[plt-scheme] Null syntax / Conditional definition

From: Jordan Johnson (jorjohns at cs.indiana.edu)
Date: Sat Jun 18 18:20:17 EDT 2005

On Saturday, June 18, 2005, at 01:17  PM, Mike wrote:
>> |(define-syntax (OS-dep stx)
>> |    (syntax-case stx ()
>> |      [(_ u w m)
>> |       (case (system-type)
>> |         [(unix)    #'u]
>> |         [(windows) #'w]
>> |         [(macosx) #'m])]))
> What's the meaning of '_' in the expression after syntax-case?

Shorthand for "name of the macro", as I read it.  In the case above 
you'd have
	(OS-dep (define foo 1) (define foo 2) (define foo 3))
for example, so
	_	binds to OS-dep
	u	binds to (define foo 1)
	w	binds to (define foo 2)
	m	binds to (define foo 3)

There's nothing magical about "_" AFAIK; you could just as easily put 
some other identifier there, and it should work.  Consider:

	(define-syntax (foo x) (syntax-case x () ((bar x) #'(cons 'bar 'x))))


Find out just what any people will quietly submit to, and you have the
exact measure of the injustice and wrong which will be imposed on them.
		-- Frederick Douglass

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