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[Emacs-diffs] Changes to variables.texi
|
From: |
Glenn Morris |
|
Subject: |
[Emacs-diffs] Changes to variables.texi |
|
Date: |
Thu, 06 Sep 2007 04:15:18 +0000 |
CVSROOT: /sources/emacs
Module name: emacs
Changes by: Glenn Morris <gm> 07/09/06 04:15:17
Index: variables.texi
===================================================================
RCS file: variables.texi
diff -N variables.texi
--- variables.texi 7 Apr 2007 01:46:25 -0000 1.94
+++ /dev/null 1 Jan 1970 00:00:00 -0000
@@ -1,1908 +0,0 @@
address@hidden -*-texinfo-*-
address@hidden This is part of the GNU Emacs Lisp Reference Manual.
address@hidden Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999,
2000,
address@hidden 2001, 2002, 2003, 2004, 2005, 2006, 2007 Free Software
Foundation, Inc.
address@hidden See the file elisp.texi for copying conditions.
address@hidden ../info/variables
address@hidden Variables, Functions, Control Structures, Top
address@hidden Variables
address@hidden variable
-
- A @dfn{variable} is a name used in a program to stand for a value.
-Nearly all programming languages have variables of some sort. In the
-text of a Lisp program, variables are written using the syntax for
-symbols.
-
- In Lisp, unlike most programming languages, programs are represented
-primarily as Lisp objects and only secondarily as text. The Lisp
-objects used for variables are symbols: the symbol name is the variable
-name, and the variable's value is stored in the value cell of the
-symbol. The use of a symbol as a variable is independent of its use as
-a function name. @xref{Symbol Components}.
-
- The Lisp objects that constitute a Lisp program determine the textual
-form of the program---it is simply the read syntax for those Lisp
-objects. This is why, for example, a variable in a textual Lisp program
-is written using the read syntax for the symbol that represents the
-variable.
-
address@hidden
-* 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.
-* Frame-Local Variables:: Variable values in effect only in one frame.
-* Future Local Variables:: New kinds of local values we might add some day.
-* File Local Variables:: Handling local variable lists in files.
-* Variable Aliases:: Variables that are aliases for other variables.
-* Variables with Restricted Values:: Non-constant variables whose value can
- @emph{not} be an arbitrary Lisp object.
address@hidden menu
-
address@hidden Global Variables
address@hidden Global Variables
address@hidden 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,
-
address@hidden
-(setq x '(a b))
address@hidden example
-
address@hidden
-gives the variable @code{x} the value @code{(a b)}. Note that
address@hidden does not evaluate its first argument, the name of the
-variable, but it does evaluate the second argument, the new value.
-
- Once the variable has a value, you can refer to it by using the symbol
-by itself as an expression. Thus,
-
address@hidden
address@hidden
-x @result{} (a b)
address@hidden group
address@hidden example
-
address@hidden
-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:
-
address@hidden
address@hidden
-x
- @result{} (a b)
address@hidden group
address@hidden
-(setq x 4)
- @result{} 4
address@hidden group
address@hidden
-x
- @result{} 4
address@hidden group
address@hidden example
-
address@hidden Constant Variables
address@hidden Variables that Never Change
address@hidden setting-constant
address@hidden keyword symbol
address@hidden variable with constant value
address@hidden constant variables
address@hidden symbol that evaluates to itself
address@hidden 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
address@hidden 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.
-
address@hidden
address@hidden
-nil @equiv{} 'nil
- @result{} nil
address@hidden group
address@hidden
-(setq nil 500)
address@hidden Attempt to set constant symbol: nil
address@hidden group
address@hidden example
-
address@hidden keywordp object
-function returns @code{t} if @var{object} is a symbol whose name
-starts with @samp{:}, interned in the standard obarray, and returns
address@hidden otherwise.
address@hidden defun
-
address@hidden Local Variables
address@hidden Local Variables
address@hidden binding local variables
address@hidden local variables
address@hidden local binding
address@hidden global binding
-
- Global variables have values that last until explicitly superseded
-with new values. Sometimes it is useful to create variable values that
-exist temporarily---only until a certain part of the program finishes.
-These values are called @dfn{local}, and the variables so used are
-called @dfn{local variables}.
-
- For example, when a function is called, its argument variables receive
-new local values that last until the function exits. The @code{let}
-special form explicitly establishes new local values for specified
-variables; these last until exit from the @code{let} form.
-
address@hidden shadowing of variables
- Establishing a local value saves away the previous value (or lack of
-one) of the variable. When the life span of the local value is over,
-the previous value is restored. In the mean time, we say that the
-previous value is @dfn{shadowed} and @dfn{not visible}. Both global and
-local values may be shadowed (@pxref{Scope}).
-
- If you set a variable (such as with @code{setq}) while it is local,
-this replaces the local value; it does not alter the global value, or
-previous local values, that are shadowed. To model this behavior, we
-speak of a @dfn{local binding} of the variable as well as a local value.
-
- The local binding is a conceptual place that holds a local value.
-Entry to a function, or a special form such as @code{let}, creates the
-local binding; exit from the function or from the @code{let} removes the
-local binding. As long as the local binding lasts, the variable's value
-is stored within it. Use of @code{setq} or @code{set} while there is a
-local binding stores a different value into the local binding; it does
-not create a new binding.
-
- We also speak of the @dfn{global binding}, which is where
-(conceptually) the global value is kept.
-
address@hidden 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
address@hidden binding} of the variable. (This rule is called
address@hidden scoping}; see @ref{Variable Scoping}.) If there are no
-local bindings, the variable's global binding is its current binding.
-We sometimes call the current binding the @dfn{most-local existing
-binding}, for emphasis. Ordinary evaluation of a symbol always returns
-the value of its current binding.
-
- The special forms @code{let} and @code{let*} exist to create
-local bindings.
-
address@hidden let (address@hidden) address@hidden
-This special form binds variables according to @var{bindings} and then
-evaluates all of the @var{forms} in textual order. The @code{let}-form
-returns the value of the last form in @var{forms}.
