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

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

Giovanni, yes, I always think it's good to think on it before putting a
stake in the ground.

I'm just excited to finally have a solution to the +x+y issue that doesn't
introduce the +x/+y/+x+y complication.

I'll put this on the agenda for tomorrow's telecon.

=Drummond 

> -----Original Message-----
> From: Giovanni Bartolomeo [mailto:giovanni.bartolomeo@uniroma2.it]
> Sent: Wednesday, May 06, 2009 2:16 PM
> To: Drummond Reed; xdi@lists.oasis-open.org
> Subject: RE: [xdi] Key implications of new metagraph $has definition
> 
> Yes, Drummond I see your argument. But as you said:
> 
> "the potential that +x/+y/+z could be referred to as +x+y+z sounds
> appelling"
> 
> so my suggestion is: let us think at it a bit more before taking a
> final decision..
> 
> Giovanni
> 
> At 00.15 06/05/2009, Drummond Reed wrote:
> Giovanni et al,
> 
> In re-reading my message below, the final paragraph may not have been
> clear.
> Let me restate it. In Giovanni's original proposal for a new definition of
> the $has metagraph predicate that would avoid the RDF interpretation we
> found troubling (as discussed in
> http://lists.oasis-open.org/archives/xdi/200904/msg00017.html), he
> proposed
> that:
> 
> #1) The relationship +x/+y can be referred to as the subject +x+y and vice
> versa, and
> 
> #2) The relationship +x/+y/+z can be referred to as the subject +x+y+z and
> vice versa.
> 
> The result of our discussions of this proposal is, as I understand it,
> that
> we are adopting #1 (with one small but important clarification), but that
> we
> are rejecting #2, since the definition that supports #1 does not support
> #2.
> 
> The small but important clarification on #1 is that it is not the XDI
> statement +x/+y that can be referred to as the XDI RDF subject +x+y, but
> the
> _reification_ of the XDI statement +x/+y, which is (+x/+y). So, restated:
> 
> #1) The relationship (+x/+y) can be referred to as the subject +x+y and
> vice
> versa.
> 
> I am fully satisfied that this gives us a precise definition of $has
> relationships, i.e., +x/$has/+y ==> (+x/+y) ==> +x+y, and that this
> definition to finally solve the problem we had with the original $has
> definition (that +x/$has/+y inferred +x/+y/+x+y, which it no longer does).
> 
> The reason #2 does not hold is that in order for #1 to be consistent, it
> must recurse. So +x+y+z refers to ((+x/+y)/+z) and vice versa.
> 
> While the potential that +x/+y/+z or (+x/+y/+z) could be referred to as
> +x+y+z sounded appealing in some respects, it struck me from the outset
> that
> this was likely to conflict with a rigorous definition of #1, which it
> turns
> out it does.
> 
> Giovanni, I am happy to give up #2 (for which I don't have any use cases)
> to
> solve #1. Did you have any particular reason for wanting +x+y+z to express
> (+x/+y/+z) as a shorthand? Is there any reason not to just use (+x/+y/+z)?
> 
> =Drummond
> 
> > -----Original Message-----
> > From: Drummond Reed [mailto:drummond.reed@cordance.net]
> > Sent: Tuesday, May 05, 2009 12:51 PM
> > To: 'Giovanni Bartolomeo'; xdi@lists.oasis-open.org
> > Subject: RE: [xdi] Key implications of new metagraph $has definition
> >
> > 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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