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Subject: RE: [xdi] More comments on XDI metagraph predicate examples (was: RE: [xdi] Minutes: XDI TC Telecon Thursday 1-2PM PT 2008-12-18)
Hi Giovanni,
I think I understand your first question but I think the reversal of +a+b+c still works I think.
Paraphrasing what you said (let me know if wrong) as the XRI +a+b+c entails either
(1) +a+b/$has/+c
or
(2) +a/$has/+b+c
or
(3) both
and the outcomes are different in each case.
It seems to me the outcomes would be the same though, as doesn't
(1) the XRI +a+b/$has/+c entail the XRI +a/$has/+b//$has/+c
(2) the XRI +a/$has/+b+c entail the XRI +a/$has/+b//$has/+c
(3) If we use the following substitutions
p : +a+b/$has/+c
q : +a/$has/+b+c
r : +a/$has/+b//$has/+c
and from (1) and (2) we know that p entails r, q entails r, then p and q -> r, so both XRI's being entailed still entails +a/$has/+b//$has/+c
What about the reverse way?
+a/$has/+b//$has/+c
a)=> +a+b//$has/+c => +a+b+c
b)=> +a/$has/+b+c => +a+b+c
I am unsure about entailments above as they involve '//' though and need to punt to Drummond:
Does +a/$has/+b//$has/+c entail +a/$has/+b+c?
Pretty sure it has to for $has to work as it's being used and have it be an inverse functional property.
Also in general can +a/P/Q//R be treated as +a/P/Q/R?
Not sure on this one.
Thanks,
=Bill.Barnhill
-----Original Message-----
From: Giovanni Bartolomeo [ mailto:giovanni.bartolomeo@uniroma2.it]
Sent: Fri 1/9/2009 5:47 AM
To: Drummond Reed
Cc: 'Nick Nicholas'; 'OASIS - XDI TC'
Subject: [xdi] More comments on XDI metagraph predicate examples (was: RE: [xdi] Minutes: XDI TC Telecon Thursday 1-2PM PT 2008-12-18)
Hello Drummond,
thank you for your answers; but I fear I've some more concerns.. please see my comments below.
Thanks,
Giovanni
At 09.09 30/12/2008, Drummond Reed wrote:
Giovanni pointed out that Statement 6 in the xdi-rdf-graphing-v1 document is
not consistent with the link contract example found on page 33 of the
xdi-rdf-model-v11 document:
http://www.oasis-open.org/committees/download.php/29748/xdi-rdf-model-v11.pd <http://www.oasis-open.org/committees/download.php/29748/xdi-rdf-model-v11.pd>
f
In that document, =drummond/$has/+friend$contract results in the XRI
=drummond+friend$contract, but it should be =drummond(+friend$contract).
Drummond agreed this should be changed in the next version.
UPDATE: In his study of the Statement 6 issue identified by Markus, Drummond
subsequently revised the proposed graph structure for compound $has
statements. This eliminates the issue identified by Markus and also removes
the inconsistency with the V11 XDI RDF Model document. V2 of the XDI RDF
Graphing document has been posted at:
[giovanni] To further clarify, I also propose to have
(1) =drummond+friend/$has/$contract
instead of
(2) =drummond/$has/+friend$contract
even if they both result in
(3) =drummond+friend$contract
statement (2) seems to me to assert that a subject +friend$contract exists regardless the context it is intended to be into (=drummond). Statement (1) instead put $contract in the context of =drummond+friend and +friend in the context of =drummond. Or maybe I'm missing something?
[=Drummond] No, I think you are correct, statement (2) implies that there is a XDI subject with the XRI +friend$contract. But I think that is implied in all cumulative $has statements. As I clarified in the page I just posted ( http://wiki.oasis-open.org/xdi/XdiOne/RdfGraphModel < http://wiki.oasis-open.org/xdi/XdiOne/RdfGraphModel> ), in an XDI context, the XRI =drummond+friend$contract infers all of the following XDI RDF statements:
(a) =drummond/$has/+friend/
(b) =drummond+friend/$has/$contract
(c) =drummond/$has/+friend$contract
[giovanni] Yes, I see that
+a/$has/+b+c ==> +a+b+c
+a+b/$has/+c ==> +a+b+c
however, if you want to REVERSE these statements, starting from +a+b+c, you can infer EITHER (1) +a/$has/+b+c OR (2) +a+b/$has/+c OR (3) both. And the outcomes are different in the three different cases.
In particular, if you have (1) +a/$has/+b+c, this implies that you should have also +b/$has/+c.
But if you have (2), then you should have also +a/$has/+b.
Coming back to the example, from =drummond+friend$contract you may infer
(1)
=drummond/$has/+friend/
=drummond+friend/$has/$contract
OR
(2)
+friend/$has/$contract
=drummond/$has/+friend$contract
OR (3)
both.
I think that (1) and (2) are asserting very different things and that (1) is what we want to say, whereas (2) is not the same.
[giovanni] In example #8 (equivalence +x/$is/+y) the arc connecting the two circles is labelled with +y, does it have a particular meaning? Maybe, for consistency's sake, it is better to maintain the graphical convention to name the arc after the predicate. Since $is is a symmetric predicate, what do you think about the following amendment?
[=Drummond] The graph you draw is 100% accurate - it is a depiction of the full metagraph statement, i.e., of the XDI RDF statement +x/$is/+y. The graph I was drawing is a depiction of the resulting statement in the XDI RDF graph, which is that the node +x has a self-referential arc of type +y. So both are correct, and I agree we should show both in the two columns. I'll make a note to revise that in the next version.
[giovanni] Ok, I see. But in the graph in example #9
+x/$has$a/+y
+x/+y
the arc +y is used to represent a property belonging to +x (this also applies to all meta-graphs depicted in the document). Thus wouldn't the picture in example #8 read as: +x has a property +y whose value is +x itself: +x/+y/+x ?
BTW I think this is strongly related to another issue which has come to my mind. I remember that in the ATI model it was stated that addresses refer to ARCs, not to NODEs. This is consistent with some OO programming languages like c++ and Java which have the concepts of pointers and objects. Pointers are represented through arcs pointing to nodes which are objects. Maybe we have a similar issues here: XDI addresses might be arcs, not nodes.
This should address also Markus' question:
[markus] BTW in your terminology, a predicate is not a node in the graph, right? So predicates themselves don't have addresses?
What about to amend this
"Every *node* in the XDI RDF graph can be addressed by at least one XRI
representing an XDI address, and every XDI address identifies a unique *node*
in the XDI RDF graph."
into
"Every *ARC* in the XDI RDF graph can be addressed by at least one XRI
representing an XDI address, and every XDI address identifies a unique *ARC*
in the XDI RDF graph."
(..but maybe I can miss some issues here...)?
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