Explain when to use make-variable-buffer-local.

[gnu-emacs] / lispref / variables.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994 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::     How to avoid bad results from quitting
                            within the code to initialize 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.
@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 another @code{setq}, 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
@vindex nil
@vindex t
@kindex setting-constant

  Emacs Lisp has two special symbols, @code{nil} and @code{t}, that
always evaluate to themselves.  These symbols cannot be rebound, nor can
their value cells be changed.  An attempt to change the value of
@code{nil} or @code{t} signals a @code{setting-constant} 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

@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 while within a certain part of the program.
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 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 also 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} any of the symbols are bound.  Here
is an example of this: @code{Z} is bound to the old value of @code{Y},
which is 2, not the new value, 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
@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{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.

  The default value is 600.

  @code{max-lisp-eval-depth} provides another limit on depth of nesting.
@xref{Eval}.
@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 binding of @var{symbol} void.
Subsequent attempts to use this symbol's value as a variable will signal
the error @code{void-variable}, unless or until you set it again.

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

@example
@group
(makunbound 'x)      ; @r{Make the global value}
                     ;   @r{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 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 programs
will change the variable.  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.

  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}.

@defspec defvar symbol [value [doc-string]]
This special form defines @var{symbol} as a value and initializes it.
The definition informs a person reading your code that @var{symbol} is
used as a variable that programs are likely to set or change.  It is
also used for all user option variables except in the preloaded parts of
Emacs.  Note that @var{symbol} is not evaluated; the symbol to be
defined must appear explicitly in the @code{defvar}.

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{symbol} is void and @var{value} is specified, @code{defvar}
evaluates it and sets @var{symbol} to the result.  (If @var{value} is
omitted, the value of @var{symbol} is not changed in any case.)

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} evaluates it as a @code{defconst}.  The purpose of
this is to make sure the variable's value is reinitialized, when you ask
for it specifically.

If @var{symbol} has a buffer-local binding in the current buffer,
@code{defvar} sets the default value, not the local value.
@xref{Buffer-Local Variables}.

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 first character of @var{doc-string} is @samp{*}, it means that
this variable is considered a user option.  This lets users set the
variable conventiently using the commands @code{set-variable} and
@code{edit-options}.

For example, this form defines @code{foo} but does not set its value:

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

The following example sets 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+ 23)} is not even
performed.)

@example
@group
(defvar bar (1+ 23)
  "*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 global
value, established here, that will not normally be changed or locally
bound by the execution of the program.  The user, however, may be
welcome to change it.  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 global value
of @var{symbol} to the result, provided @var{value} is given.  If
@var{symbol} has a buffer-local binding in the current buffer,
@code{defconst} sets the default value, not the local value.

@strong{Please note:} Don't use @code{defconst} for user option
variables in libraries that are not standardly preloaded.  The user
should be able to specify a value for such a variable in the
@file{.emacs} file, so that it will be in effect if and when the library
is loaded later.

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 by the
first character of the @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.
@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
to @code{interactive} (@pxref{Using Interactive}).

  @strong{Warning:} If the @code{defconst} and @code{defvar} special
forms are used while the variable has a local binding, they set the
local binding's value; the global binding is not changed.  This is not
what we really 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 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 my-mode-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 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})
(if my-mode-map
    nil
  (let ((map (make-sparse-keymap)))
    (define-key my-mode-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})
(if my-mode-map
    nil
  (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 only if the variable
had been @code{ni}.  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 @var{symbol} has neither a
local binding nor a global value.
@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 value, ignoring any lexical bindings.  In Emacs Lisp,
all variables and all bindings are (in effect) special, so @code{set}
always affects the most local existing binding.
@end quotation
@end defun

  One other function for setting a variable is designed to add
an element to a list if it is not already present in the list.

@defun add-to-list symbol element
This function sets the variable @var{symbol} by consing @var{element}
onto the old value, if @var{element} is not already a member of that
value.  It returns the resulting list, whether updated or not.  The
value of @var{symbol} had better be a list already before the call.

The argument @var{symbol} is not implicitly quoted; @code{add-to-list}
is an ordinary function, like @code{set} and unlike @code{setq}.  Quote
the argument yourself if that is what you want.

Here's a scenario showing how to use @code{add-to-list}:

@example
(setq foo '(a b))
     @result{} (a b)

(add-to-list 'foo 'c)     ;; @r{Add @code{c}.}
     @result{} (c a b)

(add-to-list 'foo 'b)     ;; @r{No effect.}
     @result{} (c a b)

foo                       ;; @r{@code{foo} was changed.}
     @result{} (c a b)
@end example
@end defun

  An equivalent expression for @code{(add-to-list '@var{var}
@var{value})} is this:

@example
(or (member @var{value} @var{var})
    (setq @var{var} (cons @var{value} @var{var})))
@end example

@node Variable Scoping
@section Scoping Rules for Variable Bindings

  A given symbol @code{foo} may 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
  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 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 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 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 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
If we define @code{foo} as follows and 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

@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 work as intended 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: variable-value pairs.  At entry
to a function or to a @code{let} form, we can push bindings on 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) on a stack, and storing the 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 variables, buffer-local
@cindex buffer-local variables

  Global and local variable bindings are found in most programming
languages in one form or another.  Emacs also supports another, unusual
kind of variable binding: @dfn{buffer-local} bindings, which apply only
to one buffer.  Emacs Lisp is meant for programming editing commands,
and having different values for a variable in different buffers is an
important customization method.  (A few variables have bindings that
are local to a given X 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 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 the global binding is unchanged; this means that the change is
visible in that buffer alone.

