Add some documentation on defining new generalized variables

[gnu-emacs] / doc / lispref / variables.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990-1995, 1998-2012 Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@node Variables
@chapter Variables
@cindex variable

  A @dfn{variable} is a name used in a program to stand for a value.
In Lisp, each variable is represented by a Lisp symbol
(@pxref{Symbols}).  The variable name is simply the symbol's name, and
the variable's value is stored in the symbol's value cell@footnote{To
be precise, under the default @dfn{dynamic binding} rules the value
cell always holds the variable's current value, but this is not the
case under @dfn{lexical binding} rules.  @xref{Variable Scoping}, for
details.}.  @xref{Symbol Components}.  In Emacs Lisp, the use of a
symbol as a variable is independent of its use as a function name.

  As previously noted in this manual, a Lisp program is represented
primarily by Lisp objects, and only secondarily as text.  The textual
form of a Lisp program is given by the read syntax of the Lisp objects
that constitute the program.  Hence, the textual form of a variable in
a Lisp program is written using the read syntax for the symbol
representing the variable.

@menu
* Global Variables::            Variable values that exist permanently, everywhere.
* Constant Variables::          Certain "variables" have values that never change.
* Local Variables::             Variable values that exist only temporarily.
* Void Variables::              Symbols that lack values.
* Defining Variables::          A definition says a symbol is used as a variable.
* Tips for Defining::           Things you should think about when you
                            define a variable.
* Accessing Variables::         Examining values of variables whose names
                            are known only at run time.
* Setting Variables::           Storing new values in variables.
* Variable Scoping::            How Lisp chooses among local and global values.
* Buffer-Local Variables::      Variable values in effect only in one buffer.
* File Local Variables::        Handling local variable lists in files.
* Directory Local Variables::   Local variables common to all files in a directory.
* Variable Aliases::            Variables that are aliases for other variables.
* Variables with Restricted Values::  Non-constant variables whose value can
                                        @emph{not} be an arbitrary Lisp object.
* Generalized Variables::       Extending the concept of variables.
@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} is a special form (@pxref{Special Forms}); it 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 itself as an expression.  Thus,

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

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

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

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

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

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

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

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

These constants are fundamentally different from the ``constants''
defined using the @code{defconst} special form (@pxref{Defining
Variables}).  A @code{defconst} form serves to inform human readers
that you do not intend to change the value of a variable, but Emacs
does not raise an error if you actually change it.

@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 give a variable a
@dfn{local value}---a value that takes effect only within a certain
part of a Lisp program.  When a variable has a local value, we say
that it is @dfn{locally bound} to that value, and that it is a
@dfn{local variable}.

  For example, when a function is called, its argument variables
receive local values, which are the actual arguments supplied to the
function call; these local bindings take effect within the body of the
function.  To take another example, the @code{let} special form
explicitly establishes local bindings for specific variables, which
take effect within the body of the @code{let} form.

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

@cindex shadowing of variables
  Establishing a local binding saves away the variable's previous
value (or lack of one).  We say that the previous value is
@dfn{shadowed}.  Both global and local values may be shadowed.  If a
local binding is in effect, using @code{setq} on the local variable
stores the specified value in the local binding.  When that local
binding is no longer in effect, the previously shadowed value (or lack
of one) comes back.

@cindex current binding
  A variable can have more than one local binding at a time (e.g.@: if
there are nested @code{let} forms that bind the variable).  The
@dfn{current binding} is the local binding that is actually in effect.
It determines the value returned by evaluating the variable symbol,
and it is the binding acted on by @code{setq}.

  For most purposes, you can think of the current binding as the
``innermost'' local binding, or the global binding if there is no
local binding.  To be more precise, a rule called the @dfn{scoping
rule} determines where in a program a local binding takes effect.  The
default scoping rule in Emacs Lisp is called @dfn{dynamic scoping},
which simply states that the current binding at any given point in the
execution of a program is the most recently-created binding for that
variable that still exists.  For details about dynamic scoping, and an
alternative scoping rule called @dfn{lexical scoping}, @xref{Variable
Scoping}.

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

@defspec let (bindings@dots{}) forms@dots{}
This special form sets up local bindings for a certain set of
variables, as specified by @var{bindings}, and then evaluates all of
the @var{forms} in textual order.  Its return value is 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 locally bound to @code{nil}; or @w{(ii) a} list of the
form @code{(@var{symbol} @var{value-form})}, in which case
@var{symbol} is locally bound to the result of evaluating
@var{value-form}.  If @var{value-form} is omitted, @code{nil} is used.

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

@example
@group
(setq y 2)
     @result{} 2
@end group

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

@defspec let* (bindings@dots{}) forms@dots{}
This special form is like @code{let}, but it binds each variable right
after computing its local value, before computing the local value for
the next variable.  Therefore, an expression in @var{bindings} can
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 Terminals}).  These kinds of bindings work somewhat
like ordinary local bindings, but they are localized depending on
``where'' you are in Emacs.

