[Haskell-cafe] Is this a "monad" for contravariant functors?
zemyla at gmail.com
Fri Mar 10 22:25:12 UTC 2017
There originally seems like there shouldn't be one, because it's impossible
to, for any permutation of the arguments of (>>=), get the
covariances/contravariances right on the arguments. However, based on the
fact that there is a mapping:
a -> ((a -> r) -> r)
and what seems like an isomorphism:
(a -> r) -> (((a -> r) -> r) -> r)
(((a -> r) -> r) -> r) -> (a -> r)
then this looks like it might be some kind of monad-like structure:
class Contravariant f => Contramonad f where
contrareturn :: a -> f (f a)
contrabind :: (a -> f (f b)) -> f b -> f a
I have no idea what the laws should be for it, though. So is this worth
pursuing, or have I just stumbled down a blind alley?
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