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RE: motivation for bNodes/existentials in RDF; note for parsers

From: Massimo Marchiori <massimo@w3.org>
Date: Thu, 4 Apr 2002 19:27:36 +0200
To: "Pat Hayes" <phayes@ai.uwf.edu>
Cc: <w3c-rdfcore-wg@w3.org>, "Lynn AndreaStein" <las@olin.edu>, "Dan Connolly" <connolly@w3.org>, "Lynn AndreaStein" <las@olin.edu>, "Jan Grant" <Jan.Grant@bristol.ac.uk>
Message-ID: <NGBBJNKIMLOPPCFHEJEMAEOLCAAA.massimo@w3.org>

Pat, thanks for your reply, which in fact helped to better
understand what what your assumptions are.

> ><disclaimer>
> >In these kind of sentences I always have to argue with Dan, as
> >he always says so and I always reply in the usual way...
> >So, restating the above, the *current version of the
> >RDF Model theory* states that the interpretation of RDF
> >ought to be ... bla.
> >This is important to remember, as it's a fundamental design choice
> >that it's going to be decided, but it's not present in the
> >"normative RDF" (M&S) document.
> ></disclaimer>
>
> Well, the normative M&S seems pretty clear about the intended status
> of anonymous nodes. I can see why, for some purposes, it would be
> convenient if anonymous nodes had hidden labels; but that
> interpretation is wishful thinking imposed on the M&S, which is about
> as unambiguous as it is possible to be without actually giving a
> formal model theory.
In fact, I profoundly disagree here. If there's something where M&S is very
vague is just anonymous nodes. Even, reading the spec carefully,
one will see that, in fact, this vagueness can be somehow justified,
as anonymous nodes are there seen just as accessories, and not as a
fundamental component of M&S: a facility to avoid having to assign
names, but just a facility. Then, we can debate at length on how the
spec should have been clearer on this (and where the architectural
pitfalls are). We can even decide M&S was not smart and that we need to
redefine and introduce first-class anonymous nodes (not a bad idea), but it
should be clear the distinction on what *we* are saying, and what's written
in the normative spec (the only normative thing we have so far).
And above all, this has to be pointed out because of the charter
implications
it has...

Anway, this is leading away from the other point of the discussion: the
existential interpretation.
Now, before we got lost here, just a note: no fundamental objections
to the existential interpretation per se, as it's in the MT, and the MT
is (citing the abstract) "a model-theoretic semantics for RDF and RDFS",
(so, "a", not "the"). But, as this point has never well been clarified so
far, if the "a" changes into a "the", then the level of criticisms
radically changes, and that's where more severe stop-over criticisms can
occur.
So, let's go on:

> Logically, skolemization is
> quite complicated. It isn't valid, for example, and it blocks several
> useful intuitive inferences. It would be impossible to express
> queries in RDF if it had no variables, for example.
Yes, skolemization can be as complex as existential quantification. Even,
mea culpa for using the word "skolemization" here. Sure, it's
better to have variables to do RDF query (not impossible, btw).
But all this is about different things: inference and RDF Query.
RDF Core was not chartered to normatively do these two, and not chartered to
do the "web logic".
There's nothing like a "variable" in RDF. The moment you put in variables,
existential quantification, entailment, you're trying to do RDF logic,
which is a different wg (and, a different spec than M&S).
What is needed is just the minimum clarifications (and patches) to M&S model
that complete the
formalization vagueness. Doing (normatively) the RDF Logic requires another
wg, or a
different charter.

So, now the higher level of criticism can be better understood.
If MT is just a proposal, fine (even more: good! as, it helps to
provide a possible good starting point for a next RDF Logic/Query wg).
If on the other hand, it aims to define now the normative RDF Logic,
then I think it's truly beyond scope.
Its formal status is unclear now, so better clarify it.

