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15.3 Defining Customization Variables

Customizable variables, also called user options, are global Lisp variables whose values can be set through the Customize interface. Unlike other global variables, which are defined with defvar (see Defining Variables), customizable variables are defined using the defcustom macro. In addition to calling defvar as a subroutine, defcustom states how the variable should be displayed in the Customize interface, the values it is allowed to take, etc.

— Macro: defcustom option standard doc [keyword value]...

This macro declares option as a user option (i.e., a customizable variable). You should not quote option.

The argument standard is an expression that specifies the standard value for option. Evaluating the defcustom form evaluates standard, but does not necessarily bind the option to that value. If option already has a default value, it is left unchanged. If the user has already saved a customization for option, the user's customized value is installed as the default value. Otherwise, the result of evaluating standard is installed as the default value.

Like defvar, this macro marks option as a special variable, meaning that it should always be dynamically bound. If option is already lexically bound, that lexical binding remains in effect until the binding construct exits. See Variable Scoping.

The expression standard can be evaluated at various other times, too—whenever the customization facility needs to know option's standard value. So be sure to use an expression which is harmless to evaluate at any time.

The argument doc specifies the documentation string for the variable.

If a defcustom does not specify any :group, the last group defined with defgroup in the same file will be used. This way, most defcustom do not need an explicit :group.

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, see Defining Variables.) Using eval-defun on a defcustom that is already defined calls the :set function (see below), if there is one.

If you put a defcustom in a pre-loaded Emacs Lisp file (see Building Emacs), the standard value installed at dump time might be incorrect, e.g., because another variable that it depends on has not been assigned the right value yet. In that case, use custom-reevaluate-setting, described below, to re-evaluate the standard value after Emacs starts up.

In addition to the keywords listed in Common Keywords, this macro accepts the following keywords:

:type type
Use type as the data type for this option. It specifies which values are legitimate, and how to display the value (see Customization Types). Every defcustom should specify a value for this keyword.
:options value-list
Specify 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.

:set setfunction
Specify setfunction as the way to change the value of this option when using the Customize interface. The function setfunction should take two arguments, a symbol (the option name) 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); preferably, though, it should not modify its value argument destructively. The default for setfunction is set-default.

If you specify this keyword, the variable's documentation string should describe how to do the same job in hand-written Lisp code.

: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 whatever customize should use as the current value for that symbol (which need not be the symbol's Lisp value). The default is default-value.

You have to really understand the workings of Custom to use :get correctly. It is meant for values that are treated in Custom as variables but are not actually stored in Lisp variables. It is almost surely a mistake to specify getfunction for a value that really is stored in a Lisp variable.

:initialize function
function should be a function used to initialize the variable when the defcustom is evaluated. It should take two arguments, the option name (a symbol) and the 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.
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). This is the default :initialize function.
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.
custom-initialize-delay
This functions behaves like custom-initialize-set, but it delays the actual initialization to the next Emacs start. This should be used in files that are preloaded (or for autoloaded variables), so that the initialization is done in the run-time context rather than the build-time context. This also has the side-effect that the (delayed) initialization is performed with the :set function. See Building Emacs.

:risky value
Set the variable's risky-local-variable property to value (see File Local Variables).
:safe function
Set the variable's safe-local-variable property to function (see File Local Variables).
:set-after variables
When setting variables according to saved customizations, make sure to set the variables variables before this one; i.e., 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.

It is useful to specify the :require keyword for an option that turns on a certain feature. This causes Emacs to load the feature, if it is not already loaded, whenever the option is set. See Common Keywords. Here is an example: 

     (defcustom frobnicate-automatically nil
       "Non-nil means automatically frobnicate all buffers."
       :type 'boolean
       :require 'frobnicate-mode
       :group 'frobnicate)

If a customization item has a type such as hook or alist, which supports :options, you can add additional values to the list from outside the defcustom declaration by calling custom-add-frequent-value. 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 reasonable values for emacs-lisp-mode-hook, but not by editing its definition. You can do it thus: 

     (custom-add-frequent-value 'emacs-lisp-mode-hook
        'my-lisp-mode-initialization)
— Function: custom-add-frequent-value symbol value

For the customization option symbol, add value to the list of reasonable values.

The precise effect of adding a value depends on the customization type of symbol.

Internally, defcustom uses the symbol property standard-value to record the expression for the standard value, saved-value to record the value saved by the user with the customization buffer, and customized-value to record the value set by the user with the customization buffer, but not saved. See Symbol Properties. These properties are lists, the car of which is an expression that evaluates to the value.

— Function: custom-reevaluate-setting symbol

This function re-evaluates the standard value of symbol, which should be a user option declared via defcustom. If the variable was customized, this function re-evaluates the saved value instead. Then it sets the user option to that value (using the option's :set property if that is defined).

This is useful for customizable options that are defined before their value could be computed correctly. For example, during startup Emacs calls this function for some user options that were defined in pre-loaded Emacs Lisp files, but whose initial values depend on information available only at run-time.

— Function: custom-variable-p arg

This function returns non-nil if arg is a customizable variable. A customizable variable is either a variable that has a standard-value or custom-autoload property (usually meaning it was declared with defcustom), or an alias for another customizable variable.