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Subject: RE: [xdi] XDI dictionary, reasoner...
Markus’ graph is a such a wonderful
visual illustration of the question being discussed here that I attached it
again so I could reference it in this reply. My personal view is that there are subtle
yet vital distinctions between the two graphs and sets of XDI statements they represent. Markus’ first graph (upper left
corner) represents the following 3 XDI statements: #1 These statements identify three DIFFERENT literals.
With the right accompanying dictionary, an XDI reasoner could prove the values
of each these literals SHOULD be the same. However, I agree with Markus that, because
each of these literals is a separate instance of a literal in the graph, it SHOULD
require three separate $mod operations to change all three. (Whether an “intelligent”
XDI endpoint could use the dictionary to determine that a single $mod to one of
them SHOULD result in changes to the other two is a different point. From a
strict graph view, the three literals are separate values.) In Markus’ second graph (lower right
corner) represents the following three 3 XDI statements: #1 From a best practices standpoint, this is MUCH
cleaner, since it eliminates the need to have to rely on a reasoner to compare
the logical equivalence of literals. It also follows the general rule of “one
authoritative source, many pointers to that source”. Furthermore, a $mod
to any of the following XDI addresses… =giovanni+phone+home/$a$xsd$string …would unambiguously result in a
modify to the literal "+39 06 4451843". Markus, Giovanni, do you agree? =Drummond From:
markus.sabadello@gmail.com [mailto:markus.sabadello@gmail.com] On Behalf Of Markus Sabadello
On Sun, Apr 27, 2008 at 8:13 PM, Giovanni Bartolomeo <giovanni.bartolomeo@uniroma2.it>
wrote: Hello Markus,
Sorry for having been quiet about this so far, but I don't understand
it. What does it mean to say that the three statements "identify the same
literal": Hello Drummond, All, as for last week's request, I'm reintroducing this
issue. After receiving your answer, I tried to figure out how to prove that two
XDI RDF statements =giovanni+home+phone/$type$xsd$string/ =giovanni/+home+phone/ does identify the same
node. In order to recall the whole issue, which is now also on the wiki
thanks to Drummond, http://wiki.oasis-open.org/xdi/XdiReasoners I report
hereafter the whole message, at the end my elaborations:
Now, I think the most
important part to understand is related to the semantics underlying the
dictionary sentences:
which in English means
$has, according to XDI RDF
model v9, is aggregation, meaning that Any two XDI subjects with
a $has relationship can be concatenated into a single XRI representing this
relationship. Thus +phone+home is the
way how aggregation is expressed. Whereas $has$a is defined
as a composition. The object in the RDF statement is therefore an
"attribute" belonging to the subject. Example similar to the
one in XDI RDF v9: =giovanni/$has$a/+hair+color; thus
=giovanni/+hair+color/+black should be a valid XDI RDF statement,
expressing composition. Quoting from wikipedia, difference between aggregation
and composition: Aggregation differs from ordinary composition in that it does
not imply ownership. In composition, when the owning object is destroyed, so
are the contained objects. In aggregation, this is not necessarily true. For
example, a university owns various departments (e.g., chemistry), and each
department has a number of professors. If the university closes, the
departments will no longer exist, but the professors in those departments will
continue to exist. Therefore, a University can be seen as a composition of
departments, whereas departments have an aggregation of professors. In
addition, a Professor could work in more than one department, but a department
could not be part of more than one university. Now, I tried to work out
a formal proof using the assertions 1)-10) but I failed in proving that all the
following XDI statements identify the same literal: ... =giovanni+home+phone/$type$xsd$string/ =giovanni/+home+phone/ ... Probably I'm missing something (probably some
underlying semantics), however, can you give some hints? Maybe we can work out
the prove once for all and report this in the wiki/deliverable, as reference? Thanks, Giovanni At 01.32 24/04/2008,
Drummond Reed wrote: 4) TECH TOPIC: XDI DICTIONARY AND XDI REASONER On last week's call
Giovanni requested that our tech topic this week be the XDI dictionary and XDI
reasoner discussion we started on the list. Wikipedia has a good article on semantic reasoners: http://en.wikipedia.org/wiki/Semantic_Reasoner Drummond created a wiki
page to capture the email thread for further discussion:
http://wiki.oasis-open.org/xdi/XdiReasoners
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