Running out of memory in a simple program

[Magnus Lindberg]

{-
  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
-}