1 665c255d 2023-08-04 jrmu (define (memo-proc proc)
 2 665c255d 2023-08-04 jrmu   (let ((already-run? false) (result false))
 3 665c255d 2023-08-04 jrmu     (lambda ()
 4 665c255d 2023-08-04 jrmu       (if already-run?
 5 665c255d 2023-08-04 jrmu 	  result
 6 665c255d 2023-08-04 jrmu 	  (begin (set! already-run? true)
 7 665c255d 2023-08-04 jrmu 		 (set! result (proc))
 8 665c255d 2023-08-04 jrmu 		 result)))))
 9 665c255d 2023-08-04 jrmu 
10 665c255d 2023-08-04 jrmu (define-syntax mydelay
11 665c255d 2023-08-04 jrmu   (rsc-macro-transformer
12 665c255d 2023-08-04 jrmu    (let ((xfmr
13 665c255d 2023-08-04 jrmu 	  (lambda (exp)
14 665c255d 2023-08-04 jrmu 	    `(memo-proc (lambda () ,exp)))))
15 665c255d 2023-08-04 jrmu      (lambda (e r)
16 665c255d 2023-08-04 jrmu        (apply xfmr (cdr e))))))
17 665c255d 2023-08-04 jrmu 
18 665c255d 2023-08-04 jrmu (define (myforce delayed-object)
19 665c255d 2023-08-04 jrmu   (delayed-object))
20 665c255d 2023-08-04 jrmu 
21 665c255d 2023-08-04 jrmu (define-syntax cons-stream
22 665c255d 2023-08-04 jrmu   (rsc-macro-transformer
23 665c255d 2023-08-04 jrmu    (let ((xfmr (lambda (x y) `(cons ,x (mydelay ,y)))))
24 665c255d 2023-08-04 jrmu      (lambda (e r)
25 665c255d 2023-08-04 jrmu        (apply xfmr (cdr e))))))
26 665c255d 2023-08-04 jrmu 
27 665c255d 2023-08-04 jrmu (define (stream-car s)
28 665c255d 2023-08-04 jrmu   (car s))
29 665c255d 2023-08-04 jrmu (define (stream-cdr s)
30 665c255d 2023-08-04 jrmu   (myforce (cdr s)))
31 665c255d 2023-08-04 jrmu 
32 665c255d 2023-08-04 jrmu (define (integers-starting-from n)
33 665c255d 2023-08-04 jrmu   (cons-stream n (integers-starting-from (+ n 1))))
34 665c255d 2023-08-04 jrmu 
35 665c255d 2023-08-04 jrmu (define (stream-ref s n)
36 665c255d 2023-08-04 jrmu   (if (= n 0)
37 665c255d 2023-08-04 jrmu       (stream-car s)
38 665c255d 2023-08-04 jrmu       (stream-ref (stream-cdr s) (- n 1))))
39 665c255d 2023-08-04 jrmu (define (stream-map proc s)
40 665c255d 2023-08-04 jrmu   (if (stream-null? s)
41 665c255d 2023-08-04 jrmu       the-empty-stream
42 665c255d 2023-08-04 jrmu       (cons-stream (proc (stream-car s))
43 665c255d 2023-08-04 jrmu 		   (stream-map proc (stream-cdr s)))))
44 665c255d 2023-08-04 jrmu (define (stream-for-each proc s)
45 665c255d 2023-08-04 jrmu   (if (stream-null? s)
46 665c255d 2023-08-04 jrmu       'done
47 665c255d 2023-08-04 jrmu       (begin (proc (stream-car s))
48 665c255d 2023-08-04 jrmu 	     (stream-for-each proc (stream-cdr s)))))
49 665c255d 2023-08-04 jrmu 
50 665c255d 2023-08-04 jrmu (define (display-stream s)
51 665c255d 2023-08-04 jrmu   (stream-for-each display-line s))
52 665c255d 2023-08-04 jrmu (define (display-line x)
53 665c255d 2023-08-04 jrmu   (newline)
54 665c255d 2023-08-04 jrmu   (display x))
55 665c255d 2023-08-04 jrmu 
56 665c255d 2023-08-04 jrmu (define (test-case actual expected)
57 665c255d 2023-08-04 jrmu   (newline)
58 665c255d 2023-08-04 jrmu   (display "Actual:   ")
59 665c255d 2023-08-04 jrmu   (display actual)
60 665c255d 2023-08-04 jrmu   (newline)
61 665c255d 2023-08-04 jrmu   (display "Expected: ")
62 665c255d 2023-08-04 jrmu   (display expected)
63 665c255d 2023-08-04 jrmu   (newline))
64 665c255d 2023-08-04 jrmu 
65 665c255d 2023-08-04 jrmu 
66 665c255d 2023-08-04 jrmu (define counting-numbers (integers-starting-from 1))
67 665c255d 2023-08-04 jrmu (test-case (stream-car (stream-cdr (stream-cdr counting-numbers))) 3)