Emacs custom major mode with multiple comment types

Question

I'm trying to write my first (very simple) major mode for Emacs. My goal is to have multiple comment types, but I can't find a way to implement this. Basically, I want my major mode to make highlighted comments as follows

• line that comes after #, exactly like in Python
• line that comes after !, same as above, but with a different color. Also, I want to be able to add a # comment at the end of this line.
• a code block starting with % and ending in end, again with a different color

So far, I have the following in my .el file:

(defvar my-mode-hook nil)

(defvar my-mode-map
  (let ((map (make-keymap)))
    map)

(defvar my-mode-syntax-table
  (let ((st (make-syntax-table)))
    ;; comment style: "# ..."
    (modify-syntax-entry ?# "<" st)
    (modify-syntax-entry ?\n ">" st)
    (setq comment-start "# ") 
    (setq comment-end "")
    st))

(defvar my-highlights
  '(("!\\|" . font-lock-function-name-face)
    ("!\\([^<]+?\\)\n" . (1 font-lock-function-name-face))))

(define-derived-mode my-mode prog-mode "my-mode"
  (interactive)
  ;; (kill-all-local-variables)
  (use-local-map my-mode-map)
  (set-syntax-table my-mode-syntax-table)
  (setq font-lock-defaults (list nil nil))
  (set (make-local-variable 'font-lock-defaults) '(my-highlights))
  ;; (set (make-local-variable 'indent-line-function) 'my-indent-line)
  (setq major-mode 'my-mode)
  (setq mode-name "MY MODE")

  (run-hooks 'my-mode-hook))

This works somewhat for # and !, but the text highlight for ! comments does not work if I put a # at the end of the line. Also, it only works after I hit enter at the end of the line, not straight after the ! as I write.

Answer 1

Here are some tips to get you started:

• The (setq comment-start/end ..) needs to be within the define-derived-mode, not within that defvar of my-mode-syntax-table (otherwise it just sets the comment style in the buffer that happens to be current when your code loaded, rather than in all the buffers that use your mode).
• You wan to add (modify-syntax-entry ?! "<" st) since ! also starts a single-line comment.
• To change the color of comments depending on the char that introduces it, you'll want to set font-lock-syntactic-face-function.

• You want to add (modify-syntax-entry ?% "< b" st) since % starts a comment, but one that's ended with something else than \n so it needs to use another style (alternatively, you could mark %, \n, and ! as style b and use the "default" style for this one). To mark end as being the matching comment ender, you'll have to use syntax-propertize-function, e.g.

  (setq-local syntax-propertize-local
              (syntax-propertize-rules
               ("e\\(n\\)\\(d\\)" (1 "w 3b") (2 "w 4b"))))
  

  note that the exact rule to use will depend on whether the end in bending also ends the comment (for example).

• You don't need (interactive), (use-local-map my-mode-map), (set-syntax-table my-mode-syntax-table), (setq major-mode 'my-mode), (setq mode-name "MY MODE"), nor (run-hooks 'my-mode-hook) since define-derived-mode already does that (tho you might then like to change "my-mode" to "MY MODE").

• Your code sets font-lock-defaults twice (and not to the same value).