fontify-face: Fontification of symbols referencing faces

Today I was updating my theme to support some additional packages. I'm a big fan of face inheritance because it makes things very consistent across the board. In particular I'm trying to reuse the faces in the font-lock- group as much as possible.

I have even created a similar hierarchy for the various markup systems such as markdown, org mode, rst, textile, latex and others. It's called, unsurprisingly, markup and contains all the common elements such as headers, code text, emphasis and so on. I then make all the other modes inherit from this set of faces and thus achieve a consistent look and feel across all the markup modes.¹. Have a look at the definitions if this makes no sense to you yet!

Anyway, while I was working on adding new faces I got annoyed because I couldn't see what I was doing. I sometimes inherit from multiple faces to make combined faces, sometimes I just add a parameter or two and it's difficult to see the results. Usually you need to switch to a different buffer (with the mode you are theming), find the desired element and look what's going on.

Why couldn't the face name itself be fontified with the face? A ha! I already use rainbow-mode which colorizes color references such as #fafafa with the color they represent. So I'm thinking along the same lines: the symbols representing faces should be fontified with that face. This will make the feedback loop much shorter and the development much faster and more convenient.

I have quickly turned this idea into a package and called it fontify-face. The code is rather unsurprising, just a couple lines. The most interesting part is the function fontify-face-colorize-matched-face which computes the face from the matched region and then passes it back to font-lock to do its thing.

This works because font-lock allows arbitrary forms in every place where you can pass a face reference. The rule is, this form must evaluate to

• a face reference,
• or a plist representing face properties,
• or nil if we don't want do do anything with the match.

The form I use is (funcall 'fontify-face-colorize-matched-face) because I dislike just passing random blobs of code around. This way I can redefine the function without having to mess around with removing the old forms from font-lock-keywords-alist. This is similar in logic to why you shouldn't put lambda forms in hooks.

So that's it folks. Finally, here are some pictures: