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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 12:27:51 +0200
Message-ID: <AANLkTimmsZxQIDKbRgcyuY_tFocAQOMCiIYrUorPew-b@mail.gmail.com>
To: RIF <public-rif-wg@w3.org>
As it seems [1] that equality and class membership formulas are not allowed
to appear in facts, I propose to change the second bullet in section 2.3 of
the RIF-Core spec [2] to:

* Equality terms and class membership terms *cannot* occur in universal
facts, variable-free atomic formulas outside of rule premises, or rule
conclusions -- they are allowed only in rule premises.


Best, Jos

[1] http://lists.w3.org/Archives/Public/public-rif-wg/2010May/0054.html

On Wed, May 12, 2010 at 11:10 AM, Jos de Bruijn <jos.debruijn@gmail.com>wrote:

>
>
> 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
>



-- 
Jos de Bruijn
 Web:          http://www.debruijn.net/
 LinkedIn:     http://at.linkedin.com/in/josdebruijn
Received on Wednesday, 12 May 2010 10:28:46 GMT

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