ref: 518415febfea39229bf8a23ad2435009a8c88147
dir: femtolisp/lib/lazy.scm
; SRFI 45: Primitives for Expressing Iterative Lazy Algorithms
; by André van Tonder
;=========================================================================
; Boxes
(define (box x) (list x))
(define unbox car)
(define set-box! set-car!)
;=========================================================================
; Primitives for lazy evaluation:
(define (eager x)
(box (cons 'eager x)))
#|
(define-syntax lazy
(syntax-rules ()
((lazy exp)
(box (cons 'lazy (lambda () exp))))))
(define-syntax delay
(syntax-rules ()
((delay exp) (lazy (eager exp)))))
|#
(define-macro (lazy exp)
`(box (cons 'lazy (lambda () ,exp))))
(define-macro (delay exp)
`(lazy (eager ,exp)))
(define (force promise)
(let ((content (unbox promise)))
(case (car content)
((eager) (cdr content))
((lazy) (let* ((promise* ((cdr content)))
(content (unbox promise))) ; *
(if (not (eqv? (car content) 'eager)) ; *
(begin (set-car! content (car (unbox promise*)))
(set-cdr! content (cdr (unbox promise*)))
(set-box! promise* content)))
(force promise))))))
; (*) These two lines re-fetch and check the original promise in case
; the first line of the let* caused it to be forced. For an example
; where this happens, see reentrancy test 3 below.