[Haskell-cafe] Functional dependence nightmare

[Stephen Tetley]

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.