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#### 22.2.4 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. 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 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.

  Here is a hypothetical example:

      (define-derived-mode hypertext-mode
        text-mode "Hypertext"
        "Major mode for hypertext.
      \\{hypertext-mode-map}"
        (setq case-fold-search nil))

      (define-key hypertext-mode-map
        [down-mouse-3] 'do-hyper-link)

  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.

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