;; 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
	  (list
	   (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))))))))))