#lang racket
(provide make-account)

;; ----------------------------------------------------------------------
;; This account object responds to messages on its own!
;; ----------------------------------------------------------------------

(define make-account        ;; Defines a procedure that creates
  (lambda (balance)    ;; a closure around balance.
    (let ((withdraw               ;; These procedures share
            (lambda (amount)      ;; the balance variable.
              (if (>= balance amount)
                  (begin
                    (set! balance (- balance amount))
                    balance)
                  (error "Insufficient funds" balance))))
          (deposit
            (lambda (amount)
              (set! balance (+ balance amount))
              balance))
          (get-balance
            (lambda ()
              balance)))
      (lambda (transaction . args)       ;; This procedure *applies*
        (case transaction                ;; the "method" corresponding
          ('withdraw                     ;; to the transaction to its
             (apply withdraw args))      ;; arguments.
          ('deposit
             (apply deposit  args))
          ('balance
             (apply get-balance  args))
          (else
             (error "Unknown request -- ACCOUNT" transaction)))))))

;; ----------------------------------------------------------------------
;; How can we make the repeated (apply <selector> args) go away?
;; ----------------------------------------------------------------------