Writing Customization Definitions
*********************************

This chapter describes how to declare user options for customization, and also customization groups for classifying them.  We use the term customization item to include both kinds of customization definitions---as well as face definitions (see Defining Faces).

Common Item Keywords
====================

All kinds of customization declarations (for variables and groups, and for faces) accept keyword arguments for specifying various information. This section describes some keywords that apply to all kinds.

All of these keywords, except :tag, can be used more than once in a given item.  Each use of the keyword has an independent effect. The keyword :tag is an exception because any given item can only display one name.

:tag LABEL
     Use LABEL, a string, instead of the item's name, to label the item in customization menus and buffers.

:group GROUP
     Put this customization item in group GROUP.  When you use :group in a defgroup, it makes the new group a subgroup of GROUP.

     If you use this keyword more than once, you can put a single item into more than one group.  Displaying any of those groups will show this item.  Please don't overdo this, since the result would be annoying.

:link LINK-DATA
     Include an external link after the documentation string for this item. This is a sentence containing an active field which references some other documentation.

     There are four alternatives you can use for LINK-DATA:

     (custom-manual INFO-NODE)
          Link to an Info node; INFO-NODE is a string which specifies the node name, as in "(emacs)Top".  The link appears as [manual] in the customization buffer.

     (info-link INFO-NODE)
          Like custom-manual except that the link appears in the customization buffer with the Info node name.

     (url-link URL)
          Link to a web page; URL is a string which specifies the URL. The link appears in the customization buffer as URL.

     (emacs-commentary-link LIBRARY)
          Link to the commentary section of a library; LIBRARY is a string which specifies the library name.

     You can specify the text to use in the customization buffer by adding :tag NAME after the first element of the LINK-DATA; for example, (info-link :tag "foo" "(emacs)Top") makes a link to the Emacs manual which appears in the buffer as foo.

     An item can have more than one external link; however, most items have none at all.

:load FILE
     Load file FILE (a string) before displaying this customization item.  Loading is done with load-library, and only if the file is not already loaded.

:require FEATURE
     Require feature FEATURE (a symbol) when installing a value for this item (an option or a face) that was saved using the customization feature.  This is done by calling require.

     The most common reason to use :require is when a variable enables a feature such as a minor mode, and just setting the variable won't have any effect unless the code which implements the mode is loaded.

Defining Custom Groups
======================

Each Emacs Lisp package should have one main customization group which contains all the options, faces and other groups in the package.  If the package has a small number of options and faces, use just one group and put everything in it.  When there are more than twelve or so options and faces, then you should structure them into subgroups, and put the subgroups under the package's main customization group.  It is OK to put some of the options and faces in the package's main group alongside the subgroups.

The package's main or only group should be a member of one or more of the standard customization groups.  (To display the full list of them, use M-x customize.)  Choose one or more of them (but not too many), and add your group to each of them using the :group keyword.

The way to declare new customization groups is with defgroup.

 -- Macro: defgroup group members doc [keyword value]...
     Declare GROUP as a customization group containing MEMBERS. Do not quote the symbol GROUP.  The argument DOC specifies the documentation string for the group.  It should not start with a * as in defcustom; that convention is for variables only.

     The argument MEMBERS is a list specifying an initial set of customization items to be members of the group.  However, most often MEMBERS is nil, and you specify the group's members by using the :group keyword when defining those members.

     If you want to specify group members through MEMBERS, each element should have the form (NAME WIDGET).  Here NAME is a symbol, and WIDGET is a widget type for editing that symbol. Useful widgets are custom-variable for a variable, custom-face for a face, and custom-group for a group.

     When a new group is introduced into Emacs, use this keyword in defgroup:

     :version VERSION
          This option specifies that the group was first introduced in Emacs version VERSION.  The value VERSION must be a string.

     Tag the group with a version like this when it is introduced, rather than the individual members (see Variable Definitions).

