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80 lines
2.2 KiB
Scheme
80 lines
2.2 KiB
Scheme
(define-library (scheme lazy)
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(import (scheme base))
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(export
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delay
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force
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delay-force
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make-promise
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;promise?
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)
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(begin
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; (define (make-promise x)
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; (delay x))
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; ;)
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; ;(begin
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; (define (promise? x)
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; (and (pair? x)
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; (null? (cdr x))
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; (pair? (car x))
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; (or (eq? #t (caar x))
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; (and (eq? #f (caar x))
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; (procedure? (cdar x))))))
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;
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; (define-syntax delay-force
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; (er-macro-transformer
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; (lambda (expr rename compare)
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; `(,(rename 'promise) #f (,(rename 'lambda) () ,(cadr expr))))))
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;
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; (define-syntax delay
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; (er-macro-transformer
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; (lambda (expr rename compare)
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; `(,(rename 'delay-force) (,(rename 'promise) #t ,(cadr expr))))))
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;
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; (define (promise done? proc)
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; (list (cons done? proc)))
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; (define (promise-done? x) (car (car x)))
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; (define (promise-value x) (cdr (car x)))
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; (define (promise-update! new old)
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; (set-car! (car old) (promise-done? new))
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; (set-cdr! (car old) (promise-value new))
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; (set-car! new (car old)))
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; (define (force promise)
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; (if (promise-done? promise)
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; (promise-value promise)
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; (let ((promise* ((promise-value promise))))
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; (if (not (promise-done? promise))
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; (promise-update! promise* promise))
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; (force promise))))))
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;; Delayed evaluation functions from husk
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(define force
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(lambda (object)
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(object)))
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(define-syntax delay
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(er-macro-transformer
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(lambda (expr rename compare)
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`(make-promise (lambda () ,(cadr expr))))))
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(define-syntax delay-force
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(er-macro-transformer
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(lambda (expr rename compare)
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`(make-promise (lambda () ,(cadr expr))))))
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;(define-syntax delay
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; (syntax-rules ()
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; ((delay expression)
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; (make-promise (lambda () expression)))))
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(define make-promise
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(lambda (proc)
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(let ((result-ready? #f)
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(result #f))
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(lambda ()
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(if result-ready?
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result
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(let ((x (proc)))
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(if result-ready?
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result
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(begin (set! result x)
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(set! result-ready? #t)
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result))))))))
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))
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