計算機プログラムの構造と解釈[第2版]
(翔泳社)
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)
開発環境
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、原書: Structure and Interpretation of Computer Programs (MIT Electrical Engineering and Computer Science)(SICP))の第3章(標準部品化力、オブジェクトおよび状態)、3.3(可変データのモデル化)、3.3.5(制約の拡散)、制約システムの使い方、実装、コネクタの表現、問題3.33.を解いてみる。
その他参考書籍
- Instructor's Manual to Accompany Structure & Interpretation of Computer Programs
- プログラミングGauche (Kahuaプロジェクト (著), 川合 史朗 (監修), オライリージャパン)
- Scheme手習い
問題3.33.
コード(Emacs)
(begin
(define print (lambda (x) (display x) (newline)))
;; kscheme に error手続き はみ実装
;; (define (error s obj)
;; (display s)
;; (display " ")
;; (print obj))
(define (adder a1 a2 sum)
(define (process-new-value)
(cond ((and (has-value? a1) (has-value? a2))
(set-value! sum (+ (get-value a1) (get-value a2)) me))
((and (has-value? a1) (has-value? sum))
(set-value! a2 (- (get-value sum) (get-value a1)) me))
((and (has-value? a2) (has-value? sum))
(set-value! a1 (- (get-value sum) (get-value a2)) me))))
(define (process-forget-value)
(forget-value! sum me)
(forget-value! a1 me)
(forget-value! a2 me)
(process-new-value))
(define (me request)
(cond ((eq? request 'I-have-a-value)
(process-new-value))
((eq? request 'I-lost-my-value)
(process-forget-value))
(else (error "Unknown request -- ADDER" request))))
(connect a1 me)
(connect a2 me)
(connect sum me)
me)
(define (inform-about-value constraint)
(constraint 'I-have-a-value))
(define (inform-about-no-value constraint)
(constraint 'I-lost-my-value))
(define (multiplier m1 m2 product)
(define (process-new-value)
(cond ((or (and (has-value? m1) (= (get-value m1) 0))
(and (has-value? m2) (= (get-value m2) 0)))
(set-value! product 0 me))
((and (has-value? m1) (has-value? m2))
(set-value! product (* (get-value m1) (get-value m2)) me))
((and (has-value? product) (has-value? m1))
(set-value! m2 (/ (get-value product) (get-value m1)) me))
((and (has-value? product) (has-value? m2))
(set-value! m1 (/ (get-value product) (get-value m2)) me))))
(define (process-forget-value)
(forget-value! product me)
(forget-value! m1 me)
(forget-value! m2 me)
(process-new-value))
(define (me request)
(cond ((eq? request 'I-have-a-value)
(process-new-value))
((eq? request 'I-lost-my-value)
(process-forget-value))
(else (error "Unknown request -- MULTIPLIER" request))))
(connect m1 me)
(connect m2 me)
(connect product me)
me)
(define (constant value connector)
(define (me request)
(error "Unknown request -- CONSTANT" request))
(connect connector me)
(set-value! connector value me)
me)
(define (probe name connector)
(define (print-probe value)
(newline)
(display "Probe: ")
(display name)
(display " = ")
(display value))
(define (process-new-value)
(print-probe (get-value connector)))
(define (process-forget-value)
(print-probe "?"))
(define (me request)
(cond ((eq? request 'I-have-a-value)
(process-new-value))
((eq? request 'I-lost-my-value)
(process-forget-value))
(else (error "Unknown request -- PROBE" request))))
(connect connector me)
me)
(define (make-connector)
(let ((value #f) (informant #f) (constraints '()))
(define (set-my-value newval setter)
(cond ((not (has-value? me))
(set! value newval)
(set! informant setter)
(for-each-except setter
inform-about-value
constraints))
((not (= value newval))
(error "Contradiction" (list value newval)))
(else 'ignored)))
(define (forget-my-value retractor)
(if (eq? retractor informant)
(begin (set! informant #f)
(for-each-except retractor
inform-about-no-value
constraints))
'ignored))
(define (connect new-constraint)
(if (not (memq new-constraint constraints))
(set! constraints
(cons new-constraint constraints)))
(if (has-value? me)
(inform-about-value new-constraint))
'done)
(define (me request)
(cond ((eq? request 'has-value?) (if informant #t #f))
((eq? request 'value) value)
((eq? request 'set-value!) set-my-value)
((eq? request 'forget) forget-my-value)
((eq? request 'connect) connect)
(else (error "Unknown operation -- CONNECTOR" request))))
me))
(define (for-each-except exception procedure lst)
(define (loop items)
(cond ((null? items) 'done)
((eq? (car items) exception) (loop (cdr items)))
(else (procedure (car items))
(loop (cdr items)))))
(loop lst))
(define (has-value? connector) (connector 'has-value?))
(define (get-value connector) (connector 'value))
(define (set-value! connector new-value informant)
((connector 'set-value!) new-value informant))
(define (forget-value! connector retractor)
((connector 'forget) retractor))
(define (connect connector new-constraint)
((connector 'connect) new-constraint))
(define (averager a b c)
(let ((s (make-connector))
(x (make-connector)))
(adder a b s)
(multiplier s x c)
(constant 0.5 x)
'ok))
(define A (make-connector))
(define B (make-connector))
(define C (make-connector))
(probe "a" A)
(probe "b" B)
(probe "average" C)
(averager A B C)
(set-value! A 5 'user)
(newline)
(set-value! B 10 'user)
(newline)
(forget-value! A 'user)
(newline)
(set-value! C 15 'user)
(newline)
)
入出力結果(Terminal(kscheme), REPL(Read, Eval, Print, Loop))
$ gosh sample33.scm Probe: a = 5 Probe: average = 7.5 Probe: b = 10 Probe: average = ? Probe: a = ? Probe: a = 20.0 Probe: average = 15 $
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