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

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

The default value is 1300.  Entry to the Lisp debugger increases the
value, if there is little room left, to make sure the debugger itself
has room to execute.
@end defopt

@node Void Variables
@section When a Variable is ``Void''
@cindex @code{void-variable} error
@cindex void variable

  We say that a variable is void if its symbol has an unassigned value
cell (@pxref{Symbol Components}).  Under Emacs Lisp's default dynamic
binding rules (@pxref{Variable Scoping}), the value cell stores the
variable's current (local or global) value.  Note that an unassigned
value cell is @emph{not} the same as having @code{nil} in the value
cell.  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 still a value.
If a variable is void, trying to evaluate the variable signals a
@code{void-variable} error rather than a value.

  Under lexical binding rules, the value cell only holds the
variable's global value, i.e.@: the value outside of any lexical
binding construct.  When a variable is lexically bound, the local value
is determined by the lexical environment; the variable may have a
local value if its symbol's value cell is unassigned.

@defun makunbound symbol
This function empties out the value cell of @var{symbol}, making the
variable void.  It returns @var{symbol}.

If @var{symbol} has a dynamic local binding, @code{makunbound} voids
the current binding, and this voidness lasts only as long as the local
binding is in effect.  Afterwards, the previously shadowed local or
global binding is reexposed; then the variable will no longer be void,
unless the reexposed binding is void too.

Here are some examples (assuming dynamic binding is in effect):

@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

@defun boundp variable
This function returns @code{t} if @var{variable} (a symbol) is not
void, and @code{nil} if it is void.

Here are some examples (assuming dynamic binding is in effect):

@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

  A @dfn{variable definition} is a construct that announces your
intention to use a symbol as a global variable.  It uses the special
forms @code{defvar} or @code{defconst}, which are documented below.

  A variable definition serves three purposes.  First, it informs
people who read the code that the symbol is @emph{intended} to be used
a certain way (as a variable).  Second, it informs the Lisp system of
this, optionally supplying an initial value and a documentation
string.  Third, it provides information to programming tools such as
@command{etags}, allowing them to find where the variable was defined.

  The difference between @code{defconst} and @code{defvar} is mainly a
matter of intent, serving to inform human readers of whether the value
should ever change.  Emacs Lisp does not actually prevent you from
changing the value of a variable defined with @code{defconst}.  One
notable difference between the two forms is that @code{defconst}
unconditionally initializes the variable, whereas @code{defvar}
initializes it only if it is originally void.

  To define a customizable variable, you should use @code{defcustom}
(which calls @code{defvar} as a subroutine).  @xref{Variable
Definitions}.

@defspec defvar symbol [value [doc-string]]
This special form defines @var{symbol} as a variable.  Note that
@var{symbol} is not evaluated; the symbol to be defined should appear
explicitly in the @code{defvar} form.  The variable is marked as
@dfn{special}, meaning that it should always be dynamically bound
(@pxref{Variable Scoping}).

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

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

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

If the @var{doc-string} argument is supplied, it specifies the
documentation string for the variable (stored in the symbol's
@code{variable-documentation} property).  @xref{Documentation}.

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

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

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

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

The @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

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

The @code{defconst} form, like @code{defvar}, marks the variable as
@dfn{special}, meaning that it should always be dynamically bound
(@pxref{Variable Scoping}).  In addition, it marks the variable as
risky (@pxref{File Local Variables}).

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

An example of the use of @code{defconst} is Emacs's definition of
@code{float-pi}---the mathematical constant @math{pi}, which ought not
to be changed by anyone (attempts by the Indiana State Legislature
notwithstanding).  As the second form illustrates, however,
@code{defconst} is only advisory.

@example
@group
(defconst float-pi 3.141592653589793 "The value of Pi.")
     @result{} float-pi
@end group
@group
(setq float-pi 3)
     @result{} float-pi
@end group
@group
float-pi
     @result{} 3
@end group
@end example
@end defspec

  @strong{Warning:} If you use a @code{defconst} or @code{defvar}
special form while the variable has a local binding (made with
@code{let}, or a function argument), it sets the local binding rather
than the global binding.  This is not what you usually want.  To
prevent this, use these special forms at top level in a file, where
normally no local binding is in effect, and make sure to load the file
before making a local binding for the variable.

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

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

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

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

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

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

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

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

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

@item @dots{}-flag
The value is significant only as to whether it is @code{nil} or not.
Since such variables often end up acquiring more values over time,
this convention is not strongly recommended.

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

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

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

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

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

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

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

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

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

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

@node Accessing Variables
@section Accessing Variable Values

  The usual way to reference a variable is to write the symbol which
names it.  @xref{Symbol Forms}.

  Occasionally, you may want to reference a variable which is only
determined at run time.  In that case, you cannot specify the variable
name in the text of the program.  You can use the @code{symbol-value}
function to extract the value.

