set!でデータの中身を変更できるようにすると参照透明性が失なわれてデータの同一性が判断できなくなるよ、というお話(かな？)
3.1
(define (make-accumulator acc)
(lambda (n)
(begin
(set! acc (+ acc n))
acc)))
3.2
(define (make-monitored proc)
(let ((calls 0))
(lambda (m)
(cond
((eq? m 'how-many-calls?) calls)
(else
(begin
(set! calls (+ calls 1))
(proc m))))
)
))
3.3 (パスワード付き口座)
(define (make-account balance password)
(define (withdraw amount)
(if (>= balance amount)
(begin (set! balance (- balance amount))
balance)
"Insufficient funds"))
(define (deposit amount)
(set! balance (+ balance amount))
balance)
(define (dispatch p m)
(cond ((not (eq? p password)) (error "Incorrect password"))
((eq? m 'withdraw) withdraw)
((eq? m 'deposit) deposit)
(else (error "Unknown request -- MAKE-ACCOUNT"
m))))
dispatch)
3.4
(define (make-account balance password)
(define (withdraw amount)
(if (>= balance amount)
(begin (set! balance (- balance amount))
balance)
"Insufficient funds"))
(define (deposit amount)
(set! balance (+ balance amount))
balance)
(define (call-the-cops)
(display "('ω`)＜お客さん、こっち来てもらえますか")
(newline))
(let ((times-wrong 0))
(define (dispatch p m)
(if (eq? p password)
(begin
(set! times-wrong 0)
(cond ((eq? m 'withdraw) withdraw)
((eq? m 'deposit) deposit)
(else (error "Unknown request -- MAKE-ACCOUNT"
m))))
(begin
(set! times-wrong (+ times-wrong 1))
(if (>= times-wrong 7)
(call-the-cops)
(error "Incorrect password")))))
dispatch))ここから3.1.2.
rand-updateは http://www.cs.tau.ac.il/~scheme/mavo-fall-2001/Software/SICP/ch3support.scm で定義されているのを。
3.5
(define (random-in-range low high)
(let ((range (- high low)))
(+ low (random range))))
(define (estimate-integral P x1 x2 y1 y2 trials)
(define (region-test)
(P (random-in-range x2 x1)
(random-in-range y2 y1)))
(* (- x1 x2) (- y1 y2) (monte-carlo trials region-test)))
3.6
(define random-init 100)
(define rand
(let ((x random-init))
(lambda (m)
(cond ((eq? m 'generate)
(begin (set! x (rand-update x))
x))
((eq? m 'reset)
(lambda (new-value)
(set! x new-value)
x))
(else
(error "Unknown method" m))
))))(rand 'generate) ; => 59 (rand 'generate) ; => 95 (rand 'generate) ; => 51 (rand 'generate) ; => 6 ((rand 'reset) 100) (rand 'generate) ; => 59 (rand 'generate) ; => 95 (rand 'generate) ; => 51 (rand 'generate) ; => 6
3.7
(define (make-joint account password new-password)
(define (dispatch p m)
(if (not (eq? p new-password))
(error "Incorrect password")
(account password m)))
dispatch)
3.8
引数の評価順序によって結果の異なる関数適用ができてしまう。
(define f
(let ((x #f))
(lambda (n)
(if x
x
(begin (set! x n)
x)))))
