upscheme/scheme-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.