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 <[email protected]> wrote: > > > > On Fri, Mar 19, 2021 at 12:02 PM Jay McCarthy <[email protected]> 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 <[email protected]> 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 [email protected]. >> > 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 [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/CAJYbDa%3DNiV7ryAnPp_KJOuyez23FY9ztqr%3Daydij7j4hHLn43g%40mail.gmail.com.
