Hi, I'm extract this fragment of code from my tesis program. I'm try to reduce more, but i can't. The concrete question is: What does {_Gc Black Hole} mean? ======================================================= ===== Concrete problem here, implementation later ===== ======================================================= I'm execute mainOk and receive the expected result Main> mainOk Dyn Hello World!! (82 reductions, 196 cells) I'm execute main and system stop, not reduction executed (27 reductions in five minutes of wait) Main> main {Interrupted!} (27 reductions, 64 cells) I'm expected Dyn I'm break and re run and receive the message Main> main Program error: {_Gc Black Hole} (8 reductions, 51 cells) What does _Gc Black Hole mean? ======================================================= ========== Implementation can look here =============== ======================================================= Tested on Hugs 98 February 2001 in Windows 98 with options -98 +som ======================================================= ================ Class Result ========================= =======================================================

class Result r where toString :: r -> String fromString :: String -> r toR :: Result r' => r -> r' fromR :: Result r' => r'-> r

======================================================= ============ Class SubResult ========================== =======================================================

class Result a => SubResult a where toHtml :: a -> Html fromHtml :: Html -> a

======================================================= ========= Html data type ============================== =======================================================

newtype Html = Html String deriving Show

======================================================= ========= Html are Result and SubResult =============== =======================================================

instance Result Html where toString (Html st) = st fromString st = Html ("<" ++ st ++ ">") toR (Html st) = toR st fromR r = toR r

instance SubResult Html where toHtml = id fromHtml = id

======================================================= =========== Dyn data type ============================= =======================================================

data Dyn = forall a . SubResult a => Dyn a

instance Show Dyn where show (Dyn x) = "Dyn " ++ toString x

======================================================= =========== Dyn are Result and SubResult ============== =======================================================

instance Result Dyn where toString (Dyn x) = toString x fromString st = Dyn st fromR = toR toR (Dyn x) = toR x

instance SubResult Dyn where toHtml (Dyn x) = toHtml x fromHtml html = Dyn html

======================================================= ============ String are Result and SubResult ========== =======================================================

instance Result String where toString = id fromString = id fromR = toString toR = fromString

instance SubResult String where toHtml st = fromString st fromHtml html = toString html

======================================================= =============== Rule Class ============================ =======================================================

class (Result r) => Rule e r where apply :: e r -> r -> r rws :: (Result r') => (r -> r') -> e r

======================================================= =============== SubRule Class ========================= =======================================================

class (Rule e r, SubResult r) => SubRule e r where rwsS :: (SubResult r') => (r -> r') -> e r

======================================================= ======== TRule data type ============================== =======================================================

newtype TRule r = TRule (r -> r)

======================================================= ========= TRule are Rule ============================== =======================================================

instance Result a => Rule TRule a where apply (TRule f) r = f r rws f = TRule (\r -> toR (f r))

======================================================= =========TRule are SubRule ============================ =======================================================

instance SubResult a => SubRule TRule a where rwsS f = TRule (\r -> toR $ toHtml (f r))

======================================================= ========== Examples =================================== =======================================================

genRule1 :: Rule e r => e r genRule1 = rws (\r -> toR r ++ "!!")

genRule2 :: SubRule e r => e r genRule2 = rwsS (\r -> toR r ++ "!!")

mainOk:: Dyn mainOk = apply (genRule1::TRule Dyn) (toR "Hello World")

main :: Dyn main = apply (genRule2::TRule Dyn) (toR "Hello World")

======================================================== ===================== The end ========================== ======================================================== Saludos Pablo "Yeti" Cardenal from La Plata, Buenos Aires, Argentina