Hello all, I've been debugging a space leak, and believe I've traced it to a chain of unevaluated calls to mappend in the bind operator for Control.Monad.Writer.Strict. I had expected these calls to be evaluated strictly by the strict Writer, but it doesn't seem to be the case. Am I understanding this correctly? If so, is this the intended behavior? Below are two example programs with runtime stats demonstrating the issue. The first uses Control.Monad.Writer.Strict, the second uses an alternative implementation that is strict in the Writer's monoid. The first runs in 78.5s with total memory 630MB. The second runs in 38.15s with total memory 237MB. FWIW, the lazy writer monad turned out to be more efficient for my program (and this example -- it runs in 9.96s with total memory 389MB, and doesn't need an increased stack size). thanks, -matt -- BEGIN writerTest.hs import Control.Monad.Writer.Strict type M = Writer [()] go :: Integer -> M () go 0 = return () go n = return () >> go (n-1) main = print $ runWriter $ go 3000000 -- END writerTest.hs -- BEGIN ./writerTest +RTS -K1G -sstderr ((),[]) 1,521,548,048 bytes allocated in the heap 654,690,520 bytes copied during GC 380,421,720 bytes maximum residency (10 sample(s)) 268,515,200 bytes maximum slop 630 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 1869 collections, 0 parallel, 77.34s, 77.40s elapsed Generation 1: 10 collections, 0 parallel, 0.40s, 0.40s elapsed INIT time 0.00s ( 0.00s elapsed) MUT time 0.76s ( 0.76s elapsed) GC time 77.75s ( 77.81s elapsed) EXIT time 0.00s ( 0.11s elapsed) Total time 78.50s ( 78.56s elapsed) %GC time 99.0% (99.0% elapsed) Alloc rate 2,008,440,129 bytes per MUT second Productivity 1.0% of total user, 1.0% of total elapsed -- END -- BEGIN writerTest2.hs import Data.Monoid data Writer w a = Writer a !w runWriter (Writer a w) = (a, w) instance (Monoid w) => Monad (Writer w) where return a = Writer a mempty m >>= k = case m of Writer a w -> case k a of Writer b w' -> Writer b (w `mappend` w') type M = Writer [()] go :: Integer -> M () go 0 = return () go n = return () >> go (n-1) main = print $ runWriter $ go 3000000 -- END writerTest2.hs -- BEGIN ./writerTest2 +RTS -K1G -sstderr ((),[]) 894,148,696 bytes allocated in the heap 221,145,720 bytes copied during GC 161,143,256 bytes maximum residency (9 sample(s)) 133,479,928 bytes maximum slop 257 MB total memory in use (11 MB lost due to fragmentation) Generation 0: 1185 collections, 0 parallel, 37.57s, 37.60s elapsed Generation 1: 9 collections, 0 parallel, 0.14s, 0.14s elapsed INIT time 0.00s ( 0.00s elapsed) MUT time 0.44s ( 0.44s elapsed) GC time 37.72s ( 37.75s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 38.15s ( 38.18s elapsed) %GC time 98.8% (98.9% elapsed) Alloc rate 2,036,159,030 bytes per MUT second Productivity 1.1% of total user, 1.1% of total elapsed -- END