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Subject: RE: [xdi] More on $has predicate (was RE: [xdi] Agenda: XDI TC Telecon Thursday 1-2PM PT 2009-01-15)
Quick follow-on to this last message: Nick
and I had another conversation about $has tonight and agreed that one aspect of
Bill’s sense of the semantics was correct – in RDF, it represents a
restriction. In fact it bears a similarity to a particular OWL restriction,
hasValue: http://www.w3.org/TR/owl-ref/#hasValue-def The W3C OWL Reference has this example of a
restriction to the set of individuals who have a parent named Clinton. <owl:Restriction> <owl:onProperty rdf:resource="#hasParent" /> <owl:hasValue rdf:resource="#Clinton" /> </owl:Restriction>
The metagraph statement +x/$has/+y that
produces the base graph statements +x+y and +x/+y/+x+y is expressing the restriction
on the class +y that restricts it to the members for whom +x/+y is true. Just
like “British car” is a restriction of the class of all cars to
those that are British, we could say: Metagraph Base
Graph +british/$has/+car +british+car The XRI +british+car identifies the node
in the XDI RDF graph representing the class of all cars ($$/$is$a/+car) that
are the object of the predicate +car from the subject +british
(+british/+car/$$). So Bill, how would you express $$/$is$a/+car
plus the restriction +british/+car/$$ in RDF? =Drummond From: Drummond Reed
[mailto:drummond.reed@cordance.net] I had a very productive 1-on-1 call with
Nick Nicholas after the XRI TC telecon today, where we went into the metagraph
model diagrams even more deeply. Several key conclusions came out of it. Nick
is doing his own writeup, which may have even more detail, but in this email I
wanted to highlight two key conclusions that directly address the questions
Bill raised this morning in his message below. Also, I posted a few minor edits
in a V2 of the metagraph/graph diagrams at:
http://www.oasis-open.org/committees/download.php/30719/xdi-rdf-graph-model-diagrams-v2.pdf
Note: I’m noticing how much we need
to refer to the NON-metagraph, but talking about a non-metagraph hurts my head,
I propose the term we use to talk about “the XDI RDF graph that does not
include any metagraph statements” is “base graph”, simply
because something can only be meta if it has a base to work against. Please
post if you prefer another term. ************* The first conclusion is that the V1
diagrams are, in fact, correct. In other words, the metagraph statement
+x/$has/+y does in fact translate into two base graph statements:
+x/+y/+x+y
+x+y Both of them are reflected in the diagram: This is a different conclusion than I gave
in my first response to Bill earlier today when I said that +x/$has/+y was a
way of asserting that the target of the outgoing arc +y originating from +x was
a blank node. Saying the object is in fact +x+y is a better solution for two
reasons:
1) There is no need to invoke the concept of a blank node (and all its
baggage).
2) There is no need to specify that a $has statement means there can be only
one arc +y originating from +x. Instead what +x/$has/+y describes is that:
There is an exactly one arc +y originating from +x terminating in the object
identified as +x+y. This is consistent with:
a) RDF, because there is exactly one RDF statement with the same subject,
predicate, and object (multiples of that same statement are ignored).
b) Our use of $has in the metagraph to define XDI addresses in the base graph:
$has identifies the singleton arcs in the base graph that serve this addressing
role. No more, no less. Well, actually, a little more. There is one other statement that can be derived
from a $has statement – but it really is just a logical conclusion that
fundamentally falls out of the logical structure of the metagraph itself. It is
the following inference:
Every metagraph statement +x/$has/+y infers the statement +x+y/$is$a/+y. Kudos to Nick for recognizing this
inference. The funny thing is that it’s plain as the nose on your face if
you study the metagraph/base graph diagrams. Diagram from section 4A/4B of the
PDF, which is the diagram of the metagraph statements +x/$a/+y and +y/$is$a/+x,
shows: Because both $a and $is$a statements are
assertions about an object having an incoming arc, it follows that any
metagraph statement that specifies an outgoing arc terminating in an object is
also a statement that the object has an incoming arc. Thus all four of the following statements infer the other
three:
+x/$has/+y
+y/$is$has/+x
+x+y/$is$a/+y
+y/$a/+x+y This jibes with the English semantics if
we substitute real words for x and y:
+dog/$has/+collar
+collar/$is$has/+dog
+dog+collar/$is$a/+collar
+collar/$a/+dog+collar So the English phrase “dog
collar” only means that the identified object is-a collar, not that it
is-a dog. However it is not just any
collar, it is-a collar in the context of being something a dog has. This finally addresses the question Bill raised
in his message below In the RDF he provided, his assumption was that for the
metagraph statement +x+y, the RDF statements were:
1) +x+y is a +x
2) +x has the property +y. What Nick and I concluded is that the RDF
statements are:
1) +x+y is a +y
2) +x has the property +y whose object is +x+y Bill, we also agreed that since you have
more RDF experience than we, it may be easier for you to express the RDF then
we can. Hope this helps. =Drummond From: Barnhill,
William [USA] [mailto:barnhill_william@bah.com] Hi Drummond, I'm unclear on the relationship between metagraph and graph. I had
thought the relationship was comparable to the relationship between the
T-Box and the A-Box in RDFS, OWL, etc. but your diagrams seem to be
saying otherwise. For an example the metagraph diagram for 2A (+x+y) leads to a
graph statement of +x/+y/+x+y, which I am having trouble relating to RDF.
