There are some serious dependeny problems with Haskell libraries, caused by two different versions of the mtl library (mtl-1, bundled with ghc, and mtl-2 from devel/hs-mtl). The latter (devel/hs-mtl) wasn't meant to be used yet, but unfortunately some ports *do* rely on it (they don't build with mtl-1).
On the other hand, we have some ports which don't build with mtl-2 but strictly need mtl-1 (that's one of the reasons why lang/ghc/patches/patch-ghc_mk exists). I hoped to fix this soon, but I'll have to pkg_delete all Haskell stuff from my machine, rebuild all of it, check the .cabal files of all the ~110 or so hs-* ports and check wether everything pulls the correct version of mtl. Bulk builders: if the breakage is too annoyint, feel free to mark lang/ghc as broken. It will take some days to fix this mess (I don't have much time before the next friday). Ciao, Kili