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### 14.3 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-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 signal no error.

    These functions are meant for options defined in pre-loaded files, where the standard expression may signal an error because some required variable or function is not yet defined. The value normally gets updated in `startup.el`, ignoring the value computed by `defcustom`. After startup, if one unsets the value and reevaluates the `defcustom`, the standard expression can be evaluated without 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; 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, 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)

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.

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