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Re: What is an RDF Query?

From: pat hayes <phayes@ai.uwf.edu>
Date: Tue, 11 Sep 2001 19:53:12 -0500
Message-Id: <v04210103b7c45e1c2b28@[205.160.76.173]>
To: Sandro Hawke <sandro@w3.org>, "Eric Prud'hommeaux" <eric@w3.org>
Cc: www-rdf-rules@w3.org
> >                         So why are you talking about this topic under
> > this thread?
>
>Because this thread started when you took exception to my agreeing
>that Eric's claim was indeed on track.  Eric had said:
>
>   In the charter terms, I make the bold statement that rules are
>   composed of queries and assertions.  This is intended to clarify why
>   both communities have been inducted into the same list.  In
>   implementing algae [2], I used the variables bindings from a query
>   to compose assertions.  Does this match the experience of others?
>
>Do you agree Eric is on track?

OK, I took a look at http://lists.w3.org/Archives/Public/www-rdf-rules/#terms
and found this:
----
rule
        Expression of query and assertion that causes the automatic 
insertion of asserted statements wherever the query is matched.
-----
which may help to explain the (my) confusion. This notion of 'rule' 
doesn't make sense to me. Why would matching a *query* produce the 
insertion of *statements*? If we were to use a rule (in the usual 
sense, now, not Eric's) in forward inference, then it would match an 
assertion and produce a new assertion (as in inferring Q from P and 
(if P then Q) ) , and if we are using it in backwards mode then it 
would match a query and produce a new query (as in  going from ?Q and 
(if P then Q) to ?P). These are really the 'same' inference, 
logically speaking - modus ponens - but done in different 
'directions'. But I can't think of any way to take a rule, match it 
to a query, and get an assertion.

BTW, another quibble:

----
assertion
        Insertion of statements into an RDF graph.
------
That kind of begs the question, since part of what we are talking 
about here is whether an RDF graph is asserted or queried. (In fact, 
RDF *is* asserted, so this definition kind of works by accident; but 
if anyone ever uses RDF to express queries then this definition of 
'assertion' will be pre-broken.) This distinction between asserting 
and querying is really fundamental. Other speech acts in the same 
category include denying, questioning, qualifying, expressing doubt 
about, and so on. These are all different relations between an agent, 
or maybe a piece of text, and some content. They can't be expressed 
as extra content, since without some assumptions about the 
relationship, one cannot even know if that extra content itself is 
being questioned, asserted, denied, etc... They have to be given in 
some extra 'layer' outside the content language. Of course this may 
be entirely implicit and universally agreed, and so doesnt need to be 
said explicitly; but it has to be there somehow, in order for any 
communication to take place. If all I can do is wave some RDF at you 
without you knowing whether Im asserting it or querying it or 
whatever, then you have no idea what you are supposed to do with it. 
So to *define* assertion as any kind of operation on RDF (or any 
other content language) is begging the question.

Pat

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Received on Tuesday, 11 September 2001 20:53:16 GMT

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