haskell - 如何在 [maybe string] 上使用 fmap 所以添加的字符串在 Just 之前？

Question

我得到了以下功能:

maybeStrings:: [Maybe String] -> [Maybe String]
maybeStrings s = fmap ("Hello, "++) <$> s

问题是，我怎样才能在 Just 之前得到那个“Hello”？
例如，用户给出以下列表: [Just "Simpson"]并且当前的输出是 [Just "Hello, Simpson"] .我希望它是: ["Hello, Just Simpson"] .

Answer 1

从更简单的函数开始

foo :: String -> String
foo x = "Hello, Just " ++ x

然后使用 fmap两次:一次映射 foo在 Maybe值，并再次映射您的 fmap ped 函数覆盖 Maybe 的列表值(value)观:

> fmap foo Nothing
Nothing
> fmap foo (Just "Simpson")
Just "Hello, Just Simpson"
> fmap (fmap foo) [Just "Simpson", Nothing]
[Just "Hello, Just Simpson", Nothing]

然后

maybeStrings :: [Maybe String] -> [Maybe String]
maybeStrings = fmap (fmap ("Hello, Just " ++))