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[emacs.git] / lispref / variables.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2000,
@c   2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008  Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../info/variables
@node Variables, Functions, Control Structures, Top
@chapter Variables
@cindex variable

  A @dfn{variable} is a name used in a program to stand for a value.
Nearly all programming languages have variables of some sort.  In the
text of a Lisp program, variables are written using the syntax for
symbols.

  In Lisp, unlike most programming languages, programs are represented
primarily as Lisp objects and only secondarily as text.  The Lisp
objects used for variables are symbols: the symbol name is the variable
name, and the variable's value is stored in the value cell of the
symbol.  The use of a symbol as a variable is independent of its use as
a function name.  @xref{Symbol Components}.

  The Lisp objects that constitute a Lisp program determine the textual
form of the program---it is simply the read syntax for those Lisp
objects.  This is why, for example, a variable in a textual Lisp program
is written using the read syntax for the symbol that represents the
variable.

@menu
* Global Variables::      Variable values that exist permanently, everywhere.
* Constant Variables::    Certain "variables" have values that never change.
* Local Variables::       Variable values that exist only temporarily.
* Void Variables::        Symbols that lack values.
* Defining Variables::    A definition says a symbol is used as a variable.
* Tips for Defining::     Things you should think about when you
                            define a variable.
* Accessing Variables::   Examining values of variables whose names
                            are known only at run time.
* Setting Variables::     Storing new values in variables.
* Variable Scoping::      How Lisp chooses among local and global values.
* Buffer-Local Variables::  Variable values in effect only in one buffer.
* Future Local Variables::  New kinds of local values we might add some day.
* File Local Variables::  Handling local variable lists in files.
* Variable Aliases::      Variables that are aliases for other variables.
* Variables with Restricted Values::  Non-constant variables whose value can
                                        @emph{not} be an arbitrary Lisp object.
@end menu

@node Global Variables
@section Global Variables
@cindex global variable

  The simplest way to use a variable is @dfn{globally}.  This means that
the variable has just one value at a time, and this value is in effect
(at least for the moment) throughout the Lisp system.  The value remains
in effect until you specify a new one.  When a new value replaces the
old one, no trace of the old value remains in the variable.

  You specify a value for a symbol with @code{setq}.  For example,

@example
(setq x '(a b))
@end example

@noindent
gives the variable @code{x} the value @code{(a b)}.  Note that
@code{setq} does not evaluate its first argument, the name of the
variable, but it does evaluate the second argument, the new value.

  Once the variable has a value, you can refer to it by using the symbol
by itself as an expression.  Thus,

@example
@group
x @result{} (a b)
@end group
@end example

@noindent
assuming the @code{setq} form shown above has already been executed.

  If you do set the same variable again, the new value replaces the old
one:

@example
@group
x
     @result{} (a b)
@end group
@group
(setq x 4)
     @result{} 4
@end group
@group
x
     @result{} 4
@end group
@end example

@node Constant Variables
@section Variables that Never Change
@kindex setting-constant
@cindex keyword symbol
@cindex variable with constant value
@cindex constant variables
@cindex symbol that evaluates to itself
@cindex symbol with constant value

  In Emacs Lisp, certain symbols normally evaluate to themselves.  These
include @code{nil} and @code{t}, as well as any symbol whose name starts
with @samp{:} (these are called @dfn{keywords}).  These symbols cannot
be rebound, nor can their values be changed.  Any attempt to set or bind
@code{nil} or @code{t} signals a @code{setting-constant} error.  The
same is true for a keyword (a symbol whose name starts with @samp{:}),
if it is interned in the standard obarray, except that setting such a
symbol to itself is not an error.

@example
@group
nil @equiv{} 'nil
     @result{} nil
@end group
@group
(setq nil 500)
@error{} Attempt to set constant symbol: nil
@end group
@end example

@defun keywordp object
function returns @code{t} if @var{object} is a symbol whose name
starts with @samp{:}, interned in the standard obarray, and returns
@code{nil} otherwise.
@end defun

@node Local Variables
@section Local Variables
@cindex binding local variables
@cindex local variables
@cindex local binding
@cindex global binding

  Global variables have values that last until explicitly superseded
with new values.  Sometimes it is useful to create variable values that
exist temporarily---only until a certain part of the program finishes.
These values are called @dfn{local}, and the variables so used are
called @dfn{local variables}.

  For example, when a function is called, its argument variables receive
new local values that last until the function exits.  The @code{let}
special form explicitly establishes new local values for specified
variables; these last until exit from the @code{let} form.

@cindex shadowing of variables
  Establishing a local value saves away the previous value (or lack of
one) of the variable.  When the life span of the local value is over,
the previous value is restored.  In the mean time, we say that the
previous value is @dfn{shadowed} and @dfn{not visible}.  Both global and
local values may be shadowed (@pxref{Scope}).

  If you set a variable (such as with @code{setq}) while it is local,
this replaces the local value; it does not alter the global value, or
previous local values, that are shadowed.  To model this behavior, we
speak of a @dfn{local binding} of the variable as well as a local value.

  The local binding is a conceptual place that holds a local value.
Entry to a function, or a special form such as @code{let}, creates the
local binding; exit from the function or from the @code{let} removes the
local binding.  As long as the local binding lasts, the variable's value
is stored within it.  Use of @code{setq} or @code{set} while there is a
local binding stores a different value into the local binding; it does
not create a new binding.

  We also speak of the @dfn{global binding}, which is where
(conceptually) the global value is kept.

@cindex current binding
  A variable can have more than one local binding at a time (for
example, if there are nested @code{let} forms that bind it).  In such a
case, the most recently created local binding that still exists is the
@dfn{current binding} of the variable.  (This rule is called
@dfn{dynamic scoping}; see @ref{Variable Scoping}.)  If there are no
local bindings, the variable's global binding is its current binding.
We sometimes call the current binding the @dfn{most-local existing
binding}, for emphasis.  Ordinary evaluation of a symbol always returns
the value of its current binding.

  The special forms @code{let} and @code{let*} exist to create
local bindings.

@defspec let (bindings@dots{}) forms@dots{}
This special form binds variables according to @var{bindings} and then
evaluates all of the @var{forms} in textual order.  The @code{let}-form
returns the value of the last form in @var{forms}.

Each of the @var{bindings} is either @w{(i) a} symbol, in which case
that symbol is bound to @code{nil}; or @w{(ii) a} list of the form
@code{(@var{symbol} @var{value-form})}, in which case @var{symbol} is
bound to the result of evaluating @var{value-form}.  If @var{value-form}
is omitted, @code{nil} is used.

All of the @var{value-form}s in @var{bindings} are evaluated in the
order they appear and @emph{before} binding any of the symbols to them.
Here is an example of this: @code{z} is bound to the old value of
@code{y}, which is 2, not the new value of @code{y}, which is 1.

@example
@group
(setq y 2)
     @result{} 2
@end group
@group
(let ((y 1)
      (z y))
  (list y z))
     @result{} (1 2)
@end group
@end example
@end defspec

@defspec let* (bindings@dots{}) forms@dots{}
This special form is like @code{let}, but it binds each variable right
after computing its local value, before computing the local value for
the next variable.  Therefore, an expression in @var{bindings} can
reasonably refer to the preceding symbols bound in this @code{let*}
form.  Compare the following example with the example above for
@code{let}.

