In Ch 25 of Real World Haskell, the authors introduce some naive code for finding the average of a big list; it has a space leak, and they present several solutions. Below are two of the solutions that successfully eliminate the space leak (though, the first one -- the one that uses foldl'rnf -- is quite a bit faster). However, if compiled with profiling, the first one (using foldl'rnf) NOW has a leak. The second solution (foldl') does not have a leak even when profiling is enabled. I have used this foldl'rnf function in my own code, as it is the only solution I have found for a space leak in my own code. But, since it leaks when profiled, it is making analysis difficult. Is this a feature, bug, or user error? If a known issue, is there a workaround? The code and some documenting output follows. thanks, Travis ------------------------------------ {-# LANGUAGE BangPatterns #-} import System.Environment import Text.Printf import Control.Parallel.Strategies import Control.DeepSeq import Data.List (foldl') main = do [d] <- map read `fmap` getArgs printf "%f\n" (mean [1..d]) foldl'rnf :: NFData a => (a -> b -> a) -> a -> [b] -> a foldl'rnf f z xs = lgo z xs where lgo z [] = z lgo z (x:xs) = lgo z' xs where z' = f z x `using` rdeepseq -- first mean fn aka foldl'rnf mean :: [Double] -> Double mean xs = s / fromIntegral n where (n, s) = foldl'rnf k (0, 0) xs k (n, s) x = (n+1, s+x) :: (Int, Double) -- second mean fn aka foldl' -- mean :: [Double] -> Double -- mean xs = s / fromIntegral n -- where -- (n, s) = foldl' k (0, 0) xs -- k (!n, !s) x = (n+1, s+x) ------------------------------------------ [NO PROFILING, NO SPACE LEAK] C:\Documents and Settings\Travis\My Documents\Haskell Code>ghc --make temp5 -O2 -fasm [1 of 1] Compiling Main ( temp5.hs, temp5.o ) Linking temp5.exe ... C:\Documents and Settings\Travis\My Documents\Haskell Code>temp5 1e7 +RTS -sstderr temp5 1e7 +RTS -sstderr 5000000.5 1,170,230,652 bytes allocated in the heap 128,876 bytes copied during GC 3,372 bytes maximum residency (1 sample(s)) 13,012 bytes maximum slop 1 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 2232 collections, 0 parallel, 0.05s, 0.05s elapsed Generation 1: 1 collections, 0 parallel, 0.00s, 0.00s elapsed INIT time 0.02s ( 0.03s elapsed) MUT time 1.52s ( 1.55s elapsed) GC time 0.05s ( 0.05s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 1.58s ( 1.63s elapsed) %GC time 3.0% (2.9% elapsed) Alloc rate 764,232,262 bytes per MUT second Productivity 96.0% of total user, 93.3% of total elapsed C:\Documents and Settings\Travis\My Documents\Haskell Code>temp5 1e8 +RTS -sstderr temp5 1e8 +RTS -sstderr 50000000.5 11,702,079,228 bytes allocated in the heap 1,253,872 bytes copied during GC 3,372 bytes maximum residency (1 sample(s)) 13,012 bytes maximum slop 1 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 22321 collections, 0 parallel, 0.38s, 0.39s elapsed Generation 1: 1 collections, 0 parallel, 0.00s, 0.00s elapsed INIT time 0.02s ( 0.00s elapsed) MUT time 15.47s ( 15.72s elapsed) GC time 0.38s ( 0.39s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 15.86s ( 16.11s elapsed) %GC time 2.4% (2.4% elapsed) Alloc rate 755,734,682 bytes per MUT second Productivity 97.5% of total user, 96.0% of total elapsed [NOW TURN ON PROFILING, GET SPACE LEAK] C:\Documents and Settings\Travis\My Documents\Haskell Code>ghc --make temp5 -O2 -fasm -prof -auto-all [1 of 1] Compiling Main ( temp5.hs, temp5.o ) Linking temp5.exe ... C:\Documents and Settings\Travis\My Documents\Haskell Code>temp5 1e6 +RTS -sstderr -p -K128M temp5 1e6 +RTS -sstderr -p -K128M 500000.5 395,774,976 bytes allocated in the heap 238,684,620 bytes copied during GC 102,906,760 bytes maximum residency (7 sample(s)) 66,283,900 bytes maximum slop 179 MB total memory in use (4 MB lost due to fragmentation) Generation 0: 493 collections, 0 parallel, 4.83s, 4.84s elapsed Generation 1: 7 collections, 0 parallel, 0.23s, 0.30s elapsed INIT time 0.02s ( 0.03s elapsed) MUT time 0.81s ( 0.91s elapsed) GC time 5.06s ( 5.14s elapsed) RP time 0.00s ( 0.00s elapsed) PROF time 0.00s ( 0.00s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 5.89s ( 6.08s elapsed) %GC time 85.9% (84.6% elapsed) Alloc rate 477,916,952 bytes per MUT second Productivity 13.8% of total user, 13.4% of total elapsed