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43 lines
1 KiB
Scheme
43 lines
1 KiB
Scheme
(define-library (scheme lazy)
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(import (scheme base))
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(export
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delay
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force
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delay-force
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make-promise
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promise?
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)
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(begin
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(define force
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(lambda (object)
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(object)))
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(define-syntax delay
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(er-macro-transformer
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(lambda (expr rename compare)
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`(make-promise (lambda () ,(cadr expr))))))
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(define-syntax delay-force
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(er-macro-transformer
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(lambda (expr rename compare)
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`(make-promise (lambda () ,(cadr expr))))))
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(define make-promise
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(lambda (proc)
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(let ((result-ready? #f)
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(result #f))
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(lambda ()
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(if result-ready?
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result
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(let ((x (proc)))
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(if result-ready?
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result
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(begin (set! result x)
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(set! result-ready? #t)
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result))))))))
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;; Not a very satisfying implementation, but would need to change
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;; how promises are stored to do better
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(define promise? procedure?)
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))
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