Is there fully-automatic fill-paragraph-mode for code comments?

Question

I'm looking for a minor mode to keep paragraphs filled at all times while typing (similar to what aggressive-indent-mode does for indentation). It also needs to be smart enough to only fill comments (and maybe strings depending on the language).

Some things that I've tried are:

• auto-fill-mode automatically fills while you are typing new paragraphs but doesn't refill when you edit paragraphs.

• refill-mode does constantly refill paragraphs but it tries to wrap code into paragraphs as well as comments.

• I tried adding fill-paragraph to the after-change-functions hook but it screws up undo and plenty of other things (this is probably all fixable but would take some effort).

Any better ideas?

Answer 1

I came up with a minimal way of implementing this functionality: just bind space bar to also call (fill-paragraph)!

(defun fill-then-insert-space ()
  (interactive)
  (fill-paragraph)
  (insert " "))
(global-set-key (kbd "SPC") #'fill-then-insert-space)

There are a couple of caveat's that I've stumbled across so far:

1. elisp-mode (possibly others) does some fancy code filling when you call fill-paragraph, this may or may not be what you want. Can probably be fixed with by testing if point is in a comment or docstring but I'm not sure how to do that.

2. You sometimes can't easily enter multiple spaces (because the fill-paragraph kills any trailing spaces). Since the space-bar now acts like just-one-space it's probably ok to replace that binding with (insert " ").

I made a minor-mode containing this functionality, it's available on github or in the melpa package aggressive-fill-paragraph.

Answer 2

If you like how refill-mode behaves (I don't :p) but not when it behaves, it should be relatively easy to fix by inhibiting it whenever you're not in the right conditions

For example

(defvar plop/refill-enabler-function nil)

(defun plop/region-in-comment (beg end)
  ;; should really be comment-only-p, but that seems really broken for me
  (not
   (loop for c from beg to end
      if (not (eq 'font-lock-comment-face (get-char-property c 'face)))
      return t)))

(defun plop/refill-after-change-function (beg end len)
  (unless undo-in-progress
    (when (and plop/refill-enabler-function
               (funcall plop/refill-enabler-function beg end))
      (setq refill-doit end))))

(defun plop/install-refill-hack ()
  (if refill-mode
      (progn
        (add-hook 'after-change-functions 'plop/refill-after-change-function nil t)
        (remove-hook 'after-change-functions 'refill-after-change-function t))
    (progn
      (remove-hook 'after-change-functions 'plop/refill-after-change-function t))))

(defun plop/refill-hook ()
  (set (make-local-variable 'plop/refill-enabler-function)
       #'plop/region-in-comment)
  (add-hook 'refill-mode-hook 'plop/install-refill-hack t t)
  (refill-mode))

(add-hook 'some-hook 'plop/refill-hook)

Basically it removes the function that triggers the refill from after-change-functions and replaces it with one that will in addition check that we're in a comment before doing exactly the same thing.