class (Typeable a) => ScalarClass a where
    scalar_iType :: a -> Type
    scalar_iType = const $ mkType "Scalar"
    scalar_fetch :: a -> Eval VScalar
    scalar_store :: a -> VScalar -> Eval ()
    scalar_const :: a -> Maybe VScalar

instance ScalarClass IScalarProxy where
    scalar_iType = const $ mkType "Scalar::Proxy"
    scalar_fetch = fst
    scalar_store = snd
    scalar_const = const Nothing

instance ScalarClass IScalar where
    scalar_fetch = liftSTM . readTVar
    scalar_store = (liftSTM .) . writeTVar
    scalar_const = const Nothing

instance ScalarClass IScalarLazy where
    scalar_iType = const $ mkType "Scalar::Lazy"
    scalar_fetch = return . maybe undef id
    scalar_store d _ = retConstError $ VStr $ show d
    scalar_const = const Nothing

instance ScalarClass IScalarCwd where
    scalar_iType = const $ mkType "Scalar::Cwd"
    scalar_fetch _ = do
        str <- liftIO $ getCurrentDirectory
	return $ VStr str
    scalar_store _ val = do
        str <- fromVal val
	tryIO () $ setCurrentDirectory str
    scalar_const = const Nothing

instance ScalarClass VScalar where
    scalar_iType = const $ mkType "Scalar::Const"
    scalar_fetch (VRef ref) = readRef ref
    scalar_fetch v = return v
    scalar_store d _ = retConstError d
    scalar_const = Just