287 evaluate the symbol, you get a @code{void-variable} error rather than
290 Note that a value of @code{nil} is not the same as void. The symbol
291 @code{nil} is a Lisp object and can be the value of a variable just as any
292 other object can be; but it is @emph{a value}. A void variable does not
295 After you have given a variable a value, you can make it void once more
296 using @code{makunbound}.
298 @defun makunbound symbol
299 This function makes the current variable binding of @var{symbol} void.
300 Subsequent attempts to use this symbol's value as a variable will signal
301 the error @code{void-variable}, unless and until you set it again.
303 @code{makunbound} returns @var{symbol}.
307 (makunbound 'x) ; @r{Make the global value of @code{x} void.}
312 @error{} Symbol's value as variable is void: x
316 If @var{symbol} is locally bound, @code{makunbound} affects the most
317 local existing binding. This is the only way a symbol can have a void
318 local binding, since all the constructs that create local bindings
319 create them with values. In this case, the voidness lasts at most as
320 long as the binding does; when the binding is removed due to exit from
321 the construct that made it, the previous local or global binding is
322 reexposed as usual, and the variable is no longer void unless the newly
323 reexposed binding was void all along.
327 (setq x 1) ; @r{Put a value in the global binding.}
329 (let ((x 2)) ; @r{Locally bind it.}
330 (makunbound 'x) ; @r{Void the local binding.}
332 @error{} Symbol's value as variable is void: x
335 x ; @r{The global binding is unchanged.}
338 (let ((x 2)) ; @r{Locally bind it.}
339 (let ((x 3)) ; @r{And again.}
340 (makunbound 'x) ; @r{Void the innermost-local binding.}
341 x)) ; @r{And refer: it's void.}
342 @error{} Symbol's value as variable is void: x
348 (makunbound 'x)) ; @r{Void inner binding, then remove it.}
349 x) ; @r{Now outer @code{let} binding is visible.}
355 A variable that has been made void with @code{makunbound} is
356 indistinguishable from one that has never received a value and has
359 You can use the function @code{boundp} to test whether a variable is
362 @defun boundp variable
363 @code{boundp} returns @code{t} if @var{variable} (a symbol) is not void;
364 more precisely, if its current binding is not void. It returns
365 @code{nil} otherwise.
369 (boundp 'abracadabra) ; @r{Starts out void.}
373 (let ((abracadabra 5)) ; @r{Locally bind it.}
374 (boundp 'abracadabra))
378 (boundp 'abracadabra) ; @r{Still globally void.}
382 (setq abracadabra 5) ; @r{Make it globally nonvoid.}
386 (boundp 'abracadabra)
392 @node Defining Variables
393 @section Defining Global Variables
394 @cindex variable definition
396 You may announce your intention to use a symbol as a global variable
397 with a @dfn{variable definition}: a special form, either @code{defconst}
400 In Emacs Lisp, definitions serve three purposes. First, they inform
401 people who read the code that certain symbols are @emph{intended} to be
402 used a certain way (as variables). Second, they inform the Lisp system
403 of these things, supplying a value and documentation. Third, they
404 provide information to utilities such as @code{etags} and
405 @code{make-docfile}, which create data bases of the functions and
406 variables in a program.
408 The difference between @code{defconst} and @code{defvar} is primarily
409 a matter of intent, serving to inform human readers of whether the value
410 should ever change. Emacs Lisp does not restrict the ways in which a
411 variable can be used based on @code{defconst} or @code{defvar}
412 declarations. However, it does make a difference for initialization:
413 @code{defconst} unconditionally initializes the variable, while
414 @code{defvar} initializes it only if it is void.
417 One would expect user option variables to be defined with
418 @code{defconst}, since programs do not change them. Unfortunately, this
419 has bad results if the definition is in a library that is not preloaded:
420 @code{defconst} would override any prior value when the library is
421 loaded. Users would like to be able to set user options in their init
422 files, and override the default values given in the definitions. For
423 this reason, user options must be defined with @code{defvar}.
426 @defspec defvar symbol [value [doc-string]]
427 This special form defines @var{symbol} as a variable and can also
428 initialize and document it. The definition informs a person reading
429 your code that @var{symbol} is used as a variable that might be set or
430 changed. Note that @var{symbol} is not evaluated; the symbol to be
431 defined must appear explicitly in the @code{defvar}.
433 If @var{symbol} is void and @var{value} is specified, @code{defvar}
434 evaluates it and sets @var{symbol} to the result. But if @var{symbol}
435 already has a value (i.e., it is not void), @var{value} is not even
436 evaluated, and @var{symbol}'s value remains unchanged. If @var{value}
437 is omitted, the value of @var{symbol} is not changed in any case.
439 If @var{symbol} has a buffer-local binding in the current buffer,
440 @code{defvar} operates on the default value, which is buffer-independent,
441 not the current (buffer-local) binding. It sets the default value if
442 the default value is void. @xref{Buffer-Local Variables}.
444 When you evaluate a top-level @code{defvar} form with @kbd{C-M-x} in
445 Emacs Lisp mode (@code{eval-defun}), a special feature of
446 @code{eval-defun} arranges to set the variable unconditionally, without
447 testing whether its value is void.
449 If the @var{doc-string} argument appears, it specifies the documentation
450 for the variable. (This opportunity to specify documentation is one of
451 the main benefits of defining the variable.) The documentation is
452 stored in the symbol's @code{variable-documentation} property. The
453 Emacs help functions (@pxref{Documentation}) look for this property.
455 If the first character of @var{doc-string} is @samp{*}, it means that
456 this variable is considered a user option. This lets users set the
457 variable conveniently using the commands @code{set-variable} and
458 @code{edit-options}. However, it is better to use @code{defcustom}
459 instead of @code{defvar} for user option variables, so you can specify
460 customization information. @xref{Customization}.
