Path: utzoo!attcan!uunet!mcsun!ukc!newcastle.ac.uk!turing!ncmh
From: Chris.Holt@newcastle.ac.uk (Chris Holt)
Newsgroups: comp.lang.misc
Subject: Re: Lets define "pointer"
Message-ID: <1990Oct25.144947.14276@newcastle.ac.uk>
Date: 25 Oct 90 14:49:47 GMT
References: <26767@megaron.cs.arizona.edu> <3808@lanl.gov>
Sender: news@newcastle.ac.uk
Organization: Computing Laboratory, U of Newcastle upon Tyne, UK NE1 7RU.
Lines: 36

In article <3808@lanl.gov> jlg@lanl.gov (Jim Giles) writes:
>David Gudeman suggested:
>> (4) pointer: an abstration of a position in a sequence
>>     ... My argument was that pointer(4) is a useful abstraction.
>
>From your previous posting of this idea, I can't see that it requires
>more than the data structures that I've been hawking.  However, I wouldn't
>call this object a "pointer" - it is clearly a "list index" or some such
>thing.  I _would_ like to know more about this abstraction though.
>Even if I don't decide it needs direct implementation in a language,
>it could be a good example of the types of things that need to be
>supported.

This one is interesting, but needs generalizing.  So let's look at
a "tree index", or even better, a graph index.  This "points" at
a component (node) of a graph, and can "move" in any direction for
which there is a field in the node (i.e. an arc from the indexed
node to another node might be described by a field name; viewing the
node as a function and applying it to the field name yields
the result consisting of the original graph, with the new node index).

At this point ( :-) there are a couple of different things we might
want to do.  Some indices will want merely to move about the graph,
looking at various nodes and the relationships among them; these
have read-access.  Others may wish to add new arcs and nodes to the
graph; these require create-access.  Still others may wish to have
the power to remove arcs and nodes, requiring destroy-access.

Perhaps the best discipline would be to create a graph attached
to an unchangeable initial index.  Then, any other index for the
graph would be declared with respect to that one, together with
its access properties.  Or perhaps not?
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 Chris.Holt@newcastle.ac.uk      Computing Lab, U of Newcastle upon Tyne, UK
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 "Who overcomes by reason sole hath overcome but half his foe..."