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 @defvar keyword-symbols-constant-flag
126 If this variable is @code{nil}, you are allowed to set and bind symbols
127 whose names start with @samp{:} however you wish. This is to make it
128 possible to run old Lisp programs which do that.
131 @node Local Variables
132 @section Local Variables
133 @cindex binding local variables
134 @cindex local variables
135 @cindex local binding
136 @cindex global binding
138 Global variables have values that last until explicitly superseded
139 with new values. Sometimes it is useful to create variable values that
140 exist temporarily---only until a certain part of the program finishes.
141 These values are called @dfn{local}, and the variables so used are
142 called @dfn{local variables}.
144 For example, when a function is called, its argument variables receive
145 new local values that last until the function exits. The @code{let}
146 special form explicitly establishes new local values for specified
147 variables; these last until exit from the @code{let} form.
149 @cindex shadowing of variables
150 Establishing a local value saves away the previous value (or lack of
151 one) of the variable. When the life span of the local value is over,
152 the previous value is restored. In the mean time, we say that the
153 previous value is @dfn{shadowed} and @dfn{not visible}. Both global and
154 local values may be shadowed (@pxref{Scope}).
156 If you set a variable (such as with @code{setq}) while it is local,
157 this replaces the local value; it does not alter the global value, or
158 previous local values, that are shadowed. To model this behavior, we
159 speak of a @dfn{local binding} of the variable as well as a local value.
161 The local binding is a conceptual place that holds a local value.
162 Entry to a function, or a special form such as @code{let}, creates the
163 local binding; exit from the function or from the @code{let} removes the
164 local binding. As long as the local binding lasts, the variable's value
165 is stored within it. Use of @code{setq} or @code{set} while there is a
166 local binding stores a different value into the local binding; it does
167 not create a new binding.
169 We also speak of the @dfn{global binding}, which is where
170 (conceptually) the global value is kept.
172 @cindex current binding
173 A variable can have more than one local binding at a time (for
174 example, if there are nested @code{let} forms that bind it). In such a
175 case, the most recently created local binding that still exists is the
176 @dfn{current binding} of the variable. (This rule is called
177 @dfn{dynamic scoping}; see @ref{Variable Scoping}.) If there are no
178 local bindings, the variable's global binding is its current binding.
179 We sometimes call the current binding the @dfn{most-local existing
180 binding}, for emphasis. Ordinary evaluation of a symbol always returns
181 the value of its current binding.
183 The special forms @code{let} and @code{let*} exist to create
186 @defspec let (bindings@dots{}) forms@dots{}
187 This special form binds variables according to @var{bindings} and then
188 evaluates all of the @var{forms} in textual order. The @code{let}-form
189 returns the value of the last form in @var{forms}.
191 Each of the @var{bindings} is either @w{(i) a} symbol, in which case
192 that symbol is bound to @code{nil}; or @w{(ii) a} list of the form
193 @code{(@var{symbol} @var{value-form})}, in which case @var{symbol} is
194 bound to the result of evaluating @var{value-form}. If @var{value-form}
195 is omitted, @code{nil} is used.
197 All of the @var{value-form}s in @var{bindings} are evaluated in the
198 order they appear and @emph{before} binding any of the symbols to them.
199 Here is an example of this: @code{Z} is bound to the old value of
200 @code{Y}, which is 2, not the new value of @code{Y}, which is 1.
216 @defspec let* (bindings@dots{}) forms@dots{}
217 This special form is like @code{let}, but it binds each variable right
218 after computing its local value, before computing the local value for
219 the next variable. Therefore, an expression in @var{bindings} can
220 reasonably refer to the preceding symbols bound in this @code{let*}
221 form. Compare the following example with the example above for
231 (Z Y)) ; @r{Use the just-established value of @code{Y}.}
238 Here is a complete list of the other facilities that create local
243 Function calls (@pxref{Functions}).
246 Macro calls (@pxref{Macros}).
249 @code{condition-case} (@pxref{Errors}).
252 Variables can also have buffer-local bindings (@pxref{Buffer-Local
253 Variables}) and frame-local bindings (@pxref{Frame-Local Variables}); a
254 few variables have terminal-local bindings (@pxref{Multiple Displays}).
255 These kinds of bindings work somewhat like ordinary local bindings, but
256 they are localized depending on ``where'' you are in Emacs, rather than
259 @defvar max-specpdl-size
260 @cindex variable limit error
261 @cindex evaluation error
262 @cindex infinite recursion
263 This variable defines the limit on the total number of local variable
264 bindings and @code{unwind-protect} cleanups (@pxref{Nonlocal Exits})
265 that are allowed before signaling an error (with data @code{"Variable
266 binding depth exceeds max-specpdl-size"}).
268 This limit, with the associated error when it is exceeded, is one way
269 that Lisp avoids infinite recursion on an ill-defined function.
270 @code{max-lisp-eval-depth} provides another limit on depth of nesting.
273 The default value is 600. Entry to the Lisp debugger increases the
274 value, if there is little room left, to make sure the debugger itself
279 @section When a Variable is ``Void''
280 @kindex void-variable
281 @cindex void variable
283 If you have never given a symbol any value as a global variable, we
284 say that that symbol's global value is @dfn{void}. In other words, the
285 symbol's value cell does not have any Lisp object in it. If you try to
286 evaluate the symbol, you get a @code{void-variable} error rather than
289 Note that a value of @code{nil} is not the same as void. The symbol
290 @code{nil} is a Lisp object and can be the value of a variable just as any
291 other object can be; but it is @emph{a value}. A void variable does not
294 After you have given a variable a value, you can make it void once more
295 using @code{makunbound}.
297 @defun makunbound symbol
298 This function makes the current variable binding of @var{symbol} void.
299 Subsequent attempts to use this symbol's value as a variable will signal
300 the error @code{void-variable}, unless and until you set it again.
302 @code{makunbound} returns @var{symbol}.
