Lambda the Ultimate Actor (2)

メッセージの行き先をreceiverで指定
 (define (make-account total)
   (lambda (msg . args)
     (case msg
       ;; (acct 'withdraw amount receiver)
       ((withdraw) (let ((amount (car args))
                         (receiver (cadr args)))
                     (dec! total amount)
                     (receiver 'deposit amount)))
       ;; (acct 'deposit amount)
       ((deposit)  (inc! total (car args)) #f)
       ;; (acct 'query receiver)
       ((query)    ((car args) total)))))