#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 args 
    ; if no param list, return the counter
    (if (null? args) counter
        (let ( [ op (first args) ] )
        (cond
          [ (eq? op 'get) counter ]
          [ (eq? op 'inc)
            (set! counter (+ 1 counter)) counter ]
          [ (eq? op 'inc-by)
            (set! counter (+ (second args) counter)) counter ]
          [ else (error (format "Invalid operation ~a" op ) ) ]
          ))
        )
  ))

; (define o1 (create-counter 0))
;
; (o1 'inc)
; (o1)
; (o1 'inc-by 2)
; (o1)