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Subject: Re: [xdi] Re: XDI subjects (was Groups - XDI RDF v8 Comments-Barnhill)


That's an excellent point, Giovanni.

My own conclusion from your observation, however, is that the change
introduced in XDI/RDF V9 that made $is reflexive and therefore
eliminated $a$is as inverse was a mistake and should be repealed.

The fact that RDF describes a directed graph, together with the fact
that a subject is not the same thing as an object, leads me to believe
that it's not meaningful to have reflexive (i.e., bidirectional)
predicates in RDF.

=Kermit

On 3/19/08, Giovanni Bartolomeo <giovanni.bartolomeo@uniroma2.it> wrote:
> Hello,
>
>  I think I've now understood why we cannot have
>  XDI documents (and possibly literals) in subjects
>  (i.e. to have a full addressable RDF graph).
>  However, with predicates like $is and their
>  inverse (which is $is) we should proceed
>  carefully. Coming back to the third example by
>  Kermit, if we use $is as predicate we've exactly what Kermit suggested:
>
>
>  <"http://example.com/number/of/hours/in/a/day";>
>  <"http://english.com/is";> <"24">
>  <"http://example.com/number/of/beers/in/a/case";>
>  <"http://english.com/is";> <"24">
>
>
> <"24"> <"http://english.com/is";>
>  <"http://example.com/number/of/hours/in/a/day";>
>  <"24"> <"http://english.com/is";>
>  <"http://example.com/number/of/beers/in/a/case";>
>
>
> This is implicit in $is which MUST be symmetric
>  ($is inverse predicate is $is). Otherwise, I
>  think, the semantics of $is is lost. But probably
>  here $is is not the best predicate...
>
>  Giovanni
>
>
>  At 20.57 18/03/2008, Kermit Snelson wrote:
>  >There are at least three reasons, in my opinion,
>  >why XDI/RDF subjects can't be literals: 1) RDF
>  >itself doesn't allow literals as subjects:
>  >http://www.w3.org/TR/rdf-concepts/#dfn-subject
>  >2) The proper XDI mapping of:   Blah blah <a
>  >href="http://example.com/some/target";>some
>  >literal text here</a> blah blah would be:   XDI
>  >subject = "http://example.com/some/target";   XDI
>  >predicate = $html$a   XDI object = "some literal
>  >text here" and the corresponding inline X3, with
>  >the additional predicates in Drummond's example,
>  >would be something like:   Blah blah
>  >[http://example.com/some/target[$html$a["some
>  >literal text here"]]
>  >[$uri$https[https://example.com/resolvable/uri]][$is[@!F83.62B1.44F.2813!1234]]
>  >blah blah 3) Allowing literals as subjects
>  >doesn't make logical sense. Consider the
>  >following:
>  ><"http://example.com/number/of/hours/in/a/day";>
>  ><http://english.com/is";>
>  ><"24">.
>  ><"http://example.com/number/of/beers/in/a/case";>
>  ><http://english.com/is";> <"24">. So far, so
>  >good. But to assert the following:   <"24">
>  ><"http://english.com/is";>
>  ><"http://example.com/number/of/hours/in/a/day";>.
>  ><"24"> <"http://english.com/is";>
>  ><"http://example.com/number/of/beers/in/a/case";>.
>  >  is to assert some substantial connection
>  >between the number of hours in a day and the
>  >number of beers in a case, which is fallacious.
>  >=Kermit On 3/18/08, Drummond Reed
>  ><drummond.reed@cordance.net> wrote: > > > > >
>  >First, let me be clear: I'm not a big fan of
>  >using literals as subjects, and > I don't have
>  >any compelling use cases for it (see below for
>  >the only one > I've been thinking about). It was
>  >Giovanni who seemed to have a reason for > using
>  >literals as subjects. > > > > Second, I agree, a
>  >literal as a subject can't be changed or it
>  >becomes a new > subject from an XDI
>  >standpoint. > > > > Now, here's the one thing
>  >that's had me thinking about
>  >literals-as-subjects > for a long time ­ take a
>  >standard HTML link
>  >tag: > > > >              Blah blah <a >
>  >href="http://example.com/some/target";>some
>  >literal text > here</a> blah blah > > > > If you
>  >wanted to turn this into an XDI statement, the
>  >only logical mapping > that seems to make sense