462 Here are some examples. This form defines @code{foo} but does not
472 This example initializes the value of @code{bar} to @code{23}, and gives
473 it a documentation string:
478 "The normal weight of a bar.")
483 The following form changes the documentation string for @code{bar},
484 making it a user option, but does not change the value, since @code{bar}
485 already has a value. (The addition @code{(1+ nil)} would get an error
486 if it were evaluated, but since it is not evaluated, there is no error.)
491 "*The normal weight of a bar.")
500 Here is an equivalent expression for the @code{defvar} special form:
504 (defvar @var{symbol} @var{value} @var{doc-string})
507 (if (not (boundp '@var{symbol}))
508 (setq @var{symbol} @var{value}))
509 (if '@var{doc-string}
510 (put '@var{symbol} 'variable-documentation '@var{doc-string}))
515 The @code{defvar} form returns @var{symbol}, but it is normally used
516 at top level in a file where its value does not matter.
519 @defspec defconst symbol [value [doc-string]]
520 This special form defines @var{symbol} as a value and initializes it.
521 It informs a person reading your code that @var{symbol} has a standard
522 global value, established here, that should not be changed by the user
523 or by other programs. Note that @var{symbol} is not evaluated; the
524 symbol to be defined must appear explicitly in the @code{defconst}.
526 @code{defconst} always evaluates @var{value}, and sets the value of
527 @var{symbol} to the result if @var{value} is given. If @var{symbol}
528 does have a buffer-local binding in the current buffer, @code{defconst}
529 sets the default value, not the buffer-local value. (But you should not
530 be making buffer-local bindings for a symbol that is defined with
533 Here, @code{pi} is a constant that presumably ought not to be changed
534 by anyone (attempts by the Indiana State Legislature notwithstanding).
535 As the second form illustrates, however, this is only advisory.
539 (defconst pi 3.1415 "Pi to five places.")
553 @defun user-variable-p variable
555 This function returns @code{t} if @var{variable} is a user option---a
556 variable intended to be set by the user for customization---and
557 @code{nil} otherwise. (Variables other than user options exist for the
558 internal purposes of Lisp programs, and users need not know about them.)
560 User option variables are distinguished from other variables either
561 though being declared using @code{defcustom}@footnote{They may also be
562 declared equivalently in @file{cus-start.el}.} or by the first character
563 of their @code{variable-documentation} property. If the property exists
564 and is a string, and its first character is @samp{*}, then the variable
568 @kindex variable-interactive
569 If a user option variable has a @code{variable-interactive} property,
570 the @code{set-variable} command uses that value to control reading the
571 new value for the variable. The property's value is used as if it were
572 specified in @code{interactive} (@pxref{Using Interactive}). However,
573 this feature is largely obsoleted by @code{defcustom}
574 (@pxref{Customization}).
576 @strong{Warning:} If the @code{defconst} and @code{defvar} special
577 forms are used while the variable has a local binding, they set the
578 local binding's value; the global binding is not changed. This is not
579 what you usually want. To prevent it, use these special forms at top
580 level in a file, where normally no local binding is in effect, and make
581 sure to load the file before making a local binding for the variable.
583 @node Tips for Defining
584 @section Tips for Defining Variables Robustly
586 When you define a variable whose value is a function, or a list of
587 functions, use a name that ends in @samp{-function} or
588 @samp{-functions}, respectively.
590 There are several other variable name conventions;
591 here is a complete list:
595 The variable is a normal hook (@pxref{Hooks}).
597 @item @dots{}-function
598 The value is a function.
600 @item @dots{}-functions
601 The value is a list of functions.
604 The value is a form (an expression).
606 @item @dots{}-functions
607 The value is a list of forms (expressions).
609 @item @dots{}-predicate
610 The value is a predicate---a function of one argument that returns
611 non-@code{nil} for ``good'' arguments and @code{nil} for ``bad''
615 The value is significant only as to whether it is @code{nil} or not.
617 @item @dots{}-program
618 The value is a program name.
620 @item @dots{}-command
621 The value is a whole shell command.
623 @item @samp{}-switches
624 The value specifies options for a command.
627 When you define a variable, always cvonsider whether you should mark
628 it as ``risky''; see @ref{File Local Variables}.
630 When defining and initializing a variable that holds a complicated
631 value (such as a keymap with bindings in it), it's best to put the
632 entire computation of the value into the @code{defvar}, like this:
636 (let ((map (make-sparse-keymap)))
637 (define-key map "\C-c\C-a" 'my-command)
644 This method has several benefits. First, if the user quits while
645 loading the file, the variable is either still uninitialized or
646 initialized properly, never in-between. If it is still uninitialized,
647 reloading the file will initialize it properly. Second, reloading the
648 file once the variable is initialized will not alter it; that is
649 important if the user has run hooks to alter part of the contents (such
650 as, to rebind keys). Third, evaluating the @code{defvar} form with
651 @kbd{C-M-x} @emph{will} reinitialize the map completely.
653 Putting so much code in the @code{defvar} form has one disadvantage:
654 it puts the documentation string far away from the line which names the
655 variable. Here's a safe way to avoid that:
658 (defvar my-mode-map nil
661 (let ((map (make-sparse-keymap)))
662 (define-key map "\C-c\C-a" 'my-command)
664 (setq my-mode-map map)))
668 This has all the same advantages as putting the initialization inside
669 the @code{defvar}, except that you must type @kbd{C-M-x} twice, once on
670 each form, if you do want to reinitialize the variable.
