haskell - Haskell 类似函数（也许）

Question

我必须实现一个功能maybemap

它具有（类似于功能图）一个功能

f :: a -> b

它应该适用于列表类型Maybe a并给出一个列表Maybe b

maybemap :: (a -> b) -> [Maybe a] -> [Maybe b]

我将如何定义maybemap函数？

Answer 1

您可以只使用Maybe的仿函数实例，其定义如下：

fmap :: (a -> b) -> Maybe a -> Maybe b -- fmap's type for Maybe
fmap f Nothing  = Nothing
fmap f (Just a) = Just (f a)

使用它，您的函数可以这样定义：

maybemap :: (a -> b) -> [Maybe a] -> [Maybe b]
maybemap f = map (fmap f)

Answer 2

最简单的解决方案，正如它所提到的，

maybemap :: (a -> b) -> [Maybe a] -> [Maybe b]
maybemap = map . fmap

但总的来说，它是更通用的功能：

maybemap :: Functor f => (a -> b) -> [f a]  -> [f b]

如我们所见，list也是一个函子，所以我们可以重写

fmap2 :: (Functor f, Functor g) => (a -> b) -> g (f a)  -> g (f b)
fmap2 = fmap . fmap

并测试：

> fmap2 (++"!") [Nothing,Just"u"]
[Nothing,Just "u!"]

更新

中最有趣的fmap2，它(.)也是a -> b函数的 Functor，并且fmap是具有相同签名的函数：f a -> f b. 我们的函数只能包含fmaps：

fmap2 = fmap fmap fmap