@example
@group
(setq y 2)
     @result{} 2
@end group
@group
(let* ((y 1)
       (z y))    ; @r{Use the just-established value of @code{y}.}
  (list y z))
     @result{} (1 1)
@end group
@end example
@end defspec

  Here is a complete list of the other facilities that create local
bindings:

@itemize @bullet
@item
Function calls (@pxref{Functions}).

@item
Macro calls (@pxref{Macros}).

@item
@code{condition-case} (@pxref{Errors}).
@end itemize

  Variables can also have buffer-local bindings (@pxref{Buffer-Local
Variables}); a few variables have terminal-local bindings
(@pxref{Multiple Displays}).  These kinds of bindings work somewhat
like ordinary local bindings, but they are localized depending on
``where'' you are in Emacs, rather than localized in time.

@defvar max-specpdl-size
@anchor{Definition of max-specpdl-size}
@cindex variable limit error
@cindex evaluation error
@cindex infinite recursion
This variable defines the limit on the total number of local variable
bindings and @code{unwind-protect} cleanups (@pxref{Cleanups,,
Cleaning Up from Nonlocal Exits}) that are allowed before signaling an
error (with data @code{"Variable binding depth exceeds
max-specpdl-size"}).

This limit, with the associated error when it is exceeded, is one way
that Lisp avoids infinite recursion on an ill-defined function.
@code{max-lisp-eval-depth} provides another limit on depth of nesting.
@xref{Definition of max-lisp-eval-depth,, Eval}.

The default value is 1000.  Entry to the Lisp debugger increases the
value, if there is little room left, to make sure the debugger itself
has room to execute.
@end defvar

@node Void Variables
@section When a Variable is ``Void''
@kindex void-variable
@cindex void variable

  If you have never given a symbol any value as a global variable, we
say that that symbol's global value is @dfn{void}.  In other words, the
symbol's value cell does not have any Lisp object in it.  If you try to
evaluate the symbol, you get a @code{void-variable} error rather than
a value.

  Note that a value of @code{nil} is not the same as void.  The symbol
@code{nil} is a Lisp object and can be the value of a variable just as any
other object can be; but it is @emph{a value}.  A void variable does not
have any value.

  After you have given a variable a value, you can make it void once more
using @code{makunbound}.

@defun makunbound symbol
This function makes the current variable binding of @var{symbol} void.
Subsequent attempts to use this symbol's value as a variable will signal
the error @code{void-variable}, unless and until you set it again.

@code{makunbound} returns @var{symbol}.

@example
@group
(makunbound 'x)      ; @r{Make the global value of @code{x} void.}
     @result{} x
@end group
@group
x
@error{} Symbol's value as variable is void: x
@end group
@end example

If @var{symbol} is locally bound, @code{makunbound} affects the most
local existing binding.  This is the only way a symbol can have a void
local binding, since all the constructs that create local bindings
create them with values.  In this case, the voidness lasts at most as
long as the binding does; when the binding is removed due to exit from
the construct that made it, the previous local or global binding is
reexposed as usual, and the variable is no longer void unless the newly
reexposed binding was void all along.

@smallexample
@group
(setq x 1)               ; @r{Put a value in the global binding.}
     @result{} 1
(let ((x 2))             ; @r{Locally bind it.}
  (makunbound 'x)        ; @r{Void the local binding.}
  x)
@error{} Symbol's value as variable is void: x
@end group
@group
x                        ; @r{The global binding is unchanged.}
     @result{} 1

(let ((x 2))             ; @r{Locally bind it.}
  (let ((x 3))           ; @r{And again.}
    (makunbound 'x)      ; @r{Void the innermost-local binding.}
    x))                  ; @r{And refer: it's void.}
@error{} Symbol's value as variable is void: x
@end group

@group
(let ((x 2))
  (let ((x 3))
    (makunbound 'x))     ; @r{Void inner binding, then remove it.}
  x)                     ; @r{Now outer @code{let} binding is visible.}
     @result{} 2
@end group
@end smallexample
@end defun

  A variable that has been made void with @code{makunbound} is
indistinguishable from one that has never received a value and has
always been void.

  You can use the function @code{boundp} to test whether a variable is
currently void.

@defun boundp variable
@code{boundp} returns @code{t} if @var{variable} (a symbol) is not void;
more precisely, if its current binding is not void.  It returns
@code{nil} otherwise.

@smallexample
@group
(boundp 'abracadabra)          ; @r{Starts out void.}
     @result{} nil
@end group
@group
(let ((abracadabra 5))         ; @r{Locally bind it.}
  (boundp 'abracadabra))
     @result{} t
@end group
@group
(boundp 'abracadabra)          ; @r{Still globally void.}
     @result{} nil
@end group
@group
(setq abracadabra 5)           ; @r{Make it globally nonvoid.}
     @result{} 5
@end group
@group
(boundp 'abracadabra)
     @result{} t
@end group
@end smallexample
@end defun

@node Defining Variables
@section Defining Global Variables
@cindex variable definition

  You may announce your intention to use a symbol as a global variable
with a @dfn{variable definition}: a special form, either @code{defconst}
or @code{defvar}.

  In Emacs Lisp, definitions serve three purposes.  First, they inform
people who read the code that certain symbols are @emph{intended} to be
used a certain way (as variables).  Second, they inform the Lisp system
of these things, supplying a value and documentation.  Third, they
provide information to utilities such as @code{etags} and
@code{make-docfile}, which create data bases of the functions and
variables in a program.

  The difference between @code{defconst} and @code{defvar} is primarily
a matter of intent, serving to inform human readers of whether the value
should ever change.  Emacs Lisp does not restrict the ways in which a
variable can be used based on @code{defconst} or @code{defvar}
declarations.  However, it does make a difference for initialization:
@code{defconst} unconditionally initializes the variable, while
@code{defvar} initializes it only if it is void.

@ignore
  One would expect user option variables to be defined with
@code{defconst}, since programs do not change them.  Unfortunately, this
has bad results if the definition is in a library that is not preloaded:
@code{defconst} would override any prior value when the library is
loaded.  Users would like to be able to set user options in their init
files, and override the default values given in the definitions.  For
this reason, user options must be defined with @code{defvar}.
@end ignore

@defspec defvar symbol [value [doc-string]]
This special form defines @var{symbol} as a variable and can also
initialize and document it.  The definition informs a person reading
your code that @var{symbol} is used as a variable that might be set or
changed.  Note that @var{symbol} is not evaluated; the symbol to be
defined must appear explicitly in the @code{defvar}.

If @var{symbol} is void and @var{value} is specified, @code{defvar}
evaluates it and sets @var{symbol} to the result.  But if @var{symbol}
already has a value (i.e., it is not void), @var{value} is not even
evaluated, and @var{symbol}'s value remains unchanged.  If @var{value}
is omitted, the value of @var{symbol} is not changed in any case.

If @var{symbol} has a buffer-local binding in the current buffer,
@code{defvar} operates on the default value, which is buffer-independent,
not the current (buffer-local) binding.  It sets the default value if
the default value is void.  @xref{Buffer-Local Variables}.

When you evaluate a top-level @code{defvar} form with @kbd{C-M-x} in
Emacs Lisp mode (@code{eval-defun}), a special feature of
@code{eval-defun} arranges to set the variable unconditionally, without
testing whether its value is void.

