data Either a =3D Num a =3D> Left a Ord a =3D> Right a
This is a mistake because the same variable is being constrained in two different ways. The only way both constrains can be satisfied is if a is both in Num and Ord. This can be written: data (Num a,Ord a) => Either a = Left a | Right a Which makes it a lot clearer that the constaints apply to both the Left and Right constructor. You have to remember that a is a polymorphic variable... now the first line [Num a => Left a] says that a has the type (forall a . Num a) but the second line [Ord a => Right a] says a has the type (forall a . Ord a) These are not the same type, so when they are combined (remember a scopes over the whole type definition) there is a type error because: (forall a . Ord a) is not the same as (forall a . Num a) Regards, Keean.