流计算模式的使用

;系统地将迭代操作方式表示为流过程(define (sqrt-stream x)  (define guesses    (cons-stream 1.0                 (stream-map                  (lambda (guess) (/ (+ guess (/ x guess)) 2))                  guesses)))  guesses)(for-each (lambda (x)            (newline)            (display x))          (show-n (sqrt-stream 2) 10));(define (pi-summands n)  (cons-stream (/ 1.0 n)               (stream-map - (pi-summands (+ n 2)))))(define pi-stream  (scale-stream (partial-sums (pi-summands 1)) 4))(define (display-n stream n)  (for-each (lambda (x)              (newline)              (display x))            (show-n stream n)))(display-n pi-stream 9)(define (euler-transform s)  (let ((s0 (stream-ref s 0))        (s1 (stream-ref s 1))        (s2 (stream-ref s 2)))    (cons-stream (- s2 (/ (square (- s2 s1))                          (+ s0 s2 (- (* 2 s1)))))                 (euler-transform (stream-cdr s)))))(display-n (euler-transform pi-stream) 10)(define (make-tableau transform s)  (cons-stream s               (make-tableau transform                             (transform s))))(define (accelerated-sequence transform s)  (stream-map stream-car              (make-tableau transform s)))(display-n (accelerated-sequence euler-transform pi-stream) 8);3.64(define (stream-limit s tolerance)  (if (< (abs (- (stream-car s) (stream-car (stream-cdr s))))         tolerance)      (stream-car (stream-cdr s))      (stream-limit (stream-cdr s) tolerance)))(define (my-sqrt x tolerance)  (stream-limit (sqrt-stream x) tolerance));3.65(define (ln2-summands n)  (cons-stream (/ 1.0 n) (scale-stream (ln2-summands (+ n 1)) -1)))(define ln2-stream  (partial-sums (ln2-summands 1)))(display-n ln2-stream 10)(display-n (accelerated-sequence euler-transform ln2-stream) 10)