14.3 Defining Customization Variables

Use defcustom to declare user-customizable 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. See Customization Types, for more information.

: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. 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). 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 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-safe-set

custom-initialize-safe-default

These functions behave like custom-initialize-set (custom-initialize-default, respectively), but catch errors. If an error occurs during initialization, they set the variable to nil using set-default, and throw no error.

These two functions are only meant for options defined in pre-loaded files, where some variables or functions used to compute the option's value may not yet be defined. The option normally gets updated in startup.el, ignoring the previously computed value. Because of this typical usage, the value which these two functions compute normally only matters when, after startup, one unsets the option's value and then reevaluates the defcustom. By that time, the necessary variables and functions will be defined, so there will not be an error.

: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; 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.

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, from the library saveplace.el:

     (defcustom save-place nil
       "Non-nil means automatically save place in each file..."
       :type 'boolean
       :require 'saveplace
       :group 'save-place)

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)

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 Property Lists. These properties are lists, the car of which is an expression that evaluates to the value.