- From: Eric Prud'hommeaux <eric@w3.org>
- Date: Fri, 4 Feb 2005 12:44:04 -0500
- To: Pat Hayes <phayes@ihmc.us>
- Cc: Dan Connolly <connolly@w3.org>, www-archive@w3.org, Jos De Roo <jos.deroo@agfa.com>
- Message-ID: <20050204174404.GD20374@w3.org>
On Fri, Feb 04, 2005 at 10:59:14AM -0600, Pat Hayes wrote:
> >Pat,
> >
> >I can't quite put my finger on it, but I'm afraid there are
> >some serious architectural use/mention issues with operators like
> >BOUND, URI-equal, isURI, and isBlank
> > http://www.w3.org/2001/sw/DataAccess/issues#useMentionOp
> >
> >As I see the architecture, a query plays the role of
> >a conjecture to be proved. A query solution is a sketch
> >of a proof.
>
> That is one way to think about it, which I find congenial myself.
> Another way is that the target graph is like a dark pool of triples,
> and the query is like fishing or casting a net into the pool to see
> what you can catch. I think some of the WG think more like the second
> than the first. Yet another way is the DB idea of thinking of a query
> as a kind of algebraic filter applied to a table, which only lets the
> answers through; this gives you a much more algebraic view of query
> languages, where they consist of 'operators'. I think SQL is partly
> based on that idea.
>
> >If I write
> >
> > SELECT ?who
> > WHERE ?who foaf:name "Dan Connolly"
> >
> >it's akin to asking a theorem prover to prove:
> >
> > exists (x) such that foaf:name(x, "Dan Connolly")
> >
> >The solution "?who binds to <danshomePage#topic>"
> >is a sketch of a proof (I think the literature
> >uses the term "witness" for this sort of thing).
>
> Right. Though in this case I think that 'sketch of a proof' might be
> a bit of an overstatement. I think of it as more like the bindings
> produced by a proof. For RDF these are almost the same thing because
> proofs are so simple, but when we move to OWL or (still more so) a
> rules reasoner which answers queries against a closure graph (like
> cwm?) the disparity between the proof and the final variable bindings
> becomes more extreme. Still, this is probably just niggling , I see
> your basic point.
>
> >
> >Now I can't imagine how to turn
> >
> > SELECT ?who
> > WHERE ?who foaf:homePage ?x
> > AND isURI(?x)
> >
> >into a conjecture to be proved by a theorem prover.
> >isURI() is not a function of objects in the
> >domain of discourse, but an operator that distinguishes
> >one sort of term from another.
>
> Right. Its a kind of meta-constraint on the TP rather than an
> assertion in the logical language. Still, its meaningful, and you
> could even axiomatize it if your language doesnt have equality
> (though that would be a hack, and would break when you extended the
> language).
>
> >We might have
Let's add some data to query (I think just querying the schema info
slightly obscures the problem):
_:somebody foaf:homePage <dansHomePage#topic>.
> > <dansHomePage#topic> owl:sameAs _:somebody .
and ask a question:
CONSTRUCT *
WHERE { (?who foaf:homePage <dansHomePage#topic>) }
A complete OWL reasoner must know that
<dansHomePage#topic> foaf:homePage <dansHomePage#topic>.
Just for giggles, it could add
_:x foaf:homePage <dansHomePage#topic>.
_:y foaf:homePage <dansHomePage#topic>.
_:y foaf:homePage <dansHomePage#topic>.
_:z foaf:homePage <dansHomePage#topic>.
Does anything but a query like
CONSTRUCT *
WHERE { (?who foaf:homePage <dansHomePage#topic>) }
AND isURI(?who)
keep the reasoner from reporting an endless series of equivlient bNode
solutions? Is it logically equivilent to substitute a bNodes for any
URI in the graph? It seems that OWL would not worry about this
limitless enumeration.
> >and by definition
> > isURI(<dansHomePage#topic>)
> >but not
> > isURI(_:somebody)
If isURI is a constraint on a set of bindings of nodes/literals to
variables, and each node/literal is only one of URI, bNode, Literal,
then it seems like we're fine. If owl:sameAs makes some node both a
URI and a bNode, then I don't understand owl:sameAs (a definite
possibility).
Also, I'm not sure why this is a use/mention problem rather than a
potential over-simplification of the RDF model.
> >This sets off flags in my mind, but I can't state,
> >in black-and-white, testable ways that matter to
> >applications and coders, why it matters.
>
> Well, one thing to say is that constraints on the form of results,
> like this, are not themselves expressible in RDF, so cannot be sent
> downstream from the query itself. Also there might be some nasty
> interactions between these and the UNSAID construction, but I only
> just thought of that one and will need to think more.
>
> >Does it worry you?
>
> Yes and no. Or maybe, I've learned to stop worrying. That is, I wish
> the world were logically neat, but I gave up years ago thinking it
> was going to actually be logically neat. And one CAN give a neat
> story about things like this, by talking about metalevel constraints,
> or constraints on the syntactic form, and so forth. And they clearly
> seem to be useful in practice: and who are we to limit what kinds of
> software might want to fiddle around with RDF query results?
>
> Also, you know, RDF has always flirted with this use/mention issue:
> reification was kind of ambivalent about it, and the idea of
> rdfs:Literal kept sliding back and forth between meaning literal
> values and actual literal forms, and as far as I can see the new XML
> Schema part 2 doesnt even talk about values in the RDF sense at all.
> If you are thinking in terms of code and structured programming
> models and so forth, its all 'form', and the model-theoretic stuff is
> just a kind of Platonic fantasy: so what's wrong with looking at the
> URIs and checking that they really are URIs (say)? I can see the
> force of this POV.
>
> >p.s. formally this is a WG issue
> > http://www.w3.org/2001/sw/DataAccess/issues#useMentionOp
> >In Helsinki, we has some relevant discussion and made
> >a nearby decision, but it wasn't explicitly a decision
> >to close this issue.
> >
> > RESOLVED: BOUND keyword and no UNSAID to address common
> > UNSAID issues. KendallC abstaining
> > http://www.w3.org/2001/sw/DataAccess/ftf4.html#item04
>
> Sigh, I havnt been keeping up with BOUND and these discussions, my
> bad. I'll try to get up to speed soon (after MOnday: we have some
> awful deadlines looming)
>
> >
> >I'm trying to figure out whether to open substantive discussion
> >of this useMentionOp issue or just say "oh... yeah, we meant to close
> >that one too, didn't we?"
>
> My guess is that it will be a lead balloon. After all, if you have
> some neat app in mind, to be told that you shouldn't do a simple
> thing because a logician thinks that it violates his dream of the way
> the world should be, isn't likely to resonate very strongly, if you
> take my meaning. And after all, these form-restrictions are perfectly
> meaningful: its just that they are syntactic rather than semantic.
> But why shouln't a query language allow syntactic constraints on
> answers? Even reasoning engines do things like that to recognize
> call-out cases and apply optimizations, after all.
>
> Pat
>
> PS all in haste, more later.
>
> >
> >
> >--
> >Dan Connolly, W3C http://www.w3.org/People/Connolly/
> >D3C2 887B 0F92 6005 C541 0875 0F91 96DE 6E52 C29E
>
>
> --
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--
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Received on Friday, 4 February 2005 17:44:05 UTC