Add implementation of exercise 3-35

Author: ramsayleung

README.org

@@ -25,7 +25,7 @@
   习题完成情况:
   - 章节一: 43/46
   - 章节二: 88/97
-  - 章节三: 34/82
+  - 章节三: 35/82
   - 章节四: TODO
   - 章节五: TODO
 * 运行

chapter3/exercise3-35.rkt

@@ -0,0 +1,74 @@
+#lang racket
+(require "constraints.rkt")
+
+(define (squarer a b)
+  (define (process-new-value)
+    ;; 原书的实现有缺陷，它假定 b 有值的时候 a 没有值，a 有值的时候 b
+    ;; 没有, 但是也存在一种可能是 a 和 b 都没有值, 比如在调用
+    ;; =(process-forget-value)= 时
+    (cond ((and (has-value? a) (not (has-value? b)))
+           (set-value! b (* (get-value a) (get-value a))
+                       me))
+          ((and (has-value? b) (not (has-value? a)))
+           (if (< (get-value b) 0)
+               (error "square less than 0 -- SQUARER" (get-value b))
+               ;; 还有一种可能就是有 (- (sqrt b)
+               (set-value! a (sqrt (get-value b))
+                           me)))))
+
+  (define (process-forget-value)
+    (forget-value! a me)
+    (forget-value! b me)
+    (process-new-value))
+
+  (define (me request)
+    (cond ((eq? request 'I-have-a-value)
+           (process-new-value))
+
+          ((eq? request 'I-lost-my-value)
+           (process-forget-value))
+
+          (else
+           (error "Unknown request -- MULTIPLIER" request))))
+
+  (connect a me)
+  (connect b me)
+  me)
+
+(module+ test
+  (require rackunit)
+
+  (test-case "Test for squarer with a set"
+             (define a (make-connector))
+             (define b (make-connector))
+             (squarer a b)
+             (probe "value a" a)
+             (probe "value b" b)
+             (check-false (has-value? a))
+             (check-false (has-value? b))
+             (set-value! a 5 'user)
+             (check-equal? (get-value b) 25)
+
+             (forget-value! a 'user)
+             (check-false (has-value? a))
+             (set-value! a -5 'user)
+             (check-equal? (get-value b) 25)
+             )
+
+  (test-case "Test for squarer with b set"
+             (define a (make-connector))
+             (define b (make-connector))
+             (squarer a b)
+             (probe "value a" a)
+             (probe "value b" b)
+             (check-false (has-value? a))
+             (check-false (has-value? b))
+             (set-value! b 25 'user)
+             (check-equal? (get-value a) 5)
+
+             (forget-value! b 'user)
+             (check-false (has-value? a))
+             (set-value! b 16 'user)
+             (check-equal? (get-value a) 4)
+             )
+  )