[racket] Traits and private data

On Wed, 2010-12-29 at 13:42 -0700, Matthew Flatt wrote:
> At Wed, 29 Dec 2010 19:23:24 +0200, Markku Rontu wrote:
> > Now what would be closer to my wishes is something like this where the
> > counter is protected and not exposed.
> > 
> > ---
> > 
> > (define protected-counted-trait
> >   (trait
> >    (define counter 0)
> >    
> >    (define/public (add!)
> >      (set! counter (add1 counter)))
> >    
> >    (define/public (current-count)
> >      counter)))
> > 
> > ---
> > 
> > This however is not possible as far as I know. There is a hint to some
> > solution in the Racket Reference 5.6 Traits:
> > 
> > "External identifiers in trait, trait-exclude, trait-exclude-field,
> > trait-alias, trait-rename, and trait-rename-field forms are subject to
> > binding via define-member-name and define-local-member-name. Although
> > private methods or fields are not allowed in a trait form, they can be
> > simulated by using a public or field declaration and a name whose scope
> > is limited to the trait form."
> 
> Like this:
> 
> (define protected-counted-trait
>   (let ()
>     (define-local-member-name counter)
> 
>     (trait
>      (field (counter 0))
>      
>      (define/public (add!)
>        (set! counter (add1 counter)))
>      
>      (define/public (current-count)
>        counter))))
> 

Thanks!

That does the trick. I wrote a macro to have a "trait/private" that
introduces "field/private" which does the magic behind the scenes.

-Markku