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Re: Comment on the Dataset proposal (syntax)

From: Pat Hayes <phayes@ihmc.us>
Date: Sat, 28 Apr 2012 21:21:45 -0500
Cc: Antoine Zimmermann <antoine.zimmermann@emse.fr>, Richard Cyganiak <richard@cyganiak.de>, RDF WG <public-rdf-wg@w3.org>
Message-Id: <2D1B0471-7CD2-4293-899D-31F135FFD088@ihmc.us>
To: Sandro Hawke <sandro@w3.org>

On Apr 26, 2012, at 11:13 AM, Sandro Hawke wrote:

> On Thu, 2012-04-26 at 17:30 +0200, Antoine Zimmermann wrote:
>> Hi,
>> 
>> 
>> (This email is mostly for Richard's attention)
>> 
>> Putting aside the discussion on dataset semantics, I have a few comments 
>> on the way the dataset proposal is described in terms of syntax:
>> 
>> 
>> "The RDF data model expresses information as graphs consisting of 
>> triples with subject, predicate and object."
>> 
>> The word "graph", in the RDF specifications, should never appear alone 
>> like this. It is well known that a graph is a pair (V,E) where V is a 
>> set of vertices and E is a set of edges. This is not what RDF Graphs 
>> are. RDF Graphs are not graphs, in any of the accepted mathematical 
>> definition of the term. 
> 
> Aren't RDF Graphs a kind of graph?   

Actually, no, if we interpret 'graph' in the mathematical sense. Graphs dont have labelled edges: they are set of pairs, not sets of triples. Back in the RDF day we once tried to state exactly what an RDF graph is, using mathematical terminology, and I think it was something like a labeled. directed pseudograph.

Pat


> The restrictions, I think, are that
> there are no unconnected vertices, the edges are directed and labeled
> with an IRI, and the nodes may be labeled with an IRI or a datatype
> expression.   If this is true, that every RDF Graph is a graph, then I
> think linguistically it's okay to sometimes use the term "graph" if it
> makes the text read better and doesn't introduce too much ambiguity.
> 
>> We already agreed that the word "graph" alone is 
>> ambiguous and we resolved to use the phrase "RDF Graph" whenever we talk 
>> about sets of triples.
>> 
>> SUGGESTION:
>> "The RDF data model expresses information as RDF Graphs consisting of a 
>> set of triples with subject, predicate and object."
>> 
>> -----
>> 
>> "Often, one wants to hold multiple RDF graphs and record information 
>> about each graph, allowing an application to work with datasets that 
>> involve information from more than one graph."
>> 
>> SUGGESTION:
>> "... each RDF Graph, ... than one RDF Graph."
>> 
>> To sound less redundent, "hold multiple RDF graphs and record 
>> information about each one, ..."
>> 
>> -----
>> 
>> "An RDF Dataset represents a collection of graphs. An RDF Dataset 
>> comprises one graph, the default graph, which does not have a name, and 
>> zero or more named graphs, where each named graph is identified by an IRI."
>> 
>> Maybe say "distinguished RDF Graph":
>> 
>> SUGGESTION:
>> "An RDF Dataset comprises one distinguished RDF Graph, the /default 
>> graph/, which does not have a name, ..."
>> 
>> Moreover, the word "identified" may be missinterpreted.
>> 
>> SUGGESTION:
>> "..., where each named graph associates an IRI with an RDF Graph."
>> 
>> -----
>> 
>> "An RDF Dataset may contain zero named graphs; an RDF Dataset always 
>> contains one default graph."
>> 
>> SUGGESTION:
>> add "The default graph MAY be empty."
>> 
>> -----
>> 
>> Maybe a definition for "named graph" could be given before the formal 
>> definition of RDF Dataset:
>> 
>> SUGGESTION:
>> "A /named graph/ is a pair (n,g) where n is an IRI called the /graph 
>> name/ and g is an RDF Graph."
>> 
>> -----
>> 
>> "Formally, an RDF dataset is a set:
>> 
>> { G, (<u1>, G1), (<u2>, G2), . . . (<un>, Gn) }
>> 
>> where G and each Gi are graphs, and each <ui> is an IRI. Each <ui> is 
>> distinct."
>> 
>> "... are RDF Graphs, ..."
>> 
>> ----
>> 
>> "G is called the default graph. The pairs (<ui>, Gi) are called named 
>> graphs."
>> 
>> If "named graph" is defined before, it could look like this:
>> 
>> SUGGESTION:
>> "G is called the default graph. The pairs (<ui>, Gi) are named graphs."
> 
> I have to say (again) that I'm not okay with calling something a "named
> graph", especially formally, when it isn't named and isn't a graph (or
> RDF Graph).   If we have to use the terms "name" and "graph", then the
> pair (ui, Gi) is a name-graph pair, and Gi is the named graph.
> 
> I don't think wordsmithing this section will productive until/unless we
> have a shared understand of what we actually want to say, though.
> 
>    -- Sandro
> 
> 
> 
> 

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Received on Sunday, 29 April 2012 02:22:24 UTC

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