On Stream in Scheme

最新推荐文章于 2025-05-06 08:00:00 发布

Lu_ming

最新推荐文章于 2025-05-06 08:00:00 发布

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文章标签： stream scheme lambda newline delay

本文链接：https://blog.youkuaiyun.com/Lu_ming/article/details/7434334

本文介绍了一个使用Scheme语言实现的无限序列生成器，包括素数序列的生成，并通过延迟求值和备忘录技术提高效率。文章展示了如何用递归定义如金钱数列等无限序列，以及如何筛选素数。

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(define (force delayed-object)
(delayed-object))

(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))))
;;(memo-proc (lambda () <exp>))
;;(delay <exp>)
;;is syntactic sugar for
;;(lambda () <exp>)

(define-syntax cons-stream
(lambda (x)
(syntax-case x ()
((_ a b) (syntax (cons a (delay b)))))))

(define-syntax car-stream
(lambda (x)
(syntax-case x ()
((_ s) (syntax (car s))))))

(define-syntax cdr-stream
(lambda (x)
(syntax-case x ()
((_ s) (syntax (force (cdr s)))))))

(define (for-each-stream pred s)
(pred (car-stream s))
(for-each-stream pred (cdr-stream s)))

(define (show-money to s)
(call/cc (lambda (end)
(for-each-stream (lambda (m)
(if (> m to)
(end m))
(display m)
(newline))
s))))

(define (money-tree n)
(cons-stream n (money-tree (* n n))))

(define salary (money-tree 2))

(define (sieve stream)
(cons-stream
(stream-car stream)
(sieve (stream-filter
(lambda (x)
(not (divisible? x (stream-car stream))))
(stream-cdr stream)))))

(define primes (sieve (integers-starting-from 2)))

Now to find a particular prime we need only ask for it:

(stream-ref primes 50)
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