Question

I’m playing with lagrest prime divisor and I have troubles with this code:

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

The error message is:

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.

It seems that my typing is lacking. What could I do to make this code work?

Answer 1

Replace sqrt n with sqrt $ fromIntegral n.

The problem is that sqrt has type (Floating a) => a -> a, so it doesn’t work on integers. The function

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

“casts” from integral types to more general Num instances.