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From: freeman@argosy.UUCP (Jay R. Freeman)
Newsgroups: comp.lang.scheme
Subject: Mutation and backquote (was something else)
Message-ID: <685@argosy.UUCP>
Date: 25 Sep 90 20:48:40 GMT
References: <KM+5+T4@xds13.ferranti.com> <10340003@hpfcso.HP.COM> <1990Sep25.112400.26991@hellgate.utah.edu>
Sender: news@argosy.UUCP
Reply-To: freeman@cleo.UUCP (Jay R. Freeman)
Organization: MasPar Computer Corporation, Sunnyvale, CA
Lines: 27

In regard to the construct `(,a b c), I thought the R3 standard
(haven't seen the later ones yet) indicates that all of what is
returned by backquote is immutable; that is, if we do

(define a 42)
(define foo `(,a b c))

making the value of foo be (42 b c), then the following are errors:

(set-car! foo 88)
(set-cdr! foo 88)
(set-car! (cdr foo) 88)
(set-cdr! (cdr foo) 88)
(set-car! (cddr foo) 88)
(set-cdr! (cddr foo) 88)

(This list doesn't go any further, because (cdddr foo) is the empty
list, which is not capable of being set-car!ed and set-cdr!ed anyway.)

If I am right, this is a somewhat stricter interpretation than in
Common Lisp.  Do people agree or disagree?


                                      -- Jay Freeman


	  <canonical disclaimer -- I speak only for myself>