From: Birte Glimm <birte.glimm@comlab.ox.ac.uk>

Date: Thu, 24 Sep 2009 15:00:11 +0100

Message-ID: <492f2b0b0909240700i69b8b32fqe967b125d8ff8884@mail.gmail.com>

To: SPARQL Working Group <public-rdf-dawg@w3.org>

Date: Thu, 24 Sep 2009 15:00:11 +0100

Message-ID: <492f2b0b0909240700i69b8b32fqe967b125d8ff8884@mail.gmail.com>

To: SPARQL Working Group <public-rdf-dawg@w3.org>

Hi all, I am reading the definitions of a pattern instance mapping and basic graph pattern matching again and again and I am not quite happy with them. Maybe I am too much of a theoretician, but I find them too imprecise and given that they are very important I would like them to be very clear and I am inclined to suggest some changes/clarifications for those definitions in the next SPARQL spec, but please correct me if I am just overlooking somehing. First, a pattern instance mapping P is defined as a composition of an RDF instance mapping sigma with a solution mapping mu, i.e., P(x)=mu(sigma(x)), see 12.3.1. The domain of an RDF instance mapping is, however, the set of blank nodes and its range is the set of literals, blank nodes, and IRIs. Now the domain of mu is the set of variables, which means we cannot really compose the two. The range of the first is disjoint from the domain of the second. What I would rather say is that P is a pair (mu, sigma). Next, in the definition of basic graph pattern matching, we suddenly use P(BGP) although the domain of P is, well it is unclear but most likely, variables and blank nodes and not basic graph patterns. It is intuitively clear what is meant, but it is an abuse of notation. To clarify this, I would prefer to write something like: Let P=(mu, sigma) be a pattern instance mapping, BGP a basic graph pattern, and G an RDF graph. We write P(BGP) to denote the result of replacing each variable ?x in BGP that is in dom(mu) with mu(?x) and each blank node b that is in dom(sigma) with sigma(b). As I understand it, mu and sigma might be undefined for some variables and blank nodes in BGP because they are just partial functions, so I should only replace variables/bnodes for which the function is defined. We can then go on to define solutions: Let V(BGP) denote the set of variables occurring in BGP. A solution mapping mu is a solution for BGP from G if dom(mu)=V(BGP) and there is a pattern instance mapping P=(mu, sigma) such that P(BGP) is *an instance of* a subgraph of G. I think that t is important to use *an instance* of a subgraph here because as is said later, we use a scoping graph SG (at least in theory) instead of the active graph and instantiations of BGP have to be subgraphs of SG and not necessarily G. Since SG is an instance of G, of course P(BGP) is an instance of a subgraph of G. Any thoughts on that? Apart from that I have a few more minor editorial errors for the SPARQL 1.0 spec... This is really minor, but there is a comma missing in the second paragraph of 12.6: ... a triple ( s, p, o ) is in the first pattern if and only if the triple ( M(s), M(p)*,* M(o) ) is in the second. Further: 12.6, Notes (c) These conditions do not impose the SPARQL requirement that SG share*s* no blank nodes with AG or BGP. Cheers, Birte -- Dr. Birte Glimm, Room 306 Computing Laboratory Parks Road Oxford OX1 3QD United Kingdom +44 (0)1865 283529Received on Thursday, 24 September 2009 14:00:51 UTC

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