emacs delete-trailing-whitespace except current line

Question

I recently added Emacs (delete-trailing-whitespace) function to my 'before-save-hook for some programming modes, but I find it rather frustrating that it deletes whitespace from the line I am currently editing. Any suggestions as to how to fix this problem?

Answer 1

This wrapper for delete-trailing-whitespace can be used to do what you want:

(defun delete-trailing-whitespace-except-current-line ()
  "do delete-trailing-whitespace, except preserve whitespace of current line"
  (interactive)
  (let ((current-line (buffer-substring (line-beginning-position) (line-end-position)))
        (backward (- (line-end-position) (point))))
    (delete-trailing-whitespace)
    (when (not (string-equal (buffer-substring (line-beginning-position) (line-end-position))
                             current-line))
      (delete-region (line-beginning-position) (line-end-position))
      (insert current-line)
      (backward-char backward))))

Answer 2

Since delete-trailing-whitespace respects narrowing, one solution is to narrow the buffer to the portion before the current line and call it, then narrow to the portion after the current line and call it again:

(defun delete-trailing-whitespace-except-current-line ()
  (interactive)
  (let ((begin (line-beginning-position))
        (end (line-end-position)))
    (save-excursion
      (when (< (point-min) begin)
        (save-restriction
          (narrow-to-region (point-min) (1- begin))
          (delete-trailing-whitespace)))
      (when (> (point-max) end)
        (save-restriction
          (narrow-to-region (1+ end) (point-max))
          (delete-trailing-whitespace))))))

Put this function on your before-save-hook instead of delete-trailing-whitespace.