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Re: [RDF-OWL] what if there are no graphs at import locations?

From: Jos de Bruijn <debruijn@inf.unibz.it>
Date: Fri, 13 Feb 2009 10:08:08 +0100
Message-ID: <49953878.9010602@inf.unibz.it>
To: Axel Polleres <axel.polleres@deri.org>
CC: RIF WG <public-rif-wg@w3.org>
Then about SPARQL:
you were citing the wrong piece of text; it does not specify how the
default data set is obtained from the FROM clause.

The following text in section 8.2.1 does seem to support your argument:
"Each FROM clause contains an IRI that indicates a graph to be used to
form the default graph. "

one wonders, though, what it means for an IRI to "indicate" a graph.
Note that we are not talking about named graphs, but about RDF graphs
(which do not have a name).

I did not find any formal definitions about evaluating SPARQL queries
including FROM clauses, so I could really verify what they mean
precisely with "indicate".


Best, Jos

Jos de Bruijn wrote:
> As a clarification:
> the OWL2 document you are referring to is merely a "normal" working
> draft; it is not in last call, and I know for a fact that there have
> already been substantial (and substantive) changes in the document since
> the working draft of December.  So, this is not something we should base
> our decisions on.
> 
> The section from the OWL semantics you are referring to is concerned
> with the abstract syntax, not with RDF documents on the web.  In fact,
> when going to the RDF world in section 5.4 [1] I read:
> "[A collection of OWL DL ontologies] O is said to be imports closed iff
> for any URI, u, in an imports directive in any ontology in O the RDF
> parsing of the document accessible on the Web at u results in T(K),
> where K is the ontology in O with name u."
> 
> This is very much in line with what we wrote in the RDF and OWL
> compatibility document.
> 
> all that said, I don't care too much about this issue.  But we need to
> get our facts straight when referring to other specifications.
> 
> 
> Best, Jos
> 
> [1] http://www.w3.org/TR/owl-semantics/rdfs.html#5.4
> 
> Axel Polleres wrote:
>> I am a bit surprised to see that we refer to a "RDF graphs accessible
>> from the locations u1,...,un" in RIF imports, which suggests that we
>> talk about URLs here, rather than URIs.
>>
>> Neither OWL, nor SPARQL, nor OWL2 do this:
>>
>> OWL [2, Section 3,4]:
>> "The imported ontology is the one, if any, that has as name the argument
>> of the imports construct. (This treatment of imports is divorced from
>> Web issues. The intended use of names for OWL ontologies is to make the
>> name be the location of the ontology on the Web, but this is outside of
>> this formal treatment.)"
>>
>> SPARQL [3, section 8]:
>> "A SPARQL query is executed against an RDF Dataset which represents a
>> collection of graphs. An RDF Dataset comprises one graph, the default
>> graph, which does not have a name, and zero or more named graphs, where
>> each named graph is identified by an IRI."
>>
>> OWL2 [4, Section 9]:
>> "Definition 3.1 (Import Closure): Let K be a collection of RDF graphs. K
>> is imports closed iff for every triple in any element of K of the form x
>> owl:imports u then K contains a graph that is referred to by u. The
>> imports closure of a collection of RDF graphs is the smallest imports
>> closed collection of RDF graphs containing the graphs."
>>
>> Neither of these specs require the URI/IRI of an ontology (or for a
>> named graph in the case of SPARQL) to be dereferenceable on the Web, but
>> this is - IMO intentionally - left open in the specs, just mentioning
>> that the URI/IRI at identifies a graph/ontology. How this identification
>> is specified is not part of the specs. While accessing the URI as a URL
>> from the Web might be the default behavior, there are use cases where
>> this may not be desirable (e.g. in a Triple store which has several
>> named graphs stored, these graphs may not be (web) accessible, but only
>> be called by these "names" within the triple store.)
>>
>> Likewise, I would be reluctant if we made any stronger assumptions here,
>> which might be restrictive. I rather suggest to adopt something similar
>> to the formulation in OWL above.
>>
>> Long written, briefly summarized:
>> I suggest to replace
>> "accessible from the locations u1,...,un"
>> by
>> "referred to by u1,...,un"
>>
>>
>> This is though not directly related to your question, I see. But we
>> could state e.g. something like "If there is no RDF graph (or, resp.
>> ontology) referred to by uri u_i in an imports statement, the respective
>>   graph SHOULD be treated as empty." (in case this is the behavior we
>> want to advocate)
>>
>> Axel
>>
>> 2. http://www.w3.org/TR/owl-semantics/direct.html
>> 3. http://www.w3.org/TR/rdf-sparql-query/
>> 4. http://www.w3.org/TR/2008/WD-owl2-rdf-based-semantics-20081202/
>> Jos de Bruijn wrote:
>>> In section 5.2 of the RDF-OWL document [1] we specify how RIF documents
>>> with two-ary import statements must be interpreted. however, we do not
>>> say anything about the case that an import statement refers to a
>>> location does not have an RDF graph.
>>>
>>> So, if
>>>
>>> Import(<u1> <p1>)
>>>   ...
>>> Import(<un> <pn>)
>>>
>>> are the 2-ary import statements and one of u1,...,un does not point to
>>> an RDF graph, what should happen?  Do we say that the document could be
>>> rejected, or do we leave this unspecified?
>>>
>>> related: what if pi denotes the OWL DL profile, but ui does not point to
>>> an OWL DL ontology?  Should the document be rejected?  I think so.
>>>
>>> Best, Jos
>>>
>>>
>>> [1] http://www.w3.org/2005/rules/wiki/SWC#Interpretation_of_Profiles
>>
> 

-- 
Jos de Bruijn            debruijn@inf.unibz.it
+390471016224         http://www.debruijn.net/
----------------------------------------------
No one who cannot rejoice in the discovery of
his own mistakes deserves to be called a
scholar.
  - Donald Foster


Received on Friday, 13 February 2009 09:08:11 GMT

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