Highlight error, undefined with font-lock-warning-face.

[ikirill]

Both are very special symbols; this is similar to emacs-lisp-mode, for
example.

[gracjan]

I am partial to this one. I understand that it is good to highlight keywords that should not appear in production code but then error and undefined aren't real keywords and also there are plenty of other functions that should not appear in production code like head or fromJust.

Meanwhile there is also throw, throwIO and friends that signal local errors in code and those would also be worth highlighting.

[ivan-m]

I think undefined is worth highlighting as a warning; I'm not so sure about error but it would probably be worth it.

I think adding every single partial function to be marked like this might be a bit excessive though.

But if this is meant to be a face for warnings (i.e. "are you really sure you want to have this here?") then throw and throwIO probably don't belong (as they are valid to use within user code, and preferable to using error).

[gracjan]

We are working on #724, so your simple fix will be obsolete. We need a more principled approach.

[themattchan]

Why have these definitions been removed from the master?

[gracjan]

I sense you would like to have these definitions back, if so please create a PR and include test cases. Thanks.

[mrBliss]

@themattchan you can add the following lines to your config to revert these changes:

(font-lock-add-keywords
  'haskell-mode
  '(("\\_<\\(error\\|undefined\\)\\_>" . 'font-lock-warning-face)))