haskell - 匹配 Maybe 和 Either 组合的模式

Question

如何匹配模式

Maybe (Either (Int, String) String)

我需要用这种输入编写函数，如何解析这种输入？

Answer 1

Maybe a类型有模式Just a和Nothing。Either a b类型有模式Left a和Right b。因此类型的值Maybe (Either (Int, String) String)可以匹配以下模式：

• Nothing
• Just (Left (x,y))哪里x是一个Int并且y是一个String
• Just (Right z)哪里z是String。

Answer 2

f :: Maybe (Either (Int, String) String) -> <SOMETHING>
f x = case x of
    Just (Left (i, s)) -> <...>
    Just (Right s) -> <...>
    Nothing -> <...>

Answer 3

matchme Nothing = "Nothing"
matchme (Just (Left (x,y)) = "Left " ++ show x ++ " " + y
matchme (Just (Right z)) = "Right " ++ z

Answer 4

也可以使用maybeandeither函数，如下所示：

matchit = maybe nothing (left `either` right)
  where
      nothing = {- value for the nothing case -}
      left (x,y) = {- code for the (Just (Left (x,y)) case -}
      right z = {- code for the (Just (Right z)) case -}