haddock 2.14.3 that ships with it seems to be quite broken. Perhaps it's a bad interaction with cabal, it's hard to say from the outside, but here are some details.
Here's the kind of error I get from haddock when I try to use cabal to install a package (primitive, in this case):
module ‘primitive-0.5.3.0:Main’ is defined in multiple files:
I can't reproduce that here with a ghc-7.8.3/haddock-2.14.3 install on Linux. However, this sounds *alot* like a Clang/CPP issue, specifically
https://github.com/haskell/cabal/issues/1740#issuecomment-39559026
This is a Clang/CPP issue. A workaround is to pass `—ghc-options=-optP-P` to `cabal haddock`. The corresponding GHC issue was closed 5 weeks ago: https://ghc.haskell.org/trac/ghc/ticket/9174 -- Miëtek