#lang racket

; example of implementing your own delay/force
; a promise is a mutable pair
; if the first is 0, the second is a lambda () that when evaluated 
;   gives the value of the promise
; if the first is 1, the second is the actual value of the promise
;
; 
(define (pdelay x) (mcons 0 (lambda () x)))
(define (pforce promise) 
    (if (= 1 (mcar promise)) 
        ; already got value memoized
        (mcdr promise) 
        ; evaluate the promise and memoize it
        (begin (set-mcar! promise 1) 
               (set-mcdr! promise ((mcdr promise))) 
               (mcdr promise))
    )
)