From: Lee Feigenbaum <lee@thefigtrees.net>

Date: Mon, 02 Apr 2012 09:36:07 -0400

Message-ID: <4F79AB47.1000504@thefigtrees.net>

To: Andy Seaborne <andy.seaborne@epimorphics.com>

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

Date: Mon, 02 Apr 2012 09:36:07 -0400

Message-ID: <4F79AB47.1000504@thefigtrees.net>

To: Andy Seaborne <andy.seaborne@epimorphics.com>

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

I'd really like to know if there is anyone who feels negatively towards this proposed design -- please take a look at it in advance of tomorrow's meeting! thanks, Lee On 3/30/2012 5:25 AM, Andy Seaborne wrote: > This message is one of two. > > This first message has a description of why option 6 is the way it is > and then describes the definitions. > > The second message gives the core of the definitions in a more formal, > but not completely formal, way with some explanations. > > Read this message; optionally read the other one. > > == Option 6 > > This takes the F&R uses cases as natural user expectations and so to > suggest the path operators needed and their semantics. > > { ?list rdf:rest*/rdf:first ?member } is a good illustration. > > If the list is (1 2 1), we want the results to be > (?member=1, ?member=2, ?member=1) i.e. 1 twice. So "/" is not distinct. > > rdf:rest* is the same for either distinct or counting * on a well-formed > list. It's a chain, you get the same results either way. > > { :X rdf:type/rdfs:subClassOf* ?C } > > Here, {:X rdf:type ?V } is only going to yield distinct answers because > an RDF graph is a set of triples. You can not get duplicate values for > ?V. The distinct/counting issue for "/" does not matter here. > > Like foaf:knows*, this use of rdfs:subClassOf* is more about which terms > can be reached, not how many times. So a definition that does distinct > result, which can be a lot faster, is all that's needed. > > So, we get: > > 6.A: /, |, ! as there are in 2LC. > 6.B: *, +, ? are non-counting > 6.C: No DISTINCT > 6.D: No {} forms: {n}, {n,m}, {n,}, {,m} > > It is less than other proposals - you can construct use cases which it > does not cover directly. The question for the WG is whether it's better > to leave those to a future WG. > > == Description > > There's the core of the formal definitions in the other message. Here, > we describe the operators. > > 6.A are done by transformations but it's useful to also have definitions > as well (this is true of 2LC as well, but it didn't have them) because > we end up talking about "evaluating a path". > > { X :p/:q Y } is { X :p ?V . ?v :q Y } > etc > > 6.C and 6.D are quite easy. Remove text from document. > > 6.B: > > For the remaining operators, we start by defining > > operator(X path ?var) > > and then define > > operator(?var1 path ?var2) > > by considering every node in the graph for ?var1. So the core definition > is operator(X path ?var) for some given RDF term (graph node) x. > > It's all based on a function PathPoints(X, P, N) as the set of points > reached from X by exactly N steps of path P. As this is a definition, > we're not worrying about the fact there may several ways to do that; the > result is a set. > > { x path? ?V } is the set of bindings (?V=y) where y is zero or one > steps from x. ?V is bound to each element of the set: > > PathPoints(x P 0) set-union PathPoints(x P 1) > > { x path* ?V } is the set of bindings (?V=y) where y is zero or more > steps from x -- all PathPoints(X, P, N) for N >= 0 > > { x path+ ?V } is the set of bindings (?V=y) where y is one or more > steps from x -- all PathPoints(X, P, N) for N >= 1 > > To help implementers, there is an non-normative description of a simple > function that will calculate path* -- it's a modified form of ALP from > 2LC -- and it's much faster. It's not the only way but it is a short > illustration of an algorithm for path*. > > Andy > >Received on Monday, 2 April 2012 13:36:47 UTC

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