How to make font-lock use my face?

Question

I'm interested in making a major mode to display PGN files nicely. I'm trying to create my own faces since the defaults don't seem super-appropriate.

My mode definition looks like this:

(defface my-r-face `((t (:foreground "red")))  "Red highlight")

(setq pgn-highlights
      '(("Pi\\|Infinity" . font-lock-constant-face)
    ("Sin\\|Cos" . font-lock-string-face)
    ("[0-9]+\\." . font-lock-keyword-face)
    ;; ("%.*" . my-r-face) ;; This doesn't show up (the relevant text appears just like the default text
    ("%.*" . font-lock-comment-face) ;; works great - relevant text looks like the comment face
    ))

(define-derived-mode pgn-mode fundamental-mode "mymath"
  "major mode for editing mymath language code."
  (setq font-lock-defaults '(pgn-highlights)))

A typical file that I'd like to use this on is:

[Event "?"]
[Site "?"]
[Date "2019.6.3"]
[Round "?"]
[White "Mike"]
[Black "Mikey"]
[Result "*"]
[WhiteELO "?"]
[BlackELO "?"]

%Created by Caissa's Web PGN Editor
1. e4 e5 2. Nf3 Nc6 3. Bc4 Bc5 4. O-O Nf6 5. d3 d6 6. Nc3 a6 7. a3 Be6 8.
Bxe6 fxe6 9. h3 O-O 10. Be3 Ba7 11. Ra2 Qe8 12. Qc1 Nh5 13. Qd2 Nf4 14. g3
Nxh3+ 15. Kg2 Qh5 16. Rh1 Qxf3+ 17. Kh2 Nxf2 18. Rf1 Qh5+ 19. Kg2 Qf3+ 20.
Kg1 Qh1#

The issue is the line with the comment (%Created by...). If I use the built-in font-lock-comment-face then everything is highlighted nicely and the comment looks like the normal comment. But if I change it to use my-r-facethen the comment is not highlighted, and instead appears be be the default font face.

What am I doing wrong here?

Answer 1

Just add this, after the defface, to define a variable of the same name, whose value is the face symbol:

(defvar my-r-face 'my-r-face
  "Variable for face `my-r-face'.)

font-lock-defaults in the case of just (pgn-highlights) defines font-lock-keywords as the list that is the value of variable pgn-highlights.

The doc for font-lock-keywords tells you about this case, which is the one you are using:

(MATCHER . FACESPEC)

In this kind of element, FACESPEC is an expression whose value specifies the face to use for highlighting. In the simplest case, FACESPEC is a Lisp variable (a symbol) whose value is a face name.

     ;; Highlight occurrences of ‘fubar’,
     ;; using the face which is the value of ‘fubar-face’.
     ("fubar" . fubar-face)