Just one more question... data Prop = forall a b. (Eq a, Eq b, Show a, Packet b, Convertible a b) => Attr a b := a deriving (Typeable) data Attr a b = Attr String (a -> Dynamic, Dynamic -> Maybe a) (a -> b, b -> a) makeAttr :: (Typeable a, Convertible a b) => String -> Attr a b makeAttr name = Attr name (toDyn, fromDynamic) (convert_AB, convert_BA) I can do this for Attr instance (Typeable a, Arbitrary a, Typeable b, Arbitrary b, Convertible a b) => Arbitrary (Attr a b) where arbitrary = makeAttr `fmap` arbitrary coarbitrary a = error "Not implemented" How do I define an arbitrary prop, though? Following does not work: arbitraryProp :: forall a b.(Arbitrary a, Arbitrary b) => Attr a b -> Gen Prop arbitraryProp = arbitrary := arbitrary Thanks, Joel -- http://wagerlabs.com/