@defun symbol-value symbol
This function returns the value stored in @var{symbol}'s value cell.
This is where the variable's current (dynamic) value is stored.  If
the variable has no local binding, this is simply its global value.
If the variable is void, a @code{void-variable} error is signaled.

If the variable is lexically bound, the value reported by
@code{symbol-value} is not necessarily the same as the variable's
lexical value, which is determined by the lexical environment rather
than the symbol's value cell.  @xref{Variable Scoping}.

@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
@end defun

@node Setting Variables
@section Setting Variable Values

  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 current 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 puts @var{value} in the value cell of @var{symbol}.
Since it is a function rather than a special form, the expression
written for @var{symbol} is evaluated to obtain the symbol to set.
The return value is @var{value}.

When dynamic variable binding is in effect (the default), @code{set}
has the same effect as @code{setq}, apart from the fact that
@code{set} evaluates its @var{symbol} argument whereas @code{setq}
does not.  But when a variable is lexically bound, @code{set} affects
its @emph{dynamic} value, whereas @code{setq} affects its current
(lexical) value.  @xref{Variable Scoping}.

@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
@end defun

@node Variable Scoping
@section Scoping Rules for Variable Bindings

  When you create a local binding for a variable, that binding takes
effect only within a limited portion of the program (@pxref{Local
Variables}).  This section describes exactly what this means.

@cindex scope
@cindex extent
  Each local binding has a certain @dfn{scope} and @dfn{extent}.
@dfn{Scope} refers to @emph{where} in the textual source code the
binding can be accessed.  @dfn{Extent} refers to @emph{when}, as the
program is executing, the binding exists.

@cindex dynamic binding
@cindex indefinite scope
@cindex dynamic extent
  By default, the local bindings that Emacs creates are @dfn{dynamic
bindings}.  Such a binding has @dfn{indefinite scope}, meaning that
any part of the program can potentially access the variable binding.
It also has @dfn{dynamic extent}, meaning that the binding lasts only
while the binding construct (such as the body of a @code{let} form) is
being executed.

@cindex lexical binding
@cindex lexical scope
@cindex indefinite extent
  Emacs can optionally create @dfn{lexical bindings}.  A lexical
binding has @dfn{lexical scope}, meaning that any reference to the
variable must be located textually within the binding construct.  It
also has @dfn{indefinite extent}, meaning that under some
circumstances the binding can live on even after the binding construct
has finished executing, by means of special objects called
@dfn{closures}.

  The following subsections describe dynamic binding and lexical
binding in greater detail, and how to enable lexical binding in Emacs
Lisp programs.

@menu
* Dynamic Binding::         The default for binding local variables in Emacs.
* Dynamic Binding Tips::    Avoiding problems with dynamic binding.
* Lexical Binding::         A different type of local variable binding.
* Using Lexical Binding::   How to enable lexical binding.
@end menu

@node Dynamic Binding
@subsection Dynamic Binding

  By default, the local variable bindings made by Emacs are dynamic
bindings.  When a variable is dynamically bound, its current binding
at any point in the execution of the Lisp program is simply the most
recently-created dynamic local binding for that symbol, or the global
binding if there is no such local binding.

  Dynamic bindings have indefinite scope and dynamic extent, as shown
by the following example:

@example
@group
(defvar x -99)  ; @r{@code{x} receives an initial value of -99.}

(defun getx ()
  x)            ; @r{@code{x} is used ``free'' in this function.}

(let ((x 1))    ; @r{@code{x} is dynamically bound.}
  (getx))
     @result{} 1

;; @r{After the @code{let} form finishes, @code{x} reverts to its}
;; @r{previous value, which is -99.}

(getx)
     @result{} -99
@end group
@end example

@noindent
The function @code{getx} refers to @code{x}.  This is a ``free''
reference, in the sense that there is no binding for @code{x} within
that @code{defun} construct itself.  When we call @code{getx} from
within a @code{let} form in which @code{x} is (dynamically) bound, it
retrieves the local value of @code{x} (i.e.@: 1).  But when we call
@code{getx} outside the @code{let} form, it retrieves the global value
of @code{x} (i.e.@: -99).

  Here is another example, which illustrates setting a dynamically
bound variable using @code{setq}:

@example
@group
(defvar x -99)      ; @r{@code{x} receives an initial value of -99.}

(defun addx ()
  (setq x (1+ x)))  ; @r{Add 1 to @code{x} and return its new value.}

(let ((x 1))
  (addx)
  (addx))
     @result{} 3           ; @r{The two @code{addx} calls add to @code{x} twice.}

;; @r{After the @code{let} form finishes, @code{x} reverts to its}
;; @r{previous value, which is -99.}

(addx)
     @result{} -98
@end group
@end example

  Dynamic binding is implemented in Emacs Lisp in a simple way.  Each
symbol has a value cell, which specifies its current dynamic value (or
absence of value).  @xref{Symbol Components}.  When a symbol is given
a dynamic local binding, Emacs records the contents of the value cell
(or absence thereof) in a stack, and stores the new local value in the
value cell.  When the binding construct finishes executing, Emacs pops
the old value off the stack, and puts it in the value cell.

