haskell - 为什么我会收到无法推断 (Ord a) 错误？

Question

我试图找到元素总和最小的列表。:

shortest :: (Num a) => [[a]] -> [a]
shortest [] = []
shortest (x:xs) = if sum x < sum (shortest xs) then x else shortest xs

这给了我以下错误:

Could not deduce (Ord a) arising from a use of `<'
from the context (Eq a)
  bound by the type signature for shortest :: Eq a => [[a]] -> [a]
  at code.hs:(8,1)-(9,71)
Possible fix:
  add (Ord a) to the context of
    the type signature for shortest :: Eq a => [[a]] -> [a]
In the expression: sum x < sum (shortest xs)
In the expression:
  if sum x < sum (shortest xs) then x else shortest xs
In an equation for `shortest':
    shortest (x : xs)
      = if sum x < sum (shortest xs) then x else shortest xs

为什么函数不进行类型检查？

Answer 1

该代码涉及两个类型类:Num和 Ord .笔记
一个类型可以是成员Num而不是 Ord ，反之亦然。
sum的类型是 Num a => [a] -> a所以输入元素到shortest需要成为 Num 的成员.您还可以执行以下操作
在您的代码中:

sum x < sum (shortest xs)

这意味着您正在使用运算符 <在 a s，但在您的类型签名中，您没有要求 a s 是 Ord 的一个实例它定义了 < :

class Eq a => Ord a where
  compare :: a -> a -> Ordering
  (<) :: a -> a -> Bool
  ...

因此，您需要将该要求添加到您的类型签名中:

shortest :: (Ord a, Num a) => [[a]] -> [a]

或者你可以省略类型签名。