[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] | [List Home]
Subject: RE: [xdi] Definition of $has metagraph predicate
Dear all,
I think we all agree on the following claim: "The $has metagraph
predicate describes the relationship between a node and an outgoing
arc in the XDI RDF graph".
Unless I really did not understand what you want to do, I cannot see
how metagraph statements could exist without the graph they describe.
Therefore, to me the following statements are all valid:
#1: +x/$has/+y INFERS:
+y/$is$has/+x
+x+y/$is$a/+y
+y/$a/+x+y
+x+y
+x/+y
The corollary is also true, i.e.:
#1A: +x+y INFERS
+x/$has/+y
+y/$is$has/+x
+x+y/$is$a/+y
+y/$a/+x+y
+x/+y
To summarize my position:
-I agree with Bill when he says "I think $has must infer a
minCardinality of 1"
-I disagree with Drummond and Bill on the FULLY associativity of $has.
An explanation of why is reported here:
http://lists.oasis-open.org/archives/xdi/201001/msg00014.html
(and in the following of this thread).
Since $has is the pillar of our specs, I would suggest these actions:
- create a summary of the two (or three) models under discussion.
- encourage members of the TC to discuss this topic
- open a poll and record decisions
Kind Regards,
Giovanni
Def. Quota "Barnhill, William [USA]" <barnhill_william@bah.com>:
> Coments inline, look for =Bill.Barnhill+comment and =Bill.Barnhill+endcomment
> Kind regards,
>
> Bill Barnhill
> Booz Allen Hamilton - Rome, NY
> 315-330-7386 |
> william.barnhill.ctr@rl.af.mil<mailto:william.barnhill.ctr@rl.af.mil> |
> barnhill_william@bah.com<mailto:barnhill_william@bah.com>
>
> ________________________________
> From: drummond.reed@gmail.com [drummond.reed@gmail.com] On Behalf Of
> Drummond Reed [drummond.reed@xdi.org]
> Sent: Wednesday, January 13, 2010 4:43 AM
> To: OASIS - XDI TC
> Subject: [xdi] Definition of $has metagraph predicate
>
>
> Giovanni's message raised the question of whether there is consensus
> on the TC about the definition of $has semantics that we reached
> last spring and documented on May 15 2009 the wiki spec
> (http://wiki.oasis-open.org/xdi/XdiOne/RdfGraphModel).
>
>
>
> =Bill.Barnhill+comment
>
>
>
> Based on the email threads I think we cannot have consensus yet.
>
>
>
> =Bill.Barnhill+endcomment
>
>
>
> In that definition, we say (emphasis in red added):
>
>
>
> ********** EXCERPT ***********
>
> The $has metagraph predicate describes the relationship between a
> node and an outgoing arc in the XDI RDF graph. The subject of a $has
> statement is the XRI of the node from which the arc originates, and
> the object is the XRI of the arc.
>
> The inverse predicate, $is$has, describes the same relationship,
> however the subject and object are reversed, i.e., the object is the
> XRI of the node from which the arc originates, and the subject is
> the XRI of the arc.
>
> When used alone as a metagraph predicate, $has is an assertion that
> reifies this subject/predicate relationship so that this reification
> can serve as a new XDI RDF subject node. The XDI address of this new
> subject node is a direct concatenation of the XRI for the subject of
> the $has statement and the XRI for the object of the $has statement.
> For example:
>
> +x/$has/+y ==> (+x/+y) ==> +x+y
>
>
>
> *********** END EXCERPT ************
>
>
>
> =Bill.Barnhill+comment
>
>
>
> I believe we may reached consense on the above. I think it would be
> helpful to hash out a list of axioms like this (the above would be 6
> separate axioms and hold an informal recorded vote (OASIS voting
> process maybe but seems heavy weight).
>
>
>
> =Bill.Barnhill+endcomment
>
>
>
> Also, at the end of the $has and $is$has definition section, we said:
>
>
>
> ********** EXCERPT ***********
>
>
>
> [TODO: add explanation that $has statements do not and cannot infer
> cardinality.]
>
> =Bill.Barnhill+comment
>
>
>
> I don't remember reaching consensus on this. I remember you and
> Markus have said that you believe the above must be the case. I
> think $has must infer a minCardinality of 1 and have said so in the
> past.
>
>
>
> =Bill.Barnhill+endcomment
>
> *********** END EXCERPT ************
>
> The red highlighted section in the first excerpt documents, in my
> recollection, the realization we had that $has statements expressed
> a reification of a subject/predicate relationship. That's why the
> parentheses on (+x/+y) are necessary.
