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Date: Mon, 23 Apr 2001 12:00:06 -0400 (EDT) From: Andrew Pochinsky <avp@xxxxxxxxxxxxx> Olin, 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 ...)) E.g., (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. -Olin