Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!zodiac!doug
From: doug@zodiac.ADS.COM (Doug Morgan)
Newsgroups: comp.lang.lisp
Subject: Re: NCONC & Functions
Message-ID: <DOUG.89Nov29163722@zodiac.ADS.COM>
Date: 30 Nov 89 00:37:22 GMT
References: <464@ntcsd1.UUCP>
Sender: news@zodiac.ADS.COM
Distribution: na
Organization: Advanced Decision Systems
Lines: 26
In-reply-to: mps@ntcsd1.UUCP's message of 28 Nov 89 22:19:26 GMT


When the lisp reader sees:

(DEFUN foo1 () '(a b c))

it turns the text (a b c) into an internal representation as a list of
conses pointing to symbols.  At run time the ' (actually (quote ...))
returns THAT PARTICULAR internal representation.  A later NCONC
munches that representation and you get exactly what you saw.

In the case of (DEFUN foo2 () (LIST 'a 'b 'c)), the reader creates a
different internal representation.  This one is 4 conses with the
first pointing to the symbol named LIST.  At run time, the evaluation
of this representation causes a 3 cons list pointing to a b and c to
be generated.  Since these lists are generated upon each evaluation of
(LIST ...), NCONC can't affect later ones.  Again exactly as you saw.

Any lisp that treats FOO2 like FOO1 is in error, no ifs ands or buts.

I use destructive operations pretty much at will on lists that are
created from nil and the operations are in the lexical scope of the
point of creation (I get to look at everything that could be building
pointers to such lists).  Never (well, hardly ever) trust a list
that's passed in from "outside."

doug