Hi, If I compile and run the code below, the file "foo" contains 10 lines of output (as I would expect), but the file "bar" contains just 9 - the final line is missing. If I add a "join", as in the comment, then all 10 lines appear. I don't understand why, completely - my best guess is that it's related laziness and the final result of the fold not being used. However, in that case, I'm worried that the "join fix" isn't guaranteed to work. Please could someone explain what is happening and how I should guarantee that the output is "complete". Thanks, Andrew $ cat test.hs main :: IO() main = do h <- openFile "foo" WriteMode foldM (printer h) 1 [1..10] hClose h h <- openFile "bar" WriteMode foldM (liftM2 (printer h)) (return 1) $ map return [1..10] -- join $ foldM (liftM2 (printer h)) (return 1) $ map return [1..10] hClose h printer :: Handle -> Int -> Int -> IO Int printer h n i = do hPutStr h (show n) hPutStr h ":" hPutStr h (show i) hPutStr h "\n" return (n+1) $ ghc test.hs $ ./a.out $ diff foo bar 10d9 < 10:10 -- personal web site: http://www.acooke.org/andrew personal mail list: http://www.acooke.org/andrew/compute.html