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Re: Mutability and graphs [was: Re: page about the term "named graphs"]

From: Andy Seaborne <andy.seaborne@epimorphics.com>
Date: Fri, 23 Jul 2010 17:56:17 +0100
Message-ID: <4C49C9B1.5020305@epimorphics.com>
To: Axel Polleres <axel.polleres@deri.org>
CC: SPARQL Working Group <public-rdf-dawg@w3.org>

 > ----------------------------------------------------------------------
 > Dataset(new): Given RDF graphs (referred to by symbols)
 > <G_0>, .... ,<G_l>,<G_n1>, ....<G_nm>  a dataset DS is the following
set of
 > (mathemetical) sets,
 >
 > DS = {<G>,<G_n1>, .... ,<G_nm>}
 >
 > where G = {<G_0>  merge .... merge<G_l>  }
 >
 > and merge is RDF merge in the sense of rdf-mt.
 > We also denote the symbol referring to a graph as "name" and particularly
 > call the graph<G>  "default graph".
 > ----------------------------------------------------------------------

Why do you need to redefine RDF dataset?  You don't make use of this 
definition anywhere.  What visible difference are you getting at? If 
there is no visible difference, why are you proposing a change? You 
don't use the definition for your graph store proposal where you just 
use "dataset".


The definition does not make sense to me. What is <G_0>? Is it an IRI? 
Or a name for the purposed of this definition?  If it's an IRI, then

 > DS = {<G>,<G_n1>, .... ,<G_nm>}

is a set of IRIs, right?.  How does the spec get a graph in order to 
query it?  In SPARQL 1.0 it's Map IRI->graph.  (A Map is a Set of pairs 
with a look function from "key" the first in a pair to "value" the 
second in a pair.  In Java you can even iterate over a Map as a set of 
pairs.)


How can RDF Graphs, which are sets, have names. Sets don't have "names" 
other than in the self-denoting sense that literals do.

Suppose:
GET http://faraway/resource -> "1"^^xsd:integer

today
and

GET http://faraway/resource -> "2"^^xsd:integer

tomorrow.

so http://faraway/resource is not a permanent name for the number 1.

 > DS = {<G>,<G_n1>, .... ,<G_nm>}

is that a set of symbols or a set of graphs? Or a set of symbols and graphs?

How does this work for GRAPH, FROM and FROM NAMED?

 > G = {<G_0>  merge .... merge<G_l>  }

This breaks SPARQL 1.0.

The default graph IS NOT the RDF merge of the named graphs.  It is a 
graph without constraints. /Some/ systems may make it the RDF merge of 
the named graphs (usually, they don't, they make it the union but that's 
OK with some wiggle room about a priori knowledge of the domains of 
discourse - /pace/ Steve).

BUT this is NOT required by the spec for SPARQL 1.0.

	Andy
Received on Friday, 23 July 2010 16:56:44 GMT

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