Emacs Filling

Filling, in Emacs, means to format a paragrah so that it only takes up a certain number of column, usually 80. There's a command to do this directly, called fill-paragraph, which is bound to M-q. Typing M-q will fill whatever paragraph you're in.

Filling is a useful thing to do and in general it's good to have it done automatically. There are two minor modes in this regard: refill-mode and auto-fill-mode. refill-mode will automatically call fill-paragraph for you every time you make a change to a paragraph. This can be distracting and can cause "action at a distance"; it can reformat your entire paragraph just by typing a letter in the middle.

auto-fill-mode is more popular, and appears to be a good compromise between no automatic filling at all and full-blown refill-mode. It is a minor, buffer local mode where every space and <RET> is followed by an attempt to break the current line if it is too long. It will not reformat your entire paragraph; auto-fill mode breaks lines but does not merge them. If your paragraph is unfilled near the bottom and you edit the top, you won't refill the whole thing. The easiest thing to do at this point is just to hit M-q.

If you want to add a space or newline without breaking, use C-q, which quotes characters. So use C-q <SPC> or C-q C-j (C-j stands for newline)

Filling, in emacs, is good when writing comments in programming modes. Filling is smart enough to prepend the # character when filling perl comment paragraphs, for example.

I currently have this in my .emacs file:

(setq default-major-mode 'text-mode)
(defun my-text-mode-hook ()
  (auto-fill-mode t)
)
(add-hook 'text-mode-hook 'my-text-mode-hook)

This sets any unknown mode to text, and turns auto-fill mode on for text mode (and its derivatives, like mail-mode (how do I figure out a mode derivative?) ). So I get auto-fill mode for text modes and unknown modes. I don't get it for programming modes, but I can turn it on for that if I really want to, or just hit M-q if I want to fill something. In general, with programming modes, I think filling is only good for comments, so it's probably best not to enable it unconditionally.

But...if you want to unconditionally set auto-fill mode in all modes, add this to your .emacs:

(setq-default auto-fill-function 'do-auto-fill)

Set the default paragraph width this way:

(setq-default fill-column 76)

Or call set-fill-column directly; it's bound to C-x f.