Re: [Haskell-cafe] Functional dependence nightmare

For the specific error at line 265 I think you should be using ScopedTypeVariables and properly qualifying the type signature at the function level with a forall. The local annotation { ::(SubUnit a)=> } is presumably introducing another type variable unrelated to 'a' in the function level type signature. Note, my suggestion is "by eye" - I haven't run the code, so it may not work or there may be other errors.