In my mind the XDI metagraph statement +x+y is comparable to the metagraph
statements stated in the following RDF/XML notation: <owl:Class rdf:about="http://plus.xri.net/x"> <rdfs:subClassOf
rdf:resource="&owl;Thing" /> <rdfs:subClassOf> <owl:Restriction>
<owl:onProperty rdf:resource=http://plus.xri.net/y
/>
<owl:minCardinality rdf:datatype="&xsd;nonNegativeInteger">1</owl:minCardinality> </rdfs:subClassOf> </owl:Class> Which then might lead an XDI reasoner to entail the implicit graph
statements expressed in <rdf:Description rdf:about="http://equals.xri.net/Bill.Barnhill"
> <rdf:type
rdf:resource="http://plus.xri.net/x" /> </rdfDescription> from the explicit graph statements expressed in: <rdf:Description
rdf:about="http://equals.xri.net/Bill.Barnhill" > <rdf:type rdf:resource="&owl;Thing"
/> <plus:y>!1234!234</plus:y> </rdfDescription> In XRI terms the following implicit XRI in the XDI graph
would be determined by an XDI reasoner: =Bill.Barnhill/$a/+x from the following explicit XRIs in the graph (playing fast and loose
with name spaces here): =Bill.Barnhill/$a/+Thing =Bill.Barnhill/+y/!1234!234 and a query on the graph of $$X/$a/+x will answer [{"$$X",
"=Bill.Barnhill"}] Could you explain how the above would be done with the metagraph
predicate framework described by your diagram and recent wiki update? Regards, =Bill.Barnhill From: Drummond
Reed Following is the agenda for the unofficial telecon of the XDI TC at:
Date: Thursday, 15 January 2009 USA Time: 1:00PM - 2:00PM Pacific Time (21:00-22:00 UTC)
TO ACCESS THE AUDIO CONFERENCE: Dial In Number: 571-434-5750 Conference ID: 5474
AGENDA
1) XDI ADDRESSER
We will start with a tour from Markus of his latest XDI RDF utility:
http://graceland.parityinc.net/xdi-addresser/XDIAddresser
2) SOLVING THE RUBIK'S CUBE OF THE XDI 1.0 RDF METAGRAPH MODEL
Based on insights from Monday's aborted XRI/XDI editor's telecon (which turned into a long informal XDI telecon) and Tuesday's special XRI Syntax 3.0 telecon (which also ended out as a long XDI discussion), Drummond had a key revelation about the metagraph model.
He has uploaded a PDF with a new set of diagrams illustrating the metagraph model and how it describes the graph model.
http://www.oasis-open.org/committees/download.php/30700/xdi-rdf-graph-model- diagrams-v1.pdf
He also updated the XDI RDF Graph Model wiki page with new text descriptions of each metagraph predicate and references to the diagrams.
http://wiki.oasis-open.org/xdi/XdiOne/RdfGraphModel
The main topic of the call will be to review this and discuss how it returns the metagraph back to a pure description of the graph with no other semantics and thus enables the metagraph predicates to be used with any description logic:
http://en.wikipedia.org/wiki/Description_logic
3) MULTI-PART SPEC DESIGN AND NEXT STEPS WITH THE FIRST WORKING DRAFT
Drummond had his questions answered by Mary McRae about how a multi-part specification should work. See the DITA example:
http://docs.oasis-open.org/dita/v1.1/OS/overview/overview.html
Given this model, Drummond has updated the specification names on:
http://wiki.oasis-open.org/xdi/XdiOneSpecs
If there is consensus on this, our next step is to choose a template and begin the first Working Draft.
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