-
-Each of the @var{bindings} is either @w{(i) a} symbol, in which case
-that symbol is bound to @code{nil}; or @w{(ii) a} list of the form
address@hidden(@var{symbol} @var{value-form})}, in which case @var{symbol} is
-bound to the result of evaluating @var{value-form}. If @var{value-form}
-is omitted, @code{nil} is used.
-
-All of the @var{value-form}s in @var{bindings} are evaluated in the
-order they appear and @emph{before} binding any of the symbols to them.
-Here is an example of this: @code{z} is bound to the old value of
address@hidden, which is 2, not the new value of @code{y}, which is 1.
-
address@hidden
address@hidden
-(setq y 2)
- @result{} 2
address@hidden group
address@hidden
-(let ((y 1)
- (z y))
- (list y z))
- @result{} (1 2)
address@hidden group
address@hidden example
address@hidden defspec
-
address@hidden let* (address@hidden) address@hidden
-This special form is like @code{let}, but it binds each variable right
-after computing its local value, before computing the local value for
-the next variable. Therefore, an expression in @var{bindings} can
-reasonably refer to the preceding symbols bound in this @code{let*}
-form. Compare the following example with the example above for
address@hidden
-
address@hidden
address@hidden
-(setq y 2)
- @result{} 2
address@hidden group
address@hidden
-(let* ((y 1)
- (z y)) ; @r{Use the just-established value of @code{y}.}
- (list y z))
- @result{} (1 1)
address@hidden group
address@hidden example
address@hidden defspec
-
- Here is a complete list of the other facilities that create local
-bindings:
-
address@hidden @bullet
address@hidden
-Function calls (@pxref{Functions}).
-
address@hidden
-Macro calls (@pxref{Macros}).
-
address@hidden
address@hidden (@pxref{Errors}).
address@hidden itemize
-
- Variables can also have buffer-local bindings (@pxref{Buffer-Local
-Variables}) and frame-local bindings (@pxref{Frame-Local Variables}); a
-few variables have terminal-local bindings (@pxref{Multiple Displays}).
-These kinds of bindings work somewhat like ordinary local bindings, but
-they are localized depending on ``where'' you are in Emacs, rather than
-localized in time.
-
address@hidden max-specpdl-size
address@hidden of max-specpdl-size}
address@hidden variable limit error
address@hidden evaluation error
address@hidden infinite recursion
-This variable defines the limit on the total number of local variable
-bindings and @code{unwind-protect} cleanups (@pxref{Cleanups,,
-Cleaning Up from Nonlocal Exits}) that are allowed before signaling an
-error (with data @code{"Variable binding depth exceeds
-max-specpdl-size"}).
-
-This limit, with the associated error when it is exceeded, is one way
-that Lisp avoids infinite recursion on an ill-defined function.
address@hidden provides another limit on depth of nesting.
address@hidden of max-lisp-eval-depth,, Eval}.
-
-The default value is 1000. Entry to the Lisp debugger increases the
-value, if there is little room left, to make sure the debugger itself
-has room to execute.
address@hidden defvar
-
address@hidden Void Variables
address@hidden When a Variable is ``Void''
address@hidden void-variable
address@hidden 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
address@hidden 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}.
-
address@hidden makunbound symbol
-This function makes the current variable binding of @var{symbol} void.
-Subsequent attempts to use this symbol's value as a variable will signal
-the error @code{void-variable}, unless and until you set it again.
-
address@hidden returns @var{symbol}.
-
address@hidden
address@hidden
-(makunbound 'x) ; @r{Make the global value of @code{x} void.}
- @result{} x
address@hidden group
address@hidden
-x
address@hidden Symbol's value as variable is void: x
address@hidden group
address@hidden example
-
-If @var{symbol} is locally bound, @code{makunbound} affects the most
-local existing binding. This is the only way a symbol can have a void
-local binding, since all the constructs that create local bindings
-create them with values. In this case, the voidness lasts at most as
-long as the binding does; when the binding is removed due to exit from
-the construct that made it, the previous local or global binding is
-reexposed as usual, and the variable is no longer void unless the newly
-reexposed binding was void all along.
-
address@hidden
address@hidden
-(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)
address@hidden Symbol's value as variable is void: x
address@hidden group
address@hidden
-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.}
address@hidden Symbol's value as variable is void: x
address@hidden group
-
address@hidden
-(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
address@hidden group
address@hidden smallexample
address@hidden 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.
-
address@hidden boundp variable
address@hidden returns @code{t} if @var{variable} (a symbol) is not void;
-more precisely, if its current binding is not void. It returns
address@hidden otherwise.
-
address@hidden
address@hidden
-(boundp 'abracadabra) ; @r{Starts out void.}
- @result{} nil
address@hidden group
address@hidden
-(let ((abracadabra 5)) ; @r{Locally bind it.}
- (boundp 'abracadabra))
- @result{} t
address@hidden group
address@hidden
-(boundp 'abracadabra) ; @r{Still globally void.}
- @result{} nil
address@hidden group
address@hidden
-(setq abracadabra 5) ; @r{Make it globally nonvoid.}
- @result{} 5
address@hidden group
address@hidden
-(boundp 'abracadabra)
- @result{} t
address@hidden group
address@hidden smallexample
address@hidden defun
-
address@hidden Defining Variables
address@hidden Defining Global Variables
address@hidden variable definition
-
- You may announce your intention to use a symbol as a global variable
-with a @dfn{variable definition}: a special form, either @code{defconst}
-or @code{defvar}.
-
- In Emacs Lisp, definitions serve three purposes. First, they inform
-people who read the code that certain symbols are @emph{intended} to be
-used a certain way (as variables). Second, they inform the Lisp system
-of these things, supplying a value and documentation. Third, they
-provide information to utilities such as @code{etags} and
address@hidden, which create data bases of the functions and
-variables in a program.
-
- The difference between @code{defconst} and @code{defvar} is primarily
-a matter of intent, serving to inform human readers of whether the value
-should ever change. Emacs Lisp does not restrict the ways in which a
-variable can be used based on @code{defconst} or @code{defvar}
-declarations. However, it does make a difference for initialization:
address@hidden unconditionally initializes the variable, while
address@hidden initializes it only if it is void.
