Haskell/Performance examples

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Goal: Explain optimizations step by step with examples that actually happened.

Tight loopEdit

CSV ParsingEdit

haskell-cafe: another Newbie performance question I hope he doesn't mind if I post his code here, I still have to ask him. -- apfeλmus 08:46, 18 May 2008 (UTC)

type CSV = [[String]]

main = do
                  args <- getArgs
                  file <- readFile (head args)
                  writeFile (head args ++ "2") (processFile (args !! 1) file)

processFile s     = writeCSV . doInteraction s . readCSV
doInteraction line csv = insertLine (show line) (length csv - 1) csv
writeCSV          = (\x -> x ++ "\n") . concat . intersperse "\n" . (map (concat . intersperse "," . (map show)))
insertLine line pos csv = (take pos csv) ++ [readCSVLine line] ++ drop pos csv
readCSVLine       = read . (\x -> "["++x++"]")
readCSV           = map readCSVLine . lines

I think there was another cvs parsing thread on the mailing list which I deemed appropriate, but I can't remember.

Space LeakEdit

jkff asked about some code in #haskell which was analyzing a logfile. Basically, it was building a histogram

foldl' (\m (x,y) -> insertWith' x (\[y] ys -> y:ys) [y] m) M.empty
  [(ByteString.copy foo, ByteString.copy bar) | (foo,bar) <- map (match regex) lines]

The input was a 1GB logfile and the program blew the available memory mainly because the ByteString.copy weren't forced and the whole file lingered around in memory.