Languguage OS 2/Languguage OS II Version 10-94 (Knowledge Media)(1994).ISO/gnu/elispman.lha/elispman/variables.texi

Lisp/Scheme | 1993-05-19 | 43KB | 1,231 lines

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993 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 for 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 whether
the same symbol has a function definition.  @xref{Symbol Components}.

  The textual form of a program is determined by its Lisp object
representation; it is the read syntax for the Lisp object which
constitutes the program.  This is why a variable in a textual Lisp
program is written as 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.
* 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, Constant Variables, Variables, 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 the
first argument of @code{setq}, the name of the variable, is not
evaluated, but the second argument, the desired value, is evaluated
normally.

  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
x
     @result{} 4
@end group
@end example

@node Constant Variables, Local Variables, Global Variables, 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, Void Variables, Constant Variables, Variables
@section Local Variables
@cindex binding local variables
@cindex local variables
@cindex local binding
@cindex global binding

  Global variables are given 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 which last until the function exits.  Similarly, 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
  When a local value is established, the previous value (or lack of one)
of the variable is saved away.  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 function 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}, except that each symbol in
@var{bindings} is bound as soon as its new value is computed, before the
computation of the values of the following local bindings.  Therefore,
an expression in @var{bindings} may 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 which create local
bindings:

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

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

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

@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, Defining Variables, Local Variables, 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.  This
causes any future attempt to use this symbol as a variable to 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, Accessing Variables, Void Variables, Variables
@section Defining Global Variables

  You may announce your intention to use a symbol as a global variable
with a definition, using @code{defconst} or @code{defvar}.

  In Emacs Lisp, definitions serve three purposes.  First, they inform
the user who reads the code that certain symbols are @emph{intended} to be
used 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 also makes 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 the option in their init
files, and override the default value given in the definition.  For this
reason, user options must be defined with @code{defvar}.

@defspec defvar symbol [value [doc-string]]
This special form informs a person reading your code that @var{symbol}
will be used as a variable that the 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, it is evaluated and
@var{symbol} is set to the result.  (If @var{value} is not specified,
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} sets the default value, not the local value.

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 to be a user option.  This affects commands
such as @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}))
  (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 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 normally loaded.  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 "Pi to one place.")
     @result{} pi
@end group
@group
(setq pi 4)
     @result{} pi
@end group
@group
pi
     @result{} 4
@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,
intended to be set by the user for customization, @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

  Note that if the @code{defconst} and @code{defvar} special forms are
used while the variable has a local binding, the local binding's value
is set, and the global binding is not changed.  This would be confusing.
But the normal way to use these special forms is at top level in a file,
where no local binding should be in effect.

@node Accessing Variables, Setting Variables, Defining Variables, 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, Variable Scoping, Accessing Variables, 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.

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

The most-local existing binding of the variable is the binding that is
set; shadowed bindings are not affected.  If @var{symbol} is not
actually a symbol, a @code{wrong-type-argument} error is signaled.

@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

Logically speaking, @code{set} is a more fundamental primitive that
@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 needed only when the
choice of variable to be set is made at run time.  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 special, so @code{set} always affects the
most local existing binding.
@end quotation
@end defun

@node Variable Scoping, Buffer-Local Variables, Setting Variables, Variables
@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
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, Extent, Variable Scoping, Variable Scoping
@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} from
@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, Impl of Scope, Scope, Variable Scoping
@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

@node Impl of Scope, Using Scoping, Extent, Variable Scoping
@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.  When a new local binding is created, the local value is
stored in the value cell, and the old value (belonging to a previous
binding) is pushed on a stack.  When a binding is eliminated, the old
value is popped off the stack and stored in 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,, Impl of Scope, Variable 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

@node Buffer-Local Variables,, Variable Scoping, 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.

@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, Creating Buffer-Local, Buffer-Local Variables, Buffer-Local Variables
@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}.  To preserve your
sanity, it is wise to avoid such situations.  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 incorrect code:

@example
@group
(setq foo 'b)
(set-buffer "a")
(make-local-variable 'foo)
@end group
@group
(setq foo 'a)
(let ((foo 'temp))
  (set-buffer "b")
  @dots{})
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")
    @dots{}))
@end group
@end example

  Local variables in a file you edit are also represented by
buffer-local bindings for the buffer that holds the file within Emacs.
@xref{Auto Major Mode}.

@node Creating Buffer-Local, Default Value, Intro to Buffer-Local, Buffer-Local Variables
@subsection Creating and Destroying 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
@end deffn

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

The value returned is @var{variable}.
@end deffn

@defun buffer-local-variables &optional buffer
This function tells you what the buffer-local variables are in buffer
@var{buffer}.  It returns an association list (@pxref{Association
Lists}) in which each association contains one buffer-local variable and
its value.  If @var{buffer} is omitted, the current buffer is used.

@example
@group
(setq lcl (buffer-local-variables))
@result{} ((fill-column . 75)
    (case-fold-search . t)
    @dots{}
    (mark-ring #<marker at 5454 in buffers.texi>)
    (require-final-newline . t))
@end group
@end example

Note that storing new values into the @sc{cdr}s of the elements 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.

It is possible to kill the local binding of a variable that automatically
becomes local when set.  This causes the variable to show its global value
in the current buffer.  However, if you set the variable again, this will
once again create a local value.

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

@defun kill-all-local-variables
This function eliminates all the buffer-local variable bindings of the
current buffer except for variables marker 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: its local keymap is set to @code{nil}, its syntax table is set
to the value of @code{standard-syntax-table}, and its abbrev table is
set 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,, Creating Buffer-Local, Buffer-Local Variables
@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} allow you
to access and change the 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 which 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 variable
The function @code{default-boundp} tells you whether @var{variable}'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}.
@var{symbol} is not evaluated, but @var{value} is.  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, this 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