306 (makunbound 'x) ; @r{Make the global value of @code{x} void.}
311 @error{} Symbol's value as variable is void: x
315 If @var{symbol} is locally bound, @code{makunbound} affects the most
316 local existing binding. This is the only way a symbol can have a void
317 local binding, since all the constructs that create local bindings
318 create them with values. In this case, the voidness lasts at most as
319 long as the binding does; when the binding is removed due to exit from
320 the construct that made it, the previous local or global binding is
321 reexposed as usual, and the variable is no longer void unless the newly
322 reexposed binding was void all along.
326 (setq x 1) ; @r{Put a value in the global binding.}
328 (let ((x 2)) ; @r{Locally bind it.}
329 (makunbound 'x) ; @r{Void the local binding.}
331 @error{} Symbol's value as variable is void: x
334 x ; @r{The global binding is unchanged.}
337 (let ((x 2)) ; @r{Locally bind it.}
338 (let ((x 3)) ; @r{And again.}
339 (makunbound 'x) ; @r{Void the innermost-local binding.}
340 x)) ; @r{And refer: it's void.}
341 @error{} Symbol's value as variable is void: x
347 (makunbound 'x)) ; @r{Void inner binding, then remove it.}
348 x) ; @r{Now outer @code{let} binding is visible.}
354 A variable that has been made void with @code{makunbound} is
355 indistinguishable from one that has never received a value and has
358 You can use the function @code{boundp} to test whether a variable is
361 @defun boundp variable
362 @code{boundp} returns @code{t} if @var{variable} (a symbol) is not void;
363 more precisely, if its current binding is not void. It returns
364 @code{nil} otherwise.
368 (boundp 'abracadabra) ; @r{Starts out void.}
372 (let ((abracadabra 5)) ; @r{Locally bind it.}
373 (boundp 'abracadabra))
377 (boundp 'abracadabra) ; @r{Still globally void.}
381 (setq abracadabra 5) ; @r{Make it globally nonvoid.}
385 (boundp 'abracadabra)
391 @node Defining Variables
392 @section Defining Global Variables
393 @cindex variable definition
395 You may announce your intention to use a symbol as a global variable
396 with a @dfn{variable definition}: a special form, either @code{defconst}
399 In Emacs Lisp, definitions serve three purposes. First, they inform
400 people who read the code that certain symbols are @emph{intended} to be
401 used a certain way (as variables). Second, they inform the Lisp system
402 of these things, supplying a value and documentation. Third, they
403 provide information to utilities such as @code{etags} and
404 @code{make-docfile}, which create data bases of the functions and
405 variables in a program.
407 The difference between @code{defconst} and @code{defvar} is primarily
408 a matter of intent, serving to inform human readers of whether the value
409 should ever change. Emacs Lisp does not restrict the ways in which a
410 variable can be used based on @code{defconst} or @code{defvar}
411 declarations. However, it does make a difference for initialization:
412 @code{defconst} unconditionally initializes the variable, while
413 @code{defvar} initializes it only if it is void.
416 One would expect user option variables to be defined with
417 @code{defconst}, since programs do not change them. Unfortunately, this
418 has bad results if the definition is in a library that is not preloaded:
419 @code{defconst} would override any prior value when the library is
420 loaded. Users would like to be able to set user options in their init
421 files, and override the default values given in the definitions. For
422 this reason, user options must be defined with @code{defvar}.
425 @defspec defvar symbol [value [doc-string]]
426 This special form defines @var{symbol} as a variable and can also
427 initialize and document it. The definition informs a person reading
428 your code that @var{symbol} is used as a variable that might be set or
429 changed. Note that @var{symbol} is not evaluated; the symbol to be
430 defined must appear explicitly in the @code{defvar}.
432 If @var{symbol} is void and @var{value} is specified, @code{defvar}
433 evaluates it and sets @var{symbol} to the result. But if @var{symbol}
434 already has a value (i.e., it is not void), @var{value} is not even
435 evaluated, and @var{symbol}'s value remains unchanged. If @var{value}
436 is omitted, the value of @var{symbol} is not changed in any case.
438 If @var{symbol} has a buffer-local binding in the current buffer,
439 @code{defvar} operates on the default value, which is buffer-independent,
440 not the current (buffer-local) binding. It sets the default value if
441 the default value is void. @xref{Buffer-Local Variables}.
443 When you evaluate a top-level @code{defvar} form with @kbd{C-M-x} in
444 Emacs Lisp mode (@code{eval-defun}), a special feature of
445 @code{eval-defun} arranges to set the variable unconditionally, without
446 testing whether its value is void.
448 If the @var{doc-string} argument appears, it specifies the documentation
449 for the variable. (This opportunity to specify documentation is one of
450 the main benefits of defining the variable.) The documentation is
451 stored in the symbol's @code{variable-documentation} property. The
452 Emacs help functions (@pxref{Documentation}) look for this property.
454 If the first character of @var{doc-string} is @samp{*}, it means that
455 this variable is considered a user option. This lets users set the
456 variable conveniently using the commands @code{set-variable} and
457 @code{edit-options}. However, it is better to use @code{defcustom}
458 instead of @code{defvar} for user option variables, so you can specify
459 customization information. @xref{Customization}.
461 Here are some examples. This form defines @code{foo} but does not
471 This example initializes the value of @code{bar} to @code{23}, and gives
472 it a documentation string:
477 "The normal weight of a bar.")
482 The following form changes the documentation string for @code{bar},
483 making it a user option, but does not change the value, since @code{bar}
484 already has a value. (The addition @code{(1+ nil)} would get an error
485 if it were evaluated, but since it is not evaluated, there is no error.)
490 "*The normal weight of a bar.")
499 Here is an equivalent expression for the @code{defvar} special form:
503 (defvar @var{symbol} @var{value} @var{doc-string})
506 (if (not (boundp '@var{symbol}))
507 (setq @var{symbol} @var{value}))
508 (if '@var{doc-string}
509 (put '@var{symbol} 'variable-documentation '@var{doc-string}))
514 The @code{defvar} form returns @var{symbol}, but it is normally used
515 at top level in a file where its value does not matter.
518 @defspec defconst symbol [value [doc-string]]
519 This special form defines @var{symbol} as a value and initializes it.
520 It informs a person reading your code that @var{symbol} has a standard
521 global value, established here, that should not be changed by the user
522 or by other programs. Note that @var{symbol} is not evaluated; the
523 symbol to be defined must appear explicitly in the @code{defconst}.
