带报警功能的 make-account 定义如下（大部分代码重用自 练习 3.3 ）：
;;; 4-make-account.scm
(define (make-account blance password)
(let ((max-try-times 7)
(try-times 0))
(define (withdraw amount)
(if (>= blance amount)
(begin (set! blance (- blance amount))
blance)
"Insufficient funds"))
(define (deposit amount)
(set! blance (+ blance amount)))
(define (password-match? given-password)
(eq? given-password password))
(define (display-wrong-password-message useless-arg)
(display "Incorrect password"))
(define (dispatch given-password mode)
(if (password-match? given-password)
(begin
(set! try-times 0) ; 成功登录之后清零计数器
(cond ((eq? mode 'withdraw)
withdraw)
((eq? mode 'deposit)
deposit)
(else
(error "Unknow request -- MAKE-ACCOUNT" mode))))
(begin
(set! try-times (+ 1 try-times)) ; 进行计数
(if (>= try-times max-try-times)
(call-the-cops)
display-wrong-password-message))))
dispatch))
(define (call-the-cops)
(error "You try too much times, calling the cops ..."))
测试：
1 ]=> (load "4-make-account.scm")
;Loading "4-make-account.scm"... done
;Value: call-the-cops
1 ]=> (define acc (make-account 100 'secret-password))
;Value: acc
1 ]=> ((acc 'wrong-password 'withdraw) 50)
Incorrect password
;Unspecified return value
1 ]=> ((acc 'wrong-password 'withdraw) 50)
Incorrect password
;Unspecified return value
1 ]=> ((acc 'wrong-password 'withdraw) 50)
Incorrect password
;Unspecified return value
1 ]=> ((acc 'wrong-password 'withdraw) 50)
Incorrect password
;Unspecified return value
1 ]=> ((acc 'wrong-password 'withdraw) 50)
Incorrect password
;Unspecified return value
1 ]=> ((acc 'wrong-password 'withdraw) 50)
Incorrect password
;Unspecified return value
1 ]=> ((acc 'wrong-password 'withdraw) 50)
;You try too much times, calling the cops ...
;To continue, call RESTART with an option number:
; (RESTART 1) => Return to read-eval-print level 1.