foldM

定義

code:hs

foldlM :: (Foldable t, Monad m) => (b -> a -> m b) -> b -> t a -> m b

foldlM f z0 xs = foldr f' return xs z0

where f' x k z = f z x >>= k

code:hs

foldlM _ acc [] = return acc

foldlM f acc (x:xs) = do

acc' <- f acc x

foldlM f acc' xs

例

code:hs

>> let f a e = do { print e ; return $ e : a }

0

1

2

3

以下2つは同じ

code:hs

code:hs

do

a2 <- f a1 x1

a3 <- f a2 x2

...

f am xm