525 @code{defconst} always evaluates @var{value}, and sets the value of
526 @var{symbol} to the result if @var{value} is given. If @var{symbol}
527 does have a buffer-local binding in the current buffer, @code{defconst}
528 sets the default value, not the buffer-local value. (But you should not
529 be making buffer-local bindings for a symbol that is defined with
532 Here, @code{pi} is a constant that presumably ought not to be changed
533 by anyone (attempts by the Indiana State Legislature notwithstanding).
534 As the second form illustrates, however, this is only advisory.
538 (defconst pi 3.1415 "Pi to five places.")
552 @defun user-variable-p variable
554 This function returns @code{t} if @var{variable} is a user option---a
555 variable intended to be set by the user for customization---and
556 @code{nil} otherwise. (Variables other than user options exist for the
557 internal purposes of Lisp programs, and users need not know about them.)
559 User option variables are distinguished from other variables by the
560 first character of the @code{variable-documentation} property. If the
561 property exists and is a string, and its first character is @samp{*},
562 then the variable is a user option.
565 @kindex variable-interactive
566 If a user option variable has a @code{variable-interactive} property,
567 the @code{set-variable} command uses that value to control reading the
568 new value for the variable. The property's value is used as if it were
569 specified in @code{interactive} (@pxref{Using Interactive}). However,
570 this feature is largely obsoleted by @code{defcustom}
571 (@pxref{Customization}).
573 @strong{Warning:} If the @code{defconst} and @code{defvar} special
574 forms are used while the variable has a local binding, they set the
575 local binding's value; the global binding is not changed. This is not
576 what you usually want. To prevent it, use these special forms at top
577 level in a file, where normally no local binding is in effect, and make
578 sure to load the file before making a local binding for the variable.
580 @node Tips for Defining
581 @section Tips for Defining Variables Robustly
583 When you define a variable whose value is a function, or a list of
584 functions, use a name that ends in @samp{-function} or
585 @samp{-functions}, respectively.
587 There are several other variable name conventions;
588 here is a complete list:
592 The variable is a normal hook (@pxref{Hooks}).
594 @item @dots{}-function
595 The value is a function.
597 @item @dots{}-functions
598 The value is a list of functions.
601 The value is a form (an expression).
603 @item @dots{}-functions
604 The value is a list of forms (expressions).
606 @item @dots{}-predicate
607 The value is a predicate---a function of one argument that returns
608 non-@code{nil} for ``good'' arguments and @code{nil} for ``bad''
612 The value is significant only as to whether it is @code{nil} or not.
614 @item @dots{}-program
615 The value is a program name.
617 @item @dots{}-command
618 The value is a whole shell command.
620 @item @samp{}-switches
621 The value specifies options for a command.
624 When you define a variable, always cvonsider whether you should mark
625 it as ``risky''; see @ref{File Local Variables}.
627 When defining and initializing a variable that holds a complicated
628 value (such as a keymap with bindings in it), it's best to put the
629 entire computation of the value into the @code{defvar}, like this:
633 (let ((map (make-sparse-keymap)))
634 (define-key map "\C-c\C-a" 'my-command)
641 This method has several benefits. First, if the user quits while
642 loading the file, the variable is either still uninitialized or
643 initialized properly, never in-between. If it is still uninitialized,
644 reloading the file will initialize it properly. Second, reloading the
645 file once the variable is initialized will not alter it; that is
646 important if the user has run hooks to alter part of the contents (such
647 as, to rebind keys). Third, evaluating the @code{defvar} form with
648 @kbd{C-M-x} @emph{will} reinitialize the map completely.
650 Putting so much code in the @code{defvar} form has one disadvantage:
651 it puts the documentation string far away from the line which names the
652 variable. Here's a safe way to avoid that:
655 (defvar my-mode-map nil
658 (let ((map (make-sparse-keymap)))
659 (define-key my-mode-map "\C-c\C-a" 'my-command)
661 (setq my-mode-map map)))
665 This has all the same advantages as putting the initialization inside
666 the @code{defvar}, except that you must type @kbd{C-M-x} twice, once on
667 each form, if you do want to reinitialize the variable.
669 But be careful not to write the code like this:
672 (defvar my-mode-map nil
675 (setq my-mode-map (make-sparse-keymap))
676 (define-key my-mode-map "\C-c\C-a" 'my-command)
681 This code sets the variable, then alters it, but it does so in more than
682 one step. If the user quits just after the @code{setq}, that leaves the
683 variable neither correctly initialized nor void nor @code{nil}. Once
684 that happens, reloading the file will not initialize the variable; it
685 will remain incomplete.
687 @node Accessing Variables
688 @section Accessing Variable Values
690 The usual way to reference a variable is to write the symbol which
691 names it (@pxref{Symbol Forms}). This requires you to specify the
692 variable name when you write the program. Usually that is exactly what
693 you want to do. Occasionally you need to choose at run time which
694 variable to reference; then you can use @code{symbol-value}.
696 @defun symbol-value symbol
697 This function returns the value of @var{symbol}. This is the value in
698 the innermost local binding of the symbol, or its global value if it
699 has no local bindings.
712 ;; @r{Here the symbol @code{abracadabra}}
713 ;; @r{is the symbol whose value is examined.}
714 (let ((abracadabra 'foo))
715 (symbol-value 'abracadabra))
720 ;; @r{Here the value of @code{abracadabra},}
721 ;; @r{which is @code{foo},}
722 ;; @r{is the symbol whose value is examined.}
723 (let ((abracadabra 'foo))
724 (symbol-value abracadabra))
729 (symbol-value 'abracadabra)
734 A @code{void-variable} error is signaled if the current binding of
735 @var{symbol} is void.
738 @node Setting Variables
739 @section How to Alter a Variable Value
741 The usual way to change the value of a variable is with the special
742 form @code{setq}. When you need to compute the choice of variable at
743 run time, use the function @code{set}.
745 @defspec setq [symbol form]@dots{}
746 This special form is the most common method of changing a variable's
747 value. Each @var{symbol} is given a new value, which is the result of
748 evaluating the corresponding @var{form}. The most-local existing
749 binding of the symbol is changed.
