Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!samsung!think!think.com!barmar
From: barmar@think.com
Newsgroups: comp.lang.lisp
Subject: Backquote vs quote (was Re: NCONC & Functions)
Message-ID: <31793@news.Think.COM>
Date: 29 Nov 89 04:04:24 GMT
References: <464@ntcsd1.UUCP> <9893@zodiac.ADS.COM>
Sender: news@Think.COM
Distribution: na
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 28

In article <9893@zodiac.ADS.COM> rshu@ads.com (Richard Shu) writes:
>I guess there's a corollary that says: make sure you create new copies of
>lists if that's what you want.  Don't use QUOTE (') to cons up data structures.
>If you don't like LIST, use the BACKQUOTE macro (`).  I've been shafted by
>variations of your bug where the problem was using QUOTE instead of BACKQUOTE.

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.

An implementation would be justified in translating

(defun foo (a)
  `(,a b c))

into

(defun foo (a)
  (cons a '(b c)))

Then (eq (foo) (foo)) => NIL, but (eq (cdr (foo)) (cdr (foo))) => T.  And
if you NCONC something to the end of (foo) you may change the value of
future calls.
Barry Margolin, Thinking Machines Corp.

barmar@think.com
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