Boxes are the closest thing to addresses you can find in Racket. You need them when you want to mimic a classical by-reference function parameter protocol. For example,
(define (return-nothing x y z a-box) (set-box! a-box ComputeSomethingWith-x-y-z) (void) ;; just to make clear that nothing else is returned ) You'd call this elsewhere with (define my-personal-box (box #f)) ... (return-nothing my-personal-box) (unbox my-personal-box) You can imagine that you have some other ways of referring to my-personal-box (say in a list of all your boxes) which you may then use to implement other services (saving all boxes to a file). Some of our old methods that go down to C++ use this protocol. In your 'p.s.' example, there is no reason whatsoever to use a box over set! and variables. -- Matthias On Oct 21, 2010, at 11:31 AM, Greg Hendershott wrote: > 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. >> > _________________________________________________ > For list-related administrative tasks: > http://lists.racket-lang.org/listinfo/users _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
