Emacs Lisp: How to Write Your Own Comment Command from Scratch

By Xah Lee. Date: . Last updated: .

This page shows you how to write a command to insert/delete comment syntax chars of a programing language.

First, you should know about emacs's builtin comment package newcomment.el. See: Emacs Lisp: How to Write Comment Command in Major Mode.

Problem

You want to write a emacs command to comment or uncomment code for your own language. You want to write it from scratch. (maybe because you don't like the newcomment.el behavior.)

Solution

The following code handles C++-style comment // …. The code can be easily modified to handle any comment syntax that starts with a comment char and ends in a newline. (for example, Python's # ….)

It has 3 user-level functions:

“my-comment-dwim” is the general command. If there are no text selection, then it will comment or uncomment the current line, depending on whether the current line is a comment (If the comment start in the middle of the line, the line is not considered a comment). If there is a text selection, then it will comment or uncomment the whole region. Which action it does depends on whether the first line in selection is a comment line.

(defun my-comment-dwim ()
  "Comment or uncomment the current line or text selection."
  (interactive)

  ;; If there's no text selection, comment or uncomment the line
  ;; depending whether the WHOLE line is a comment. If there is a text
  ;; selection, using the first line to determine whether to
  ;; comment/uncomment.
  (let (p1 p2)
    (if (use-region-p)
        (save-excursion
          (setq p1 (region-beginning) p2 (region-end))
          (goto-char p1)
          (if (wholeLineIsCmt-p)
              (my-uncomment-region p1 p2)
            (my-comment-region p1 p2)
            ))
      (progn
        (if (wholeLineIsCmt-p)
            (my-uncomment-current-line)
          (my-comment-current-line)
          )) )))

(defun wholeLineIsCmt-p ()
  (save-excursion
    (beginning-of-line 1)
    (looking-at "[ \t]*//")
    ))

(defun my-comment-current-line ()
  (interactive)
  (beginning-of-line 1)
  (insert "//")
  )

(defun my-uncomment-current-line ()
  "Remove “//” (if any) in the beginning of current line."
  (interactive)
  (when (wholeLineIsCmt-p)
    (beginning-of-line 1)
    (search-forward "//")
    (delete-backward-char 2)
    ))

(defun my-comment-region (p1 p2)
  "Add “//” to the beginning of each line of selected text."
  (interactive "r")
  (let ((deactivate-mark nil))
    (save-excursion
      (goto-char p2)
      (while (>= (point) p1)
        (my-comment-current-line)
        (previous-line)
        ))))

(defun my-uncomment-region (p1 p2)
  "Remove “//” (if any) in the beginning of each line of selected text."
  (interactive "r")
  (let ((deactivate-mark nil))
    (save-excursion
      (goto-char p2)
      (while (>= (point) p1)
        (my-uncomment-current-line)
        (previous-line) )) ))
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