Add exercise 4.51

mceval/amb-natural-language.rkt

@@ -352,3 +352,42 @@ try-again
 (compound-sentence (compound-sentence (compound-sentence (compound-sentence (simple-sentence (simple-noun-phrase (article the) ((noun professor))) (verb studies)) (connective and) (simple-sentence (simple-noun-phrase (article a) ((noun cat))) (verb studies))) (connective and) (simple-sentence (simple-noun-phrase (article the) ((noun professor))) (verb studies))) (connective and) (simple-sentence (simple-noun-phrase (article the) ((noun cat))) (verb sleeps))) (connective and) (simple-sentence (simple-noun-phrase (article the) ((noun student))) (verb studies)))
 
 ;;; Amb-Eval input:
+
+
+;; Exercise 4.51:
+
+;;; Amb-Eval input:
+(define count 0)
+
+          (let ((x (an-element-of '(a b c)))
+                (y (an-element-of '(a b c))))
+            (permanent-set! count (+ count 1))
+            (require (not (eq? x y)))
+            (list x y count))
+
+;;; Starting a new problem 
+
+
+;;; Amb-Eval value:
+0
+ok
+
+;;; Amb-Eval input:
+
+;;; Starting a new problem 
+
+
+;;; Amb-Eval value:
+0
+(a b 2)
+
+;;; Amb-Eval input:
+try-again
+
+
+;;; Amb-Eval value:
+3
+(a c 3)
+
+;; Using set! instead of permanent-set! resets the counter back to 0
+;; on each backtrack of x or y, so will always be 1.

mceval/ambeval.rkt

@@ -29,6 +29,7 @@
         ((quoted? exp) (analyze-quoted exp))
         ((variable? exp) (analyze-variable exp))
         ((assignment? exp) (analyze-assignment exp))
+        ((permanent-assignment? exp) (analyze-permanent-assignment exp))
         ((definition? exp) (analyze-definition exp))
         ((if? exp) (analyze-if exp))
         ((lambda? exp) (analyze-lambda exp))
@@ -132,6 +133,19 @@
                             (fail2)))))
              fail))))
 
+(define (analyze-permanent-assignment exp)
+  (let ((var (assignment-variable exp))
+        (vproc (analyze (assignment-value exp))))
+    (lambda (env succeed fail)
+      (vproc env
+             (lambda (val fail2)        ; *1*
+               (let ((old-value
+                      (lookup-variable-value var env))) 
+                 (set-variable-value! var val env)
+                 (succeed 'ok
+                          fail2)))
+             fail))))
+
 ;;;Procedure applications
 
 (define (analyze-application exp)

mceval/syntax.rkt

@@ -83,6 +83,8 @@
 ;; Assignment
 (define (assignment? exp)
   (tagged-list? exp 'set!))
+(define (permanent-assignment? exp)
+  (tagged-list? exp 'permanent-set!))
 (define (assignment-variable exp) (cadr exp))
 (define (assignment-value exp) (caddr exp))
 (define (make-assignment variable value)