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+;;; mode-clone.el (alpha version) -- allow inheritance of major modes.
+;;; $Id: mode-clone.el,v 1.5 1993/12/25 14:02:33 david Exp $
+
+;; Copyright (C) 1993 Free Software Foundation, Inc.
+
+;; Author: David Megginson (dmeggins@aix1.uottawa.ca)
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation; either version 2, or (at your option)
+;; any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs; see the file COPYING.  If not, write to
+;; the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
+
+;;; Commentary:
+
+;; GNU Emacs is already, in a sense, object oriented -- each object
+;; (buffer) belongs to a class (major mode), and that class defines
+;; the relationship between messages (input events) and methods
+;; (commands) by means of a keymap.
+;;
+;; The only thing missing is a good scheme of inheritance.  It is
+;; possible to simulate a single level of inheritance with generous
+;; use of hooks and a bit of work -- sgml-mode, for example, also runs
+;; the hooks for text-mode, and keymaps can inherit from other keymaps
+;; -- but generally, each major mode ends up reinventing the wheel.
+;; Ideally, someone should redesign all of Emacs's major modes to
+;; follow a more conventional object-oriented system: when defining a
+;; new major mode, the user should need only to name the existing mode
+;; it is most similar to, then list the (few) differences.
+;;
+;; In the mean time, this package offers most of the advantages of
+;; full inheritance with the existing major modes.  The function
+;; `mode-clone' allows the user to make a clone of an existing
+;; major mode, with its own keymap.  The new mode will inherit the key
+;; bindings of its parent, and will, in fact, run its parent first
+;; every time it is called.  For example, the commands
+;;
+;;  (mode-clone text-mode hypertext-mode "Hypertext"
+;;    "Major mode for hypertext.\n\n\\{hypertext-mode-map}"
+;;    (setq case-fold-search nil))
+;;
+;;  (define-key hypertext-mode-map [down-mouse-3] 'do-hyper-link)
+;;
+;; will create a function `hypertext-mode' with its own (sparse)
+;; keymap `hypertext-mode-map.'  The command M-x hypertext-mode will
+;; perform the following actions:
+;;
+;; - run the command (text-mode) to get its default setup
+;; - replace the current keymap with 'hypertext-mode-map,' which will
+;;   inherit from 'text-mode-map'.
+;; - replace the current syntax table with
+;;   'hypertext-mode-syntax-table', which will borrow its defaults
+;;   from the current text-mode-syntax-table.
+;; - if 'hypertext-mode-abbrev-table' exists, it will become the
+;;   current abbrev table.
+;; - change the mode line to read "Hypertext"
+;; - assign the value 'hypertext-mode' to the 'major-mode' variable
+;; - run the body of commands provided in the macro -- in this case,
+;;   set the local variable `case-fold-search' to nil.
+;; - **run the command (hypertext-mode-setup), which is empty by
+;;   default, but may be redefined by the user to contain special
+;;   commands (ie. setting local variables like 'outline-regexp')
+;;   **NOTE: do not use this option -- it will soon be obsolete.
+;; - run anything assigned to 'hypertext-mode-hooks' (obsolete, but
+;;   supported for the sake of compatibility).
+;;
+;; The advantages of this system are threefold.  First, text mode is
+;; untouched -- if you had added the new keystroke to `text-mode-map,'
+;; possibly using hooks, you would have added it to all text buffers
+;; -- here, it appears only in hypertext buffers, where it makes
+;; sense.  Second, it is possible to build even further, and clone the
+;; clone.  The commands
+;;
+;;   (mode-clone hypertext-mode html-mode "HTML")
+;;   [various key definitions]
+;; 
+;; will add a new major mode for HTML with very little fuss.
+;;
+;; Note also the function `clone-class,' which returns the non-clone
+;; major mode which a clone is based on (ie. NOT necessarily the
+;; immediate parent).
