Please satisfy my curiosity, what is it about data WrappedDisplayType = forall displayType . DisplayType displayType => WrappedDisplayType displayType deriving (Typeable) that causes GHC 6.0 to utter DisplayTypes.hs:135: Can't make a derived instance of `Typeable WrappedDisplayType' (`WrappedDisplayType' has existentially-quantified constructor(s)) When deriving instances for type `WrappedDisplayType' Obviously it doesn't like the existential quantifier, but why should that be a problem for Typeable?