-
address@hidden
- One would expect user option variables to be defined with
address@hidden, since programs do not change them. Unfortunately, this
-has bad results if the definition is in a library that is not preloaded:
address@hidden would override any prior value when the library is
-loaded. Users would like to be able to set user options in their init
-files, and override the default values given in the definitions. For
-this reason, user options must be defined with @code{defvar}.
address@hidden ignore
-
address@hidden defvar symbol [value [doc-string]]
-This special form defines @var{symbol} as a variable and can also
-initialize and document it. The definition informs a person reading
-your code that @var{symbol} is used as a variable that might be set or
-changed. Note that @var{symbol} is not evaluated; the symbol to be
-defined must appear explicitly in the @code{defvar}.
-
-If @var{symbol} is void and @var{value} is specified, @code{defvar}
-evaluates it and sets @var{symbol} to the result. But if @var{symbol}
-already has a value (i.e., it is not void), @var{value} is not even
-evaluated, and @var{symbol}'s value remains unchanged. If @var{value}
-is omitted, the value of @var{symbol} is not changed in any case.
-
-If @var{symbol} has a buffer-local binding in the current buffer,
address@hidden 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
address@hidden arranges to set the variable unconditionally, without
-testing whether its value is void.
-
-If the @var{doc-string} argument appears, it specifies the documentation
-for the variable. (This opportunity to specify documentation is one of
-the main benefits of defining the variable.) The documentation is
-stored in the symbol's @code{variable-documentation} property. The
-Emacs help functions (@pxref{Documentation}) look for this property.
-
-If the variable is a user option that users would want to set
-interactively, you should use @samp{*} as the first character of
address@hidden This lets users set the variable conveniently using
-the @code{set-variable} command. Note that you should nearly always
-use @code{defcustom} instead of @code{defvar} to define these
-variables, so that users can use @kbd{M-x customize} and related
-commands to set them. @xref{Customization}.
-
-Here are some examples. This form defines @code{foo} but does not
-initialize it:
-
address@hidden
address@hidden
-(defvar foo)
- @result{} foo
address@hidden group
address@hidden example
-
-This example initializes the value of @code{bar} to @code{23}, and gives
-it a documentation string:
-
address@hidden
address@hidden
-(defvar bar 23
- "The normal weight of a bar.")
- @result{} bar
address@hidden group
address@hidden example
-
-The following form changes the documentation string for @code{bar},
-making it a user option, but does not change the value, since @code{bar}
-already has a value. (The addition @code{(1+ nil)} would get an error
-if it were evaluated, but since it is not evaluated, there is no error.)
-
address@hidden
address@hidden
-(defvar bar (1+ nil)
- "*The normal weight of a bar.")
- @result{} bar
address@hidden group
address@hidden
-bar
- @result{} 23
address@hidden group
address@hidden example
-
-Here is an equivalent expression for the @code{defvar} special form:
-
address@hidden
address@hidden
-(defvar @var{symbol} @var{value} @var{doc-string})
address@hidden
-(progn
- (if (not (boundp '@var{symbol}))
- (setq @var{symbol} @var{value}))
- (if '@var{doc-string}
- (put '@var{symbol} 'variable-documentation '@var{doc-string}))
- '@var{symbol})
address@hidden group
address@hidden 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.
address@hidden defspec
-
address@hidden 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}.
-
address@hidden always evaluates @var{value}, and sets the value of
address@hidden 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
address@hidden)
-
-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.
-
address@hidden
address@hidden
-(defconst pi 3.1415 "Pi to five places.")
- @result{} pi
address@hidden group
address@hidden
-(setq pi 3)
- @result{} pi
address@hidden group
address@hidden
-pi
- @result{} 3
address@hidden group
address@hidden example
address@hidden defspec
-
address@hidden user-variable-p variable
address@hidden user option
-This function returns @code{t} if @var{variable} is a user option---a
-variable intended to be set by the user for customization---and
address@hidden otherwise. (Variables other than user options exist for the
-internal purposes of Lisp programs, and users need not know about them.)
-
-User option variables are distinguished from other variables either
-though being declared using @address@hidden may also be
-declared equivalently in @file{cus-start.el}.} or by the first character
-of their @code{variable-documentation} property. If the property exists
-and is a string, and its first character is @samp{*}, then the variable
-is a user option. Aliases of user options are also user options.
address@hidden defun
-
address@hidden variable-interactive
- If a user option variable has a @code{variable-interactive} property,
-the @code{set-variable} command uses that value to control reading the
-new value for the variable. The property's value is used as if it were
-specified in @code{interactive} (@pxref{Using Interactive}). However,
-this feature is largely obsoleted by @code{defcustom}
-(@pxref{Customization}).
-
- @strong{Warning:} If the @code{defconst} and @code{defvar} special
-forms are used while the variable has a local binding (made with
address@hidden, or a function argument), they set the local-binding's
-value; the top-level binding is not changed. This is not what you
-usually want. To prevent it, use these special forms at top level in
-a file, where normally no local binding is in effect, and make sure to
-load the file before making a local binding for the variable.
-
address@hidden Tips for Defining
address@hidden 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
address@hidden, respectively.
-
- There are several other variable name conventions;
-here is a complete list:
-
address@hidden @samp
address@hidden @dots{}-hook
-The variable is a normal hook (@pxref{Hooks}).
-
address@hidden @dots{}-function
-The value is a function.
-
address@hidden @dots{}-functions
-The value is a list of functions.
-
address@hidden @dots{}-form
-The value is a form (an expression).
-
address@hidden @dots{}-forms
-The value is a list of forms (expressions).
-
address@hidden @dots{}-predicate
-The value is a predicate---a function of one argument that returns
address@hidden for ``good'' arguments and @code{nil} for ``bad''
-arguments.
-
address@hidden @dots{}-flag
-The value is significant only as to whether it is @code{nil} or not.
-
address@hidden @dots{}-program
-The value is a program name.
-
address@hidden @dots{}-command
-The value is a whole shell command.
-
address@hidden @dots{}-switches
-The value specifies options for a command.
address@hidden table
-
- When you define a variable, always consider whether you should mark
-it as ``risky''; see @ref{File Local Variables}.
