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(elisp)Variable Definitions
Defining Customization Variables
================================
Use `defcustom' to declare user-editable variables.
- Macro: defcustom option default doc [keyword value]...
Declare OPTION as a customizable user option variable. Do not
quote OPTION. The argument DOC specifies the documentation string
for the variable. It should often start with a `*' to mark it as
a "user option" (Note: Defining Variables). Do not start the
documentation string with `*' for options which cannot or normally
should not be set with `set-variable'; examples of the former are
global minor mode options such as `global-font-lock-mode' and
examples of the latter are hooks.
If OPTION is void, `defcustom' initializes it to DEFAULT. DEFAULT
should be an expression to compute the value; be careful in
writing it, because it can be evaluated on more than one occasion.
You should normally avoid using backquotes in DEFAULT because
they are not expanded when editing the value, causing list values
to appear to have the wrong structure.
When you evaluate a `defcustom' form with `C-M-x' in Emacs Lisp
mode (`eval-defun'), a special feature of `eval-defun' arranges to
set the variable unconditionally, without testing whether its
value is void. (The same feature applies to `defvar'.) Note:
Defining Variables.
`defcustom' accepts the following additional keywords:
`:type TYPE'
Use TYPE as the data type for this option. It specifies which
values are legitimate, and how to display the value. Note:
Customization Types, for more information.
`:options LIST'
Specify LIST as the list of reasonable values for use in this
option. The user is not restricted to using only these values,
but they are offered as convenient alternatives.
This is meaningful only for certain types, currently including
`hook', `plist' and `alist'. See the definition of the individual
types for a description of how to use `:options'.
`:version VERSION'
This option specifies that the variable was first introduced, or
its default value was changed, in Emacs version VERSION. The value
VERSION must be a string. For example,
(defcustom foo-max 34
"*Maximum number of foo's allowed."
:type 'integer
:group 'foo
:version "20.3")
`:set SETFUNCTION'
Specify SETFUNCTION as the way to change the value of this option.
The function SETFUNCTION should take two arguments, a symbol and
the new value, and should do whatever is necessary to update the
value properly for this option (which may not mean simply setting
the option as a Lisp variable). The default for SETFUNCTION is
`set-default'.
`:get GETFUNCTION'
Specify GETFUNCTION as the way to extract the value of this
option. The function GETFUNCTION should take one argument, a
symbol, and should return the "current value" for that symbol
(which need not be the symbol's Lisp value). The default is
`default-value'.
`:initialize FUNCTION'
FUNCTION should be a function used to initialize the variable when
the `defcustom' is evaluated. It should take two arguments, the
symbol and value. Here are some predefined functions meant for
use in this way:
`custom-initialize-set'
Use the variable's `:set' function to initialize the
variable, but do not reinitialize it if it is already
non-void. This is the default `:initialize' function.
`custom-initialize-default'
Like `custom-initialize-set', but use the function
`set-default' to set the variable, instead of the variable's
`:set' function. This is the usual choice for a variable
whose `:set' function enables or disables a minor mode; with
this choice, defining the variable will not call the minor
mode function, but customizing the variable will do so.
`custom-initialize-reset'
Always use the `:set' function to initialize the variable.
If the variable is already non-void, reset it by calling the
`:set' function using the current value (returned by the
`:get' method).
`custom-initialize-changed'
Use the `:set' function to initialize the variable, if it is
already set or has been customized; otherwise, just use
`set-default'.
`:set-after VARIABLES'
When setting variables according to saved customizations, make
sure to set the variables VARIABLES before this one; in other
words, delay setting this variable until after those others have
been handled. Use `:set-after' if setting this variable won't
work properly unless those other variables already have their
intended values.
The `:require' option is useful for an option that turns on the
operation of a certain feature. Assuming that the package is coded to
check the value of the option, you still need to arrange for the package
to be loaded. You can do that with `:require'. Note: Common
Keywords. Here is an example, from the library `paren.el':
(defcustom show-paren-mode nil
"Toggle Show Paren mode..."
:set (lambda (symbol value)
(show-paren-mode (or value 0)))
:initialize 'custom-initialize-default
:type 'boolean
:group 'paren-showing
:require 'paren)
If a customization item has a type such as `hook' or `alist', which
supports `:options', you can add additional options to the item,
outside the `defcustom' declaration, by calling `custom-add-option'.
For example, if you define a function `my-lisp-mode-initialization'
intended to be called from `emacs-lisp-mode-hook', you might want to
add that to the list of options for `emacs-lisp-mode-hook', but not by
editing its definition. You can do it thus:
(custom-add-option 'emacs-lisp-mode-hook
'my-lisp-mode-initialization)
- Function: custom-add-option symbol option
To the customization SYMBOL, add OPTION.
The precise effect of adding OPTION depends on the customization
type of SYMBOL.
Internally, `defcustom' uses the symbol property `standard-value' to
record the expression for the default value, and `saved-value' to
record the value saved by the user with the customization buffer. The
`saved-value' property is actually a list whose car is an expression
which evaluates to the value.
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