@node Dynamic Binding Tips
@subsection Proper Use of Dynamic Binding

  Dynamic binding is a powerful feature, as it allows programs to
refer to variables that are not defined within their local textual
scope.  However, if used without restraint, this can also make
programs hard to understand.  There are two clean ways to use this
technique:

@itemize @bullet
@item
If a variable has no global definition, use it as a local variable
only within a binding construct, e.g.@: the body of the @code{let}
form where the variable was bound, or the body of the function for an
argument variable.  If this convention is followed consistently
throughout a program, the value of the variable will not affect, nor
be affected by, any uses of the same variable symbol elsewhere in the
program.

@item
Otherwise, define the variable with @code{defvar}, @code{defconst}, or
@code{defcustom}.  @xref{Defining Variables}.  Usually, the definition
should be at top-level in an Emacs Lisp file.  As far as possible, it
should include a documentation string which explains the meaning and
purpose of the variable.  You should also choose the variable's name
to avoid name conflicts (@pxref{Coding Conventions}).

Then you can bind the variable anywhere in a program, knowing reliably
what the effect will be.  Wherever you encounter the variable, it will
be easy to refer back to the definition, e.g.@: via the @kbd{C-h v}
command (provided the variable definition has been loaded into Emacs).
@xref{Name Help,,, emacs, The GNU Emacs Manual}.

For example, it is common to use local bindings for customizable
variables like @code{case-fold-search}:

@example
@group
(defun search-for-abc ()
  "Search for the string \"abc\", ignoring case differences."
  (let ((case-fold-search nil))
    (re-search-forward "abc")))
@end group
@end example
@end itemize

@node Lexical Binding
@subsection Lexical Binding

Optionally, you can create lexical bindings in Emacs Lisp.  A
lexically bound variable has @dfn{lexical scope}, meaning that any
reference to the variable must be located textually within the binding
construct.

  Here is an example
@iftex
(see the next subsection, for how to actually enable lexical binding):
@end iftex
@ifnottex
(@pxref{Using Lexical Binding}, for how to actually enable lexical binding):
@end ifnottex

@example
@group
(let ((x 1))    ; @r{@code{x} is lexically bound.}
  (+ x 3))
     @result{} 4

(defun getx ()
  x)            ; @r{@code{x} is used ``free'' in this function.}

(let ((x 1))    ; @r{@code{x} is lexically bound.}
  (getx))
@error{} Symbol's value as variable is void: x
@end group
@end example

@noindent
Here, the variable @code{x} has no global value.  When it is lexically
bound within a @code{let} form, it can be used in the textual confines
of that @code{let} form.  But it can @emph{not} be used from within a
@code{getx} function called from the @code{let} form, since the
function definition of @code{getx} occurs outside the @code{let} form
itself.

@cindex lexical environment
  Here is how lexical binding works.  Each binding construct defines a
@dfn{lexical environment}, specifying the symbols that are bound
within the construct and their local values.  When the Lisp evaluator
wants the current value of a variable, it looks first in the lexical
environment; if the variable is not specified in there, it looks in
the symbol's value cell, where the dynamic value is stored.

@cindex closures, example of using
  Lexical bindings have indefinite extent.  Even after a binding
construct has finished executing, its lexical environment can be
``kept around'' in Lisp objects called @dfn{closures}.  A closure is
created when you define a named or anonymous function with lexical
binding enabled.  @xref{Closures}, for details.

  When a closure is called as a function, any lexical variable
references within its definition use the retained lexical environment.
Here is an example:

@example
(defvar my-ticker nil)   ; @r{We will use this dynamically bound}
                         ; @r{variable to store a closure.}

(let ((x 0))             ; @r{@code{x} is lexically bound.}
  (setq my-ticker (lambda ()
                    (setq x (1+ x)))))
    @result{} (closure ((x . 0) t) ()
          (1+ x))

(funcall my-ticker)
    @result{} 1

(funcall my-ticker)
    @result{} 2

(funcall my-ticker)
    @result{} 3

x                        ; @r{Note that @code{x} has no global value.}
@error{} Symbol's value as variable is void: x
@end example

@noindent
The @code{let} binding defines a lexical environment in which the
variable @code{x} is locally bound to 0.  Within this binding
construct, we define a lambda expression which increments @code{x} by
one and returns the incremented value.  This lambda expression is
automatically turned into a closure, in which the lexical environment
lives on even after the @code{let} binding construct has exited.  Each
time we evaluate the closure, it increments @code{x}, using the
binding of @code{x} in that lexical environment.

  Note that functions like @code{symbol-value}, @code{boundp}, and
@code{set} only retrieve or modify a variable's dynamic binding
(i.e.@: the contents of its symbol's value cell).  Also, the code in
the body of a @code{defun} or @code{defmacro} cannot refer to
surrounding lexical variables.