672 But be careful not to write the code like this:
675 (defvar my-mode-map nil
678 (setq my-mode-map (make-sparse-keymap))
679 (define-key my-mode-map "\C-c\C-a" 'my-command)
684 This code sets the variable, then alters it, but it does so in more than
685 one step. If the user quits just after the @code{setq}, that leaves the
686 variable neither correctly initialized nor void nor @code{nil}. Once
687 that happens, reloading the file will not initialize the variable; it
688 will remain incomplete.
690 @node Accessing Variables
691 @section Accessing Variable Values
693 The usual way to reference a variable is to write the symbol which
694 names it (@pxref{Symbol Forms}). This requires you to specify the
695 variable name when you write the program. Usually that is exactly what
696 you want to do. Occasionally you need to choose at run time which
697 variable to reference; then you can use @code{symbol-value}.
699 @defun symbol-value symbol
700 This function returns the value of @var{symbol}. This is the value in
701 the innermost local binding of the symbol, or its global value if it
702 has no local bindings.
715 ;; @r{Here the symbol @code{abracadabra}}
716 ;; @r{is the symbol whose value is examined.}
717 (let ((abracadabra 'foo))
718 (symbol-value 'abracadabra))
723 ;; @r{Here the value of @code{abracadabra},}
724 ;; @r{which is @code{foo},}
725 ;; @r{is the symbol whose value is examined.}
726 (let ((abracadabra 'foo))
727 (symbol-value abracadabra))
732 (symbol-value 'abracadabra)
737 A @code{void-variable} error is signaled if the current binding of
738 @var{symbol} is void.
741 @node Setting Variables
742 @section How to Alter a Variable Value
744 The usual way to change the value of a variable is with the special
745 form @code{setq}. When you need to compute the choice of variable at
746 run time, use the function @code{set}.
748 @defspec setq [symbol form]@dots{}
749 This special form is the most common method of changing a variable's
750 value. Each @var{symbol} is given a new value, which is the result of
751 evaluating the corresponding @var{form}. The most-local existing
752 binding of the symbol is changed.
754 @code{setq} does not evaluate @var{symbol}; it sets the symbol that you
755 write. We say that this argument is @dfn{automatically quoted}. The
756 @samp{q} in @code{setq} stands for ``quoted.''
758 The value of the @code{setq} form is the value of the last @var{form}.
765 x ; @r{@code{x} now has a global value.}
769 (setq x 6) ; @r{The local binding of @code{x} is set.}
773 x ; @r{The global value is unchanged.}
777 Note that the first @var{form} is evaluated, then the first
778 @var{symbol} is set, then the second @var{form} is evaluated, then the
779 second @var{symbol} is set, and so on:
783 (setq x 10 ; @r{Notice that @code{x} is set before}
784 y (1+ x)) ; @r{the value of @code{y} is computed.}
790 @defun set symbol value
791 This function sets @var{symbol}'s value to @var{value}, then returns
792 @var{value}. Since @code{set} is a function, the expression written for
793 @var{symbol} is evaluated to obtain the symbol to set.
795 The most-local existing binding of the variable is the binding that is
796 set; shadowed bindings are not affected.
801 @error{} Symbol's value as variable is void: one
812 (set two 2) ; @r{@code{two} evaluates to symbol @code{one}.}
816 one ; @r{So it is @code{one} that was set.}
818 (let ((one 1)) ; @r{This binding of @code{one} is set,}
819 (set 'one 3) ; @r{not the global value.}
829 If @var{symbol} is not actually a symbol, a @code{wrong-type-argument}
834 @error{} Wrong type argument: symbolp, (x y)
837 Logically speaking, @code{set} is a more fundamental primitive than
838 @code{setq}. Any use of @code{setq} can be trivially rewritten to use
839 @code{set}; @code{setq} could even be defined as a macro, given the
840 availability of @code{set}. However, @code{set} itself is rarely used;
841 beginners hardly need to know about it. It is useful only for choosing
842 at run time which variable to set. For example, the command
843 @code{set-variable}, which reads a variable name from the user and then
844 sets the variable, needs to use @code{set}.
846 @cindex CL note---@code{set} local
848 @b{Common Lisp note:} In Common Lisp, @code{set} always changes the
849 symbol's ``special'' or dynamic value, ignoring any lexical bindings.
850 In Emacs Lisp, all variables and all bindings are dynamic, so @code{set}
851 always affects the most local existing binding.
855 One other function for setting a variable is designed to add
856 an element to a list if it is not already present in the list.
858 @defun add-to-list symbol element
859 This function sets the variable @var{symbol} by consing @var{element}
860 onto the old value, if @var{element} is not already a member of that
861 value. It returns the resulting list, whether updated or not. The
862 value of @var{symbol} had better be a list already before the call.
864 The argument @var{symbol} is not implicitly quoted; @code{add-to-list}
865 is an ordinary function, like @code{set} and unlike @code{setq}. Quote
866 the argument yourself if that is what you want.
