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GNU Emacs Lisp Reference Manual

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23.1.2 Major Mode Examples

Text mode is perhaps the simplest mode besides Fundamental mode. Here are excerpts from `text-mode.el' that illustrate many of the conventions listed above:

 
;; Create mode-specific tables.
(defvar text-mode-syntax-table nil 
  "Syntax table used while in text mode.")

(if text-mode-syntax-table
    ()              ; Do not change the table if it is already set up.
  (setq text-mode-syntax-table (make-syntax-table))
  (modify-syntax-entry ?\" ".   " text-mode-syntax-table)
  (modify-syntax-entry ?\\ ".   " text-mode-syntax-table)
  (modify-syntax-entry ?' "w   " text-mode-syntax-table))

(defvar text-mode-abbrev-table nil
  "Abbrev table used while in text mode.")
(define-abbrev-table 'text-mode-abbrev-table ())

(defvar text-mode-map nil    ; Create a mode-specific keymap.
  "Keymap for Text mode.
Many other modes, such as Mail mode, Outline mode and Indented Text mode,
inherit all the commands defined in this map.")

(if text-mode-map
    ()              ; Do not change the keymap if it is already set up.
  (setq text-mode-map (make-sparse-keymap))
  (define-key text-mode-map "\e\t" 'ispell-complete-word)
  (define-key text-mode-map "\t" 'indent-relative)
  (define-key text-mode-map "\es" 'center-line)
  (define-key text-mode-map "\eS" 'center-paragraph))

Here is the complete major mode function definition for Text mode:

 
(defun text-mode ()
  "Major mode for editing text intended for humans to read...
 Special commands: \\{text-mode-map}
Turning on text-mode runs the hook `text-mode-hook'."
  (interactive)
  (kill-all-local-variables)
  (use-local-map text-mode-map)
  (setq local-abbrev-table text-mode-abbrev-table)
  (set-syntax-table text-mode-syntax-table)
  (make-local-variable 'paragraph-start)
  (setq paragraph-start (concat "[ \t]*$\\|" page-delimiter))
  (make-local-variable 'paragraph-separate)
  (setq paragraph-separate paragraph-start)
  (make-local-variable 'indent-line-function)
  (setq indent-line-function 'indent-relative-maybe)
  (setq mode-name "Text")
  (setq major-mode 'text-mode)
  (run-hooks 'text-mode-hook))      ; Finally, this permits the user to
                                    ;   customize the mode with a hook.

The three Lisp modes (Lisp mode, Emacs Lisp mode, and Lisp Interaction mode) have more features than Text mode and the code is correspondingly more complicated. Here are excerpts from `lisp-mode.el' that illustrate how these modes are written.

 
;; Create mode-specific table variables.
(defvar lisp-mode-syntax-table nil "")  
(defvar emacs-lisp-mode-syntax-table nil "")
(defvar lisp-mode-abbrev-table nil "")

(if (not emacs-lisp-mode-syntax-table) ; Do not change the table
                                       ;   if it is already set.
    (let ((i 0))
      (setq emacs-lisp-mode-syntax-table (make-syntax-table))

      ;; Set syntax of chars up to 0 to class of chars that are
      ;;   part of symbol names but not words.
      ;;   (The number 0 is 48 in the ASCII character set.)
      (while (< i ?0) 
        (modify-syntax-entry i "_   " emacs-lisp-mode-syntax-table)
        (setq i (1+ i)))
      ...
      ;; Set the syntax for other characters.
      (modify-syntax-entry ?  "    " emacs-lisp-mode-syntax-table)
      (modify-syntax-entry ?\t "    " emacs-lisp-mode-syntax-table)
      ...
      (modify-syntax-entry ?\( "()  " emacs-lisp-mode-syntax-table)
      (modify-syntax-entry ?\) ")(  " emacs-lisp-mode-syntax-table)
      ...))
;; Create an abbrev table for lisp-mode.
(define-abbrev-table 'lisp-mode-abbrev-table ())

Much code is shared among the three Lisp modes. The following function sets various variables; it is called by each of the major Lisp mode functions:

 
(defun lisp-mode-variables (lisp-syntax)
  (cond (lisp-syntax
	  (set-syntax-table lisp-mode-syntax-table)))
  (setq local-abbrev-table lisp-mode-abbrev-table)
  ...

Functions such as forward-paragraph use the value of the paragraph-start variable. Since Lisp code is different from ordinary text, the paragraph-start variable needs to be set specially to handle Lisp. Also, comments are indented in a special fashion in Lisp and the Lisp modes need their own mode-specific comment-indent-function. The code to set these variables is the rest of lisp-mode-variables.

 
  (make-local-variable 'paragraph-start)
  (setq paragraph-start (concat page-delimiter "\\|$" ))
  (make-local-variable 'paragraph-separate)
  (setq paragraph-separate paragraph-start)
  ...
  (make-local-variable 'comment-indent-function)
  (setq comment-indent-function 'lisp-comment-indent))
  ...

Each of the different Lisp modes has a slightly different keymap. For example, Lisp mode binds C-c C-z to run-lisp, but the other Lisp modes do not. However, all Lisp modes have some commands in common. The following code sets up the common commands:

 
(defvar shared-lisp-mode-map ()
  "Keymap for commands shared by all sorts of Lisp modes.")

(if shared-lisp-mode-map
    ()
   (setq shared-lisp-mode-map (make-sparse-keymap))
   (define-key shared-lisp-mode-map "\e\C-q" 'indent-sexp)
   (define-key shared-lisp-mode-map "\177"
               'backward-delete-char-untabify))

And here is the code to set up the keymap for Lisp mode:

 
(defvar lisp-mode-map ()
  "Keymap for ordinary Lisp mode...")

(if lisp-mode-map
    ()
  (setq lisp-mode-map (make-sparse-keymap))
  (set-keymap-parent lisp-mode-map shared-lisp-mode-map)
  (define-key lisp-mode-map "\e\C-x" 'lisp-eval-defun)
  (define-key lisp-mode-map "\C-c\C-z" 'run-lisp))

Finally, here is the complete major mode function definition for Lisp mode.

 
(defun lisp-mode ()
  "Major mode for editing Lisp code for Lisps other than GNU Emacs Lisp.
Commands:
Delete converts tabs to spaces as it moves back.
Blank lines separate paragraphs.  Semicolons start comments.
\\{lisp-mode-map}
Note that `run-lisp' may be used either to start an inferior Lisp job
or to switch back to an existing one.

Entry to this mode calls the value of `lisp-mode-hook'
if that value is non-nil."
  (interactive)
  (kill-all-local-variables)
  (use-local-map lisp-mode-map)          ; Select the mode's keymap.
  (setq major-mode 'lisp-mode)           ; This is how describe-mode
                                         ;   finds out what to describe.
  (setq mode-name "Lisp")                ; This goes into the mode line.
  (lisp-mode-variables t)                ; This defines various variables.
  (setq imenu-case-fold-search t)
  (set-syntax-table lisp-mode-syntax-table)
  (run-hooks 'lisp-mode-hook))           ; This permits the user to use a
                                         ;   hook to customize the mode.


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