48 lines
1.4 KiB
Scheme
48 lines
1.4 KiB
Scheme
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; SRFI 45: Primitives for Expressing Iterative Lazy Algorithms
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; by André van Tonder
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;=========================================================================
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; Boxes
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(define (box x) (list x))
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(define unbox car)
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(define set-box! set-car!)
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;=========================================================================
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; Primitives for lazy evaluation:
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(define (eager x)
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(box (cons 'eager x)))
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#|
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(define-syntax lazy
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(syntax-rules ()
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((lazy exp)
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(box (cons 'lazy (lambda () exp))))))
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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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|#
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(define-macro (lazy exp)
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`(box (cons 'lazy (lambda () ,exp))))
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(define-macro (delay exp)
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`(lazy (eager ,exp)))
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(define (force promise)
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(let ((content (unbox promise)))
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(case (car content)
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((eager) (cdr content))
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((lazy) (let* ((promise* ((cdr content)))
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(content (unbox promise))) ; *
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(if (not (eqv? (car content) 'eager)) ; *
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(begin (set-car! content (car (unbox promise*)))
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(set-cdr! content (cdr (unbox promise*)))
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(set-box! 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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