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 (define stream-null? null?)
 32 665c255d 2023-08-04 jrmu (define the-empty-stream '())
 33 665c255d 2023-08-04 jrmu 
 34 665c255d 2023-08-04 jrmu (define (integers-starting-from n)
 35 665c255d 2023-08-04 jrmu   (cons-stream n (integers-starting-from (+ n 1))))
 36 665c255d 2023-08-04 jrmu 
 37 665c255d 2023-08-04 jrmu (define (stream-ref s n)
 38 665c255d 2023-08-04 jrmu   (if (= n 0)
 39 665c255d 2023-08-04 jrmu       (stream-car s)
 40 665c255d 2023-08-04 jrmu       (stream-ref (stream-cdr s) (- n 1))))
 41 665c255d 2023-08-04 jrmu (define (stream-map proc s)
 42 665c255d 2023-08-04 jrmu   (if (stream-null? s)
 43 665c255d 2023-08-04 jrmu       the-empty-stream
 44 665c255d 2023-08-04 jrmu       (cons-stream (proc (stream-car s))
 45 665c255d 2023-08-04 jrmu 		   (stream-map proc (stream-cdr s)))))
 46 665c255d 2023-08-04 jrmu (define (stream-for-each proc s)
 47 665c255d 2023-08-04 jrmu   (if (stream-null? s)
 48 665c255d 2023-08-04 jrmu       'done
 49 665c255d 2023-08-04 jrmu       (begin (proc (stream-car s))
 50 665c255d 2023-08-04 jrmu 	     (stream-for-each proc (stream-cdr s)))))
 51 665c255d 2023-08-04 jrmu 
 52 665c255d 2023-08-04 jrmu (define (stream-enumerate-interval low high)
 53 665c255d 2023-08-04 jrmu   (if (> low high)
 54 665c255d 2023-08-04 jrmu       the-empty-stream
 55 665c255d 2023-08-04 jrmu       (cons-stream 
 56 665c255d 2023-08-04 jrmu        low
 57 665c255d 2023-08-04 jrmu        (stream-enumerate-interval (+ low 1) high))))
 58 665c255d 2023-08-04 jrmu (define (stream-filter pred s)
 59 665c255d 2023-08-04 jrmu   (if (stream-null? s)
 60 665c255d 2023-08-04 jrmu       the-empty-stream
 61 665c255d 2023-08-04 jrmu       (let ((scar (stream-car s)))
 62 665c255d 2023-08-04 jrmu 	(if (pred scar)
 63 665c255d 2023-08-04 jrmu 	    (cons-stream scar (stream-filter pred (stream-cdr s)))
 64 665c255d 2023-08-04 jrmu 	    (stream-filter pred (stream-cdr s))))))
 65 665c255d 2023-08-04 jrmu 
 66 665c255d 2023-08-04 jrmu (define (display-stream s)
 67 665c255d 2023-08-04 jrmu   (stream-for-each display-line s))
 68 665c255d 2023-08-04 jrmu (define (display-line x)
 69 665c255d 2023-08-04 jrmu   (newline)
 70 665c255d 2023-08-04 jrmu   (display x))
 71 665c255d 2023-08-04 jrmu 
 72 665c255d 2023-08-04 jrmu (define (test-case actual expected)
 73 665c255d 2023-08-04 jrmu   (newline)
 74 665c255d 2023-08-04 jrmu   (display "Actual:   ")
 75 665c255d 2023-08-04 jrmu   (display actual)
 76 665c255d 2023-08-04 jrmu   (newline)
 77 665c255d 2023-08-04 jrmu   (display "Expected: ")
 78 665c255d 2023-08-04 jrmu   (display expected)
 79 665c255d 2023-08-04 jrmu   (newline))
 80 665c255d 2023-08-04 jrmu 
 81 665c255d 2023-08-04 jrmu ;; Exercise 3.52.  Consider the sequence of expressions
