Scheme closure balance example

balance.scm

(define (make-balance)
  (let ((balance 0))
    (define add-method
      (lambda (value) (set! balance (+ balance value))))
    (define get-method
      (lambda () balance))
    (define sub-method
      (lambda (value) 
        (if (<= value balance)
            (set! balance (- balance value))
            (begin
            	(display "INSUFFICIENT FUNDS")
	        (newline)))))
    (lambda (arg . rest)
      (cond ((equal? arg "get") (get-method))
	    ((equal? arg "add") (add-method (car rest)))
	    ((equal? arg "sub") (sub-method (car rest)))
	    (#t (begin (display "UNKNOWN COMMAND") (newline)))))))

;; in the REPL...

> (define balance (make-balance))
> (balance "get")
0
> (balance "add" 10)
> (balance "get")
10
> (balance "add" 1)
> (balance "get")
11
> (balance "sub" 20)
INSUFFICIENT FUNDS
> (balance "whagdakjdadsl" 92)
UNKNOWN COMMAND
> (balance "dlakjdkaljd")
UNKNOWN COMMAND
> (balance "sub" 6)
> (balance "get")
5