从[Maybe a]中提取第一个Just值

Question

说我有一个列表,如:

[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/ 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:

一系列动作的总和,概括concat.

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实例Alternative而表现如上.