sicp exercise 3.3

;; Exercise 3.3.  Modify the make-account procedure so that it creates password-protected accounts. That is, make-account should take a symbol as an additional argument, as in

;; (define acc (make-account 100 'secret-password))

;; The resulting account object should process a request only if it is accompanied by the password with which the account was created, and should otherwise return a complaint:

;; ((acc 'secret-password 'withdraw) 40)
;; 60

;; ((acc 'some-other-password 'deposit) 50)
;; "Incorrect password"

(define (make-account balance password)
  (define (wrong-passwd arg) "Incorrect password")
  (define (withdraw amount)
    (if (>= balance amount)
        (begin (set! balance (- balance amount))
               balance)
        "Insufficient funds"))
  (define (deposit amount)
    (set! balance (+ balance amount))
    balance)
  (define (dispatch passwrd m)
    (if (eq? passwrd password)
      (cond ((eq? m 'withdraw) withdraw)
            ((eq? m 'deposit) deposit)
            (else (error "Unknown request -- MAKE-ACCOUNT"
                         m)))
      wrong-passwd))
  dispatch)

(define acc (make-account 100 'secret-password))

(display ((acc 'secret-password 'withdraw) 40)) (newline)
(display ((acc 'secret-password 'withdraw) 40)) (newline)
(display ((acc 'secret-password 'withdraw) 40)) (newline)
(display ((acc 'some-other-password 'deposit) 50)) (newline)