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]

我想获取第一个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/monad-generic 方法来做到这一点？

Answer 1

import Data.Foldable (find)
import Data.Maybe (isJust)
import Control.Monad (join)

firstJust = join . find isJust

用法：

> firstJust [Nothing, Just 1, Nothing, Just 2]
Just 1

> firstJust [Nothing, Nothing]
Nothing

Answer 2

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：

动作集合的总和，泛化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。