Does anybody use Haskell who can test this?

Just tried this. :CtrlPBufTag doesn't work(always shows empty list of tags) but :CtrlPBufTagAll works.

@osa1 thanks for testing!

I've not tested this PR, but FWIW I've personally not used lushtags because its maintenance seems to be stalled, its dependencies are now rather out-of-date. I'm using hasktags successfully for :CtrlPBufTag and :CtrlPTag with this custom type setting:

  let g:ctrlp_buftag_types.haskell = {
    \ 'bin'  : 'hasktags',
    \ 'args' : '--ctags --extendedctag --output=-',
  \ }

and in Tagbar with the hasktags definition on their wiki.

If there's interest in a pull request similar to this one but supporting the hasktags executable, I can whip one up.

FWIW, the mkasa/lushtags fork of lushtags works well and I'm using it for both ctrlp and tagbar. I don't know the relative pros and cons of this version of lushtags vs hasktags.

EDIT: Also @d11wtq, what is the [buffer] argument of :CtrlPBufTag supposed to be? The path of the file opened in the buffer? At first I assumed it was the buffer number, but that didn't work. This pull request seems to work for me for both :CtrlPBufTag % and :CtrlPBufTagAll.