{- Hi! I have a problem with monads. When a monadic function calls itself many times (see below) Hugs and GHCi runs out of memory/stack, and I can't see why. This is my program (well, I've kind of sipmlified it a lot :): -} newtype M a = M (Int -> (a,Int)) instance Monad M where return x = M $ \i -> (x,i) M f >>= k = M $ \i -> let (x,i2) = f i M f2 = k x in f2 i2 inc 0 = return () inc n = do inc'; inc (n-1) where inc' = M $ \i -> ((), i+1) runM :: M () -> IO () runM (M m) = print (m 0) -- in print i run n = runM (inc n) -- crashes for large n !! {- Shouldn't `run n' work for any n? But if I use n over 20 000 (or something) Hugs says "error: couldn't retreive enough memory when garabae-collecting" (or something), and other times Windows 2000 says "unknown software exception 0xc000000fd at 0x00040ec01". I tested to add "continuation passing style" (I don't really know what I should do) but that didn't work either. Does anyone have an idea of what the error is and what I should do / not do? Best regards, Magnus Lindberg -}