-
- When defining and initializing a variable that holds a complicated
-value (such as a keymap with bindings in it), it's best to put the
-entire computation of the value into the @code{defvar}, like this:
-
address@hidden
-(defvar my-mode-map
- (let ((map (make-sparse-keymap)))
- (define-key map "\C-c\C-a" 'my-command)
- @dots{}
- map)
- @var{docstring})
address@hidden example
-
address@hidden
-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
address@hidden @emph{will} reinitialize the map completely.
-
- Putting so much code in the @code{defvar} form has one disadvantage:
-it puts the documentation string far away from the line which names the
-variable. Here's a safe way to avoid that:
-
address@hidden
-(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)))
address@hidden example
-
address@hidden
-This has all the same advantages as putting the initialization inside
-the @code{defvar}, except that you must type @kbd{C-M-x} twice, once on
-each form, if you do want to reinitialize the variable.
-
- But be careful not to write the code like this:
-
address@hidden
-(defvar my-mode-map nil
- @var{docstring})
-(unless my-mode-map
- (setq my-mode-map (make-sparse-keymap))
- (define-key my-mode-map "\C-c\C-a" 'my-command)
- @dots{})
address@hidden example
-
address@hidden
-This code sets the variable, then alters it, but it does so in more than
-one step. If the user quits just after the @code{setq}, that leaves the
-variable neither correctly initialized nor void nor @code{nil}. Once
-that happens, reloading the file will not initialize the variable; it
-will remain incomplete.
-
address@hidden Accessing Variables
address@hidden 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}.
-
address@hidden 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.
-
address@hidden
address@hidden
-(setq abracadabra 5)
- @result{} 5
address@hidden group
address@hidden
-(setq foo 9)
- @result{} 9
address@hidden group
-
address@hidden
-;; @r{Here the symbol @code{abracadabra}}
-;; @r{is the symbol whose value is examined.}
-(let ((abracadabra 'foo))
- (symbol-value 'abracadabra))
- @result{} foo
address@hidden group
-
address@hidden
-;; @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
address@hidden group
-
address@hidden
-(symbol-value 'abracadabra)
- @result{} 5
address@hidden group
address@hidden example
-
-A @code{void-variable} error is signaled if the current binding of
address@hidden is void.
address@hidden defun
-
address@hidden Setting Variables
address@hidden 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}.
-
address@hidden setq [symbol address@hidden
-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.
-
address@hidden does not evaluate @var{symbol}; it sets the symbol that you
-write. We say that this argument is @dfn{automatically quoted}. The
address@hidden in @code{setq} stands for ``quoted.''
-
-The value of the @code{setq} form is the value of the last @var{form}.
-
address@hidden
address@hidden
-(setq x (1+ 2))
- @result{} 3
address@hidden group
-x ; @address@hidden now has a global value.}
- @result{} 3
address@hidden
-(let ((x 5))
- (setq x 6) ; @r{The local binding of @code{x} is set.}
- x)
- @result{} 6
address@hidden group
-x ; @r{The global value is unchanged.}
- @result{} 3
address@hidden example
-
-Note that the first @var{form} is evaluated, then the first
address@hidden is set, then the second @var{form} is evaluated, then the
-second @var{symbol} is set, and so on:
-
address@hidden
address@hidden
-(setq x 10 ; @r{Notice that @code{x} is set before}
- y (1+ x)) ; @r{the value of @code{y} is computed.}
- @result{} 11
address@hidden group
address@hidden example
address@hidden defspec
-
address@hidden set symbol value
-This function sets @var{symbol}'s value to @var{value}, then returns
address@hidden Since @code{set} is a function, the expression written for
address@hidden is evaluated to obtain the symbol to set.
-
-The most-local existing binding of the variable is the binding that is
-set; shadowed bindings are not affected.
-
address@hidden
address@hidden
-(set one 1)
address@hidden Symbol's value as variable is void: one
address@hidden group
address@hidden
-(set 'one 1)
- @result{} 1
address@hidden group
address@hidden
-(set 'two 'one)
- @result{} one
address@hidden group
address@hidden
-(set two 2) ; @address@hidden evaluates to symbol @code{one}.}
- @result{} 2
address@hidden group
address@hidden
-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
address@hidden group
address@hidden
-one
- @result{} 2
address@hidden group
address@hidden example
-
-If @var{symbol} is not actually a symbol, a @code{wrong-type-argument}
-error is signaled.
-
address@hidden
-(set '(x y) 'z)
address@hidden Wrong type argument: symbolp, (x y)
address@hidden example
-
-Logically speaking, @code{set} is a more fundamental primitive than
address@hidden Any use of @code{setq} can be trivially rewritten to use
address@hidden; @code{setq} could even be defined as a macro, given the
-availability of @code{set}. However, @code{set} itself is rarely used;
-beginners hardly need to know about it. It is useful only for choosing
-at run time which variable to set. For example, the command
address@hidden, which reads a variable name from the user and then
-sets the variable, needs to use @code{set}.
-
address@hidden CL address@hidden local
address@hidden
address@hidden Lisp note:} In Common Lisp, @code{set} always changes the
-symbol's ``special'' or dynamic value, ignoring any lexical bindings.
-In Emacs Lisp, all variables and all bindings are dynamic, so @code{set}
-always affects the most local existing binding.
address@hidden quotation
address@hidden defun
-
address@hidden Variable Scoping
address@hidden Scoping Rules for Variable Bindings
-
- A given symbol @code{foo} can have several local variable bindings,
-established at different places in the Lisp program, as well as a global
-binding. The most recently established binding takes precedence over
-the others.
-
address@hidden scope
address@hidden extent
address@hidden dynamic scoping
address@hidden lexical scoping
- Local bindings in Emacs Lisp have @dfn{indefinite scope} and
address@hidden 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
address@hidden scoping}. By contrast, most programming languages use
address@hidden scoping}, in which references to a local variable must be
-located textually within the function or block that binds the variable.
