Hi, I'm trying to improve a small haskell program of mine. A more extended description with full source code is here: http://codereview.stackexchange.com/questions/26107/how-to-improve-readabili... The script transforms CSV files into other CSV files but looks like it's reading the whole input files before writing output files. I guess that the script can be improved in many ways, in readability and efficiency, thus any suggestion is wellcome as an occasion to learn. But what I can't understand is why this design doesn't work: transformFile :: FilePath -> ([String] -> a) -> (a -> IO r) -> IO r transformFile file operation continuation = withFile file ReadMode (\h -> hGetContents h >>= (continuation.operation.lines)) This function recieves a path, a function to left fold lines to a new list of objects and a function to persist the fold output to files. Here the relevant parts: importTrades :: FilePath -> FilePath -> IO () importTrades outDir csvFile = transformFile csvFile (foldTradingSample.getTickWriteTrades) (saveTradingSamples outDir) where getTickWriteTrades = filter (isBetween (9, 0) (18, 0)).(catMaybes.(map fromCSVLine)) foldTradingSample = foldl toTradingSample [] This is the folding function: toTradingSample :: [TradingSample] -> Tw.Trade -> [TradingSample] toTradingSample (current:others) twTrade | newEqt == equity current && newDay == day current = (current { trades = newTrades }):others | otherwise = current : toTradingSample others twTrade where newEqt = Tw.tSimbol twTrade newDay = Tw.tDate twTrade newTrade = fromTickWrite twTrade newTrades = trades current ++ [newTrade] toTradingSample [] twTrade = [TradingSample { equity = Tw.tSimbol twTrade , day = Tw.tDate twTrade , trades = [fromTickWrite twTrade] }] And this is the function that safe the fold results to files saveTradingSamples :: String -> [TradingSample] -> IO () saveTradingSamples folder samples = mapM_ (saveTradingSample folder) samples saveTradingSample :: String -> TradingSample -> IO () saveTradingSample folder sample = writeFile fileName contents where fileName = folder ++ "\\" ++ (equity sample) ++ "_" ++ (formatTime defaultTimeLocale "%F" $ day sample) ++ ".CSV" contents = tradingSampleToCSV sample What's wrong here? My insight is that the problem is in the signature of transform files, that requires to completely compute the list of TradingSample before calling saveTradingSamples. Is this the problem? How can I fix this? Giacomo