Try this simple program: import Text.ParserCombinators.Parsec ppAny = tokenPrim show (\p _ _ -> p) (\t -> Just t) ppTest = many ppAny p s = case runParser ppTest True "" s of Left error -> show error Right result -> result main = do let fname = "C:/main.i" f <- readFile fname let tokens = p f putStrLn . show $ head tokens This is the simplest expression of using Parsec, it just returns the input unchanged. But it already seems to have a big memory leak. When parsing a 2 MB file, I'm seeing the program grow in size up to about 160 MB before it ends. I assume this is known behavior. I believe it's because of the way Parsec deals with errors: when you chain two parsers in sequence, the result of the first one (in this case the head of the list) is not available until it knows if the second one failed or not. Is there any known alternative that doesn't exhibit this behavior? It would have to somehow return errors inline or on a "side channel". I'll be toying with this sort of thing for a while. Thanx! JCAB