751 @code{setq} does not evaluate @var{symbol}; it sets the symbol that you
752 write. We say that this argument is @dfn{automatically quoted}. The
753 @samp{q} in @code{setq} stands for ``quoted.''
755 The value of the @code{setq} form is the value of the last @var{form}.
762 x ; @r{@code{x} now has a global value.}
766 (setq x 6) ; @r{The local binding of @code{x} is set.}
770 x ; @r{The global value is unchanged.}
774 Note that the first @var{form} is evaluated, then the first
775 @var{symbol} is set, then the second @var{form} is evaluated, then the
776 second @var{symbol} is set, and so on:
780 (setq x 10 ; @r{Notice that @code{x} is set before}
781 y (1+ x)) ; @r{the value of @code{y} is computed.}
787 @defun set symbol value
788 This function sets @var{symbol}'s value to @var{value}, then returns
789 @var{value}. Since @code{set} is a function, the expression written for
790 @var{symbol} is evaluated to obtain the symbol to set.
792 The most-local existing binding of the variable is the binding that is
793 set; shadowed bindings are not affected.
798 @error{} Symbol's value as variable is void: one
809 (set two 2) ; @r{@code{two} evaluates to symbol @code{one}.}
813 one ; @r{So it is @code{one} that was set.}
815 (let ((one 1)) ; @r{This binding of @code{one} is set,}
816 (set 'one 3) ; @r{not the global value.}
826 If @var{symbol} is not actually a symbol, a @code{wrong-type-argument}
831 @error{} Wrong type argument: symbolp, (x y)
834 Logically speaking, @code{set} is a more fundamental primitive than
835 @code{setq}. Any use of @code{setq} can be trivially rewritten to use
836 @code{set}; @code{setq} could even be defined as a macro, given the
837 availability of @code{set}. However, @code{set} itself is rarely used;
838 beginners hardly need to know about it. It is useful only for choosing
839 at run time which variable to set. For example, the command
840 @code{set-variable}, which reads a variable name from the user and then
841 sets the variable, needs to use @code{set}.
843 @cindex CL note---@code{set} local
845 @b{Common Lisp note:} In Common Lisp, @code{set} always changes the
846 symbol's ``special'' or dynamic value, ignoring any lexical bindings.
847 In Emacs Lisp, all variables and all bindings are dynamic, so @code{set}
848 always affects the most local existing binding.
852 One other function for setting a variable is designed to add
853 an element to a list if it is not already present in the list.
855 @defun add-to-list symbol element
856 This function sets the variable @var{symbol} by consing @var{element}
857 onto the old value, if @var{element} is not already a member of that
858 value. It returns the resulting list, whether updated or not. The
859 value of @var{symbol} had better be a list already before the call.
861 The argument @var{symbol} is not implicitly quoted; @code{add-to-list}
862 is an ordinary function, like @code{set} and unlike @code{setq}. Quote
863 the argument yourself if that is what you want.
866 Here's a scenario showing how to use @code{add-to-list}:
872 (add-to-list 'foo 'c) ;; @r{Add @code{c}.}
875 (add-to-list 'foo 'b) ;; @r{No effect.}
878 foo ;; @r{@code{foo} was changed.}
882 An equivalent expression for @code{(add-to-list '@var{var}
883 @var{value})} is this:
886 (or (member @var{value} @var{var})
887 (setq @var{var} (cons @var{value} @var{var})))
890 @node Variable Scoping
891 @section Scoping Rules for Variable Bindings
893 A given symbol @code{foo} can have several local variable bindings,
894 established at different places in the Lisp program, as well as a global
895 binding. The most recently established binding takes precedence over
900 @cindex dynamic scoping
901 Local bindings in Emacs Lisp have @dfn{indefinite scope} and
902 @dfn{dynamic extent}. @dfn{Scope} refers to @emph{where} textually in
903 the source code the binding can be accessed. ``Indefinite scope'' means
904 that any part of the program can potentially access the variable
905 binding. @dfn{Extent} refers to @emph{when}, as the program is
906 executing, the binding exists. ``Dynamic extent'' means that the binding
907 lasts as long as the activation of the construct that established it.
909 The combination of dynamic extent and indefinite scope is called
910 @dfn{dynamic scoping}. By contrast, most programming languages use
911 @dfn{lexical scoping}, in which references to a local variable must be
912 located textually within the function or block that binds the variable.
914 @cindex CL note---special variables
916 @b{Common Lisp note:} Variables declared ``special'' in Common Lisp are
917 dynamically scoped, like all variables in Emacs Lisp.
921 * Scope:: Scope means where in the program a value is visible.
922 Comparison with other languages.
923 * Extent:: Extent means how long in time a value exists.
924 * Impl of Scope:: Two ways to implement dynamic scoping.
925 * Using Scoping:: How to use dynamic scoping carefully and avoid problems.
931 Emacs Lisp uses @dfn{indefinite scope} for local variable bindings.
932 This means that any function anywhere in the program text might access a
933 given binding of a variable. Consider the following function
938 (defun binder (x) ; @r{@code{x} is bound in @code{binder}.}
939 (foo 5)) ; @r{@code{foo} is some other function.}
943 (defun user () ; @r{@code{x} is used ``free'' in @code{user}.}
948 In a lexically scoped language, the binding of @code{x} in
949 @code{binder} would never be accessible in @code{user}, because
950 @code{user} is not textually contained within the function
951 @code{binder}. However, in dynamically-scoped Emacs Lisp, @code{user}
952 may or may not refer to the binding of @code{x} established in
953 @code{binder}, depending on the circumstances:
957 If we call @code{user} directly without calling @code{binder} at all,
958 then whatever binding of @code{x} is found, it cannot come from
962 If we define @code{foo} as follows and then call @code{binder}, then the
963 binding made in @code{binder} will be seen in @code{user}:
973 However, if we define @code{foo} as follows and then call @code{binder},
974 then the binding made in @code{binder} @emph{will not} be seen in
983 Here, when @code{foo} is called by @code{binder}, it binds @code{x}.
984 (The binding in @code{foo} is said to @dfn{shadow} the one made in
985 @code{binder}.) Therefore, @code{user} will access the @code{x} bound
986 by @code{foo} instead of the one bound by @code{binder}.
