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Re: To embed or combine

From: Dave Reynolds <der@hplb.hpl.hp.com>
Date: Mon, 10 Sep 2007 13:34:20 +0100
Message-ID: <46E539CC.9090103@hplb.hpl.hp.com>
To: Chris Welty <cawelty@gmail.com>
CC: "Public-Rif-Wg (E-mail)" <public-rif-wg@w3.org>

On reflection I think we have to slice this slightly differently.

There's the question of how RIF rule sets can simply access RDF data. I 
believe that requires parts of the "Embedding triples and Graphs" 
section to be normative (see below).

Then there's the question of the various RDF(S) entailment regimes and 
how those interact with RIF. I think those are specified better by means 
of the model theoretic semantics (see below).

I also think we need a separate informative document (or document part) 
which offers a "Guide to using RIF with the semantic web" that is more 
accessible to implementers and users and which opens up the options of 
non-normative subset semantics.


** Accessing RDF data

We have a UCR requirement that RIF rules should be able to "cover RDF 
triples as data where compatible with phase 1 semantics". To me this 
means it should be possible write RIF rules which can access RDF 
statements.

This requires the syntax section (how literals and URIs are mapped) and 
the tr and tr\sub(s) mappings from "Embedding triples and Graphs" to be 
normative.

Further, as I've argued several times, in practice RDF rule languages 
include builtins like "isBlankNode". Supporting those requires a 
modification to the tr mapping to distinguish the skolem constants as 
referenced by the blue text in Jos' document.

** Entailment regimes

Since RDF is not simply data but also comes with a set of entailment 
regimes we also have to specify how RIF rule sets that access RDF data 
interact with those regimes.

[The RIF processor will also need to know what the required RDF 
entailment regime is - hence the Data Sets section in Arch.]

I think the interaction of the entailment regimes is best specified 
using a model theoretic approach as Jos has done. I say that for the 
following reasons:

o In the RDF specification the rule sets for RDF and RDFS entailment are 
purely informative, it is preferable for the RIF specification to 
reference the normative parts of the RDF specification.

o We agreed in RIF that dialect semantics should be specified model 
theoretically where possible, in preference to proof-theoretic or 
operational semantics. That seems consistent with preferring a model 
theoretic formulation of how the RDF and RIF entailment regimes interact.

o If we made the RIF rule sets for RDF and RDFS entailment normative we 
would be directing implementers to use this approach. Whereas in 
practice RDF implementations will already have solutions to especially 
the RDF entailment and should be free to use that and connect to a RIF 
processor in "black box" style if desired.

o The model theoretic approach seems more extensible towards defining 
the interaction with OWL (which we also have to do) than one based on 
translation rules.

At first I had been concerned that Jos' proposal forces the full 
RDF/RDFS semantics on implementations whereas in practice many people 
implement subsets of the entailments. However, when people want to 
support subset semantics they can do this by specifying just simple 
entailment and conveying the subset semantics by imported RIF rulesets, 
such as that for rho-df. That practice is sufficiently important that I 
think we should enable it by providing a rule import mechanism and 
document it in a non-normative "Guide to using RIF with the semantic web".

We could simply duck the question of how RDF and RIF semantics interact 
altogether. We could just specify the data access embedding and give 
neither the model theoretic nor the rule-based-embedding normative 
status. I don't think that would be ideal but would probably be 
acceptable to me.

So my conclusion is to restructure the document slightly:
   o RDF graph embedding (normative) combining current syntax section 
and definition of tr and tr\sub(s)
   o Semantics of RIF rules combined with RDF entailment, model 
theoretic, normative
   o Embedding RDF semantics, informative

Note that putting the tr definition in the first section also clarifies 
for implementers what is actually going on and may help to alleviate 
some of Michael's concerns.

Dave
-- 
Hewlett-Packard Limited
Registered Office: Cain Road, Bracknell, Berks RG12 1HN
Registered No: 690597 England

Chris Welty wrote:
> 
> 
> <chair>
> The status of the discussion regarding Jos' RDF compatibility section 
> appears to be mired in whether the normative semantics of RDF in RIF 
> should be specified in the model theory through a "combination" of RIF 
> and RDF semantics, or through an "embedding" of RDF semantics in RIF (as 
> rules).  The two approaches have been shown by Jos to be equivalent.
> 
> At the moment I have not seen any technical arguments supporting one 
> approach or the other.  Michael prefers the "embedding" on the basis that:
> 
> (1) the "combination" is more complicated than the "embedding" and thus 
> more difficult to understand.
> 
> (2) it is not our job viz. our charter to specify a model theoretic 
> approach to the RDF/RIF combination
> 
> Jos seems to prefer the "combination" and argues re: (1) that:
> 
> (3) it is no more difficult to understand the "combination" than the RIF 
> model theory.
> 
> As chair, my own read of the charter does not provide any particular 
> help on (2), I'm not quite sure what Michael is referring to there.  It 
> is certainly our job to specify how RIF and RDF should be used together, 
> and as chair I interpret this as meaning we should have a normative 
> standard for that.
> 
> Thus, as suggested by Michael, it seems to me we are at a difference in 
> preference only, and I see no alternative other than to call a vote.  It 
> seems to me the vote is about which approaches to make *normative*:
> 
> 1) The model-theoretic "combination" of RIF and RDF is normative
> 2) The "embedding" of RDF semantics as RIF rules is normative
> 3) Both the "combination" and "embedding" are normative (What would that 
> mean?)
> </chair>
> 
> -Chris
> 
> 
Received on Monday, 10 September 2007 12:34:39 GMT

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