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elisp: Derived Modes

 
 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’.  Basic Major Modes.  If none of
 these are suitable, you can inherit from ‘fundamental-mode’ (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.  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 (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:
 
           (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.
 

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