+;;
+;; (clone-class 'text-mode) ==> text-mode
+;; (clone-class 'hypertext-mode) ==> text-mode
+;; (clone-class 'html-mode) ==> text-mode
+
+;;; Code:
+
+;; PUBLIC: define a new major mode which inherits from an existing one.
+
+;;;###autoload
+(defmacro mode-clone (parent child name &optional docstring &rest body)
+  "Create a new mode which is similar to an old one.
+
+The arguments to this command are as follow:
+
+parent:    the name of the command for the parent mode (ie. text-mode)
+child:     the name of the command for the clone
+name:      a string which will appear in the status line (ie. \"Hypertext\")
+docstring: an optional documentation string -- if you do not supply one,
+           the function will attempt to invent something useful.  If this
+           argument is not a string, it will be added to body automatically.
+body:      a body of commands to execute just before running the
+           hooks for the new mode.
+
+The following simple command would clone LaTeX-mode into
+LaTeX-thesis-mode:
+
+  (mode-clone LaTeX-mode LaTeX-thesis-mode \"LaTeX-Thesis\")
+
+It would then be possible to assign commands to keystrokes in
+`LaTeX-thesis-mode-map' without changing the interface in the regular
+LaTeX-mode.  The function (LaTeX-thesis-mode-setup), if it exists,
+will contain commands which will run whenever (LaTeX-thesis-mode) is
+run (just before 'LaTeX-thesis-mode-hooks).
+
+On a more complicated level, the following command would clone
+sgml-mode and change the variable `case-fold-search' to nil:
+
+  (mode-clone sgml-mode article-mode \"Article\"
+    \"Major mode for editing technical articles.\"
+    (setq case-fold-search nil))
+
+Note that if the documentation string had been left out, it would have
+been generated automatically, with a reference to the keymap."
+
+					; Some trickiness, since what
+					; appears to be the docstring
+					; may really be the first
+					; element of the body.
+  (if (and docstring (not (stringp docstring)))
+      (progn (setq body (cons docstring body))
+	     (setq docstring nil)))
+  (setq docstring (or docstring (clone-make-docstring parent child)))
+
+  (` (progn 
+       (clone-init-mode-variables (quote (, child)))
+       (defun (, child) ()
+	 (, docstring)
+	 (interactive)
+					; Run the parent.
+	 ((, parent))
+					; Identify special modes.
+	 (if (get (quote (, parent)) 'special)
+	     (put (quote (, child)) 'special t))
+					; Identify the child mode.
+	 (setq major-mode (quote (, child)))
+	 (setq mode-name (, name))
+					; Set up maps and tables.
+	 (clone-set-keymap (quote (, child)))
+	 (clone-set-syntax-table (quote (, child)))
+	 (clone-set-abbrev-table (quote (, child)))
+					; Splice in the body (if any).
+	 (,@ body)
+					; Run the setup function, if
+					; any -- this will soon be
+					; obsolete.
+	 (clone-run-setup-function (quote (, child)))
+					; Run the hooks, if any.
+	 (clone-run-hooks (quote (, child)))))))
+
+
+;; PUBLIC: find the ultimate class of a clone mode.
+
+(defun clone-class (mode)
+  "Find the class of a major mode.
+A mode's class is the first ancestor which is NOT a clone.
+Use the `clone-parent' property of the symbol to trace backwards."
+  (while (get mode 'clone-parent)
+    (setq mode (get mode 'clone-parent)))
+  mode)
+
+
+;; Inline functions to construct various names from a mode name.
+
+(defsubst clone-setup-function-name (mode)
+  "Construct a setup-function name based on a mode name."
+  (intern (concat (symbol-name mode) "-setup")))
+
+(defsubst clone-hooks-name (mode)
+  "Construct a hooks name based on a mode name."
+  (intern (concat (symbol-name mode) "-hooks")))
+
+(defsubst clone-map-name (mode)
+  "Construct a map name based on a mode name."
+  (intern (concat (symbol-name mode) "-map")))
+
+(defsubst clone-syntax-table-name (mode)
+  "Construct a syntax-table name based on a mode name."
