On 8/13/06, Lennart Augustsson
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!
There was a proposal (Re: [Haskell] A collection of related proposals
regarding monads) to allow instance definitions to be combined,
allowing a more fine-grained hierarchy (Functor, PointedFunctor,
Applicative, Monad) without changing the instance definition
requirements.
In my opinion, an instance definition of a subclass should allow the
superclass's methods to be defined as if they were part of the
subclass, e.g.:
instance Monad [] where
fmap = map
return x = [x]
join = concat
It's so pretty! (But a little inefficient. You'd probably want to
define ap/lift2 in there.)
--
Taral