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? > >> > > >> > -- > >> > You received this message because you are subscribed to the Google > Groups "Racket Users" group. > >> > To unsubscribe from this group and stop receiving emails from it, > send an email to racket-users+unsubscr...@googlegroups.com. > >> > To view this discussion on the web visit > https://groups.google.com/d/msgid/racket-users/CAE8gKocbPgjcFAF_o2g6mhZBEH8PpeGyJ4CwznKc3DZkMjY%3DGw%40mail.gmail.com > . > -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/CAE8gKofsHVJfBZCX12Sm%3DeNSeEj4AmehPYhsS2wJf8G3q%3DRXXA%40mail.gmail.com.