>  >is: > > > >             XDI subject = "some
>  >literal text here" > >             XDI predicate
>  >= $uri > >             XDI object =
>  >"http://example.com/some/target"; > > > > In
>  >other words, were you to replace HTML <a> tags
>  >with X3 within an HTML > document, the above
>  >link would look like: > > > >             Blah
>  >blah ["some literal text >
>  >here"[$uri["http://example.com/some/target";]]]
>  >blah blah > > > > That's pretty cool, because
>  >now you have a way of embedding really rich >
>  >semantics into ordinary web pages and web links.
>  >As a simple example, image > being able to make
>  >the above simple link into a compound statement,
>  >which > includes: a) an alternate HTTPS URL for
>  >the target resource, and b) a > persistent XRI
>  >synonym for the
>  >resource: > > > >             Blah blah ["some
>  >literal text >
>  >here"[$uri["http://example.com/some/target";]] >
>  >[$uri$https["https://example.com/some/target"]][$is[@!F83.62B1.44F.2813!1234]]
>  > > blah blah > > > > Net net: it's the ability
>  >to put XDI statements inline in ordinary HTML
>  >and > other markup formats that's the strongest
>  >use case I've seen so far for > being able to
>  >treat literals as XDI subjects. > > > >
>  >=Drummond > > > > > > >
>  >________________________________ > > > From:
>  >markus.sabadello@gmail.com
>  >[mailto:markus.sabadello@gmail.com] On > Behalf
>  >Of Markus Sabadello >  Sent: Tuesday, March 18,
>  >2008 9:30 AM >  To: Drummond Reed >  Cc:
>  >Giovanni Bartolomeo; barnhill_william@bah.com;
>  >xdi@lists.oasis-open.org >  Subject: [xdi] Re:
>  >XDI subjects (was Groups - XDI RDF v8
>  >Comments-Barnhill) > > > > > >  One aspect that
>  >seems strange with using literals as subjects is
>  >that you > can't modify them with XDI messages
>  >(I think). > >  If you
>  >have > >  =drummond >     +name >
>  >"Drummond" > >  You can modify the literal like
>  >this: > >  =drummond >     $mod >        / >
>  >       =drummond >              +name >
>  >        "D.Reed" > >  But you can't modify a
>  >subject. > >  What again was a use case for
>  >literals in subjects? (I'm not against it, >
>  >just asking) > >  Markus > > > On Mon, Mar 17,
>  >2008 at 7:34 PM, Drummond Reed
>  ><drummond.reed@cordance.net> > wrote: > > > >
>  >[renaming this thread to something more
>  >relevant] > > > > Giovanni, > > > > I agree with
>
> >Markus ­ I can't make sensee of having an XDI
>
> >document as an XDI > subject. I'm not sure my
>  >point from my earlier message came across, but
>  >I > was saying that when you use XDI context
>  >syntax ­ the  // syntax ­ it does > _not_ assert
>
> >that the previous XDDI document is the subject
>
> >of an XDI > statement. It says that the previous
>  >XDI statement _contains_ another XDI >
>  >statement. For example, in the following X3
>
> >Simple graph… > > > >
>
> >=drummond > >             +email > >
>  >             / > >
>  >       =drummond > >
>  >                     +email+home > > >
>  >"dsr.example@gmail.com" > >
>  >                            +email+work > > >
>  >"drummond.example@cordance.net" > > > > …you can
>
> >makke the following "compound XDI
>
> >statement": > > > >
>  >=drummond/+email//=drummond/+email+home > > > >
>  >This compound statement does not assert an XDI
>  >document as a subject. It > asserts the
>  >following: > > > > 1) =drummond is an XDI
>  >subject > > 2) +email is an XDI predicate of
>  >this subject > > 3) The object is another XDI
>  >document > > 4) =drummond is an XDI subject in
>  >this contained XDI document > > 5) +email+home
>  >is an XDI predicate of that XDI subject > > 6)
>  >"dsr.example@gmail.com" is the literal value of
>  >that XDI object > > > > If you wanted to have an
>  >entire XDI document as the subject of an XDI >
>  >statement, I think the syntax you are looking
>  >for is: > > > > >
>  >(xdi-subject/$context$xdi)/xdi-predicate/xdi-object
>  > > > > > In this XDI statement: > > > > 1)
