Best way to say 'block until true'?

[David Storrs]

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?

[Jay McCarthy]

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.

--
Jay McCarthy
Associate Professor @ CS @ UMass Lowell
http://jeapostrophe.github.io
Vincit qui se vincit.

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[Sam Tobin-Hochstadt]

Another possibility is to send a message on a channel when the user is
set, and then just wait with `sync` for a message to appear on the
channel.

Sam

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[David Storrs]

Cool.  Thank you both.

[David Storrs]

On Fri, Mar 19, 2021 at 12:02 PM Jay McCarthy <jay.mc...@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]

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.

[David Storrs]

Ah. Thanks.