In my search for a haskell library that can process large (300-1000mb) xml files i came across hexpat. There is an example in the Haskell Wiki that claims to -- Process document before handling error, so we get lazy processing. For testing purposes i have redirected the output to /dev/null and throw a 300mb file at it. Memory consumption kept rising until i had to kill the process. Now i removed the error handling from the process function: process :: String -> IO () process filename = do inputText <- L.readFile filename let (xml, mErr) = parse defaultParseOptions inputText :: (UNode String, Maybe XMLParseError) hFile <- openFile "/dev/null" WriteMode L.hPutStr hFile $ format xml hClose hFile return () As a result the function now uses constant memory. Why does the error handling result in massive memory consumption? As far as i understand xml and mErr are two seperate unevaluated thunks after the call to parse. Does format xml evaluate xml and build the evaluation tree of 'mErr'? If yes is there a way to handle the error while using constant memory? http://www.haskell.org/haskellwiki/Hexpat/