[racket] Scheme Design Patterns

Thanks for your help.  I was sure it would have something to do with
capturing bindings in a closure, but the code in question seems to
lack a closure.  Maybe I'm missing something, though.

Here's the code:
https://github.com/plt/racket/blob/v5.3.4/collects/web-server/http/request.rkt#L102.

Thanks again,
Chad


--
Chad Albers
http://www.neomantic.com
(pgp signature available on request)


On Tue, Sep 10, 2013 at 10:16 PM, Jay McCarthy <jay.mccarthy at gmail.com> wrote:
> I think more context would be useful to know why a specific case is useful.
> But, I'll do my best with none...
>
> By using functions like this, you can control when things happen and what
> they can see. For instance...
>
> (define (make-worker)
>   (define a (build-list 10 add1)
>   (define (worker)
>     ;; a is visible to 'worker'
>     (+ (first a) 10))
>   worker)
>
> ;; a is not visible here
>
> ;; .... you do some other work ...
>
> ;; and only now do you run the construction of a
>
> (define some-worker
>   (make-worker))
>
> ;; .... you do some other work before getting the contents
>
> (define answer
>   (some-worker))
>
> This is all because closures are a basic form of objects---their environment
> values are their "private fields" and "apply" is their one method. Thus, any
> time that you would find OO useful, you might find a pattern like this
> useful. Especially when you don't want to go whole hog and use the
> object/class system from racket/class
>
> Jay
>
>
>
>
> On Mon, Sep 9, 2013 at 8:24 PM, Chad A. <chad at neomantic.com> wrote:
>>
>> Hi,
>>
>> I'm wondering if someone can help me understand a design-pattern that
>> I have noticed in the Racket code base. (I'm teaching myself, so
>> review other people's code alot).
>>
>> I've seen this pattern..which is a bit contrived.
>>
>> (define (say-hello)
>>   (define (display-hello)
>>      (display "hello")))
>>
>> So this produces a producer, and to evaluate it I would need to write
>> ((hello)) => "hello"
>>
>> But then I see this...
>>
>> (define say-hi (say-hello)).  When I evaluate (say-hello), I get
>> "hello".  Obviously, the definition of "say-hi" contains the evaluated
>> (say-hello) than returns the 'display-hello' procedure to be
>> evaluated.
>>
>> My question is...why would I want to write the code like this? 3
>> definitions have been created which could easily be compressed into
>> one.
>>
>>  (I also see nothing in the code I'm looking at that appears to
>> capture a binding via a closure).
>>
>> Thanks in advance for any insights someone can share.
>>
>> Chad
>> ____________________
>>   Racket Users list:
>>   http://lists.racket-lang.org/users
>
>
>
>
> --
> Jay McCarthy <jay at cs.byu.edu>
> Assistant Professor / Brigham Young University
> http://faculty.cs.byu.edu/~jay
>
> "The glory of God is Intelligence" - D&C 93