truncate的麻烦:：(RealFrac a，Integral b) => a -> b

Question

我使用的是lagrest素除数，但我对下面的代码有问题：

lpd :: Integer -> Integer
lpd n = helper n (2:[3,5..ceiling])
  where
    helper n divisors@(d:ds)
      | n == d         = n
      | n `rem` d == 0 = helper (n `div` d) divisors
      | otherwise      = helper n ds
    ceiling = truncate $ sqrt n

错误消息为：

problems.hs:52:15:
    No instance for (RealFrac Integer)
      arising from a use of `truncate'
    Possible fix: add an instance declaration for (RealFrac Integer)
    In the expression: truncate
    In the expression: truncate $ sqrt n
    In an equation for `ceiling': ceiling = truncate $ sqrt n

problems.hs:52:26:
    No instance for (Floating Integer)
      arising from a use of `sqrt'
    Possible fix: add an instance declaration for (Floating Integer)
    In the second argument of `($)', namely `sqrt n'
    In the expression: truncate $ sqrt n
    In an equation for `ceiling': ceiling = truncate $ sqrt n
Failed, modules loaded: none.

我的打字好像不太好。我该怎么做才能让这段代码工作呢？

Answer 1

用sqrt $ fromIntegral n替换sqrt n。

问题是sqrt的类型是(Floating a) => a -> a，所以它不能处理整数。函数

fromIntegral :: (Integral a, Num b) => a -> b

从整数类型到更通用的Num实例的“强制转换”。