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Urgent: Issue with RIF-Core EBNF Grammar?

From: Axel Polleres <axel.polleres@deri.org>
Date: Tue, 11 May 2010 17:15:01 +0100
To: RIF <public-rif-wg@w3.org>
Message-Id: <7A7B5D25-8407-4DBA-B374-FFFC8D32C9E1@deri.org>
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
Received on Tuesday, 11 May 2010 16:15:35 GMT

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