How do I get Emacs to fill sentences, but not paragraphs?

Question

I\'ve seen at least two recommendations on StackOverflow to insert newlines between sentences when editing LaTeX documents. The reason being that the practice facilitates so

Answer 1

May not work in all circumstances, but:

(defun my-fill-sentence ()
  "Fill sentence separated by punctuation or blank lines."
  (interactive)
  (let (start end)
    (save-excursion
      (re-search-backward "\\(^\\s-*$\\|[.?!]\\)" nil t)
      (skip-syntax-forward "^w")
      (setq start (point-at-bol)))
    (save-excursion
      (re-search-forward "\\(^\\s-*$\\|[.?!]\\)" nil t)
      (setq end (point-at-eol)))
    (save-restriction
      (narrow-to-region start end)
      (fill-paragraph nil))))

To make it work with auto-fill-mode, add (setq normal-auto-fill-function 'my-fill-sentence) to your LaTeX mode hook (I think).

Answer 2

Here's what I use, which was mostly cribbed from Luca de Alfaro:

(defun fill-sentence ()
  (interactive)
  (save-excursion
    (or (eq (point) (point-max)) (forward-char))
    (forward-sentence -1)
    (indent-relative t)
    (let ((beg (point))
          (ix (string-match "LaTeX" mode-name)))
      (forward-sentence)
      (if (and ix (equal "LaTeX" (substring mode-name ix)))
          (LaTeX-fill-region-as-paragraph beg (point))
        (fill-region-as-paragraph beg (point))))))

I bind this to M-j with

(global-set-key (kbd "M-j") 'fill-sentence)

The references to "LaTeX" are for AUCTeX support. If you don't use AUCTeX, the let can be simplified to

(let (beg (point))
  (forward-sentence)
  (fill-region-as-paragraph beg (point)))

Answer 3

I am assuming you know elisp.

There are a few approaches you can take:

• Hook into auto-fill-mode. There are a lot of hard-coded conditionals there, so it might not work for you. You can potentially play with auto-fill-function and see if you have the hook you need there.

• Make a character (probably .) "electric" so that when you press it, it inserts itself and then calls a function to determine how to fill the line you're on.

• Set an after-change-hook to call a function that determines how to fill the sentence. This function will be called after every change to the buffer, so do it efficiently. (This mechanism is used by font-lock, so don't worry about it too much. It sounds slow, but really isn't -- people type slowly.)

Once you have hooked in at the right place, you just have to implement the filling logic. The source for sentence-at-point (from thingatpt) may be instructive.

Anyway, I've never heard of anyone doing this... but it is definitely possible. Like most things in Emacs, it's just a Simple Matter Of Programming.

Answer 4

I have been meaning to do this forever and I recently found this blog post which worked fairly well for me. So here is (a slightly modified version of) what I have been using for a few days.

(defun auto-fill-by-sentences ()
  (if (looking-back (sentence-end))
      ;; Break at a sentence
      (progn
        (LaTeX-newline)
        t)
    ;; Fall back to the default
    (do-auto-fill)))
(add-hook 'LaTeX-mode-hook (lambda () (setq auto-fill-function 'auto-fill-by-sentences)))

;; Modified from http://pleasefindattached.blogspot.com/2011/12/emacsauctex-sentence-fill-greatly.html
(defadvice LaTeX-fill-region-as-paragraph (around LaTeX-sentence-filling)
  "Start each sentence on a new line."
  (let ((from (ad-get-arg 0))
        (to-marker (set-marker (make-marker) (ad-get-arg 1)))
        tmp-end)
    (while (< from (marker-position to-marker))
      (forward-sentence)
      ;; might have gone beyond to-marker---use whichever is smaller:
      (ad-set-arg 1 (setq tmp-end (min (point) (marker-position to-marker))))
      ad-do-it
      (ad-set-arg 0 (setq from (point)))
      (unless (or (looking-back "^\\s *")
                  (looking-at "\\s *$"))
        (LaTeX-newline)))
    (set-marker to-marker nil)))
(ad-activate 'LaTeX-fill-region-as-paragraph)