Writing GNU Emacs Extensions

Mode Meat

With the basic structure in place, let's start defining the guts of Refill mode.

We've already identified the basic feature of refill-mode: each insertion and deletion must ensure that the current paragraph is correctly filled. The correct way to execute code when the buffer is changed, as you may recall from Chapter 4, is by adding a function to the hook variable after-change-functions when refill-mode is turned on (and removing it when it is turned off). We'll add a function called refill (which does not yet exist) that will do all the work of making sure the current paragraph remains correctly filled.

(defun refill-mode (&optional arg)
  "Refill minor mode."
  (interactive "P")
  (setq refill-mode
        (if (null arg)
            (not refill-mode)
          (> (prefix-numeric-value arg) 0)))
  (make-local-hook 'after-change-functions)
  (if refill-mode
      (add-hook 'after-change-functions 'refill nil t)
    (remove-hook 'after-change-functions 'refill t)))

The extra arguments to add-hook and remove-hook ensure that only the buffer-local copies of after-change-functions are altered. Whether refill-mode is being turned on or off when this function is called, we call make-local-hook on after-change-functions to make it buffer-local. This is because in both cases—turning refill-mode on or turning it off—we need to manipulate after-change-functions separately in each buffer. Unconditionally calling make-local-hook first is the simplest way to do this, especially since make-local-hook has no effect if the named hook variable ...