RE: [xdi] Key implications of new metagraph $has definition

["Drummond Reed" <drummond.reed@cordance.net>]

Giovanni,

The proposed definition didn't say that +x+y and +x/+y are synonyms - they
are not, and cannot be, since the first one -- +x+y -- is a single-segment
XRI representing an XDI RDF subject and the second one -- +x/+y -- is a two
segment XRI representing an XDI RDF subject/predicate relationship.

The proposed definition said that +x+y and (+x/+y) are synonyms. The
cross-reference parentheses around the latter are critical. It is only by
turning +x/+y into the cross-reference (+x/+y) that it becomes a single XDI
RDF subject.

RE the three part statement +x/+y/+z, you are correct that, based on the
proposed $has definition, +x+y+z cannot infer either +x/+y/+z OR (+x/+y/+z).
Rather +x+y+z can only infer ((+x/+y)/+z).

The proposal that +x+y+z could refer to +x/+y/+z, or even to (+x/+y/+z), was
the one part of your initial proposed definition that I didn't fully
understand. Now that we have a clear proposed definition of +x+y as defined
above, I am satisifed. If you needs to refer to the full statement +x/+y/+z
as an XDI RDF subject, then you would use the cross-reference (+x/+y/+z).

Do you see any issues with that?

=Drummond 

> -----Original Message-----
> From: Giovanni Bartolomeo [mailto:giovanni.bartolomeo@uniroma2.it]
> Sent: Tuesday, May 05, 2009 9:28 AM
> To: Drummond Reed; xdi@lists.oasis-open.org
> Subject: Re: [xdi] Key implications of new metagraph $has definition
> 
> Hello Drummond,
> 
> I'm still thinking at all possible implications, however I'd like to
> express one first comment on this: if we assume that +x+y and +x/+y
> are synonyms, we have two implications, as you properly describe:
> 
> from point 1) we have two ways of saying the same thing
> 
> from point 2) we have an asymmetry in case of a three part statement,
> cannot use +x+y+z to refer to +x/+y/+z
> 
> Right now +x/+y and +x+y have been thought for different purposes: the
> first one to be used in query, e.g.
> 
> =markus
>    $get
>      /
>        =drummond
>          +friend
> 
> whereas the second one has been introduced in order to have the
> possibility to assert something about +x/+y, having a way to reference
> it as a new subject in the graph.
> 
> Are we sure that we really want to use them as synonyms?
> 
> Thanks,
> Giovanni
> 
> At 08.16 05/05/2009, Drummond Reed wrote:
> It was an extremely productive XDI TC telecon this last week (see the
> minutes at http://lists.oasis-open.org/archives/xdi/200905/msg00000.html)
> because it resulted in, I believe, a precise definition of Giovanni's
> proposal for the definition of $has statements. I want to reiterate that
> definition here and discuss two key implications that need to be reflected
> in the XDI Addressing & RDF Graph Model spec.
> 
> 1) REVISED DEFINITION OF $HAS PREDICATE
> 
> An XDI RDF $has statement between +x and +y, i.e., +x/$has/+y, asserts
> that
> +y is a predicate on the subject +x. It infers the following two XDI RDF
> subjects exist:
> 
> (+x/+y)
> +x+y
> 
> These two XDI RDF subjects are synonyms, i.e. this means the following two
> XDI RDF statements are true:
> 
> +x+y/$is/(+x/+y)
> (+x/+y)/$is/+x+y
> 
> In addition, both the subjects (+x/+y) and +x+y identify the set of all
> XDI
> RDF nodes that are objects of the XDI RDF statement +x/+y.
> 
> Lastly, this definition is recursive. So the XDI RDF statement
> +x+y/$has/+z
> identifies the set of all XDI RDF nodes that are objects of the XDI RDF
> statement +x+y/+z, and that this set can be identified by either of the
> following two XDI RDF subjects:
> 
> ((+x/+y)/+z)
> +x+y+z
> 
> This recursion repeats to any depth; ordering is always left-to-right.
> 
> 
> 2) THREE PART XDI RDF STATEMENTS
> 
> Giovanni's email
> (http://lists.oasis-open.org/archives/xdi/200904/msg00017.html) also
> proposed that +x/+y/+z infers +x+y+z. However the above definition does
> not
> allow this. +x+y+z expresses +x+y/$has/+z, which is equivalent to
> ((+x/+y)/+z). Rather, if there is a need to refer to a complete three-part
> XDI RDF statement such as +x/+y/+z, the entire statement becomes a
> cross-reference (+x/+y/+z). There is no shorthand for this statement.
> 
> 
> 3) $HAS$A STATEMENTS NOT NEEDED
> 
> Another key implication of this new definition is profound: $has$a
> statements no longer appear to be necessary. Rather $, $a, $is, and $has
> appear to be the complete set of metagraph predicates needed to express
> the
> fundamental relationships in an RDF graph:
> 
> 	$a is an incoming arc relationship (inverse: $is$a)
> 	$is is a self-referential arc relationship (and is its own inverse)
> 	$has is an outgoing arc relationship (inverse: $is$has)
> 
> This actually solves some longstanding issues around clarifying the
> relationship of $has and $has$a
> 
> If everyone is in agreement with these conclusions, I will update
> http://wiki.oasis-open.org/xdi/XdiOne/RdfGraphModel to reflect them, which
> will move us one step closer to publishing it as a spec.
> 
> =Drummond
> 
> 
> 
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