All Monads are Functors
ashley at semantic.org
Fri Aug 25 22:09:11 EDT 2006
Jon Fairbairn wrote:
> There has been discussion in the past about whether Monad
> should be defined as
>> class Functor m => Monad m where ...
It's more complicated now that we have Ross Patterson's "Applicative".
The correct decl I think is this:
class Applicative m => Monad m where
-- remove "return"
and changing the names of the Applicative functions:
class Functor f => Applicative f where
return :: a -> f a
ap :: f (a -> b) -> f a -> f b
This would mean moving Applicative into the Prelude.
I think "joining up the classes" is a good idea, and there may be other
cases too. "Alternative" and "MonadPlus"?
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