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Re: Dark triples, motivating examples

From: Dan Connolly <connolly@w3.org>
Date: 16 Apr 2002 11:12:59 -0500
To: Pat Hayes <phayes@ai.uwf.edu>
Cc: Jeremy Carroll <jjc@hplb.hpl.hp.com>, w3c-rdfcore-wg@w3.org, "Peter F. "Patel-Schneider <pfps@research.bell-labs.com>
Message-Id: <1018973581.1666.42.camel@dirk>
On Mon, 2002-04-15 at 20:07, Pat Hayes wrote:
> >ACTION 2002-04-12#7, jjc: post message to rdfcore with motivating example
> >(for dark triples?)
> >
> >The basic problem can be expressed in DAML+OIL as:
> >
> ><rdf:Description rdf:about="#John">
> >    <rdf:type>
[...]

> Um. I don't think this quite captures the essential point.

Would you please given an example that does?

> The 
> entailment given is indeed wanted, but its only one of a whole class 
> of entailments. Now, those entailments themselves are not the 
> problem: after all, OWL can impose its own semantic conditions on the 
> graphs, to make sure its entailments come out the way it wants. In 
> these cases the RDF in effect says less than OWL requires, but that 
> is to be expected. The problem (that dark triples is intended to 
> address) is almost the inverse of this: it is that the RDF graphs 
> that encode DAML (more generally, OWL)  might say *more* than what is 
> intended: in particular, they assert the *existence* of certain 
> containers, corresponding to the syntax of some DAML (OWL) 
> expressions, eg the use of daml:collection to encode some DAML 
> expressions in RDF. When DAML was designed, RDF had no semantics,

I disagree; RDF has had existential/conjunctive semantics all along.

> so 
> it was quite OK for DAML to use RDF triples freely and assign its own 
> meaning to them. But now that RDF does have a semantics, those 
> triples have a definite RDF meaning (they assert the existence of a 
> thing called a daml:collection, with relationships to its contents) 
> and since DAML (OWL) is supposed to be a 'layered' extension of RDF, 
> they should have the same meaning in DAML as they have in RDF.

quite.

>  But 
> DAML doesn't want them to have that meaning

no? I think it does. (and I helped design DAML+OIL).

> (the DAML MT gives them a 
> different meaning);

I don't think that's entirely clear. The DAML axiomatic
semantics gives them exactly this meaning.

> and in OWL, Peter has shown

I'm not convinced.

> that if we insist 
> that OWL interpretations preserve all the meanings that the RDF MT 
> assigns to all the 'syntax-encoding' triples that will be needed to 
> ensure an adequate layering, then those extra things that are 
> asserted (by RDF) to exist, can actually produce inconsistencies in 
> the OWL, basically rendering what would otherwise be perfectly good 
> OWL assertions into nonsense.

I still think we should work on fixing that; i.e. being careful
about what entailments follow in OWL so that paradoxes don't arise.


> In general, it usually isn't a good 
> idea to impose, as a semantic condition on a language (particularly a 
> 'class', ie set-theoretic, language like DAML or OWL),  that it is 
> obliged to talk about its own expression syntax:

I'm not sure what you mean by that.

DAML+OIL is just a collection of terms for use in RDF syntax, no?

> but that is what the 
> 'layering' conventions basically do, if the higher layers are obliged 
> to take the RDF semantics seriously. So, we need a way to say 
> something like: *these* triples are being asserted (and DAML accepts 
> them as assertions and agrees with their meanings) but *these other* 
> triples are just being used to encode syntax, and are not being 
> asserted. The 'dark' triples are the latter kind, and all that is 
> required is that DAML (and OWL, and probably almost every other 
> layered extension of RDF) has some way to make the distinction.
> 
> I would emphasize that this issue really is a semantic non-issue for 
> RDF: it doesn't fundamentally change the language at all, other than 
> asking it to provide a way for a 'higher' layer to make this 
> distinction, and be willing to record it and preserve it under 
> inferences.

Until I can look at some specific proposals and examples, I don't
have any way of evaluating that claim.

> The distinction is already in the RDF MT, in fact ('dark' 
> = 'unasserted'). So it just requires some kind of syntactic 
> convention or addition to allow an RDF graph/document to actually 
> record the distinction so that it can be accessed and preserved. This 
> would be easy in N-triples, but seems to require some new idea in 
> RDF/XML. (Eg one idea that was mooted was to allow <rdf> ...</rdf> 
> brackets inside an RDF/XML document, the understanding being that 
> anything inside them was 'dark'.)
> 
> If the WG accepts this task, the basic decision to be taken is 
> whether an RDF document with dark triples should be designed to be 
> acceptable to current parsers or whether it should, as a matter of 
> policy, be designed to break current parsers. I would suggest the 
> former, if some way can be found to do this while preserving the 
> distinction. I would also suggest the easiest way to go would be to 
> provide some way to say that a given namespace is 'dark'; Peter and I 
> agree that, while somewhat less flexible than allowing an arbitrary 
> distinction, this would be sufficient to allow WOWG to proceed.
> 
> >In the discussion WOWG appeared to like this entailment so much, that it was
> >worth paying the cost of having two separate RDF documents to express the
> >light and dark triples. This was the mechanism discussed at greatest length
> >in the webont f2f. (I took it that the motive for discussing that mechanism
> >is that this was the smallest possible change required of RDFCore: simply
> >permission to have an RDF/XML document that is interpreted as a "dark
> >document").
> 
> That would indeed be the minimal change to RDF, ie no change at all, 
> and I did seriously suggest it to the WOWG, since that is what Euler 
> does. But it was felt that mechanism would be rather hard to use in 
> OWL, since it would probably lead to a proliferation of multiple dark 
> documents from any reasonably complex piece of OWL.
> 
> >In terms of RDF collections, we could imagine trying to make the same
> >construct with a closed bag.
> 
> That would not achieve the required goal, however, since the triples 
> describing the bag would be present in the graph and hence the bag 
> would be asserted to exist in the MT, and hence would be required to 
> exist in any OWL interpretation; but that is exactly what we are 
> trying to avoid.
> 
> Pat
> 
> PS. A pre-emptive strike: reification doesn't do it, either.
> 
> -- 
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-- 
Dan Connolly, W3C http://www.w3.org/People/Connolly/
Received on Tuesday, 16 April 2002 12:14:12 EDT

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