haskell 错误: "No instance fo (Eq Time) arising from a use of ' =='

Question

我对 Haskell 还很陌生，我不明白为什么我不能比较这两个变量。我有这个功能:

nextMinute :: Date -> Time -> DateTime
nextMinute date time = if time == Time 23 59
                       then DateTime (tomorrow date) newDay
                       else DateTime date (timeSucc time)

我收到此错误消息:

Time.hs:88:32:
    No instance for (Eq Time) arising from a use of ‘==’
    In the expression: time == Time 23 59
    In the expression:
      if time == Time 23 59 then
          DateTime date newDay
      else
          DateTime date (timeSucc time)
    In an equation for ‘nextMinute’:
        nextMinute date time
          = if time == Time 23 59 then
                DateTime date newDay
            else
                DateTime date (timeSucc time)

Answer 1

您可以只使用模式匹配，而不是使用 ==:

nextMinute :: Date -> Time -> DateTime
nextMinute date (Time 23 59) = DateTime (tomorrow date) newDay
nextMinute date time         = DateTime date            (timeSucc time)

将 Eq 添加到您的 Time 类型中可能没问题，但是当存在同样好的模式匹配解决方案时，没有特别需要。