Ah. Thanks.

On Fri, Mar 19, 2021 at 1:33 PM Jay McCarthy <jay.mccar...@gmail.com> wrote:

> It is not a built-in thing. I am talking about the use-pattern of a
> condition variable:
> https://en.wikipedia.org/wiki/Monitor_(synchronization)#Condition_variables
>
> --
> Jay McCarthy
> Associate Professor @ CS @ UMass Lowell
> http://jeapostrophe.github.io
> Vincit qui se vincit.
>
> On Fri, Mar 19, 2021 at 1:02 PM David Storrs <david.sto...@gmail.com>
> wrote:
> >
> >
> >
> > On Fri, Mar 19, 2021 at 12:02 PM Jay McCarthy <jay.mccar...@gmail.com>
> wrote:
> >>
> >> The best thing is to use a semaphore instead of a mutable reference.
> >> If you can't do that, then I think that you should combine the mutable
> >> reference with a signaling semaphore. If you can't do that, then I
> >> can't think of anything but a poll.
> >
> >
> > Is this the kind of thing you meant by 'signalling semaphore'?
> >
> > #lang racket
> >
> > (define x #f)
> > (define sema (make-semaphore 0))
> >
> > (define (wait-on-x) (sync (semaphore-peek-evt sema)) always-evt)
> > (define (set-x! val)
> >   (void (thread ; don't print the thread object when running in the repl
> >          (thunk
> >           (sleep 3)
> >           (set! x val)
> >           (semaphore-post sema)))))
> >
> > (define (check-x)
> >   (match (sync/timeout 10 (wait-on-x))
> >     [#f (displayln "timeout")]
> >     [_ (displayln "success")]))
> >
> > (set-x! 7)
> > (check-x)  ; pauses for 3 seconds, then outputs "success"
> > (check-x)  ; outputs "success" immediately
> > (check-x)  ; ibid
> >>
> >>
> >> --
> >> Jay McCarthy
> >> Associate Professor @ CS @ UMass Lowell
> >> http://jeapostrophe.github.io
> >> Vincit qui se vincit.
> >>
> >> On Fri, Mar 19, 2021 at 11:59 AM David Storrs <david.sto...@gmail.com>
> wrote:
> >> >
> >> > Suppose I have a function that tests for some condition, e.g.
> >> >
> >> > (define current-user (make-parameter #f))
> >> > (define (current-user-set?) (not (false? (current-user)))
> >> >
> >> > What is the best way to say "wait until 'current-user-set?' returns
> true"?  I've been through the Events chapter in the Reference and nothing
> seems like a great fit.  I could do polling via sleep or alarm-evt but that
> seems inefficient.  Is there a better way?
> >> >
> >> > --
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> .
>

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