[GUI] Are windows just ordinary widgets?

[Axel Simon]

On Tue, Apr 29, 2003 at 09:43:18AM +0100, Alastair Reid wrote:
> 
> Wolfgang Thaller <wolfgang.thaller@gmx.net> writes:
> > 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. 
> 
> By itself, there's no problem having a Widget type and to/fromWidget
> operations.  After all, with _only_ those two operations, there's not
> that much you can do with them.
> 
> But, the moment we add additional operations which act on objects of
> type Widget and not on objects in the Widget class, my gut tells me
> there will be problems.  I was keeping quiet until such operations
> were proposed since it'd be easier to point out problems with concrete
> examples in front of us.

IMHO the classes should just reflect the object hierarchy. If we have e.g.

class Button b where...
class ToggleButton b where...
instance Button ToggleButton where...

then we can have the simple function

onActivate :: Button b => b -> IO () -> IO ()

and use it with ToggleButton as well (but not with a plain Widget or 
anything on the other branches of the tree). Having onActivate as a class 
member doesn't give any improvement in type safety, is less efficient 
(bigger dictionaries) and doesn't lend itself to automatic creation of 
this type hierarchy.

Axel.