Promises (and dataflow programming in general) are really useful with
interacting with physical devices, especially robots. Suppose you
have a 200 ton press, with a safety sensor. You have the following
code:
(def is-it-safe? (promise))
(defn activate-press []
(if @is-it-safe?
(go!)))
(defn poll-sensor []
(loop []
(if (= (read-sensor) :safe)
(deliver is-it-safe :safe)
(do (Thread/sleep 50) recur))))
Now, the poll-sensor fn is usually running in a daemon thread
somewhere else. The call to activate-press will happen at a seemingly
random time, at least as far as poll-sensor is concerned. The promise
forces the press to wait unit it's safe to activate.
There are all sort of other conditions where this makes sense when
dealing with mechanics. This is just one.
HTH,
Sean
On Jan 21, 6:40 am, Laurent PETIT <laurent.pe...@gmail.com> wrote:
> In a nutshell, those are the building blocks for the "dataflow
> programming paradigm".
>
> It's an easy way to make a computation done in thread A (and using a
> pre-declared promise) block until thread B has delivered the promise
> (given it its value).
>
> The book CTM covers dataflow programming
> :http://www.info.ucl.ac.be/~pvr/book.html
>
> Now, to be honest, i still haven't read the related parts of the book,
> and I'm unable to give more concrete examples yet :-)
>
> 2010/1/21 Baishampayan Ghose <b.gh...@ocricket.com>:
>
> > 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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