  Currently, lexical binding is not much used within the Emacs
sources.  However, we expect its importance to increase in the future.
Lexical binding opens up a lot more opportunities for optimization, so
Emacs Lisp code that makes use of lexical binding is likely to run
faster in future Emacs versions.  Such code is also much more friendly
to concurrency, which we want to add to Emacs in the near future.

@node Using Lexical Binding
@subsection Using Lexical Binding

  When loading an Emacs Lisp file or evaluating a Lisp buffer, lexical
binding is enabled if the buffer-local variable @code{lexical-binding}
is non-@code{nil}:

@defvar lexical-binding
If this buffer-local variable is non-@code{nil}, Emacs Lisp files and
buffers are evaluated using lexical binding instead of dynamic
binding.  (However, special variables are still dynamically bound; see
below.)  If @code{nil}, dynamic binding is used for all local
variables.  This variable is typically set for a whole Emacs Lisp
file, as a file local variable (@pxref{File Local Variables}).
Note that unlike other such variables, this one must be set in the
first line of a file.
@end defvar

@noindent
When evaluating Emacs Lisp code directly using an @code{eval} call,
lexical binding is enabled if the @var{lexical} argument to
@code{eval} is non-@code{nil}.  @xref{Eval}.

@cindex special variables
  Even when lexical binding is enabled, certain variables will
continue to be dynamically bound.  These are called @dfn{special
variables}.  Every variable that has been defined with @code{defvar},
@code{defcustom} or @code{defconst} is a special variable
(@pxref{Defining Variables}).  All other variables are subject to
lexical binding.

@defun special-variable-p SYMBOL
This function returns non-@code{nil} if @var{symbol} is a special
variable (i.e.@: it has a @code{defvar}, @code{defcustom}, or
@code{defconst} variable definition).  Otherwise, the return value is
@code{nil}.
@end defun

  The use of a special variable as a formal argument in a function is
discouraged.  Doing so gives rise to unspecified behavior when lexical
binding mode is enabled (it may use lexical binding sometimes, and
dynamic binding other times).

  Converting an Emacs Lisp program to lexical binding is pretty easy.
First, add a file-local variable setting of @code{lexical-binding} to
@code{t} in the Emacs Lisp source file.  Second, check that every
variable in the program which needs to be dynamically bound has a
variable definition, so that it is not inadvertently bound lexically.

  A simple way to find out which variables need a variable definition
is to byte-compile the source file.  @xref{Byte Compilation}.  If a
non-special variable is used outside of a @code{let} form, the
byte-compiler will warn about reference or assignment to a ``free
variable''.  If a non-special variable is bound but not used within a
@code{let} form, the byte-compiler will warn about an ``unused lexical
variable''.  The byte-compiler will also issue a warning if you use a
special variable as a function argument.

  (To silence byte-compiler warnings about unused variables, just use
a variable name that start with an underscore.  The byte-compiler
interprets this as an indication that this is a variable known not to
be used.)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

  A buffer-local variable cannot be made terminal-local
(@pxref{Multiple Terminals}).

@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}.

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.}
(with-current-buffer "b2"
  foo)
     @result{} 5
@end group
@end example

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

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

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

@deffn Command make-variable-buffer-local variable
This function marks @var{variable} (a symbol) automatically
buffer-local, so that any subsequent attempt to set it will make it
local to the current buffer at the time.  Unlike
@code{make-local-variable}, with which it is often confused, this
cannot be undone, and affects the behavior of the variable in all
buffers.

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

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

The value returned is @var{variable}.

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

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

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

@defun local-variable-if-set-p variable &optional buffer
This returns @code{t} if @var{variable} either has a buffer-local
value in buffer @var{buffer}, or is automatically buffer-local.
Otherwise, it returns @code{nil}.  If omitted or @code{nil},
@var{buffer} defaults to the current buffer.
@end defun

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

@defun buffer-local-variables &optional buffer
This function returns a list describing the buffer-local variables in
buffer @var{buffer}.  (If @var{buffer} is omitted, the current buffer
is used.)  Normally, each list element has the form
@w{@code{(@var{sym} . @var{val})}}, where @var{sym} is a buffer-local
variable (a symbol) and @var{val} is its buffer-local value.  But when
a variable's buffer-local binding in @var{buffer} is void, its list
element is just @var{sym}.

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

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

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

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

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

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

@defun kill-all-local-variables
This function eliminates all the buffer-local variable bindings of the
current buffer except for variables marked as ``permanent'' and local
hook functions that have a non-@code{nil} @code{permanent-local-hook}
property (@pxref{Setting Hooks}).  As a result, the buffer will see
the default values of most variables.

