Prevent Emacs from linebreaking in TeX inline math

Question

I use Emacs in combination with AUCTeX to edit manuscripts with a lot of inline maths using $. When automatic filling (e.g., with M-q), Emacs often breaks these inline math environments at positions which disturbs the fluency of reading (e.g., in subscripts or the like).

Is there a way to tell Emacs to prefer putting the whole $…$ environment in a new line if that would prevent breaking? More specifically, if breaking in maths would occur, the whole environment should be moved to a new line which should not be broken apart.

An example:

Lorem ipsum dolor sit amet, consectetur adipisici elit, sed $a^2 + b^2 = c^2$ eiusmod tempor incidunt ut labore et dolore magna aliqua. Ut enim ad minim veniam.

should result in

Lorem ipsum dolor sit amet, consectetur adipisici elit, sed
$a^2 + b^2 = c^2$ eiusmod tempor incidunt ut labore et dolore magna
aliqua. Ut enim ad minim veniam.

Penicillium

Answer 1

Try:

(add-hook 'LaTeX-mode-hook
          (lambda ()
            (add-to-list 'fill-nobreak-predicate 'texmathp)))

Place it in your .emacs and restart emacs.

Stylistic

Thanks, that's close! It's nice that it gathers short math environments spread over multiple lines. This comes at the cost, however, that very long lines cannot be broken explicitely (I mean that the explicit breaks are deleted when pressing M-q). Do you know how to incorporate this. (It's tricky though; I even cannot state it more precisely in a concise way.) – Penicillium

I'm not sure I understand what you mean by "explicit breaks". If you just mean positions of the text where you pressed RET, well, there's no way of distinguishing those from the automatic line breaks done by the emacs filling functions. You could, however, add the comment symbol % at the end of a line to try to prevent that line break from being changed by M-q. – Stylistic

What I mean is that if long math (longer than one line) already is on a seperate line and broken (by whomsoever), then I do not want to pull Emacs subsequent lines up (i.e., don't remove a linebreak if that would lead to too long a line). I'm already using % but it is not very convenient. – Penicillium

I think I understand. However, what you want does not seem possible using the fill-nobreak-predicate variable. Before even calling the functions from this list, M-q already collapses the whole paragraph to a single line, making it impossible to figure out where the previous line breaks occurred. – Stylistic

Thank you for this clarification. There may be a different solution, not using fill-nobreak-predicate, though… – Penicillium

Answer 2

You can do something like:

(add-hook 'LaTeX-mode-hook
          (lambda ()
            (add-hook 'fill-nobreak-predicate 'my-latex-fill-nobreak)))

and then define a function my-latex-fill-nobreak which returns non-nil if you're inside a short math environment, e.g. something like:

(defun my-latex-fill-nobreak ()
  (save-excursion
    (when (search-backward "$" (- (point) 20) t)
      ;; We found a nearby $.  Now let's see if it's an opening $
      ;; rather than a closing one.  Using font-lock properties is
      ;; a hack, but it might be good enough.  Apparently (texmathp)
      ;; could be a better choice.
      (eq (get-text-property (point) 'face) 'font-latex-math-face))))

Taler

You might use texmathp to detect whether point is in math mode. – Stylistic

I tried to add this to ~/.emacs but that didn't seem to have any effect. – Penicillium

Probably because of the unquoted $ in the regexp search. I changed it to use a non-regexp search instead, which might fix your problem. – Taler

Thanks! It's nice that it puts long maths onto its own line. There's an issue with that, however, as it seems to erroneously put the last part (including the closing $) to the next line. What's more, can you explain where the threshold comes from (trial-and-error gave me something around 47 but I don't see where this comes from). – Penicillium