On Oct 21, 2010, at 5:48 PM, Greg Hendershott wrote:
> 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).
Racket is call-by-value. Period. Always. -- Matthias
> 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).
p.s. Amen.
>
> 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.
>>>>
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