Haskell语言学习笔记（47）Arrow（2）

Function, Monad, Arrow

f :: Int -> (Int, Int)f = \x ->  let y  = 2 * x      z1 = y + 3      z2 = y - 5  in (z1, z2)-- ghci> f 10-- (23, 15)fM :: Int -> Identity (Int, Int)fM = \x -> do  y  <- return (2 * x)  z1 <- return (y + 3)  z2 <- return (y - 5)  return (z1, z2)-- ghci> runIdentity (fM 10)-- (23,15)fA :: Int -> (Int, Int)fA = proc x -> do  y  <- (2 *) -< x  z1 <- (+ 3) -< y  z2 <- (subtract 5) -< y  returnA -< (z1, z2)-- ghci> fA 10-- (23,15)

24 Days of GHC Extensions: Arrows

ArrowZero, ArrowPlus, ArrowChoice

class Arrow a => ArrowZero a where    zeroArrow :: a b cclass ArrowZero a => ArrowPlus a where    (<+>) :: a b c -> a b c -> a b cclass Arrow a => ArrowChoice a where    left :: a b c -> a (Either b d) (Either c d)    left = (+++ id)    right :: a b c -> a (Either d b) (Either d c)    right = (id +++)    (+++) :: a b c -> a b' c' -> a (Either b b') (Either c c')    f +++ g = left f >>> arr mirror >>> left g >>> arr mirror      where        mirror :: Either x y -> Either y x        mirror (Left x) = Right x        mirror (Right y) = Left y    (|||) :: a b d -> a c d -> a (Either b c) d    f ||| g = f +++ g >>> arr untag      where        untag (Left x) = x        untag (Right y) = y

instance MonadPlus m => ArrowZero (Kleisli m) where    zeroArrow = Kleisli (\_ -> mzero)instance MonadPlus m => ArrowPlus (Kleisli m) where    Kleisli f <+> Kleisli g = Kleisli (\x -> f x `mplus` g x)instance ArrowChoice (->) where    left f = f +++ id    right f = id +++ f    f +++ g = (Left . f) ||| (Right . g)    (|||) = eitherinstance Monad m => ArrowChoice (Kleisli m) where    left f = f +++ arr id    right f = arr id +++ f    f +++ g = (f >>> arr Left) ||| (g >>> arr Right)    Kleisli f ||| Kleisli g = Kleisli (either f g)

Prelude Control.Arrow> runKleisli ((Kleisli (\x -> [x * 2])) <+> (Kleisli (\x -> [x, -x]))) 2[4,2,-2]Prelude Control.Arrow> either (+2) (*3) (Left 3)5Prelude Control.Arrow> either (+2) (*3) (Right 3)9Prelude Control.Arrow> (+2) ||| (*3) $ (Left 3)5Prelude Control.Arrow> (+2) +++ (*3) $ (Left 3)Left 5Prelude Control.Arrow> (+2) ||| (*3) $ (Right 3)9Prelude Control.Arrow> (+2) +++ (*3) $ (Right 3)Right 9Prelude Control.Arrow> left (+2) (Left 3)Left 5Prelude Control.Arrow> right (*3) (Right 3)Right 9Prelude Control.Arrow> left (+2) (Right 3)Right 3Prelude Control.Arrow> right (*3) (Left 3)Left 3Prelude Control.Arrow> runKleisli ((Kleisli (\x -> [x * 2])) ||| (Kleisli (\x -> [x, -x]))) (Left 3)[6]Prelude Control.Arrow> runKleisli ((Kleisli (\x -> [x * 2])) ||| (Kleisli (\x -> [x, -x]))) (Right 3)[3,-3]Prelude Control.Arrow> runKleisli ((Kleisli (\x -> [x * 2])) +++ (Kleisli (\x -> [x, -x]))) (Left 3)[Left 6]Prelude Control.Arrow> runKleisli ((Kleisli (\x -> [x * 2])) +++ (Kleisli (\x -> [x, -x]))) (Right 3)[Right 3,Right (-3)]