Haskell语言学习笔记（13）Monadic Functions

Monadic Functions

liftM

liftM :: (Monad m) => (a1 -> r) -> m a1 -> m rliftM f m1 = do { x1 <- m1; return (f x1) }
PRelude Control.Monad> liftM (*3) (Just 8)Just 24Prelude Control.Monad> liftM (*3) [1,2,3][3,6,9]
ap
Monad 的 ap 相当于 Applicative Functor 的 <*>。
ap :: (Monad m) => m (a -> b) -> m a -> m bap m1 m2 = do { x1 <- m1; x2 <- m2; return (x1 x2) }
Prelude Control.Monad> ap (Just (*3)) (Just 8)Just 24Prelude Control.Monad> ap [(*3),(+3)] [1,2,3][3,6,9,4,5,6]
liftM2 liftM3 liftM4 liftM5
Monad 的 liftM2 相当于 Applicative Functor 的 liftA2。
liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m rliftM2 f m1 m2 = do { x1 <- m1; x2 <- m2; return (f x1 x2) }liftM3 :: (Monad m) => (a1 -> a2 -> a3 -> r) -> m a1 -> m a2 -> m a3 -> m rliftM3 f m1 m2 m3 = do { x1 <- m1; x2 <- m2; x3 <- m3; return (f x1 x2 x3) }
Prelude Control.Monad> liftM2 (*) (Just 3) (Just 8)Just 24Prelude Control.Monad> liftM2 (*) [1,2,3] [4,5,6][4,5,6,8,10,12,12,15,18]
join
join 函数移除一层 Monadic structure, 使其平坦化。
join :: (Monad m) => m (m a) -> m ajoin x = x >>= id
Prelude Control.Monad> join (Just (Just 9))Just 9Prelude Control.Monad> join [[1,2,3],[4,5,6]][1,2,3,4,5,6]
filterM
filterM 是列表操作函数 filter 的一般化。
filterM :: (Applicative m) => (a -> m Bool) -> [a] -> m [a]
import Control.Monadimport Control.Monad.Trans.WriterkeepSmall :: Int -> Writer [String] BoolkeepSmall x    | x < 4 = do        tell ["Keeping " ++ show x]        return True    | otherwise = do        tell [show x ++ " is too large, throwing it away"]        return Falsemain = do    let (a, w) = runWriter $ filterM keepSmall [9,1,5,2,10,3]    mapM_ putStrLn w; print a{-9 is too large, throwing it awayKeeping 15 is too large, throwing it awayKeeping 210 is too large, throwing it awayKeeping 3[1,2,3]-}
Prelude Control.Monad> filterM (/x -> [False, True]) [1,2,3][[],[3],[2],[2,3],[1],[1,3],[1,2],[1,2,3]]
foldM
foldM 是列表操作函数 foldl 的一般化。
foldM :: (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m b
Prelude Control.Monad> let binSmalls acc x = if x > 9 then Nothing else Just (acc + x)Prelude Control.Monad> foldM binSmalls 0 [2,8,3,1]Just 14Prelude Control.Monad> foldM binSmalls 0 [2,11,3,1]Nothing