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From: jbn@glacier.STANFORD.EDU (John B. Nagle)
Newsgroups: comp.lang.lisp
Subject: Re: Circular Lists - (nf)
Message-ID: <16946@glacier.STANFORD.EDU>
Date: Tue, 17-Mar-87 00:31:12 EST
Article-I.D.: glacier.16946
Posted: Tue Mar 17 00:31:12 1987
Date-Received: Wed, 18-Mar-87 06:07:02 EST
References: <2700001@uicsg.UUCP> <6900003@iaoobelix.UUCP> <5044@shemp.ucla-cs.UCLA.EDU>
Organization: Stanford University
Lines: 17
Summary: Could circularity be avoided by better packaging?


      Worth considering is whether a slightly different set of primitives
might provide the functionality needed for back-pointers and true circularity
without making refererence-count storage management impossible.

      Suppose we call the existing type of list pointer an "asymmetrical
pointer" and introduce the notion of a "symmetrical pointer".  A 
symmetrical pointer connects two objects and can be traversed in either
direction.  Symmetrical pointers can be altered by atomic operations which
specify a pair of objects to be connected.  Asymmetrical pointers cannot
be altered once created.

      This is not a totally novel idea.  See SLIP and Safe Mesa for examples.
It's not necessarily LISP oriented, either.


						John Nagle