Scheme - オブジェクトおよび状態(set!と代入と局所状態、局所状態変数)

計算機プログラムの構造と解釈
Gerald Jay Sussman, Julie Sussman
Harold Abelson (原著) 和田 英一 (翻訳)

開発環境

• OS X Lion - Apple(OS)
• Emacs、BBEdit - Bare Bones Software, Inc. (Text Editor)
• プログラミング言語: MIT/GNU Scheme

計算機プログラムの構造と解釈(Gerald Jay Sussman(原著)、Julie Sussman(原著)、Harold Abelson(原著)、和田 英一(翻訳)、ピアソンエデュケーション、原書: Structure and Interpretation of Computer Programs (MIT Electrical Engineering and Computer Science)(SICP))の3(標準部品化力, オブジェクトおよび状態)、3.1(代入と局所状態)、3.1.1(局所状態変数)の問題 3.4を解いてみる。

その他参考書籍

• Instructor's Manual to Accompany Structure & Interpretation of Computer Programs

問題 3.4

コード(BBEdit)

sample.scm

(define (make-account balance secret-password)
  (define (withdraw amount)
    (if (>= balance amount)
        (begin (set! balance (- balance amount))
               balance)
        "Insufficient funds"))
  (define (deposit amount)
    (set! balance (+ balance amount))
    balance)
  (define count 0)
  (define (call-the-cops a) "Call the cops!")
  (define (dispatch password m)
    (if (eq? secret-password password)
        (begin (set! count 0)
               (cond ((eq? m 'withdraw) withdraw)
                     ((eq? m 'deposit) deposit)
                     (else (error "Unkown request -- MAKE-ACCOUNT"
                                  m))))
        (begin (set! count (+ count 1))
               (if (> count 6)
                   call-the-cops
                   (lambda (a) "Incorrect password")))))
                   
  dispatch)

(define acc (make-account 100 'secret-password))

入出力結果(Terminal, REPL(Read, Eval, Print, Loop))

1 ]=> ((acc 'some-other-password 'deposit) 50)

;Value 2: "Incorrect password"

1 ]=> ((acc 'some-other-password 'deposit) 50)

;Value 2: "Incorrect password"

1 ]=> ((acc 'some-other-password 'deposit) 50)

;Value 2: "Incorrect password"

1 ]=> ((acc 'some-other-password 'deposit) 50)

;Value 2: "Incorrect password"

1 ]=> ((acc 'some-other-password 'deposit) 50)

;Value 2: "Incorrect password"

1 ]=> ((acc 'some-other-password 'deposit) 50)

;Value 2: "Incorrect password"

1 ]=> ((acc 'secret-password 'withdraw) 40)

;Value: 60

1 ]=> ((acc 'some-other-password 'deposit) 50)

;Value 2: "Incorrect password"

1 ]=> ((acc 'secret-password 'withdraw) 40)

;Value: 20

1 ]=> ((acc 'some-other-password 'deposit) 50)

;Value 2: "Incorrect password"

1 ]=> ((acc 'some-other-password 'deposit) 50)

;Value 2: "Incorrect password"

1 ]=> ((acc 'some-other-password 'deposit) 50)

;Value 2: "Incorrect password"

1 ]=> ((acc 'some-other-password 'deposit) 50)

;Value 2: "Incorrect password"

1 ]=> ((acc 'some-other-password 'deposit) 50)

;Value 2: "Incorrect password"

1 ]=> ((acc 'some-other-password 'deposit) 50)

;Value 2: "Incorrect password"

1 ]=> ((acc 'some-other-password 'deposit) 50)

;Value 3: "Call the cops!"

7回連続じゃないといけないことが確認できた。