On Tue, 2009-02-03 at 15:03 -0200, Marco Túlio Gontijo e Silva wrote: > Hello, > > I was kind of bothered in having to pass castTo* functions to > xmlGetWidget, and I thought that this could be done easier with a type > class: > > > class Cast widget where > > cast :: GObjectClass obj => obj -> widget > > > instance Cast Window where > > cast = castToWindow > > > instance Cast ImageMenuItem where > > cast = castToImageMenuItem > > > getWidget > > :: (Cast widget, WidgetClass widget) => GladeXML -> String -> IO > widget > > getWidget glade name = xmlGetWidget glade cast name > > After having the GladeXML, I'll do something like: > > let > getWidget' :: (Cast widget, WidgetClass widget) => String -> IO > widget > getWidget' = getWidget glade > > Is it bad to have this Cast class in gtk2hs?
Well, it's a bit of a hen-and-egg thing. You got to tell the compiler eventually what widget you'd like 'widget' to be. Since most (all?) methods in Gtk2Hs are able to take more derived widgets, (e.g. even ComboBoxEntry which is a leaf has functions like, uh, comboBoxEntryDoSomething :: ComboBoxEntryClass self => self -> Int -> IO () simply calling a method will not be enough to pin down the type. It's like trying to compile (show . read) where the intermediate type is also ambiguous. Thus, using your class Cast, you'd still need to add a type signature somewhere. This might be less of a hassle than using a specific castToComboBoxEntry, but only in some cases. Unless you have a very good example where the Cast class would be simpler than a castToFoo function, I'd rather not add yet another layer of type classes. Cheers, Axel. ------------------------------------------------------------------------------ Create and Deploy Rich Internet Apps outside the browser with Adobe(R)AIR(TM) software. With Adobe AIR, Ajax developers can use existing skills and code to build responsive, highly engaging applications that combine the power of local resources and data with the reach of the web. Download the Adobe AIR SDK and Ajax docs to start building applications today-http://p.sf.net/sfu/adobe-com _______________________________________________ Gtk2hs-devel mailing list Gtk2hs-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/gtk2hs-devel