-M


ps
Replies to the other more precise points that have been asked, become
by the above discussion just secondary (and trivial), as they
deal with the "existential vs skolem" within a full RDF logic/query
context, which is another level of discussion (interesting, but another
one).
However, they are included for completeness:

> >That is, what are the pro's and con's that favour the existential
> >approach vs the skolem one?
> >AFAIK the second one has been so far the natural choice (the
> >"understood standard" if you want ;), for some good reasons.
>
> Which are? (I know some, but I wonder if you have others.)
Just one (others too, but not pertinent to this discussion): doing
existential
interpretation, you're introducing variables and more powerful logic. Not
what M&S does.

> >>  At the first WG F2F we had a long (and, i think, productive) argument*
> >>  about this. Sergei produced a good set of pros and cons; my arguments
> >>  for this are ...
> >>
> >>  - supports "non-assertional" mode, ie, RDF querying by turning around
> >>    the "X entails what?" into "what entails X?"
> >>
> >>  - aesthetic reasons, and those of transparency. When I write an
> >>    assertion with a blank node, I intend it to mean "there exists...".
> >>
> >>  - DanC also claimed that skolemisation was too much of a general
> >>    impediment to getting software written :-) I think he may have
> >>    been dramatising for, well, dramatic effect, but I've some sympathy
> >>    with this POV. In other words, supporting anonymous nodes requires
> >>    some API fiddling, but is not necessarily a "simpler mechanism".
> >
> >Thanks for the initial reply, Jan.
> >I don't want to start a complex debate without first having seen
> a complete
> >reply, just noticing that this is such a fundamental
> architectural decision
> >that a complete and careful pro/con analysis is due. The above
> three "pro"
> >reasons
> >all have good counterarguments,
>
> I'd be interested in hearing them.

"non-assertional": this doesn't makes a difference for M&S, as this
functionality goes into the RDF Query, not in RDF.

"aesthetic": sure, but this doesn't imply you have to impose a normative
existential quantification already at the M&S level.

"general impediment": this refers to the semweb/RDF logic and query, not to
the M&S
(in fact, without variables and quantifications, it's much easier to write
software ;)

> For me, the fundamental motivation for not skolemizing blank nodes
> concerns entailment. Consider an RDF graph G1 and another graph G2
> got from G1 by erasing some of the node labels, ie replacing urirefs
> with blank nodes. Does G1 entail G2? Seems to me that the answer has
> to be 'yes' in order to capture the intended meaning of blank nodes
> as described in the M&S. If blank nodes are hidden skolemizations,
> however, the answer is 'no'. So skolemized graphs do not adequately
> support the proper RDF entailment conditions.
You're talking about entailment and RDF logic here, not M&S. Nobody
prevents you from introducing existentials at a higher level (and,
within the RDF logic wg...).

> It would be fine for blank nodes to have some kind of
> implementation-dependent hidden labels - in effect, that is what the
> bnode labels in Ntriples are - but the critical point is that these
> labels are not treated like urirefs; they have no global scope, and
> are not meaningful outside the graph, and are not treated
> semantically as names.
Yes, and that's what needs to be clarified in the M&S. But no need to
introduce existentials and variables to do so...

> RDF could be redefined without blank nodes, of course, but it would
> be a different, and much weaker, language. In effect, it would be a
> simple positive propositional logic, with no quantification.
> Skolemized nodes are not blank, so the whole concept of anonymous
> nodes would be eliminated from the language, rendering it simpler in
> both its syntax and its semantics.
Again, here the discussion is on how to better do a powerful RDF logic.
And as it's not in M&S, the architectural possibility to drop blank nodes
had in fact to be considered (..!)

> I agree that this is a fundamental
> architectural decision, but it seems to me that the M&S has already
> clearly made the decision: the language does contain blank nodes.
> Given that decision, skolemization is not an option. The milestone
> decision was taken long ago, and all we are doing is stating it more
> precisely.
As said at the beginning, this is not true. M&S introduces blank nodes
as a facility. Then, it can be convenient to give them first-class
status in the model. But that's all, the moment you go beyond with
variables,
it's no M&S any more, it's not RDF core any more.
Received on Thursday, 4 April 2002 12:28:45 EST

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