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Re: RDF Dataset semantics

From: Peter F. Patel-Schneider <pfpschneider@gmail.com>
Date: Tue, 17 Sep 2013 05:42:51 -0700
Message-ID: <52384E4B.8000905@gmail.com>
To: Antoine Zimmermann <antoine.zimmermann@emse.fr>
CC: RDF Working Group WG <public-rdf-wg@w3.org>
This document promotes the notion that graph names denote. Technically this is 
now benign, as in the new semantics all IRIs and blank nodes denote, but there 
is the implicit assumption that graph names in RDF datasets are supposed to 
denote something as graph names.  I'm not sure just how to change the wording 
- "IRIs or blank nodes used as graph names" is rather cumbersome.

There are lots of other natural choices for the denotation of graph names 
(assuming that one wants these to denote something of interest).  They could 
denote the set of interpretations of the graph, they could denote the mapping 
from the graph to its interpretations, they could denote the set of graphs 
equivalent to the graph, they could denote the inferential closure of the 
graph. Of course, these are all covered in the second-last possibility in the 
document, but that is so general that it could include something like the 
graph obtained by swapping subjects and objects in the named graph, so it 
seems to me that some semantic denotations should be called out to balance the 
numerous syntactic denotations.

The 2004 RDF semantics defines the meaning of sets of RDF graphs by providing 
definitions of entailment from sets of RDF graphs to an RDF graph.    The 
merge of a set of RDF graphs is then shown to have a certain special 
relationship to this definition.  I can find no place in the 2004 RDF 
semantics where merging is used to define the meaning of RDF graphs.  The 2013 
RDF semantics does not define entailment for sets of RDF graphs because one 
has to first determine what to do with blank nodes that are shared between the 
graphs.

It is misleading to imply that there has been a significant change in the 
semantics of blank nodes between 2004 and 2013.  In 2004 it was assumed that 
blank nodes would not be shared between RDF graphs.  However, the semantics 
needed to say something about what would happen in this case, and the choice 
was to go with separating the illegally-shared blank nodes.   (Perhaps a 
better choice would have been to disallow this situation in the semantics.)

There are conditions on the pairs in an RDF dataset that are not captured in 
the document.

I would not say that it is problematic to provide a meaning for RDF 
datasets.   There are very many unproblematic reasonable semantics for RDF 
datasets.  It is just that many of these are incompatible so picking one 
generally rules out others.


It is not the case that the semantics for RDF datasets that ignores the named 
graphs has "no added value".  The simple ability to carry around the 
uninterpreted named graphs can be used to decorate RDF graphs.


I can't find a place where the actual meaning of RDF datasets is mentioned or 
described.




There should be a reference to the graph names paper by Jeremy et al. where 
the paper is first mentioned.

There are a number of minor grammatical issues that need to be fixed.

peter


On 09/17/2013 04:13 AM, Antoine Zimmermann wrote:
> I've updated dataset semantics [1].
>
> I especially invite Pat, Peter and certainly Sandro to take a look at it. 
> I've tried to be comprehensive but there are many parameters that can vary 
> and produce different formalisations, and different entailments.
>
>
> [1] https://dvcs.w3.org/hg/rdf/raw-file/default/rdf-dataset/index.html
Received on Tuesday, 17 September 2013 12:43:20 UTC

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