在Haskell中键入问题

Question

我一直在努力奋斗半个多小时.我知道这很简单,但我对Haskell中的类型很糟糕,即使在阅读了与我非常相似的问题的公认答案之后,我仍然无法解决我的问题 - 更不用说了解它!

代码:

p108 = [filter (\[a,b] -> a>0 && b>0) (diophantinepairs n) | n <- [1..]]

diophantinepairs :: Integer -> [[Integer]]
diophantinepairs n = nub$map sort b
    where
        a = divisors n
        b = [[(n-d), n - (n^2)/d] | d <- a]

错误 :

249:39:
    No instance for (Fractional Integer)
      arising from a use of `/'
    Possible fix: add an instance declaration for (Fractional Integer)
    In the second argument of `(-)', namely `(n ^ 2) / d'
    In the expression: n - (n ^ 2) / d
    In the expression: [(n - d), n - (n ^ 2) / d]

谢谢,山姆.

Answer 1

以下是您阅读这些错误的方法:

No instance for (Fractional Integer)

翻译:你的程序有一个Integer,但你正在使用其中一个方法Fractional.

arising from a use of `/'

翻译:涉及的方法是/,这是Fractional课程的一部分. Integer不是Fractional,所以你不能申请/一个整数.

解决方案:使用div或quot替代.

我可以ghci很容易地得到同样的错误:

Prelude> (1 :: Integer) / (2 :: Integer)

<interactive>:2:16:
    No instance for (Fractional Integer)
      arising from a use of `/'
    Possible fix: add an instance declaration for (Fractional Integer)
    In the expression: (1 :: Integer) / (2 :: Integer)
    In an equation for `it': it = (1 :: Integer) / (2 :: Integer)

替代修复:使用Fractional诸如a之类的类型Rational而不是Integer:

Prelude> (1 :: Integer) `div` (2 :: Integer)
0
Prelude> :m + Data.Ratio
Prelude Data.Ratio> (1 :: Rational) / (2 :: Rational)
1 % 2