如果两个参数都是 Just ，则 Haskell 函数返回 Just 值对，否则返回任何值

Question

Define a function

   pairMaybe :: Maybe a -> Maybe b -> Maybe (a,b)

that produces a Just result only if both arguments are Just, and a Nothing if either argument is Nothing.

我想出了:

pairMaybe (Just a) (Just b) = Just (a,b)
pairMaybe (Just a) Nothing = Nothing
pairMaybe Nothing (Just b) = Nothing

我不确定这是否是正确的写作方式。这有什么问题吗，或者这是定义这个函数的方式吗？

另外我想我可能想要更好地解释这个函数实际上可以做什么，所以如果我调用 pairMaybe有两个参数，它们可以是什么参数？当然，它们必须是 Maybe 类型的，但什么是一个很好的例子？

Answer 1

通过模式匹配来做到这一点很好；你可以通过使用来简化你的代码

pairMaybe :: Maybe a -> Maybe b -> Maybe (a,b)
pairMaybe (Just a) (Just b) = Just (a,b)
pairMaybe _        _        = Nothing

话虽如此，您的函数实际上只是解除了 (,)函数(创建 2 元组)到 Maybe monad，所以你也可以写

pairMaybe :: Maybe a -> Maybe b -> Maybe (a,b)
pairMaybe = liftM2 (,)