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)
