Re: [xdi] Regrets

On Thu, Jan 24, 2013 at 4:47 AM, Giovanni Bartolomeo <giovanni.bartolomeo@uniroma2.it> wrote:

Tomorrow (Jan. 25) I'll take part at the Open Data Day preparation event in Rome so I'll not be able to join the call this Friday. Once more my apologies.

Best Regards,
Giovanni

Def. Quota Giovanni Bartolomeo <giovanni.bartolomeo@uniroma2.it>:

Thanks Drummond for your answer.

Sorry, last week the flu didn't allow me to work properly so I'm reading your mail only now.

I'm starting understanding your point, and agree that $is and $ref serve different purposes. My *personal* feeling is however we do not need canonical addressing but just addressing - but this is somehow longer to discuss and I think I can live with $ref and canonical addressing for now.

Nevertheless I have still problems with this sentence:

For this reason, the subject of a *$is* statement SHOULD NOT be a
parent context of the object of the statement, and vice versa. This avoids
the logical entanglement of a graph "containing a copy of itself".

Here is why: I've always assumed that XDI contexts somehow map to graph names in RDF/Linked Data/Named Graph. But this latter has a nice feature which allows an URIRef to be used as name of a graph and as a subject or an object of an RDF statement within the graph. So for example:

ex:G1 { _:Monica ex:name "Monica Murphy" .
_:Monica ex:email <mailto:monica@murphy.org> .
ex:G1 pr:disallowedUsage pr:Marketing }

where ex:G1 is both the name of the graph and a subject inside an inner statement. And this works with equivalence statements as well.

ex:G1 {
ex:G1 owl:sameAs my:alias .
my:alias pr:disallowedUsage pr:Marketing }

So this seems to me in contrast with the sentence "the subject of a *$is* statement SHOULD NOT be a parent context of the object of the statement" (in the example above ex:G1 is a parent context of the object my:alias). Does it make sense or Am I missing something here?

Best Regards,
Giovanni

Def. Quota Drummond Reed <drummond.reed@xdi.org>:

Giovanni, thanks for sending this, and sorry it's taken me a week to
respond. Because the Equivalence Links proposal is on the decision queue
again tomorrow, I wanted to give you a detailed reply. Hopefully it will
save us time in discussion tomorrow (especially if you are able to join
us). See inline.

On Fri, Jan 11, 2013 at 5:34 AM, Giovanni Bartolomeo <
giovanni.bartolomeo@uniroma2.it> wrote:

Hello All,

I will not be able to make the call this week as well. However, the good
news is that I'm happy with having two properties $is and $ref. The bad one
is that I'm not happy with their semantics.

However, I understand I'm quite late. I've been far away from the TC for
several calls, and might have missed agreements and decisions. Thus if you,
TC, think this is the right way, go ahead, no worry about my opinion.

For those who are interested, instead, a longer explanation follows.

Best Regards.
Giovanni

* Longer explanation *

To me $is is the XDI counterpart of owl:sameAs, logical equivalence
following the definition of identity by Leibnitz (see Halpin's paper). I do
not agree with the sentence: "a local root node MUST NOT have a is
statement. This avoids the logical entanglement of a graph containing a
copy of itself". Because it just violates the universality of this
semantics (Leibnitz's law should hold for the root node as well!).

Actually, that was my mistake, and Markus corrected it in the proposal. It
now says:

For this reason, the subject of a *$is* statement SHOULD NOT be a
parent context of the object of the statement, and vice versa. This avoids
the logical entanglement of a graph "containing a copy of itself".

It's not peer equivalence that's a logical problem, it's
container/containee equivalence. Again, my apologies for my mistake.

The nonsense you have expressed in this sentence comes from the fact that
you read addresses directly from the graph. We discussed this in middle 2011

https://lists.oasis-open.org/**archives/xdi/201107/msg00007.**html<https://lists.oasis-open.org/archives/xdi/201107/msg00007.html>

Unfortunately we suspended this thread in November 2011 (
https://lists.oasis-open.org/**archives/xdi/201110/msg00002.**html<https://lists.oasis-open.org/archives/xdi/201110/msg00002.html>)
when it disappeared and was never resumed again...

Ironically, as I review those threads now, it appears to be exactly the
issue we are covering in this proposal. So we're finally resolving it.

It is correct that "in RDF, you must use an owl:sameAs statement to create
an equivalence statement between two distinct RDF graph nodes". But not
that "every [RDF] graph node has exactly one arc (one URI) that identifies
it". RDF graphs are not addressable (although you can assign an URI to each
node, you cannot create a path to a node following the connections of arcs,
as we do in XDI). If you want to have this in XDI, the only way you can is
to have the two nodes drawn as one single node (otherwise you have the
nonsense above). In the thread above, the main argument against this
representation was canonical identifiers, which are not in RDF/Linked Data
but appear in the definition of $ref. Therefore although I like $ref to be
distinguished from $is, I would rather prefer to read another semantics in
it. According to Halpin's paper: "Individuals could be thought of as being
composed of differing aspects at different levels of granularity rather
than the notion of individuals traditionally used in semantics [that is in
owl:sameAs]". So $ref to me should be only "a simple similarity property
[...] sub-property of rdfs:seeAlso."

So I read Halpin's paper in its entirety, and I agree that semantics for
describing other types of similarity (those that don't satisfy Leibnitz's
law) are needed in XDI. I very much like the three-dimensional matrix he
and his co-authors present for this, and I think we should consider
adapting it for the $ dictionary.

However this is a different issue from the problem that $ref solves, which
is one of being able to specify canonical references. RDF does not have the
problem of canonical referencing because it doesn't have addressable
graphs, as you say. But in an addressable graph, you need a way to specify
at a node that it serves *only* as a reference to another node and *has no
subgraph*.

Given the correction above, and given that canonical referencing has been
an issue for both the XRI and XDI Technical Committees for 9 years now
(since 2004), it seems that there is a extremely strong case for having
clear and unambiguous semantics for a canonical reference. That is the
semantics that the Equivalence Link
proposal<https://wiki.oasis-open.org/xdi/EquivalenceLinks>proposes for
$ref (and why it is now completely distinct from $is).

I hope this helps,

=Drummond

* End of explanation *

Def. Quota Drummond Reed <drummond.reed@xdi.org>:

--- EQUIVALENCE LINKS AND REFERENCE LINKS (DRUMMOND & MARKUS)

https://wiki.oasis-open.org/**xdi/EquivalenceLinks<https://wiki.oasis-open.org/xdi/EquivalenceLinks>

This proposal has been updated to reflect the consensus to use two
different XDI verbs - $ref and $is - to express two different kinds of
equivalence.

Note also this paper referenced by Giovanni Bartolomeo:

http://iswc2010.semanticweb.**org/pdf/261.pdf<http://iswc2010.semanticweb.org/pdf/261.pdf>

We are ready to move this into Last Call provided there is consensus on
the
call.

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