If the @var{doc-string} argument appears, it specifies the documentation
for the variable.  (This opportunity to specify documentation is one of
the main benefits of defining the variable.)  The documentation is
stored in the symbol's @code{variable-documentation} property.  The
Emacs help functions (@pxref{Documentation}) look for this property.

If the variable is a user option that users would want to set
interactively, you should use @samp{*} as the first character of
@var{doc-string}.  This lets users set the variable conveniently using
the @code{set-variable} command.  Note that you should nearly always
use @code{defcustom} instead of @code{defvar} to define these
variables, so that users can use @kbd{M-x customize} and related
commands to set them.  @xref{Customization}.

Here are some examples.  This form defines @code{foo} but does not
initialize it:

@example
@group
(defvar foo)
     @result{} foo
@end group
@end example

This example initializes the value of @code{bar} to @code{23}, and gives
it a documentation string:

@example
@group
(defvar bar 23
  "The normal weight of a bar.")
     @result{} bar
@end group
@end example

The following form changes the documentation string for @code{bar},
making it a user option, but does not change the value, since @code{bar}
already has a value.  (The addition @code{(1+ nil)} would get an error
if it were evaluated, but since it is not evaluated, there is no error.)

@example
@group
(defvar bar (1+ nil)
  "*The normal weight of a bar.")
     @result{} bar
@end group
@group
bar
     @result{} 23
@end group
@end example

Here is an equivalent expression for the @code{defvar} special form:

@example
@group
(defvar @var{symbol} @var{value} @var{doc-string})
@equiv{}
(progn
  (if (not (boundp '@var{symbol}))
      (setq @var{symbol} @var{value}))
  (if '@var{doc-string}
    (put '@var{symbol} 'variable-documentation '@var{doc-string}))
  '@var{symbol})
@end group
@end example

The @code{defvar} form returns @var{symbol}, but it is normally used
at top level in a file where its value does not matter.
@end defspec

@defspec defconst symbol value [doc-string]
This special form defines @var{symbol} as a value and initializes it.
It informs a person reading your code that @var{symbol} has a standard
global value, established here, that should not be changed by the user
or by other programs.  Note that @var{symbol} is not evaluated; the
symbol to be defined must appear explicitly in the @code{defconst}.

@code{defconst} always evaluates @var{value}, and sets the value of
@var{symbol} to the result.  If @var{symbol} does have a buffer-local
binding in the current buffer, @code{defconst} sets the default value,
not the buffer-local value.  (But you should not be making
buffer-local bindings for a symbol that is defined with
@code{defconst}.)

Here, @code{pi} is a constant that presumably ought not to be changed
by anyone (attempts by the Indiana State Legislature notwithstanding).
As the second form illustrates, however, this is only advisory.

@example
@group
(defconst pi 3.1415 "Pi to five places.")
     @result{} pi
@end group
@group
(setq pi 3)
     @result{} pi
@end group
@group
pi
     @result{} 3
@end group
@end example
@end defspec

@defun user-variable-p variable
@cindex user option
This function returns @code{t} if @var{variable} is a user option---a
variable intended to be set by the user for customization---and
@code{nil} otherwise.  (Variables other than user options exist for the
internal purposes of Lisp programs, and users need not know about them.)

User option variables are distinguished from other variables either
though being declared using @code{defcustom}@footnote{They may also be
declared equivalently in @file{cus-start.el}.} or by the first character
of their @code{variable-documentation} property.  If the property exists
and is a string, and its first character is @samp{*}, then the variable
is a user option.  Aliases of user options are also user options.
@end defun

@kindex variable-interactive
  If a user option variable has a @code{variable-interactive} property,
the @code{set-variable} command uses that value to control reading the
new value for the variable.  The property's value is used as if it were
specified in @code{interactive} (@pxref{Using Interactive}).  However,
this feature is largely obsoleted by @code{defcustom}
(@pxref{Customization}).

  @strong{Warning:} If the @code{defconst} and @code{defvar} special
forms are used while the variable has a local binding (made with
@code{let}, or a function argument), they set the local-binding's
value; the top-level binding is not changed.  This is not what you
usually want.  To prevent it, use these special forms at top level in
a file, where normally no local binding is in effect, and make sure to
load the file before making a local binding for the variable.

@node Tips for Defining
@section Tips for Defining Variables Robustly

  When you define a variable whose value is a function, or a list of
functions, use a name that ends in @samp{-function} or
@samp{-functions}, respectively.

  There are several other variable name conventions;
here is a complete list:

@table @samp
@item @dots{}-hook
The variable is a normal hook (@pxref{Hooks}).

@item @dots{}-function
The value is a function.

@item @dots{}-functions
The value is a list of functions.

@item @dots{}-form
The value is a form (an expression).

@item @dots{}-forms
The value is a list of forms (expressions).

@item @dots{}-predicate
The value is a predicate---a function of one argument that returns
non-@code{nil} for ``good'' arguments and @code{nil} for ``bad''
arguments.

@item @dots{}-flag
The value is significant only as to whether it is @code{nil} or not.

@item @dots{}-program
The value is a program name.

@item @dots{}-command
The value is a whole shell command.

@item @dots{}-switches
The value specifies options for a command.
@end table

  When you define a variable, always consider whether you should mark
it as ``risky''; see @ref{File Local Variables}.

  When defining and initializing a variable that holds a complicated
value (such as a keymap with bindings in it), it's best to put the
entire computation of the value into the @code{defvar}, like this:

@example
(defvar my-mode-map
  (let ((map (make-sparse-keymap)))
    (define-key map "\C-c\C-a" 'my-command)
    @dots{}
    map)
  @var{docstring})
@end example

@noindent
This method has several benefits.  First, if the user quits while
loading the file, the variable is either still uninitialized or
initialized properly, never in-between.  If it is still uninitialized,
reloading the file will initialize it properly.  Second, reloading the
file once the variable is initialized will not alter it; that is
important if the user has run hooks to alter part of the contents (such
as, to rebind keys).  Third, evaluating the @code{defvar} form with
@kbd{C-M-x} @emph{will} reinitialize the map completely.

  Putting so much code in the @code{defvar} form has one disadvantage:
it puts the documentation string far away from the line which names the
variable.  Here's a safe way to avoid that:

@example
(defvar my-mode-map nil
  @var{docstring})
(unless my-mode-map
  (let ((map (make-sparse-keymap)))
    (define-key map "\C-c\C-a" 'my-command)
    @dots{}
    (setq my-mode-map map)))
@end example

@noindent
This has all the same advantages as putting the initialization inside
the @code{defvar}, except that you must type @kbd{C-M-x} twice, once on
each form, if you do want to reinitialize the variable.

  But be careful not to write the code like this:

@example
(defvar my-mode-map nil
  @var{docstring})
(unless my-mode-map
  (setq my-mode-map (make-sparse-keymap))
  (define-key my-mode-map "\C-c\C-a" 'my-command)
  @dots{})
@end example

@noindent
This code sets the variable, then alters it, but it does so in more than
one step.  If the user quits just after the @code{setq}, that leaves the
variable neither correctly initialized nor void nor @code{nil}.  Once
that happens, reloading the file will not initialize the variable; it
will remain incomplete.

