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Re: LIST-LENGTH & circular lists
Date: Mon, 23 Apr 2001 12:00:06 -0400 (EDT)
From: Andrew Pochinsky <avp@xxxxxxxxxxxxx>
There is actually a difference between 3-element cicrular list
(define three-element (1 7 2 1 7 2 ...))
and a similary looking 6-element circular list
(define six-element (1 7 2 1 7 2 ...))
(eq? three-elements (cdddr three-elements)) ==> #t
(eq? six-elements (cdddr six-elements)) ==> #f
unless, of course, one considers those pesky eq?'s side-effect....
Exactly. The whole notion of EQ? is tied up with the notion of side-effects.
For example, if I tell you that two pairs P1 & P2 are EQ?, that means that
if you SET-CAR! P1, P2 will change.
If I don't allow side-effects to list cells (as in ML), then you can't
tell the difference between two cons cells with identical cars & cdrs.
So, there are basically these two levels: pure lists and side-effectable lists.
In the pure-list world, it is not interesting or even well-defined to ask
the how-many-elements-in-this-circular-list question. In the side-effectable
list world, it makes sense.