Hello Taral, Wednesday, August 16, 2006, 1:25:03 AM, you wrote:
in this case we lose "class Functor a => Monad a" base class declaration. so what will be the meaning of this:
I don't see why that is the case.
class Functor m => Monad m where return :: a -> m a (>>=) :: m a -> (a -> m b) -> m b instance Functor m where fmap f = (>>= return . f)
What's wrong with this? All Monads are Functors. If you don't provide a Functor, it gets defined for you. The problem is working out whether to use the default Functor or an external Functor.
you deleted context of my note, where you wrote something opposite to "All Monads are Functors":
Not necessarily. If A doesn't have any Functor declarations, it could be considered just a Monad without a Functor.
is it possible to declare Monad Foo without Functor Foo with the above class definition? i think no. Functor instance will be either defaulted or explicitly defined. so both modules, A and B, actually defines _both_ instances, although A defines Functor Foo implicitly while B does it explicitly. importing both modules will mean importing two different declaration of both instances, Functor Foo and Monad Foo -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com