From: Stefan Decker (stefan@db.stanford.edu)
Date: 05/30/01
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.
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.
>>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.
[1] Knowledge Representation with Logic Program
Handbook of Philosophical Logic, 2nd edition, Volume 6,
Chapter 6, Oxford University Press, 2001.
(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.
Model identifier and skolem functions enable this.
>>>>This needs to be reflected in the rule language - it is not sufficient
>>>>to just query if a certain
>>>>fact is present. To distinguish between different sources would be
>>>>enabled by model identifiers
>>>>
>>>>subject[predicate->object]@model
>>>
>>>I have no idea what you are talking about. What is a 'model' in this sense?
>>
>>A set of RDF statements (triples).
>
>Ah, I see. What is the difference between saying, in M, that
>subject[predicate->object] is true, and saying that
>subject[predicate->object]@M
Thats identical.
>? Can I say subject[predicate->object]@M in N where N is different from M?
no.
(actually, I think we could if we define vocabulary representing the "true
in a model"
property and use reification. There are reasons not to do this (see above).
Would it be useful? )
Stefan
>Pat
>
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