I needed a lens function that works like over
, but with monadic operations:
overM :: (Monad m) => Lens s t a b -> (a -> m b) -> (s -> m t)
While this function is easy to define (it's actually just an identity modulo WrappedMonad
), I wonder are such functions defined somewhere in lens?
{-# LANGUAGE RankNTypes #-}
import Control.Applicative
import Control.Lens
overF :: (Functor f) => Lens s t a b -> (a -> f b) -> (s -> f t)
overF l = l
overM :: (Monad m) => Lens s t a b -> (a -> m b) -> (s -> m t)
overM l = (unwrapMonad .) . l . (WrapMonad .)
in Control.Lens.Traversal:
traverseOf :: Over p f s t a b -> p a (f b) -> s -> f t
traverseOf = id
mapMOf :: Profunctor p =>
Over p (WrappedMonad m) s t a b -> p a (m b) -> s -> m t
mapMOf l cmd = unwrapMonad #. l (WrapMonad #. cmd)
Example:
Prelude Control.Lens> traverseOf _1 (Just . (+2)) (2,2)
Just (4,2)
Prelude Control.Lens> mapMOf _1 (Just . (+2)) (2,2)
Just (4,2)
来源:https://stackoverflow.com/questions/22209982/how-to-modify-using-a-monadic-function-with-lenses