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23.2.4 Defining Derived Modes

The recommended way to define a new major mode is to derive it from an existing one using define-derived-mode. If there is no closely related mode, you should inherit from either text-mode, special-mode, or prog-mode. See Basic Major Modes. If none of these are suitable, you can inherit from fundamental-mode (see Major Modes).

macro define-derived-mode variant parent name docstring keyword-args… body…​

This macro defines variant as a major mode command, using name as the string form of the mode name. variant and parent should be unquoted symbols.

The new command variant is defined to call the function parent, then override certain aspects of that parent mode:

  • The new mode has its own sparse keymap, named variant-map. define-derived-mode makes the parent mode’s keymap the parent of the new map, unless variant-map is already set and already has a parent.
  • The new mode has its own syntax table, kept in the variable variant-syntax-table, unless you override this using the :syntax-table keyword (see below). define-derived-mode makes the parent mode’s syntax-table the parent of variant-syntax-table, unless the latter is already set and already has a parent different from the standard syntax table.
  • The new mode has its own abbrev table, kept in the variable variant-abbrev-table, unless you override this using the :abbrev-table keyword (see below).
  • The new mode has its own mode hook, variant-hook. It runs this hook, after running the hooks of its ancestor modes, with run-mode-hooks, as the last thing it does, apart from running any :after-hook form it may have. See Mode Hooks.

In addition, you can specify how to override other aspects of parent with body. The command variant evaluates the forms in body after setting up all its usual overrides, just before running the mode hooks.

If parent has a non-nil mode-class symbol property, then define-derived-mode sets the mode-class property of variant to the same value. This ensures, for example, that if parent is a special mode, then variant is also a special mode (see Major Mode Conventions).

You can also specify nil for parent. This gives the new mode no parent. Then define-derived-mode behaves as described above, but, of course, omits all actions connected with parent.

The argument docstring specifies the documentation string for the new mode. define-derived-mode adds some general information about the mode’s hook, followed by the mode’s keymap, at the end of this documentation string. If you omit docstring, define-derived-mode generates a documentation string.

The keyword-args are pairs of keywords and values. The values, except for :after-hook’s, are evaluated. The following keywords are currently supported:

:syntax-table​

You can use this to explicitly specify a syntax table for the new mode. If you specify a nil value, the new mode uses the same syntax table as parent, or the standard syntax table if parent is nil. (Note that this does not follow the convention used for non-keyword arguments that a nil value is equivalent with not specifying the argument.)

:abbrev-table​

You can use this to explicitly specify an abbrev table for the new mode. If you specify a nil value, the new mode uses the same abbrev table as parent, or fundamental-mode-abbrev-table if parent is nil. (Again, a nil value is not equivalent to not specifying this keyword.)

:group​

If this is specified, the value should be the customization group for this mode. (Not all major modes have one.) The command customize-mode uses this. define-derived-mode does not automatically define the specified customization group.

:after-hook​

This optional keyword specifies a single Lisp form to evaluate as the final act of the mode function, after the mode hooks have been run. It should not be quoted. Since the form might be evaluated after the mode function has terminated, it should not access any element of the mode function’s local state. An :after-hook form is useful for setting up aspects of the mode which depend on the user’s settings, which in turn may have been changed in a mode hook.

Here is a hypothetical example:

(defvar hypertext-mode-map
  (let ((map (make-sparse-keymap)))
    (define-key map [down-mouse-3] 'do-hyper-link)
    map))

(define-derived-mode hypertext-mode
  text-mode "Hypertext"
  "Major mode for hypertext."
  (setq-local case-fold-search nil))

Do not write an interactive spec in the definition; define-derived-mode does that automatically.

function derived-mode-p \&rest modes​

This function returns non-nil if the current major mode is derived from any of the major modes given by the symbols modes.