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Kaushal Modi
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I prefer to use a generic "comment/uncomment line or region DWIM" kind of function that works in any major mode.

Generic comment DWIM function

Here is the function I use:

(defun endless/comment-line-or-region (n)
  "Comment or uncomment current line and proceed to the next line.
With positive prefix, apply to N lines including current one.
With negative prefix, apply to -N lines above.
If region is active, apply to active region instead."
  (interactive "p")
  (if (use-region-p)
      (comment-or-uncomment-region
       (region-beginning) (region-end))
    (let ((range
           (list (line-beginning-position)
                 (goto-char (line-end-position n)))))
      (comment-or-uncomment-region
       (apply #'min range)
       (apply #'max range)))
    (forward-line 1)
    (back-to-indentation)))

Tweaking the bindings for matlab-mode

;; Unbind the M-; key from `matlab-mode-map'
;; `with-eval-after-load' will work in emacs 24.4 and newer versions
(with-eval-after-load 'matlab
  (define-key matlab-mode-map (kbd "M-;") nil)) 

;; For emacs 24.3 and older versions use `eval-after-load' instead
;; of `with-eval-after-load'. Note that `eval-after-load' accepts
;; only a single quoted form.
;; (eval-after-load 'matlab
;;   '(define-key matlab-mode-map (kbd "M-;") nil)) 

;; Define the M-; binding globally
;; This binding will be effective in any major-mode that does not
;; have that major-mode-specific M-; binding
(global-set-key (kbd "M-;") #'endless/comment-line-or-region) 

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Kaushal Modi
  • 26k
  • 4
  • 83
  • 191