Chapter 3, Modularity, Objects, and State
Exercise 3.50
I added required stream preocedures from this answer. Here goes the code:
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(define (stream-map proc . argstreams)
(if (stream-null? (car argstreams))
the-empty-stream
(cons-stream
(apply proc (map stream-car argstreams))
(apply stream-map
(cons proc (map stream-cdr argstreams))))))
(define (stream-enumerate-interval low high)
(if (> low high)
the-empty-stream
(cons-stream
low
(stream-enumerate-interval (+ low 1) high))))
(define (stream-car stream) (car stream))
(define (stream-cdr stream) (force (cdr stream)))
(define (force exp) (exp))
(define-syntax cons-stream
(syntax-rules ()
((_ a b) (cons a (delay b)))))
(define-syntax delay
(syntax-rules ()
((_ exp) (memo-proc (lambda () exp)))))
(define (memo-proc proc)
(let ((already-run? false) (result false))
(lambda ()
(if (not already-run?)
(begin (set! result (proc))
(set! already-run? true)
result)
result))))
Output:
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1 ]=> (define s1 (stream-enumerate-interval 1 5))
;Value: s1
1 ]=> (define s2 (stream-enumerate-interval 1 5))
;Value: s2
1 ]=> (define sum-stream (stream-map + s1 s2))
;Value: sum-stream
1 ]=> (stream-car sum-stream)
;Value: 2
1 ]=> (stream-car (stream-car sum-stream))
;The object 2, passed as the first argument to car, is not the correct type.
;To continue, call RESTART with an option number:
; (RESTART 2) => Specify an argument to use in its place.
; (RESTART 1) => Return to read-eval-print level 1.
2 error> (restart 1)
;Abort!
1 ]=> (stream-car (stream-cdr sum-stream))
;Value: 4
1 ]=> (stream-car (stream-cdr (stream-cdr sum-stream)))
;Value: 6
1 ]=> (stream-car (stream-cdr (stream-cdr (stream-cdr sum-stream))))
;Value: 8
1 ]=> (stream-car (stream-cdr (stream-cdr (stream-cdr (stream-cdr sum-stream)))))
;Value: 10
1 ]=> (stream-car (stream-cdr (stream-cdr (stream-cdr (stream-cdr (stream-cdr sum-stream))))))
;The object (), passed as the first argument to car, is not the correct type.
;To continue, call RESTART with an option number:
; (RESTART 2) => Specify an argument to use in its place.
; (RESTART 1) => Return to read-eval-print level 1.
2 error> (stream-cdr (stream-cdr (stream-cdr (stream-cdr (stream-cdr sum-stream)))))
;Value: ()
1 ]=> (stream-cdr sum-stream)
;Value 4: (4 . #[compound-procedure 5])