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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'."
  (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"

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.
Delete converts tabs to spaces as it moves back.
Blank lines separate paragraphs.  Semicolons start comments.
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."
  (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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