このレッスンのコードをオンラインで見つけました (http://groups.csail.mit.edu/mac/ftpdir/6.001-fall91/ps4/matcher-from-lecture.scm)。それをデバッグしようとしています。このコードは、Sussman が書いたものとかなり似ています。
;;; Scheme code from the Pattern Matcher lecture
;; Pattern Matching and Simplification
(define (match pattern expression dictionary)
(cond ((eq? dictionary 'failed) 'failed)
((atom? pattern)
(if (atom? expression)
(if (eq? pattern expression)
dictionary
'failed)
'failed))
((arbitrary-constant? pattern)
(if (constant? expression)
(extend-dictionary pattern expression dictionary)
'failed))
((arbitrary-variable? pattern)
(if (variable? expression)
(extend-dictionary pattern expression dictionary)
'failed))
((arbitrary-expression? pattern)
(extend-dictionary pattern expression dictionary))
((atom? expression) 'failed)
(else
(match (cdr pattern)
(cdr expression)
(match (car pattern)
(car expression)
dictionary)))))
(define (instantiate skeleton dictionary)
(cond ((atom? skeleton) skeleton)
((skeleton-evaluation? skeleton)
(evaluate (evaluation-expression skeleton)
dictionary))
(else (cons (instantiate (car skeleton) dictionary)
(instantiate (cdr skeleton) dictionary)))))
(define (simplifier the-rules)
(define (simplify-exp exp)
(try-rules (if (compound? exp)
(simplify-parts exp)
exp)))
(define (simplify-parts exp)
(if (null? exp)
'()
(cons (simplify-exp (car exp))
(simplify-parts (cdr exp)))))
(define (try-rules exp)
(define (scan rules)
(if (null? rules)
exp
(let ((dictionary (match (pattern (car rules))
exp
(make-empty-dictionary))))
(if (eq? dictionary 'failed)
(scan (cdr rules))
(simplify-exp (instantiate (skeleton (car rules))
dictionary))))))
(scan the-rules))
simplify-exp)
;; Dictionaries
(define (make-empty-dictionary) '())
(define (extend-dictionary pat dat dictionary)
(let ((vname (variable-name pat)))
(let ((v (assq vname dictionary)))
(cond ((null? v)
(cons (list vname dat) dictionary))
((eq? (cadr v) dat) dictionary)
(else 'failed)))))
(define (lookup var dictionary)
(let ((v (assq var dictionary)))
(if (null? v)
var
(cadr v))))
;; Expressions
(define (compound? exp) (pair? exp))
(define (constant? exp) (number? exp))
(define (variable? exp) (atom? exp))
;; Rules
(define (pattern rule) (car rule))
(define (skeleton rule) (cadr rule))
;; Patterns
(define (arbitrary-constant? pattern)
(if (pair? pattern) (eq? (car pattern) '?c) false))
(define (arbitrary-expression? pattern)
(if (pair? pattern) (eq? (car pattern) '? ) false))
(define (arbitrary-variable? pattern)
(if (pair? pattern) (eq? (car pattern) '?v) false))
(define (variable-name pattern) (cadr pattern))
;; Skeletons & Evaluations
(define (skeleton-evaluation? skeleton)
(if (pair? skeleton) (eq? (car skeleton) ':) false))
(define (evaluation-expression evaluation) (cadr evaluation))
;; Evaluate (dangerous magic)
(define (evaluate form dictionary)
(if (atom? form)
(lookup form dictionary)
(apply (eval (lookup (car form) dictionary)
user-initial-environment)
(mapcar (lambda (v) (lookup v dictionary))
(cdr form)))))
;;
;; A couple sample rule databases...
;;
;; Algebraic simplification
(define algebra-rules
'(
( ((? op) (?c c1) (?c c2)) (: (op c1 c2)) )
( ((? op) (? e ) (?c c )) ((: op) (: c) (: e)) )
( (+ 0 (? e)) (: e) )
( (* 1 (? e)) (: e) )
( (* 0 (? e)) 0 )
( (* (?c c1) (* (?c c2) (? e ))) (* (: (* c1 c2)) (: e)) )
( (* (? e1) (* (?c c ) (? e2))) (* (: c ) (* (: e1) (: e2))) )
( (* (* (? e1) (? e2)) (? e3)) (* (: e1) (* (: e2) (: e3))) )
( (+ (?c c1) (+ (?c c2) (? e ))) (+ (: (+ c1 c2)) (: e)) )
( (+ (? e1) (+ (?c c ) (? e2))) (+ (: c ) (+ (: e1) (: e2))) )
( (+ (+ (? e1) (? e2)) (? e3)) (+ (: e1) (+ (: e2) (: e3))) )
( (+ (* (?c c1) (? e)) (* (?c c2) (? e))) (* (: (+ c1 c2)) (: e)) )
( (* (? e1) (+ (? e2) (? e3))) (+ (* (: e1) (: e2))
(* (: e1) (: e3))) )
))
(define algsimp (simplifier algebra-rules))
;; Symbolic Differentiation
(define deriv-rules
'(
( (dd (?c c) (? v)) 0 )
( (dd (?v v) (? v)) 1 )
( (dd (?v u) (? v)) 0 )
( (dd (+ (? x1) (? x2)) (? v)) (+ (dd (: x1) (: v))
(dd (: x2) (: v))) )
( (dd (* (? x1) (? x2)) (? v)) (+ (* (: x1) (dd (: x2) (: v)))
(* (dd (: x1) (: v)) (: x2))) )
( (dd (** (? x) (?c n)) (? v)) (* (* (: n) (+ (: x) (: (- n 1))))
(dd (: x) (: v))) )
))
(define dsimp (simplifier deriv-rules))
(define scheme-rules
'(( (square (?c n)) (: (* n n)) )
( (fact 0) 1 )
( (fact (?c n)) (* (: n) (fact (: (- n 1)))) )
( (fib 0) 0 )
( (fib 1) 1 )
( (fib (?c n)) (+ (fib (: (- n 1)))
(fib (: (- n 2)))) )
( ((? op) (?c e1) (?c e2)) (: (op e1 e2)) ) ))
(define scheme-evaluator (simplifier scheme-rules))
DrRacket で R5RS を使用して実行していますが、最初に遭遇した問題はそのアトムでしたか? 未定義の識別子でした。そのため、次を追加できることがわかりました。
(define (atom? x) ; atom? is not in a pair or null (empty)
(and (not (pair? x))
(not (null? x))))
次に、この獣を実際に実行する方法を理解しようとしたので、ビデオをもう一度見て、彼が次のように使用しているのを見ました。
(dsimp '(dd (+ x y) x))
Sussman が述べたように、(+ 1 0) に戻る必要があります。代わりに、R5RS を使用すると、次の行で拡張辞書プロシージャが壊れているようです。
((eq? (cadr v) dat) dictionary)
返される特定のエラーは次のとおりです。 mcdr: 可変ペア型の引数が必要です。与えられた #f
neil/sicp を使用する場合、次の行で評価手順を中断しています。
(apply (eval (lookup (car form) dictionary)
user-initial-environment)
返される特定のエラーは次のとおりです: モジュール内のバインドされていない識別子: ユーザー初期環境
以上のことを踏まえて、私は何らかの助け、または正しい方向への良いナッジをいただければ幸いです。ありがとう！