Common Lisp の labels を Scheme の衛生的マクロで書いてみる

こんな感じでどうでしょうか。

	;; Common Lisp : labels
	(define-syntax labels
	(syntax-rules ()
	((_ ((name (var)
	proc-body))
	body)
	(letrec
	((name (lambda (var)
	proc-body)))
	body))))
	;; test
	(labels ((fact (n)
	(if (zero? n)
	1
	(* n (fact (- n 1))))))
	(fact 5))
	; -> 120
	(define-syntax labels
	(syntax-rules ()
	((_ ((name (var ...)
	proc-body ...))
	body ...)
	(letrec
	((name (lambda (var ...)
	proc-body ...)))
	body ...))))
	(labels ((fact (n)
	(if (zero? n)
	1
	(* n (fact (- n 1))))))
	(fact 5))
	; -> 120
	;; test
	(labels ((fact (n acc)
	(if (zero? n)
	acc
	(fact (- n 1)(* n acc)))))
	(fact 5 1))
	; -> 120
	;; test
	(labels ((fact (n acc)
	(format #t "n = ~a, acc = ~a\n" n acc)
	(if (zero? n)
	acc
	(fact (- n 1)(* n acc)))))
	(fact 5 1))
	;; test
	(for-each (lambda (e)
	(print e))
	(list (odd? 11)
	(odd? 22)
	(even? 11)
	(even? 22)))
	;; #t
	;; #f
	;; #f
	;; #t
	;; #<undef>
	(labels ((odd? (n)
	(if (zero? n)
	#f
	(even? (- n 1))))
	(even? (n)
	(if (zero? n)
	#t
	(odd? (- n 1)))))
	(for-each (lambda (e)
	(print e))
	(list (odd? 11)
	(odd? 22)
	(even? 11)
	(even? 22))))
	; -> error
	(define-syntax labels
	(syntax-rules ()
	((_ ((name (var ...)
	proc-body ...) ...)
	body ...)
	(letrec
	((name (lambda (var ...)
	proc-body ...)) ...)
	body ...))))
	;; test
	(labels ((my-odd? (n)
	(if (zero? n)
	#f
	(my-even? (- n 1))))
	(my-even? (n)
	(if (zero? n)
	#t
	(my-odd? (- n 1)))))
	(display (list (my-odd? 11)
	(my-odd? 22)
	(my-even? 11)
	(my-even? 22))))
	; -> (#t #f #f #t)#<undef>

view raw labels.scm hosted with ❤ by GitHub

うーん、どうだろ。