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 section Defining Faces).
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 name
:group group
: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. Be careful not to overdo this!
:link link-data
(custom-manual info-node)
"(emacs)Top". The link appears as
`[manual]' in the customization buffer.
(info-link info-node)
custom-manual except that the link appears
in the customization buffer with the Info node name.
(url-link url)
: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-library, and only if the file is
not already loaded.
:require feature
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.
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.
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.
In addition to the common keywords (see section Common Keywords for All Kinds of Items), you can
use this keyword in defgroup:
:prefix prefix
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.
Use defcustom to declare user-editable variables.
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.
defcustom accepts the following additional keywords:
:type type
:options list
hook. In that
case, the elements of list should be functions that are useful as
elements of the hook value. The user is not restricted to using only
these functions, but they are offered as convenient alternatives.
:version version
(defcustom foo-max 34 "*Maximum number of foo's allowed." :type 'integer :group 'foo :version "20.3")
:set setfunction
set-default.
:get getfunction
default-value.
:initialize function
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
: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
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
: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
:set function to initialize the variable, if it is
already set or has been customized; otherwise, just use
set-default.
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 section Common Keywords for All Kinds of Items. Here is an example, from the library `paren.el':
(defcustom show-paren-mode nil
"Toggle Show Paren mode@enddots{}"
:set (lambda (symbol value)
(show-paren-mode (or value 0)))
:initialize 'custom-initialize-default
:type 'boolean
:group 'paren-showing
:require 'paren)
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.
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 section 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).
This section describes all the simple customization types.
sexp
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
number
string
regexp
string except that the string must be a valid regular
expression.
character
file
(file :must-match t)
directory
hook
:options keyword in a hook variable's
defcustom to specify a list of functions recommended for use in
the hook; see section Defining Customization Variables.
symbol
function
variable
face
boolean
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.
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)
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.
'object. This sort of element
in the list says that object itself is an acceptable value.
(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)
(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...)
(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...)
list except that the value must be a vector instead of a
list. The elements work the same as in list.
(choice alternative-types...)
(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 section Type Keywords.
(const value)
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)
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)
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)
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 elements...)
(repeat element-type)
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.
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
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
: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-face.
:tag
keyword.
:action action
:button-face face
:button-prefix prefix
:button-suffix suffix
nil
:tag tag
:doc doc
: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
widget-forward or
widget-backward, it will display the string motion-doc
in the echo area.
:match function
nil if
the value is acceptable.
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