http://projecteuler.net

gistfile1.hs

factor :: Integer -> [Integer]
factor n = factor_iter n 2 []

factor_iter :: Integer -> Integer -> [Integer] -> [Integer]
factor_iter n x xs | n < x          = reverse xs
                   | n `mod` x == 0 = factor_iter (n `div` x) x (x:xs)
                   | otherwise      = factor_iter n (x + 1) xs

main = print (maximum((factor 600851475143)))