Project Euler - Problem 3

gistfile1.hs

import System.IO

main = do
    putStrLn $ show $ last $ filter ((== 0) . (mod 600851475143)) (takeWhile (< (truncate (sqrt 600851475143))) prime)

prime = 2:f [3] [3,5..]
    where f (x:xs) ys   =   let (ps, qs) = span (< x^2) ys
                            in  ps ++ f (xs ++ ps) [z | z <- qs, z `mod` x /= 0]