- From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
- Date: Tue, 02 Oct 2001 12:48:49 -0400
- To: drew.mcdermott@yale.edu
- Cc: Patrick.Stickler@nokia.com, www-rdf-logic@w3.org
From: Drew McDermott <drew.mcdermott@yale.edu> Subject: Re: Literals (Re: model theory for RDF/S) Date: Tue, 2 Oct 2001 10:10:19 -0400 (EDT) > > [Peter Patel-Schneider] > Yes, in DAML+OIL you can equate two different URIs, via equivalentTo. > However, that is certainly not how DAML+OIL handles datatypes, like > integers. A DAML+OIL (March 2001) processor has to understand a portion of > XML Schema, not just the syntax but also the semantics. > > This is a basic difference between DAML and RDF, correct? Well not actually between DAML+OIL and RDF but between the datatype extension (also called the March 2001 version) of DAML+OIL and RDF. [...] > This has consequences with respect to the *meaning* of answers returned by > an RDF query system. In particular, the notion of cardinality becomes more > than a little suspect. (Consider asking how many favorite integers Susan > has. If an RDF query system answers two, then it is not just incomplete, > it is wrong. This can be fixed by some rather technical means, but must be > done carefully.) > > I thought that these issues all had a standard answer on the wide-open > world-wide semantic web: you *can't* answer cardinality questions just > by enumerating the answers you get back. If an RDF query system > responds with "int:05, int:5" as an answer to the query about Susan's > favorite integers, it isn't wrong; what would be wrong is concluding > that 'int:05' and 'int:5' are different objects from the fact that > they both occur in such a query answer. Agreed, for some things. However, for integers, a DAML+OIL (March 2001) processor needs to know that the integers 5 and 6 are different, and, similarly, that the literals "5" and "05" both map to the integer 5, and thus are the same when considered as integers. > Still, I appreciate the point that literals name objects that are > "manipulable" by the computer in a way that the object named by > 'famouscriminals:JackTheRipper' is not. What if we adopt a pragmatic > scheme that does one of the following with a literal (in order from > less to more manipulability): > > * Treat it as an uninterpretable URI > > * As Patrick suggested, view the "hostname" in the literal as a > "literal server" for the characters in it. The server can be > asked questions such as, "Do these two strings denote the same > object?" and "Is the object denoted by this literal convertable > to an object of type 'dt:int' and if so what would its > representation as an int be?" > > * Transform it into an object that the RDF parser knows how to > handle. Numbers, strings, and such would presumably fall into > this category. This is one way to go, but may have certain other consequences. In particular, the second choice may require more power from the ``literal server'' to support reasoners for formalisms that are more expressive than RDF(S). It may not even be possible to provide ``literal servers'' for some formalisms. (Consider the problems with combining multiple theories that have been partially addressed by theory resolution or concrete domains for description logics.) > This all may contradict RDF Schema in some way. I hope not. Who knows? > -- Drew McDermott Peter F. Patel-Schneider
Received on Tuesday, 2 October 2001 12:49:31 UTC