Hello Haskellers, I'm running ghc 6.8.2 on vista 64. Consider the following program, which is compiled with -02 -prof -auto-all: module Main where import System.IO (openFile, IOMode(..), hPutStr) testlst = let ls = [(i, [(j, (fromIntegral j)::Float) | j <- [1..5]::[Int]]) | i <- [1..500000]::[Int]] in ls main2 = do h <- openFile "bardump" WriteMode mapM_ ((hPutStr h) . show) testlst main = do h <- openFile "bardump2" WriteMode mapM ((hPutStr h) . show) testlst return () main and main2 are different in only that mapM_ versus mapM_ are used. But the mapM version runs about 20x slower! I'm running with +RTS -p -hc -RTS and I see that the amount of memory allocated is about the same, and I think the resident memory is about the same too. But the mapM_ version runs in about 8.7 seconds, and the mapM version takes 167 seconds. You may ask, why use mapM if you're discarding the values? Unfortunately in my real app I need the values, which are more interesting than IO (). ? Take care, Ben