Fill and unfill paragraphs with a single key

fill-paragraph is probably among the most underappreciated Emacs commands. I use it dozens of times a day, and never stop to think of just how awesome and practical it is. Still, we can make it a little bit better. Every once in a while I need to “unfill” (or “unwrap”) a paragraph that’s broken over many lines.

By being clever enough, we can make this into a free feature. There’s never any reason to hit M-q twice on the same paragraph, so we can use that as our keybind for the “unfill” command.

(defun endless/fill-or-unfill ()
  "Like `fill-paragraph', but unfill if used twice."
  (interactive)
  (let ((fill-column
         (if (eq last-command 'endless/fill-or-unfill)
             (progn (setq this-command nil)
                    (point-max))
           fill-column)))
    (call-interactively #'fill-paragraph)))

(global-set-key [remap fill-paragraph]
                #'endless/fill-or-unfill)

With this, M-q will act as a toggle. Hitting it once will do its usual thing (even if the paragraph is already filled), but hitting it twice will completely unwrap the current paragraph into a single line.