Question

我想要一个类型为

的函数

f :: [Maybe a] -> Maybe [a]

e.g。

f [Just 3, Just 5] == Just [3, 5]
f [Just 3, Nothing] == Nothing
f [] == Just []

与catMaybes :: [Maybe a] -> [a]中的Data.Maybe相似，但catMaybes忽略Nothing，而f对Nothing非常认真。我可以以天真的方式实现f（如下所示），但想知道是否有更多的惯用方式（如“applicative functor”）：

f :: [Maybe a] -> Maybe [a]
f xs = let ys = catMaybes xs
        in if length ys == length xs 
           then Just ys
           else Nothing

或

f :: [Maybe a] -> Maybe [a]
f xs = if all isJust xs
        then catMaybes xs
        else Nothing

Answer 1

您要搜索的功能称为序列：

sequence :: (Monad m) => [m a] -> m [a]

您可以使用hoogle找到此功能：link。

示例：

>>> sequence [Just 3, Just 5]
Just [3,5]

>>> sequence [] :: Maybe [Int]
Just []

注意：Data.Traversable中的sequenceA也有点概括，但对于您的用例，来自Control.Monad的序列就足够了。

Answer 2

您希望来自Control.Monad的{{3}}。

（这也在sequence中以有用的方式推广。）

haskell中的函数类似于catMaybes，但有类型[Maybe a] - ＆gt;也许[a]

2 个答案: