I'm a Haskell neophyte, so may be missing something obvious in the problem outlined below. I'm fairly proficient in Python + have some limited experience in OCaml and F#, so know just enough to be be dangerous, but not nearly enough to really know what I'm doing here. OK, I have text files containing 'compressed lists' Compressed lists look like this: 8+12,11,7+13,10 1+2+3,1+9,3+6,4 . . Sublists are comma-delimited, and sublist elements are separated by '+' character. I parse these to look like so: [[[8,12],[11],[7,13],[10]], [[1,2,3],[1,9],[3,6],[4]], . . ] I need to explode these and produce a lex-sorted list of exploded lists: [[1,1,3,4],[1,1,6,4],[1,9,3,4],[1,9,6,4],[2,1,3,4],[2,1,6,4],[2,9,3,4], [2,9,6,4],[3,1,3,4],[3,1,6,4],[3,9,3,4],[3,9,6,4], [8,11,7,10],[8,11,13,10],[12,11,7,10],[12,11,13,10]] I then output this data as comma-delimited lists: 1,1,3,4 1,1,6,4 . . 12,11,13,10 I can do all this in my (no doubt fairly naive) program shown below. In real life, one of my test data files contains compressed lists which all contain 7 sublists. This data (correctly) explodes to a list containing ~3,700,000 exploded lists. All good as far as correctly transforming input to output goes. However, sorting the data uses a lot of memory: Partial output from ./explode +RTS -sstderr: 540 MB total memory in use (4 MB lost due to fragmentation) If I do not do any sorting on the exploded lists, i.e. modify the main function to be main = interact (unlines . toCSV . explode . fromCSV . lines) I then see this partial output from ./explode +RTS --stderr: 2 MB total memory in use (0 MB lost due to fragmentation) I can guess that there might be be less laziness and more instantiation when sorting is introduced, but my questions are: (1) Am I doing anything terribly stupid/naive here? (2) If so, what can I do to improve space efficiency? TIA! import Data.List (sort) import Data.List.Split (splitOn) -- Cartesian Product over a List of Lists -- cp [[1,2],[3],[4,5,6]] --> [[1,3,4],[1,3,5],[1,3,6],[2,3,4],[2,3,5], [2,3,6]] cp :: [[a]] -> [[a]] cp [] = [[]] cp (xs:xss) = [y:ys | y <- xs, ys <- cp xss] -- fromCSV ["8+12,11,7+13,10", "1+2+3,1+9,3+6,4"] --> -- [[[8,12],[11],[7,13],[10]],[[1,2,3],[1,9],[3,6],[4]]] fromCSV :: [String] -> [[[Int]]] fromCSV = map parseOneLine where parseOneLine = map parseGroup . splitOn "," where parseGroup = map read . splitOn "+" -- explode [[[1,2],[3],[4,5,6]], [[1, 2], [14,15], [16]]] --> [[1,3,4], [1,3,5], -- [1,3,6],[2,3,4],[2,3,5],[2,3,6],[1,14,16],[1,15,16],[2,14,16], [2,15,16]] explode :: [[[a]]] -> [[a]] explode = concatMap cp -- toCSV [[8,11,7,10,12],[8,11,7,10,12],[8,11,7,10,12]] --> -- ["8,11,7,10,12","8,11,7,10,12","8,11,7,10,12"] toCSV :: (Show a) => [[a]] -> [String] toCSV = map $ tail . init . show --toCSV = map (intercalate "," . map show) main = interact (unlines . toCSV . sort . explode . fromCSV . lines)