989 Emacs Lisp uses dynamic scoping because simple implementations of
990 lexical scoping are slow. In addition, every Lisp system needs to offer
991 dynamic scoping at least as an option; if lexical scoping is the norm,
992 there must be a way to specify dynamic scoping instead for a particular
993 variable. It might not be a bad thing for Emacs to offer both, but
994 implementing it with dynamic scoping only was much easier.
999 @dfn{Extent} refers to the time during program execution that a
1000 variable name is valid. In Emacs Lisp, a variable is valid only while
1001 the form that bound it is executing. This is called @dfn{dynamic
1002 extent}. ``Local'' or ``automatic'' variables in most languages,
1003 including C and Pascal, have dynamic extent.
1005 One alternative to dynamic extent is @dfn{indefinite extent}. This
1006 means that a variable binding can live on past the exit from the form
1007 that made the binding. Common Lisp and Scheme, for example, support
1008 this, but Emacs Lisp does not.
1010 To illustrate this, the function below, @code{make-add}, returns a
1011 function that purports to add @var{n} to its own argument @var{m}. This
1012 would work in Common Lisp, but it does not do the job in Emacs Lisp,
1013 because after the call to @code{make-add} exits, the variable @code{n}
1014 is no longer bound to the actual argument 2.
1018 (function (lambda (m) (+ n m)))) ; @r{Return a function.}
1020 (fset 'add2 (make-add 2)) ; @r{Define function @code{add2}}
1021 ; @r{with @code{(make-add 2)}.}
1022 @result{} (lambda (m) (+ n m))
1023 (add2 4) ; @r{Try to add 2 to 4.}
1024 @error{} Symbol's value as variable is void: n
1027 @cindex closures not available
1028 Some Lisp dialects have ``closures'', objects that are like functions
1029 but record additional variable bindings. Emacs Lisp does not have
1033 @subsection Implementation of Dynamic Scoping
1034 @cindex deep binding
1036 A simple sample implementation (which is not how Emacs Lisp actually
1037 works) may help you understand dynamic binding. This technique is
1038 called @dfn{deep binding} and was used in early Lisp systems.
1040 Suppose there is a stack of bindings, which are variable-value pairs.
1041 At entry to a function or to a @code{let} form, we can push bindings
1042 onto the stack for the arguments or local variables created there. We
1043 can pop those bindings from the stack at exit from the binding
1046 We can find the value of a variable by searching the stack from top to
1047 bottom for a binding for that variable; the value from that binding is
1048 the value of the variable. To set the variable, we search for the
1049 current binding, then store the new value into that binding.
1051 As you can see, a function's bindings remain in effect as long as it
1052 continues execution, even during its calls to other functions. That is
1053 why we say the extent of the binding is dynamic. And any other function
1054 can refer to the bindings, if it uses the same variables while the
1055 bindings are in effect. That is why we say the scope is indefinite.
1057 @cindex shallow binding
1058 The actual implementation of variable scoping in GNU Emacs Lisp uses a
1059 technique called @dfn{shallow binding}. Each variable has a standard
1060 place in which its current value is always found---the value cell of the
1063 In shallow binding, setting the variable works by storing a value in
1064 the value cell. Creating a new binding works by pushing the old value
1065 (belonging to a previous binding) onto a stack, and storing the new
1066 local value in the value cell. Eliminating a binding works by popping
1067 the old value off the stack, into the value cell.
1069 We use shallow binding because it has the same results as deep
1070 binding, but runs faster, since there is never a need to search for a
1074 @subsection Proper Use of Dynamic Scoping
1076 Binding a variable in one function and using it in another is a
1077 powerful technique, but if used without restraint, it can make programs
1078 hard to understand. There are two clean ways to use this technique:
1082 Use or bind the variable only in a few related functions, written close
1083 together in one file. Such a variable is used for communication within
1086 You should write comments to inform other programmers that they can see
1087 all uses of the variable before them, and to advise them not to add uses
1091 Give the variable a well-defined, documented meaning, and make all
1092 appropriate functions refer to it (but not bind it or set it) wherever
1093 that meaning is relevant. For example, the variable
1094 @code{case-fold-search} is defined as ``non-@code{nil} means ignore case
1095 when searching''; various search and replace functions refer to it
1096 directly or through their subroutines, but do not bind or set it.
1098 Then you can bind the variable in other programs, knowing reliably what
1102 In either case, you should define the variable with @code{defvar}.
1103 This helps other people understand your program by telling them to look
1104 for inter-function usage. It also avoids a warning from the byte
1105 compiler. Choose the variable's name to avoid name conflicts---don't
1106 use short names like @code{x}.
1108 @node Buffer-Local Variables
1109 @section Buffer-Local Variables
1110 @cindex variables, buffer-local
1111 @cindex buffer-local variables
1113 Global and local variable bindings are found in most programming
1114 languages in one form or another. Emacs, however, also supports additional,
1115 unusual kinds of variable binding: @dfn{buffer-local} bindings, which
1116 apply only in one buffer, and @dfn{frame-local} bindings, which apply only in
1117 one frame. Having different values for a variable in different buffers
1118 and/or frames is an important customization method.
1120 This section describes buffer-local bindings; for frame-local
1121 bindings, see the following section, @ref{Frame-Local Variables}. (A few
1122 variables have bindings that are local to each terminal; see
1123 @ref{Multiple Displays}.)
1126 * Intro to Buffer-Local:: Introduction and concepts.
1127 * Creating Buffer-Local:: Creating and destroying buffer-local bindings.
1128 * Default Value:: The default value is seen in buffers
1129 that don't have their own buffer-local values.
1132 @node Intro to Buffer-Local
1133 @subsection Introduction to Buffer-Local Variables
1135 A buffer-local variable has a buffer-local binding associated with a
1136 particular buffer. The binding is in effect when that buffer is
1137 current; otherwise, it is not in effect. If you set the variable while
1138 a buffer-local binding is in effect, the new value goes in that binding,
1139 so its other bindings are unchanged. This means that the change is
1140 visible only in the buffer where you made it.
