Ignore the previous message, wrong code, here I come again Ok, instead of pushing about why I want to use unsafeCoerce (which I know it's not a good thing) I decided (as suggested by Taral) to paste a simplified example of my code. If anyone finds a way of implementing something equivalent to this code without unsafeCoerce# and ... * Not changing chooseDesc or finding an equivalent * Not splitting or changing Descriptor type (I already found an equivalent way which uses existentials and in which the type is splitted in two) ... I'll give up on my risky campaign on unsafeCoerce and you won't won't have to stand my questions about it again ;) --------------------- {-# OPTIONS_GHC -fglasgow-exts #-} import GHC.Base import Foreign -- Fake instantiation Data -- To make it simple, lets assume it doesn't need to be marshaled from C type InstanceInitData = Int -- Descriptor, equivalent to a C struct with function pointers -- hd is the handler of the callbacks (void *) in C data Descriptor hd = Descriptor { -- create a new instance and return its handler instantiate :: InstanceInitData -> hd, -- Run and return a new handler run :: hd -> IO hd} descInt:: Descriptor Int descInt = Descriptor (\_ -> 1) (\hd -> putStrLn (show hd) >> (return $ hd*2)) descChar :: Descriptor Char descChar = Descriptor (\_ -> 'a') (\hd -> putStrLn (show hd) >> (return $ succ hd)) data Dyn = Dyn toDyn :: Descriptor hd -> Dyn toDyn = unsafeCoerce# fromDyn :: Dyn -> Descriptor hd fromDyn = unsafeCoerce# descList :: [Dyn] descList = [toDyn descInt, toDyn descChar] -- Choose a descriptor, (called from C) chooseDesc :: Int -> IO (StablePtr (Descriptor a)) chooseDesc n = newStablePtr (fromDyn (descList !! n)) foreign export ccall "chooseDesc" chooseDesc :: Int -> IO (StablePtr (Descriptor hd)) -- Descriptor functions called from C -- once the descriptor is obtanied through chooseDesc cInstantiate :: StablePtr (Descriptor hd) -> InstanceInitData -> IO (StablePtr hd) cInstantiate ptr iid = do desc <- deRefStablePtr ptr (newStablePtr.(instantiate desc)) iid cRun :: StablePtr (Descriptor hd) -> StablePtr hd -> IO (StablePtr hd) cRun dptr hdptr = do desc <- deRefStablePtr dptr hd <- deRefStablePtr hdptr newhd <- (run desc) hd newStablePtr newhd -----------