     In addition to the common keywords (see Common Keywords), you can also use this keyword in defgroup:

     :prefix PREFIX
          If the name of an item in the group starts with PREFIX, then the tag for that item is constructed (by default) by omitting PREFIX.

          One group can have any number of prefixes.

The prefix-discarding feature is currently turned off, which means that :prefix currently has no effect.  We did this because we found that discarding the specified prefixes often led to confusing names for options.  This happened because the people who wrote the defgroup definitions for various groups added :prefix keywords whenever they make logical sense---that is, whenever the variables in the library have a common prefix.

In order to obtain good results with :prefix, it would be necessary to check the specific effects of discarding a particular prefix, given the specific items in a group and their names and documentation.  If the resulting text is not clear, then :prefix should not be used in that case.

It should be possible to recheck all the customization groups, delete the :prefix specifications which give unclear results, and then turn this feature back on, if someone would like to do the work.

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 (see 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.) See 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. See 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.  See 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.

Customization Types
===================

When you define a user option with defcustom, you must specify its customization type.  That is a Lisp object which describes (1) which values are legitimate and (2) how to display the value in the customization buffer for editing.

You specify the customization type in defcustom with the :type keyword.  The argument of :type is evaluated; since types that vary at run time are rarely useful, normally you use a quoted constant.  For example:

     (defcustom diff-command "diff"
       "*The command to use to run diff."
       :type '(string)
       :group 'diff)

In general, a customization type is a list whose first element is a symbol, one of the customization type names defined in the following sections.  After this symbol come a number of arguments, depending on the symbol.  Between the type symbol and its arguments, you can optionally write keyword-value pairs (see Type Keywords).

Some of the type symbols do not use any arguments; those are called simple types.  For a simple type, if you do not use any keyword-value pairs, you can omit the parentheses around the type symbol.  For example just string as a customization type is equivalent to (string).

All customization types are implemented as widgets; see Introduction (The Emacs Widget Library) for details.

Simple Types
------------

This section describes all the simple customization types.

sexp
     The value may be any Lisp object that can be printed and read back.  You can use sexp as a fall-back for any option, if you don't want to take the time to work out a more specific type to use.

integer
     The value must be an integer, and is represented textually in the customization buffer.

number
     The value must be a number, and is represented textually in the customization buffer.

string
     The value must be a string, and the customization buffer shows just the contents, with no delimiting " characters and no quoting with \.

regexp
     Like string except that the string must be a valid regular expression.

character
     The value must be a character code.  A character code is actually an integer, but this type shows the value by inserting the character in the buffer, rather than by showing the number.

file
     The value must be a file name, and you can do completion with M-TAB.

(file :must-match t)
     The value must be a file name for an existing file, and you can do completion with M-TAB.

directory
     The value must be a directory name, and you can do completion with M-TAB.

hook
     The value must be a list of functions (or a single function, but that is obsolete usage).  This customization type is used for hook variables. You can use the :options keyword in a hook variable's defcustom to specify a list of functions recommended for use in the hook; see Variable Definitions.

alist
     The value must be a list of cons-cells, the CAR of each cell representing a key, and the CDR of the same cell representing an associated value.  The user can add and delete key/value pairs, and edit both the key and the value of each pair.

     You can specify the key and value types like this:

          (alist :key-type KEY-TYPE :value-type VALUE-TYPE)

     where KEY-TYPE and VALUE-TYPE are customization type specifications.  The default key type is sexp, and the default value type is sexp.

     The user can add any key matching the specified key type, but you can give some keys a preferential treatment by specifying them with the :options (see Variable Definitions).  The specified keys will always be shown in the customize buffer (together with a suitable value), with a checkbox to include or exclude or disable the key/value pair from the alist.  The user will not be able to edit the keys specified by the :options keyword argument.

     The argument to the :options keywords should be a list of option specifications.  Ordinarily, the options are simply atoms, which are the specified keys.  For example:

          :options '("foo" "bar" "baz")

     specifies that there are three ``known'' keys, namely "foo", "bar" and "baz", which will always be shown first.

