Re: [Gtk2hs-devel] Generalizing castTo* Functions

[Axel S. <Axe...@en...>]

On May 12, 2009, at 20:27, Maurício wrote:

>>>> class Cast widget where
>>>>   cast :: GObjectClass obj => obj -> widget
>>>> instance Cast Window where
>>>>   cast = castToWindow
>>>> (...)
>
>> (...) 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.
>
> My package 'gtk2hs-rpn' would use that. Since it's a RPN
> calculator for widgets, it uses a stack (list, actually)
> of widgets, casted at runtime. I do that using a CanBeCasted
> module inside the package.
>
> I don't know if this qualify as a very good example, though.
>

One drawback of these classes is that they are highly ambiguous: You  
cannot use the 'cast' function without also providing an explicit  
type signature. In particular, you might as well insert the  
castToWindow function directly wherever you use 'cast w :: Window'.  
In your implementation you have, e.g.:

pBPED = RPN $
      let { bped :: Box -> Widget -> IO () ; bped = boxPackEndDefaults }
          in pullWidget bped ;

If I'm not mistaken, you could change pullWidget to take a castToBox  
function to achieve the same effect.

Cheers,
Axel.