Il giorno 25/mag/2015, alle ore 21.56, Matthias Felleisen ha scritto:
...
>> Yes, I was thinking about things like these in my other message. But
>> this alreaddy depends on the evaluation order which can be "reversed".
>> Right in this example one would like to mix in lazy evaluation.
>>
>> (define the-view
>> {with-lazy-evaluation (new view% #model: (new model% #view: the-view)) })
>
> There is a difference between lazy evaluation and cyclic references.
> And it's huge.
>
In the specific example lazy evaluation would do the job, though.
>
>> One might even think about an interpreter that automatically tries ...
>
> See shared. That's NOT laziness. Repeat, it's NOT laziness.
>
Cyclic references or mutual hosting objects are not a problem at all.
It's only when you want the evualator handle that one line definition. But
a lazy evaluator solves this problem which a lot of people stumble over
as long as they have not yet adapted their way of thinking to the machine.
>
> Here is what it takes (in principle) to extend it to objects:
>
> (define-syntax (object-shared stx)
> (syntax-case stx (new)
> [(_ [(x x-field (new x% [x-field_ x0]))
> (y y-field (new y% [y-field_ y0]))]
> body)
> #'(letrec ([x (new x% [x-field '*undefined-x*])]
> [y (new y% [y-field y0])])
> (set-field! x-field x x0)
> #;
> (set-field! y-field y y0)
> body)]))
>
> (define the-view
> (object-shared [(the-model view (new model% [view the-view]))
> (the-view model (new view% [model the-model]))]
> the-view))
>
>
Sorry, ut I call it a workaround.
(define my-view (new view% #model: null))
(define my-model (new model% #view: null))
(send my-view set-model my-model)
(send my-model set-view my-view)
Every view should be able to handle a null or void model and every
model should be detachable from the view anyway.
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