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From: barmar@think.COM (Barry Margolin)
Newsgroups: comp.lang.lisp
Subject: Re: Use of backquote & macro question
Message-ID: <39782@think.UUCP>
Date: 28 Apr 89 16:06:10 GMT
References: <2277@perseus.sw.mcc.com>
Sender: news@think.UUCP
Reply-To: barmar@kulla.think.com (Barry Margolin)
Organization: Thinking Machines Corporation, Cambridge, MA
Lines: 39


You are correct that backquote only works in an evaluation context.
On p.349 of CltL, it says "Note that the [resulting] form is
equivalent only in the sense that when it is evaluated it will
calculate the correct result."  Backquote expands into a form that
constructs the specified data structure, but it has to be invoked
somehow.  If it's used in a non-evaluation context, it will simply be
left there.

For instance, a possible expansion of `(a ,b c) is (list (quote a) b
(quote c)).  If this were used in a let-binding, e.g.

	(let `(a ,b c)
	  ...)

the result will be

	(let (list
	      (quote a)
	      b)
	  ...)

which will bind the variables LIST and B to NIL, and bind the variable
QUOTE to the value of A.

Note, however, that you can frequently solve this problem by
restructuring the expression.  What was probably mean above was
something like

	`(let (a ,b c) ...)

which uses the value (at macro-expansion time) of B as the
let-binding.

Barry Margolin
Thinking Machines Corp.

barmar@think.com
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