Need help with search based font-locking

[Tassilo Horn]

Hi all,

in my greql-mode [1], I setup font-locking like that:

--8<---------------cut here---------------start------------->8---
(define-derived-mode greql-mode text-mode "GReQL"
  "A major mode for GReQL2."
  [...]
  (setq font-lock-defaults
        '((greql-fontlock-keywords-1
           greql-fontlock-keywords-2
           greql-fontlock-keywords-3)))
  (add-hook 'after-save-hook 'greql-set-fontlock-keywords-3 t t)
  [...]
  (define-key greql-mode-map (kbd "C-c C-f") 'greql-format))
--8<---------------cut here---------------end--------------->8---

`greql-fontlock-keywords-3's value is not static, it frequently changes.
So I update it after saving (as you can see), and at several other
places.  The updating of the variable works, but how do I tell font-lock
that it should use the new value and re-fontify the current buffer?

As a brute-force workaround, I can do

   (set (make-local-variable 'font-lock-keywords)
        greql-fontlock-keywords-3)
   (redisplay t)

but I guess this is very bad style.  And even then some things that
should be highlighted are not.  For those, I need to delete a char and
add it back to apply the new fontification.

I really tried to dig into the elisp manual, but I couldn't find any
help with font-lock-defaults KEYWORDS that aren't fixed.

Thanks for help!
Tassilo
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[1] GReQL is a graph query language developed at our institute.