Hello. I am trying to write some thing in haskell and i need fast storage to store and select this things from storage. https://github.com/s9gf4ult/projs/tree/master/haskell/teststorage I am writing simple testing package to determine my needs and select the fastest storage and i have encountered problems. Firstly i decided to check out postgresql-simple, but when i am doing "executeMany" i see space leaks, here is the picture http://bayimg.com/NAdjHAaeA http://bayimg.com/NADjKAAea I dont fully understand what is happening here, but i belive this is because of lazy consuming of "executeMany" or something. So things im trying to insert do not calculate one by one, but this is creating many thunks for calculate them. I would understand how to narrow the cause of this problem and create more strict function "executeMany" which would work in constant space (or space of data). Here the insertMany from postgresql-simple executeMany :: (ToRow q) => Connection -> Query -> [q] -> IO Int64 executeMany _ _ [] = return 0 executeMany conn q qs = do result <- exec conn =<< formatMany conn q qs finishExecute conn q result And this is not looks like a problem case go to formatMany formatMany :: (ToRow q) => Connection -> Query -> [q] -> IO ByteString formatMany _ q [] = fmtError "no rows supplied" q [] formatMany conn q@(Query template) qs = do case parseTemplate template of Just (before, qbits, after) -> do bs <- mapM (buildQuery conn q qbits . toRow) qs return . toByteString . mconcat $ fromByteString before : intersperse (fromChar ',') bs ++ [fromByteString after] Nothing -> fmtError "syntax error in multi-row template" q [] Here bs is the map of bs and must stay lazy evaluated and here we see mconcat which must be strict i think, and if i am right so problem must disapear whan i replace it with strict mconcat mconcat' :: (Monod a) => [a] -> a mconcat' [] = mempty mconcat' (x:xs) = x `seq` (mappend x $ mconcat' xs) but it doesnt. I assume i must write the same for "deepseq" to realy calculate each parameter up to value. But if so, this must touch several projects such as blaze and maybe bytestring because of the need to make NFData instances. How to solve this problem right - way ? Maybe haskell have elegant solution for this ?