RE: Anomalous Class Fundep Inference

| In Hugs 98 Feb2001 (as hugs -98), this piece of code gives an error: | | class X a b | a -> b where x :: a; | class (X a b) => Y a b | a -> b where y :: a; | instance (X a b) => Y a b where y = x; | instance (Num a) => X a a where x = 0; -- line A Quite right too! The final instance declaration says that the two arguments of X must always be the same: * the class says a->b * the instance matches any first arg and says that the second must be equal. So when typechecking the instance for Y a b, a and b are forced to be equal, and that's less general than the instance claims to be. Simon