#lang racket

; the constructor for a counter object, returns a closure with
; the initial value of the counter set to init.

(define (create-counter init)
  (define counter init)
  
  ; the lambda captures the local defintion of counter
  (lambda (op)
    (cond
      [ (eq? op 'get) counter ]
      [ (eq? op 'inc)
        (set! counter (+ 1 counter)) counter ]
      [ else (error (format "Invalid operation ~a" op) ) ]
      ))
  )
          
; (define c (create-counter 43))
; (c 'get)
; (c 'inc)
; (c 'get)
; (c 'dec)