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15.2 Defining Customization 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 twenty 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 Do not quote the symbol group.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.

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 you introduce a new group into Emacs, use the :version keyword in the defgroup; then you need not use it for the individual members of the group.

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, and the customizable variable custom-unlispify-remove-prefixes is non-nil, the item’s tag will omit prefix. A group can have any number of prefixes.

The variables and subgroups of a group are stored in the custom-group property of the group’s symbol. See Symbol Plists. The value of that property is a list of pairs whose car is the variable or subgroup symbol and the cdr is either custom-variable or custom-group.

user option custom-unlispify-remove-prefixes

If this variable is non-nil, the prefixes specified by a group’s :prefix keyword are omitted from tag names, whenever the user customizes the group.

The default value is nil, i.e., the prefix-discarding feature is disabled. This is because discarding prefixes often leads to confusing names for options and faces.