I would like to have a function for either mapping a pure function to a container or sequencing applicative/monadic action through it. For pure mapping we have
fmap :: Functor f => (a -> b) -> (f a -> f b)
For monadic sequencing we have (from Data.Taversable)
mapM :: (Traversable f, Monad m) => (a -> m b) -> (f a -> m (f b))
Which is similar to
mapKleisli :: (Traversable f, Monad m) => Kleisli m a b -> Kleisli m (f a) (f b)
mapKleisli = Kleisli . mapM . runKleisli
We know both (->) and (Kleisli m) are categories (and arrows). So it's naturally to make a generalization:
mapCategory :: (X f, Category c) => c a b -> c (f a) (f b)
Do you know such a class X with similar method? Maybe, somewhere on hackage? I tried to hoogle/hayoo but haven't found anything appropriate.
Update:
Now I know better what I need. Both Kleisli arrows and (->) are instances of ArrowApply which is as powerful as Monad. I came up with this arrow-based version of Travesable:
{-# LANGUAGE TypeOperators #-}
import Prelude hiding (id, (.), mapM)
import Control.Arrow
import Control.Category
class Traversable f where
traverse :: ArrowApply (~>) => f a -> (a ~> b) ~> f b
mapArrow :: (ArrowApply (~>), Traversable f) => a ~> b -> f a ~> f b
mapArrow a = arr (\x -> (traverse x, a)) >>> app
instance Traversable Maybe where
traverse Nothing = arr (const Nothing)
traverse (Just x) = arr (\a -> (a, x)) >>> app >>> arr Just
instance Traversable [] where
traverse [] = arr (const [])
traverse (x : xs) = undefined -- this is hard!
I could use just usual Applicative-based Traversable, with Identity for pure functions, but I'm not sure it is good. Considering pure functions as special case of monadic actions is weird. Interpreting both pure functions and monadic actions as instances of some action class (Category/Arrow/ArrowApply) looks more straightforward to me.
Questions: would you like to finish instance for []
? Has my opinion about ArrowApply vs Monad any sense?