Re: Literals (Re: model theory for RDF/S)

>    [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?

Right. RDF has no notion of equality or equivalence (well, it does 
now for class extensions since it allows subclass loops, but never 
mind that).

>
>    Consider
>
>    #Susan #favorite-integer int:05 .
>    #Susan #favorite-integer int:5 .
>
>    how is an RDF query system supposed to respond when asked about Susan's
>    favorite integers?

It should say : int:05 and int:5 . If you could ask it how many 
integers that is (you can't in RDF, but if you could) it ought to say 
it doesn't know.

>    ...
>
>    It is true that you can make a consistent view of all this from this
>    ``RDF'' viewpoint, but you do have to be a bit careful.  In particular, if
>    you want to allow RDF to be consistent with different URI schemes, you have
>    to modify the "one-URI, one-Resource" philosophy to a "one-URI, possibly
>    one-Resource". 
>
>    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.

Right, I agree.

>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.
>
>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 all may contradict RDF Schema in some way.  I hope not.

I don't think it does.

Pat

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Received on Wednesday, 3 October 2001 17:09:20 UTC