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Re: blank node scope - ISSUE-107 - resolve as in Semantics - hopefully on 20 March

From: Antoine Zimmermann <antoine.zimmermann@emse.fr>
Date: Wed, 13 Mar 2013 17:32:07 +0100
Message-ID: <5140AA07.3080506@emse.fr>
To: public-rdf-wg@w3.org, "Peter F. Patel-Schneider" <pfpschneider@gmail.com>, Pat Hayes <phayes@ihmc.us>
Please, could you show the mathematical definitions of all this. I do 
not understand what is a scope with the text of Semantics. I can see 
several intepretations:

1) there is a mapping s from the set of all blank nodes to the set of 
scopes (and what's a scope is not specified beyond that there is a set 
of them). So, given a bnode b, I can say what's its scope by s(b).

I am very much against this design, but it's not clear that the ED of 
RDF 1.1 Semantics is rejecting this one (especially given the remarks 
that Pat made during previous discussions on the topic).
I would object formally to such a design.

2) scopes form a partition of the RDF triples, so a triple belong to a 
single scope. A set in the partition is a complete graph.
The problem is that the union of two different complete graphs is not a 
complete graph. I don't like this design at all, although it is already 
much better than the first one.

3) a scope corresponds to an RDF graph, and scopes can overlap 
(mathematically, there is a mapping M from the set of scopes to the set 
of graphs). A graph in a scope is a scoped graph (or mathematically, 
there exists a scope s such that M(s) contains the graph). The set of 
triples in the graph of a scope form a complete graph (M(s) is a 
complete graph). Possibly, the set of complete graphs is closed under 
set union (so that the union of two complete graphs is still a complete 
This would be much better, yet not completely up to my expectation, but 
there are indications that the chosen design in the current ED is not 
this one.

There are probably other ways to interpret the current text. I would be 
curious to know what would be your respective formalisation, Peter and 
Pat, if you had to write it independently of one another. I had the 
impression, reading some of your emails, that your understanding of 
scope was different.

In any case, I fail to understand why scope should have any consequences 
on the truth of a set of triples. Thus my plea to revert to the 
semantics of bnodes as in Semantics 2004.

If scope impacts the semantics at all, then there should be a separate 
definition of the truth of a scoped graphs, as opposed to the truth of a 
set of triples. Something like:

"A scoped graph G in scope s is true in interpretation I iff there 
exists a mapping A from the bnodes in s to resources of I such that 
[I+A](M(s)) is true, otherwise it's false."

Note that A is independent of the graph G, it only depends on the 
complete graph M(s).


PS: this is a bit redundent with my complete review that will follow 
(tomorrow I hope).

Le 13/03/2013 16:57, Peter Patel-Schneider a écrit :
> ISSUE-107 concerns what to do with blank nodes.  This includes cross-graph
> blank node scopes.
> The current draft of Semantics
> https://dvcs.w3.org/hg/rdf/raw-file/default/rdf-mt/index.html includes a
> solution to blank node scoping.  I propose that this solution be adopted by
> the WG as the result of the issue.
> The basic idea is to introduce the notion of a blank node scope.  RDF
> graphs within a single scope can share blank nodes, graphs not in the same
> scope cannot!  This makes blank-node-renaming unnecessary during graph
> merging.  (Of course, in a surface syntax, different blank nodes may have
> the same b-node name, so these names may have to be changed when merging in
> a particular syntax.)
> For graphs not in the same scope, nothing changes.   For graphs in the same
> scope not sharing blank nodes, nothing changes.
> For graphs in the same scope sharing blank nodes, these blank nodes are
> interpreted uniformly.
> This last breaks a feature of RDF, that a set of graphs entails their
> merge.  There is a new definition in Semantics (complete graphs) that shows
> when this feature is retained.
> This solution needs changes in Concepts, minimally introducing the notion
> of a blank node scope, but maybe also talking about how blank node scope
> can be determined by different surface syntaxes.
> I suppose that there is also the issue of whether all the RDF graphs in a
> dataset are always in the same blank node scope.  It may be that it is not
> reasonable to say that this is the case, because datasets are already
> sometimes used as if they do not share blank nodes.
> peter

Antoine Zimmermann
ISCOD / LSTI - Institut Henri Fayol
École Nationale Supérieure des Mines de Saint-Étienne
158 cours Fauriel
42023 Saint-Étienne Cedex 2
Tél:+33(0)4 77 42 66 03
Fax:+33(0)4 77 42 66 66
Received on Wednesday, 13 March 2013 16:32:40 UTC

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