-
address@hidden CL note---special variables
address@hidden
address@hidden Lisp note:} Variables declared ``special'' in Common Lisp are
-dynamically scoped, like all variables in Emacs Lisp.
address@hidden quotation
-
address@hidden
-* 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.
address@hidden menu
-
address@hidden Scope
address@hidden 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:
-
address@hidden
address@hidden
-(defun binder (x) ; @address@hidden is bound in @code{binder}.}
- (foo 5)) ; @address@hidden is some other function.}
address@hidden group
-
address@hidden
-(defun user () ; @address@hidden is used ``free'' in @code{user}.}
- (list x))
address@hidden group
address@hidden example
-
- In a lexically scoped language, the binding of @code{x} in
address@hidden would never be accessible in @code{user}, because
address@hidden is not textually contained within the function
address@hidden However, in dynamically-scoped Emacs Lisp, @code{user}
-may or may not refer to the binding of @code{x} established in
address@hidden, depending on the circumstances:
-
address@hidden @bullet
address@hidden
-If we call @code{user} directly without calling @code{binder} at all,
-then whatever binding of @code{x} is found, it cannot come from
address@hidden
-
address@hidden
-If we define @code{foo} as follows and then call @code{binder}, then the
-binding made in @code{binder} will be seen in @code{user}:
-
address@hidden
address@hidden
-(defun foo (lose)
- (user))
address@hidden group
address@hidden example
-
address@hidden
-However, if we define @code{foo} as follows and then call @code{binder},
-then the binding made in @code{binder} @emph{will not} be seen in
address@hidden:
-
address@hidden
-(defun foo (x)
- (user))
address@hidden example
-
address@hidden
-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
address@hidden) Therefore, @code{user} will access the @code{x} bound
-by @code{foo} instead of the one bound by @code{binder}.
address@hidden itemize
-
-Emacs Lisp uses dynamic scoping because simple implementations of
-lexical scoping are slow. In addition, every Lisp system needs to offer
-dynamic scoping at least as an option; if lexical scoping is the norm,
-there must be a way to specify dynamic scoping instead for a particular
-variable. It might not be a bad thing for Emacs to offer both, but
-implementing it with dynamic scoping only was much easier.
-
address@hidden Extent
address@hidden Extent
-
- @dfn{Extent} refers to the time during program execution that a
-variable name is valid. In Emacs Lisp, a variable is valid only while
-the form that bound it is executing. This is called @dfn{dynamic
-extent}. ``Local'' or ``automatic'' variables in most languages,
-including C and Pascal, have dynamic extent.
-
- One alternative to dynamic extent is @dfn{indefinite extent}. This
-means that a variable binding can live on past the exit from the form
-that made the binding. Common Lisp and Scheme, for example, support
-this, but Emacs Lisp does not.
-
- To illustrate this, the function below, @code{make-add}, returns a
-function that purports to add @var{n} to its own argument @var{m}. This
-would work in Common Lisp, but it does not do the job in Emacs Lisp,
-because after the call to @code{make-add} exits, the variable @code{n}
-is no longer bound to the actual argument 2.
-
address@hidden
-(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.}
address@hidden Symbol's value as variable is void: n
address@hidden example
-
address@hidden closures not available
- Some Lisp dialects have ``closures,'' objects that are like functions
-but record additional variable bindings. Emacs Lisp does not have
-closures.
-
address@hidden Impl of Scope
address@hidden Implementation of Dynamic Scoping
address@hidden deep binding
-
- A simple sample implementation (which is not how Emacs Lisp actually
-works) may help you understand dynamic binding. This technique is
-called @dfn{deep binding} and was used in early Lisp systems.
-
- Suppose there is a stack of bindings, which are variable-value pairs.
-At entry to a function or to a @code{let} form, we can push bindings
-onto the stack for the arguments or local variables created there. We
-can pop those bindings from the stack at exit from the binding
-construct.
-
- We can find the value of a variable by searching the stack from top to
-bottom for a binding for that variable; the value from that binding is
-the value of the variable. To set the variable, we search for the
-current binding, then store the new value into that binding.
-
- As you can see, a function's bindings remain in effect as long as it
-continues execution, even during its calls to other functions. That is
-why we say the extent of the binding is dynamic. And any other function
-can refer to the bindings, if it uses the same variables while the
-bindings are in effect. That is why we say the scope is indefinite.
-
address@hidden shallow binding
- The actual implementation of variable scoping in GNU Emacs Lisp uses a
-technique called @dfn{shallow binding}. Each variable has a standard
-place in which its current value is always found---the value cell of the
-symbol.
-
- In shallow binding, setting the variable works by storing a value in
-the value cell. Creating a new binding works by pushing the old value
-(belonging to a previous binding) onto a stack, and storing the new
-local value in the value cell. Eliminating a binding works by popping
-the old value off the stack, into the value cell.
-
- We use shallow binding because it has the same results as deep
-binding, but runs faster, since there is never a need to search for a
-binding.
-
address@hidden Using Scoping
address@hidden 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:
-
address@hidden @bullet
address@hidden
-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.
-
address@hidden
-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
address@hidden is defined as address@hidden 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.
address@hidden itemize
-
- In either case, you should define the variable with @code{defvar}.
-This helps other people understand your program by telling them to look
-for inter-function usage. It also avoids a warning from the byte
-compiler. Choose the variable's name to avoid name conflicts---don't
-use short names like @code{x}.
-
address@hidden Buffer-Local Variables
address@hidden Buffer-Local Variables
address@hidden variable, buffer-local
address@hidden 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: @dfn{buffer-local} bindings, which
-apply only in one buffer, and @dfn{frame-local} bindings, which apply only in
-one frame. Having different values for a variable in different buffers
-and/or frames is an important customization method.
-
- This section describes buffer-local bindings; for frame-local
-bindings, see the following section, @ref{Frame-Local Variables}. (A few
-variables have bindings that are local to each terminal; see
address@hidden Displays}.)
-
address@hidden
-* 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.
address@hidden menu
-
address@hidden Intro to Buffer-Local
address@hidden 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
-(assuming there are no frame-local bindings to complicate the matter),
-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
address@hidden, 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.
