haskell - truncate::(RealFrac a, Integral b) => a -> b 的问题

Question

我正在玩 lagrest prime divisor 并且我在使用这段代码时遇到了麻烦:

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

错误信息是:

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.

看来我的打字有欠缺。我该怎么做才能使这段代码正常工作？

Answer 1

将 sqrt n 替换为 sqrt $ fromIntegral n。

问题是 sqrt 的类型是 (Floating a) => a -> a，所以它不适用于整数。功能

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

从整数类型“转换”为更一般的 Num 实例。