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 ____________________ Racket Users list: http://lists.racket-lang.org/users
