25 lines
759 B
Scheme
25 lines
759 B
Scheme
;; All implementations here are "borrowed" from
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;; husk-scheme (github.com/justinethier/husk-scheme).
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(define (force) "force execution of a delayed function"
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(lambda (object)
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(object)))
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(define-syntax delay
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(syntax-rules ()
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((delay expression)
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(make-promise (lambda () expression)))))
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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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