(define (make-wire)
(let ((signal-value 0)
(action-procedures '()))
(define (set-my-signal! new-value)
(if (not (= signal-value new-value))
(begin (set! signal-value new-value)
(call-each action-procedures))
'done))
(define (accept-action-procedure! proc)
(set! action-procedures (cons proc action-procedures))
(proc))
(define (dispatch m)
(cond ((eq? m 'get-signal) signal-value)
((eq? m 'set-signal!) set-my-signal!)
((eq? m 'add-action!) accept-action-procedure!)
(else (error "Unknown operation -- WIRE" m))))
dispatch))
(define (and-gate a1 a2 output)
(define (and-action-procedure)
(let ((new-value
(logical-and (get-signal a1) (get-signal a2))))
(after-delay and-gate-delay
(lambda ()
(set-signal! output new-value)))))
(add-action! a1 and-action-procedure)
(add-action! a2 and-action-procedure)
'ok)
(define (after-delay delay action)
(add-to-agenda! (+ delay (current-time the-agenda))
action
the-agenda))
(define (propagate)
(if (empty-agenda? the-agenda)
'done
(let ((first-item (first-agenda-item the-agenda)))
(first-item)
(remove-first-agenda-item! the-agenda)
(propagate))))