Glynn Clements wrote:
Wolfgang Thaller wrote:
Now perhaps we want a common datatype "Widget" with functions toWidget :: Widget w => w -> Widget and fromWidget :: Widget w => Widget -> Maybe w
Perhaps we want a datatype "Pane" (or "Control" or whatever) with toPane :: Pane p => p -> Pane and so on.
I don't see the point in this. If you have classes, then surely you don't need the types?
The example implementation already had a "Widget" type; a Widget type would be necessary when you want to have heterogenous lists of widgets. That's why somebody brought up the idea of having the toWidget and fromWidget functions; Nobody protested when the idea was first proposed, so I assumed that it was agreed. If such a generic type is used, that has certain implications for the implementation; the fltk-based example uses one generic type "Widget" along with specialized types "Window", "Button" and "Box", each of which is an instance of Widget (with methods toWidget and fromWidget). I was just pointing out that this specific choice of just one generic type for both Windows and other Widgets was impractical for the Mac OS implementation. Cheers, Wolfgang