From: pat hayes (phayes@ai.uwf.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.
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.
>>>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.
>(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? )
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.
Melnik seems to be almost proactive in ignoring meaning. He says in
one email (referred to in that paper):
IMO people who are worried about "logical" consistency forget about the
"crystal-palace" nature of the mathematics: the set theory is formalized
using the predicate logic which relies upon the set theory. At some
point, people as well as machines need a built-in understanding of a few
basic concepts to communicate meaningfully. Mathematicians needed
centuries to develop such a "built-in understanding". For machines,
executable code can be a good approximation.
There isn't space here to document all that is wrong with this, but
let me just say that one really shouldnt venture into philosophy of
mathematics until they know what they are talking about.
>Model identifier and skolem functions enable this.
Skolem functions just create new names; they don't create new
languages. Model identifiers denote, you tell me, sets of RDF
triples. But since, for you, these are just triple-encodings of
uninterpreted graphs, I fail to see how they can define a language
either.
>>>>>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.
Then I fail to see the utility of adding the '@M', since this
information is redundant.
>>? 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? )
I have no idea; I was just probing to find out what the '@' notation meant.
Pat
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