On Tue, Apr 15, 2003 at 09:34:29PM +0100, Alastair Reid wrote:
Since Haskell has a different type system from OO languages, we simply cannot do things the 'typical GUI toolkit' way. Instead, we have to find a way to encode as much as possible of the foreign type system in Haskell's type system.
One way to do this is the one Wolfgang advocates: we provide a bunch of separate datatypes for each leaf node in the OO class hierarchy, we provide a class for each non-leaf node (or 'abstract class' if you prefer) in the OO class hierarchy and we use inheritance between the Haskell classes and instance declarations to encode the inheritance relations in the OO class hierarchy. This approach suffers a bit from being tedious to specify all the instances you need for each datatype but otherwise works pretty well.
Manuel uses type classes to enforce the inheritance relationship. I construct these classes automatically for gtk2hs. I think this matches very closely what Glynn and you propose. One question which I asked earlier is: a) Shall the classes containt the widget's functionality as well? (Or, alternatively: b) Shall we have a class for each attribute (like "onActivate") and create instances for all widgets that implement it.) The type definition for a Button in gtk2hs looks like this (and as you might guess, it is generated automatically): Cheers, Axel. -- ********************************************************* Button newtype Button = Button (ForeignPtr (Button)) mkButton = Button unButton (Button o) = o class BinClass o => ButtonClass o where toButton :: o -> Button fromButton :: Button -> o instance ButtonClass Button where toButton = id fromButton = id instance BinClass Button where toBin = mkBin.castForeignPtr.unButton fromBin = mkButton.castForeignPtr.unBin instance ContainerClass Button where toContainer = mkContainer.castForeignPtr.unButton fromContainer = mkButton.castForeignPtr.unContainer instance WidgetClass Button where toWidget = mkWidget.castForeignPtr.unButton fromWidget = mkButton.castForeignPtr.unWidget instance ObjectClass Button where toObject = mkObject.castForeignPtr.unButton fromObject = mkButton.castForeignPtr.unObject instance GObjectClass Button where toGObject = mkGObject.castForeignPtr.unButton fromGObject = mkButton.castForeignPtr.unGObject