It does seem unnecessary. The Data instance for Proxy is manually written in Data.Data: proxyConstr :: ConstrproxyConstr = mkConstr proxyDataType "Proxy" [] PrefixproxyDataType :: DataTypeproxyDataType = mkDataType "Data.Proxy.Proxy" [proxyConstr] instance (Data t) => Data (Proxy t) where gfoldl _ z Proxy = z Proxy toConstr Proxy = proxyConstr gunfold _ z c = case constrIndex c of 1 -> z Proxy _ -> error "Data.Data.gunfold(Proxy)" dataTypeOf _ = proxyDataType dataCast1 f = gcast1 f If dataCast1 weren't defined to be gcast1, then that instance would only require a Typeable t constraint, which would make it polykinded. To be honest, I'm not sure why gcast1 requires a Data t constraint (I don't use Data that often, so hopefully someone more knowledgeable than me can answer this).