@node Accessing Variables
@section Accessing Variable Values

  The usual way to reference a variable is to write the symbol which
names it (@pxref{Symbol Forms}).  This requires you to specify the
variable name when you write the program.  Usually that is exactly what
you want to do.  Occasionally you need to choose at run time which
variable to reference; then you can use @code{symbol-value}.

@defun symbol-value symbol
This function returns the value of @var{symbol}.  This is the value in
the innermost local binding of the symbol, or its global value if it
has no local bindings.

@example
@group
(setq abracadabra 5)
     @result{} 5
@end group
@group
(setq foo 9)
     @result{} 9
@end group

@group
;; @r{Here the symbol @code{abracadabra}}
;;   @r{is the symbol whose value is examined.}
(let ((abracadabra 'foo))
  (symbol-value 'abracadabra))
     @result{} foo
@end group

@group
;; @r{Here, the value of @code{abracadabra},}
;;   @r{which is @code{foo},}
;;   @r{is the symbol whose value is examined.}
(let ((abracadabra 'foo))
  (symbol-value abracadabra))
     @result{} 9
@end group

@group
(symbol-value 'abracadabra)
     @result{} 5
@end group
@end example

A @code{void-variable} error is signaled if the current binding of
@var{symbol} is void.
@end defun

@node Setting Variables
@section How to Alter a Variable Value

  The usual way to change the value of a variable is with the special
form @code{setq}.  When you need to compute the choice of variable at
run time, use the function @code{set}.

@defspec setq [symbol form]@dots{}
This special form is the most common method of changing a variable's
value.  Each @var{symbol} is given a new value, which is the result of
evaluating the corresponding @var{form}.  The most-local existing
binding of the symbol is changed.

@code{setq} does not evaluate @var{symbol}; it sets the symbol that you
write.  We say that this argument is @dfn{automatically quoted}.  The
@samp{q} in @code{setq} stands for ``quoted.''

The value of the @code{setq} form is the value of the last @var{form}.

@example
@group
(setq x (1+ 2))
     @result{} 3
@end group
x                   ; @r{@code{x} now has a global value.}
     @result{} 3
@group
(let ((x 5))
  (setq x 6)        ; @r{The local binding of @code{x} is set.}
  x)
     @result{} 6
@end group
x                   ; @r{The global value is unchanged.}
     @result{} 3
@end example

Note that the first @var{form} is evaluated, then the first
@var{symbol} is set, then the second @var{form} is evaluated, then the
second @var{symbol} is set, and so on:

@example
@group
(setq x 10          ; @r{Notice that @code{x} is set before}
      y (1+ x))     ;   @r{the value of @code{y} is computed.}
     @result{} 11
@end group
@end example
@end defspec

@defun set symbol value
This function sets @var{symbol}'s value to @var{value}, then returns
@var{value}.  Since @code{set} is a function, the expression written for
@var{symbol} is evaluated to obtain the symbol to set.

The most-local existing binding of the variable is the binding that is
set; shadowed bindings are not affected.

@example
@group
(set one 1)
@error{} Symbol's value as variable is void: one
@end group
@group
(set 'one 1)
     @result{} 1
@end group
@group
(set 'two 'one)
     @result{} one
@end group
@group
(set two 2)         ; @r{@code{two} evaluates to symbol @code{one}.}
     @result{} 2
@end group
@group
one                 ; @r{So it is @code{one} that was set.}
     @result{} 2
(let ((one 1))      ; @r{This binding of @code{one} is set,}
  (set 'one 3)      ;   @r{not the global value.}
  one)
     @result{} 3
@end group
@group
one
     @result{} 2
@end group
@end example

If @var{symbol} is not actually a symbol, a @code{wrong-type-argument}
error is signaled.

@example
(set '(x y) 'z)
@error{} Wrong type argument: symbolp, (x y)
@end example

Logically speaking, @code{set} is a more fundamental primitive than
@code{setq}.  Any use of @code{setq} can be trivially rewritten to use
@code{set}; @code{setq} could even be defined as a macro, given the
availability of @code{set}.  However, @code{set} itself is rarely used;
beginners hardly need to know about it.  It is useful only for choosing
at run time which variable to set.  For example, the command
@code{set-variable}, which reads a variable name from the user and then
sets the variable, needs to use @code{set}.

@cindex CL note---@code{set} local
@quotation
@b{Common Lisp note:} In Common Lisp, @code{set} always changes the
symbol's ``special'' or dynamic value, ignoring any lexical bindings.
In Emacs Lisp, all variables and all bindings are dynamic, so @code{set}
always affects the most local existing binding.
@end quotation
@end defun

@node Variable Scoping
@section Scoping Rules for Variable Bindings

  A given symbol @code{foo} can have several local variable bindings,
established at different places in the Lisp program, as well as a global
binding.  The most recently established binding takes precedence over
the others.

@cindex scope
@cindex extent
@cindex dynamic scoping
@cindex lexical scoping
  Local bindings in Emacs Lisp have @dfn{indefinite scope} and
@dfn{dynamic extent}.  @dfn{Scope} refers to @emph{where} textually in
the source code the binding can be accessed.  ``Indefinite scope'' means
that any part of the program can potentially access the variable
binding.  @dfn{Extent} refers to @emph{when}, as the program is
executing, the binding exists.  ``Dynamic extent'' means that the binding
lasts as long as the activation of the construct that established it.

  The combination of dynamic extent and indefinite scope is called
@dfn{dynamic scoping}.  By contrast, most programming languages use
@dfn{lexical scoping}, in which references to a local variable must be
located textually within the function or block that binds the variable.

@cindex CL note---special variables
@quotation
@b{Common Lisp note:} Variables declared ``special'' in Common Lisp are
dynamically scoped, like all variables in Emacs Lisp.
@end quotation

@menu
* Scope::          Scope means where in the program a value is visible.
                     Comparison with other languages.
* Extent::         Extent means how long in time a value exists.
* Impl of Scope::  Two ways to implement dynamic scoping.
* Using Scoping::  How to use dynamic scoping carefully and avoid problems.
@end menu

@node Scope
@subsection Scope

  Emacs Lisp uses @dfn{indefinite scope} for local variable bindings.
This means that any function anywhere in the program text might access a
given binding of a variable.  Consider the following function
definitions:

@example
@group
(defun binder (x)   ; @r{@code{x} is bound in @code{binder}.}
   (foo 5))         ; @r{@code{foo} is some other function.}
@end group

@group
(defun user ()      ; @r{@code{x} is used ``free'' in @code{user}.}
  (list x))
@end group
@end example

  In a lexically scoped language, the binding of @code{x} in
@code{binder} would never be accessible in @code{user}, because
@code{user} is not textually contained within the function
@code{binder}.  However, in dynamically-scoped Emacs Lisp, @code{user}
may or may not refer to the binding of @code{x} established in
@code{binder}, depending on the circumstances:

@itemize @bullet
@item
If we call @code{user} directly without calling @code{binder} at all,
then whatever binding of @code{x} is found, it cannot come from
@code{binder}.

