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#lang racket
;; 1
(define (make-accumulator amount)
(lambda (add-this)
(set! amount (+ amount add-this))
amount))
(define A (make-accumulator 5))
(A 10)
(A 15)
(define B (make-accumulator 5))
(B 10)
(B 15)
;; 2
(define (make-monitored fn)
(define calls (make-accumulator 0))
(lambda (arg-or-message)
(cond ((equal? arg-or-message 'how-many-calls?) (calls 0))
((equal? arg-or-message 'reset-count) (set! calls (make-accumulator 0)))
(else (begin
(calls 1)
(fn arg-or-message))))))
(define s (make-monitored sqrt))
(s 100)
(s 'how-many-calls?)
;; 3
(define (make-account balance secret)
(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))))
(define (args->dispatch fn)
(lambda (arg-list)
(let ((password (car arg-list))
(m (cadr arg-list)))
(if (eq? password secret)
(fn m)
(lambda (a . n)
"Wrong password")))))
(define monitored-dispatch (make-monitored (args->dispatch dispatch)))
(define (call-the-cops)
(lambda (a . n)
"Calling the cops"))
(define (safe-dispatch . args)
(if (> 7 (monitored-dispatch 'how-many-calls?))
(monitored-dispatch args)
(call-the-cops)))
safe-dispatch)
(define acc (make-account 100 'secret-password))
((acc 'secret-password 'withdraw) 10)
((acc 'some-other-password 'deposit) 50)
((acc 'some-other-password 'deposit) 50)
((acc 'some-other-password 'deposit) 50)
((acc 'some-other-password 'deposit) 50)
((acc 'some-other-password 'deposit) 50)
((acc 'some-other-password 'deposit) 50)
((acc 'some-other-password 'deposit) 50)
((acc 'some-other-password 'deposit) 50)
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