Project Euler Problem #3

euler3.hs

leastDivisor :: Integer -> Integer -> Integer
leastDivisor divisor n
  | rem n divisor == 0 = divisor
  | divisor^2 > n      = n
  | otherwise          = leastDivisor (divisor+1) n

factors :: Integer -> [Integer]
factors 1 = []
factors n = ld : factors (n `div` ld)
  where ld = leastDivisor 2 n

euler3 :: Integer
euler3 = maximum $ factors 600851475143