@item
If we define @code{foo} as follows and then call @code{binder}, then the
binding made in @code{binder} will be seen in @code{user}:

@example
@group
(defun foo (lose)
  (user))
@end group
@end example

@item
However, if we define @code{foo} as follows and then call @code{binder},
then the binding made in @code{binder} @emph{will not} be seen in
@code{user}:

@example
(defun foo (x)
  (user))
@end example

@noindent
Here, when @code{foo} is called by @code{binder}, it binds @code{x}.
(The binding in @code{foo} is said to @dfn{shadow} the one made in
@code{binder}.)  Therefore, @code{user} will access the @code{x} bound
by @code{foo} instead of the one bound by @code{binder}.
@end itemize

Emacs Lisp uses dynamic scoping because simple implementations of
lexical scoping are slow.  In addition, every Lisp system needs to offer
dynamic scoping at least as an option; if lexical scoping is the norm,
there must be a way to specify dynamic scoping instead for a particular
variable.  It might not be a bad thing for Emacs to offer both, but
implementing it with dynamic scoping only was much easier.

@node Extent
@subsection Extent

  @dfn{Extent} refers to the time during program execution that a
variable name is valid.  In Emacs Lisp, a variable is valid only while
the form that bound it is executing.  This is called @dfn{dynamic
extent}.  ``Local'' or ``automatic'' variables in most languages,
including C and Pascal, have dynamic extent.

  One alternative to dynamic extent is @dfn{indefinite extent}.  This
means that a variable binding can live on past the exit from the form
that made the binding.  Common Lisp and Scheme, for example, support
this, but Emacs Lisp does not.

  To illustrate this, the function below, @code{make-add}, returns a
function that purports to add @var{n} to its own argument @var{m}.  This
would work in Common Lisp, but it does not do the job in Emacs Lisp,
because after the call to @code{make-add} exits, the variable @code{n}
is no longer bound to the actual argument 2.

@example
(defun make-add (n)
    (function (lambda (m) (+ n m))))  ; @r{Return a function.}
     @result{} make-add
(fset 'add2 (make-add 2))  ; @r{Define function @code{add2}}
                           ;   @r{with @code{(make-add 2)}.}
     @result{} (lambda (m) (+ n m))
(add2 4)                   ; @r{Try to add 2 to 4.}
@error{} Symbol's value as variable is void: n
@end example

@cindex closures not available
  Some Lisp dialects have ``closures,'' objects that are like functions
but record additional variable bindings.  Emacs Lisp does not have
closures.

@node Impl of Scope
@subsection Implementation of Dynamic Scoping
@cindex deep binding

  A simple sample implementation (which is not how Emacs Lisp actually
works) may help you understand dynamic binding.  This technique is
called @dfn{deep binding} and was used in early Lisp systems.

  Suppose there is a stack of bindings, which are variable-value pairs.
At entry to a function or to a @code{let} form, we can push bindings
onto the stack for the arguments or local variables created there.  We
can pop those bindings from the stack at exit from the binding
construct.

  We can find the value of a variable by searching the stack from top to
bottom for a binding for that variable; the value from that binding is
the value of the variable.  To set the variable, we search for the
current binding, then store the new value into that binding.

  As you can see, a function's bindings remain in effect as long as it
continues execution, even during its calls to other functions.  That is
why we say the extent of the binding is dynamic.  And any other function
can refer to the bindings, if it uses the same variables while the
bindings are in effect.  That is why we say the scope is indefinite.

@cindex shallow binding
  The actual implementation of variable scoping in GNU Emacs Lisp uses a
technique called @dfn{shallow binding}.  Each variable has a standard
place in which its current value is always found---the value cell of the
symbol.

  In shallow binding, setting the variable works by storing a value in
the value cell.  Creating a new binding works by pushing the old value
(belonging to a previous binding) onto a stack, and storing the new
local value in the value cell.  Eliminating a binding works by popping
the old value off the stack, into the value cell.

  We use shallow binding because it has the same results as deep
binding, but runs faster, since there is never a need to search for a
binding.

@node Using Scoping
@subsection Proper Use of Dynamic Scoping

  Binding a variable in one function and using it in another is a
powerful technique, but if used without restraint, it can make programs
hard to understand.  There are two clean ways to use this technique:

@itemize @bullet
@item
Use or bind the variable only in a few related functions, written close
together in one file.  Such a variable is used for communication within
one program.

You should write comments to inform other programmers that they can see
all uses of the variable before them, and to advise them not to add uses
elsewhere.

@item
Give the variable a well-defined, documented meaning, and make all
appropriate functions refer to it (but not bind it or set it) wherever
that meaning is relevant.  For example, the variable
@code{case-fold-search} is defined as ``non-@code{nil} means ignore case
when searching''; various search and replace functions refer to it
directly or through their subroutines, but do not bind or set it.

Then you can bind the variable in other programs, knowing reliably what
the effect will be.
@end itemize

  In either case, you should define the variable with @code{defvar}.
This helps other people understand your program by telling them to look
for inter-function usage.  It also avoids a warning from the byte
compiler.  Choose the variable's name to avoid name conflicts---don't
use short names like @code{x}.

@node Buffer-Local Variables
@section Buffer-Local Variables
@cindex variable, buffer-local
@cindex buffer-local variables

  Global and local variable bindings are found in most programming
languages in one form or another.  Emacs, however, also supports
additional, unusual kinds of variable binding, such as
@dfn{buffer-local} bindings, which apply only in one buffer.  Having
different values for a variable in different buffers is an important
customization method.  (A few variables have bindings that are local
to each terminal; see @ref{Multiple Displays}.)

@menu
* Intro to Buffer-Local::      Introduction and concepts.
* Creating Buffer-Local::      Creating and destroying buffer-local bindings.
* Default Value::              The default value is seen in buffers
                                 that don't have their own buffer-local values.
@end menu

@node Intro to Buffer-Local
@subsection Introduction to Buffer-Local Variables

  A buffer-local variable has a buffer-local binding associated with a
particular buffer.  The binding is in effect when that buffer is
current; otherwise, it is not in effect.  If you set the variable while
a buffer-local binding is in effect, the new value goes in that binding,
so its other bindings are unchanged.  This means that the change is
visible only in the buffer where you made it.

  The variable's ordinary binding, which is not associated with any
specific buffer, is called the @dfn{default binding}.  In most cases,
this is the global binding.

  A variable can have buffer-local bindings in some buffers but not in
other buffers.  The default binding is shared by all the buffers that
don't have their own bindings for the variable.  (This includes all
newly-created buffers.)  If you set the variable in a buffer that does
not have a buffer-local binding for it, this sets the default binding,
so the new value is visible in all the buffers that see the default
binding.

  The most common use of buffer-local bindings is for major modes to change
variables that control the behavior of commands.  For example, C mode and
Lisp mode both set the variable @code{paragraph-start} to specify that only
blank lines separate paragraphs.  They do this by making the variable
buffer-local in the buffer that is being put into C mode or Lisp mode, and
then setting it to the new value for that mode.  @xref{Major Modes}.

  The usual way to make a buffer-local binding is with
@code{make-local-variable}, which is what major mode commands typically
use.  This affects just the current buffer; all other buffers (including
those yet to be created) will continue to share the default value unless
they are explicitly given their own buffer-local bindings.

@cindex automatically buffer-local
  A more powerful operation is to mark the variable as
@dfn{automatically buffer-local} by calling
@code{make-variable-buffer-local}.  You can think of this as making the
variable local in all buffers, even those yet to be created.  More
precisely, the effect is that setting the variable automatically makes
the variable local to the current buffer if it is not already so.  All
buffers start out by sharing the default value of the variable as usual,
but setting the variable creates a buffer-local binding for the current
buffer.  The new value is stored in the buffer-local binding, leaving
the default binding untouched.  This means that the default value cannot
be changed with @code{setq} in any buffer; the only way to change it is
with @code{setq-default}.

