Ian Lynagh schrieb:
readValue' :: Field -> Maybe (forall s. SqlBind s => s) -> Value readValue' fld s = if isJust s then readValue fld (fromJust s) else emptyValue fld
Thank you very much, that's exactly what I wanted. After reading in the GHC users guide about rank 2 polymorphism I thought that this is not possible. Chapter "7.4.8. Arbitrary-rank polymorphism" says: There is one place you cannot put a forall: you cannot instantiate a type variable with a forall-type. So you cannot make a forall-type the argument of a type constructor. So these types are illegal: x1 :: [forall a. a->a] x2 :: (forall a. a->a, Int) x3 :: Maybe (forall a. a->a) Maybe the users guide is not precise enough at this point. Regards, Martin.