869 Here's a scenario showing how to use @code{add-to-list}:
875 (add-to-list 'foo 'c) ;; @r{Add @code{c}.}
878 (add-to-list 'foo 'b) ;; @r{No effect.}
881 foo ;; @r{@code{foo} was changed.}
885 An equivalent expression for @code{(add-to-list '@var{var}
886 @var{value})} is this:
889 (or (member @var{value} @var{var})
890 (setq @var{var} (cons @var{value} @var{var})))
893 @node Variable Scoping
894 @section Scoping Rules for Variable Bindings
896 A given symbol @code{foo} can have several local variable bindings,
897 established at different places in the Lisp program, as well as a global
898 binding. The most recently established binding takes precedence over
903 @cindex dynamic scoping
904 Local bindings in Emacs Lisp have @dfn{indefinite scope} and
905 @dfn{dynamic extent}. @dfn{Scope} refers to @emph{where} textually in
906 the source code the binding can be accessed. ``Indefinite scope'' means
907 that any part of the program can potentially access the variable
908 binding. @dfn{Extent} refers to @emph{when}, as the program is
909 executing, the binding exists. ``Dynamic extent'' means that the binding
910 lasts as long as the activation of the construct that established it.
912 The combination of dynamic extent and indefinite scope is called
913 @dfn{dynamic scoping}. By contrast, most programming languages use
914 @dfn{lexical scoping}, in which references to a local variable must be
915 located textually within the function or block that binds the variable.
917 @cindex CL note---special variables
919 @b{Common Lisp note:} Variables declared ``special'' in Common Lisp are
920 dynamically scoped, like all variables in Emacs Lisp.
924 * Scope:: Scope means where in the program a value is visible.
925 Comparison with other languages.
926 * Extent:: Extent means how long in time a value exists.
927 * Impl of Scope:: Two ways to implement dynamic scoping.
928 * Using Scoping:: How to use dynamic scoping carefully and avoid problems.
934 Emacs Lisp uses @dfn{indefinite scope} for local variable bindings.
935 This means that any function anywhere in the program text might access a
936 given binding of a variable. Consider the following function
941 (defun binder (x) ; @r{@code{x} is bound in @code{binder}.}
942 (foo 5)) ; @r{@code{foo} is some other function.}
946 (defun user () ; @r{@code{x} is used ``free'' in @code{user}.}
951 In a lexically scoped language, the binding of @code{x} in
952 @code{binder} would never be accessible in @code{user}, because
953 @code{user} is not textually contained within the function
954 @code{binder}. However, in dynamically-scoped Emacs Lisp, @code{user}
955 may or may not refer to the binding of @code{x} established in
956 @code{binder}, depending on the circumstances:
960 If we call @code{user} directly without calling @code{binder} at all,
961 then whatever binding of @code{x} is found, it cannot come from
965 If we define @code{foo} as follows and then call @code{binder}, then the
966 binding made in @code{binder} will be seen in @code{user}:
976 However, if we define @code{foo} as follows and then call @code{binder},
977 then the binding made in @code{binder} @emph{will not} be seen in
986 Here, when @code{foo} is called by @code{binder}, it binds @code{x}.
987 (The binding in @code{foo} is said to @dfn{shadow} the one made in
988 @code{binder}.) Therefore, @code{user} will access the @code{x} bound
989 by @code{foo} instead of the one bound by @code{binder}.
992 Emacs Lisp uses dynamic scoping because simple implementations of
993 lexical scoping are slow. In addition, every Lisp system needs to offer
994 dynamic scoping at least as an option; if lexical scoping is the norm,
995 there must be a way to specify dynamic scoping instead for a particular
996 variable. It might not be a bad thing for Emacs to offer both, but
997 implementing it with dynamic scoping only was much easier.
1002 @dfn{Extent} refers to the time during program execution that a
1003 variable name is valid. In Emacs Lisp, a variable is valid only while
1004 the form that bound it is executing. This is called @dfn{dynamic
1005 extent}. ``Local'' or ``automatic'' variables in most languages,
1006 including C and Pascal, have dynamic extent.
1008 One alternative to dynamic extent is @dfn{indefinite extent}. This
1009 means that a variable binding can live on past the exit from the form
1010 that made the binding. Common Lisp and Scheme, for example, support
1011 this, but Emacs Lisp does not.
1013 To illustrate this, the function below, @code{make-add}, returns a
1014 function that purports to add @var{n} to its own argument @var{m}. This
1015 would work in Common Lisp, but it does not do the job in Emacs Lisp,
1016 because after the call to @code{make-add} exits, the variable @code{n}
1017 is no longer bound to the actual argument 2.
1021 (function (lambda (m) (+ n m)))) ; @r{Return a function.}
1023 (fset 'add2 (make-add 2)) ; @r{Define function @code{add2}}
1024 ; @r{with @code{(make-add 2)}.}
1025 @result{} (lambda (m) (+ n m))
1026 (add2 4) ; @r{Try to add 2 to 4.}
1027 @error{} Symbol's value as variable is void: n
1030 @cindex closures not available
1031 Some Lisp dialects have ``closures'', objects that are like functions
1032 but record additional variable bindings. Emacs Lisp does not have
1036 @subsection Implementation of Dynamic Scoping
1037 @cindex deep binding
1039 A simple sample implementation (which is not how Emacs Lisp actually
1040 works) may help you understand dynamic binding. This technique is
1041 called @dfn{deep binding} and was used in early Lisp systems.
1043 Suppose there is a stack of bindings, which are variable-value pairs.
1044 At entry to a function or to a @code{let} form, we can push bindings
1045 onto the stack for the arguments or local variables created there. We
1046 can pop those bindings from the stack at exit from the binding
1049 We can find the value of a variable by searching the stack from top to
1050 bottom for a binding for that variable; the value from that binding is
1051 the value of the variable. To set the variable, we search for the
1052 current binding, then store the new value into that binding.
