;; Swap-Channels

(define-record-type :swap-channel
  (really-make-swap-channel channel)
  swap-channel?
  (channel swap-channel-channel))

(define (make-swap-channel)
  (really-make-swap-channel (make-channel)))

(define (swap-rv swap-channel message-out)
  (let ((channel (swap-channel-channel swap-channel)))
    (guard
     (lambda ()
       (let ((in-channel (make-channel)))
	 (choose
	  (wrap (receive-rv channel)
                (lambda (pair)
                  (let ((message-in (car pair))
                        (out-channel (cdr pair)))
                    (send out-channel message-out)
                    message-in)))
          (wrap (send-rv channel (cons message-out in-channel))
                (lambda (ignore)
                  (receive in-channel)))))))))


(define (make-const-reply x ch)
  (let ((rv (swap-rv ch x)))
    (let lp ()
      (display (sync rv))
      (newline)
      (lp))))

(define c1 (make-swap-channel))

(spawn (lambda ()
         (make-const-reply 42 c1)))

(spawn (lambda ()
         (make-const-reply 67 c1)))