default class method types... (unification)

[MR K P SCHUPKE]

If I have a class:

class A m n where
    f :: m a -> n a

I can declare instances

instance A Maybe Maybe where
    f = id

instance A [] [] where
    f = id

but if I try and put a default method:
    f = id

I get a unification error (less polymorphic than expected).This seems
a little odd - is there a work around for this?

    Regards,
    Keean Schupke,
    Department of Electrical & Electronic Engineering,
    Imperial College London.