1054 As you can see, a function's bindings remain in effect as long as it
1055 continues execution, even during its calls to other functions. That is
1056 why we say the extent of the binding is dynamic. And any other function
1057 can refer to the bindings, if it uses the same variables while the
1058 bindings are in effect. That is why we say the scope is indefinite.
1060 @cindex shallow binding
1061 The actual implementation of variable scoping in GNU Emacs Lisp uses a
1062 technique called @dfn{shallow binding}. Each variable has a standard
1063 place in which its current value is always found---the value cell of the
1066 In shallow binding, setting the variable works by storing a value in
1067 the value cell. Creating a new binding works by pushing the old value
1068 (belonging to a previous binding) onto a stack, and storing the new
1069 local value in the value cell. Eliminating a binding works by popping
1070 the old value off the stack, into the value cell.
1072 We use shallow binding because it has the same results as deep
1073 binding, but runs faster, since there is never a need to search for a
1077 @subsection Proper Use of Dynamic Scoping
1079 Binding a variable in one function and using it in another is a
1080 powerful technique, but if used without restraint, it can make programs
1081 hard to understand. There are two clean ways to use this technique:
1085 Use or bind the variable only in a few related functions, written close
1086 together in one file. Such a variable is used for communication within
1089 You should write comments to inform other programmers that they can see
1090 all uses of the variable before them, and to advise them not to add uses
1094 Give the variable a well-defined, documented meaning, and make all
1095 appropriate functions refer to it (but not bind it or set it) wherever
1096 that meaning is relevant. For example, the variable
1097 @code{case-fold-search} is defined as ``non-@code{nil} means ignore case
1098 when searching''; various search and replace functions refer to it
1099 directly or through their subroutines, but do not bind or set it.
1101 Then you can bind the variable in other programs, knowing reliably what
1105 In either case, you should define the variable with @code{defvar}.
1106 This helps other people understand your program by telling them to look
1107 for inter-function usage. It also avoids a warning from the byte
1108 compiler. Choose the variable's name to avoid name conflicts---don't
1109 use short names like @code{x}.
1111 @node Buffer-Local Variables
1112 @section Buffer-Local Variables
1113 @cindex variables, buffer-local
1114 @cindex buffer-local variables
1116 Global and local variable bindings are found in most programming
1117 languages in one form or another. Emacs, however, also supports additional,
1118 unusual kinds of variable binding: @dfn{buffer-local} bindings, which
1119 apply only in one buffer, and @dfn{frame-local} bindings, which apply only in
1120 one frame. Having different values for a variable in different buffers
1121 and/or frames is an important customization method.
1123 This section describes buffer-local bindings; for frame-local
1124 bindings, see the following section, @ref{Frame-Local Variables}. (A few
1125 variables have bindings that are local to each terminal; see
1126 @ref{Multiple Displays}.)
1129 * Intro to Buffer-Local:: Introduction and concepts.
1130 * Creating Buffer-Local:: Creating and destroying buffer-local bindings.
1131 * Default Value:: The default value is seen in buffers
1132 that don't have their own buffer-local values.
1135 @node Intro to Buffer-Local
1136 @subsection Introduction to Buffer-Local Variables
1138 A buffer-local variable has a buffer-local binding associated with a
1139 particular buffer. The binding is in effect when that buffer is
1140 current; otherwise, it is not in effect. If you set the variable while
1141 a buffer-local binding is in effect, the new value goes in that binding,
1142 so its other bindings are unchanged. This means that the change is
1143 visible only in the buffer where you made it.
1145 The variable's ordinary binding, which is not associated with any
1146 specific buffer, is called the @dfn{default binding}. In most cases,
1147 this is the global binding.
1149 A variable can have buffer-local bindings in some buffers but not in
1150 other buffers. The default binding is shared by all the buffers that
1151 don't have their own bindings for the variable. (This includes all
1152 newly-created buffers.) If you set the variable in a buffer that does
1153 not have a buffer-local binding for it, this sets the default binding
1154 (assuming there are no frame-local bindings to complicate the matter),
1155 so the new value is visible in all the buffers that see the default
1158 The most common use of buffer-local bindings is for major modes to change
1159 variables that control the behavior of commands. For example, C mode and
1160 Lisp mode both set the variable @code{paragraph-start} to specify that only
1161 blank lines separate paragraphs. They do this by making the variable
1162 buffer-local in the buffer that is being put into C mode or Lisp mode, and
1163 then setting it to the new value for that mode. @xref{Major Modes}.
1165 The usual way to make a buffer-local binding is with
1166 @code{make-local-variable}, which is what major mode commands typically
1167 use. This affects just the current buffer; all other buffers (including
1168 those yet to be created) will continue to share the default value unless
1169 they are explicitly given their own buffer-local bindings.
1171 @cindex automatically buffer-local
1172 A more powerful operation is to mark the variable as
1173 @dfn{automatically buffer-local} by calling
1174 @code{make-variable-buffer-local}. You can think of this as making the
1175 variable local in all buffers, even those yet to be created. More
1176 precisely, the effect is that setting the variable automatically makes
1177 the variable local to the current buffer if it is not already so. All
1178 buffers start out by sharing the default value of the variable as usual,
1179 but setting the variable creates a buffer-local binding for the current
1180 buffer. The new value is stored in the buffer-local binding, leaving
1181 the default binding untouched. This means that the default value cannot
1182 be changed with @code{setq} in any buffer; the only way to change it is
1183 with @code{setq-default}.
