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From: folta@tove.umd.edu (Wayne Folta)
Newsgroups: comp.lang.lisp
Subject: Re: Backquote vs quote (was Re: NCONC & Functions)
Message-ID: <20990@mimsy.umd.edu>
Date: 29 Nov 89 08:53:07 GMT
References: <464@ntcsd1.UUCP> <9893@zodiac.ADS.COM> <31793@news.Think.COM>
Sender: news@mimsy.umd.edu
Reply-To: folta@tove.umd.edu (Wayne Folta)
Distribution: usa
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 16

In article <31793@news.Think.COM> barmar@think.com writes:
>You are assuming that backquote always creates new lists.  The definition
>of backquote doesn't say what it produces.  It may do as much sharing as it
>can.  In particular, if a backquoted expression contains no comma, then the
>backquote is equivalent to a quote.

Exactly!  In fact, that is what happened to me when using Apple's ACL.
I am new to LISP and so I lost several hours trying to figure out why certain
data were mysteriously changing.  (Of course, the problem was that they were
'eq'.)  [As I remember it, I created a backquoted list of two sublists, one
with comma and one without.  The sublist without was reused, the part with
was not.  It sure helped disguise the problem--not that it would have been
clear to this novice anyhow :-)]


Wayne Folta          (folta@cs.umd.edu  128.8.128.8)