Sieve Of Eratosthenes

import qualified Data.IntSet as S
primesUpTo :: Int -> [Int]
primesUpTo n = 2 : put S.empty [3,5..n]
    where put :: S.IntSet -> [Int] -> [Int]
          put _ [] = []
          put comps (x:xs) =
            if S.member x comps
            then put comps xs
            else x : put (S.union comps multiples) xs
                where multiples = S.fromList [x*2, x*3 .. n]
primesUpTo' :: Int -> [Int]
primesUpTo' n = takeWhile (<n) $ sieve [2..]
    where sieve (p:xs) = p : sieve [x | x <- xs, mod x p /= 0]
main = putStrLn . show . last . primesUpTo . read $ "2000000" -- =<< getLine