-
address@hidden automatically buffer-local
- A more powerful operation is to mark the variable as
address@hidden buffer-local} by calling
address@hidden 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 or frame-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 or frame-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
address@hidden while still in the other buffer, you won't see the
-unbinding occur (though it will occur properly). Here is an example
-to illustrate:
-
address@hidden
address@hidden
-(setq foo 'g)
-(set-buffer "a")
-(make-local-variable 'foo)
address@hidden 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}}
- @address@hidden)
address@hidden
-foo @result{} 'g ; @r{exiting restored the local value in buffer
@samp{a},}
- ; @r{but we don't see that in buffer @samp{b}}
address@hidden group
address@hidden
-(set-buffer "a") ; @r{verify the local value was restored}
-foo @result{} 'a
address@hidden group
address@hidden example
-
- Note that references to @code{foo} in @var{body} access the
-buffer-local binding of buffer @samp{b}.
-
- When a file specifies local variable values, these become buffer-local
-values when you visit the file. @xref{File Variables,,, emacs, The
-GNU Emacs Manual}.
-
address@hidden Creating Buffer-Local
address@hidden Creating and Deleting Buffer-Local Bindings
-
address@hidden Command make-local-variable variable
-This function creates a buffer-local binding in the current buffer for
address@hidden (a symbol). Other buffers are not affected. The value
-returned is @var{variable}.
-
address@hidden Emacs 19 feature
-The buffer-local value of @var{variable} starts out as the same value
address@hidden previously had. If @var{variable} was void, it remains
-void.
-
address@hidden
address@hidden
-;; @r{In buffer @samp{b1}:}
-(setq foo 5) ; @r{Affects all buffers.}
- @result{} 5
address@hidden group
address@hidden
-(make-local-variable 'foo) ; @r{Now it is local in @samp{b1}.}
- @result{} foo
address@hidden group
address@hidden
-foo ; @r{That did not change}
- @result{} 5 ; @r{the value.}
address@hidden group
address@hidden
-(setq foo 6) ; @r{Change the value}
- @result{} 6 ; @r{in @samp{b1}.}
address@hidden group
address@hidden
-foo
- @result{} 6
address@hidden group
-
address@hidden
-;; @r{In buffer @samp{b2}, the value hasn't changed.}
-(save-excursion
- (set-buffer "b2")
- foo)
- @result{} 5
address@hidden group
address@hidden example
-
-Making a variable buffer-local within a @code{let}-binding for that
-variable does not work reliably, unless the buffer in which you do this
-is not current either on entry to or exit from the @code{let}. This is
-because @code{let} does not distinguish between different kinds of
-bindings; it knows only which variable the binding was made for.
-
-If the variable is terminal-local, this function signals an error. Such
-variables cannot have buffer-local bindings as well. @xref{Multiple
-Displays}.
-
address@hidden:} 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
address@hidden
address@hidden deffn
-
address@hidden Command make-variable-buffer-local variable
-This function marks @var{variable} (a symbol) automatically
-buffer-local, so that any subsequent attempt to set it will make it
-local to the current buffer at the time.
-
-A peculiar wrinkle of this feature is that binding the variable (with
address@hidden or other binding constructs) does not create a buffer-local
-binding for it. Only setting the variable (with @code{set} or
address@hidden), 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}.
-
address@hidden:} Don't assume that you should use
address@hidden 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
address@hidden can be the best solution.
address@hidden deffn
-
address@hidden local-variable-p variable &optional buffer
-This returns @code{t} if @var{variable} is buffer-local in buffer
address@hidden (which defaults to the current buffer); otherwise,
address@hidden
address@hidden defun
-
address@hidden local-variable-if-set-p variable &optional buffer
-This returns @code{t} if @var{variable} will become buffer-local in
-buffer @var{buffer} (which defaults to the current buffer) if it is
-set there.
address@hidden defun
-
address@hidden 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.
address@hidden defun
-
address@hidden buffer-local-variables &optional buffer
-This function returns a list describing the buffer-local variables in
-buffer @var{buffer}. (If @var{buffer} is omitted, the current buffer is
-used.) It returns an association list (@pxref{Association Lists}) in
-which each element contains one buffer-local variable and its value.
-However, when a variable's buffer-local binding in @var{buffer} is void,
-then the variable appears directly in the resulting list.
-
address@hidden
address@hidden
-(make-local-variable 'foobar)
-(makunbound 'foobar)
-(make-local-variable 'bind-me)
-(setq bind-me 69)
address@hidden group
-(setq lcl (buffer-local-variables))
- ;; @r{First, built-in variables local in all buffers:}
address@hidden ((mark-active . nil)
- (buffer-undo-list . nil)
- (mode-name . "Fundamental")
- @dots{}
address@hidden
- ;; @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))
address@hidden group
address@hidden 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.
address@hidden defun
-
address@hidden Command kill-local-variable variable
-This function deletes the buffer-local binding (if any) for
address@hidden (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.
-
address@hidden 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.
address@hidden deffn
-
address@hidden kill-all-local-variables
-This function eliminates all the buffer-local variable bindings of the
-current buffer except for variables marked as ``permanent.'' As a
-result, the buffer will see the default values of most variables.
-
-This function also resets certain other information pertaining to the
-buffer: it sets the local keymap to @code{nil}, the syntax table to the
-value of @code{(standard-syntax-table)}, the case table to
address@hidden(standard-case-table)}, and the abbrev table to the value of
address@hidden
-
-The very first thing this function does is run the normal hook
address@hidden (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.
-
address@hidden returns @code{nil}.
address@hidden defun
-
address@hidden 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}.
address@hidden defvar
-
address@hidden Emacs 19 feature
address@hidden permanent local variable
-A buffer-local variable is @dfn{permanent} if the variable name (a
-symbol) has a @code{permanent-local} property that is address@hidden
-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.
-
address@hidden Default Value
address@hidden The Default Value of a Buffer-Local Variable
address@hidden 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
address@hidden to change the default setting of
address@hidden 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.
-
address@hidden 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 or frame-local value.
-
address@hidden 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}).
address@hidden defun
-
address@hidden Emacs 19 feature
address@hidden default-boundp symbol
-The function @code{default-boundp} tells you whether @var{symbol}'s
-default value is nonvoid. If @code{(default-boundp 'foo)} returns
address@hidden, then @code{(default-value 'foo)} would get an error.
