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;;;"promise.scm" promise for force and delay
;;; From Revised^5 Report on the Algorithmic Language Scheme
;;; Editors: William Clinger and Jonathon Rees
;
; We intend this report to belong to the entire Scheme community, and so
; we grant permission to copy it in whole or in part without fee. In
; particular, we encourage implementors of Scheme to use this report as
; a starting point for manuals and other documentation, modifying it as
; necessary.
;@
(define force (lambda (object) (object)))
;@
(define make-promise
(lambda (proc)
(let ((result-ready? #f)
(result #f))
(lambda ()
(if result-ready?
result
(let ((x (proc)))
(if result-ready?
result
(begin (set! result-ready? #t)
(set! result x)
result))))))))
;;; change occurences of (DELAY <expression>) to
;;; (MAKE-PROMISE (LAMBDA () <expression>))
;@
(define-syntax delay
(syntax-rules ()
((delay expression)
(make-promise (lambda () expression)))))
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