练习 3.50

;;; 50-stream-map.scm

(define (stream-map proc . list-of-stream)
    (if (null? (car list-of-stream))
        '()
        (cons-stream
            (apply proc 
                   (map (lambda (s)
                            (stream-car s))
                        list-of-stream))
            (apply stream-map 
                   (cons proc (map (lambda (s)
                                       (stream-cdr s))
                                   list-of-stream))))))

测试：

1 ]=> (load "50-stream-map.scm")

;Loading "50-stream-map.scm"... done
;Value: stream-map

1 ]=> (load "p223-stream-enumerate-interval.scm")

;Loading "p223-stream-enumerate-interval.scm"... done
;Value: stream-enumerate-interval

1 ]=> (define one-to-ten (stream-enumerate-interval 1 10))      ; 一个 1 至 10 的整数序列

;Value: one-to-ten

1 ]=> (stream-map + one-to-ten one-to-ten)                      ; 直接调用还是会返回流

;Value 17: (2 . #[promise 18])

1 ]=> (load "p222-display-stream.scm")                          ; 所以需要 display-stream 的帮助
                                                                ; 将流求值并打印出来
;Loading "p222-display-stream.scm"... done
;Value: display-line

1 ]=> (display-stream (stream-map + one-to-ten one-to-ten))

2
4
6
8
10
12
14
16
18
20
;Unspecified return value

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