     You may want to restrict the value type for specific keys, for example, the value associated with the "bar" key can only be an integer. You can specify this by using a list instead of an atom in the option specification.  The first element will specify the key, like before, while the second element will specify the value type.

          :options '("foo" ("bar" integer) "baz")

     Finally, you may want to change how the key is presented.  By default, the key is simply shown as a const, since the user cannot change the special keys specified with the :options keyword.  However, you may want to use a more specialized type for presenting the key, like function-item if you know it is a symbol with a function binding. This is done by using a customization type specification instead of a symbol for the key.

          :options '("foo" ((function-item some-function) integer) "baz")

     Many alists use lists with two elements, instead of cons cells.  For example,

          (defcustom list-alist '(("foo" 1) ("bar" 2) ("baz" 3))
            "Each element is a list of the form (KEY VALUE).")

     instead of

          (defcustom cons-alist '(("foo" . 1) ("bar" . 2) ("baz" . 3))
            "Each element is a cons-cell (KEY . VALUE).")

     Because of the way lists are implemented on top of cons cells, you can treat list-alist in the example above as a cons cell alist, where the value type is a list with a single element containing the real value.

          (defcustom list-alist '(("foo" 1) ("bar" 2) ("baz" 3))
            "Each element is a list of the form (KEY VALUE)."
            :type '(alist :value-type (group integer)))

     The group widget is used here instead of list only because the formatting is better suited for the purpose.

     Similarily, you can have alists with more values associated with each key, using variations of this trick:

          (defcustom person-data '(("brian"  50 t)
                                   ("dorith" 55 nil)
                                   ("ken"    52 t))
            "Alist of basic info about people.
          Each element has the form (NAME AGE MALE-FLAG)."
            :type '(alist :value-type (group age boolean)))

          (defcustom pets '(("brian")
                            ("dorith" "dog" "guppy")
                            ("ken" "cat"))
            "Alist of people's pets.
          In an element (KEY . VALUE), KEY is the person's name,
          and the VALUE is a list of that person's pets."
            :type '(alist :value-type (repeat string)))

plist
     The plist custom type is similar to the alist (see above), except that the information is stored as a property list, i.e. a list of this form:

          (KEY VALUE KEY VALUE KEY VALUE ...)

     The default :key-type for plist is symbol, rather than sexp.

symbol
     The value must be a symbol.  It appears in the customization buffer as the name of the symbol.

function
     The value must be either a lambda expression or a function name.  When it is a function name, you can do completion with M-TAB.

variable
     The value must be a variable name, and you can do completion with M-TAB.

face
     The value must be a symbol which is a face name, and you can do completion with M-TAB.

boolean
     The value is boolean---either nil or t.  Note that by using choice and const together (see the next section), you can specify that the value must be nil or t, but also specify the text to describe each value in a way that fits the specific meaning of the alternative.

coding-system
     The value must be a coding-system name, and you can do completion with M-TAB.

color
     The value must be a valid color name, and you can do completion with M-TAB.  A sample is provided,

Composite Types
---------------

When none of the simple types is appropriate, you can use composite types, which build new types from other types.  Here are several ways of doing that:

(restricted-sexp :match-alternatives CRITERIA)
     The value may be any Lisp object that satisfies one of CRITERIA. CRITERIA should be a list, and each element should be one of these possibilities:

       * A predicate---that is, a function of one argument that has no side effects, and returns either nil or non-nil according to the argument.  Using a predicate in the list says that objects for which the predicate returns non-nil are acceptable.

       * A quoted constant---that is, 'OBJECT.  This sort of element in the list says that OBJECT itself is an acceptable value.

     For example,

          (restricted-sexp :match-alternatives
                           (integerp 't 'nil))

     allows integers, t and nil as legitimate values.

     The customization buffer shows all legitimate values using their read syntax, and the user edits them textually.

