SICP 読み (150) 4.1.2 式の表現

ナチュラル全開だったらどうしようと思いつつ、そのまま進めてしまう。

問題 4.10

誤読かもしれませんがとりあえず問題 4.3 なソレを作ってみます。以下が現時点での問題 4.3 な解。

(define (eval exp env)
  (cond ((self-evaluating? exp) exp)
	((variable? exp) (lookup-variable-value exp env))
	((not (pair? exp))
	 (error "Unknown expression type -- EVAL" exp))
	(else
	 (let ((x (get (car exp) 'eval)))
	   (if x
	       (x exp env)
	       (apply (eval (operator exp) env)
		      (list-of-values (operands exp) env)))))))

ちょっと長くなりそう。試験ってできるのかなぁ。

(define (make-if predicate consequent alternative)
  (list 'if predicate consequent alternative))
(define (make-lambda parameters body)
  (cons 'lambda (cons parameters body)))
(define (make-procedure parameters body env)
  )
(define (make-begin seq) (cons 'begin seq))

(define (install-quote)
  (define (eval-quote exp env)
    (cadr exp))
  (put 'quote 'eval eval-quote)
  'done)

(define (install-assignment)
  (define (assignment-variable exp)
    (cadr exp))
  (define (assignment-value exp)
    (caddr exp))
  (define (eval-assignment exp env)
    (set-variable-value! (assignment-variable exp)
			 (eval (assignment-value exp) env)
			 env))
  (put 'set! 'eval eval-assignment)
  'done)

(define (install-definition)
  (define (definition-variable exp)
    (if (symbol? (cadr exp))
	(cadr exp)
	(caadr exp)))
  (define (definition-value exp)
    (if (symbol? (cadr exp))
	(caddr exp)
	(make-lambda (cdadr exp)
		     (cddr exp))))
  (define (eval-definition exp env)
    (define-variable! (definition-variable exp)
      (eval (definition-value exp) env)
      env))
  (put 'define 'eval eval-definition)
  'done)

(define (install-if)
  (define (if-predicate exp) (cadr exp))
  (define (if-consequent exp) (caddr exp))
  (define (if-alternative exp)
    (if (not (null? (cdddr exp)))
	(cadddr exp)
	'false))
  (define (eval-if exp env)
    (if (true? (eval (if-predicate exp) env))
	(eval (if-consequent exp) env)
	(eval (if-alternative exp) env)))
  (put 'if 'eval eval-if)
  'done)

(define (install-lambda)
  (define (lambda-parameters exp)
    (cadr exp))
  (define (lambda-body exp)
    (cddr exp))
  (define (eval-lambda exp env)
    (make-procedure (lambda-parameters exp)
		    (lambda-body exp)
		    env))
  (put 'lambda 'eval eval-lambda)
  'done)

(define (install-begin)
  (define (begin-actions exp)
    (cdr exp))
  (define (last-exp? seq) (null? (cdr seq)))
  (define (first-exp seq) (car seq))
  (define (rest-exps seq) (cdr seq))
  (define (eval-sequence exps env)
    (cond ((last-exp? exps) (eval (first-exp exps) env))
	  (else (eval (first-exp exps) env)
		(eval-sequence (rest-exps exps) env))))
  (define (eval-begin exp env)
    (eval-sequence (begin-actions exp) env))
  (put 'begin 'eval eval-begin)
  'done)

(define (install-cond)
  (define (cond->clauses exp) (cdr exp))
  (define (cond-else-clause? clause)
    (eq? (cond-predicate clause) 'else))
  (define (cond-predicate clause) (car clause))
  (define (cond-actions clause) (cdr clause))

  (define (expand-clauses clauses)
    (if (null? clauses)
	'false
	(let ((first (car clauses))
	      (rest (cdr clauses)))
	  (if (cond-else-clause? first)
	      (if (null? rest)
		  (sequence->exp (cond-actions first))
		  (error "ELSE clause isn't last -- COND->IF" clauses))
	      (make-if (cond-predicate first)
		       (sequence->exp (cond-actions first))
		       (expand-clauses rest))))))

  (define (sequence->exp seq)
    (cond ((null? seq) seq)
	  ((last-exp? seq) (first-exp seq))
	  (else (make-begin seq))))
  (define (cond->if exp)
    (expand-clause (cond-clauses exp)))

  (define (eval-cond exp env)
    (eval (cond->if exp) env))
  (put 'cond 'eval eval-cond)
  'done)

eval をもう一度。

(define (eval exp env)
  (define (list-of-values exps env)
    (if (no-operands? exps)
	'()
	(cons (eval (first-operand exps) env)
	      (list-of-values (rest-operands exps) env))))
  (define (operator exp) (car exp))
  (define (operands exp) (cdr exp))
  (define (no-operands? ops) (null? ops))
  (define (first-operand ops) (car ops))
  (define (rest-operands ops) (cdr ops))
  (define (self-evaluating? exp)
    (cond ((number? exp) true)
	  ((string? exp) true)
	  (else false)))

  (cond ((self-evaluating? exp) exp)
	((variable? exp) (lookup-variable-value exp env))
	((not (pair? exp))
	 (error "Unknown expression type -- EVAL" exp))
	(else
	 (let ((x (get (car exp) 'eval)))
	   (if x
	       (x exp env)
	       (apply (eval (operator exp) env)
		      (list-of-values (operands exp) env)))))))

とりあえず上記のソレをどう試験したものか。

試験

って試験を書きかけたんですが、eval がきちんとデキないと駄目じゃん。