#lang racket

  ;;; balance-global.rkt
  ;;; ==================
  ;;; A naive implementation of a back-account using a global
  ;;; variable *balance* to store the bank account's balance.

  (provide
    *balance*
    deposit
    withdraw)

  ;;; *balance* : nat?
  (define *balance* 100)  ;; opening balance in account

  ;;; deposit : nat? -> nat?   
  (define (deposit m)   ;; deposit an amount m into the account
    (set! *balance* (+ *balance* m)))

  ;;; withdraw : nat? -> void?   
  (define (withdraw m)
    (if (< *balance* m)
      (error 'account "balance ~a less than  requested withdrawal ~a" *balance* m)
      (set! *balance* (- *balance* m))))

(set! *balance* -500)