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, but only once when the
defcustom is executed, so it isn’t useful for the value to vary. Normally we use a quoted constant. For example:
(defcustom diff-command "diff"
"The command to use to run 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 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
All customization types are implemented as widgets; see Introduction in The Emacs Widget Library, for details.
|• Simple Types||Simple customization types: sexp, integer, etc.|
|• Composite Types||Build new types from other types or data.|
|• Splicing into Lists||Splice elements into list with |
|• Type Keywords||Keyword-argument pairs in a customization type.|
|• Defining New Types||Give your type a name.|