"type" introduce a type synonym, and Haskell98 forbids these in instances, so GHC complains. GHC also lifts this restriction when invoked with -fglasgow-exts . http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html...
Flexible Instances will probably be added to HaskellPrime: http://hackage.haskell.org/trac/haskell-prime/wiki/FlexibleInstances This page: http://cvs.haskell.org/Hugs/pages/users_guide/class-extensions.html#FLEXIBLE... says that ultimately you would turn the type language pretty much into Prolog (which would allow more expressive power---and less inconvenience as we both would like--but make general type checking undecidable). Instead they do a more conservative extension with a fixed depth of constraints it will check so the compiler will terminate. Jared.