  @strong{Warning:} When a variable has buffer-local
bindings in one or more buffers, @code{let} rebinds the binding that's
currently in effect.  For instance, if the current buffer has a
buffer-local value, @code{let} temporarily rebinds that.  If no
buffer-local bindings are in effect, @code{let} rebinds
the default value.  If inside the @code{let} you then change to a
different current buffer in which a different binding is in effect,
you won't see the @code{let} binding any more.  And if you exit the
@code{let} while still in the other buffer, you won't see the
unbinding occur (though it will occur properly).  Here is an example
to illustrate:

@example
@group
(setq foo 'g)
(set-buffer "a")
(make-local-variable 'foo)
@end group
(setq foo 'a)
(let ((foo 'temp))
  ;; foo @result{} 'temp  ; @r{let binding in buffer @samp{a}}
  (set-buffer "b")
  ;; foo @result{} 'g     ; @r{the global value since foo is not local in @samp{b}}
  @var{body}@dots{})
@group
foo @result{} 'g        ; @r{exiting restored the local value in buffer @samp{a},}
                 ; @r{but we don't see that in buffer @samp{b}}
@end group
@group
(set-buffer "a") ; @r{verify the local value was restored}
foo @result{} 'a
@end group
@end example

  Note that references to @code{foo} in @var{body} access the
buffer-local binding of buffer @samp{b}.

  When a file specifies local variable values, these become buffer-local
values when you visit the file.  @xref{File Variables,,, emacs, The
GNU Emacs Manual}.

@node Creating Buffer-Local
@subsection Creating and Deleting Buffer-Local Bindings

@deffn Command make-local-variable variable
This function creates a buffer-local binding in the current buffer for
@var{variable} (a symbol).  Other buffers are not affected.  The value
returned is @var{variable}.

@c Emacs 19 feature
The buffer-local value of @var{variable} starts out as the same value
@var{variable} previously had.  If @var{variable} was void, it remains
void.

@example
@group
;; @r{In buffer @samp{b1}:}
(setq foo 5)                ; @r{Affects all buffers.}
     @result{} 5
@end group
@group
(make-local-variable 'foo)  ; @r{Now it is local in @samp{b1}.}
     @result{} foo
@end group
@group
foo                         ; @r{That did not change}
     @result{} 5                   ;   @r{the value.}
@end group
@group
(setq foo 6)                ; @r{Change the value}
     @result{} 6                   ;   @r{in @samp{b1}.}
@end group
@group
foo
     @result{} 6
@end group

@group
;; @r{In buffer @samp{b2}, the value hasn't changed.}
(save-excursion
  (set-buffer "b2")
  foo)
     @result{} 5
@end group
@end example

Making a variable buffer-local within a @code{let}-binding for that
variable does not work reliably, unless the buffer in which you do this
is not current either on entry to or exit from the @code{let}.  This is
because @code{let} does not distinguish between different kinds of
bindings; it knows only which variable the binding was made for.

If the variable is terminal-local, this function signals an error.  Such
variables cannot have buffer-local bindings as well.  @xref{Multiple
Displays}.

@strong{Warning:} do not use @code{make-local-variable} for a hook
variable.  The hook variables are automatically made buffer-local as
needed if you use the @var{local} argument to @code{add-hook} or
@code{remove-hook}.
@end deffn

@deffn Command make-variable-buffer-local variable
This function marks @var{variable} (a symbol) automatically
buffer-local, so that any subsequent attempt to set it will make it
local to the current buffer at the time.

A peculiar wrinkle of this feature is that binding the variable (with
@code{let} or other binding constructs) does not create a buffer-local
binding for it.  Only setting the variable (with @code{set} or
@code{setq}), while the variable does not have a @code{let}-style
binding that was made in the current buffer, does so.

If @var{variable} does not have a default value, then calling this
command will give it a default value of @code{nil}.  If @var{variable}
already has a default value, that value remains unchanged.
Subsequently calling @code{makunbound} on @var{variable} will result
in a void buffer-local value and leave the default value unaffected.

The value returned is @var{variable}.

@strong{Warning:} Don't assume that you should use
@code{make-variable-buffer-local} for user-option variables, simply
because users @emph{might} want to customize them differently in
different buffers.  Users can make any variable local, when they wish
to.  It is better to leave the choice to them.

The time to use @code{make-variable-buffer-local} is when it is crucial
that no two buffers ever share the same binding.  For example, when a
variable is used for internal purposes in a Lisp program which depends
on having separate values in separate buffers, then using
@code{make-variable-buffer-local} can be the best solution.
@end deffn

@defun local-variable-p variable &optional buffer
This returns @code{t} if @var{variable} is buffer-local in buffer
@var{buffer} (which defaults to the current buffer); otherwise,
@code{nil}.
@end defun

@defun local-variable-if-set-p variable &optional buffer
This returns @code{t} if @var{variable} will become buffer-local in
buffer @var{buffer} (which defaults to the current buffer) if it is
set there.
@end defun

@defun buffer-local-value variable buffer
This function returns the buffer-local binding of @var{variable} (a
symbol) in buffer @var{buffer}.  If @var{variable} does not have a
buffer-local binding in buffer @var{buffer}, it returns the default
value (@pxref{Default Value}) of @var{variable} instead.
@end defun

@defun buffer-local-variables &optional buffer
This function returns a list describing the buffer-local variables in
buffer @var{buffer}.  (If @var{buffer} is omitted, the current buffer is
used.)  It returns an association list (@pxref{Association Lists}) in
which each element contains one buffer-local variable and its value.
However, when a variable's buffer-local binding in @var{buffer} is void,
then the variable appears directly in the resulting list.

@example
@group
(make-local-variable 'foobar)
(makunbound 'foobar)
(make-local-variable 'bind-me)
(setq bind-me 69)
@end group
(setq lcl (buffer-local-variables))
    ;; @r{First, built-in variables local in all buffers:}
@result{} ((mark-active . nil)
    (buffer-undo-list . nil)
    (mode-name . "Fundamental")
    @dots{}
@group
    ;; @r{Next, non-built-in buffer-local variables.}
    ;; @r{This one is buffer-local and void:}
    foobar
    ;; @r{This one is buffer-local and nonvoid:}
    (bind-me . 69))
@end group
@end example

Note that storing new values into the @sc{cdr}s of cons cells in this
list does @emph{not} change the buffer-local values of the variables.
@end defun

@deffn Command kill-local-variable variable
This function deletes the buffer-local binding (if any) for
@var{variable} (a symbol) in the current buffer.  As a result, the
default binding of @var{variable} becomes visible in this buffer.  This
typically results in a change in the value of @var{variable}, since the
default value is usually different from the buffer-local value just
eliminated.

If you kill the buffer-local binding of a variable that automatically
becomes buffer-local when set, this makes the default value visible in
the current buffer.  However, if you set the variable again, that will
once again create a buffer-local binding for it.

@code{kill-local-variable} returns @var{variable}.

This function is a command because it is sometimes useful to kill one
buffer-local variable interactively, just as it is useful to create
buffer-local variables interactively.
@end deffn

@defun kill-all-local-variables
This function eliminates all the buffer-local variable bindings of the
current buffer except for variables marked as ``permanent.''  As a
result, the buffer will see the default values of most variables.

