Question

我已经发现从这一课代码在线（http://groups.csail.mit.edu/mac/ftpdir/6.001-fall91/ps4/matcher-from-lecture.scm），并且我有一个赫克一段时间试图调试它。该代码看起来与Sussman所写的相当：

;;; 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))

我在DrRacket中使用R5RS运行它，我遇到的第一个问题是那个原子？是一个未定义的标识符。所以，我发现我可以添加以下内容：

    (define (atom? x) ; atom? is not in a pair or null (empty)
    (and (not (pair? x))
    (not (null? x))))

然后我试图找出如何实际运行这个野兽，所以我再次观看视频并看到他使用以下内容：

(dsimp '(dd (+ x y) x))

如Sussman所述，我应该回来（+ 1 0）。相反，使用R5RS我似乎在扩展字典过程中打破了这一行：

((eq? (cadr v) dat) dictionary)

它返回的具体错误是：mcdr：expect参数类型为mutable-pair;给定#f

当使用neil / sicp时，我在评估程序中打破了这一行：

(apply (eval (lookup (car form) dictionary)
                   user-initial-environment)

它返回的具体错误是：模块中的未绑定标识符：user-initial-environment

所以，尽管如此，我会感谢一些帮助，或者是正确方向的推动。谢谢！

Answer 1

您的代码来自1991年。自R5RS于1998年问世以来，代码必须为R4RS（或更早版本）编写。 R4RS和后来的方案之间的区别之一是，空列表在R4RS中被解释为假，在R5RS中被解释为真。

示例：

  (if '() 1 2)

在R5RS中给出1，在R4RS中给出2。

例如assq等程序因此可以返回'（）而不是false。这就是您需要将extend-directory的定义更改为：

的原因

(define (extend-dictionary pat dat dictionary)
  (let ((vname (variable-name pat)))
    (let ((v (assq vname dictionary)))
      (cond ((not v)
             (cons (list vname dat) dictionary))
            ((eq? (cadr v) dat) dictionary)
            (else 'failed)))))

同样在那些日子里，地图被称为mapcar。只需用地图替换mapcar。

您在DrRacket中看到的错误是：

mcdr: expects argument of type <mutable-pair>; given '()

这意味着cdr得到一个空列表。由于空列表有没有cdr这会给出错误消息。现在DrRacket写了mcdr 而不是cdr，但暂时忽略它。

最佳建议：一次完成一项功能并进行测试 REPL中的一些表达式。这比计算更容易一切都在外面。

最后以：

开始你的程序

(define user-initial-environment (scheme-report-environment 5))

R4RS（或1991年的麻省理工学院计划？）的另一个变化。

<强>附录：

此代码http://pages.cs.brandeis.edu/~mairson/Courses/cs21b/sym-diff.scm几乎已运行。使用以下命令在DrRacket中进行前缀：

#lang r5rs
(define false #f)
(define user-initial-environment (scheme-report-environment 5))
(define mapcar map)

在extend-directory中将（null？v）更改为（不是v）。这至少适用于简单的表达方式。

Answer 2

Here是使用mit-scheme（版本9.1.1）的代码。

Answer 3

您也可以使用this code。它在Racket上运行。

为了无误地运行“eval”，需要添加以下内容

(define ns (make-base-namespace))
(apply (eval '+ ns) '(1 2 3))

运行基于SICP模式匹配规则的替代码

3 个答案: