Add exercise 4.38

mceval/amb-utilities.rkt

@@ -14,5 +14,11 @@
       (amb (car items) (an-element-of (cdr items))))
     
     (define (an-integer-starting-from n)
-      (amb n (an-integer-starting-from (+ n 1))))))
+      (amb n (an-integer-starting-from (+ n 1))))
+
+    (define (distinct? items)
+      (cond ((null? items) true)
+            ((null? (cdr items)) true)
+            ((member (car items) (cdr items)) false)
+            (else (distinct? (cdr items)))))))
 

mceval/multiple-dwelling.rkt

@@ -0,0 +1,64 @@
+#lang sicp
+
+(#%require "ambeval.rkt")
+
+(define (multiple-dwelling)
+  (let ((baker (amb 1 2 3 4 5))
+        (cooper (amb 1 2 3 4 5))
+        (fletcher (amb 1 2 3 4 5))
+        (miller (amb 1 2 3 4 5))
+        (smith (amb 1 2 3 4 5)))
+    (require
+     (distinct? (list baker cooper fletcher miller smith)))
+    (require (not (= baker 5)))
+    (require (not (= cooper 1)))
+    (require (not (= fletcher 5)))
+    (require (not (= fletcher 1)))
+    (require (> miller cooper))
+    ;; (require (not (= (abs (- smith fletcher)) 1)))
+    (require (not (= (abs (- fletcher cooper)) 1)))
+    (list (list 'baker baker)
+          (list 'cooper cooper)
+          (list 'fletcher fletcher)
+          (list 'miller miller)
+          (list 'smith smith))))
+
+;; Exercise 4.38: without the constraint that smith and fletcher do
+;; not live on adjacent floors, there are 5 solutions:
+
+;;; Amb-Eval input:
+(multiple-dwelling)
+
+;;; Starting a new problem 
+;;; Amb-Eval value:
+((baker 1) (cooper 2) (fletcher 4) (miller 3) (smith 5))
+
+;;; Amb-Eval input:
+try-again
+
+;;; Amb-Eval value:
+((baker 1) (cooper 2) (fletcher 4) (miller 5) (smith 3))
+
+;;; Amb-Eval input:
+try-again
+
+;;; Amb-Eval value:
+((baker 1) (cooper 4) (fletcher 2) (miller 5) (smith 3))
+
+;;; Amb-Eval input:
+try-again
+
+;;; Amb-Eval value:
+((baker 3) (cooper 2) (fletcher 4) (miller 5) (smith 1))
+
+;;; Amb-Eval input:
+try-again
+
+;;; Amb-Eval value:
+((baker 3) (cooper 4) (fletcher 2) (miller 5) (smith 1))
+
+;;; Amb-Eval input:
+try-again
+
+;;; There are no more values of
+(multiple-dwelling)