1185 @strong{Warning:} When a variable has buffer-local values in one or
1186 more buffers, you can get Emacs very confused by binding the variable
1187 with @code{let}, changing to a different current buffer in which a
1188 different binding is in effect, and then exiting the @code{let}. This
1189 can scramble the values of the buffer-local and default bindings.
1191 To preserve your sanity, avoid using a variable in that way. If you
1192 use @code{save-excursion} around each piece of code that changes to a
1193 different current buffer, you will not have this problem
1194 (@pxref{Excursions}). Here is an example of what to avoid:
1200 (make-local-variable 'foo)
1207 foo @result{} 'a ; @r{The old buffer-local value from buffer @samp{a}}
1208 ; @r{is now the default value.}
1212 foo @result{} 'temp ; @r{The local @code{let} value that should be gone}
1213 ; @r{is now the buffer-local value in buffer @samp{a}.}
1218 But @code{save-excursion} as shown here avoids the problem:
1229 Note that references to @code{foo} in @var{body} access the
1230 buffer-local binding of buffer @samp{b}.
1232 When a file specifies local variable values, these become buffer-local
1233 values when you visit the file. @xref{File Variables,,, emacs, The
1236 @node Creating Buffer-Local
1237 @subsection Creating and Deleting Buffer-Local Bindings
1239 @deffn Command make-local-variable variable
1240 This function creates a buffer-local binding in the current buffer for
1241 @var{variable} (a symbol). Other buffers are not affected. The value
1242 returned is @var{variable}.
1245 The buffer-local value of @var{variable} starts out as the same value
1246 @var{variable} previously had. If @var{variable} was void, it remains
1251 ;; @r{In buffer @samp{b1}:}
1252 (setq foo 5) ; @r{Affects all buffers.}
1256 (make-local-variable 'foo) ; @r{Now it is local in @samp{b1}.}
1260 foo ; @r{That did not change}
1261 @result{} 5 ; @r{the value.}
1264 (setq foo 6) ; @r{Change the value}
1265 @result{} 6 ; @r{in @samp{b1}.}
1273 ;; @r{In buffer @samp{b2}, the value hasn't changed.}
1281 Making a variable buffer-local within a @code{let}-binding for that
1282 variable does not work reliably, unless the buffer in which you do this
1283 is not current either on entry to or exit from the @code{let}. This is
1284 because @code{let} does not distinguish between different kinds of
1285 bindings; it knows only which variable the binding was made for.
1287 If the variable is terminal-local, this function signals an error. Such
1288 variables cannot have buffer-local bindings as well. @xref{Multiple
1291 @strong{Note:} Do not use @code{make-local-variable} for a hook
1292 variable. Instead, use @code{make-local-hook}. @xref{Hooks}.
1295 @deffn Command make-variable-buffer-local variable
1296 This function marks @var{variable} (a symbol) automatically
1297 buffer-local, so that any subsequent attempt to set it will make it
1298 local to the current buffer at the time.
1300 A peculiar wrinkle of this feature is that binding the variable (with
1301 @code{let} or other binding constructs) does not create a buffer-local
1302 binding for it. Only setting the variable (with @code{set} or
1303 @code{setq}) does so.
1305 The value returned is @var{variable}.
1307 @strong{Warning:} Don't assume that you should use
1308 @code{make-variable-buffer-local} for user-option variables, simply
1309 because users @emph{might} want to customize them differently in
1310 different buffers. Users can make any variable local, when they wish
1311 to. It is better to leave the choice to them.
1313 The time to use @code{make-variable-buffer-local} is when it is crucial
1314 that no two buffers ever share the same binding. For example, when a
1315 variable is used for internal purposes in a Lisp program which depends
1316 on having separate values in separate buffers, then using
1317 @code{make-variable-buffer-local} can be the best solution.
1320 @defun local-variable-p variable &optional buffer
1321 This returns @code{t} if @var{variable} is buffer-local in buffer
1322 @var{buffer} (which defaults to the current buffer); otherwise,
1326 @defun buffer-local-variables &optional buffer
1327 This function returns a list describing the buffer-local variables in
1328 buffer @var{buffer}. (If @var{buffer} is omitted, the current buffer is
1329 used.) It returns an association list (@pxref{Association Lists}) in
1330 which each element contains one buffer-local variable and its value.
1331 However, when a variable's buffer-local binding in @var{buffer} is void,
1332 then the variable appears directly in the resulting list.
1336 (make-local-variable 'foobar)
1337 (makunbound 'foobar)
1338 (make-local-variable 'bind-me)
1341 (setq lcl (buffer-local-variables))
1342 ;; @r{First, built-in variables local in all buffers:}
1343 @result{} ((mark-active . nil)
1344 (buffer-undo-list . nil)
1345 (mode-name . "Fundamental")
1348 ;; @r{Next, non-built-in buffer-local variables.}
1349 ;; @r{This one is buffer-local and void:}
1351 ;; @r{This one is buffer-local and nonvoid:}
1356 Note that storing new values into the @sc{cdr}s of cons cells in this
1357 list does @emph{not} change the buffer-local values of the variables.
1360 @deffn Command kill-local-variable variable
1361 This function deletes the buffer-local binding (if any) for
1362 @var{variable} (a symbol) in the current buffer. As a result, the
1363 default binding of @var{variable} becomes visible in this buffer. This
1364 typically results in a change in the value of @var{variable}, since the
1365 default value is usually different from the buffer-local value just
1368 If you kill the buffer-local binding of a variable that automatically
1369 becomes buffer-local when set, this makes the default value visible in
1370 the current buffer. However, if you set the variable again, that will
1371 once again create a buffer-local binding for it.
