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  1. (define-syntax lazy-cons
  2.   (syntax-rules ()
  3.     ((_ head tail) (cons (delay head) (delay tail)))))
  4.  
  5. (define (lazy-head ll)
  6.   (force (car ll)))
  7.  
  8. (define (lazy-tail ll)
  9.   (force (cdr ll)))
  10.  
  11. (define-syntax lazy-list
  12.   (syntax-rules ()
  13.     ((_ x)      (lazy-cons x '()))
  14.     ((_ x . xs) (lazy-cons x (lazy-list . xs)))))
  15.  
  16. ;;; ----------------------------------------------------------------
  17.  
  18. (define (lazy-take n xs)
  19.   (cond ((<= n 0)
  20.          '())
  21.         ((null? xs)
  22.          '())
  23.         (else
  24.          (lazy-cons (lazy-head xs) (lazy-take (- n 1) (lazy-tail xs))))))
  25.  
  26. (define (lazy-zip f xs ys)
  27.   (cond ((null? xs) '())
  28.         ((null? ys) '())
  29.         (else (lazy-cons (f (lazy-head xs) (lazy-head ys))
  30.                          (lazy-zip f (lazy-tail xs) (lazy-tail ys))))))
  31.  
  32. (define (force-list xs)
  33.   (if (null? xs)
  34.       '()
  35.       (cons (lazy-head xs) (force-list (lazy-tail xs)))))
  36.  
  37. ;;; ----------------------------------------------------------------
  38.  
  39. (define fibs (lazy-cons 0 (lazy-cons 1 (lazy-zip + fibs (lazy-tail fibs)))))
  40.  
  41. (print (force-list (lazy-take 10 fibs)))
Success #stdin #stdout 0.01s 7924KB
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https://ideone.com/ysmTzd
language:
Scheme (chicken 4.13)
created:
3 years ago
visibility:
public

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