vallog: 遅延評価はマクロで、多値は継続とマクロで実装できるんですって

2010/05/31

そういえば、伝統的マクロなら継続も実装できるんでしたっけ？

	;; delay, force
	;; Control Operations http://www.scheme.com/tspl4/control.html#./control:h7

	;; (define delay (with-module gauche delay))
	;; (define force (with-module gauche force))

	(define-syntax let/
	(syntax-rules ()
	((_ val (var ...) body ...)
	(let ((v val))
	(let ((var v) ...)
	body ...)))))



	(define-syntax my-delay
	(syntax-rules ()
	((_ expr)
	(make-promise (lambda ()
	expr)))))

	(define (make-promise p)
	(let/ #f (val set?)
	(lambda ()
	(unless set?
	(let ((x (p)))
	(unless set?
	(set! val x)
	(set! set? #t))))
	val)))

	(define (force promise)
	(promise))

	(define (stream-car s)
	(car (my-force s)))

	(define (stream-cdr s)
	(cdr (my-force s)))

	(define counters
	(let next ((n 1))
	(my-delay (cons n (next (+ n 1))))))

	(stream-car counters)
	; -> 1

	(stream-car (stream-cdr counters))
	; -> 2

	(define (stream-add s1 s2)
	(my-delay (cons
	(+ (stream-car s1)
	(stream-car s2))
	(stream-add (stream-cdr s1)
	(stream-cdr s2)))))

	(define even-counters
	(stream-add counters counters))

	(stream-car even-counters)
	; -> 2

	(stream-car (stream-cdr even-counters))
	; -> 4

	;; values
	;; 多値を学ぶ - ひげぽん OSとか作っちゃうかMona- http://d.hatena.ne.jp/higepon/20080608/1212937386

	(define (values . things)
	(call-with-current-continuation
	(lambda (cont) (apply cont things))))

view raw values.scm hosted with ❤ by GitHub

vallog