After Shersh code review

10_v2.hs

import Data.List
import Data.IntMap.Strict(IntMap, (!))
import qualified Data.IntMap.Strict as IntMap

problem10 :: Int -> Int
problem10 n = (sieve (IntMap.fromList [(i, i * (i + 1) `div` 2 - 1) | i <- vs]) 2 r vs) ! n
              where vs = [n `div` i | i <- [1..r]] ++ [n', n' - 1 .. 1]
                    r  = floor (sqrt (fromIntegral n))
                    n' = n `div` r - 1

sieve :: IntMap Int -> Int -> Int -> [Int] -> IntMap Int
sieve m p r vs | p > r               = m
               | m ! p > m ! (p - 1) = sieve (update m vs p) (p + 1) r vs
               | otherwise           = sieve m (p + 1) r vs

update :: IntMap Int -> [Int] -> Int -> IntMap Int
update m vs p = foldl' decrease m (takeWhile (>= p*p) vs)
                where sumOfSieved v = p * (m ! (v `div` p) - m ! (p - 1))
                      decrease m  v = IntMap.adjust (subtract $ sumOfSieved v) v m

main = print $ problem10 $ 2*10^9

-- $ time ./10
-- 95673602693282040

-- real	0m3.279s
-- user	0m3.231s
-- sys	0m0.041s