Custom implementation for Widget class in gtkhs

Question

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 LuaWidget where
    ....

Is it possible on Haskell level?

Answer 1

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 #-}