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From: shiffman%basselope@Sun.COM (Hank Shiffman)
Newsgroups: comp.lang.lisp
Subject: Re: Are "destructive" functions really destructive?
Message-ID: <128536@sun.Eng.Sun.COM>
Date: 29 Nov 89 17:37:53 GMT
References: <20991@mimsy.umd.edu>
Sender: news@sun.Eng.Sun.COM
Reply-To: shiffman@sun.UUCP (Hank Shiffman)
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Organization: Sun Microsystems, Mountain View
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In article <20991@mimsy.umd.edu> folta@tove.umd.edu (Wayne Folta) writes:
>For example, if I want to delete something from a large datastructure, I
>assume that I would use "delete":
>
>   (delete item huge-list :key #'my-key)
>
>Or do I need to do:
>
>   (setq huge-list (delete item huge-list :key #'my-key))

Either one of these will work AS LONG AS THE DELETED ITEM WASN'T THE
FIRST ELEMENT OF THE LIST!  You need to use the second version to deal
with the fact that using DELETE to remove the first element leaves the
rest of the list unchanged.  Since the deleted element will not be
modified, it will still point to the rest of the list.

DELETE is a function.  By the time it's called, the HUGE-LIST argument
has been evaluated to the first CONS in the list.  DELETE can't do the
SETQ for you, since it doesn't know what variable to SETQ.

>
>So destructiveness is an option for the convenience of LISP
>implementation creators, not users?
>

On the contrary.  Destructiveness saves the user a lot of extra
consing by reusing the same list.  By and large, ease of
implementation was *not* a major consideration in the design of Common
Lisp.  And it shows.



-- 
Hank Shiffman                                     (415) 336-4658
Marketing Technical Specialist
Software Engineering Technologies               ...!sun!shiffman
Sun Microsystems, Inc.                          shiffman@Sun.com

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