;;; derived.el --- allow inheritance of major modes -*- lexical-binding: t; -*-
;; (formerly mode-clone.el)
;; Copyright (C) 1993-2024 Free Software Foundation, Inc.
;; Author: David Megginson <dmeggins@aix1.uottawa.ca>
;; Maintainer: emacs-devel@gnu.org
;; Keywords: extensions
;; Package: emacs
;; 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 3 of the License, 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. If not, see <https://www.gnu.org/licenses/>.
;;; 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 macro
;; `define-derived-mode' allows the user to make a variant 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
;;
;; (define-derived-mode hypertext-mode text-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.
;; - replace the current abbrev table with
;; 'hypertext-mode-abbrev-table', which will borrow its defaults
;; from the current text-mode-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.
;;
;; 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 make
;; a derived mode from a derived mode. The commands
;;
;; (define-derived-mode html-mode hypertext-mode "HTML")
;; [various key definitions]
;;
;; will add a new major mode for HTML with very little fuss.
;;
;; Note also the function `derived-mode-p' which can tell if the current
;; mode derives from another. In a hypertext-mode, buffer, for example,
;; (derived-mode-p 'text-mode) would return non-nil. This should always
;; be used in place of (eq major-mode 'text-mode).
;;; Code:
;;; PRIVATE: defsubst must be defined before they are first used
(defsubst derived-mode-hook-name (mode)
"Construct a mode-hook name based on the symbol MODE."
(intern (concat (symbol-name mode) "-hook")))
(defsubst derived-mode-map-name (mode)
"Construct a map name based on the symbol MODE."
(intern (concat (symbol-name mode) "-map")))
(defsubst derived-mode-syntax-table-name (mode)
"Construct a syntax-table name based on the symbol MODE."
(intern (concat (symbol-name mode) "-syntax-table")))
(defsubst derived-mode-abbrev-table-name (mode)
"Construct an abbrev-table name based on the symbol MODE."
(intern (concat (symbol-name mode) "-abbrev-table")))
;; PUBLIC: define a new major mode which inherits from an existing one.
;;;###autoload
(defmacro define-derived-mode (child parent name &optional docstring &rest body)
"Create a new mode CHILD which is a variant of an existing mode PARENT.
The arguments are as follows:
CHILD: the name of the command for the derived mode.
PARENT: the name of the command for the parent mode (e.g. `text-mode')
or nil if there is no parent.
NAME: a string that will appear in the mode line (e.g. \"HTML\")
DOCSTRING: an optional documentation string--if you do not supply one,
the function will attempt to invent something useful.
KEYWORD-ARGS:
optional arguments in the form of pairs of keyword and value.
The following keyword arguments are currently supported:
:group GROUP
Declare the customization group that corresponds
to this mode. The command `customize-mode' uses this.
:syntax-table TABLE
Use TABLE instead of the default (CHILD-syntax-table).
A nil value means to simply use the same syntax-table
as the parent.
:abbrev-table TABLE
Use TABLE instead of the default (CHILD-abbrev-table).
A nil value means to simply use the same abbrev-table
as the parent.
:after-hook FORM
A single Lisp form which is evaluated after the mode
hooks have been run. It should not be quoted.
:interactive BOOLEAN
Whether the derived mode should be `interactive' or not.
The default is t.
BODY: forms to execute just before running the
hooks for the new mode. Do not use `interactive' here.
Here is how you could define LaTeX-Thesis mode as a variant of LaTeX mode:
(define-derived-mode LaTeX-thesis-mode LaTeX-mode \"LaTeX-Thesis\")
You could then make new key bindings for `LaTeX-thesis-mode-map'
without changing regular LaTeX mode. In this example, BODY is empty,
and DOCSTRING is generated by default.
As a more complex example, the following command uses `sgml-mode' as
the parent, and then sets the variable `case-fold-search' to nil:
(define-derived-mode article-mode sgml-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.
The new mode runs the hook named MODE-hook. For `foo-mode',
the hook will be named `foo-mode-hook'.
See Info node `(elisp)Derived Modes' for more details.
\(fn CHILD PARENT NAME [DOCSTRING] [KEYWORD-ARGS...] &rest BODY)"
(declare (debug (&define name symbolp sexp [&optional stringp]
[&rest keywordp sexp] def-body))
(doc-string 4)
(indent defun))
(when (and docstring (not (stringp docstring)))
;; Some trickiness, since what appears to be the docstring may really be
;; the first element of the body.
(push docstring body)
(setq docstring nil))
(when (eq parent 'fundamental-mode) (setq parent nil))
(let ((map (derived-mode-map-name child))
(syntax (derived-mode-syntax-table-name child))
(abbrev (derived-mode-abbrev-table-name child))
(declare-abbrev t)
(declare-syntax t)
(hook (derived-mode-hook-name child))
(group nil)
(interactive t)
(after-hook nil))
;; Process the keyword args.
(while (keywordp (car body))
(pcase (pop body)
(:group (setq group (pop body)))
(:abbrev-table (setq abbrev (pop body)) (setq declare-abbrev nil))
(:syntax-table (setq syntax (pop body)) (setq declare-syntax nil))
(:after-hook (setq after-hook (pop body)))
(:interactive (setq interactive (pop body)))
(_ (pop body))))
(setq docstring (derived-mode-make-docstring
parent child docstring syntax abbrev))
`(progn
(defvar ,hook nil)
(unless (get ',hook 'variable-documentation)
(put ',hook 'variable-documentation
,(format "Hook run after entering `%S'.
