From: pat hayes (phayes@ai.uwf.edu)
Date: 05/30/01
>>>>>(2) A single semantics for one kind of language. We will have
>>>>>multiple languages represented in RDF, e.g.
>>>>>UML (see http://www-db.stanford.edu/~melnik/rdf/uml/ ) and it
>>>>>would be nice if the rule language we
>>>>>design is able to deal with multiple languages at the same time.
>>>>
>>>>Again, I have no idea what you mean. How can *a* (note singular)
>>>>rule language deal with multiple languages? ("Deal with" in what
>>>>sense? )
>>>
>>>Be able to compute the deductive closure of a single set of RDF
>>>statements with respect to a set of rules.
>>
>>Stefan, according to you there is no such thing as the deductive
>>closure of a set of RDF statements. You have said earlier in this
>>thread that RDF triples have no logical interpretation, and in this
>>message you say that RDF is a datastructuring language. Concepts
>>like 'deductive closure' only apply to languages with a proof
>>theory. What are the inference rules of RDF?
>
>I meant the deductive closure of the Facts + Rules. Sorry for the sloppiness.
>RDF does not have inference rules, in the same way that a set of tuples has
>no inference rules. Together with a rule language (e.g. SQL) I might
>have inference
>rules.
Maybe we are still not following one another. As I understand your
position, the rules themselves do not have a semantics (right?). So
they are not *inference* rules, and it still seems to me not to make
sense to speak of a deductive closure. In fact, if the RDF triples
don't have a semantics (ie if RDF is a datastructuring language) then
the triples are not even properly called 'facts'. A triple without
any meaning assigned to it is just a datastructure, not a fact. It
might be intended to represent a fact, or a picture, or a diagram, or
a mathematical expression, or just about anything, or even nothing.
Without a semantics, there is no way to tell.
>>>>And in any case what does it mean to say that there will be
>>>>multiple languages "represented" in RDF? In the example in
>>>>http://www-db.stanford.edu/~melnik/rdf/uml/ , the RDF graph is
>>>>being used simply as a graph to encode a state diagram,
>>>>in a way that completely ignores the RDF semantics (such as it
>>>>is). If that is 'representation', then RDF is just being used as
>>>>a datastructuring language.
>>>
>>>If I store a statemachine in a database, do I ignore the logical
>>>semantics of a database?
>>>Probably yes, but I can live with it and I guess other people can also.
>>
>>I guess at this point we should just declare that we are working in
>>different worlds. I feel like someone who is trying to design
>>better cars, arguing with someone who insists that an internal
>>combustion engine can be used to cook hamburgers. Of course you are
>>free to you store a statemachine in a database, but it seems to me
>>that the designers of the database language are then under no
>>obligation to support you or accomodate to what you are doing. If
>>you do things like that, you are on your own.
>
>If a database vendor wants to restrict in me what kind of data I'm
>allowed to store in my database,
>I don't buy the database.
No, of course, but that is a different issue. You didn't say that you
were storing *data about* a statemachine; you said you were storing
the statemachine. That is what Melnik is doing, seems to me: he is
using the 'data' not as data at all, but as an encoding for something
entirely different. Now, one can do that, of course. You CAN use
predicate logic syntax to encode musical notation (use depth of
function nesting for pitch, the constant at the bottom of the term
for length of note, and clause ordering to indicate time-sequence)
but if you do that, then you have no legitimate complaint if a Prolog
interpreter fails to handle key shifts, say. The Prolog isnt meant to
be used that way, and that way of using the notation does not conform
to its declared semantics. Similarly, if you take a relational
assertion language and use it to encode finite-state transition
diagrams, then you cannot complain if the rule processor fails to
conform to the conventions of your idiosyncratic usage; and, more to
the present point, the designer of the rule language is not obliged
to consider such idiosyncratic uses. If people want to write
arbitrary encodings with perfect freedom, then they should be writing
code, not putting data into datbases; and they can't expect to get
interoperability.
>An example: look at http://www.bpmi.org
>These people are building an Ontology for representing business processes.
>This incorporates states, events, dynamics etc.
>Are you saying they are not allowed to express these concepts in an
>ontology language
Of course not
>and
>express it in RDF? (Actually, they use XML Schema).
Sure, though I think their chances of being able to do so in RDF are
remote. [Added later: did you really mean *express* it in RDF? Or did
you mean encode the datastructures of the ontology language in RDF?
Of course they can do the latter, but that's not what I would call
expressing the ontology in RDF.]
But look: there is a distinction between building an ontology in an
ontology langauge, and constructing datastructures in a datastructure
language. In the first case the language has a semantics; in the
second case, it does not (or at any rate, if it does, it is a
semantics about datastructures, not about the concepts in the
ontology.)
>>>And yes, RDF is a datastructuring language.
>>
>>Well, Stefan, that is not what the rest of the RDF community seem
>>to be saying. I wish y'all would get your act together and come out
>>with a single consistent story. If RDF is just a datastructuring
>>language, then what has it got to do with the Semantic Web? And
>>what advantages does it have over LISP (say), or XML ?
>
>It as consistent as every community where different people with
>different opinions and different
>background try to work together.
That says nothing about the utility of RDF over XML (or LISP, for that matter).
Pat
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