72 lines
2.6 KiB
Scheme
72 lines
2.6 KiB
Scheme
#!r6rs
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;; Copyright André van Tonder. All Rights Reserved.
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;;
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;; Permission is hereby granted, free of charge, to any person
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;; obtaining a copy of this software and associated documentation
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;; files (the "Software"), to deal in the Software without
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;; restriction, including without limitation the rights to use, copy,
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;; modify, merge, publish, distribute, sublicense, and/or sell copies
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;; of the Software, and to permit persons to whom the Software is
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;; furnished to do so, subject to the following conditions:
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;;
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;; The above copyright notice and this permission notice shall be
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;; included in all copies or substantial portions of the Software.
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;;
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;; THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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;; EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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;; MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
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;; NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
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;; BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
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;; ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
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;; CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
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;; SOFTWARE.
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;; Modified by Andreas Rottmann to use records instead of mutable pairs.
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(library (srfi :45 lazy)
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(export delay
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lazy
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force
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eager)
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(import (rnrs base)
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(rnrs records syntactic))
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(define-record-type promise
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(fields (mutable val)))
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(define-record-type value
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(fields (mutable tag)
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(mutable proc)))
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(define-syntax lazy
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(syntax-rules ()
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((lazy exp)
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(make-promise (make-value 'lazy (lambda () exp))))))
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(define (eager x)
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(make-promise (make-value 'eager x)))
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(define-syntax delay
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(syntax-rules ()
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((delay exp) (lazy (eager exp)))))
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(define (force promise)
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(let ((content (promise-val promise)))
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(case (value-tag content)
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((eager) (value-proc content))
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((lazy) (let* ((promise* ((value-proc content)))
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(content (promise-val promise))) ; *
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(if (not (eqv? (value-tag content) 'eager)) ; *
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(begin (value-tag-set! content
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(value-tag (promise-val promise*)))
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(value-proc-set! content
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(value-proc (promise-val promise*)))
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(promise-val-set! promise* content)))
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(force promise))))))
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;; (*) These two lines re-fetch and check the original promise in case
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;; the first line of the let* caused it to be forced. For an example
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;; where this happens, see reentrancy test 3 below.
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)
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