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From: koomen@cs.rochester.edu (Hans Koomen)
Newsgroups: comp.lang.lisp
Subject: Re: Are "destructive" functions really destructive?
Message-ID: <1989Nov29.191106.10329@cs.rochester.edu>
Date: 29 Nov 89 19:11:06 GMT
References: <20991@mimsy.umd.edu> <1989Nov29.173139.26165@Neon.Stanford.EDU>
Reply-To: koomen@cs.rochester.edu (Hans Koomen)
Distribution: usa
Organization: University of Rochester Computer Science Department
Lines: 28

Michael Greenwald <michaelg@Neon.Stanford.EDU> writes:
...
>(let ((x (copy-list '(a b c d e f))))
>  (setq y (delete 'a x)))
>
>There is no reasonable way to destructively "modify" x so that delete
>will work and you maitain EQness, since in this case DELETE == CDR.
>In other words, you lose the first cons cell, and the rest of the list
>is untouched.
>
>(I'm ignoring the pathological implementation of altering the CAR of
>every sublist, and knocking off the last cons.)
>

You don't need to walk down the entire list to knock off the last cons.  When
deleting the first element of a 2+ element list, the Interlisp equivalent of
Common Lisp delete, dremove, effectively does
	(progn (rplaca x (cadr x)) (rplacd x (cddr x)))
i.e., splices out the second cons after stuffing value of the second car into
the first car.  No pathology required here to maintain eq-ness.

The real problem lies in removing the first element of a 1-element list, e.g.
if x is bound to '(a) then (delete 'a x) returns nil, but there's no way for
the delete function to know that its second argument, (a), was the binding of
the variable x, and that x should be set to nil!
-- 

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