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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 11:10:43 +0200
Message-ID: <AANLkTinv-PBIEOLWJqTRSd4tNdBvN9W65WJ-nBxg-zJX@mail.gmail.com>
To: RIF <public-rif-wg@w3.org>
On Wed, May 12, 2010 at 10:32 AM, Jos de Bruijn <jos.debruijn@gmail.com>wrote:

> 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.
>

s/allowing the/allowed in/


> 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 09:11:36 GMT

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