-- What is the largest prime factor of the number 600851475143 ?
n `divides` m = m `mod` n == 0
primes = go [2..] where go (p:ps) = p : go (filter (\n -> not (p `divides` n)) ps)

factors n = go primes n
   where go (p:ps) n | n < p         = []
                     | p `divides` n = p : go (p:ps) (n `div` p)
                     | otherwise     = go ps n

main = print $ last $ factors 600851475143