Hello, I've been playing with the INTEST problem on SPOJ which demonstrates the ability to write a program which processes large quantities of input data. http://www.spoj.pl/problems/INTEST/ I came across some curious behavior while cleaning up the program. The original program, which runs fast (enough), is: module Main(main) where import Control.Monad import Data.Maybe import qualified Data.ByteString.Char8 as B divisibleBy :: Int -> Int -> Bool a `divisibleBy` n = a `rem` n == 0 main :: IO () main = do [n,k] <- (map int . B.split ' ') `fmap` B.getLine :: IO [Int] let doLine :: Int -> Int -> IO Int doLine r _ = B.getLine >>= testDiv r testDiv r l | int l `divisibleBy` k = return (r + 1) | otherwise = return r foldM doLine 0 [1..n] >>= print where int :: B.ByteString -> Int int = fst . fromJust . B.readInt But when I make a slight modification, the program chews up a ton more memory and takes more time: module Main(main) where import Control.Monad import Data.Maybe import qualified Data.ByteString.Char8 as B divisibleBy :: Int -> Int -> Bool a `divisibleBy` n = a `rem` n == 0 main :: IO () main = do [n,k] <- (map int . B.split ' ') `fmap` B.getLine :: IO [Int] let doLine :: Int -> Int -> IO Int doLine r _ = B.getLine >>= return . testDiv r -- 'return' moved here ^^ testDiv r l | int l `divisibleBy` k = r + 1 | otherwise = r foldM doLine 0 [1..n] >>= print where int :: B.ByteString -> Int int = fst . fromJust . B.readInt This program will generate sample data: import System.Random import System.Environment import Control.Monad main = do [n] <- map read `fmap` getArgs :: IO [Int] k <- randomRIO (1, 100) putStrLn $ unwords [show n, show k] replicateM_ n $ randomRIO (1, 10^9) >>= print Note that the same behavior occurs even if I manually inline the local function and try: return (if .. then .. else). Some sample runs: $ ghc/compiler/ghc-inplace ghc-6.7.20070601: no input files $ ghc/compiler/ghc-inplace --make -O2 intest.hs [1 of 1] Compiling Main ( intest.hs, intest.o ) Linking intest ... $ ghc/compiler/ghc-inplace --make -O2 intest_slow.hs [1 of 1] Compiling Main ( intest_slow.hs, intest_slow.o ) Linking intest_slow ... $ time ./intest +RTS -tstderr -RTS < test1 ./intest +RTS -tstderr 8830 <<ghc: 134876896 bytes, 248 GCs, 28672/28672 avg/max bytes residency (1 samples), 2M in use, 0.00 INIT (0.00 elapsed), 0.12 MUT (0.12 elapsed), 0.00 GC (0.00 elapsed) :ghc>> real 0m0.129s user 0m0.124s sys 0m0.006s $ time ./intest_slow +RTS -tstderr -RTS < test1 ./intest_slow +RTS -tstderr 8830 <<ghc: 144278584 bytes, 269 GCs, 7030784/21843968 avg/max bytes residency (6 samples), 38M in use, 0.00 INIT (0.00 elapsed), 0.13 MUT (0.14 elapsed), 0.10 GC (0.15 elapsed) :ghc>> real 0m0.296s user 0m0.238s sys 0m0.058s -- -- Matthew Danish -- user: mrd domain: cmu.edu -- OpenPGP public key: C24B6010 on keyring.debian.org