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This system provides a simple thread synchronization primitive called Rendezvous. It allows exchanging pieces of data between threads.

Here is how it works. You create a rendezvous object. Then you might create one or many threads and each of them can either to "call" rendezvous and pass it some value or to "accept" value and return a result.

Accept blocks the calling thread until some other thread will not call and vice-versa. This is similar to a thread-safe blocking queue of size 1:

POFTHEDAY> (defparameter *r*

POFTHEDAY> (bt:make-thread
            (lambda ()
              (log:info "Waiting for value")
              (let ((value (thread.comm.rendezvous:accept-rendezvous *r*)))
                (log:info "Value received: ~S" value))))

<INFO> [2020-07-21T23:06:56.836061+03:00] Waiting for value

POFTHEDAY> (thread.comm.rendezvous:call-rendezvous

<INFO> [2020-07-21T23:07:46.642640+03:00] Value received: :THE-VALUE-TO-BE-SENT-TO-THE-THREAD

I wasn't able to imagine a more complex but short illustration of the case where this synchronization primitive can be useful. If you know one, please share your ideas in comments.

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