In article <200307102002.31955.wolfgang@jeltsch.net>,
Wolfgang Jeltsch
By the way, I strongly vote for Functor being a superclass of Monad in Haskell 2.
I recently created my own Monad class in HBase instead of using the Prelude one. The hierarchy looks something like this: class HasReturn f where return :: a -> f a class Functor f where fmap :: (a -> b) -> f a -> f b class (Functor f) => FunctorApply f where fApply :: f (a -> b) -> f a -> f b fPassTo :: f a -> f (a -> b) -> f b (>>) :: f a -> f b -> f b class (FunctorApply f,HasReturn f) => FunctorApplyReturn f instance (FunctorApply f,HasReturn f) => FunctorApplyReturn f class (FunctorApplyReturn f) => Monad f where (>>=) :: f a -> (a -> f b) -> f b class (Functor f) => ExtractableFunctor f where fExtract :: (FunctorApplyReturn g) => f (g a) -> g (f a) fToList :: f a -> [a] -- has default definition (exercise for the reader) Certain standard monadic functions, such as LiftM2, actually apply to classes higher up the hierarchy (FunctorApply, in this case). FunctorApplyReturn is a particularly useful class for "manipulating things in a box" when the box isn't quite a Monad. I'm glad to hear there isn't a _serious_ cost (i.e. performance penalty) for fine-grained hierarchies. Yeah, so if you want to define your own Monad, you have to define all the other instances too. I ease this by providing functions such as monad_fmap and monad_fApply etc. that you can use for your instances. -- Ashley Yakeley, Seattle WA