P.S. Maybe a simpler example I should have used:
(define next-web-parameter-id
(let ([i (box 0)])
(lambda ()
(begin0 (unbox i)
(set-box! i (add1 (unbox i)))))))
In both examples the box is involved in a closure lambda pattern.
This example, my old C/C++ brain wants to interpret it as a kind of
thread-safe increment of a variable. But what's really going on?
On Thu, Oct 21, 2010 at 11:22 AM, Greg Hendershott
<greghendersh...@gmail.com> wrote:
> Where can I find more information about the use scenarios for "box"?
>
> The Guide is terse:
>
>>>
> 3.11 Boxes
>
> A box is like a single-element vector. It can print as a quoted #&
> followed by the printed form of the boxed value. A #& form can also be
> used as an expression, but since the resulting box is constant, it has
> practically no use.
> <<
>
> This explains what a box is, but not why or when you would want to use one.
>
> I see box used as in this example from servlet-dispatch.rkt:
>
> (define (dispatch/servlet
> ...
> (define servlet-box (box #f))
> ...
> (filter:make
> ...
> (lambda (url)
> (or (unbox servlet-box)
> (let ([servlet
> (parameterize ([current-custodian (make-custodian)]
> [current-namespace
> (make-servlet-namespace
> #:additional-specs
> default-module-specs)])
> (if stateless?
> (make-stateless.servlet
> servlet-current-directory stuffer manager start)
> (make-v2.servlet servlet-current-directory
> manager start)))])
> (set-box! servlet-box servlet)
> servlet))))))
>
> And I'm scratching my head, not understanding the point of using a box
> as opposed to a simple non-boxed value.
>
> My question isn't about this code per se; only an example. Generally,
> in what situations would you use a box, and why?
>
> Thank you.
>
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