haskell - 如何将文字 1 作为 Maybe Integer 返回？

Question

我正在学习 Haskell，但无法将文字 1 返回为 Maybe Integer

alwaysOne:: Integer -> Maybe Integer
alwaysOne n = 1

加薪

error:
    • No instance for (Num (Maybe Integer))
        arising from the literal ‘1’
    • In the expression: 1
      In an equation for ‘alwaysOne’: alwaysOne n = 1
   |
14 | alwaysOne n = 1
   |             ^

Answer 1

having trouble returning literal 1 as Maybe Integer

也许类型有两个数据构造函数:

data Maybe a = Nothing | Just a

如果您想生成Maybe Integer 值，则需要使用这些数据构造函数之一。您想要的是将 1 传递给 Just 数据构造函数:

alwaysOne:: Integer -> Maybe Integer
alwaysOne n = Just 1

也就是说，Just 接受任何类型的值并生成为该类型参数化的Maybe 值:

Just :: a -> Maybe a

在您的情况下，上面的

a 被推断为 Integer。

类似地，如果您想定义alwaysNothing:

alwaysNothing :: Integer -> Maybe Integer
alwaysNothing _ = Nothing