#lang racket

(define balance 100)

;; -----------------------------------------------------------------
;; Step 0 toward a bank account
;; -----------------------------------------------------------------

(define withdraw
  (lambda (amount)
    (if (>= balance amount)         ;; balance is free
        (begin (define balance (- balance amount))
               balance)
        (error "Insufficient funds" balance))))

;; -----------------------------------------------------------------
;; Step 1: use set! to modify existing object
;; Step 2: make balance a local variable
;; Step 3: swap the lambda and the let
;; Step 4: convert the let into a lambda
;; -----------------------------------------------------------------