RE: a blank node issue

Hi all!

Within the last few days my mailbox must have been hit by more mails about blank node issues as there are different blank node identifiers on the Web. It's these old discussions again and again: Should the syntactic scope of blank nodes be graph-local or global? Should the semantics of blank nodes be that of existentially quantified variables or skolem constants? Should there be blank node identifiers in the abstract RDF model or not? Some people seem to think that what was meant to be a syntactic property of blank nodes is actually a semantic one. Etc.

As anyone else, I have strong opinions on all these questions, but it's Friday afternoon and I don't like to add just more comments here. Instead, I have created a small questionnaire based on some simple examples that take both syntactic and semantic aspects into account. It should be easy for anyone, depending on his or her preferences, to put the cross at the right place. Enjoy!  :-)

Cheers,
Michael

===== BEGIN BLANK NODE QUESTIONNAIRE ===

Section 1: Syntactic Scope of Blank Nodes
-----------------------------------------

When merging the two below RDF graphs G1 and G2 into a single graph, what would be your /preferred/ result, regardless what the current RDF spec sais (all examples given in Turtle syntax)?

G1: {
    ex:FoafNamedPerson rdfs:subClassOf _:x .
    _:x rdf:type owl:Restriction .
    _:x owl:onProperty foaf:name .
    _:x owl:allValuesFrom xsd:string .
}

G2: {
    :alice foaf:knows _:x .
    _:x rdf:type foaf:Person .
    _:x foaf:name "Bob" .
}

[ ] Pro graph-local blank nodes: 
    The blank node identifiers should be /renamed/.
    That is, after merging there will be /two/ distinct blank 
    node identifiers, one (e.g. "_:x1") substituting the "_:x" coming 
    from G1, and another one ("e.g. "_:x2") substituting the "_:x" 
    from G2.

[ ] Pro global blank nodes: 
    The blank node identifiers should /not/ be renamed.
    That is, the result graph will still contain
    one single blank node identifier named "_:x".

[ ] I don't know / I have no opinion

Section 2: Semantics of Blank Nodes
-----------------------------------

Which of the following proposed "Simple" entailments should hold ("Yes") or not ("No") according to your opinion, regardless what the current RDF spec sais about Simple entailment in Sec. 2 of the RDF Semantics (format: "PREMISE-GRAPH |= CONCLUSION-GRAPH"; all examples given in Turtle syntax)?

* Example 1 (graph serializations are exactly the same):

{  :alice foaf:knows _:x . _:x foaf:name "Bob" . }
|=
{  :alice foaf:knows _:x . _:x foaf:name "Bob" . }

[ ] Yes, this should be an entailment
[ ] No, this should be a non-entailment
[ ] I don't know / I have no opinion

* Example 2 (graph serializations use different bnode identifiers):

{  :alice foaf:knows _:x . _:x foaf:name "Bob" . }
|=
{  :alice foaf:knows _:y . _:y foaf:name "Bob" . }

[ ] Yes, this should be an entailment
[ ] No, this should be a non-entailment
[ ] I don't know / I have no opinion

* Example 3 (graph serializations use "anonymous bnodes"):

{  :alice foaf:knows [ foaf:name "Bob" ] . }
|=
{  :alice foaf:knows [ foaf:name "Bob" ] . }

[ ] Yes, this should be an entailment
[ ] No, this should be a non-entailment
[ ] I don't know / I have no opinion

===== END BLANK NODE QUESTIONNAIRE ===

--
Dipl.-Inform. Michael Schneider
Research Scientist, Information Process Engineering (IPE)
Tel  : +49-721-9654-726
Fax  : +49-721-9654-727
Email: michael.schneider@fzi.de
WWW  : http://www.fzi.de/michael.schneider

==============================================================================
FZI Forschungszentrum Informatik an der Universität Karlsruhe
Haid-und-Neu-Str. 10-14, D-76131 Karlsruhe
Tel.: +49-721-9654-0, Fax: +49-721-9654-959
Stiftung des bürgerlichen Rechts
Stiftung Az: 14-0563.1 Regierungspräsidium Karlsruhe
Vorstand: Dipl. Wi.-Ing. Michael Flor, Prof. Dr. rer. nat. Ralf Reussner,
Prof. Dr. rer. nat. Dr. h.c. Wolffried Stucky, Prof. Dr. rer. nat. Rudi Studer
Vorsitzender des Kuratoriums: Ministerialdirigent Günther Leßnerkraus
==============================================================================

Received on Friday, 4 March 2011 13:52:52 UTC