49 lines
1.3 KiB
Scheme
49 lines
1.3 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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;; type 'a <promise> = cached of 'a | chained of 'a promise | pending of () -> 'a promise
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(define-record-type <promise>
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(promise state value)
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promise?
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(state promise-state set-promise-state!)
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(value promise-value set-promise-value!))
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(define (make-promise obj)
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(if (promise? obj)
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obj
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(promise 'cached obj)))
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(define-syntax delay-force
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(syntax-rules ()
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((_ expr)
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(promise (string->symbol "pending") (lambda () expr)))))
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(define-syntax delay
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(syntax-rules ()
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((_ expr)
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(delay-force (make-promise expr)))))
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(define (force p)
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(let ((v (promise-value p)))
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(case (promise-state p)
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((cached) v)
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((chained) (let ()
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(when (eq? 'cached (promise-state v))
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(set-promise-state! p 'cached)
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(set-promise-value! p (promise-value v)))
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(force v)))
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((pending) (let ((q (v)))
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(when (eq? 'pending (promise-state p))
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(set-promise-state! p 'chained)
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(set-promise-value! p q))
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(force p))))))
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(export delay-force
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delay
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force
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make-promise
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promise?))
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