从 [Maybe a] 中提取第一个 Just 值

Question

假设我有一个这样的列表：

[Nothing, Just 1, Nothing, Just 2]

我想获取第一个Just（非错误）值；在本例中，它是 Just 1。我唯一能想到的是：

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

是否有 better/monad-generic 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.

有签名：

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

表现如上

Answer 2

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

Extracting the first Just value from [Maybe a]