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Re: What we mean by "graph" / Named Graphs in SD

From: Andy Seaborne <andy.seaborne@talis.com>
Date: Wed, 21 Jul 2010 16:05:00 +0100
Message-ID: <4C470C9C.2060002@talis.com>
To: Sandro Hawke <sandro@w3.org>
CC: SPARQL Working Group <public-rdf-dawg@w3.org>


On 20/07/2010 7:22 PM, Sandro Hawke wrote:
 > Meanwhile, I've been meaning to send a question about our use of the
 > term "Graph", which is connected here.
 >
 > It seems to me there are two different common meanings for the term
 > "RDF Graph".  To use the AI terms for each of them:
 >
 >          1. A Knowledge Base (KB); a specific repository or store of RDF
 >          triples.  As in, "Please update your graph to remove the triple
 >          <a>  <b>  <c>."
 >
 >          2. A Formula; a mathematical set of RDF triples.   As in, "Graph
 >          G1 entails infinite other graphs".
 >
 > The most crisp distinction may be around identity.   Two formulas are
 > identical if and only if they contain the same triples.  Meanwhile, KBs
 > can have the same triples while remaining distinct.   It also makes
 > sense to talk about the state of a KB, and a KB changing over time.  It
 > makes no sense to say such things about a formula; it's just a pure
 > mathematical set.
 >
 > I think we can agree that formally, technically, only definition 2
 > (formulas) is correct.  But I think meaning two is in common use; I
 > expect most of us use it often.    When I say "graph" in the sense of
 > definition 1, I mean it as shorthand for "graph storage location",
 > "graph data structure", or "graph store".   In spoken language, the
 > context usually makes it clear whether people mean KB or formula.

I think it's helpful to go back tot he work already done:
The term "RDF graph" is defined in RDF Concepts:

http://www.w3.org/TR/rdf-concepts/#section-rdf-graph

RDF does not talk about mutability but on the web things can change - 
the (web) resource is changing from one (def 2) graph to another.

I think I know what you are describing by formula, but the term is used 
in as somethign specific and maybe has different aspects.

http://www.w3.org/TeamSubmission/n3/#Quoting

[[
Formulae

An RDF document parses to a set of statements, or graph. However RDF 
itelf has no datatype allowing a graph as a literal value. N3 extends 
RDF allows a graph itself to be referred to within the language, where 
it is known as a formula.
]]

As well as allowing variables, it also is the value of a graph, not just 
a set of triples.  Earlier writing on N3 explicit describes it as a literal.

When comprised of ground terms, it behaves like a literal with datatype 
(the datatype needs to imply the syntax so there is a mapping from 
lexcial form to value space).  Equality seems to be defined as 
bNode-isomorphic - or possibly RDF equivalent (which would involved 
leaning as well).

	Andy
Received on Wednesday, 21 July 2010 15:05:29 GMT

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