Given an applicative functor f
, I had an idea of making a new applicative functor Rev f
like f
but with the order of effects reversed. Here it is:
import Control.Applicative
newtype Rev f a = Rev {unRev :: f a}
instance Functor f => Functor (Rev f) where
fmap f (Rev fx) = Rev (fmap f fx)
instance Applicative f => Applicative (Rev f) where
pure x = Rev (pure x)
(Rev ff) <*> (Rev fx) = Rev (pure (\x f -> f x) <*> fx <*> ff)
My questions are
- Is that a valid
Applicative
instance (does it obey theApplicative
laws)? - Does this construction have a name? Is there a module somewhere this is hiding in?