Haskell（也许，x，也许 y，也许 z）->也许（x，y，z）

Question

我有一组 Haskell IO 绑定调用 return Maybe Z，Maybe Y，Maybe Z，其中 X /= Y /= Z。

我想要一个函数 returns Just (X,Y,Z) where ALL are Just n, or else Nothing.

一个天真的实现是

fn :: (Maybe x, Maybe y, Maybe z) -> Maybe (x,y,z)
fn (Just x, Just y, Just z) = Just (x,y,z)
fn _ = Nothing

但我想知道是否有更优雅的解决方案？也许是某种映射？

据我所知，比较普通

let x = do x <- fn1
           y <- fn2
           z <- fn3
           return (x,y,z) :: Maybe (X,Y,Z)

不会工作，因为 fn1-3 是 IO 绑定的？

谢谢。

Answer 1

您不需要指定输出类型。您可以使用：

f :: Monad m => m a -> m b -> m c -> m (a, b, c)
f mx my mz = do
    x <- mx
    y <- my
    z <- mz
    return (x, y, z)

但在这里您可以使用：

f :: Applicative f => f a -> f b -> f c -> f (a, b, c)
f fa fb fc = (,,) <$> fa <*> fb <*> fc

import Control.Applicative(liftA3)

f :: Applicative f => f a -> f b -> f c -> f (a, b, c)
f = liftA3 (,,)

Haskell (Maybe, x, Maybe y, Maybe z) -> Maybe (x,y,z)