Re: Review XML Data in RIF

Hi Michael,

Michael Kifer <kifer@cs.stonybrook.edu> wrote on 19/10/2010 06:03:54:
> 
> I was tied up, so I took a rather superficial look at the new document. 
It is
> in a much better shape than before. In particular, I am happy that the
> infomodel stuff is gone.

Great. Although the infomodel is not gone, of course. Only, now, you have 
to read XDM by yourself :-)

Michael Kifer <kifer@cs.stonybrook.edu> wrote on 19/10/2010 06:18:55:
> 
> Sorry, didn't have a chance to look at it before, but the whole purpose 
of
> defining the models is so that then one could define entailment. Without 
the
> entailment there is not much use for the notion of a model.
> 
> Regarding the notion of entailment, I think it is not quite correct 
> (Definition
> Logical entailment in a RIF BLD+XML data combination). We should be 
> talking about one BLD+XML combo <phi,E,S> entailing another, <psi,E',S>.
> Perhaps actually just entailing psi. Just phi entailing psi seems 
incorrect.

Without earlier response to my email about entailment, here is what I had 
put in the spec. I think that it goes in the direction you say:

Definition (Logical entailment in a RIF BLD+XML data combination). Let φ 
and ψ be (document or non-document) formulas. We say that φ entails ψ in a 
RIF BLD+XML data combination <φ, E, S> if and only if every model of the 
RIF BLD+XML data combination <φ, E, S> is also a model of the RIF BLD+XML 
data combination <ψ, E, S>.

I just rewrote it, as follows, to make the intent clearer (and the 
definition less ambiguous):

Definition (Logical entailment of a RIF BLD+XML data combination). Let φ 
and ψ be (document or non-document) formulas. We say that a RIF BLD+XML 
data combination <φ, E, S> entails the RIF BLD+XML data combination <ψ, E, 
S> if and only if every model of the RIF BLD+XML data combination <φ, E, 
S> is also a model of the RIF BLD+XML data combination <ψ, E, S>.


> My understanding is that the XML part, E, is just another form of data, 
ie, a
> bunch of facts.

My problem with the above definition is, indeed, that entailment is 
defined wrt a specific set of data only, which might be too restrictive to 
be useful.

I mean, combination with XML data adds semantics from XPath, and 
combination with an XML schema adds semantics as well...

So, really, we could have entailment wrt a specific combination of data E 
and schema S, entailment wrt any combinations including a specific schema 
S, and entailment in a combination with XML data, in general.

I mean, if expr is an XPath expression, then ?x["fn:data(expr)"->?z] :- 
?x["expr"->?y] entails ?y["fn:data(.)"->?z] :- ?x["expr"->?y] in the 
context of any RIF BLD+XML data combination, due to the semantics of XPath 
expressions, but not in RIF BLD in general (which, btw, proves that the 
semantics of a combination of RIF with empty XML data and schema is not 
equivalent to the semantics of pure RIF: the simple fact of stating that a 
RIF doc is interpreted as in a combination with XML data, even in the 
absence of data and associated schema, changes its semantics. Notice, 
however, that a RIF doc that imports no XML data nor XML schema is _not_ 
the same as a RIF combination with empty XML data and schema).

> So, we should be definitely talking about <phi,E,S> entailing something.
> Now, the question is what exactly do we want to entail? I think psi 
> should be a
> query, ie, a BLD body formula. Do we want to entail a piece of XML as 
well?
> I am not sure. We need to see a use case of what exactly the intent is.

See the example, above, where psi is a rule...

> So, I take the approval in my previous message back. I think we needto 
clarify
> this point.

Since we do not have the WG to discuss it, either it is easy to clarify, 
and we can do that quickly among you, Adrian, Gary and myself; of we 
publish with an editor's note.

In the latter case, I propose to include the most general definition (the 
one that accounts for the example, above), which seems more useful than 
the most restrictive one (the one currently in the document):

Definition (Logical entailment in a RIF BLD+XML data combination). Let φ 
and ψ be (document or non-document) formulas. We say that φ entails ψ in a 
RIF BLD+XML data combination if and only if every model (E, ?, S) of any 
RIF BLD+XML data combination <φ, E, S> is also a model of the RIF BLD+XML 
data combination <ψ, E, S>.

And an editor's note to the effect that other definitions are possible, 
and that this need be clarified before moving to last call.

What do you think?

Cheers,

Christian


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