Scheme - 標準部品化力、オブジェクト及び状態(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 第2版
原著

開発環境

• macOS Sierra - Apple (OS)
• Emacs (Text Editor)
• Scheme (プログラミング言語)
• kscheme (ksi)(github) (処理系)

計算機プログラムの構造と解釈[第2版](ハロルド エイブルソン (著)、ジュリー サスマン (著)、ジェラルド・ジェイ サスマン (著)、Harold Abelson (原著)、Julie Sussman (原著)、Gerald Jay Sussman (原著)、和田 英一 (翻訳)、翔泳社、原著: Structure and Interpretation of Computer Programs (MIT Electrical Engineering and Computer Science)(SICP))の第3章(標準部品化力、オブジェクト及び状態)、3.1(代入と局所状態)、3.1.1(局所状態変数)、問題3.1、3.2、3.3、3.4.を取り組んでみる。

その他参考書籍

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

問題3.1、3.2、3.3、3.4.

コード(Emacs)

((lambda ()
  (load "procedures.scm")
  (newline)
  (define (p obj) (display obj) (newline))

  (p 3.1)
  (define (make-accumulator sum)
    (lambda (n)
      (set!  sum (+ sum n))
      sum))

  (define A (make-accumulator 5))
  (define B (make-accumulator 5))

  (p (A 10))
  (p (A 10))
  (p (B 10))
  (p (B 10))

  (p 3.2)
  (define (make-monitored f)
    (define counter 0)
    (define (mf o)
      (if (eq? o 'how-many-calls?)
          counter
          (if (eq? o 'reset-count)
              (set! counter 0)
              ((lambda ()
                 (set! counter (+ counter 1))
                 (f o))))))
    mf)

  (define s (make-monitored sqrt))
  (p (s 100))
  (p (s 'how-many-calls?))
  (p (s 100))
  (p (s 'how-many-calls?))
  (s 'reset-count)
  (p (s 'how-many-calls?))
  (p (s 100))
  (p (s 'how-many-calls?))

  (p 3.3)
  (define (make-account balance password)
    (define (withdraw amount)
      (if (>= balance amount)
          ((lambda ()
             (set! balance (- balance amount))
             balance))
          "insufficient funds"))
    (define (deposit amount)
      (set! balance (+ balance amount))
      balance)
    (define (dispatch pwd m)
      (if (eq? pwd password)
          (if (eq? m 'withdraw)
              withdraw
              (if (eq? m 'deposit)
                  deposit
                  (error "Unknown request -- MAKE-ACCOUNT" m)))
          (lambda (o) (p "Incorrect password"))))
    dispatch)

  (define acc (make-account 100 'secret-password))
  (p ((acc 'secret-password 'withdraw) 40))
  (p ((acc 'some-other-password 'deposit) 50))
  
  (p 3.4)
  (define (call-the-cops o) 'call-the-cops)
  (define (make-account balance password)
    (define counter 0)
    (define (withdraw amount)
      (if (>= balance amount)
          ((lambda ()
             (set! balance (- balance amount))
             balance))
          "insufficient funds"))
    (define (deposit amount)
      (set! balance (+ balance amount))
      balance)
    (define (dispatch pwd m)
      (if (eq? pwd password)
          ((lambda ()
             (set! counter 0)
             (if (eq? m 'withdraw)
                 withdraw
                 (if (eq? m 'deposit)
                     deposit
                     (error "Unknown request -- MAKE-ACCOUNT" m)))))
          (if (= counter 6)
              call-the-cops
              ((lambda ()
                 (set! counter (+ counter 1))
                 (lambda (o) (p "Incorrect password")))))))
    dispatch)

  (define acc (make-account 100 'secret-password))
  (p ((acc 'secret-password 'withdraw) 40))
  (p ((acc 'some-other-password 'deposit) 50))
  (p ((acc 'some-other-password 'deposit) 50))
  (p ((acc 'some-other-password 'deposit) 50))
  (p ((acc 'some-other-password 'deposit) 50))
  (p ((acc 'some-other-password 'deposit) 50))
  (p ((acc 'some-other-password 'deposit) 50))
  (p ((acc 'secret-password 'withdraw) 40))  
  (p ((acc 'some-other-password 'deposit) 50))
  (p ((acc 'some-other-password 'deposit) 50))
  (p ((acc 'some-other-password 'deposit) 50))
  (p ((acc 'some-other-password 'deposit) 50))
  (p ((acc 'some-other-password 'deposit) 50))
  (p ((acc 'some-other-password 'deposit) 50))
  (p ((acc 'some-other-password 'deposit) 50))  
  'done))

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

$ ksi < sample1.scm
ksi> 
3.1
15
25
15
25
3.2
10
1
10
2
0
10
1
3.3
60
Incorrect password
#<unspecified>
3.4
60
Incorrect password
#<unspecified>
Incorrect password
#<unspecified>
Incorrect password
#<unspecified>
Incorrect password
#<unspecified>
Incorrect password
#<unspecified>
Incorrect password
#<unspecified>
20
Incorrect password
#<unspecified>
Incorrect password
#<unspecified>
Incorrect password
#<unspecified>
Incorrect password
#<unspecified>
Incorrect password
#<unspecified>
Incorrect password
#<unspecified>
call-the-cops
=> done
ksi> $