Use font-lock-fontify-region-function directly

[gracjan]

Emacs multipass regexp based font lock functionality as defined by font-lock-syntactic-kaywords has a couple of issues with Haskell code, mostly due to heavily overloaded meanings for dot and apostrophe. Meaning of dot as either a module name separator or composition or part of other operator is hard to get right with multipass syntax highlighter.

The solution is to use font-lock-fontify-region-function. It should go over specified fragment of source code using haskell-lexeme-looking-at-token and directly apply faces to token and/or fragments of tokens.

Example how this can be done is in haskell-syntax-propertize-function defined in PR #1039.

Summary:

• set font-lock-fontify-region-function to haskell-font-lock-fontify-region
• haskell-font-lock-fontify-region should use haskell-lexeme-looking-at-token
• haskell-font-lock-fontify-region should apply faces directly
• haskell-font-lock-fontify-region should also apply compose directly (subsuming prettify-symbols that has similar issues)
• haskell-font-lock-fontify-region should define more faces, for example modules could use different face than constructors
• haskell-font-lock-keywords should go away completely

[gracjan]

@deepfire, I'm pinging you about this one issue as I know you were good with large overhauls. Can you fix this one?

[gracjan]

Step in this direction is in #1157.

[gracjan]

Hmm, there might be an issue with rainbow-identifiers. Investigate.