Hello, I have been trying to do some CSV-style processing. My code works fine for small input (up to 10MB), but performs poorly for moderate to large input (it can't seem to finish 100MB of input with 700MB heap space). I have gone through several optimization passes with profiler help, and now I am hoping someone else can point out some other approaches to improving the code's performance (both space and time). The code breaks a large file into smaller files all of whose entries have the same date. thanks, Jeff ------------------------ module Main where import Debug.Trace import Control.Monad import Data.List import qualified Data.ByteString.Lazy.Char8 as B import qualified Data.Map as M import System.Environment (getArgs) dataDir = "dataH/" myRead file = do v <- B.readFile file let (cols' : rows) = map (B.split ',') $ B.lines v let cols = foldl' (\mp (k,v) -> M.insert k v mp) M.empty (zip cols' [0 ..]) return (cols, rows) getColId cols col = M.lookup col cols getCol cols col row = do i <- getColId cols col return $! row!!i dates file nRows = do (cols, rows) <- myRead file let addDate mp row | mp `seq` row `seq` False = undefined | otherwise = do date <- getCol cols (B.pack "\"Date\"") row let old = M.findWithDefault [] date mp return $ M.insert date (row:old) mp res <- foldM addDate M.empty $ take nRows rows mapM_ writeDate $ M.toList res where fmt = B.unpack . B.map (\x -> if x == '-' then '_' else x) . B.takeWhile (/= ' ') writeDate (date,rows) = B.writeFile (dataDir++fmt date) (B.unlines $ map (B.join (B.pack ",")) rows) main = do args <- getArgs case args of ["dates",file,nRows] -> dates file (read nRows)