equivalence and reference (Re: [xdi] Agenda: XDI TC Telecon Friday 9:00 - 10:30AM PT 2013-01-11)

[Giovanni Bartolomeo <giovanni.bartolomeo@uniroma2.it>]

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!). 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

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

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."

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


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