Some more progress. I really should read more of this book.

sec-3-3-4.scm

@@ -0,0 +1,52 @@
+
+;; The digital circuit program is very interesting,
+;; not just because it is a good demonstration of scheme,
+;; but also because I really think this is a practical
+;; program.
+
+;; anyway, here are the exercises
+
+;; so, the book does not provide an implementation for make-wire,
+;; get-signal, set-signal!, add-action! and after-delay
+;; so if we want this code to run, we need to implement them.
+
+;; I chose to use a closure for this, because this is something
+;; I really learned with SICP so may as well. We could also use
+;; a box (in r6rs I believe, or at least chez)
+;; and in all fairness I believe that is more efficient, however this was funny
+
+;; EDIT: later on in the chapter the book actually provides a similar implementation
+;; so I guess I'm prescient now
+(define (make-wire)
+  (let ((val #f) (actions '()))
+    (lambda (x param)
+      (cond
+       ((eq? x 'get) val)
+       ((eq? x 'set!)
+	(if (not (boolean=? val param))
+	    (begin (set! val param)
+		   (map (lambda (x) (x))
+			actions))))
+       ((eq? x 'add-action!)
+	(set! actions (cons param actions))
+	(param))))))
+
+(define (get-signal w)
+  (w 'get 'USELESS))
+(define (set-signal! w val)
+  (w 'set! val))
+(define (add-action! w action)
+  (w 'add-action! action))
+
+
+(define a (make-wire))
+(define b (make-wire))
+(define c (make-wire))
+(define d (make-wire))
+(define e (make-wire))
+(define s (make-wire))
+
+
+
+
+