-
address@hidden is to @code{default-value} as @code{boundp} is to
address@hidden
address@hidden defun
-
address@hidden setq-default [symbol address@hidden
-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
address@hidden 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.
-
address@hidden
address@hidden
-;; @r{In buffer @samp{foo}:}
-(make-local-variable 'buffer-local)
- @result{} buffer-local
address@hidden group
address@hidden
-(setq buffer-local 'value-in-foo)
- @result{} value-in-foo
address@hidden group
address@hidden
-(setq-default buffer-local 'new-default)
- @result{} new-default
address@hidden group
address@hidden
-buffer-local
- @result{} value-in-foo
address@hidden group
address@hidden
-(default-value 'buffer-local)
- @result{} new-default
address@hidden group
-
address@hidden
-;; @r{In (the new) buffer @samp{bar}:}
-buffer-local
- @result{} new-default
address@hidden group
address@hidden
-(default-value 'buffer-local)
- @result{} new-default
address@hidden group
address@hidden
-(setq buffer-local 'another-default)
- @result{} another-default
address@hidden group
address@hidden
-(default-value 'buffer-local)
- @result{} another-default
address@hidden group
-
address@hidden
-;; @r{Back in buffer @samp{foo}:}
-buffer-local
- @result{} value-in-foo
-(default-value 'buffer-local)
- @result{} another-default
address@hidden group
address@hidden example
address@hidden defspec
-
address@hidden set-default symbol value
-This function is like @code{setq-default}, except that @var{symbol} is
-an ordinary evaluated argument.
-
address@hidden
address@hidden
-(set-default (car '(a b c)) 23)
- @result{} 23
address@hidden group
address@hidden
-(default-value 'a)
- @result{} 23
address@hidden group
address@hidden example
address@hidden defun
-
address@hidden Frame-Local Variables
address@hidden Frame-Local Variables
address@hidden frame-local variables
-
- Just as variables can have buffer-local bindings, they can also have
-frame-local bindings. These bindings belong to one frame, and are in
-effect when that frame is selected. Frame-local bindings are actually
-frame parameters: you create a frame-local binding in a specific frame
-by calling @code{modify-frame-parameters} and specifying the variable
-name as the parameter name.
-
- To enable frame-local bindings for a certain variable, call the function
address@hidden
-
address@hidden Command make-variable-frame-local variable
-Enable the use of frame-local bindings for @var{variable}. This does
-not in itself create any frame-local bindings for the variable; however,
-if some frame already has a value for @var{variable} as a frame
-parameter, that value automatically becomes a frame-local binding.
-
-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.
-
-If the variable is terminal-local, this function signals an error,
-because such variables cannot have frame-local bindings as well.
address@hidden Displays}. A few variables that are implemented
-specially in Emacs can be buffer-local, but can never be frame-local.
-
-This command returns @var{variable}.
address@hidden deffn
-
- Buffer-local bindings take precedence over frame-local bindings. Thus,
-consider a variable @code{foo}: if the current buffer has a buffer-local
-binding for @code{foo}, that binding is active; otherwise, if the
-selected frame has a frame-local binding for @code{foo}, that binding is
-active; otherwise, the default binding of @code{foo} is active.
-
- Here is an example. First we prepare a few bindings for @code{foo}:
-
address@hidden
-(setq f1 (selected-frame))
-(make-variable-frame-local 'foo)
-
-;; @r{Make a buffer-local binding for @code{foo} in @samp{b1}.}
-(set-buffer (get-buffer-create "b1"))
-(make-local-variable 'foo)
-(setq foo '(b 1))
-
-;; @r{Make a frame-local binding for @code{foo} in a new frame.}
-;; @r{Store that frame in @code{f2}.}
-(setq f2 (make-frame))
-(modify-frame-parameters f2 '((foo . (f 2))))
address@hidden example
-
- Now we examine @code{foo} in various contexts. Whenever the
-buffer @samp{b1} is current, its buffer-local binding is in effect,
-regardless of the selected frame:
-
address@hidden
-(select-frame f1)
-(set-buffer (get-buffer-create "b1"))
-foo
- @result{} (b 1)
-
-(select-frame f2)
-(set-buffer (get-buffer-create "b1"))
-foo
- @result{} (b 1)
address@hidden example
-
address@hidden
-Otherwise, the frame gets a chance to provide the binding; when frame
address@hidden is selected, its frame-local binding is in effect:
-
address@hidden
-(select-frame f2)
-(set-buffer (get-buffer "*scratch*"))
-foo
- @result{} (f 2)
address@hidden example
-
address@hidden
-When neither the current buffer nor the selected frame provides
-a binding, the default binding is used:
-
address@hidden
-(select-frame f1)
-(set-buffer (get-buffer "*scratch*"))
-foo
- @result{} nil
address@hidden example
-
address@hidden
-When the active binding of a variable is a frame-local binding, setting
-the variable changes that binding. You can observe the result with
address@hidden:
-
address@hidden
-(select-frame f2)
-(set-buffer (get-buffer "*scratch*"))
-(setq foo 'nobody)
-(assq 'foo (frame-parameters f2))
- @result{} (foo . nobody)
address@hidden example
-
address@hidden Future Local Variables
address@hidden Possible Future Local Variables
-
- We have considered the idea of bindings that are local to a category
-of frames---for example, all color frames, or all frames with dark
-backgrounds. We have not implemented them because it is not clear that
-this feature is really useful. You can get more or less the same
-results by adding a function to @code{after-make-frame-functions}, set up to
-define a particular frame parameter according to the appropriate
-conditions for each frame.
-
- It would also be possible to implement window-local bindings. We
-don't know of many situations where they would be useful, and it seems
-that indirect buffers (@pxref{Indirect Buffers}) with buffer-local
-bindings offer a way to handle these situations more robustly.
-
- If sufficient application is found for either of these two kinds of
-local bindings, we will provide it in a subsequent Emacs version.
-
address@hidden File Local Variables
address@hidden File Local Variables
address@hidden file local variables
-
- A file can specify local variable values; Emacs uses these to create
-buffer-local bindings for those variables in the buffer visiting that
-file. @xref{File variables, , Local Variables in Files, emacs, The
-GNU Emacs Manual}, for basic information about file local variables.
