If I remember right, Functor was a superclass of Monad in Haskell early on, but it was taken away. I think this was the wrong decision, but I seem to remember that the rationale was that it would be too onerous to require programmers to write a Functor instance every time they want a Monad instance. Bah! -- Lennart On Aug 13, 2006, at 21:39 , Donald Bruce Stewart wrote:
jon.fairbairn:
I find myself dismayed that the mathematical relationship between Monads and Functors isn't available in Haskell98; if I use fmap in a Monad m=>... typed function, I get an extra Functor m required in the context, but not only are all mathematical monads functors, any instance of Monad has fmap in the form (>>= return . f), so it's annoying.
For interest. Here's the defn in the Gofer prelude from 1994:
class Functor f where map :: (a -> b) -> (f a -> f b)
class Functor m => Monad m where result :: a -> m a join :: m (m a) -> m a bind :: m a -> (a -> m b) -> m b
join x = bind x id x `bind` f = join (map f x)
class Monad m => Monad0 m where zero :: m a
http://www.cse.unsw.edu.au/~dons/data/cc.prelude
-- Don _______________________________________________ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime