... P.S.: sth like nested instances (FFunctor) should be possible, too. and a way to select the defaults in external inteface definitions: "instance Monad M . Functor M where". maybe additional syntactic sugar (instances)? [code] class FFunctor m where ffmap :: ... class FFunctor m => Functor m where fmap :: ... instance FFunctor m where ffmap = fmap class Gunctor m where gmap :: ... class Hunctor m where hmap :: ... hmap = unsafePerformIO $ putStrLn "class Hunctor" >> undefined class FFunctor m => Junctor m where jmap :: ... jmap = unsafePerformIO $ putStrLn "class Junctor" >> undefined instance FFunctor m where ffmap = jmap class FFunctor m, Functor m, Gunctor m, Hunctor m, Junctor m => Monad m where return :: a -> m a (>>=) :: m a -> (a -> m b) -> m b instance Functor m where fmap f = unsafePerformIO $ putStrLn "class Monad" >> undefined --instance FFunctor m where instance Hunctor m where hmap f = unsafePerformIO $ putStrLn "class Monad" >> undefined instance Junctor m . FFunctor m where -- to define the functions in the *unctor, using the defaults defined in the class _*unctor_ (assumed there would be any default): instance Monad M where return = ... (>>=) = ... instance Functor M where gmap = unsafePerformIO $ putStrLn "instance Functor" >> undefined instance FFunctor M where instance Gunctor M where gmap = unsafePerformIO $ putStrLn "instance Gunctor" >> undefined instance Hunctor M where instance Junctor M where -- to define the functions in the *unctor, using the defaults defined in the class _Monad_: instance Monad M where return = ... (>>=) = ... instance Functor M where instance FFunctor M where instance Gunctor M where gmap = unsafePerformIO $ putStrLn "instance Monad" >> undefined instance Hunctor M where instance Junctor M where --or instance Monad M where return = ... (>>=) = ... instance Monad M . Functor M where instance Functor M . FFunctor M where --instance Monad M . FFunctor M where instance Monad M . Gunctor M where gmap = unsafePerformIO $ putStrLn "instance Monad" >> undefined instance Monad M . Hunctor M where instance Monad M . Junctor M where --syntactic sugar: instance Monad M where return = ... (>>=) = ... instances Functor M, Gunctor M, Junctor M where gmap = unsafePerformIO $ putStrLn "instance Monad" >> undefined instances Hunctor M, Junctor M . FFunctor M where [/code] - marc