On Fri, Jan 22, 2010 at 11:41 AM, Laurent PETIT <laurent.pe...@gmail.com>wrote:
> David,
>
> maybe I don't understand your example well, but anyway, here are my
> remarks / concerns:
>
> * You're example is not at all exempt from multithreading: you use
> futures, and in the real scenario, the calls to deliver would have to
> occur from a thread able to pull from or be notified by the progress
> of the tests
>
Sure deliver would happen on the main thread. Basically the idea is that the
main thread is never blocked. Blocking happens on other threads.
> * If one wants all the graphical values to be update "on the fly",
> then one will have to place the calls to deref in independent threads,
> wouldn't it be too heavy to have so many threads (perhaps 100, 10000
> ?) just for updating a single node of a tree in the overall
> application ?
>
My technique certainly isn't efficient but it does let you engage in a
certain style of programming that I think can sometimes be useful. My
example is actually taken from a real use case I wrote in JavaScript after
understanding how promises worked in Clojure.
;; 3 ms
(time
(reduce + (range 1000)))
;; 53ms
(time
(let [r (take 1000 (repeatedly promise))]
(future-out
(println "value:" (reduce + (map deref r))))
(dotimes [x 1000]
(deliver (nth r x) x))))
If we're just pushing stuff out to UI, 53ms is certainly acceptable no?
>
>
>
> 2010/1/22 David Nolen <dnolen.li...@gmail.com>:
> > They are actually useful even outside the context of concurrency. For
> > example perhaps you have a recursive data structure that represents a
> series
> > of unit tests and unit test suites. You would like to do two things:
> > 1. Render out the nested structure of the tests that will be executed for
> > the user
> > 2. Render out the test results as they complete
> > With out promises you would have to traverse this data structure twice.
> Once
> > to render it for the user and a second time to show the results. With
> > promises you only need to traverse the data structure one time. In the
> > traversal you create promises for the test results as well as created a
> > flattened list of the tests to run.
> > In the following imagine that each test returns 0 or 1 depending on
> > pass/fail (I haven't bothered to make a recursive data structure here,
> but
> > the general point should be clear I hope).
> > (def x (promise))
> > (def y (promise))
> > (def z (promise))
> > (def passed (promise))
> > (defmacro future-out [& body]
> > `(let [out# *out*]
> > (future
> > (binding [*out* out#]
> > ~...@body))))
> > (future-out
> > (deliver passed (reduce + (map deref [x y z])))
> > (println "passed:" passed))
> > (future-out (println "x:" @x))
> > (future-out (println "y:" @y))
> > (future-out (println "z:" @z))
> > (deliver x 1)
> > (deliver y 0)
> > (deliver z 1)
> > Pretty useful I say :)
> > On Thu, Jan 21, 2010 at 6:19 AM, Baishampayan Ghose <
> b.gh...@ocricket.com>
> > wrote:
> >>
> >> Hello,
> >>
> >> I am trying to understand the use-cases of the new promise/deliver
> >> feature in Clojure. I have tried using them, and they seem to be
> >> pretty straight-forward to use, but unfortunately I haven't been able
> >> to understand its use-cases.
> >>
> >> It would be great if someone pointed out some example usage of
> >> promise/deliver.
> >>
> >> Regards,
> >> BG
> >>
> >> --
> >> Baishampayan Ghose
> >>
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