  A variable may have buffer-local bindings in some buffers but not in
others.  The global binding is shared by all the buffers that don't have
their own bindings.  Thus, if you set the variable in a buffer that does
not have a buffer-local binding for it, the new value is visible in all
buffers except those with buffer-local bindings.  (Here we are assuming
that there are no @code{let}-style local bindings to complicate the issue.)

  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.

  The usual way to make a buffer-local binding is with
@code{make-local-variable}, which is what major mode commands use.  This
affects just the current buffer; all other buffers (including those yet to
be created) continue to share the global value.

@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 global value of the variable as usual,
but any @code{setq} creates a buffer-local binding for the current
buffer.  The new value is stored in the buffer-local binding, leaving
the (default) global binding untouched.  The global value can no longer
be changed with @code{setq}; you need to use @code{setq-default} to do
that.

  @strong{Warning:} When a variable has local values in one or more
buffers, you can get Emacs very confused by binding the variable with
@code{let}, changing to a different current buffer in which a different
binding is in effect, and then exiting the @code{let}.  This can
scramble the values of the global and local bindings.

  To preserve your sanity, avoid that series of actions.  If you use
@code{save-excursion} around each piece of code that changes to a
different current buffer, you will not have this problem.  Here is an
example of what to avoid:

@example
@group
(setq foo 'b)
(set-buffer "a")
(make-local-variable 'foo)
@end group
(setq foo 'a)
(let ((foo 'temp))
  (set-buffer "b")
  @var{body}@dots{})
@group
foo @result{} 'a      ; @r{The old buffer-local value from buffer @samp{a}}
               ;   @r{is now the default value.}
@end group
@group
(set-buffer "a")
foo @result{} 'temp   ; @r{The local value that should be gone}
               ;   @r{is now the buffer-local value in buffer @samp{a}.}
@end group
@end example

@noindent
But @code{save-excursion} as shown here avoids the problem:

@example
@group
(let ((foo 'temp))
  (save-excursion
    (set-buffer "b")
    @var{body}@dots{}))
@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{Auto Major Mode}.

@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.  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{Note:} do not use @code{make-local-variable} for a hook
variable.  Instead, use @code{make-local-hook}.  @xref{Hooks}.
@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.

The value returned is @var{variable}.

@strong{Note:} It is a mistake to 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.

The main use of @code{make-variable-buffer-local} is when a variable is
used for internal purposes, and the Lisp program depends on having
separate values in separate buffers.
@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 buffer-local-variables &optional buffer
This function returns a list describing the buffer-local variables in
buffer @var{buffer}.  It returns an association list (@pxref{Association
Lists}) in which each association contains one buffer-local variable and
its value.  When a buffer-local variable is void in @var{buffer}, then
it appears directly in the resulting list.  If @var{buffer} is omitted,
the current buffer is used.

@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 local variables.} 
    ;; @r{This one is local and void:}
    foobar
    ;; @r{This one is 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 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
global (default) binding of @var{variable} becomes visible in this
buffer.  Usually this results in a change in the value of
@var{variable}, since the global value is usually different from the
buffer-local value just eliminated.

If you kill the local binding of a variable that automatically becomes
local when set, this makes the global value visible in the current
buffer.  However, if you set the variable again, that will once again
create a 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}, and the abbrev table to the value
of @code{fundamental-mode-abbrev-table}.

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

@c Emacs 19 feature
@cindex permanent local variable
A 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 except when specifically overridden.

  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 local
value.

@defun default-value symbol
This function returns @var{symbol}'s default value.  This is the value
that is seen in buffers 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 value
This sets the default value of @var{symbol} to @var{value}.  It does not
evaluate @var{symbol}, but does evaluate @var{value}.  The value of the
@code{setq-default} form is @var{value}.

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 'local)
     @result{} local
@end group
@group
(setq local 'value-in-foo)
     @result{} value-in-foo
@end group
@group
(setq-default local 'new-default)
     @result{} new-default
@end group
@group
local
     @result{} value-in-foo
@end group
@group
(default-value 'local)
     @result{} new-default
@end group

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

@group
;; @r{Back in buffer @samp{foo}:}
local
     @result{} value-in-foo
(default-value '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
evaluated.

@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