1373 @code{kill-local-variable} returns @var{variable}.
1375 This function is a command because it is sometimes useful to kill one
1376 buffer-local variable interactively, just as it is useful to create
1377 buffer-local variables interactively.
1380 @defun kill-all-local-variables
1381 This function eliminates all the buffer-local variable bindings of the
1382 current buffer except for variables marked as ``permanent''. As a
1383 result, the buffer will see the default values of most variables.
1385 This function also resets certain other information pertaining to the
1386 buffer: it sets the local keymap to @code{nil}, the syntax table to the
1387 value of @code{(standard-syntax-table)}, the case table to
1388 @code{(standard-case-table)}, and the abbrev table to the value of
1389 @code{fundamental-mode-abbrev-table}.
1391 The very first thing this function does is run the normal hook
1392 @code{change-major-mode-hook} (see below).
1394 Every major mode command begins by calling this function, which has the
1395 effect of switching to Fundamental mode and erasing most of the effects
1396 of the previous major mode. To ensure that this does its job, the
1397 variables that major modes set should not be marked permanent.
1399 @code{kill-all-local-variables} returns @code{nil}.
1402 @defvar change-major-mode-hook
1403 The function @code{kill-all-local-variables} runs this normal hook
1404 before it does anything else. This gives major modes a way to arrange
1405 for something special to be done if the user switches to a different
1406 major mode. For best results, make this variable buffer-local, so that
1407 it will disappear after doing its job and will not interfere with the
1408 subsequent major mode. @xref{Hooks}.
1412 @cindex permanent local variable
1413 A buffer-local variable is @dfn{permanent} if the variable name (a
1414 symbol) has a @code{permanent-local} property that is non-@code{nil}.
1415 Permanent locals are appropriate for data pertaining to where the file
1416 came from or how to save it, rather than with how to edit the contents.
1419 @subsection The Default Value of a Buffer-Local Variable
1420 @cindex default value
1422 The global value of a variable with buffer-local bindings is also
1423 called the @dfn{default} value, because it is the value that is in
1424 effect whenever neither the current buffer nor the selected frame has
1425 its own binding for the variable.
1427 The functions @code{default-value} and @code{setq-default} access and
1428 change a variable's default value regardless of whether the current
1429 buffer has a buffer-local binding. For example, you could use
1430 @code{setq-default} to change the default setting of
1431 @code{paragraph-start} for most buffers; and this would work even when
1432 you are in a C or Lisp mode buffer that has a buffer-local value for
1436 The special forms @code{defvar} and @code{defconst} also set the
1437 default value (if they set the variable at all), rather than any
1438 buffer-local or frame-local value.
1440 @defun default-value symbol
1441 This function returns @var{symbol}'s default value. This is the value
1442 that is seen in buffers and frames that do not have their own values for
1443 this variable. If @var{symbol} is not buffer-local, this is equivalent
1444 to @code{symbol-value} (@pxref{Accessing Variables}).
1448 @defun default-boundp symbol
1449 The function @code{default-boundp} tells you whether @var{symbol}'s
1450 default value is nonvoid. If @code{(default-boundp 'foo)} returns
1451 @code{nil}, then @code{(default-value 'foo)} would get an error.
1453 @code{default-boundp} is to @code{default-value} as @code{boundp} is to
1454 @code{symbol-value}.
1457 @defspec setq-default [symbol form]@dots{}
1458 This special form gives each @var{symbol} a new default value, which is
1459 the result of evaluating the corresponding @var{form}. It does not
1460 evaluate @var{symbol}, but does evaluate @var{form}. The value of the
1461 @code{setq-default} form is the value of the last @var{form}.
1463 If a @var{symbol} is not buffer-local for the current buffer, and is not
1464 marked automatically buffer-local, @code{setq-default} has the same
1465 effect as @code{setq}. If @var{symbol} is buffer-local for the current
1466 buffer, then this changes the value that other buffers will see (as long
1467 as they don't have a buffer-local value), but not the value that the
1468 current buffer sees.
1472 ;; @r{In buffer @samp{foo}:}
1473 (make-local-variable 'buffer-local)
1474 @result{} buffer-local
1477 (setq buffer-local 'value-in-foo)
1478 @result{} value-in-foo
1481 (setq-default buffer-local 'new-default)
1482 @result{} new-default
1486 @result{} value-in-foo
1489 (default-value 'buffer-local)
1490 @result{} new-default
1494 ;; @r{In (the new) buffer @samp{bar}:}
1496 @result{} new-default
1499 (default-value 'buffer-local)
1500 @result{} new-default
1503 (setq buffer-local 'another-default)
1504 @result{} another-default
1507 (default-value 'buffer-local)
1508 @result{} another-default
1512 ;; @r{Back in buffer @samp{foo}:}
1514 @result{} value-in-foo
1515 (default-value 'buffer-local)
1516 @result{} another-default
1521 @defun set-default symbol value
1522 This function is like @code{setq-default}, except that @var{symbol} is
1523 an ordinary evaluated argument.
1527 (set-default (car '(a b c)) 23)
1537 @node Frame-Local Variables
1538 @section Frame-Local Variables
1540 Just as variables can have buffer-local bindings, they can also have
1541 frame-local bindings. These bindings belong to one frame, and are in
1542 effect when that frame is selected. Frame-local bindings are actually
1543 frame parameters: you create a frame-local binding in a specific frame
1544 by calling @code{modify-frame-parameters} and specifying the variable
1545 name as the parameter name.
