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Re: Problem with RIF-Core specification [was Re: Urgent: Issue with RIF-Core EBNF Grammar?]

From: Jos de Bruijn <jos.debruijn@gmail.com>
Date: Wed, 12 May 2010 10:47:15 +0200
Message-ID: <AANLkTilKqCq58fnNbnEkCDuxIJhtYc8Uj3E_1J6DNC-0@mail.gmail.com>
To: Adrian Paschke <adrian.paschke@gmx.de>
Cc: Axel Polleres <axel.polleres@deri.org>, RIF <public-rif-wg@w3.org>
I am NOT proposing anything.
I am saying the spec in ambiguous and needs to be fixed.
If you can provide any pointer to any decision related to the use of class
membership in Core, this would be useful.


Best, Jos

On Wed, May 12, 2010 at 10:44 AM, Adrian Paschke <adrian.paschke@gmx.de>wrote:

>  Hi,
>
>
>
> We discussed that in RIF Core class membership cannot occur in rule
> conclusions. Since facts can be seen as rules without premises, we probably
> did not include class membership in ATOMIC terms in the CORE EBNF.
>
>
>
> However, I see the benefit of explicitly adding class membership facts in
> Core in addition to the not materialized facts which you can import from
> external data sources.
>
> If we allow class membership facts to be represented in a Core document, we
> should clearly distinguish rules from facts in the Core spec. as Jos
> proposes it.
>
>
>
> But this would mean a change of the Core document which we should avoid at
> this point in time.
>
> So, probably we have to live with it  only imports of external class
> membership facts are possible in Core.
>
>
>
> Best, Adrian
>
>
>
> *Von:* public-rif-wg-request@w3.org [mailto:public-rif-wg-request@w3.org]
> *Im Auftrag von *Jos de Bruijn
> *Gesendet:* Mittwoch, 12. Mai 2010 10:32
> *An:* Axel Polleres
> *Cc:* RIF
> *Betreff:* Problem with RIF-Core specification [was Re: Urgent: Issue with
> RIF-Core EBNF Grammar?]
>
>
>
> Axel, all,
>
> I think there are a number of basic problems in the specification of RIF
> Core formulas. In particular:
>
> 1- the notion of "rule conclusion" is never defined. In fact, neither the
> notion "rule" nor "conclusion" is defined anywhere. This leads to several
> ambiguities: e.g., is a variable-free rule implication a rule? perhaps. Is a
> variable-free atomic formula a rule? there is no wording in BLD that would
> suggest this.
>
> 2- if we were to assume that "rule" means "RIF-BLD rule", which is the
> assumption I would naturally make from the BLD spec, then I read the
> restriction
> "Equality terms and class membership terms *cannot* occur in rule
> conclusions -- they are allowed only in rule premises."
>
> in [1] as saying that equality terms and class membership terms are not
> allowing the the conclusions of RIF-BLD rules. Full-stop.
> This means they are allowed in variable-free rule implications, universal
> facts (although some text in BLD may suggest these are a kind of RIF-BLD
> rules), and variable-free atomic formulas.
> I am quite sure we decided not to allow the assertion of equality. I do not
> recall exactly what we decided about facts concerning class membership
> (i.e., a#b). Does anybody recall what we decided here?
>
> In any case, this ambiguity needs to be resolved. Notice that the EBNF
> grammar does not help us here, since it is non-normative.
>
>
> Best, Jos
>
> [1] http://www.w3.org/TR/2010/PR-rif-core-20100511/#Formulas_of_RIF-Core
>
> On Tue, May 11, 2010 at 6:15 PM, Axel Polleres <axel.polleres@deri.org>
> wrote:
>
> Hi folks,
>
> We are working on a parser with some students and I am afraid my student
> found something awkward in the RIF Core grammar, see mail below.
>
> Indeed, I think he poked into a quite weird issue:
> It doesn't make sense to allow class membership terms in rule bodies, if
> they can't appear at all in *any* facts.
> The current grammar and the restrictions in Section 2.3 though only allows
> uniterms and frames as facts.
>
>
> To repair this
>
> 1) We'd need to change in Section 2.3 Formulas of RIF-Core:
>
>  * Equality terms and class membership terms cannot occur in rule
> conclusions -- they are allowed only in rule premises.
> -->
>  * Equality terms cannot occur in rule conclusions -- they are allowed only
> in rule premises.
>  * Class membership terms can only occur in rule premises or as ground
> facts.
>
> 2) a proposal to fix the grammar in Section 2.6 would  be:
>
> In the Rule Language grammar:
>
>  CLAUSE         ::= Implies | ATOMIC
>  -->
>  CLAUSE         ::= Implies | ATOMIC | GROUNDTERM '#' GROUNDTERM
>
>
>
> sorry for spotting this now only, but I am afraid this is severe.
> the fix is not very problematic, though.
>
> Axel
>
>
>
>
> Begin forwarded message:
>
> > From: "Obermeier, Philipp" <philipp.obermeier@deri.org>
> > Date: 11 May 2010 16:26:50 GMT+01:00
> > To: "Polleres, Axel" <axel.polleres@deri.org>
> > Cc: "Marco Marano" <marcomarano83@gmail.com>
> > Subject: RIF-Core: EBNF, equality/memberhip facts
> >
> > Hi Axel,
> >
> > I found a minor error in the EBNF grammar [1] for RIF-Core (Altough,
> > this grammar is informative due to the lack of well-formedness checks,
> > it is also defined as strict superset of RIF-Core.).  Within this
> > grammar you cannot derive Equality or Membership (ground) facts since
> > the ATOMIC rule's rhs is restricted to atomic formulas excluding
> > Equality/Membership formulas. Apparently, this restriction is well
> > justified since ATOMIC may appear in rule heads (cf. IMPLIES rule's
> > rhs), for which Core forbids Equality and Membership formulas. In
> > conclusion, an introduction of a new ATOMIC_FACTS grammar rule extending
> > ATOMIC by Membership/Equality  would solve this issue w/o breaking the
> > restriction for atoms in rule heads.
> >
> > Best
> > Philipp
> >
> > [1]
> >
> http://www.w3.org/2005/rules/wiki/Core#EBNF_Grammar_for_the_Presentation_Syntax_of_RIF-Core
> >
> > --
> > Philipp Obermeier
> > Digital Enterprise Research Institute, National University of Ireland,
> Galway
> > email: philipp.obermeier@deri.org
>
>
>
>
> --
> Jos de Bruijn
>  Web:          http://www.debruijn.net/
>  LinkedIn:     http://at.linkedin.com/in/josdebruijn
>



-- 
Jos de Bruijn
 Web:          http://www.debruijn.net/
 LinkedIn:     http://at.linkedin.com/in/josdebruijn
Received on Wednesday, 12 May 2010 08:48:12 GMT

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