Hi Marc. This is a bug in ghc-7.2.* (http://hackage.haskell.org/trac/ghc/ticket/5709). It's fixed in ghc-7.4.*. I don't think any version of ghc-7.2.* will ever become the default version in a platform and/or nixpkgs. Therefore, I'm not very motivated to patch ghc. Cheers, Andres