Hi,
following a long discussion about ghc version and haskell-platform, I started a repo builded with ghc-7.2.2. I compiled about 140 packages from [extra], [community] and [haskell] for the x86_64 arch. Now I'm using this and all seems okay. If anybody want to have a look here is a copy:
To make this work you should put the repo before extra and community and make a pacman -Rcs ghc before installing anything.
Cheers
Fabio