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14.4.4 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

Provide a default value.

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

If you use this 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.

: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:


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.


Show sample in a special face specified by :sample-face.


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


Substitute the item’s documentation string.


Like ‘%d’, but if the documentation string is more than one line, add a button to control whether to show all of it or just the first line.


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:


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. If it is a function, it is called with one argument, the widget.

: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.

:validate function

Specify a validation function for input. function takes a widget as an argument, and should return nil if the widget’s current value is valid for the widget. Otherwise, it should return the widget containing the invalid data, and set that widget’s :error property to a string explaining the error.

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