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Re: decisions about RDF semantics

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Fri, 08 Nov 2002 06:58:34 -0500 (EST)
Message-Id: <20021108.065834.17901507.pfps@research.bell-labs.com>
To: phayes@ai.uwf.edu
Cc: www-webont-wg@w3.org

From: pat hayes <phayes@ai.uwf.edu>
Subject: Re: decisions about RDF semantics
Date: Thu, 7 Nov 2002 11:06:15 -0600

> >From: pat hayes <phayes@ai.uwf.edu>
> >Subject: decisions about RDF semantics
> >Date: Wed, 6 Nov 2002 23:01:48 -0600

[...]

> >Typed literals of the form "lll"^^ddd, where ddd is a
> >known datatype but lll is not in the value space for ddd are given
> >specific denotations, thus preventing them from being anything else.  For
> >example, in an XML Schema-interpretation,
> >"033333333333333333333"^^xsd:float is different from
> >"0033333333333333333333"^^xsd:float.
> 
> For D-interpretations, what you say is correct: but 
> I am writing this part now, so if you have any strong feelings about 
> what badly-formed typed literals should denote, let me know. I only 
> require that they not denote a normal literal value, so that the 
> inference
> 
> aaa ppp "sillynumber"^^xsd:number .
> --->
> aaa ppp _:xxx .
> _:xxx rdf:type _:yyy .
> _:yyy rdf:type rdfs:Datatype .
> 
> is blocked.

I think that it would be better to not pin down the denotation of such
literals.  That should be sufficient for your purposes.  In fact, what
happens if I make a datatype, ex:trip, whose value space is triples
consisting of a string, another string, and a resource?  Doesn't that mean
that you then get

 aaa ppp "sillynumber"^^xsd:number .
 --->
 aaa ppp _:xxx .
 _:xxx rdf:type ex:trip .
 ex:trip rdf:type rdfs:Datatype .

> >>  Bare literals always denote
> >>  themselves (they can be a string or a pair of strings) and this is
> >>  not influenced by datatyping. Datatype names used as class names
> >>  refer to the class of elements of the value space of the datatype.
> >>  There is no range datatyping and no provision for linking a bnode to
> >>  a literal with a datatyping property, but such usage would not
> >>  violate RDF, if OWL wanted to impose it. RDF *never* assumes that two
> >>  items in different value spaces are equal, even when they are.
> >
> >I believe that this contradicts both the current version of the RDF MT
> 
> Well, I'm writing the current version, so I hope it doesn't. But in 
> any case I don't see how not making an assumption can possibly 
> contradict anything. And RDF has no way to express equality, so this 
> isnt much of an assertion.

I don't see any place in the current model theory that requires that the
value spaces of different datatypes be distinct.  In fact, there is nothing
to prevent me from having two completely different datatypes that have
exactly the same value space.

This does matter in RDF, as, as far as I can see.  For example is there a
XML Schema-datatyped satisfying interpretation of 

rrr rdfs:range xsd:float .
john rrr "3"^^xsd:double .

or of 

rrr rdfs:range xsd:decimal .
john rrr "3"^^xsd:integer .

> >and
> >clear wording in the XML Schema datatyping document.
> 
> I think you will find that that clear wording does not mean what you 
> take it to mean. I was certainly surprised to find out what it means. 
> XSD says that any two things in different basic datatype value spaces 
> are unequal.  As far as I can tell, this uses a sense of 'equal' 
> which is unknown outside the XSD literature, but whatever xsd-'equal' 
> means, it does not mean 'identical'. Rather than try to unpick this 
> confusion, I think it is best if RDF stays quietly on the sidelines 
> and lets the XSD community try to make sense of their own pigsty.

Well, leaving aside potential problems here, such as whether the value
spaces of xsd:float and xsd:double are disjoint, what about xsd:decimal and
xsd:int?  It is certainly the case that an xsd:ind is acceptable where an
xsd:decimal is expected.

> In any case, I didnt say that they WERE unequal, I just said that RDF 
> takes no stance on the matter.

But the examples above, I think, show that a stance has to be taken on the
matter to determine how datatyped interpretations work.

[...]

> Pat

peter
Received on Friday, 8 November 2002 06:58:47 GMT

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