haskell - 从 [Maybe a] 中提取第一个 Just 值

Question

假设我有一个类似的列表:

[Nothing, Just 1, Nothing, Just 2]

我想得到第一个Just (非错误)值；在这种情况下，它是 Just 1 .我唯一能想到的是:

firstJust xs = case filter isJust xs of
                 []     -> Nothing
                 Just x -> Just x

有没有更好的/单子(monad)通用的方法来做到这一点？

Answer 1

msum 来自 Control.Monad :

\> msum [Nothing, Just 1, Nothing, Just 2]
Just 1

或 asum 来自 Data.Foldable :

\> asum [Nothing, Just 1, Nothing, Just 2]
Just 1

两者都记录为:

The sum of a collection of actions, generalizing concat.

带签名:

msum :: (Foldable t, MonadPlus m) => t (m a) -> m a
asum :: (Foldable t, Alternative f) => t (f a) -> f a

并且由于 Maybe instance of Alternative 而表现如上.