default class method types... (unification)

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.