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Re: RDF-ISSUE-5 (Graph Literals): Should we define Graph Literal datatypes? [RDF Graphs]

From: Pat Hayes <phayes@ihmc.us>
Date: Fri, 4 Mar 2011 19:26:05 -0600
Message-Id: <064E932E-12B6-4E8B-B5F2-26A628929AD7@ihmc.us>
To: RDF Working Group WG <public-rdf-wg@w3.org>

On Mar 4, 2011, at 3:59 PM, RDF Working Group Issue Tracker wrote:

> RDF-ISSUE-5 (Graph Literals): Should we define Graph Literal datatypes? [RDF Graphs]
> http://www.w3.org/2011/rdf-wg/track/issues/5
> Raised by: Sandro Hawke
> On product: RDF Graphs
> We could define datatypes, such as ser:rdfxml and ser:turtle, whose
> lexical space is the set of valid document strings in RDF/XML, Turtle,
> etc, and whose value space contains the corresponding RDF graphs.
> This would allow people to use ordinary RDF tools to express facts involving RDF graphs, such as that some graph was obtained from some URI at some point in time, or that some person claims some graph is true or false.

Allow me to cast doubt on this claim. I do not believe that graph literals (in contrast to named graphs) would in fact provide such functionality in practice. For several reasons.

1. This would allow such 'metaRDF' descriptions only for the case where the object graph - the one being described - is completely specified by its full textual representation. This would make such metaRDF almost unusable for large object graphs, and exceedingly awkward, at best, for all but toy object graphs. For any graph, the g-text is a much more verbose way to refer to it than a URI would be. 

2. The full textual representation of a graph does not, ironically, serve to "identify" it in the sense required. Suppose I publish some RDF in a box with a URI. The URI identifies the box, but it does not identify the graph. The very same graph might be a snapshot of a different box with a different provenance and history and authority claiming it to be true. It is the box, not the graph, which will be asserted or will have a history or be deprecated, etc.. But a graph literal of a snap of a box does not identify the box. Even if we say that such a literal identifies any box whose snap is equivalent to the literal, the task of checking such equivalence is NP-complete (an old result of Jeremy's) so we have hamstrung our implementations ahead of time. And this is probably not a good rule to adopt, in any case, even if it were computationally cheap.

3. It is completely unnecessary, if we have named graphs. A named graph has a name which refers to it and identifies its box. Most descriptive languages, including RDF, use names in this way to make assertions about the things named. AFAIKS, nothing is gained by making such a graph into a literal instead of simply using its name to refer to it. And this use of graph names requires no changes to any RDF syntax (or indeed semantics.)

Pat Hayes

> This would address some of the use cases for quads, reification, named
> graphs, etc, with a mechanism that is very simple to understand and
> relatively easy to implement.
> Languages (like Turtle and RDF/XML) could be extended to provide
> syntactic sugar for these literals, much as Turtle provides a nicer
> syntax for numbers, but that is not necessary for these literals to be
> useful and is not part of this proposal.
> Some discussion in http://lists.w3.org/Archives/Public/public-rdf-wg/2011Mar/0130.html

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Received on Saturday, 5 March 2011 01:27:57 UTC

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