There is another way to solve this problem with haddock: to add an Opt_Haddock at the parser stage: in this case all haddock comments will be attached to the correct places and also we'll have an option to reformat comment (if we want). This seems nice for me. However, there are some problems at this way: I've tried to enable this option in my fork and many test cases from Data and Import step failed. For Data I fixed them, but for Import there are some problems. However, nothing seems to be impossible and I can try to resolve all isues here, if we'll chose this road.

Also, if we'll chose the "right" way for haddock processing, issue for common comments still remains. So we'll have to provide two different ways for dealing with comments: one for haddock and another for other comments. It doesn't seem to be pretty enough. Nevertheless, work with haddock should be more easy this way

I found a much worse issue in this PR, than the original one. If we have a prefix comment after the data declaration like this:

data A = A

-- | foo
foo = undefined

Now stylish attaches it to the data:

data A = A
  -- | foo

-- | foo
foo = undefined

That's indeed awful. However, I don't see any good way for working around it without enabling Opt_Haddock flag. The reason is that in current parser's behavior -- | foo comment appears at the same place, where post-comment to the data constructor is. Of course, we could filter only comments starting with -- ^, but this doesn't seems to be a good and clean idea.

That's why I think we should use Opt_Haddock flag and to work haddock as ghc expects us to work with them. This will require some work, though. I'll try to carry it out. It would be helpful to know, why this hasn't been done already? Were there any problems in this way?

Anyway, it would be great to take your opinion about this problem, @jaspervdj. If my description is not clear enough, I can try to clarify it

Yes, I'm happy to adopt Opt_Haddock -- I suspect this option wasn't around when the majority of the comment-handling code was written...