(cons CAR-TYPE CDR-TYPE)
     The value must be a cons cell, its CAR must fit CAR-TYPE, and its CDR must fit CDR-TYPE.  For example, (cons string symbol) is a customization type which matches values such as ("foo" . foo).

     In the customization buffer, the CAR and the CDR are displayed and edited separately, each according to the type that you specify for it.

(list ELEMENT-TYPES...)
     The value must be a list with exactly as many elements as the ELEMENT-TYPES you have specified; and each element must fit the corresponding ELEMENT-TYPE.

     For example, (list integer string function) describes a list of three elements; the first element must be an integer, the second a string, and the third a function.

     In the customization buffer, each element is displayed and edited separately, according to the type specified for it.

(vector ELEMENT-TYPES...)
     Like list except that the value must be a vector instead of a list.  The elements work the same as in list.

(choice ALTERNATIVE-TYPES...)
     The value must fit at least one of ALTERNATIVE-TYPES. For example, (choice integer string) allows either an integer or a string.

     In the customization buffer, the user selects one of the alternatives using a menu, and can then edit the value in the usual way for that alternative.

     Normally the strings in this menu are determined automatically from the choices; however, you can specify different strings for the menu by including the :tag keyword in the alternatives.  For example, if an integer stands for a number of spaces, while a string is text to use verbatim, you might write the customization type this way,

          (choice (integer :tag "Number of spaces")
                  (string :tag "Literal text"))

     so that the menu offers Number of spaces and Literal Text.

     In any alternative for which nil is not a valid value, other than a const, you should specify a valid default for that alternative using the :value keyword.  See Type Keywords.

(radio ELEMENT-TYPES...)
     This is similar to choice, except that the choices are displayed using `radio buttons' rather than a menu.  This has the advantage of displaying documentation for the choices when applicable and so is often a good choice for a choice between constant functions (function-item customization types).

(const VALUE)
     The value must be VALUE---nothing else is allowed.

     The main use of const is inside of choice.  For example, (choice integer (const nil)) allows either an integer or nil.

     :tag is often used with const, inside of choice. For example,

          (choice (const :tag "Yes" t)
                  (const :tag "No" nil)
                  (const :tag "Ask" foo))

     describes a variable for which t means yes, nil means no, and foo means ``ask.''

(other VALUE)
     This alternative can match any Lisp value, but if the user chooses this alternative, that selects the value VALUE.

     The main use of other is as the last element of choice. For example,

          (choice (const :tag "Yes" t)
                  (const :tag "No" nil)
                  (other :tag "Ask" foo))

     describes a variable for which t means yes, nil means no, and anything else means ``ask.''  If the user chooses Ask from the menu of alternatives, that specifies the value foo; but any other value (not t, nil or foo) displays as Ask, just like foo.

(function-item FUNCTION)
     Like const, but used for values which are functions.  This displays the documentation string as well as the function name. The documentation string is either the one you specify with :doc, or FUNCTION's own documentation string.

(variable-item VARIABLE)
     Like const, but used for values which are variable names.  This displays the documentation string as well as the variable name.  The documentation string is either the one you specify with :doc, or VARIABLE's own documentation string.

(set TYPES...)
     The value must be a list, and each element of the list must match one of the TYPES specified.

     This appears in the customization buffer as a checklist, so that each of TYPES may have either one corresponding element or none.  It is not possible to specify two different elements that match the same one of TYPES.  For example, (set integer symbol) allows one integer and/or one symbol in the list; it does not allow multiple integers or multiple symbols.  As a result, it is rare to use nonspecific types such as integer in a set.

     Most often, the TYPES in a set are const types, as shown here:

          (set (const :bold) (const :italic))

     Sometimes they describe possible elements in an alist:

          (set (cons :tag "Height" (const height) integer)
               (cons :tag "Width" (const width) integer))

     That lets the user specify a height value optionally and a width value optionally.

