Re: Common GUI API : What would the code look like?

--- Glynn Clements <[email protected]> wrote:
> > > 3. Personally, I would prefer a generic widget
> > > creation function,
> > > rather than a different function for each type
> of
> > > widget. E.g. Xt has
> > > XtCreateWidget(), with the widget class being
> passed
> > > as an argument.
> > > A more Haskell-ish approach would be to use
> > > typeclasses, e.g.:
> > > 
> > > 	class Widget a where
> > > 		create :: String -> a -> [Property] -> IO a
> > > 	--	create name parent properties = ...
> 
> That should be:
> 
> 		create :: (Widget b) => String -> b -> [Property]
> -> IO a
> 
> A widget's parent must be a widget, but not
> necessarily the same type
> of widget.
> 

Ahh, I thought you wanted to create widgets based on
the String parameter.

So in the following code, you would get an ambiguity
error, right?

do
  -- mainWindow defined above
  a <- create "Some_Button" mainWindow [ label :=
"Okay" ]
  runGUI mainWindow -- or whatever it is.

The system does not know if a is a button, a label, or
whatever.  It would have to look like this to remove
the ambiguity

do
  -- mainWindow defined above
  a <- create "Some_Button" mainWindow [ label :=
"Okay" ] :: Button
  runGUI mainWindow -- or whatever it is.

It seems like this would defeat the purpose of having
a generic create function since Button needs to be in
there somewhere.  Although, I do find your argument
very credible.

David J. Sankel