Queue Concurrency

queue.scm

(define ready-to-call (make-queue))

(define (call-soon proc)
  ((ready-to-call 'insert!) proc))

(define (run)
  (cond ((ready-to-call 'empty?) 'done)
        (else
         (let ((proc (ready-to-call 'front)))
           (proc) ; execute the 0-argument lambda
           (ready-to-call 'delete!)))))

(define (countdown n)
  (cond ((= n 0) 'countdown-done)
        (else
         (display "Down ")
         (displayln n)
         (call-soon (lambda () (countdown (- n 1)))))))

(define (up stop)
  (define (iter x)
    (cond ((> x stop) 'up-done)
          (else
           (display "Up ")
           (displayln x)
           (call-soon (lambda () (iter (+ x 1)))))))
  (iter 0))

(call-soon (lambda () (countdown 3)))
(call-soon (lambda () (up 3)))
(run)

; Down 3
; Up 1
; Down 2
; Up 2
; Down 1
; Up 3