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RE: Subjects as Literals, [was Re: The Ordered List Ontology]

From: Michael Schneider <schneid@fzi.de>
Date: Fri, 2 Jul 2010 11:26:32 +0200
Message-ID: <0EF30CAA69519C4CB91D01481AEA06A001F07C0C@judith.fzi.de>
To: "Pat Hayes" <phayes@ihmc.us>
Cc: "Linked Data community" <public-lod@w3.org>, "Semantic Web" <semantic-web@w3.org>
Pat Hayes wrote:

>Just to clarify, this is a purely syntactic restriction. Allowing
>literals in subject position would require **no change at all** to the
>RDF semantics. 

Indeed.

And this is probably one of the reasons why several RDF-related standards
have already adopted literal subjects. Some examples:

* For the OWL 2 RDF-Based Semantics, being an extension of the RDF
Semantics, it has been explicitly allowed (in the "MAY" sense) to apply it
to "generalized RDF", where literals and bNodes may occur in any position of
a triple:

    <http://www.w3.org/TR/owl2-rdf-based-semantics/#topic-ont-generalrdf>

(However, OWL 2 Full is still defined to be the language that consists of
the RDF-Based Semantics and the official RDF Syntax, i.e. where subjects are
not allowed to be literals.)

* The OWL 2 RL/RDF Ruleset also allows for generalized RDF, see the second
paragraph of 

 
<http://www.w3.org/2007/OWL/wiki/Profiles#Reasoning_in_OWL_2_RL_and_RDF_Grap
hs_using_Rules>

* The RIF RDF and OWL Compatibility specification (just became a W3C
Recommendation) also uses generalized RDF, see the note at

 
<http://www.w3.org/TR/2010/REC-rif-rdf-owl-20100622/#note-generalized-rdf-gr
aphs>

and the definitions at

 
<http://www.w3.org/TR/2010/REC-rif-rdf-owl-20100622/#RDF_Vocabularies_and_Gr
aphs>
    
* SPARQL 1.1 (still work in progress) allows literal subjects in Triple
Patterns:

    <http://www.w3.org/TR/sparql11-query/#defn_TriplePattern> 

I could imagine that this list of important technologies allowing for
literal subjects will grow over time, and maybe there even exist some more
today that I missed. If RDF2 would also adopt generalized RDF, all these
RDF-dependent standards could then be updated by dropping all these "MAY"s
and similar statements.

Michael

--
Dipl.-Inform. Michael Schneider
Research Scientist, Information Process Engineering (IPE)
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Email: michael.schneider@fzi.de
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Received on Friday, 2 July 2010 09:27:12 UTC

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