But that is already used for existential datatypes: data Test = forall a . Num a => Test a Maybe that lack of forall is enough to distinguish the two though. Your example would currently be written: data (Ord a,Num b) => Either a b = Left a | Right b I dont have any problem with this, just remember these are constraints on function arguments... It my superficially look like a class system, but in reality it is more like a constraint based programming language (for example Prolog) at the type level... Regards, Keean.