Project Euler Problem 069

primes :: [Integer]
primes = 2:f [3] [3,5..] where
  f (x:xs) ys = ps ++ f (xs++ps) [z | z<-qs,z`rem`x/=0] where
    (ps,qs) = span (< x*x) ys
ans1 :: Integer
ans1 = last . takeWhile (<1000000) . scanl1 (*) $ primes
main :: IO ()
main = print ans1