Hi, I am trying out existential type, some sample code works well. Well, my own code could not be compiled with message: Grid.hs:45:11: Kind error: `GridWidget' is applied to too many type arguments In the type `GridWidget widget' In the type `(GridWidget widget) -> (widget -> t) -> t' In the type signature for `liftGW': liftGW :: (GridWidget widget) -> (widget -> t) -> t The code is: {-# OPTIONS -fglasgow-exts #-} module Grid where import Graphics.UI.Gtk data GridWidgetType = GridLabel | GridTextView data GridWidget = forall widget. (WidgetClass widget) => GridWidget widget --GWLabel Label -- | GWTextView TextView gridNew defaultWidget = do self <- fixedNew -- gw <- gridWidgetNew defaultWidget -- gridAddWidget self gw (0, 0) -- self `on` realize $ do -- (ww, wh) <- liftGW gw widgetGetSize -- (w, h) <- widgetGetSize self -- mapM_ (\x -> -- mapM_ (\y -> do -- gw <- gridWidgetNew defaultWidget -- liftGW gw $ \gw -> fixedPut self gw (x * ww, y * wh) -- ) [0..floor (h / wh)] -- ) [0..floor (w / ww)] return self -- gridSetWidget self (x, y) widget = do -- w <- gridGetWidget self (x, y) -- if w == widget -- then return () -- else do -- (w, h) <- widgetGetSize w -- gw <- gridWidgetNew widget -- fixedPut self gw (x * w, y * h) -- widgetDestroy w -- gridWidgetNew GridLabel = labelNew Nothing >>= return . GW -- gridWidgetNew GridTextView = textViewNew >>= return . GW -- gridAddWidget grid (GWLabel label) (x, y) = fixedPut grid label (x, y) -- gridAddWidget grid (GWTextView textView) (x, y) = fixedPut grid textView (x, y) liftGW :: (GridWidget widget) -> (widget -> t) -> t liftGW (GridWidget label) f = f label liftGW (GridWidget textView) f = f textView -- 竹密岂妨流水过 山高哪阻野云飞