#lang racket
(define make-stack
  (lambda ()
    (let ([stk '()])
      (lambda (msg . args)
        (case msg
          [(empty?) (null? stk)]
          [(push) (set! stk (cons (car args) stk))]
          [(pop) (if (not (null? stk))
                     (let ([top (car stk)])
                       (set! stk (cdr stk))
                       top)
                     (error "stack is empty"))]
          [else (error 'stack "illegal message to stack object: ~a" msg)])))))




(define lazy_evaluation
  (lambda (t)
    (let ([val #f]
          [flag #f])
      (lambda ()
        (if (not flag)
            (begin (set! val (t))
                   (set! flag #t)
                   (write "Yoohoo")
                   val)
            val)))))

; (define foo (lazy_evaluation (lambda () 3)))





(define make-counter
  (let ([supply 5])
    (lambda ()
      (let ([num 0])
        (lambda ()
          (if (zero? supply) 'supply_empty
              (begin
                (set! num (add1 num))
                (set! supply (sub1 supply))
                (list supply num))))))))

; (define wc1 (make-counter))
; (define wc2 (make-counter))