(repeat ELEMENT-TYPE)
     The value must be a list and each element of the list must fit the type ELEMENT-TYPE.  This appears in the customization buffer as a list of elements, with [INS] and [DEL] buttons for adding more elements or removing elements.

Splicing into Lists
-------------------

The :inline feature lets you splice a variable number of elements into the middle of a list or vector.  You use it in a set, choice or repeat type which appears among the element-types of a list or vector.

Normally, each of the element-types in a list or vector describes one and only one element of the list or vector.  Thus, if an element-type is a repeat, that specifies a list of unspecified length which appears as one element.

But when the element-type uses :inline, the value it matches is merged directly into the containing sequence.  For example, if it matches a list with three elements, those become three elements of the overall sequence.  This is analogous to using ,@ in the backquote construct.

For example, to specify a list whose first element must be t and whose remaining arguments should be zero or more of foo and bar, use this customization type:

     (list (const t) (set :inline t foo bar))

This matches values such as (t), (t foo), (t bar) and (t foo bar).

When the element-type is a choice, you use :inline not in the choice itself, but in (some of) the alternatives of the choice.  For example, to match a list which must start with a file name, followed either by the symbol t or two strings, use this customization type:

     (list file
           (choice (const t)
                   (list :inline t string string)))

If the user chooses the first alternative in the choice, then the overall list has two elements and the second element is t.  If the user chooses the second alternative, then the overall list has three elements and the second and third must be strings.

Type Keywords
-------------

You can specify keyword-argument pairs in a customization type after the type name symbol.  Here are the keywords you can use, and their meanings:

:value DEFAULT
     This is used for a type that appears as an alternative inside of choice; it specifies the default value to use, at first, if and when the user selects this alternative with the menu in the customization buffer.

     Of course, if the actual value of the option fits this alternative, it will appear showing the actual value, not DEFAULT.

     If nil is not a valid value for the alternative, then it is essential to specify a valid default with :value.

:format FORMAT-STRING
     This string will be inserted in the buffer to represent the value corresponding to the type.  The following % escapes are available for use in FORMAT-STRING:

     %[BUTTON%]
          Display the text BUTTON marked as a button.  The :action attribute specifies what the button will do if the user invokes it; its value is a function which takes two arguments---the widget which the button appears in, and the event.

          There is no way to specify two different buttons with different actions.

     %{SAMPLE%}
          Show SAMPLE in a special face specified by :sample-face.

     %v
          Substitute the item's value.  How the value is represented depends on the kind of item, and (for variables) on the customization type.

     %d
          Substitute the item's documentation string.

     %h
          Like %d, but if the documentation string is more than one line, add an active field to control whether to show all of it or just the first line.

     %t
          Substitute the tag here.  You specify the tag with the :tag keyword.

     %%
          Display a literal %.

:action ACTION
     Perform ACTION if the user clicks on a button.

:button-face FACE
     Use the face FACE (a face name or a list of face names) for button text displayed with %[...%].

:button-prefix PREFIX
:button-suffix SUFFIX
     These specify the text to display before and after a button. Each can be:

     nil
          No text is inserted.

     a string
          The string is inserted literally.

     a symbol
          The symbol's value is used.

:tag TAG
     Use TAG (a string) as the tag for the value (or part of the value) that corresponds to this type.

:doc DOC
     Use DOC as the documentation string for this value (or part of the value) that corresponds to this type.  In order for this to work, you must specify a value for :format, and use %d or %h in that value.

     The usual reason to specify a documentation string for a type is to provide more information about the meanings of alternatives inside a :choice type or the parts of some other composite type.

:help-echo MOTION-DOC
     When you move to this item with widget-forward or widget-backward, it will display the string MOTION-DOC in the echo area.  In addition, MOTION-DOC is used as the mouse help-echo string and may actually be a function or form evaluated to yield a help string as for help-echo text properties.

:match FUNCTION
     Specify how to decide whether a value matches the type.  The corresponding value, FUNCTION, should be a function that accepts two arguments, a widget and a value; it should return non-nil if the value is acceptable.