[racket] Scheme Design Patterns

Well, that was a happy coincidence.  Thank you for the explanation!
--
Chad Albers
http://www.neomantic.com
(pgp signature available on request)


On Wed, Sep 11, 2013 at 10:40 AM, Jay McCarthy <jay.mccarthy at gmail.com> wrote:
> Luckily, you've asked the person who wrote that code!
>
> There's a few things going on here...
>
> 1. read-request is just provided for backwards compatibility, the Web server
> itself ONLY use make-ext:read-request (as renamed to make-read-request on
> line 23); for instance, see
> https://github.com/plt/racket/blob/v5.3.4/collects/web-server/web-server.rkt#L62
>
> The reason is that we want to give you the option to never use HTTP/1.1 and
> always close a connection and that is part of request handling.
>
> That's the first part of make-ext:read-request, or rather why we have
> make-read-request at all.
>
> 2. Next, we want to make sure that whenever read-request throws an error, it
> kills the connection (which will in turn kill the Web server thread and free
> resources). However, we don't want to clutter the definition of
> make-read-request with these details, particularly because the test suite
> wants to turn this behavior off for various reasons and uses require/expose
> to get the /real/ make-read-request.
>
> So, in all, I wouldn't call this a use of a 'pattern' but rather a weird
> hodge-podge of the stuff from the past decade of maintaining the Web server
> :)
>
> Jay
>
>
>
>
> On Tue, Sep 10, 2013 at 7:28 PM, Chad A. <chad at neomantic.com> wrote:
>>
>> 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
>
>
>
>
> --
> 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