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Re: [ratholes, reification, risk] poison-URIref testcases

From: Graham Klyne <Graham.Klyne@MIMEsweeper.com>
Date: Mon, 25 Mar 2002 09:24:54 +0000
Message-Id: <5.1.0.14.2.20020325085047.038d4ec0@joy.songbird.com>
To: Dan Brickley <danbri@w3.org>
Cc: <w3c-rdfcore-wg@w3.org>, Pat Hayes <phayes@ai.uwf.edu>
Never one to resist an attractive rathole

At 09:18 AM 3/22/02 -0500, Dan Brickley wrote:
>I'm yet to see a W3C or IETF spec that guarantees all urirefs refer; so in
>general I proceed with caution and assume that some (eg. typos, some
>UUIDs) might not be names for anything. The MT explicitly disowns this
>problem; it's not clear which spec(s) should own it. So I tried to write
>some test cases as a start towards understanding how we might (at least in
>Primer) flag up the awkward corner cases and strategies for avoiding them.

My thoughts on this are based on:  I'm not sure that referring and denoting 
are the same thing here.

In Web architecture terms, referring relates in some way to a value you get 
when you do a Web retrieval (dereference), right?  In RDF MT terms, URIref 
denotations are required of any interpretation.  Somewhere between these, 
there is the idea of some intended interpretation that (a) satisfies the 
model theoretic aspects of RDF, and (b) is consistent with what you get 
when dereferencing Web URIs.  I think there will just about always be 
several interpretations that satisfy these tests.


>[rat1.rdf]
>
><rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
>         xmlns:foaf="http://xmlns.com/foaf/0.1/">
>  <foaf:Person foaf:name="Jan Grant">
>   <foaf:mbox rdf:resource="mailto:jan.grant@bristol.ac.uk"/>
>  </foaf:Person>
></rdf:RDF>
>
>
>[rat2.rdf]
>
><rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
>         xmlns:foaf="http://xmlns.com/foaf/0.1/">
>  <foaf:Person foaf:name="Jan Grant">
>   <foaf:mbox rdf:resource="mailto:jan.grant@bristol.ac.uk"/>
>  </foaf:Person>
>  <rdf:Statement>
>   <rdf:object rdf:resource="poison:uriref-like-string-naming-nothing"/>
>   <!-- note that rdf:subject used here wouldn't be so interesting -->
>  </rdf:Statement>
></rdf:RDF>
>
>
>In brief, I think we need to decide whether to seek assurance from the W3C
>Technical Architecture Group (TAG) that there are no circumstances in
>which a URIref identifier (or URIref-formatted-string) fails to refer.
>If we don't get that assurance, some RDF/XML documents might not be
>carriers for propositional content (since --loosly put-- bits of them
>will be meaningless).

I don't see that a statement being meaningless takes away from the meaning 
of the other statements.  So if there are truly no constraints on the 
denotation of the poison URI, I think it might as well be a blank node.

>We can think about this in terms of the test cases above, by asking
>ourselves what the world would have to be like if assertions of [rat1.rdf]
>and [rat2.rdf] were to be true.
>
>If there are no worlds in which the uriref 
>poison:uri-like-string-naming-nothing
>actually refers, [rat2.rdf]'s truth prospects are dismal compared to
>[rat1.rdf]'s.

They seem pretty much the same to me.  Rat2 may not add any new 
constraints, but any interpretation that satisfies Rat1 can be adjusted to 
also satisfy Rat2 (by including the necessary values in  the relational 
extension of rdf:object) without introducing any possible conflict with 
observations based on URI referral in the Web, if the poison URI cannot refer.


>ps. a summary from IRC chat with JanG (don't blame him for any goofs in my
>presentation of this though; we only manage to agree every other day)
>
><jang> the "meaning" of a piece of RDF is the conjunction of the 
>meaning  of individual triples
><jang> so if a piece of RDF contains just _one_ triple that 
>is  "meaningless", it renders the whole thing meaningless.
><jang> by using a de re quoting mechanism, the reification of such a 
>piece  of RDF is meaningless
><jang> and so is any RDF that contains it
><jang> we avoid this by ensuring that we have a default "meaning" for 
>any  piece of RDF

I would argue that the "meaning" of some RDF is not the "conjunction of the 
meaning of individual triples".  Conjunction of "meaning" seems to be a 
meaningless (er, ill-defined) idea.  It is the _truth_ under some 
interpretation that is a conjunction.


 From your quotes, these are the ones I think are relevant:

>I've excerpted here the main things the MT says about URI references:
>
>         http://www.w3.org/TR/2002/WD-rdf-mt-20020214/
>
>
>         Asserting an RDF graph amounts to claiming that it is true, which is
>         another way of saying that the world it describes is, in fact, so 
> arranged
>         as to be an interpretation which makes it true. In other words, 
> asserting
>         a piece of RDF amounts to asserting a constraint on the possible 
> ways the
>         world might be. Notice that there is no presumption here that any RDF
>         graph contains enough information to specify a single unique
>         interpretation. It is very difficult, and usually impossible, to 
> assert
>         enough in any language to completely constrain the 
> interpretations to a
>         single possible world, so there is no such thing as 'the' unique RDF
>         interpretation. In general, increasing the size of a graph 
> decreases the
>         set of interpretations that an assertion of the graph allows to 
> be true.
>
>         The use of 'public' URIs in an RDF graph is often taken to imply 
> that an
>         assertion of the graph implicitly assents to the truth of other 
> RDF graphs
>         that define the meaning of that URI. To apply the model theory to 
> this
>         kind of situation, one should think of the restriction on the world
>         represented by an assertion of the merge of the asserted graph 
> together
>         with whatever RDF graphs are assumed to define the public 
> vocabulary, in
>         order to fully convey the intended meaning of the 'public' assertion.
>
>
>         This only applies to uses of RDF that are intended to be the 
> assertion of
>         propositional content. A fully adequate account of what it means 
> to make
>         an assertion in a Web context is a research problem that is 
> beyond the
>         scope of this document.

In summary:  there are indeed research issues here, but I'm not sure that 
non-referring URIs are a problem (other than not being very useful).

#g

PS:  Here's my "test case" -- can I be sued for defaming a Unicorn?



-------------------
Graham Klyne
<GK@NineByNine.org>
Received on Monday, 25 March 2002 04:40:41 EST

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