1 (define (memo-proc proc)
2   (let ((already-run? false) (result false))
3     (lambda ()
4       (if already-run?
5 	  result
6 	  (begin (set! already-run? true)
7 		 (set! result (proc))
8 		 result)))))
9 
10 (define-syntax mydelay
11   (rsc-macro-transformer
12    (let ((xfmr
13 	  (lambda (exp)
14 	    `(memo-proc (lambda () ,exp)))))
15      (lambda (e r)
16        (apply xfmr (cdr e))))))
17 
18 (define (myforce delayed-object)
19   (delayed-object))
20 
21 (define-syntax cons-stream
22   (rsc-macro-transformer
23    (let ((xfmr (lambda (x y) `(cons ,x (mydelay ,y)))))
24      (lambda (e r)
25        (apply xfmr (cdr e))))))
26 
27 (define (stream-car s)
28   (car s))
29 (define (stream-cdr s)
30   (myforce (cdr s)))
31 (define stream-null? null?)
32 (define the-empty-stream '())
33 
34 (define (integers-starting-from n)
35   (cons-stream n (integers-starting-from (+ n 1))))
36 
37 (define (stream-ref s n)
38   (if (= n 0)
39       (stream-car s)
40       (stream-ref (stream-cdr s) (- n 1))))
41 (define (stream-map proc s)
42   (if (stream-null? s)
43       the-empty-stream
44       (cons-stream (proc (stream-car s))
45 		   (stream-map proc (stream-cdr s)))))
46 (define (stream-for-each proc s)
47   (if (stream-null? s)
48       'done
49       (begin (proc (stream-car s))
50 	     (stream-for-each proc (stream-cdr s)))))
51 
52 (define (stream-enumerate-interval low high)
53   (if (> low high)
54       the-empty-stream
55       (cons-stream 
56        low
57        (stream-enumerate-interval (+ low 1) high))))
58 
59 (define (display-stream s)
60   (stream-for-each display-line s))
61 (define (display-line x)
62   (newline)
63   (display x))
64 
65 (define (test-case actual expected)
66   (newline)
67   (display "Actual:   ")
68   (display actual)
69   (newline)
70   (display "Expected: ")
71   (display expected)
72   (newline))
73 
74  ;; Exercise 3.51.  In order to take a closer look at delayed evaluation, we will use the following procedure, which simply returns its argument after printing it:
75 
76 (define (show x)
77   (display-line x)
78   x)
79 
80 ;; What does the interpreter print in response to evaluating each expression in the following sequence?59
81 
82 (define x (stream-map show (stream-enumerate-interval 0 10)))
83 ;; 0
84 (newline)
85 (stream-ref x 5)
86 ;; 1->5
87 (newline)
88 (stream-ref x 7)
89 ;; 6->7