 82 665c255d 2023-08-04 jrmu 
 83 665c255d 2023-08-04 jrmu (define sum 0)
 84 665c255d 2023-08-04 jrmu (define (accum x)
 85 665c255d 2023-08-04 jrmu   (set! sum (+ x sum))
 86 665c255d 2023-08-04 jrmu   sum)
 87 665c255d 2023-08-04 jrmu (define seq (stream-map accum (stream-enumerate-interval 1 20)))
 88 665c255d 2023-08-04 jrmu (test-case sum 1)
 89 665c255d 2023-08-04 jrmu (define y (stream-filter even? seq))
 90 665c255d 2023-08-04 jrmu (test-case sum 6)
 91 665c255d 2023-08-04 jrmu (define z (stream-filter (lambda (x) (= (remainder x 5) 0)) 
 92 665c255d 2023-08-04 jrmu 			 seq))
 93 665c255d 2023-08-04 jrmu (test-case sum 10)
 94 665c255d 2023-08-04 jrmu (stream-ref y 7)
 95 665c255d 2023-08-04 jrmu ;; 1 3 6 10 15 21 28 36 45 55 66 78 91 105 120 136
 96 665c255d 2023-08-04 jrmu (test-case sum 136)
 97 665c255d 2023-08-04 jrmu (display-stream z)
 98 665c255d 2023-08-04 jrmu ;; 1 3 6 10 15 21 28 36 45 55 66 78 91 105 120 136 153 171 190 210
 99 665c255d 2023-08-04 jrmu (newline)
100 665c255d 2023-08-04 jrmu (display '(10 15 45 55 105 120 190 210))
101 665c255d 2023-08-04 jrmu 
102 665c255d 2023-08-04 jrmu ;; What is the value of sum after each of the above expressions is evaluated? What is the printed response to evaluating the stream-ref and display-stream expressions? Would these responses differ if we had implemented (delay <exp>) simply as (lambda () <exp>) without using the optimization provided by memo-proc ? Explain. 
103 665c255d 2023-08-04 jrmu 
104 665c255d 2023-08-04 jrmu (define (memo-proc proc)
105 665c255d 2023-08-04 jrmu   proc) ;; get rid of the optimization
106 665c255d 2023-08-04 jrmu (define sum 0)
107 665c255d 2023-08-04 jrmu (define (accum x)
108 665c255d 2023-08-04 jrmu   (set! sum (+ x sum))
109 665c255d 2023-08-04 jrmu   sum)
110 665c255d 2023-08-04 jrmu (define seq (stream-map accum (stream-enumerate-interval 1 20)))
111 665c255d 2023-08-04 jrmu (test-case sum 1)
112 665c255d 2023-08-04 jrmu (define y (stream-filter even? seq))
113 665c255d 2023-08-04 jrmu (test-case sum 6)
114 665c255d 2023-08-04 jrmu (define z (stream-filter (lambda (x) (= (remainder x 5) 0)) 
115 665c255d 2023-08-04 jrmu 			 seq))
116 665c255d 2023-08-04 jrmu ;; 1 8 11 15
117 665c255d 2023-08-04 jrmu (test-case sum 15)
118 665c255d 2023-08-04 jrmu (stream-ref y 7)
119 665c255d 2023-08-04 jrmu ;; y
120 665c255d 2023-08-04 jrmu ;; *6 19 *24 *30 37 45 *54 *64 75 87 *100 *114 129 145 *162
121 665c255d 2023-08-04 jrmu (test-case sum 162)
122 665c255d 2023-08-04 jrmu (display-stream z)
123 665c255d 2023-08-04 jrmu ;; *15 167 173 *180 188 197 207 218 *230 243 257 272 288 *305 323 342 362
124 665c255d 2023-08-04 jrmu (newline)
125 665c255d 2023-08-04 jrmu (display '(15 180 230 305))
126 665c255d 2023-08-04 jrmu