vallog: TSS rember1*, let/cc, try

2010/03/22

rember1*はThe Seasoned Schemerに何度も出てくる練習問題です。

例えば

(rember1* 'meat '((pasta meat) pasta (noodles meat sauce) meat tomatoes))

; -> ((pasta) pasta (noodles meat sauce) meat tomatoes)

というようなものです。

The Seasoned Schemerでは、一般的な再帰やlet/ccを使った例が出てきますが、いまいちです。いまいち、というかすごくわかりにくいです。

取り合えず、以下のコードが14章にでてくるlet/ccとtryを使った例です。

	; rember1*

	(define rm
	(lambda (a l oh)
	(cond
	((null? l)(oh (quote no)))
	((atom? (car l))
	(if (eq? (car l) a)
	(cdr l)
	(cons (car l)
	(rm a (cdr l) oh))))
	(else
	(if (atom?
	(let/cc oh
	(rm a (car l) oh)))
	(cons (car l)
	(rm a (cdr l) oh))
	(cons (rm a (car l) 0)
	(cdr l)))))))

	(let/cc Say
	(rm 'noodles '((food) more (food)) Say))
	; -> no

	(let/cc oh
	(rm 'a '(1 2 3 (4 5 (6)(7)) 8 (9 (10 (((((11 12 13) a))b c ) d)) e)) oh))
	; -> (1 2 3 (4 5 (6) (7)) 8 (9 (10 (((((11 12 13))) b c) d)) e))

	(define rember1*
	(lambda (a l)
	(if (atom? (let/cc oh (rm a l oh)))
	l
	(rm a l (quote ())))))

	(rember1* 'noodles '((food) more (food)))
	; -> ((food) more (food))


	; again
	(define rember1*
	(lambda (a l)
	(let ((new-l (let/cc oh (rm a l oh))))
	(if (atom? new-l)
	l
	new-l))))

	; again
	(define rm
	(lambda (a l oh)
	(cond
	((null? l)(oh (quote no)))
	((atom? (car l))
	(if (eq? (car l) a)
	(cdr l)
	(cons (car l)
	(rm a (cdr l) oh))))
	(else
	(let ((new-car
	(let/cc oh
	(rm a (car l) oh))))
	(if (atom? new-car)
	(cons (car l)
	(rm a (cdr l) oh))
	(cons new-car (cdr l))))))))

	; try
	(define-syntax try
	(syntax-rules ()
	((try var a . b)
	(let/cc success
	(let/cc var (success a)) . b))))


	; again
	(define rember1*
	(lambda (a l)
	(try oh (rm a l oh) l)))

	(rember1* 'noodles '((food) more (food)))
	; -> ((food) more (food))
	(rember1* 'a '(1 2 3 (4 5 (6)(7)) 8 (9 (10 (((((11 12 13) a))b c ) d)) e)))
	; -> (1 2 3 (4 5 (6) (7)) 8 (9 (10 (((((11 12 13))) b c) d)) e))

view raw rember1star.scm hosted with ❤ by GitHub

vallog