Method C: Define fmap in terms of bind
instance Monad m => Functor (Iteratee el m) where fmap = liftM
Now you need to do the inspection of Iteratee only once: in the definition of the bind. However, to use liftM as implementation of fmap the superclass constraint of the Functor instance has changed from Functor to Monad. Is this a problem? If so, method A seems the way to go: you could argue that `Functor m => fmap :: (a -> b) -> Iteratee el m a -> Iteratee el m b' is more general than the `Monad m =>' version (works for more `m's) and therefore deserves to redo the analysis of Iteratee. Martijn. On 1/4/11 11:24, oleg@okmij.org wrote:
I'd like to argue in opposition of making Functor a super-class of Monad. I would argue that superclass constraints are not the right tool for expressing mathematical relationship such that all monads are functors and applicatives.