1547 To enable frame-local bindings for a certain variable, call the function
1548 @code{make-variable-frame-local}.
1550 @deffn Command make-variable-frame-local variable
1551 Enable the use of frame-local bindings for @var{variable}. This does
1552 not in itself create any frame-local bindings for the variable; however,
1553 if some frame already has a value for @var{variable} as a frame
1554 parameter, that value automatically becomes a frame-local binding.
1556 If the variable is terminal-local, this function signals an error,
1557 because such variables cannot have frame-local bindings as well.
1558 @xref{Multiple Displays}. A few variables that are implemented
1559 specially in Emacs can be (and usually are) buffer-local, but can never
1563 Buffer-local bindings take precedence over frame-local bindings. Thus,
1564 consider a variable @code{foo}: if the current buffer has a buffer-local
1565 binding for @code{foo}, that binding is active; otherwise, if the
1566 selected frame has a frame-local binding for @code{foo}, that binding is
1567 active; otherwise, the default binding of @code{foo} is active.
1569 Here is an example. First we prepare a few bindings for @code{foo}:
1572 (setq f1 (selected-frame))
1573 (make-variable-frame-local 'foo)
1575 ;; @r{Make a buffer-local binding for @code{foo} in @samp{b1}.}
1576 (set-buffer (get-buffer-create "b1"))
1577 (make-local-variable 'foo)
1580 ;; @r{Make a frame-local binding for @code{foo} in a new frame.}
1581 ;; @r{Store that frame in @code{f2}.}
1582 (setq f2 (make-frame))
1583 (modify-frame-parameters f2 '((foo . (f 2))))
1586 Now we examine @code{foo} in various contexts. Whenever the
1587 buffer @samp{b1} is current, its buffer-local binding is in effect,
1588 regardless of the selected frame:
1592 (set-buffer (get-buffer-create "b1"))
1597 (set-buffer (get-buffer-create "b1"))
1603 Otherwise, the frame gets a chance to provide the binding; when frame
1604 @code{f2} is selected, its frame-local binding is in effect:
1608 (set-buffer (get-buffer "*scratch*"))
1614 When neither the current buffer nor the selected frame provides
1615 a binding, the default binding is used:
1619 (set-buffer (get-buffer "*scratch*"))
1625 When the active binding of a variable is a frame-local binding, setting
1626 the variable changes that binding. You can observe the result with
1627 @code{frame-parameters}:
1631 (set-buffer (get-buffer "*scratch*"))
1633 (assq 'foo (frame-parameters f2))
1634 @result{} (foo . nobody)
1637 @node Future Local Variables
1638 @section Possible Future Local Variables
1640 We have considered the idea of bindings that are local to a category
1641 of frames---for example, all color frames, or all frames with dark
1642 backgrounds. We have not implemented them because it is not clear that
1643 this feature is really useful. You can get more or less the same
1644 results by adding a function to @code{after-make-frame-hook}, set up to
1645 define a particular frame parameter according to the appropriate
1646 conditions for each frame.
1648 It would also be possible to implement window-local bindings. We
1649 don't know of many situations where they would be useful, and it seems
1650 that indirect buffers (@pxref{Indirect Buffers}) with buffer-local
1651 bindings offer a way to handle these situations more robustly.
1653 If sufficient application is found for either of these two kinds of
1654 local bindings, we will provide it in a subsequent Emacs version.
1656 @node File Local Variables
1657 @section File Local Variables
1659 This section describes the functions and variables that affect
1660 processing of local variables lists in files.
1662 @defopt enable-local-variables
1663 This variable controls whether to process file local variables lists. A
1664 value of @code{t} means process the local variables lists
1665 unconditionally; @code{nil} means ignore them; anything else means ask
1666 the user what to do for each file. The default value is @code{t}.
1669 @defun hack-local-variables &optional force
1670 This function parses, and binds or evaluates as appropriate, any local
1671 variables specified by the contents of the current buffer. The variable
1672 @code{enable-local-variables} has its effect here.
1674 The argument @var{force} usually comes from the argument @var{find-file}
1675 given to @code{normal-mode}.
1678 If a file local variable list could specify the a function that will
1679 be called later, or an expression that will be executed later, simply
1680 visiting a file could take over your Emacs. To prevent this, Emacs
1681 takes care not to allow local variable lists to set such variables.
1683 For one thing, any variable whose name ends in @samp{-function},
1684 @samp{-functions}, @samp{-hook}, @samp{-hooks}, @samp{-form},
1685 @samp{-forms}, @samp{-program}, @samp{-command} or @samp{-predicate}
1686 cannot be set in a local variable list. In general, you should use such
1687 a name whenever it is appropriate for the variable's meaning.
1689 In addition, any variable whose name has a non-@code{nil}
1690 @code{risky-local-variable} property is also ignored. So are
1691 all variables listed in @code{ignored-local-variables}:
1693 @defvar ignored-local-variables
1694 This variable holds a list of variables that should not be
1695 set by a file's local variables list. Any value specified
1696 for one of these variables is ignored.
1699 The @samp{Eval:} ``variable'' is also a potential loophole, so Emacs
1700 normally asks for confirmation before handling it.
1702 @defopt enable-local-eval
1703 This variable controls processing of @samp{Eval:} in local variables
1704 lists in files being visited. A value of @code{t} means process them
1705 unconditionally; @code{nil} means ignore them; anything else means ask
1706 the user what to do for each file. The default value is @code{maybe}.