syntax-rules: when, unless

define-macroとdefine-syntax。

• Scheme 入門 15. 構文の定義

	;; syntax
	(define-syntax my-when
	(syntax-rules ()
	((_ pred b1 ...)
	(if pred (begin b1 ...)))))
	(my-when (> 0 -1)
	(display "hello")
	(newline))
	;; hello
	;; #<undef>
	(macroexpand
	'(my-when (> 0 -1)
	(display "hello")
	(newline)))
	; -> (#<identifier user#if> (> 0 -1) (#<identifier user#begin> (display "hello") (newline)))
	;; (#<identifier user#if> (> 0 -1)
	;; (#<identifier user#begin>
	;; (display "hello")
	;; (newline)))
	(define-syntax my-unless
	(syntax-rules ()
	((_ pred b1 ...)
	(if (not pred) (begin b1 ...)))))
	(my-unless (> 0 1)
	(display "world")
	(newline))
	;; world
	;; #<undef>
	(macroexpand '(my-unless (> 0 1)
	(display "world")
	(newline)))
	; -> (#<identifier user#if> (#<identifier user#not> (> 0 1)) (#<identifier user#begin> (display "world") (newline)))
	;; (#<identifier user#if> (#<identifier user#not> (> 0 1))
	;; (#<identifier user#begin>
	;; (display "world")
	;; (newline)))

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