1142 The variable's ordinary binding, which is not associated with any
1143 specific buffer, is called the @dfn{default binding}. In most cases,
1144 this is the global binding.
1146 A variable can have buffer-local bindings in some buffers but not in
1147 other buffers. The default binding is shared by all the buffers that
1148 don't have their own bindings for the variable. (This includes all
1149 newly-created buffers.) If you set the variable in a buffer that does
1150 not have a buffer-local binding for it, this sets the default binding
1151 (assuming there are no frame-local bindings to complicate the matter),
1152 so the new value is visible in all the buffers that see the default
1155 The most common use of buffer-local bindings is for major modes to change
1156 variables that control the behavior of commands. For example, C mode and
1157 Lisp mode both set the variable @code{paragraph-start} to specify that only
1158 blank lines separate paragraphs. They do this by making the variable
1159 buffer-local in the buffer that is being put into C mode or Lisp mode, and
1160 then setting it to the new value for that mode. @xref{Major Modes}.
1162 The usual way to make a buffer-local binding is with
1163 @code{make-local-variable}, which is what major mode commands typically
1164 use. This affects just the current buffer; all other buffers (including
1165 those yet to be created) will continue to share the default value unless
1166 they are explicitly given their own buffer-local bindings.
1168 @cindex automatically buffer-local
1169 A more powerful operation is to mark the variable as
1170 @dfn{automatically buffer-local} by calling
1171 @code{make-variable-buffer-local}. You can think of this as making the
1172 variable local in all buffers, even those yet to be created. More
1173 precisely, the effect is that setting the variable automatically makes
1174 the variable local to the current buffer if it is not already so. All
1175 buffers start out by sharing the default value of the variable as usual,
1176 but setting the variable creates a buffer-local binding for the current
1177 buffer. The new value is stored in the buffer-local binding, leaving
1178 the default binding untouched. This means that the default value cannot
1179 be changed with @code{setq} in any buffer; the only way to change it is
1180 with @code{setq-default}.
1182 @strong{Warning:} When a variable has buffer-local values in one or
1183 more buffers, you can get Emacs very confused by binding the variable
1184 with @code{let}, changing to a different current buffer in which a
1185 different binding is in effect, and then exiting the @code{let}. This
1186 can scramble the values of the buffer-local and default bindings.
1188 To preserve your sanity, avoid using a variable in that way. If you
1189 use @code{save-excursion} around each piece of code that changes to a
1190 different current buffer, you will not have this problem
1191 (@pxref{Excursions}). Here is an example of what to avoid:
1197 (make-local-variable 'foo)
1204 foo @result{} 'a ; @r{The old buffer-local value from buffer @samp{a}}
1205 ; @r{is now the default value.}
1209 foo @result{} 'temp ; @r{The local @code{let} value that should be gone}
1210 ; @r{is now the buffer-local value in buffer @samp{a}.}
1215 But @code{save-excursion} as shown here avoids the problem:
1226 Note that references to @code{foo} in @var{body} access the
1227 buffer-local binding of buffer @samp{b}.
1229 When a file specifies local variable values, these become buffer-local
1230 values when you visit the file. @xref{File Variables,,, emacs, The
1233 @node Creating Buffer-Local
1234 @subsection Creating and Deleting Buffer-Local Bindings
1236 @deffn Command make-local-variable variable
1237 This function creates a buffer-local binding in the current buffer for
1238 @var{variable} (a symbol). Other buffers are not affected. The value
1239 returned is @var{variable}.
1242 The buffer-local value of @var{variable} starts out as the same value
1243 @var{variable} previously had. If @var{variable} was void, it remains
1248 ;; @r{In buffer @samp{b1}:}
1249 (setq foo 5) ; @r{Affects all buffers.}
1253 (make-local-variable 'foo) ; @r{Now it is local in @samp{b1}.}
1257 foo ; @r{That did not change}
1258 @result{} 5 ; @r{the value.}
1261 (setq foo 6) ; @r{Change the value}
1262 @result{} 6 ; @r{in @samp{b1}.}
1270 ;; @r{In buffer @samp{b2}, the value hasn't changed.}
1278 Making a variable buffer-local within a @code{let}-binding for that
1279 variable does not work reliably, unless the buffer in which you do this
1280 is not current either on entry to or exit from the @code{let}. This is
1281 because @code{let} does not distinguish between different kinds of
1282 bindings; it knows only which variable the binding was made for.
1284 If the variable is terminal-local, this function signals an error. Such
1285 variables cannot have buffer-local bindings as well. @xref{Multiple
1288 @strong{Note:} Do not use @code{make-local-variable} for a hook
1289 variable. Instead, use @code{make-local-hook}. @xref{Hooks}.
1292 @deffn Command make-variable-buffer-local variable
1293 This function marks @var{variable} (a symbol) automatically
1294 buffer-local, so that any subsequent attempt to set it will make it
1295 local to the current buffer at the time.
1297 A peculiar wrinkle of this feature is that binding the variable (with
1298 @code{let} or other binding constructs) does not create a buffer-local
1299 binding for it. Only setting the variable (with @code{set} or
1300 @code{setq}) does so.
1302 The value returned is @var{variable}.
1304 @strong{Warning:} Don't assume that you should use
1305 @code{make-variable-buffer-local} for user-option variables, simply
1306 because users @emph{might} want to customize them differently in
1307 different buffers. Users can make any variable local, when they wish
1308 to. It is better to leave the choice to them.
1310 The time to use @code{make-variable-buffer-local} is when it is crucial
1311 that no two buffers ever share the same binding. For example, when a
1312 variable is used for internal purposes in a Lisp program which depends
1313 on having separate values in separate buffers, then using
1314 @code{make-variable-buffer-local} can be the best solution.
1317 @defun local-variable-p variable &optional buffer
1318 This returns @code{t} if @var{variable} is buffer-local in buffer
1319 @var{buffer} (which defaults to the current buffer); otherwise,
1323 @defun buffer-local-variables &optional buffer
1324 This function returns a list describing the buffer-local variables in
1325 buffer @var{buffer}. (If @var{buffer} is omitted, the current buffer is
1326 used.) It returns an association list (@pxref{Association Lists}) in
1327 which each element contains one buffer-local variable and its value.