This function also resets certain other information pertaining to the
buffer: it sets the local keymap to @code{nil}, the syntax table to the
value of @code{(standard-syntax-table)}, the case table to
@code{(standard-case-table)}, and the abbrev table to the value of
@code{fundamental-mode-abbrev-table}.

The very first thing this function does is run the normal hook
@code{change-major-mode-hook} (see below).

Every major mode command begins by calling this function, which has the
effect of switching to Fundamental mode and erasing most of the effects
of the previous major mode.  To ensure that this does its job, the
variables that major modes set should not be marked permanent.

@code{kill-all-local-variables} returns @code{nil}.
@end defun

@defvar change-major-mode-hook
The function @code{kill-all-local-variables} runs this normal hook
before it does anything else.  This gives major modes a way to arrange
for something special to be done if the user switches to a different
major mode.  It is also useful for buffer-specific minor modes
that should be forgotten if the user changes the major mode.

For best results, make this variable buffer-local, so that it will
disappear after doing its job and will not interfere with the
subsequent major mode.  @xref{Hooks}.
@end defvar

@c Emacs 19 feature
@cindex permanent local variable
A buffer-local variable is @dfn{permanent} if the variable name (a
symbol) has a @code{permanent-local} property that is non-@code{nil}.
Permanent locals are appropriate for data pertaining to where the file
came from or how to save it, rather than with how to edit the contents.

@node Default Value
@subsection The Default Value of a Buffer-Local Variable
@cindex default value

  The global value of a variable with buffer-local bindings is also
called the @dfn{default} value, because it is the value that is in
effect whenever neither the current buffer nor the selected frame has
its own binding for the variable.

  The functions @code{default-value} and @code{setq-default} access and
change a variable's default value regardless of whether the current
buffer has a buffer-local binding.  For example, you could use
@code{setq-default} to change the default setting of
@code{paragraph-start} for most buffers; and this would work even when
you are in a C or Lisp mode buffer that has a buffer-local value for
this variable.

@c Emacs 19 feature
  The special forms @code{defvar} and @code{defconst} also set the
default value (if they set the variable at all), rather than any
buffer-local value.

@defun default-value symbol
This function returns @var{symbol}'s default value.  This is the value
that is seen in buffers and frames that do not have their own values for
this variable.  If @var{symbol} is not buffer-local, this is equivalent
to @code{symbol-value} (@pxref{Accessing Variables}).
@end defun

@c Emacs 19 feature
@defun default-boundp symbol
The function @code{default-boundp} tells you whether @var{symbol}'s
default value is nonvoid.  If @code{(default-boundp 'foo)} returns
@code{nil}, then @code{(default-value 'foo)} would get an error.

@code{default-boundp} is to @code{default-value} as @code{boundp} is to
@code{symbol-value}.
@end defun

@defspec setq-default [symbol form]@dots{}
This special form gives each @var{symbol} a new default value, which is
the result of evaluating the corresponding @var{form}.  It does not
evaluate @var{symbol}, but does evaluate @var{form}.  The value of the
@code{setq-default} form is the value of the last @var{form}.

If a @var{symbol} is not buffer-local for the current buffer, and is not
marked automatically buffer-local, @code{setq-default} has the same
effect as @code{setq}.  If @var{symbol} is buffer-local for the current
buffer, then this changes the value that other buffers will see (as long
as they don't have a buffer-local value), but not the value that the
current buffer sees.

@example
@group
;; @r{In buffer @samp{foo}:}
(make-local-variable 'buffer-local)
     @result{} buffer-local
@end group
@group
(setq buffer-local 'value-in-foo)
     @result{} value-in-foo
@end group
@group
(setq-default buffer-local 'new-default)
     @result{} new-default
@end group
@group
buffer-local
     @result{} value-in-foo
@end group
@group
(default-value 'buffer-local)
     @result{} new-default
@end group

@group
;; @r{In (the new) buffer @samp{bar}:}
buffer-local
     @result{} new-default
@end group
@group
(default-value 'buffer-local)
     @result{} new-default
@end group
@group
(setq buffer-local 'another-default)
     @result{} another-default
@end group
@group
(default-value 'buffer-local)
     @result{} another-default
@end group

@group
;; @r{Back in buffer @samp{foo}:}
buffer-local
     @result{} value-in-foo
(default-value 'buffer-local)
     @result{} another-default
@end group
@end example
@end defspec

@defun set-default symbol value
This function is like @code{setq-default}, except that @var{symbol} is
an ordinary evaluated argument.

@example
@group
(set-default (car '(a b c)) 23)
     @result{} 23
@end group
@group
(default-value 'a)
     @result{} 23
@end group
@end example
@end defun

@node Future Local Variables
@section Possible Future Local Variables

  We have considered the idea of bindings that are local to a category
of frames---for example, all color frames, or all frames with dark
backgrounds.  We have not implemented them because it is not clear that
this feature is really useful.  You can get more or less the same
results by adding a function to @code{after-make-frame-functions}, set up to
define a particular frame parameter according to the appropriate
conditions for each frame.

  It would also be possible to implement window-local bindings.  We
don't know of many situations where they would be useful, and it seems
that indirect buffers (@pxref{Indirect Buffers}) with buffer-local
bindings offer a way to handle these situations more robustly.

  If sufficient application is found for either of these two kinds of
local bindings, we will provide it in a subsequent Emacs version.

@node File Local Variables
@section File Local Variables
@cindex file local variables

  A file can specify local variable values; Emacs uses these to create
buffer-local bindings for those variables in the buffer visiting that
file.  @xref{File variables, , Local Variables in Files, emacs, The
GNU Emacs Manual}, for basic information about file local variables.
This section describes the functions and variables that affect
processing of file local variables.

@defopt enable-local-variables
This variable controls whether to process file local variables.
The possible values are:

@table @asis
@item @code{t} (the default)
Set the safe variables, and query (once) about any unsafe variables.
@item @code{:safe}
Set only the safe variables and do not query.
@item @code{:all}
Set all the variables and do not query.
@item @code{nil}
Don't set any variables.
@item anything else
Query (once) about all the variables.
@end table
@end defopt

@defun hack-local-variables &optional mode-only
This function parses, and binds or evaluates as appropriate, any local
variables specified by the contents of the current buffer.  The variable
@code{enable-local-variables} has its effect here.  However, this
function does not look for the @samp{mode:} local variable in the
@w{@samp{-*-}} line.  @code{set-auto-mode} does that, also taking
@code{enable-local-variables} into account (@pxref{Auto Major Mode}).

If the optional argument @var{mode-only} is non-@code{nil}, then all
this function does is return @code{t} if the @w{@samp{-*-}} line or
the local variables list specifies a mode and @code{nil} otherwise.
It does not set the mode nor any other file local variable.
@end defun

  If a file local variable could specify a function that would
be called later, or an expression that would be executed later, simply
visiting a file could take over your Emacs.  Emacs takes several
measures to prevent this.

@cindex safe local variable
  You can specify safe values for a variable with a
@code{safe-local-variable} property.  The property has to be
a function of one argument; any value is safe if the function
returns non-@code{nil} given that value.  Many commonly encountered
file variables standardly have @code{safe-local-variable} properties,
including @code{fill-column}, @code{fill-prefix}, and
@code{indent-tabs-mode}.  For boolean-valued variables that are safe,
use @code{booleanp} as the property value.  Lambda expressions should
be quoted so that @code{describe-variable} can display the predicate.

