[racket] Box: When and why?

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
<greghendershott at 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.
>