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

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

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

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

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

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

@c Emacs 19 feature
@cindex permanent local variable
A buffer-local variable is @dfn{permanent} if the variable name (a
symbol) has a @code{permanent-local} property that is non-@code{nil}.
Such variables are unaffected by @code{kill-all-local-variables}, and
their local bindings are therefore not cleared by changing major modes.
Permanent locals are appropriate for data pertaining to where the file
came from or how to save it, rather than with how to edit the contents.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

  If a file-local variable could specify an arbitrary function or Lisp
expression that would be called later, visiting a file could take over
your Emacs.  Emacs protects against this by automatically setting only
those file-local variables whose specified values are known to be
safe.  Other file-local variables are set only if the user agrees.

  For additional safety, @code{read-circle} is temporarily bound to
@code{nil} when Emacs reads file-local variables (@pxref{Input
Functions}).  This prevents the Lisp reader from recognizing circular
and shared Lisp structures (@pxref{Circular Objects}).

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

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

@defvar inhibit-local-variables-regexps
This is a list of regular expressions.  If a file has a name
matching an element of this list, then it is not scanned for
any form of file-local variable.  For examples of why you might want
to use this, @pxref{Auto Major Mode}.
@end defvar

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

This function works by walking the alist stored in
@code{file-local-variables-alist} and applying each local variable in
turn.  It calls @code{before-hack-local-variables-hook} and
@code{hack-local-variables-hook} before and after applying the
variables, respectively.  It only calls the before-hook if the alist
is non-@code{nil}; it always calls the other hook.  This
function ignores a @samp{mode} element if it specifies the same major
mode as the buffer already has.

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

@defvar file-local-variables-alist
This buffer-local variable holds the alist of file-local variable
settings.  Each element of the alist is of the form
@w{@code{(@var{var} . @var{value})}}, where @var{var} is a symbol of
the local variable and @var{value} is its value.  When Emacs visits a
file, it first collects all the file-local variables into this alist,
and then the @code{hack-local-variables} function applies them one by
one.
@end defvar

@defvar before-hack-local-variables-hook
Emacs calls this hook immediately before applying file-local variables
stored in @code{file-local-variables-alist}.
@end defvar

@defvar hack-local-variables-hook
Emacs calls this hook immediately after it finishes applying
file-local variables stored in @code{file-local-variables-alist}.
@end defvar

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

  When defining a user option using @code{defcustom}, you can set its
@code{safe-local-variable} property by adding the arguments
@code{:safe @var{function}} to @code{defcustom} (@pxref{Variable
Definitions}).

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

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

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

@c @cindex risky local variable   Duplicates risky-local-variable
  Some variables are considered @dfn{risky}.  If a variable is risky,
it is never entered automatically into
@code{safe-local-variable-values}; Emacs always queries before setting
a risky variable, unless the user explicitly allows a value by
customizing @code{safe-local-variable-values} directly.

  Any variable whose name has a non-@code{nil}
@code{risky-local-variable} property is considered risky.  When you
define a user option using @code{defcustom}, you can set its
@code{risky-local-variable} property by adding the arguments
@code{:risky @var{value}} to @code{defcustom} (@pxref{Variable
Definitions}).  In addition, any variable whose name ends in any of
@samp{-command}, @samp{-frame-alist}, @samp{-function},
@samp{-functions}, @samp{-hook}, @samp{-hooks}, @samp{-form},
@samp{-forms}, @samp{-map}, @samp{-map-alist}, @samp{-mode-alist},
@samp{-program}, or @samp{-predicate} is automatically considered
risky.  The variables @samp{font-lock-keywords},
@samp{font-lock-keywords} followed by a digit, and
@samp{font-lock-syntactic-keywords} are also considered risky.

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

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

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

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

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

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

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

@node Directory Local Variables
@section Directory Local Variables
@cindex directory local variables

  A directory can specify local variable values common to all files in
that directory; Emacs uses these to create buffer-local bindings for
those variables in buffers visiting any file in that directory.  This
is useful when the files in the directory belong to some @dfn{project}
and therefore share the same local variables.

  There are two different methods for specifying directory local
variables: by putting them in a special file, or by defining a
@dfn{project class} for that directory.

@defvr Constant dir-locals-file
This constant is the name of the file where Emacs expects to find the
directory-local variables.  The name of the file is
@file{.dir-locals.el}@footnote{
The MS-DOS version of Emacs uses @file{_dir-locals.el} instead, due to
limitations of the DOS filesystems.
}.  A file by that name in a directory causes Emacs to apply its
settings to any file in that directory or any of its subdirectories
(optionally, you can exclude subdirectories; see below).
If some of the subdirectories have their own @file{.dir-locals.el}
files, Emacs uses the settings from the deepest file it finds starting
from the file's directory and moving up the directory tree.  The file
specifies local variables as a specially formatted list; see
@ref{Directory Variables, , Per-directory Local Variables, emacs, The
GNU Emacs Manual}, for more details.
@end defvr

