{- Hi! I have problems with monads and memory. I have a monad through which I thread output. If I do the concatenation of the output-strings in one way Hugs runs out of memory, but if I do it in another way everything works. I can't see why the first way doesn't work but the second is OK. I woudl appreciate if someone could tell me what I am doing wrong. Here is the non-working monad: -} data M a = M (String -> (a, String)) instance Monad M where return x = M $ \o -> (x, o) M io >>= fio = M $ \o0 -> let (r1, o1) = o0 `seq` io o0 M io2 = r1 `seq` fio r1 in io2 o1 putCharM c = M $ \o -> ((), o ++ [c]) -- Is this stupid in some way? runM :: M a -> IO() runM (M io) = let (r, outp) = io "" in putStr outp crashHugs 0 = return () crashHugs n = do putCharM 'x'; crashHugs (n-1) r = runM (crashHugs 50000) {- Besides, the output generated when one runs `r' (with a smaller number, for example r = 4000) above is in the beginning written in a very slow pace but then the delay between each char decreases and finally everything is written very fast (but not for the working monad below) Well, here is the working monad: -} {- data M a = M (a, String) instance Monad M where return x = M (x, "") M f >>= k = M $ let (x, o) = f M f2 = k x (x', o') = f2 in (x', o ++ o') putCharM c = M ((), [c]) runM :: M a -> IO() runM (M f) = let (r, outp) = f in putStr outp thisIsOk 0 = return () thisIsOk n = do putCharM 'x'; thisIsOk (n-1) r = runM (thisIsOk 50000) -} {- So, it works when I concatenate the output in `>>=' but not (as in the first monad) in putChar... I can't see why :( Keen regards, Magnus Lidnberg -}