@defopt safe-local-variable-values
This variable provides another way to mark some variable values as
safe.  It is a list of cons cells @code{(@var{var} . @var{val})},
where @var{var} is a variable name and @var{val} is a value which is
safe for that variable.

When Emacs asks the user whether or not to obey a set of file local
variable specifications, the user can choose to mark them as safe.
Doing so adds those variable/value pairs to
@code{safe-local-variable-values}, and saves it to the user's custom
file.
@end defopt

@defun safe-local-variable-p sym val
This function returns non-@code{nil} if it is safe to give @var{sym}
the value @var{val}, based on the above criteria.
@end defun

@c @cindex risky local variable   Duplicates risky-local-variable
  Some variables are considered @dfn{risky}.  A variable whose name
ends in any of @samp{-command}, @samp{-frame-alist}, @samp{-function},
@samp{-functions}, @samp{-hook}, @samp{-hooks}, @samp{-form},
@samp{-forms}, @samp{-map}, @samp{-map-alist}, @samp{-mode-alist},
@samp{-program}, or @samp{-predicate} is considered risky.  The
variables @samp{font-lock-keywords}, @samp{font-lock-keywords}
followed by a digit, and @samp{font-lock-syntactic-keywords} are also
considered risky.  Finally, any variable whose name has a
non-@code{nil} @code{risky-local-variable} property is considered
risky.

@defun risky-local-variable-p sym
This function returns non-@code{nil} if @var{sym} is a risky variable,
based on the above criteria.
@end defun

  If a variable is risky, it will not be entered automatically into
@code{safe-local-variable-values} as described above.  Therefore,
Emacs will always query before setting a risky variable, unless the
user explicitly allows the setting by customizing
@code{safe-local-variable-values} directly.

@defvar ignored-local-variables
This variable holds a list of variables that should not be given local
values by files.  Any value specified for one of these variables is
completely ignored.
@end defvar

  The @samp{Eval:} ``variable'' is also a potential loophole, so Emacs
normally asks for confirmation before handling it.

@defopt enable-local-eval
This variable controls processing of @samp{Eval:} in @samp{-*-} lines
or local variables
lists in files being visited.  A value of @code{t} means process them
unconditionally; @code{nil} means ignore them; anything else means ask
the user what to do for each file.  The default value is @code{maybe}.
@end defopt

@defopt safe-local-eval-forms
This variable holds a list of expressions that are safe to
evaluate when found in the @samp{Eval:} ``variable'' in a file
local variables list.
@end defopt

  If the expression is a function call and the function has a
@code{safe-local-eval-function} property, the property value
determines whether the expression is safe to evaluate.  The property
value can be a predicate to call to test the expression, a list of
such predicates (it's safe if any predicate succeeds), or @code{t}
(always safe provided the arguments are constant).

  Text properties are also potential loopholes, since their values
could include functions to call.  So Emacs discards all text
properties from string values specified for file local variables.

@node Variable Aliases
@section Variable Aliases
@cindex variable aliases

  It is sometimes useful to make two variables synonyms, so that both
variables always have the same value, and changing either one also
changes the other.  Whenever you change the name of a
variable---either because you realize its old name was not well
chosen, or because its meaning has partly changed---it can be useful
to keep the old name as an @emph{alias} of the new one for
compatibility.  You can do this with @code{defvaralias}.

@defun defvaralias new-alias base-variable &optional docstring
This function defines the symbol @var{new-alias} as a variable alias
for symbol @var{base-variable}. This means that retrieving the value
of @var{new-alias} returns the value of @var{base-variable}, and
changing the value of @var{new-alias} changes the value of
@var{base-variable}.  The two aliased variable names always share the
same value and the same bindings.

If the @var{docstring} argument is non-@code{nil}, it specifies the
documentation for @var{new-alias}; otherwise, the alias gets the same
documentation as @var{base-variable} has, if any, unless
@var{base-variable} is itself an alias, in which case @var{new-alias} gets
the documentation of the variable at the end of the chain of aliases.

This function returns @var{base-variable}.
@end defun

  Variable aliases are convenient for replacing an old name for a
variable with a new name.  @code{make-obsolete-variable} declares that
the old name is obsolete and therefore that it may be removed at some
stage in the future.

@defun make-obsolete-variable obsolete-name current-name &optional when
This function makes the byte-compiler warn that the variable
@var{obsolete-name} is obsolete.  If @var{current-name} is a symbol, it is
the variable's new name; then the warning message says to use
@var{current-name} instead of @var{obsolete-name}.  If @var{current-name}
is a string, this is the message and there is no replacement variable.

If provided, @var{when} should be a string indicating when the
variable was first made obsolete---for example, a date or a release
number.
@end defun

  You can make two variables synonyms and declare one obsolete at the
same time using the macro @code{define-obsolete-variable-alias}.

@defmac define-obsolete-variable-alias obsolete-name current-name &optional when docstring
This macro marks the variable @var{obsolete-name} as obsolete and also
makes it an alias for the variable @var{current-name}.  It is
equivalent to the following:

@example
(defvaralias @var{obsolete-name} @var{current-name} @var{docstring})
(make-obsolete-variable @var{obsolete-name} @var{current-name} @var{when})
@end example
@end defmac

@defun indirect-variable variable
This function returns the variable at the end of the chain of aliases
of @var{variable}.  If @var{variable} is not a symbol, or if @var{variable} is
not defined as an alias, the function returns @var{variable}.

This function signals a @code{cyclic-variable-indirection} error if
there is a loop in the chain of symbols.
@end defun

@example
(defvaralias 'foo 'bar)
(indirect-variable 'foo)
     @result{} bar
(indirect-variable 'bar)
     @result{} bar
(setq bar 2)
bar
     @result{} 2
@group
foo
     @result{} 2
@end group
(setq foo 0)
bar
     @result{} 0
foo
     @result{} 0
@end example

@node Variables with Restricted Values
@section Variables with Restricted Values

  Ordinary Lisp variables can be assigned any value that is a valid
Lisp object.  However, certain Lisp variables are not defined in Lisp,
but in C.  Most of these variables are defined in the C code using
@code{DEFVAR_LISP}.  Like variables defined in Lisp, these can take on
any value.  However, some variables are defined using
@code{DEFVAR_INT} or @code{DEFVAR_BOOL}.  @xref{Defining Lisp
variables in C,, Writing Emacs Primitives}, in particular the
description of functions of the type @code{syms_of_@var{filename}},
for a brief discussion of the C implementation.

  Variables of type @code{DEFVAR_BOOL} can only take on the values
@code{nil} or @code{t}.  Attempting to assign them any other value
will set them to @code{t}:

@example
(let ((display-hourglass 5))
  display-hourglass)
     @result{} t
@end example

@defvar byte-boolean-vars
This variable holds a list of all variables of type @code{DEFVAR_BOOL}.
@end defvar

  Variables of type @code{DEFVAR_INT} can only take on integer values.
Attempting to assign them any other value will result in an error:

@example
(setq window-min-height 5.0)
@error{} Wrong type argument: integerp, 5.0
@end example

@ignore
   arch-tag: 5ff62c44-2b51-47bb-99d4-fea5aeec5d3e
@end ignore