1 (define (celsius-fahrenheit-converter c f)
2   (let ((u (make-connector))
3         (v (make-connector))
4         (w (make-connector))
5         (x (make-connector))
6         (y (make-connector)))
7     (multiplier c w u)
8     (multiplier v x u)
9     (adder v y f)
10     (constant 9 w)
11     (constant 5 x)
12     (constant 32 y)
13     'ok))
14 
15 (define (adder a1 a2 sum)
16   (define (process-new-value)
17     (cond ((and (has-value? a1) (has-value? a2))
18            (set-value! sum
19                        (+ (get-value a1) (get-value a2))
20                        me))
21           ((and (has-value? a1) (has-value? sum))
22            (set-value! a2
23                        (- (get-value sum) (get-value a1))
24                        me))
25           ((and (has-value? a2) (has-value? sum))
26            (set-value! a1
27                        (- (get-value sum) (get-value a2))
28                        me))))
29   (define (process-forget-value)
30     (forget-value! sum me)
31     (forget-value! a1 me)
32     (forget-value! a2 me)
33     (process-new-value))
34   (define (me request)
35     (cond ((eq? request 'I-have-a-value)  
36            (process-new-value))
37           ((eq? request 'I-lost-my-value) 
38            (process-forget-value))
39           (else 
40            (error "Unknown request -- ADDER" request))))
41   (connect a1 me)
42   (connect a2 me)
43   (connect sum me)
44   me)
45 
46 (define (inform-about-value constraint)
47   (constraint 'I-have-a-value))
48 (define (inform-about-no-value constraint)
49   (constraint 'I-lost-my-value))
50 
51 (define (multiplier m1 m2 product)
52   (define (process-new-value)
53     (cond ((or (and (has-value? m1) (= (get-value m1) 0))
54                (and (has-value? m2) (= (get-value m2) 0)))
55            (set-value! product 0 me))
56           ((and (has-value? m1) (has-value? m2))
57            (set-value! product
58                        (* (get-value m1) (get-value m2))
59                        me))
60           ((and (has-value? product) (has-value? m1))
61            (set-value! m2
62                        (/ (get-value product) (get-value m1))
63                        me))
64           ((and (has-value? product) (has-value? m2))
65            (set-value! m1
66                        (/ (get-value product) (get-value m2))
67                        me))))
68   (define (process-forget-value)
69     (forget-value! product me)
70     (forget-value! m1 me)
71     (forget-value! m2 me)
72     (process-new-value))
73   (define (me request)
74     (cond ((eq? request 'I-have-a-value)
75            (process-new-value))
76           ((eq? request 'I-lost-my-value)
77            (process-forget-value))
78           (else
79            (error "Unknown request -- MULTIPLIER" request))))
80   (connect m1 me)
81   (connect m2 me)
82   (connect product me)
83   me)
84 
85 (define (constant value connector)
86   (define (me request)
87     (error "Unknown request -- CONSTANT" request))
88   (connect connector me)
89   (set-value! connector value me)
90   me)
91 
92 (define (probe name connector)
93   (define (print-probe value)
94     (newline)
95     (display "Probe: ")
96     (display name)
97     (display " = ")
98     (display value))
99   (define (process-new-value)
100     (print-probe (get-value connector)))
101   (define (process-forget-value)
102     (print-probe "?"))
103   (define (me request)
104     (cond ((eq? request 'I-have-a-value)
105            (process-new-value))
106           ((eq? request 'I-lost-my-value)
107            (process-forget-value))
108           (else
109            (error "Unknown request -- PROBE" request))))
110   (connect connector me)
111   me)
112 
113 (define (make-connector)
114   (let ((value false) (informant false) (constraints '()))
115     (define (set-my-value newval setter)
116       (cond ((not (has-value? me))
117              (set! value newval)
118              (set! informant setter)
119              (for-each-except setter
120                               inform-about-value
121                               constraints))
122             ((not (= value newval))
123              (error "Contradiction" (list value newval)))
124             (else 'ignored)))
125     (define (forget-my-value retractor)
126       (if (eq? retractor informant)
127           (begin (set! informant false)
128                  (for-each-except retractor
129                                   inform-about-no-value
130                                   constraints))
131           'ignored))
132     (define (connect new-constraint)
133       (if (not (memq new-constraint constraints))
134           (set! constraints 
135                 (cons new-constraint constraints)))
136       (if (has-value? me)
137           (inform-about-value new-constraint))
138       'done)
139     (define (me request)
140       (cond ((eq? request 'has-value?)
141              (if informant true false))
142             ((eq? request 'value) value)
143             ((eq? request 'set-value!) set-my-value)
144             ((eq? request 'forget) forget-my-value)
145             ((eq? request 'connect) connect)
146             (else (error "Unknown operation -- CONNECTOR"
147                          request))))
148     me))
149 
150 (define (for-each-except exception procedure list)
151   (define (loop items)
152     (cond ((null? items) 'done)
153           ((eq? (car items) exception) (loop (cdr items)))
154           (else (procedure (car items))
155                 (loop (cdr items)))))
156   (loop list))
157 
158 (define (has-value? connector)
159   (connector 'has-value?))
160 (define (get-value connector)
161   (connector 'value))
162 (define (set-value! connector new-value informant)
163   ((connector 'set-value!) new-value informant))
164 (define (forget-value! connector retractor)
165   ((connector 'forget) retractor))
166 (define (connect connector new-constraint)
167   ((connector 'connect) new-constraint))
168 
169 (define (test-case actual expected)
170   (newline)
171   (display "Actual:   ")
172   (display actual)
173   (newline)
174   (display "Expected: ")
175   (display expected)
176   (newline))
177 
178 (define (averager a b c)
179   (let ((sum (make-connector))
180 	(two (make-connector)))
181     (adder a b sum)
182     (constant 2 two)
183     (multiplier two c sum)))
184 
185 ;; Exercise 3.34.  Louis Reasoner wants to build a squarer, a constraint device with two terminals such that the value of connector b on the second terminal will always be the square of the value a on the first terminal. He proposes the following simple device made from a multiplier:
186 
187 (define (squarer a b)
188   (multiplier a a b))
189 
190 ;; There is a serious flaw in this idea. Explain. 
191 
192 ;; This constraint only works in one direction. If a has a value, then the value is propagated to b. But, if b has a value, the value is not propagated to a because a multiplier normally needs to know one product plus one factor in orer to figure out the value of the second factor. The problem is that the multiplier is unaware that the two factors are referring to the same connector.