Writing GNU Emacs Extensions

Defining a Minor Mode

These are the steps involved in defining a minor mode.

1. Choose a name. The name for our mode is refill.

2. Define a variable named name-mode. Make it buffer-local. The minor mode is "on" in a buffer if that buffer's value of name-mode is non-nil, "off" otherwise.

   (defvar refill-mode nil
     "Mode variable for refill minor mode.")
   (make-variable-buffer-local 'refill-mode)

3. Define a command called name-mode.^[28] The command should take one optional argument. With no arguments, it should toggle the mode on or off. With an argument, it should turn the mode on if the prefix-numeric-value of the argument is greater than zero, off otherwise. Thus, C-u M-x name-mode RET always turns it on, and C-u — M-x name-mode RET always turns it off (refer to the section entitled Addendum: Raw Prefix Argument in Chapter 2). Here's the command for toggling Refill mode:

   (defun refill-mode (&optional arg)
     "Refill minor mode."
     (interactive "P")
     (setq refill-mode
           (if (null arg)
               (not refill-mode)
             (> (prefix-numeric-value arg) 0)))
     (if refill-mode
         code for turning on refill-mode
      code for turning off refill-mode))

   That setq is a little hairy, but it's a common idiom in minor mode definitions. If arg is nil (because no prefix argument was given), it sets refill-mode to (not refill-mode)—i.e., the opposite of refill-mode's previous value, t or nil. Otherwise, it sets refill-mode to the truth value of

   (> (prefix-numeric-value arg) 0)

   which is t if arg has a numeric value greater than 0, nil otherwise. ...