Question

I’d like (in TeX-related modes) the tilde key to insert itself as usual if point is on anything (in particular a line end), but if a point is on space, I’d like the tilde to overwrite it. (This would be a quite useful feature after pasting something into TeX source file.) I hacked something like this:

(defun electric-tie ()
  "Inserts a tilde at point unless the point is at a space
character, in which case it deletes the space first."
  (interactive)
  (while (equal (char-after) 32) (delete-char 1))
  (while (equal (char-before) 32) (delete-char -1))
  (insert "~"))

(add-hook 'TeX-mode-hook (lambda () (local-set-key "~" 'electric-tie)))

My questions are simple: is it correct (it seems to work) and can it be done better? (I assume that if the answer to the first question is in the affirmative, the latter is a question of style.)

Answer 1

Some points:

1. Instead of 32 use ?  (question-mark space) to express character literal.

2. Instead of defining keys in the major-mode hooks, do it in an eval-after-load block. The difference is that major-mode hook runs every time you use the major-mode, but there is only one keymap per major-mode. So there is no point in repeatedly redefining a key in it.

   see: https://stackoverflow.com/a/8139587/903943

3. It looks like this command should not take a numeric argument, but it’s worth understanding interactive specs to know how other commands you write can be made to be more flexible by taking numeric arguments into consideration.

One more note about your new modifications:

Your way to clear spaces around point is not wrong, but I’d do this:

(defun foo ()
  (interactive)
  (skip-chars-forward " ")
  (delete-region (point) (+ (point) (skip-chars-backward " "))))