39 lines
1.1 KiB
Scheme
39 lines
1.1 KiB
Scheme
; SRFI 45
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; Zhu Chongkai mrmathematica@yahoo.com
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; 25-May-2005
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(module lazy mzscheme
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(provide lazy
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eager
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s:delay
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s:force
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srfi-45-promise?)
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(define-struct srfi-45-promise (content))
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(define-syntax lazy
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(syntax-rules ()
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((_ exp)
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(make-srfi-45-promise (mcons #f (lambda () exp))))))
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(define (eager x)
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(make-srfi-45-promise (mcons #t x)))
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(define-syntax s:delay
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(syntax-rules ()
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((_ exp) (lazy (eager exp)))))
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(define (s:force promise)
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(if (srfi-45-promise? promise)
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(let ((content (srfi-45-promise-content promise)))
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(if (mcar content)
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(mcdr content)
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(let* ((promise* ((mcdr content)))
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(content (srfi-45-promise-content promise)))
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(unless (car content)
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(set-mcar! content (mcar (srfi-45-promise-content promise*)))
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(set-mcdr! content (mcdr (srfi-45-promise-content promise*)))
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(set-srfi-45-promise-content! promise* content))
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(s:force promise))))
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(raise-type-error 'force "srfi-45-promise" promise))))
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