对类型类约束感到困惑

Question

我是Haskell的初学者,我目前很困惑,我确信这与我使用的约束有关我一直都会遇到错误

代码

averageThreeNumbers:: Floating a=> (a, a, a) -> a
averageThreeNumbers (x,y,z) = ((x+y+z)/3)

howManyBelowAverage:: Floating a=> (a, a, a) -> [a]
howManyBelowAverage (b,c,d) = [x|x <- [b,c,d], x > averageThreeNumbers(b,c,d)]

错误

Could not deduce (Ord a) arising from a use of `>'
from the context: Floating a
bound by the type signature for:
howManyBelowAverage :: Floating a=> (a, a, a) -> [a]
Possible fix: add (Ord a) to the context of the type signature

虽然当我使用相同的列表但在控制台中使用原始浮动时,它可以正常工作.我在这里错过了一些大事吗？任何帮助表示赞赏.

编译好:

[x|x <- [1.2, 3.2, 4.6], x > averageThreeNumbers (1.2, 3.2, 4.6)]

Answer 1

因为您将值输出值与结果进行比较averageThreeNumbers,所以需要Ord在howManyBelowAverage函数中包含约束.我还认为你的意思是检查是否x小于函数返回值(在上面的代码片段中,你做的相反).

修改约束和比较检查,我们最终得到:

howManyBelowAverage :: (Ord a, Floating a) => (a,a,a) -> [a]
howManyBelowAverage (b,c,d) = [ x | x <- [b,c,d], x < averageThreeNumbers(b,c,d)]