+  (intern (concat (symbol-name mode) "-syntax-table")))
+
+(defsubst clone-abbrev-table-name (mode)
+  "Construct an abbrev-table name based on a mode name."
+  (intern (concat (symbol-name mode) "-abbrev-table")))
+
+
+;; Utility functions for defining a clone mode.
+
+(defun clone-init-mode-variables (mode)
+  "Initialise variables for a new mode.
+Right now, just set up a blank keymap and an empty syntax table."
+
+  (eval (` (defvar (, (clone-map-name mode)) 
+	     (make-sparse-keymap)
+	     (, (format "Keymap for %s." mode)))))
+  (put (clone-map-name mode) 'clone-merged nil)
+
+  (eval (` (defvar (, (clone-syntax-table-name mode))
+	     (make-vector 256 nil)
+	     (, (format "Syntax table for %s." mode)))))
+  (put (clone-syntax-table-name mode) 'clone-merged nil)
+
+  (eval (` (defvar (, (clone-abbrev-table-name mode))
+	     nil
+	     (, (format "Abbrev table for %s." mode)))))
+  (define-abbrev-table (clone-abbrev-table-name mode) ()))
+
+(defun clone-make-docstring (parent child)
+  "Construct a docstring for a new mode if none is provided."
+
+  (format "This major mode is a clone of (%s), created by (mode-clone).
+It inherits all of the parent's attributes, but has its own keymap
+and syntax table:
+
+  '%s-map' and '%s-syntax-table'
+
+which more-or-less shadow
+
+  '%s-map' and '%s-syntax-table'
+
+\\{%s-map}" parent child child parent parent child))
+
+
+;; Utility functions for running a clone mode.
+
+(defun clone-set-keymap (mode)
+  "Set the keymap of the new mode, maybe merging with the parent."
+  (let* ((map-name (clone-map-name mode))
+	 (new-map (eval map-name))
+	 (old-map (current-local-map)))
+    (if (get map-name 'clone-merged)
+	(use-local-map new-map)
+      (put map-name 'clone-merged t)
+      (use-local-map (set map-name (clone-merge-keymaps old-map new-map))))))
+
+(defun clone-set-syntax-table (mode) 
+  "Set the syntax table of the new mode, maybe merging with the parent."
+  (let* ((table-name (clone-syntax-table-name mode))
+	 (old-table (syntax-table))
+	 (new-table (eval table-name)))
+    (if (get table-name 'clone-merged)
+	t
+      (clone-merge-syntax-tables old-table new-table))
+    (set-syntax-table new-table)
+    (put table-name 'clone-merged t)))
+
+(defun clone-set-abbrev-table (mode)
+  "Set the abbrev table if it exists."
+  (let* ((table-name (clone-abbrev-table-name mode))
+	 (table (and (boundp table-name) (eval table-name))))
+    (if table
+	(setq local-abbrev-table table))))
+
+(defun clone-run-setup-function (mode)
+  "Run the setup function if it exists."
+
+  (let ((fname (clone-setup-function-name mode)))
+    (if (fboundp fname)
+	(funcall fname))))
+
+(defun clone-run-hooks (mode)
+  "Run the hooks if they exist."
+
+  (let ((hooks-name (clone-hooks-name mode)))
+    (if (boundp hooks-name)
+	(run-hooks hooks-name))))
+
+;; Functions to merge maps and tables.
+
+(defun clone-merge-keymaps (old new)
+  "Merge a new keymap into an old one.
+The old keymap is set to be the cdr of the new one, so that there will
+be automatic inheritance."
+  (append new old))
+
+(defun clone-merge-syntax-tables (old new)
+  "Merge a new syntax table into an old one.
+Where the new table already has an entry, nothing is copied from the old one."
+  (let ((idx 0)
+	(end (min (length new) (length old))))
+    (while (< idx end)
+      (if (not (aref new idx))
+	  (aset new idx (aref old idx)))
+      (setq idx (1+ idx)))))
+    
+(provide 'mode-clone)
+
+;;; mode-clone.el ends here