Ex3_32.scm

(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 (call-each procedures)
  (if (null? procedures)
      'done
      (begin ((car procedures))
	     (call-each (cdr procedures)))))

(define (get-signal wire)
  (wire 'get-signal))

(define (set-signal! wire new-value)
  ((wire 'set-signal!) new-value))

(define (add-action! wire action-procedure)
  ((wire 'add-action!) action-procedure))