問題3-25 (3.3.2 Representing Tables)

任意の次元のテーブルに一般化する!!

(define (make-table)
  (let ((local-table (list '*table*)))
    (define (lookup keys pointer)
      (let ((record (assoc (car keys) (cdr pointer))))
	(if record
	    (if (null? (cdr keys))
		(cdr record)
		(lookup (cdr keys) record))
	    #f)))
    (define (insert! keys value pointer)
      (let ((record (assoc (car keys) (cdr pointer))))
	(if record
	    (if (null? (cdr keys))
		(ser-cdr! record value) ; overwrite
		(insert! (cdr keys) value (cdr record)))
	    (if (null? (cdr keys))
		(set-cdr! pointer
			  (cons (cons (car keys) value)
				(cdr pointer)))
		(let ((new-record (cons (car keys) (cdr pointer))))
		  (set-cdr! pointer
			    (cons new-record 
				  (cdr pointer)))
		  (insert! (cdr keys) value new-record))))))
    (define (dispatch m)
      (cond ((eq? m 'lookup-proc) lookup)
            ((eq? m 'insert-proc!) insert!)
	    ((eq? m 'local-table) local-table)
            (else (error "Unknown operation -- TABLE" m))))
    dispatch))

(define table (make-table))
(define insert! 
  (lambda (keys value)
    ((table 'insert-proc!) keys value (table 'local-table))))
(define lookup
  (lambda (keys)
    ((table 'lookup-proc) keys (table 'local-table))))

(insert! (list 'key-1 'key-2 'key-3) 40) 
(insert! (list 'key-4 'key-5 'key-6 'key-7) 300)
(lookup (list 'key-1 'key-2 'key-3)) ; 40
(lookup (list 'key-4 'key-5 'key-6 'key-7)) ; 300