From: pat hayes (phayes@ai.uwf.edu)
Date: 05/30/01
>Pat,
>
>At 11:00 AM 5/30/2001 -0500, you wrote:
>>>Pat,
>>>
>>>At 07:51 PM 5/29/2001 -0500, pat hayes wrote:
>>>>>Hi Pat,
>>>>>
>>>>>At 05:58 PM 5/29/2001 -0500, pat hayes wrote:
>>>>>>>Hi,
>>>>>>>
>>>>>>>some additions to Pats suggestion:
>>>>>>>
>>>>>>>1) In many applications it is important to distinguish between
>>>>>>>different kind of RDF data, eg.
>>>>>>>between different sources of RDF data, one is trustworthy, the other one
>>>>>>>not.
>>>>>>
>>>>>>That seems to me to be an assertion about the source rather
>>>>>>than the data (?) But in any case it goes well beyond the RDF
>>>>>>or DAML semantics.
>>>>>
>>>>>If we built a rule language for processing of RDF data it is
>>>>>necessary to distinguish
>>>>>different sets of RDF data.
>>>>
>>>>Well OK, but I don't see why this topic comes up particularly
>>>>when we talk of rules. If you want to distinguish sets of RDF
>>>>data, why not do it in RDF? After all, RDF has the ability to
>>>>describe its own expressions, so this ought to fit into it
>>>>naturally. Why not treat an set of RDF data as something that is
>>>>referrable to by a URI, ie a resource?
>>>It is indeed possible, but very cumbersome. All implementations I
>>>am aware of have opted to represent
>>>the context explicitly, rather than encoding it into RDF itself.
>>
>>Of course RDF is cumbersome, by its very nature. The RDF encoding
>>of lists that you invented for us is cumbersome; the use of
>>reification for encoding propositional structure is cumbersome; the
>>encoding of any relation with arity >2 is cumbersome. How can you
>>be so enthusiastic about the use of RDF in all its cumbersomeness
>>for all that, and suddenly get all concerned with memory counting
>>when it comes to contexts? I do not understand what your motivation
>>is here.
>>
>>>But there are two different aspects: the query and rule language,
>>>and the actual representation in terms
>>>of tuples.
>>>For both I vote to make the context explicit:
>>>the context of RDF data is necessary in almost all applications
>>>which take different sources into account, so
>>>a rule and query language should have a convenient access to the
>>>context information.
>>>
>>>If not an explicit representation is chosen and reification is
>>>used to represent context, the
>>>amount of storage necessary multiplies by 5 - which is not
>>>acceptable for large amounts of data.
>>
>>Why by 5? It seems only to add one extra triple per entry, which is
>>a multiple of 2 at worst.
>4 for representing the reification, one for the isTrueInModel
>property. That means 5 triples instead of one.
I thought you meant reification of the context, not the assertion.
Why is there any need to reify the assertion?
>>>>>One example where it is important to distinguish between
>>>>>different kind of sets of RDF data is when it comes from
>>>>>different sources, and that
>>>>>it originates from different sources is a property of the data.
>>>>>There are other applications areas, e.g. the computation of
>>>>>different semantics (see my RDF Schema
>>>>>example).
>>>>
>>>>We really must speak different languages.
>>>>(1) To speak of "computing" semantics doesnt make sense to me;
>>>>and (2) why would we want to be using different semantics in any
>>>>case? Isnt the whole idea to have a single semantics?
>>>f(1) E.g. Dix and Brewka in [1] define semantics as follows:
>>>
>>>"A semantics SEM is a mapping from the class of all programs into
>>>the powerset of the
>>>set of all 3 valued structured. SEM assigns to every program P a
>>>set of 3-valued models of P:
>>>SEM(P) \subseteq MOD^LP_{3-val}(P)"
>>>
>>>My remark above is (admittedly loosely) based on this notion of
>>>semantics as the deductive closure.
>>
>>Oh, come on. This is a misreading of an idiosyncratic usage in a
>>minor subarea. I don't have the book in front of me, but from the
>>quote they seem to be making a distinction between the model and
>>the deductive closure in any case.
>
>Taking only herbrand models into account (also common in the minor
>subarea) there is a direct correspondence
>between the deductive closure of a logic program and models of that program.
Well, you never mentioned Herbrand models. But in any case, while
there is a correspondence, but its hardly an isomorphism. A logic
program terminates when it runs out of Herbrand models, not when it
constructs one.
>>>(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?
>>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.
>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 ?
Pat
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