Hi, The FFI doesn't allow using type signatures in exported functions. Imagine this simplified example class Foo a where action1 :: a -> IO() action2 :: a -> IO() exportable :: Foo a => StablePtr a -> IO() exportable ptr = do deref <- deRefStablePtr ptr action1 deref Currently the FFI doesn't allow to export exportable due to the type signature. But I don't really understand why isn't it feisable. In my code I solved the problem by quantifying existentially data CFoo = forall a.Foo a => CFoo a instance Foo CFoo where action1 (CFoo a) = action1 a action2 (CFoo a) = action2 a exportable' :: StablePtr CFoo -> IO() exportable' ptr = do deref <- deRefStablePtr ptr action1 a Another way to fix it would be, unlike standard Haskell98, allowing dictionaries to be packaged in data constructors data CFoo' a = Foo a => CFoo' a instance Foo (CFoo' a) where action1 (CFoo' x) = action1 x action2 (CFoo' x) = action2 x exportable'' :: StablePtr (CFoo' a) -> IO() exportable'' ptr = do deref <- deRefStablePtr ptr action1 deref As far as I know, the later option has only been recently made available for GADTs in GHC, does anyone know away of doing it with normal ADT? Does anyone know a better workaround? Thanks in advance, Alfonso Acosta