haskell - 与分数相关的类型错误

Question

我正在尝试找出问题所在。看起来类型有问题，但单独使用相同的表达式就可以正常工作。

这是代码:

a = [9, 4, 12, 0, -6, 16] :: [Int]

qsort:: [Int] -> [Int] 
qsort [] = []
qsort [x] = [x]
qsort xs = (qsort l)++(qsort r)
    where   m = (realToFrac(sum xs)) / (realToFrac(length xs))
            l = filter (<=m) xs
            r = filter (>m) xs

main::IO()
main = do
        print (show (qsort a))

它抛出:

main.hs:7:36:
    No instance for (Fractional Int)
      arising from a use of `/'
    Possible fix: add an instance declaration for (Fractional Int)
    In the expression: (realToFrac (sum xs)) / (realToFrac (length xs))
    In an equation for `m':
        m = realToFrac (sum xs) / (realToFrac (length xs))
    In an equation for `qsort':
        qsort xs
          = (qsort l) ++ (qsort r)
          where
              m = realToFrac (sum xs) / (realToFrac (length xs))
              l = filter (<= m) xs
              r = filter (> m) xs

但是效果很好:

a = [9, 4, 12, 0, -6, 16] :: [Int]
main::IO()
main = do
    print (show (   (realToFrac(sum a)) / (realToFrac(length a))   ))

Answer 1

您的m定义给出了一些Fractional类型。但是，您在 l 和 r 的定义中将 m 与 Int 进行比较，这会让编译器感到困惑。下面的方法就可以解决这个问题:

a = [9, 4, 12, 0, -6, 16] :: [Int]

qsort:: [Int] -> [Int]
qsort [] = []
qsort [x] = [x]
qsort xs = (qsort l)++(qsort r)
    where   m = realToFrac(sum xs) / (realToFrac(length xs))
            l = filter ((<=m) . fromIntegral) xs
            r = filter ((>m) . fromIntegral) xs

main::IO()
main = do
        print (show (qsort a))