-This section describes the functions and variables that affect
-processing of file local variables.
-
address@hidden enable-local-variables
-This variable controls whether to process file local variables.
-The possible values are:
-
address@hidden @asis
address@hidden @code{t} (the default)
-Set the safe variables, and query (once) about any unsafe variables.
address@hidden @code{:safe}
-Set only the safe variables and do not query.
address@hidden @code{:all}
-Set all the variables and do not query.
address@hidden @code{nil}
-Don't set any variables.
address@hidden anything else
-Query (once) about all the variables.
address@hidden table
address@hidden defopt
-
address@hidden 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
address@hidden has its effect here. However, this
-function does not look for the @samp{mode:} local variable in the
address@hidden@samp{-*-}} line. @code{set-auto-mode} does that, also taking
address@hidden into account (@pxref{Auto Major Mode}).
-
-If the optional argument @var{mode-only} is address@hidden, then all
-this function does is return @code{t} if the @address@hidden line or
-the local variables list specifies a mode and @code{nil} otherwise.
-It does not set the mode nor any other file local variable.
address@hidden defun
-
- If a file local variable could specify a function that would
-be called later, or an expression that would be executed later, simply
-visiting a file could take over your Emacs. Emacs takes several
-measures to prevent this.
-
address@hidden safe local variable
- You can specify safe values for a variable with a
address@hidden property. The property has to be
-a function of one argument; any value is safe if the function
-returns address@hidden given that value. Many commonly encountered
-file variables standardly have @code{safe-local-variable} properties,
-including @code{fill-column}, @code{fill-prefix}, and
address@hidden 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.
-
address@hidden 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
address@hidden, and saves it to the user's custom
-file.
address@hidden defopt
-
address@hidden safe-local-variable-p sym val
-This function returns address@hidden if it is safe to give @var{sym}
-the value @var{val}, based on the above criteria.
address@hidden defun
-
address@hidden @cindex risky local variable Duplicates risky-local-variable
- Some variables are considered @dfn{risky}. A variable whose name
-ends in any of @samp{-command}, @samp{-frame-alist}, @samp{-function},
address@hidden, @samp{-hook}, @samp{-hooks}, @samp{-form},
address@hidden, @samp{-map}, @samp{-map-alist}, @samp{-mode-alist},
address@hidden, or @samp{-predicate} is considered risky. The
-variables @samp{font-lock-keywords}, @samp{font-lock-keywords}
-followed by a digit, and @samp{font-lock-syntactic-keywords} are also
-considered risky. Finally, any variable whose name has a
address@hidden @code{risky-local-variable} property is considered
-risky.
-
address@hidden risky-local-variable-p sym
-This function returns address@hidden if @var{sym} is a risky variable,
-based on the above criteria.
address@hidden defun
-
- If a variable is risky, it will not be entered automatically into
address@hidden as described above. Therefore,
-Emacs will always query before setting a risky variable, unless the
-user explicitly allows the setting by customizing
address@hidden directly.
-
address@hidden 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.
address@hidden defvar
-
- The @samp{Eval:} ``variable'' is also a potential loophole, so Emacs
-normally asks for confirmation before handling it.
-
address@hidden 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}.
address@hidden defopt
-
address@hidden 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.
address@hidden defopt
-
- If the expression is a function call and the function has a
address@hidden 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.
-
address@hidden Variable Aliases
address@hidden Variable Aliases
address@hidden 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}.
-
address@hidden 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
address@hidden The two aliased variable names always share the
-same value and the same bindings.
-
-If the @var{docstring} argument is address@hidden, it specifies the
-documentation for @var{new-alias}; otherwise, the alias gets the same
-documentation as @var{base-variable} has, if any, unless
address@hidden 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}.
address@hidden 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.
-
address@hidden make-obsolete-variable obsolete-name current-name &optional when
-This function makes the byte-compiler warn that the variable
address@hidden is obsolete. If @var{current-name} is a symbol, it is
-the variable's new name; then the warning message says to use
address@hidden instead of @var{obsolete-name}. If @var{current-name}
-is a string, this is the message and there is no replacement variable.
-
-If provided, @var{when} should be a string indicating when the
-variable was first made obsolete---for example, a date or a release
-number.
address@hidden defun
-
- You can make two variables synonyms and declare one obsolete at the
-same time using the macro @code{define-obsolete-variable-alias}.
-
address@hidden 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:
-
address@hidden
-(defvaralias @var{obsolete-name} @var{current-name} @var{docstring})
-(make-obsolete-variable @var{obsolete-name} @var{current-name} @var{when})
address@hidden example
address@hidden defmac
-
address@hidden 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.
address@hidden defun
-
address@hidden
-(defvaralias 'foo 'bar)
-(indirect-variable 'foo)
- @result{} bar
-(indirect-variable 'bar)
- @result{} bar
-(setq bar 2)
-bar
- @result{} 2
address@hidden
-foo
- @result{} 2
address@hidden group
-(setq foo 0)
-bar
- @result{} 0
-foo
- @result{} 0
address@hidden example
-
address@hidden Variables with Restricted Values
address@hidden 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
address@hidden Like variables defined in Lisp, these can take on
-any value. However, some variables are defined using
address@hidden or @code{DEFVAR_BOOL}. @xref{Defining Lisp
-variables in C,, Writing Emacs Primitives}, in particular the
-description of functions of the type @address@hidden,
-for a brief discussion of the C implementation.
-
- Variables of type @code{DEFVAR_BOOL} can only take on the values
address@hidden or @code{t}. Attempting to assign them any other value
-will set them to @code{t}:
-
address@hidden
-(let ((display-hourglass 5))
- display-hourglass)
- @result{} t
address@hidden example
-
address@hidden byte-boolean-vars
-This variable holds a list of all variables of type @code{DEFVAR_BOOL}.
address@hidden 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:
-
address@hidden
-(setq window-min-height 5.0)
address@hidden Wrong type argument: integerp, 5.0
address@hidden example
-
address@hidden
- arch-tag: 5ff62c44-2b51-47bb-99d4-fea5aeec5d3e
address@hidden ignore
- [Emacs-diffs] Changes to variables.texi,
Glenn Morris <=