Elisp: How to Write Comment Command in Major Mode

By Xah Lee. Date: . Last updated: .

This page shows you how to write a command to comment/uncomment a region of text in a major mode.

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


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: How to Write Your Own Comment Command from Scratch.

thanks to Daniel for correction.

Writing Major Mode

  1. How to Write a Emacs Major Mode for Syntax Coloring
  2. Elisp: Font Lock Mode Basics
  3. Elisp: How to Define Face
  4. Elisp: How to Color Comment in Major Mode
  5. Elisp: How to Write Comment Command in Major Mode
  6. Elisp: How to Write Your Own Comment Command from Scratch
  7. Elisp: How to Write Keyword Completion Command
  8. Elisp: How to Create Keymap for Major Mode
  9. Elisp: Abbrev, Template, for Major Mode
  10. Elisp: Text Properties
  11. Elisp: Overlay Highlighting
  12. Emacs: Lookup Google, Dictionary, Documentation
  13. Elisp: Syntax Table Tutorial

  1. Elisp: How to Name Your Major Mode
  2. Elisp: What's “feature”?
  3. Elisp: require, load, load-file, autoload, feature

Syntax Table

  1. Elisp: Syntax Table Tutorial
  2. Elisp: How to Find Syntax of a Character?
  3. Elisp: How to Modify Syntax Table Temporarily
  4. Elisp: How to Determine If Cursor is Inside String or Comment
  5. Elisp: Regex Patterns and Syntax Table
  6. Elisp: Find Matching Bracket Character
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