Hi, I have tried to implement a memo function using stable names and weak pointers like it is presented in the paper "stretching the storage manager". There is an abstract datatype SNMap a b which implements a map that maps values of type StableName a to values of type b. The map is located in an IORef. {-# NOINLINE memo #-} memo :: (a -> b) -> a -> b memo f = unsafePerformIO (do tref <- newSNMap return (applyWeak f tref)) {-# NOINLINE applyWeak #-} applyWeak :: (a -> b) -> SNMap a (Weak b) -> a -> b applyWeak f tbl arg = unsafePerformIO (do sn <- makeStableName arg lkp <- lookupSNMap tbl sn case lkp of Nothing -> not_found sn Just weak -> do val <- deRefWeak weak case val of Just result -> return result Nothing -> not_found sn) where not_found sn = do let res = f arg weak <- mkWeak arg res Nothing insertSNMap tbl sn weak return res Using this memo function I have implemented a fibonacci function like it is defined in the paper. fib :: Int -> Int fib = memo ufib ufib :: Int -> Int ufib 0 = 1 ufib 1 = 1 ufib n = fib (n-1) + fib (n-2) When I compile this program with 6.10.3 fib yields the correct results up to 18 but for 19 it detects a loop. Can anybody give me a hint how I can avoid this behaviour (and still use unsafePerformIO ; ) )? Cheers, Jan