No problems result if this variable is not bound.
`add-hook' automatically binds it. (This is true for all hook variables.)"
child)))
(unless (boundp ',map)
(put ',map 'definition-name ',child))
(with-no-warnings (defvar ,map (make-sparse-keymap)))
(unless (get ',map 'variable-documentation)
(put ',map 'variable-documentation
(purecopy ,(format "Keymap for `%s'." child))))
,(if declare-syntax
`(progn
(defvar ,syntax)
(unless (boundp ',syntax)
(put ',syntax 'definition-name ',child)
(defvar ,syntax (make-syntax-table)))
(unless (get ',syntax 'variable-documentation)
(put ',syntax 'variable-documentation
(purecopy ,(format "Syntax table for `%s'." child))))))
,(if declare-abbrev
`(progn
(defvar ,abbrev)
(unless (boundp ',abbrev)
(put ',abbrev 'definition-name ',child)
(defvar ,abbrev
(progn (define-abbrev-table ',abbrev nil) ,abbrev)))
(unless (get ',abbrev 'variable-documentation)
(put ',abbrev 'variable-documentation
(purecopy ,(format "Abbrev table for `%s'." child))))))
(if (fboundp 'derived-mode-set-parent) ;; Emacs≥30.1
(derived-mode-set-parent ',child ',parent)
(put ',child 'derived-mode-parent ',parent))
,(if group `(put ',child 'custom-mode-group ,group))
(defun ,child ()
,docstring
,(and interactive '(interactive))
; Run the parent.
(delay-mode-hooks
(,(or parent 'kill-all-local-variables))
; Identify the child mode.
(setq major-mode (quote ,child))
(setq mode-name ,name)
; Identify special modes.
,(when parent
`(progn
(if (get (quote ,parent) 'mode-class)
(put (quote ,child) 'mode-class
(get (quote ,parent) 'mode-class)))
; Set up maps and tables.
(unless (keymap-parent ,map)
;; It would probably be better to set the keymap's parent
;; at the toplevel rather than inside the mode function,
;; but this is not easy for at least the following reasons:
;; - the parent (and its keymap) may not yet be loaded.
;; - the parent's keymap name may be called something else
;; than <parent>-mode-map.
(set-keymap-parent ,map (current-local-map)))
,(when declare-syntax
`(let ((parent (char-table-parent ,syntax)))
(unless (and parent
(not (eq parent (standard-syntax-table))))
(set-char-table-parent ,syntax (syntax-table)))))
,(when declare-abbrev
`(unless (or (abbrev-table-get ,abbrev :parents)
;; This can happen if the major mode defines
;; the abbrev-table to be its parent's.
(eq ,abbrev local-abbrev-table))
(abbrev-table-put ,abbrev :parents
(list local-abbrev-table))))))
(use-local-map ,map)
,(when syntax `(set-syntax-table ,syntax))
,(when abbrev `(setq local-abbrev-table ,abbrev))
; Splice in the body (if any).
,@body
)
,@(when after-hook
`((push (lambda () ,after-hook) delayed-after-hook-functions)))
;; Run the hooks (and delayed-after-hook-functions), if any.
(run-mode-hooks ',hook)))))
;;; PRIVATE
(defun derived-mode-make-docstring (parent child &optional
docstring syntax abbrev)
"Construct a docstring for a new mode if none is provided."
(let ((map (derived-mode-map-name child))
(hook (derived-mode-hook-name child)))
(unless (stringp docstring)
;; Use a default docstring.
(setq docstring
(if (null parent)
(concat
"Major-mode.\n"
(internal--format-docstring-line
"Uses keymap `%s'%s%s." map
(if abbrev (format "%s abbrev table `%s'"
(if syntax "," " and") abbrev) "")
(if syntax (format " and syntax-table `%s'" syntax) "")))
(format "Major mode derived from `%s' by `define-derived-mode'.
It inherits all of the parent's attributes, but has its own keymap%s:
%s
which more-or-less shadow%s %s's corresponding table%s."
parent
(cond ((and abbrev syntax)
",\nabbrev table and syntax table")
(abbrev "\nand abbrev table")
(syntax "\nand syntax table")
(t ""))
(internal--format-docstring-line
" `%s'%s"
map
(cond ((and abbrev syntax)
(format ", `%s' and `%s'" abbrev syntax))
((or abbrev syntax)
(format " and `%s'" (or abbrev syntax)))
(t "")))
(if (or abbrev syntax) "" "s")
parent
(if (or abbrev syntax) "s" "")))))
(unless (string-match (regexp-quote (symbol-name hook)) docstring)
;; Make sure the docstring mentions the mode's hook.
(setq docstring
(concat docstring "\n\n"
(internal--format-docstring-line
"%s%s%s"
(if (null parent)
"This mode "
(concat
"In addition to any hooks its parent mode "
(if (string-match (format "[`‘]%s['’]"
(regexp-quote
(symbol-name parent)))
docstring)
nil
(format "`%s' " parent))
"might have run, this mode "))
(format "runs the hook `%s'" hook)
", as the final or penultimate step during initialization."))))
(unless (string-match "\\\\[{[]" docstring)
;; And don't forget to put the mode's keymap.
(setq docstring (concat docstring "\n\n\\{" (symbol-name map) "}")))
docstring))
(provide 'derived)
;;; derived.el ends here