Ok thanks it all makes sense now.
Still I am wondering what such an XDI reasoner would actually DO. What's its logical input and output? What's an application that uses it?
- Given an XDI graph and a dictionary, the reasoner tells you if the graph is consistent with the dictionary or not?
- Given an XDI graph and a dictionary, the reasoner constructs new statements that are equivalent to existing statements in the graph?
Something like this?
Also, another question is, if $mod on a cross-reference modifies the referenced value, how do you modify the cross-reference itself? You probably would have to $del and $add it again?
Markus
On Mon, Apr 28, 2008 at 6:47 PM, Drummond Reed <> 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
=giovanni+phone+home
$a$xsd$string
"+39 06 4451843"
#2
=giovanni
+phone+home
"+39 06 4451843"
#3
=giovanni+phone
+home
"+39 06 4451843"
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$string
"+39 06 4451843"
#2
=giovanni
+phone+home
(=giovanni+phone+home/$a$xsd$string) <--xref to #1-->
#3
=giovanni+phone
+home
(=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
=giovanni/+phone+home
=giovanni+phone/+home
…would unambiguously result in a
modify to the literal "+39 06 4451843".
Markus, Giovanni, do you agree?
=Drummond
From:
[mailto:] On Behalf Of Markus Sabadello
Sent: Monday, April 28, 2008 7:20
AM
To: Giovanni Bartolomeo
Cc: Drummond Reed;
Subject: Re: [xdi] XDI dictionary,
reasoner...
Hmm my current understanding is that literals can not be shared in the graph,
therefore a $mod would only modify one of them.
But let's wait for Drummond to clarify.
Markus
On Sun, Apr 27, 2008 at 8:13 PM, Giovanni Bartolomeo <>
wrote:
Hello Markus,
to my understanding, the underlying idea is that there may be one node (literal
in this case) which can be accessed with different XDI RDF statements provided that, through a reasoner, all the statements
are expressing exactly the same concept. Let's consider them
synonyms. A "mod" message simply changes the node's value, so that
all three statements are automatically modified. I think this is a good
feature, I see that e.g. any object oriented programming language allows you to
define more than one pointer to the same object, and the benefits are huge.
What do you think?
Giovanni
At 05.44 26/04/2008, Markus Sabadello wrote:
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":
#1
=giovanni+phone+home
$type$xsd$string
"+39 06 4451843"
#2
=giovanni
+phone+home
"+39 06 4451843"
#3
=giovanni+phone
+home
"+39 06 4451843"
Assuming they do "identify the same literal", does that mean that
there is just one literal node in the graph, which is shared by three
predicates? If I apply a $mod message to one of the statements, will they all
change?
And is this what you are trying to prove with the dictionary?
Also, the obvious question I have to ask, what would an application look like
that demonstrates this functionality? What problems would you expect an XDI
library to solve here?
Markus
On Thu, Apr 24, 2008 at 12:09 PM, Giovanni Bartolomeo <
> wrote:
Hello Drummond, All,
as for last week's request, I'm reintroducing this
issue. After receiving your answer, I tried to figure out how to prove that two
XDI RDF statements
=giovanni+home+phone/$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:
**********
Drummond replies **********
I am so glad
someone finally asked this question in black and white – I have been
thinking about this issue of "XDI expressivity" for months now given
the fundamental capabilities of XDI RDF. Forgive me for providing an extended
answer, but I think the question can be so revealing about the power of XDI
RDF.
First, to
answer your question right up front, "Do those three XDI addresses
identify the same literal node?", my answer is: "You cannot know
deterministically without reference to the XDI dictionaries used by the
statements."
The reason is
that the three statements represent three different paths through the XDI RDF
graph, and as you pointed out, each path represents different semantics. So you
you really have to drill all the way down into the graphs, and the statements
represented by each graph, in order to answer your question.
To do this,
let's start by looking at the three graphs visually in X3 Simple (one reason I
love X3 Simple is that it lets you see the visual pattern of the graph while at
the same time seeing the addresses).
