Custom implementation for Widget class in gtkhs

Gtk2hs has various widget datatypes that implements Widget class. Is it possible to write custom datatype which does the same?

Say I want to have widget for displaying and running Lua code like this.

data LuaWidget = LuaWidget { text :: TextView, package :: HBox } deriving Eq
instance Widget LuaWidg开发者_运维问答et where
    ....

Is it possible on Haskell level?

It is not possible to create new widget 'classes' with Haskell in gtk.

What you can do is give custom attributes to an existing widget type. For example, in the package plot-gtk a custom data field (System.Glib.GObject) is added to a drawingArea widget:

import System.Glib.GObject
import Graphics.UI.Gtk

-- | create a new 'Figure' plot
plotNew :: FigureHandle -> IO DrawingArea
plotNew f = do
   canvas <- drawingAreaNew

   set canvas [maybeFigure := (Just f)]

   _ <- on canvas exposeEvent $ tryEvent $ liftIO $ do 
           s <- widgetGetSize canvas
           drw <- widgetGetDrawWindow canvas
           fig <- get canvas figure 
           renderWithDrawable drw (renderFigureState fig s)

   return canvas

-- | the figure attribute
figure :: Attr DrawingArea FigureState
figure = newAttr getFigure setFigure
   where getFigure o = do
              Just f <- get o maybeFigure 
              readMVar f 
         setFigure o f = set o [maybeFigure :~> (\(Just h) -> do
              modifyMVar_ h (\_ -> return f)
              return $ Just h)]

maybeFigure :: Attr DrawingArea (Maybe FigureHandle)
maybeFigure = unsafePerformIO $ objectCreateAttribute
{-# NOINLINE maybeFigure #-}