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; Copyright (c) 1993-2008 by Richard Kelsey and Jonathan Rees. See file COPYING.
; Placeholders (single-assignment cells for use with threads)
(define-synchronized-record-type placeholder :placeholder
(really-make-placeholder value queue id)
(value queue) ; synchronize on this
placeholder?
(queue placeholder-queue set-placeholder-queue!) ; #f means VALUE has been set
(value placeholder-real-value set-placeholder-value!)
(id placeholder-id))
(define-record-discloser :placeholder
(lambda (placeholder)
(cons 'placeholder
(if (placeholder-id placeholder)
(list (placeholder-id placeholder))
'()))))
(define (make-placeholder . id-option)
(really-make-placeholder (unspecific)
(make-queue)
(if (null? id-option) #f (car id-option))))
(define (placeholder-value placeholder)
(with-new-proposal (lose)
(let ((queue (placeholder-queue placeholder)))
(if queue
(or (maybe-commit-and-block-on-queue queue)
(lose)))))
(placeholder-real-value placeholder))
(define (placeholder-set! placeholder new-value)
(with-new-proposal (lose)
(let ((queue (placeholder-queue placeholder)))
(cond (queue
(set-placeholder-value! placeholder new-value)
(set-placeholder-queue! placeholder #f)
(or (maybe-commit-and-make-ready queue)
(lose)))
(else
;; We only read queue and value and they are set atomically,
;; so there is no need to commit here.
(error "placeholder is already assigned"
placeholder
(placeholder-real-value placeholder)))))))
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