Question

这有什么问题？我试着写一个定义如下的排序函数：

let orderFunction:: Num b => (a, [b]) -> (a, [b]) -> Ordering; 
orderFunction a1 a2 = if sum $ snd a1 > sum $ snd a2 then GT else LT

但是我收到了错误：

Could not deduce (Ord b) arising from a use of `>'
    from the context (Num b)
      bound by the type signature for
                 orderFunction :: Num b => (a, [b]) -> (a, [b]) -> Ordering
      at <interactive>:110:21-61
    Possible fix:
      add (Ord b) to the context of
        the type signature for
          orderFunction :: Num b => (a, [b]) -> (a, [b]) -> Ordering
    In the expression: sum (snd a1) > sum (snd a2)
    In the expression: if sum (snd a1) > sum (snd a2) then GT else LT
    In an equation for `orderFunction':
        orderFunction a1 a2
          = if sum (snd a1) > sum (snd a2) then GT else LT

是否有更多面向Haskell的方法来编写函数？

谢谢， FB

Answer 1

正如@ymonad所指出的，Num类型类并不意味着支持订购功能。所以我在函数定义中添加了另一个约束，如下所示：

let orderFunction::(Ord b, Num b) => (a, [b]) -> (a, [b]) -> Ordering; 
orderFunction a1 a2 = if sum (snd a1) > sum (snd a2) then GT else LT

Haskell排序函数

1 个答案: