PPL (201-1289101)
Message passing (standard example)
;; The standard "account" object
;; This is the typical example of message-passing.
;;
(define (make-account balance)
(define (withdraw amount)
(if (>= balance amount)
(begin (set! balance (- balance amount))
balance)
"Insufficient funds"))
(define (deposit amount)
(set! balance (+ balance amount))
balance)
(define (dispatch m)
(cond ((eq? m 'withdraw) withdraw)
((eq? m 'deposit) deposit)
(else (error "Unknown request -- MAKE-ACCOUNT"
m))))
dispatch)
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;; That's all folks
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Last modified May 27th, 1997
Michael Elhadad