@defun hack-dir-local-variables
This function reads the @code{.dir-locals.el} file and stores the
directory-local variables in @code{file-local-variables-alist} that is
local to the buffer visiting any file in the directory, without
applying them.  It also stores the directory-local settings in
@code{dir-locals-class-alist}, where it defines a special class for
the directory in which @file{.dir-locals.el} file was found.  This
function works by calling @code{dir-locals-set-class-variables} and
@code{dir-locals-set-directory-class}, described below.
@end defun

@defun hack-dir-local-variables-non-file-buffer
This function looks for directory-local variables, and immediately
applies them in the current buffer.  It is intended to be called in
the mode commands for non-file buffers, such as Dired buffers, to let
them obey directory-local variable settings.  For non-file buffers,
Emacs looks for directory-local variables in @code{default-directory}
and its parent directories.
@end defun

@defun dir-locals-set-class-variables class variables
This function defines a set of variable settings for the named
@var{class}, which is a symbol.  You can later assign the class to one
or more directories, and Emacs will apply those variable settings to
all files in those directories.  The list in @var{variables} can be of
one of the two forms: @code{(@var{major-mode} . @var{alist})} or
@code{(@var{directory} . @var{list})}.  With the first form, if the
file's buffer turns on a mode that is derived from @var{major-mode},
then the all the variables in the associated @var{alist} are applied;
@var{alist} should be of the form @code{(@var{name} . @var{value})}.
A special value @code{nil} for @var{major-mode} means the settings are
applicable to any mode.  In @var{alist}, you can use a special
@var{name}: @code{subdirs}.  If the associated value is
@code{nil}, the alist is only applied to files in the relevant
directory, not to those in any subdirectories.

With the second form of @var{variables}, if @var{directory} is the
initial substring of the file's directory, then @var{list} is applied
recursively by following the above rules; @var{list} should be of one
of the two forms accepted by this function in @var{variables}.
@end defun

@defun dir-locals-set-directory-class directory class &optional mtime
This function assigns @var{class} to all the files in @code{directory}
and its subdirectories.  Thereafter, all the variable settings
specified for @var{class} will be applied to any visited file in
@var{directory} and its children.  @var{class} must have been already
defined by @code{dir-locals-set-class-variables}.

Emacs uses this function internally when it loads directory variables
from a @code{.dir-locals.el} file.  In that case, the optional
argument @var{mtime} holds the file modification time (as returned by
@code{file-attributes}).  Emacs uses this time to check stored
local variables are still valid.  If you are assigning a class
directly, not via a file, this argument should be @code{nil}.
@end defun

@defvar dir-locals-class-alist
This alist holds the class symbols and the associated variable
settings.  It is updated by @code{dir-locals-set-class-variables}.
@end defvar

@defvar dir-locals-directory-cache
This alist holds directory names, their assigned class names, and
modification times of the associated directory local variables file
(if there is one).  The function @code{dir-locals-set-directory-class}
updates this list.
@end defvar

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

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

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

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

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

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

@defun make-obsolete-variable obsolete-name current-name when &optional access-type
This function makes the byte compiler warn that the variable
@var{obsolete-name} is obsolete.  If @var{current-name} is a symbol,
it is the variable's new name; then the warning message says to use
@var{current-name} instead of @var{obsolete-name}.  If
@var{current-name} is a string, this is the message and there is no
replacement variable.  @var{when} should be a string indicating when
the variable was first made obsolete (usually a version number
string).

The optional argument @var{access-type}, if non-@code{nil}, should
should specify the kind of access that will trigger obsolescence
warnings; it can be either @code{get} or @code{set}.
@end defun

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

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

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

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

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

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

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

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

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

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

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

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

@example
(setq undo-limit 1000.0)
@error{} Wrong type argument: integerp, 1000.0
@end example

@c FIXME?  Not sure this is the right place for this section.
@node Generalized Variables
@section Generalized Variables

A @dfn{generalized variable} or @dfn{place form} is one of the many places
in Lisp memory where values can be stored.  The simplest place form is
a regular Lisp variable.  But the @sc{car}s and @sc{cdr}s of lists, elements
of arrays, properties of symbols, and many other locations are also
places where Lisp values are stored.

@c FIXME?  Not sure this is a useful analogy...
Generalized variables are analogous to ``lvalues'' in the C
language, where @samp{x = a[i]} gets an element from an array
and @samp{a[i] = x} stores an element using the same notation.
Just as certain forms like @code{a[i]} can be lvalues in C, there
is a set of forms that can be generalized variables in Lisp.

@menu
* Setting Generalized Variables::   The @code{setf} macro.
* Adding Generalized Variables::    Defining new @code{setf} forms.
@end menu

@node Setting Generalized Variables
@subsection The @code{setf} Macro

The @code{setf} macro is the most basic way to operate on generalized
variables.  The @code{setf} form is like @code{setq}, except that it
accepts arbitrary place forms on the left side rather than just
symbols.  For example, @code{(setf (car a) b)} sets the car of
@code{a} to @code{b}, doing the same operation as @code{(setcar a b)},
but without having to remember two separate functions for setting and
accessing every type of place.

