43 lines
1.1 KiB
Scheme
43 lines
1.1 KiB
Scheme
;;; Appendix A. Standard Libraries Lazy
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(define-library (scheme lazy)
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(import (scheme base)
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(picrin macro))
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(define-record-type <promise>
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(make-promise% done obj)
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promise?
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(done promise-done? promise-done!)
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(obj promise-value promise-value!))
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(define-syntax delay-force
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(ir-macro-transformer
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(lambda (form rename compare?)
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(let ((expr (cadr form)))
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`(make-promise% #f (lambda () ,expr))))))
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(define-syntax delay
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(ir-macro-transformer
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(lambda (form rename compare?)
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(let ((expr (cadr form)))
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`(delay-force (make-promise% #t ,expr))))))
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(define (promise-update! new old)
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(promise-done! old (promise-done? new))
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(promise-value! old (promise-value new)))
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(define (force promise)
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(if (promise-done? promise)
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(promise-value promise)
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(let ((promise* ((promise-value promise))))
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(unless (promise-done? promise)
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(promise-update! promise* promise))
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(force promise))))
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(define (make-promise obj)
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(if (promise? obj)
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obj
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(make-promise% #t obj)))
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(export delay-force delay force make-promise promise?))
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