I have the below program, and I'm trying to run it on an input of about 90MB. It eats RAM like crazy, and I can't figure out why. I do know that the problem is not my custwords function (as you can see, I replaced the call to it with a call to the standard words function on the last line). It seems to be wordfreq, but I don't know why. For information about the problem, see http://changelog.complete.org/posts/535-A-Haskell-solution-to-Lars-Problem.h... Here's the code: import System.Environment import Data.List import Data.Char import qualified Data.Map as Map custwords = filter (/= "") . lines . map (conv . toLower) where iswordchar x = isAlphaNum x && isAscii x conv x = if iswordchar x then x else '\n' wordfreq inp = Map.toList $ foldl' updatemap (Map.empty::Map.Map String Int) inp where updatemap nm word = Map.insertWith updatefunc word 1 nm updatefunc _ x = x + 1 freqsort (w1, c1) (w2, c2) = if c1 == c2 then compare w1 w2 else compare c2 c1 showit (word, count) = show count ++ " " ++ word main = do args <- getArgs interact $ unlines . map showit . take (read . head $ args) . sortBy freqsort . wordfreq . words