>
> Secondly, the fact that a $has statement resulted in reification
> that produce a new XDI subject node explains why $has statements
> have no cardinality – a conclusion that as I recall clearly had
> consensus across all of us that were active in the discussion.
>
>
>
> =Bill.Barnhill+comment
>
>
>
> Drummond I do not see how the first part explains the second, can
> you or Markus do a contradiction proof that shows that if $has
> statements have a minCardinality of 1 then reification can't work?
>
>
>
> Sounds like we didn't have enough people active in the discussion.
> Was the discussion on the mailing list?
>
>
>
> =Bill.Barnhill+endcomment
>
> To use an analogy to English, I can say "ball color", but that
> statement has no cardinality about ball colors – it is just defining
> a TYPE of color. Type has no cardinality.
>
> If we wanted to talk about the specific colors a specific balls has,
> we would say, "a ball has a color" (i.e., a $has$a relationship).
> With a $has$a relationship, you can now add cardinality, i.e., "a
> ball has 3 colors".
>
> But "ball color" as a concept (a noun in English), identifying a
> type of color, is very different than the statement "a ball has a
> color", which is not a noun, but a complete statement about the
> relationship of a ball (a noun) and "color" (another noun,
> representing a property). In XDI, I believe this is illustrated by
> the difference between the following:
>
>
>
>
> $HAS STATEMENT
>
> ball color
>
> +ball/$has/+color
> (+ball/+color)
> +ball+color
>
> $HAS$A STATEMENT
> ball has a color
>
> +ball/$has$a/+color
> +ball/+color
>
> In conclusion, I believe the current definition of $has is what is
> currently documented in the spec, as specifically illustrated in the
> example:
>
>
> +x/$has/+y ==> (+x/+y) ==> +x+y
>
> The only revision I would make to this example is that the arrows
> may imply the inferences are only unidirectional, while I think we
> all believe they are bidirectional. So the revised diagram would be:
>
>
> +x/$has/+y <==> (+x/+y) <==> +x+y
>
> If this is true, then I believe it follows that $has statements
> cannot be either only left or only right associative, but only full
> associative, because if the inferences above are true, then both of
> the following are true:
>
>
> +x+y/$has/+z <==> (+x+y/+z) <==> +x+y+z
>
> +x/$has/+y+z <==> (+x/+y+z) <==> +x+y+z
>
> Again, I believe this is entirely logically consistent, because it
> also means:
>
>
> +x+y+z/$is$a/+z
>
> +x+y+z/$is$a/+y+z
>
> =Bill.Barnhill+comment
>
>
>
> I changed my mind on this one and agree on fully associative
>
>
>
> =Bill.Barnhill+endcomment
> Now, it is important to point out (which we do not currently do in
> the spec – I think this should be fixed) that left or right
> associativity CAN be specified through the use of XDI reification
> (cross-references). In other words, to make +x+y+z either left or
> right associative, the XDI statements would be:
>
>
> (+x+y)/$has/+z <==> ((+x+y)/+z) <==> (+x+y)+z
>
> +x/$has/(+y+z) <==> (+x/(+y+z)) <==> +x(+y+z)
>
>
> =Bill.Barnhill+comment
>
> To me the above just doesn't hunt as they say in the south. Or
> rather it does, but doesn't say what I think you mean to say. In
> english I would translate (+x+y)+z as saying the following: the
> statement +x+y has a metadata property +z.
>
>
>
> This discussion and above example may mean we need to revisit
> discussion of a grouping operator, which kind of got dropped after
> the last face to face.
>
>
>
> =Bill.Barnhill+endcomment
> I believe this provides the expressivity that Giovanni believes is
> required to accurately reflect semantics where two XDI RDF nodes can
> only be considered in their own context (e.g., his "Markus contract
> sig" example, where "Markus contract" is to be treated as a unit).
>
> The result, in my understanding, is a definition that has no logical
> inconsistencies or ambiguities.
>
> So, the question to the TC is: does anyone disagree with these
> definitions? If so, why?
>
>
>
> =Bill.Barnhill+comment
>
>
>
> Yes, for the above reasons and the reasons in the previous email
> today. I think we can't address these definitions as a single unit.
> We need to look at each in turn starting with the most fundamental
> ones, get agreement, and build from there. I'd suggest starting with
> $has implying minCardinality of 1.
>
>
>
> =Bill.Barnhill+endcomment
>
> =Drummond
>
>
----------------------------------------------------------------
This message was sent using IMP, the Internet Messaging Program.
[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] | [List Home]