@defmac setf [place form]@dots{}
This macro evaluates @var{form} and stores it in @var{place}, which
must be a valid generalized variable form.  If there are several
@var{place} and @var{form} pairs, the assignments are done sequentially
just as with @code{setq}.  @code{setf} returns the value of the last
@var{form}.
@end defmac

The following Lisp forms will work as generalized variables, and
so may appear in the @var{place} argument of @code{setf}:

@itemize
@item
A symbol naming a variable.  In other words, @code{(setf x y)} is
exactly equivalent to @code{(setq x y)}, and @code{setq} itself is
strictly speaking redundant given that @code{setf} exists.  Many
programmers continue to prefer @code{setq} for setting simple
variables, though, purely for stylistic or historical reasons.
The macro @code{(setf x y)} actually expands to @code{(setq x y)},
so there is no performance penalty for using it in compiled code.

@item
A call to any of the following standard Lisp functions:

@smallexample
car            cdr            nth            nthcdr
caar           cadr           cdar           cddr
aref           elt            get            gethash
symbol-function        symbol-value          symbol-plist
@end smallexample

@item
The following Emacs-specific functions are also @code{setf}-able:

@smallexample
default-value                 process-get
frame-parameter               process-sentinel
terminal-parameter            window-buffer
keymap-parent                 window-display-table
match-data                    window-dedicated-p
overlay-get                   window-hscroll
overlay-start                 window-parameter
overlay-end                   window-point
process-buffer                window-start
process-filter
@end smallexample
@end itemize

@noindent
Using any forms other than these in the @var{place} argument to
@code{setf} will signal an error.

@c And for cl-lib's cl-getf.
Note that for @code{nthcdr}, the list argument of the function must
itself be a valid @var{place} form.  For example, @code{(setf (nthcdr
0 foo) 7)} will set @code{foo} itself to 7.
@c The use of @code{nthcdr} as a @var{place} form is an extension
@c to standard Common Lisp.

@c FIXME I don't think is a particularly good way to do it,
@c but these macros are introduced before generalized variables are.
The macros @code{push} (@pxref{List Variables}) and @code{pop}
(@pxref{List Elements}) can manipulate generalized variables,
not just lists.  @code{(pop @var{place})} removes and returns the first
element of the list stored in @var{place}.  It is analogous to
@code{(prog1 (car @var{place}) (setf @var{place} (cdr @var{place})))},
except that it takes care to evaluate all subforms only once.
@code{(push @var{x} @var{place})} inserts @var{x} at the front of
the list stored in @var{place}.  It is analogous to @code{(setf
@var{place} (cons @var{x} @var{place}))}, except for evaluation of the
subforms.  Note that @code{push} and @code{pop} on an @code{nthcdr}
place can be used to insert or delete at any position in a list.

The @file{cl-lib} library defines various extensions for generalized
variables, including additional @code{setf} places.
@xref{Generalized Variables,,, cl, Common Lisp Extensions}.


@node Adding Generalized Variables
@subsection Defining new @code{setf} forms

This section describes how to define new forms that @code{setf} can
operate on.

@defmac gv-define-simple-setter name setter &optional fix-return
This macro enables you to easily define @code{setf} methods for simple
cases.  @var{name} is the name of a function, macro, or special form.
You can use this macro whenever @var{name} has a directly
corresponding @var{setter} function that updates it, e.g.,
@code{(gv-define-simple-setter car setcar)}.

This macro translates a call of the form

@example
(setf (@var{name} @var{args}@dots{}) @var{value})
@end example

into
@example
(@var{setter} @var{args}@dots{} @var{value})
@end example

@noindent
Such a @code{setf} call is documented to return @var{value}.  This is
no problem with, e.g., @code{car} and @code{setcar}, because
@code{setcar} returns the value that it set.  If your @var{setter}
function does not return @var{value}, use a non-@code{nil} value for
the @var{fix-return} argument of @code{gv-define-simple-setter}.  This
wraps the @code{setf} expansion in @code{(prog1 @var{value} @dots{})}
so that it returns the correct result.
@end defmac


@defmac gv-define-setter name arglist &rest body
This macro allows for more complex @code{setf} expansions than the
previous form.  You may need to use this form, for example, if there
is no simple setter function to call, or if there is one but it
requires different arguments to the place form.

This macro expands the form
@code{(setf (@var{name} @var{args}@dots{}) @var{value})} by
first binding the @code{setf} argument forms
@code{(@var{value} @var{args}@dots{})} according to @var{arglist},
and then executing @var{body}.  @var{body} should return a Lisp
form that does the assignment.  Remember that it should return the
value that was set.  An example of using this macro is:

@example
(gv-define-setter caar (val x) `(setcar (car ,x) ,val))
@end example
@end defmac

@c FIXME?  Not sure what, if anything, to say about this.
@ignore
@defmac gv-define-expander name handler
This is the most general way to define a new @code{setf} expansion.
@end defmac
@end ignore