Thank you, I understand now. It's interesting how little I've need to consider, so far, whether Racket parameters are call-by-value or call-by-reference.
If you don't use set! on parameters, one reason to consider it disappears. I've been sticking to a functional style (after an initial awkward "imperative un-training" period). Also whenever I've needed to return > 1 value, it's been very natural to use values (for 2 or 3) or a struct (for more). On Thu, Oct 21, 2010 at 11:41 AM, Matthias Felleisen <matth...@ccs.neu.edu> wrote: > > 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
