Emacs Lisp: How to Write Comment/Uncomment Command for a Major Mode
There are 2 ways to write a command to comment/uncomment code:
- Use the builtin package
newcomment.el
- Write your own command. [see Emacs Lisp: Write Comment Command from Scratch]
This page will show how to use the newcomment.el
package to define comment/uncomment commands.
Writing a Comment Command Using newcomment.el
comment-dwim
Emacs has a standard command to insert or delete comment, named comment-dwim
【Alt+;】.
When there is a text selection, comment-dwim
will comment or uncomment the region in a smart way.
If there is no text selection, comment-dwim
will insert a comment syntax at the end of line, or move to the beginning of comment if already exists.
The comment-dwim
is the standard
command to comment/uncomment code. Your major mode should support it. When
a user press the keyboard shortcut for it while in your major
mode, it should work.
comment-region, uncomment-region
comment-region
and uncomment-region
are lower level commands that do the actual work.
comment-dwim
simply determine whether to comment or uncomment, and on what region boundary, then call
comment-region
or uncomment-region
.
Make comment-dwim Work for Your Major Mode
All you have to do, is to set 2 buffer-local variables.
For example, here's code for emacs lisp:
(setq-local comment-start "; ") (setq-local comment-end "")
Here's another example.
In CSS, comment has this syntax:
/* css comment syntax */
Here's code from emacs's css-mode:
(define-derived-mode css-mode fundamental-mode "CSS" "Major mode to edit Cascading Style Sheets." (setq-local font-lock-defaults css-font-lock-defaults) (setq-local comment-start "/*") (setq-local comment-start-skip "/\\*+[ \t]*") (setq-local comment-end "*/") (setq-local comment-end-skip "[ \t]*\\*+/") ;; ... )
[see Emacs Lisp: Regex Tutorial]
Write Your Own Comment Command from Scratch
If your language's comment syntax is more complex, different from any of the form in C, Java, python, ruby, php, etc. You'll need to write your own comment/uncomment command.
See: Emacs Lisp: Write Comment Command from Scratch.
thanks to Daniel for correction.