#1
=giovanni+phone+home
$type$xsd$string
"+39 06 4451843"
#2
=giovanni
+phone+home
"+39 06 4451843"
#3
=giovanni+phone
+home
"+39 06 4451843"
Now, let's
"fully explode" each of these into ALL the XDI statements they
represent.
#1 (5
statements)
=giovanni
$has
+phone
=giovanni+phone
$has
+home
$type
$has
$xsd
$type$xsd
$has
$string
=giovanni+phone+home
$type$xsd$string
"+39 06 4451843"
#2 (3
statements)
=giovanni
$has
+phone+home
+phone
$has
+home
=giovanni
+phone+home
"+39 06 4451843"
#3 (2
statements)
=giovanni
$has
+phone
=giovanni+phone
+home
"+39 06 4451843"
Isn't it
fascinating that the first graph represents five statements, the second three,
and the final one only two?
In any case,
all three options here (plus more - see below) appear to be completely valid
chains of XDI statements between the XDI subject =giovanni and the XDI literal
"+39 06 4451843". But none of them asserts exactly the same semantics
(the only way to do that would be to use XDI synonyms, i.e., $is statements).
However using an XDI reasoner and an XDI dictionary (which itself is just a set
of XDI statements comprising definitions), you could verify that according to
that dictionary, all these statements identify the same literal node.
In fact the
dictionary is pretty short. Here it is:
+phone
$is$a
<--1-->
+
$type$xsd$string
$has
<--2-->
+home
$a$has
<--3-->
+home
$has$a
<--4-->
+home
$a$has$a
<--5-->
+home
+home
$is$a
<--6-->
+
$has
<--7-->
+phone
$a$has
<--8-->
+phone
$has$a
<--9-->
+phone
$a$has$a
<--10-->
+phone
$type
$has
$xsd
$type$xsd
$has
$string
As short as it
is, the semantics represented by those statements – while crystal clear
from a pure XDI semantics point-of-view – represent a surprising range of
grammatical relationships a human POV. Roughly translated to English, the first
ten statements say:
1) A phone is
both a subject (noun) and a label for a type of data (typically called a
"phone number", but often abbreviated in English as just
"phone").
2) A phone can
have a home, i.e., as a noun, it can have a possessive relationship with
another noun, home.
3) A phone is
something a home can have (possessive relationship).
4) A phone has
an attribute of home.
5) Phone is an
attribute of a home.
6) Home is a
subject.
7) A home can
have a phone (possessive relationship – inverse of #3).
8) A home is
something a phone can have (possessive relationship – inverse of #2).
9) A home has
an attribute of phone (inverse of #5).
10) Home is an
attribute of a phone (inverse of #4).
Armed with
that dictionary, an XDI reasoner can quickly prove that all the following XDI
statements identify the same literal:
=giovanni+phone+home/$type$xsd$string/
=giovanni+home+phone/$type$xsd$string/
=giovanni/+home+phone/
=giovanni/+phone+home/
=giovanni+phone/+home/
=giovanni+home/+phone/
However, if
you remove any of those XDI dictionary statements, some of the above are no
longer provably true. For example, if you remove the statements:
+phone
$has
$home
+home
$a$has
+phone
Then you could
no longer prove that the following two statements identify the same XDI object
as the rest:
=giovanni+phone+home/$type$xsd$string/
=giovanni/+phone+home/
They MIGHT
identify the same literal as the rest, but now you have nothing you can prove
that with.
There's much
more I'd like to say about this simple dictionary, but I'm out of time. However
I encourage everyone who is interested to study it closely. To test your knowledge,
here's a quiz question:
"Given the dictionary above, how can you prove that it is valid for the
XDI object of all six statements to be a literal string?"
=Drummond
Now, I think the most
important part to understand is related to the semantics underlying the
dictionary sentences:
+phone
$has
<--2-->
+home
$has$a
<--4-->
+home
which in English means
2) A phone can
have a home, i.e., as a noun, it can have a possessive relationship with
another noun, home.
4) A phone has
an attribute of home.
$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
