jlongster

Emacs font locking: highlighting comment lines

I recently tried an package called Quack which adds a bunch of commands for writing and running Scheme code in Emacs. Although I don't use it anymore, one thing I really like was how it highlighted comment lines with a depth of 3 or more (meaning ";;;" in Scheme, or "///" in C or Javascript). It turns out this was really nice, since it ends up highlighting file-level comments at the top and section headers throughout the code.

I think this looks much nicer than other techniques, such as repeating the comment character 40 times to indicate a line (e.g. ";;;;;;;;;;;;" over and over). If everyone on a project wrote code this simple way, each person would get to choose how to view those kinds of comments.

I wanted to separate this functionality into its own package so that I could use it in languages other than Scheme. I have never written any Emacs Lisp before, so although it was pretty brutal, I ended up with something that works rather nicely. Use it if you want.

(defconst jwl-title-face 'jwl-title-face)
(defface jwl-title-face
  '((((class color) (background light))
     (:background "DarkSeaGreen1" :foreground "grey25"))
    (((class color) (background dark))
     (:background "DarkGrey")))
  "Face used for titles.")

(defun jwl-generate-highlight-keywords (comment-char)
  `((,(concat "^\\([ \t]*"
              comment-char
              comment-char
              comment-char
              "[ \t]*"
              "\\("
              "[^\r\n]*"
              "\\)"
              "\r?\n?\\)")
     1 jwl-title-face prepend)))

(add-hook
 'scheme-mode-hook
 (lambda ()
   (font-lock-add-keywords
    nil
    (jwl-generate-highlight-keywords ";"))))

(add-hook
 'python-mode-hook
 (lambda ()
   (font-lock-add-keywords
    nil
    (jwl-generate-highlight-keywords "#"))))

(add-hook
 'js2-mode-hook
 (lambda ()
   (font-lock-add-keywords
    nil
    (jwl-generate-highlight-keywords "/"))))