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 (display-stream s)
53   (stream-for-each display-line s))
54 (define (display-line x)
55   (newline)
56   (display x))
57 
58 (define (test-case actual expected)
59   (newline)
60   (display "Actual:   ")
61   (display actual)
62   (newline)
63   (display "Expected: ")
64   (display expected)
65   (newline))
66 
67 ;; Exercise 3.50.  Complete the following definition, which generalizes stream-map to allow procedures that take multiple arguments, analogous to map in section 2.2.3, footnote 12.
68 
69 (define (stream-map proc . argstreams)
70   (if (stream-null? (car argstreams))
71       the-empty-stream
72       (cons-stream
73        (apply proc (map stream-car argstreams))
74        (apply stream-map
75               (cons proc (map stream-cdr argstreams))))))
76 
77 (define integers (integers-starting-from 1))
78 (test-case (stream-car (stream-cdr (stream-cdr (stream-cdr (stream-map * integers integers))))) 16)