1328 However, when a variable's buffer-local binding in @var{buffer} is void,
1329 then the variable appears directly in the resulting list.
1333 (make-local-variable 'foobar)
1334 (makunbound 'foobar)
1335 (make-local-variable 'bind-me)
1338 (setq lcl (buffer-local-variables))
1339 ;; @r{First, built-in variables local in all buffers:}
1340 @result{} ((mark-active . nil)
1341 (buffer-undo-list . nil)
1342 (mode-name . "Fundamental")
1345 ;; @r{Next, non-built-in buffer-local variables.}
1346 ;; @r{This one is buffer-local and void:}
1348 ;; @r{This one is buffer-local and nonvoid:}
1353 Note that storing new values into the @sc{cdr}s of cons cells in this
1354 list does @emph{not} change the buffer-local values of the variables.
1357 @deffn Command kill-local-variable variable
1358 This function deletes the buffer-local binding (if any) for
1359 @var{variable} (a symbol) in the current buffer. As a result, the
1360 default binding of @var{variable} becomes visible in this buffer. This
1361 typically results in a change in the value of @var{variable}, since the
1362 default value is usually different from the buffer-local value just
1365 If you kill the buffer-local binding of a variable that automatically
1366 becomes buffer-local when set, this makes the default value visible in
1367 the current buffer. However, if you set the variable again, that will
1368 once again create a buffer-local binding for it.
1370 @code{kill-local-variable} returns @var{variable}.
1372 This function is a command because it is sometimes useful to kill one
1373 buffer-local variable interactively, just as it is useful to create
1374 buffer-local variables interactively.
1377 @defun kill-all-local-variables
1378 This function eliminates all the buffer-local variable bindings of the
1379 current buffer except for variables marked as ``permanent''. As a
1380 result, the buffer will see the default values of most variables.
1382 This function also resets certain other information pertaining to the
1383 buffer: it sets the local keymap to @code{nil}, the syntax table to the
1384 value of @code{(standard-syntax-table)}, the case table to
1385 @code{(standard-case-table)}, and the abbrev table to the value of
1386 @code{fundamental-mode-abbrev-table}.
1388 The very first thing this function does is run the normal hook
1389 @code{change-major-mode-hook} (see below).
1391 Every major mode command begins by calling this function, which has the
1392 effect of switching to Fundamental mode and erasing most of the effects
1393 of the previous major mode. To ensure that this does its job, the
1394 variables that major modes set should not be marked permanent.
1396 @code{kill-all-local-variables} returns @code{nil}.
1399 @defvar change-major-mode-hook
1400 The function @code{kill-all-local-variables} runs this normal hook
1401 before it does anything else. This gives major modes a way to arrange
1402 for something special to be done if the user switches to a different
1403 major mode. For best results, make this variable buffer-local, so that
1404 it will disappear after doing its job and will not interfere with the
1405 subsequent major mode. @xref{Hooks}.
1409 @cindex permanent local variable
1410 A buffer-local variable is @dfn{permanent} if the variable name (a
1411 symbol) has a @code{permanent-local} property that is non-@code{nil}.
1412 Permanent locals are appropriate for data pertaining to where the file
1413 came from or how to save it, rather than with how to edit the contents.
1416 @subsection The Default Value of a Buffer-Local Variable
1417 @cindex default value
1419 The global value of a variable with buffer-local bindings is also
1420 called the @dfn{default} value, because it is the value that is in
1421 effect whenever neither the current buffer nor the selected frame has
1422 its own binding for the variable.
1424 The functions @code{default-value} and @code{setq-default} access and
1425 change a variable's default value regardless of whether the current
1426 buffer has a buffer-local binding. For example, you could use
1427 @code{setq-default} to change the default setting of
1428 @code{paragraph-start} for most buffers; and this would work even when
1429 you are in a C or Lisp mode buffer that has a buffer-local value for
1433 The special forms @code{defvar} and @code{defconst} also set the
1434 default value (if they set the variable at all), rather than any
1435 buffer-local or frame-local value.
1437 @defun default-value symbol
1438 This function returns @var{symbol}'s default value. This is the value
1439 that is seen in buffers and frames that do not have their own values for
1440 this variable. If @var{symbol} is not buffer-local, this is equivalent
1441 to @code{symbol-value} (@pxref{Accessing Variables}).
1445 @defun default-boundp symbol
1446 The function @code{default-boundp} tells you whether @var{symbol}'s
1447 default value is nonvoid. If @code{(default-boundp 'foo)} returns
1448 @code{nil}, then @code{(default-value 'foo)} would get an error.
1450 @code{default-boundp} is to @code{default-value} as @code{boundp} is to
1451 @code{symbol-value}.
1454 @defspec setq-default [symbol form]@dots{}
1455 This special form gives each @var{symbol} a new default value, which is
1456 the result of evaluating the corresponding @var{form}. It does not
1457 evaluate @var{symbol}, but does evaluate @var{form}. The value of the
1458 @code{setq-default} form is the value of the last @var{form}.
1460 If a @var{symbol} is not buffer-local for the current buffer, and is not
1461 marked automatically buffer-local, @code{setq-default} has the same
1462 effect as @code{setq}. If @var{symbol} is buffer-local for the current
1463 buffer, then this changes the value that other buffers will see (as long
1464 as they don't have a buffer-local value), but not the value that the
1465 current buffer sees.
1469 ;; @r{In buffer @samp{foo}:}
1470 (make-local-variable 'buffer-local)
1471 @result{} buffer-local
1474 (setq buffer-local 'value-in-foo)
1475 @result{} value-in-foo
1478 (setq-default buffer-local 'new-default)
1479 @result{} new-default
1483 @result{} value-in-foo
1486 (default-value 'buffer-local)
1487 @result{} new-default
1491 ;; @r{In (the new) buffer @samp{bar}:}
1493 @result{} new-default
1496 (default-value 'buffer-local)
1497 @result{} new-default
1500 (setq buffer-local 'another-default)
1501 @result{} another-default
1504 (default-value 'buffer-local)
1505 @result{} another-default
1509 ;; @r{Back in buffer @samp{foo}:}
1511 @result{} value-in-foo
1512 (default-value 'buffer-local)
1513 @result{} another-default
1518 @defun set-default symbol value
1519 This function is like @code{setq-default}, except that @var{symbol} is
1520 an ordinary evaluated argument.
