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  1. ; van eck sequence
  2.  
  3. (define-syntax define-generator
  4.   (lambda (x)
  5.     (syntax-case x (lambda)
  6.       ((stx name (lambda formals e0 e1 ...))
  7.          (with-syntax ((yield (datum->syntax (syntax stx) 'yield)))
  8.            (syntax (define name
  9.              (lambda formals
  10.                (let ((resume #f) (return #f))
  11.                  (define yield
  12.                    (lambda args
  13.                      (call-with-current-continuation
  14.                       (lambda (cont)
  15.                         (set! resume cont)
  16.                         (apply return args)))))
  17.                  (lambda ()
  18.                    (call-with-current-continuation
  19.                     (lambda (cont)
  20.                       (set! return cont)
  21.                       (cond (resume (resume))
  22.                       (else (let () e0 e1 ...)
  23.                             (error 'name "unexpected return"))))))))))))
  24.         ((stx (name . formals) e0 e1 ...)
  25.           (syntax (stx name (lambda formals e0 e1 ...)))))))
  26.  
  27. (define (take-gen n gen)
  28.   (let loop ((n n) (gs (list)))
  29.     (if (zero? n) (reverse gs)
  30.       (loop (- n 1) (cons (gen) gs)))))
  31.  
  32. (define-generator (van-eck)
  33.   (let loop ((v 0) (i 0) (prev (list)))
  34.     (yield v)
  35.     (let ((t v))
  36.       (let ((v (- i (cond ((assoc v prev) => cdr) (else i)))))
  37.         (loop v (+ i 1) (cons (cons t i) prev))))))
  38.  
  39. (display (take-gen 25 (van-eck)))
Success #stdin #stdout 0.02s 51096KB
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Standard input is empty
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(0 0 1 0 2 0 2 2 1 6 0 5 0 2 6 5 4 0 5 3 0 3 2 9 0)
https://ideone.com/tKuo7T
language:
Scheme (guile 2.2.4)
created:
4 years ago
visibility:
public

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