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Ninety-Nine Lisp Problems - P09 の解答

lisp, gauche | 23:16 |

L-99: Ninety-Nine Lisp Problemsの問題を解答してみたよ。なんか、エレガントではないなぁ:

;; P09 (**) Pack consecutive duplicates of list elements into sublists.
;;     If a list contains repeated elements they should be placed in separate sublists.
;;
;;     Example:
;;     * (pack '(a a a a b c c a a d e e e e))
;;     ((A A A A) (B) (C C) (A A) (D) (E E E E))

(define (pack-seed front sublist result l)
  (cond
   [(null? l) (append result (list sublist))]
   [(eq? front (car l)) (pack-seed front
				   (append sublist
					   (list front))
				   result
				   (cdr l))]
   [else (pack-seed (car l)
		    (list (car l))
		    (if (null? result)
			(if (null? sublist)
			    '()
			    (append result (list sublist)))
			(append result (list sublist)))
		    (cdr l))]))

(define (pack l)
  (pack-seed #f '() '() l))

(pack '(a a a a b c c a a d e e e e))