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Re: minimal requirements for Arch document

From: Axel Polleres <axel.polleres@deri.org>
Date: Fri, 12 Oct 2007 15:44:19 +0100
Message-ID: <470F8843.4080500@deri.org>
To: Sandro Hawke <sandro@w3.org>
CC: public-rif-wg@w3.org

Sandro Hawke wrote:
> I'm trying to think in minimalist, practical terms about Arch.  What do
> we really need to say?  I see a few things:
>     1.  What is required of all systems which take RIF documents as
>         input?   ("Minimal Requirements for all RIF Systems").

I earlier suggested for  this requirement

"BLD should define a minimal dialect and other dialects MUST agree
  with the semantics of BLD on the part of the abstract model they share
  with BLD"

(Michael later suggested to correct this to logic-based dialect only...
though I thought this was covered by "on the part of the abstract model 
they share with BLD")

>         Until we have a RIF Core, there's not very much to say here.
>         Maybe if we end up with BLD and PRD but not Core, we'll say it
>         has to conform to at least one of the two of them.

Well, that would make PRD also kind of a "core" then.

>         One thing we do need to mandate here is forward compatibility;
>         how must a system behave when given a RIF document which does
>         not conform to the syntax of a dialect it implements?  This
>         section could get long if we go with a powerful fallback
>         mechanism.

Indeed, that will need some work.
There are several fallback options we discussed (soundness-preserving
ignoring of rule parts which don't conform, soundness-preserving
ignoring of whole rules which don't conform, rejecting the ruleset) and 
which we need to define and order.

>         I think the BLD document needs a conformance clause.  Or maybe
>         that goes in the BLD Test Cases document (as it did with OWL).

Syntactic conformance and semantic conformance in the sense of the OWL 
document should be doable.

However, in what sense would we define conformance of *engines*, or, do 
we want to define this at all?  (This seems to be non-trivial for me, 
and maybe not necessary)

>     2.  What does one need to do to define a proper RIF dialect?
>         ("Publishing a New RIF Dialect")
>         I suggest that the basic rules are:
>             * No Language Conflict: every dialect MUST give the same
>               semantics as each prior dialect does to any document
>               which has a defined meaning in both dialects.

Michael wrote here:
"This may be too strict. An extension dialect should be allowed to make 
more inferences, but should not invalidate existing inferences of the 

This reminds me of what Jos called "loose and strict language layering" 
[1], which I try to write here a bit simplified/adapted (although this 
notion might only be useful for logic dialects):

Let D1, D2 be dialects with semantics S1 ,S2 such that D2 syntactically 
extends D1. Dialect D2 is strictly layered on top of D1, if for any 
ruleset r1 in D1 and every condition c1 in L1, c1 is entailed by r1 wrt. 
semantics S1 if and only if c1 is entailed by r1 wrt. semantics S2.
Otherwise, D2 is loosely layered on top of D1, if for any ruleset r1 in 
D1 all he entailed conditions in S1 are also entailed in S2.

However, we can decide for either solution, but we need to pick one, it 

1. Jos de Bruijn and Stijn Heymans. A semantic framework for language 
layering in WSML. In Proceedings of the First International Conference 
on Web Reasoning and Rule Systems (RR2007), pages 103-117, Innsbruck, 
Austria, June 7-8 2007. Springer.

>             * Maximize Overlap: every dialect SHOULD reuse as much of
>               the the syntax as possible from prior dialects. 

Yes, although this is indeed more a principle only than something which 
we can formally check and which we can only state by examples, I guess.

>         I think we should try defining some dialects using these
>         principles, coordinating loosely as we like, but eventually I
>         think we need to figure out how to open the process to 3rd
>         parties.  That's going to involve some careful work around
>         defining "prior dialect".

indeed, it wasn't clear what "prior" means. If such a thing should be 
kept clean, there would need to be an outhority (in W3C?) which would
approve dialects and assess whether the principles have been followed 
and compare among existing, ie. prior, dialects. Without such a body, 
that seems to be as unlikely though as enforcing reuse of ontologies and 
RDF vocabularies, honestly. But such a thing is not in the charter of 
the working group, we can only define guidelines and principles here at 
best, probably.

>     3.  What does one need to do to define a RIF extension?
>         As I see it, an extensions is a "delta" between dialects where
>         one dialect is a superset of the other.
>              NewDialect = OldDialect + Extension

As mentioned earlier, a diealect shouls also be allowed to restrict an 
old one syntactically, or build upon a restricted subset. (example 
function-free normal logic programms are an extension of a reestriction 
of BLD)

>         What is challenging about extensions is that we want them to
>         be orthogonal; we want users to be able to combine extensions
>         which were developed independently.  For this example, I'll
>         assume Lists end up in an extension, instead of in BLD.  

If we allow restrictions and extensions, we don't run into this problem, 
we could leave lists, slots, etc. in BLD, without forbidding restricted 
dialects, as long as they comply with rif on the syntactic intersection.

> I don't
>         want to do that, but it makes a good example:
>              BLD_with_NAF = BLD + NAF_Extension
>                     The BLD_with_NAF dialect should be fully specified
>                     by the NAF_Extension spec read in combination with
>                     BLD.
>              BLD_with_Lists = BLD + Lists_Extension
>                     The BLD_with_Lists dialect should be fully specified
>                     by the Lists_Extension spec read in combination with
>                     BLD.
>              BLD_with_Lists_and_NAF = BLD + NAF_Extension + Lists_Extension
>                     We would like the semantics here to be fully
>                     determined by the two extension specs and BLD.  This
>                     is the challenge.  How can the documents be written
>                     such that this is the case?
>         At the syntactic level this is clear enough, if you think of the
>         sets of strings/documents conforming to the syntax.  An
>         extension provides a set of strings, and "+" above is set-union.
>         The question is how do we address this at the semantic level?
>         Is there a way to address it across all approaches to defining
>         semantics, or is this easy to do for model-theoretic semantics
>         and impossible for procedural semantics?  (My sense is that it's
>         trivial for proof-theoretic semantics; I'm unclear on the
>         others.  I think Michael Kifer has in mind how to do this with
>         MT semantics on BLD, but I don't understand that part yet.)
>         It may well be that some extensions are incompatible (such as
>         NAF and classical negation?), in which case the combination
>         procedure should fail, I would hope.
> Note that I see no need to mention abstract syntaxes or presentation
> syntaxes.  For these purposes, all we care about is the XML.  (I'm not
> thrilled about it, but I can't find a compelling reason to address
> more than XML in this material.  At least, not yet.)

I thought only that each dialect should be free to define it's own 
syntax, as long as it has an XML syntax and two-way
(bijective or semantic equivalent for the dialect, to be decided?)
mapping for the dialect.

> Oh yeah, and external data and data models.  I keep forgetting about
> that.  Or is that in an extension?  :-) [the charter puts it in Phase 2,
> remember.]
>      -- Sandro

Dr. Axel Polleres
email: axel@polleres.net  url: http://www.polleres.net/
Received on Friday, 12 October 2007 14:44:33 UTC

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