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 
32 (define (integers-starting-from n)
33   (cons-stream n (integers-starting-from (+ n 1))))
34 
35 (define (stream-ref s n)
36   (if (= n 0)
37       (stream-car s)
38       (stream-ref (stream-cdr s) (- n 1))))
39 (define (stream-map proc s)
40   (if (stream-null? s)
41       the-empty-stream
42       (cons-stream (proc (stream-car s))
43 		   (stream-map proc (stream-cdr s)))))
44 (define (stream-for-each proc s)
45   (if (stream-null? s)
46       'done
47       (begin (proc (stream-car s))
48 	     (stream-for-each proc (stream-cdr s)))))
49 
50 (define (display-stream s)
51   (stream-for-each display-line s))
52 (define (display-line x)
53   (newline)
54   (display x))
55 
56 (define (test-case actual expected)
57   (newline)
58   (display "Actual:   ")
59   (display actual)
60   (newline)
61   (display "Expected: ")
62   (display expected)
63   (newline))
64 
65 
66 (define counting-numbers (integers-starting-from 1))
67 (test-case (stream-car (stream-cdr (stream-cdr counting-numbers))) 3)