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+;;;"promise.scm" promise for force and delay
+;;; From Revised^4 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 promise: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>))
+;;; and (define force promise:force)