Add amb utilities that are loaded at startup

mceval/amb-pythagorean-triples.rkt

@@ -54,3 +54,19 @@
 (define (a-triple-with-sum-from n)
   (amb (a-triple-with-sum-between n (+ n 1))
        (a-triple-with-sum-from (+ n 1))))
+
+;; Exercise 4.37
+
+;; Alternative version from the book.  I think this will be more
+;; efficient because it only has to explore the values of k that are
+;; sums of the squares of i and j, rather than all integers from j up
+;; to high.
+(define (a-pythagorean-triple-between* low high)
+  (let ((i (an-integer-between low high))
+        (hsq (* high high)))
+    (let ((j (an-integer-between i high)))
+      (let ((ksq (+ (* i i) (* j j))))
+        (require (>= hsq ksq))
+        (let ((k (sqrt ksq)))
+          (require (integer? k))
+          (list i j k))))))

mceval/amb-utilities.rkt

@@ -0,0 +1,18 @@
+;; Lisp code to be evaluated by amb evaluator to implement utility procedures.
+
+#lang sicp
+
+(#%provide amb-utilities-program)
+
+(define amb-utilities-program
+
+  '((define (require p)
+      (if (not p) (amb)))
+    
+    (define (an-element-of items)
+      (require (not (null? items)))
+      (amb (car items) (an-element-of (cdr items))))
+    
+    (define (an-integer-starting-from n)
+      (amb n (an-integer-starting-from (+ n 1))))))
+

mceval/ambeval.rkt

@@ -2,7 +2,8 @@
 
 #lang sicp
 
-(#%require "syntax.rkt"
+(#%require "amb-utilities.rkt"
+           "syntax.rkt"
            "environment.rkt"
            "common.rkt"
            "special-forms.rkt")
@@ -192,6 +193,18 @@
 
 (define (driver-loop)
   (define env (setup-environment))
+  ;; Need to pass success and failure continuations to ambeval, so
+  ;; will probably need a different eval-program.
+  (eval-program amb-utilities-program
+                (lambda (exp env)       ; Evaluate exp using ambeval
+                                        ; passing in appropriate
+                                        ; continuations
+                  (ambeval exp
+                           env
+                           (lambda (value fail) value)
+                           (lambda () 'failed)))
+                user-print
+                env)
   (define (internal-loop try-again)
     (prompt-for-input input-prompt)
     (let ((input (read)))