filter*

Twitter / のどっち (原村 和): ああ、お題。ネストを潜っていくfilter* を書け ...

ごちゃごちゃ。

	;; filter*
	; fold-right
	(define (filter* pred? ls)
	(fold-right (lambda (e acc)
	(if (pair? e)
	(cons (filter* pred? e)
	acc)
	(if (pred? e)
	(cons e acc)
	acc)))
	'() ls))
	(filter* number? '(1 2 (3 a) (b 4)))
	;; -> (1 2 (3) (4))
	; not fold-right
	(define (filter* pred? l)
	(if (null? l)
	'()
	(let ((kar (car l))
	(kdr (cdr l)))
	(if (pair? kar)
	(cons (filter* pred? kar)
	(filter* pred? kdr))
	(if (pred? kar)
	(cons kar (filter* pred? kdr))
	(filter* pred? kdr))))))
	(filter* number? '(1 2 (3 a) (b 4)))
	;; -> (1 2 (3) (4))
	; again
	(define (filter* pred? l)
	(if (null? l)
	'()
	(let ((kar (car l))
	(kdr (cdr l)))
	(let ((fd (filter* pred? kdr)))
	(if (pair? kar)
	(cons (filter* pred? kar)
	fd)
	(if (pred? kar)
	(cons kar fd)
	fd))))))
	(filter* number? '(1 2 (3 a) (b 4)))
	;; -> (1 2 (3) (4))
	; again letrec
	(define (filter* pred? l)
	(letrec
	((f* (lambda (l)
	(if (null? l)
	'()
	(let ((kar (car l))
	(kdr (cdr l)))
	(let ((fd (f* kdr)))
	(if (pair? kar)
	(cons (f* kar)
	fd)
	(if (pred? kar)
	(cons kar fd)
	fd))))))))
	(f* l)))
	(filter* number? '(1 2 (3 a) (b 4)))
	;; -> (1 2 (3) (4))
	; again named let
	(define (filter* pred? l)
	(let loop ((l l))
	(if (null? l)
	'()
	(let ((kar (car l))
	(kdr (cdr l)))
	(let ((fd (loop kdr)))
	(if (pair? kar)
	(cons (loop kar)
	fd)
	(if (pred? kar)
	(cons kar fd)
	fd)))))))
	(filter* number? '(1 2 (3 a) (b 4)))
	;; -> (1 2 (3) (4))

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