Monad Arrow Notes

a.hs

import Control.Arrow
import Control.Monad


-- Prelude Control.Monad Control.Arrow> :t (|||)
-- (|||) :: ArrowChoice a => a b d -> a c d -> a (Either b c) d

-- Prelude Control.Monad Control.Arrow> :k ArrowChoice
-- ArrowChoice :: (* -> * -> *) -> Constraint

-- Prelude Control.Monad Control.Arrow> :k Kleisli
-- Kleisli :: (* -> *) -> * -> * -> *

-- provide a function to be run on 'left' and 'right', loosely speaking
example :: Kleisli IO Int String ->
           Kleisli IO () String ->
           Kleisli IO (Either Int ()) String
example x y = x ||| y

run :: (Either Int ())                   -> 
       Kleisli IO (Either Int ()) String -> 
       IO String
run input k = (runKleisli k) input

-- always returns "a!" when Kleisli + IO are run
a :: Kleisli IO Int String 
a = Kleisli $ const $ return "a!"

-- always returns "b!" when Kleisli + IO are run
b :: Kleisli IO () String
b = Kleisli $ const $ return "b!"

-- will output "b!"
leftResult :: IO String
leftResult = run (Right ()) (example a b)

-- will	 output "a!"
rightResult :: IO String
rightResult = run (Left 666) (example a b)

-- from REPL
-- >rightResult
-- "a!"
-- >leftResult
-- "b!"