Elisp: How to Write Comment Command in Major Mode

By Xah Lee. Date: . Last updated: .

There are 2 ways to write a command to comment/uncomment code:

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-dwimAlt+;】.

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 types the keyboard shortcut for comment-dwim 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 Elisp: 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: Elisp: Write Comment Command from Scratch.

thanks to Daniel for correction.

Elisp, Writing Major Mode

  1. Syntax Coloring
  2. Font Lock Mode
  3. Define Face
  4. Color Comment
  5. Comment Command
  6. Your Own Comment Command
  7. Keyword Completion Command
  8. Create Keymap
  9. Abbrev/Template
  10. Text Properties
  11. Overlay Highlighting
  12. Lookup Doc
  13. Syntax Table

  1. How to Name Your Major Mode
  2. provide, require, features
  3. load, load-file, autoload

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