1524 (set-default (car '(a b c)) 23)
1534 @node Frame-Local Variables
1535 @section Frame-Local Variables
1537 Just as variables can have buffer-local bindings, they can also have
1538 frame-local bindings. These bindings belong to one frame, and are in
1539 effect when that frame is selected. Frame-local bindings are actually
1540 frame parameters: you create a frame-local binding in a specific frame
1541 by calling @code{modify-frame-parameters} and specifying the variable
1542 name as the parameter name.
1544 To enable frame-local bindings for a certain variable, call the function
1545 @code{make-variable-frame-local}.
1547 @deffn Command make-variable-frame-local variable
1548 Enable the use of frame-local bindings for @var{variable}. This does
1549 not in itself create any frame-local bindings for the variable; however,
1550 if some frame already has a value for @var{variable} as a frame
1551 parameter, that value automatically becomes a frame-local binding.
1553 If the variable is terminal-local, this function signals an error,
1554 because such variables cannot have frame-local bindings as well.
1555 @xref{Multiple Displays}. A few variables that are implemented
1556 specially in Emacs can be (and usually are) buffer-local, but can never
1560 Buffer-local bindings take precedence over frame-local bindings. Thus,
1561 consider a variable @code{foo}: if the current buffer has a buffer-local
1562 binding for @code{foo}, that binding is active; otherwise, if the
1563 selected frame has a frame-local binding for @code{foo}, that binding is
1564 active; otherwise, the default binding of @code{foo} is active.
1566 Here is an example. First we prepare a few bindings for @code{foo}:
1569 (setq f1 (selected-frame))
1570 (make-variable-frame-local 'foo)
1572 ;; @r{Make a buffer-local binding for @code{foo} in @samp{b1}.}
1573 (set-buffer (get-buffer-create "b1"))
1574 (make-local-variable 'foo)
1577 ;; @r{Make a frame-local binding for @code{foo} in a new frame.}
1578 ;; @r{Store that frame in @code{f2}.}
1579 (setq f2 (make-frame))
1580 (modify-frame-parameters f2 '((foo . (f 2))))
1583 Now we examine @code{foo} in various contexts. Whenever the
1584 buffer @samp{b1} is current, its buffer-local binding is in effect,
1585 regardless of the selected frame:
1589 (set-buffer (get-buffer-create "b1"))
1594 (set-buffer (get-buffer-create "b1"))
1600 Otherwise, the frame gets a chance to provide the binding; when frame
1601 @code{f2} is selected, its frame-local binding is in effect:
1605 (set-buffer (get-buffer "*scratch*"))
1611 When neither the current buffer nor the selected frame provides
1612 a binding, the default binding is used:
1616 (set-buffer (get-buffer "*scratch*"))
1622 When the active binding of a variable is a frame-local binding, setting
1623 the variable changes that binding. You can observe the result with
1624 @code{frame-parameters}:
1628 (set-buffer (get-buffer "*scratch*"))
1630 (assq 'foo (frame-parameters f2))
1631 @result{} (foo . nobody)
1634 @node Future Local Variables
1635 @section Possible Future Local Variables
1637 We have considered the idea of bindings that are local to a category
1638 of frames---for example, all color frames, or all frames with dark
1639 backgrounds. We have not implemented them because it is not clear that
1640 this feature is really useful. You can get more or less the same
1641 results by adding a function to @code{after-make-frame-hook}, set up to
1642 define a particular frame parameter according to the appropriate
1643 conditions for each frame.
1645 It would also be possible to implement window-local bindings. We
1646 don't know of many situations where they would be useful, and it seems
1647 that indirect buffers (@pxref{Indirect Buffers}) with buffer-local
1648 bindings offer a way to handle these situations more robustly.
1650 If sufficient application is found for either of these two kinds of
1651 local bindings, we will provide it in a subsequent Emacs version.
1653 @node File Local Variables
1654 @section File Local Variables
1656 This section describes the functions and variables that affect
1657 processing of local variables lists in files.
1659 @defopt enable-local-variables
1660 This variable controls whether to process file local variables lists. A
1661 value of @code{t} means process the local variables lists
1662 unconditionally; @code{nil} means ignore them; anything else means ask
1663 the user what to do for each file. The default value is @code{t}.
1666 @defun hack-local-variables &optional force
1667 This function parses, and binds or evaluates as appropriate, any local
1668 variables specified by the contents of the current buffer. The variable
1669 @code{enable-local-variables} has its effect here.
1671 The argument @var{force} usually comes from the argument @var{find-file}
1672 given to @code{normal-mode}.
1675 If a file local variable list could specify the a function that will
1676 be called later, or an expression that will be executed later, simply
1677 visiting a file could take over your Emacs. To prevent this, Emacs
1678 takes care not to allow local variable lists to set such variables.
1680 For one thing, any variable whose name ends in @samp{-function},
1681 @samp{-functions}, @samp{-hook}, @samp{-hooks}, @samp{-form},
1682 @samp{-forms}, @samp{-program}, @samp{-command} or @samp{-predicate}
1683 cannot be set in a local variable list. In general, you should use such
1684 a name whenever it is appropriate for the variable's meaning.
1686 In addition, any variable whose name has a non-@code{nil}
1687 @code{risky-local-variable} property is also ignored. So are
1688 all variables listed in @code{ignored-local-variables}:
1690 @defvar ignored-local-variables
1691 This variable holds a list of variables that should not be
1692 set by a file's local variables list. Any value specified
1693 for one of these variables is ignored.
1696 The @samp{Eval:} ``variable'' is also a potential loophole, so Emacs
1697 normally asks for confirmation before handling it.
1699 @defopt enable-local-eval
1700 This variable controls processing of @samp{Eval:} in local variables
1701 lists in files being visited. A value of @code{t} means process them
1702 unconditionally; @code{nil} means ignore them; anything else means ask
1703 the user what to do for each file. The default value is @code{maybe}.