Answer 3.31

ex-3_28-xx.scm

@@ -19,8 +19,8 @@
                  (call-each action-procedures))
           'done))
     (define (accept-action-procedure! proc)
-      (set! action-procedures (cons proc action-procedures))
-      (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!)
@@ -200,6 +200,9 @@
   (assert (get-signal s3) 0)
   (assert (get-signal co) 1))
 
+
+(display "\nex-3.31 - accept-action-procedure!\n")
+
 (define input-1 (make-wire))
 (define input-2 (make-wire))
 (define sum (make-wire))
@@ -207,14 +210,21 @@
 ; (probe 'sum sum)
 ; (probe 'carry carry)
 
-(half-adder input-1 input-2 sum carry)
 (set-signal! input-1 1)
 (set-signal! input-2 1)
+(half-adder input-1 input-2 sum carry)
 (propagate)
 
 (assert (get-signal sum) 0)
 (assert (get-signal carry) 1)
 
-(display "\nex-3.31\n")
+; It is necessary to call the action procedure upon the initialization of the
+; gate to make sure that all wires are set to their correct value initially.
 
+; For the example with the half-adder the carry-bit stays at zero because the
+; AND-gate is not initialized to the correct value for two 1s as input. To
+; avoid the issue we would have to make sure that each signal changes at least
+; one.
+
+(display "\nex-3.32\n")