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Subject: RE: [xdi] XDI dictionary, reasoner...
No, I agree we don’t want them as “dumb as
that” ;-) We’ve talked for a long time that an XDI endpoint should resolve any
cross-references in the same context. Perhaps you’re suggesting that we should
apply the same rule to $mod, i.e., a $mod with a literal as the object, when
applied to an XDI statement where the existing object is a cross-reference, should
automatically update the cross-reference ***if it is within the same
context***? That could work. As you suggest, it would
just be the mirror of the “same context” rule for $get. =Drummond From:
markus.sabadello@gmail.com [mailto:markus.sabadello@gmail.com] On Behalf Of Markus Sabadello
On Tue, Apr 29, 2008 at 1:18 AM, Drummond Reed <drummond.reed@cordance.net>
wrote: See [=Drummond] inline. From: markus.sabadello@gmail.com
[mailto:markus.sabadello@gmail.com]
On Behalf Of Markus Sabadello
Ok thanks it all makes sense now. [=Drummond] Yes – in that
sense it works just like XML schema validation, just smarter.
[=Drummond] Yes again –
this is one of the ways it can be smarter.
[=Drummond] As usual,
Markus, you bring up a great point. Let's take an example using the same set of
example addresses we have been discussing: #1 If you then sent the
following XDI $mod message… =giovanni …the normal behavior
would be to change the value of =giovanni/+phone+home the new literal value. So
we would go from… BEFORE THE $MOD =giovanni AFTER THE $MOD =giovanni Since you have to be able
to make such a change, i.e., take a subject/predicate XDI address who object is
current a cross-reference and change it so it's object is now a literal, my
conclusion is that I should rescind my earlier suggestion that a $mod to either
of the three addresses (#1, #2, or #3) should result in changing the
cross-referenced literal. Instead, only a $mod to #1 should change the literal.
There should be no reason then to change #2 or #3 since they cross-reference #1
anyway. So if we go down that
track, the conclusion is that a $mod to an XDI statement whose object is a
literal modifies the literal, but a $mod to an XDI statement whose object is a
cross-reference (XRI) changes the cross-reference itself (not the target of the
cross-reference). Markus, do you agree? =Drummond On Mon,
Apr 28, 2008 at 6:47 PM, Drummond Reed <drummond.reed@cordance.net>
wrote: 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 $mod to one of them SHOULD result in changes to the other two is a
different point. From a strict graph view, the three literals are separate
values.) In Markus' second graph (lower right corner) represents the
following three 3 XDI statements: #1
(=giovanni+phone+home/$a$xsd$string) <--xref to
#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
Hmm my
current understanding is that literals can not be shared in the graph,
therefore a $mod would only modify one of them. On Sun,
Apr 27, 2008 at 8:13 PM, Giovanni Bartolomeo <giovanni.bartolomeo@uniroma2.it>
wrote: Hello
Markus,
Sorry for
having been quiet about this so far, but I don't understand it. What does it
mean to say that the three statements "identify the same literal": Hello Drummond, All, as for last week's request, I'm reintroducing this issue. After
receiving your answer, I tried to figure out how to prove that two XDI RDF
statements =giovanni+home+phone/$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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