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. <c...@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.mccar...@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. <c...@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 <j...@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 <j...@cs.byu.edu> Assistant Professor / Brigham Young University http://faculty.cs.byu.edu/~jay "The glory of God is Intelligence" - D&C 93
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