;emmulating OO programing using non-functional features of Scheme
(define (BankAccount balance)
  (define (getBalance) balance)
  (define (deposit amt)
    (set! balance (+ balance amt)))
  (define (withdraw amt)
    (set! balance (- balance amt)))
  (lambda (message)
    (cond
      ((eq? message 'getBalance) getBalance)
      ((eq? message 'deposit) deposit)
      ((eq? message 'withdraw) withdraw)
      (else (error "unknown message" message)))))

;examples from the slides
> (define acct1 (BankAccount 50))
> (define acct2 (BankAccount 100))
> ((acct1 'getbalance))
50
> ((acct2 'getbalance))
100
> ((acct1 'withdraw) 40)
> ((acct2 'deposit) 50)
> ((acct1 'getbalance))
10
> ((acct2 'getbalance))
150
> ((acct1 'setbalance) 100)