>  >(xdi-subject/$context$xdi) is a cross reference
>  >that uniquely identifies > an XDI
>  >document: > >             a) xdi-subject is the
>  >XDI subject authoritative for a reference > to
>  >the XDI document > >             b) $context$xdi
>  >is the context type > > 2) xdi-predicate is the
>  >XDI predicate whose subject is the entire
>  >previous > cross-reference > > 3) xdi-object is
>  >whatever the XDI object is (literal, another XDI
>  >subject, > or another XDI document) > > > >
>  >**************** > > > > As for the issue of
>  >whether a literal can be an XDI subject, my
>  >primary > concern about that is how to treat it
>  >under XDI addressing rules. In every > XDI
>  >context, the XRIs must be unique. So there are
>  >two directions we could > take: > > > > 1) Allow
>  >literals to be XDI subjects, but ignore them
>  >from an XDI addressing > perspective (i.e., they
>  >would be "invisible" from an addressing
>  >standpoint.) > > > > 2) Allow literals as XDI
>  >subjects in syntax, but for addressing
>  >purposes, > have a specified transformation into
>  >relative XRI. For example: > > > > ["Drummond
>  >Reed"[+email[""dsr.example@gmail.com"]]]   <==X3
>  >with literal as > non-addressable subject > >
>  >[%44rummond%20%52eed[+email[""dsr.example@gmail.com"]]]
>  ><==X3 with literal > as addressable XRI
>  >subject > > > > Thoughts? > > > >
>  >=Drummond > > > > > > >
>  >________________________________ > > > From:
>  >markus.sabadello@gmail.com
>  >[mailto:markus.sabadello@gmail.com] On > Behalf
>  >Of Markus Sabadello >  Sent: Monday, March 17,
>  >2008 9:10 AM >  To: Giovanni Bartolomeo >  Cc:
>  >Drummond Reed; barnhill_william@bah.com;
>  >xdi@lists.oasis-open.org >  Subject: Re: [xdi]
>  >Groups - XDI RDF v8 Comments-Barnhill (pdf) (XDI
>  >RDF v8 > Comments-Barnhill.pdf)
>  >uploaded > > > > >  From the standpoint of
>  >having already implemented this, the proposal
>  >of > allowing a subject to be an XDI document
>  >(aka subcontext aka inner graph) is > a
>  >nightmare.. > >  I can think of at least the
>  >following immediate problems: >  - Some of the
>  >serialization formats may not be able to express
>  >this. >  - We always said that subjects in a
>  >graph must be unique. Can this still be >
>  >enforced with inner graphs as subjects? >  -
>  >What will XDI messages look like that make
>  >changes to these subjects? > >  I don't feel too
>  >secure about allowing literals as subjects
>  >either, but I > can't really argue why at this
>  >point.. > >  But I'm just thinking loud.. Of
>  >course all this is not necessarily a reason >
>  >not to do it :) > >  Markus > > > On Sat, Mar
>  >15, 2008 at 1:39 PM, Giovanni Bartolomeo >
>  ><giovanni.bartolomeo@uniroma2.it> wrote: > > > >
>  >At 20.52 13/03/2008, Drummond Reed wrote: > >
>  >Giovanni, > >  It's a subtle point, but when you
>  >use subcontext syntax (//), the parent > XDI
>  >document is not the subject of the child XDI
>  >document. It is the > container ("context") for
>  >the child. Thus I don't think the ABNF should >
>  >change. > > > > > Ok, I see; so if my
>  >understanding is correct, we have both the
>  >possibility > to have a whole XDI document as an
>  >RDF object as well as a "contained" > object
>  >("subcontext"). Thus, the original question: in
>  >these ABNF excerpts, > how could we specify that
>  >a subject can be an XDI document?
>  >E.g. > > > >  X3 = *( "[" sub *( "[" pred *( "["
>  >obj "]" ) "]" ) "]" ) >  sub = [ comment ]
>  >xri-reference [ comment ] >  sub = [ comment ]
>  >(xri-reference / X3) [ comment ] >  pred = [
>  >comment ] xri [ comment ] >  obj = [ comment ] (
>  >xri-reference / literal / X3 ) [ comment
>  >] >  literal = """ *char """ >  comment = "<--"
>  >*c-char "-->" > > > > RE the question of whether
>  >to allow a literal as an XDI subject, yes, I
>  >have > thought about, for reasons which I'll
>  >explain on today's call (if we have >
>  >time). > > > > I would allow this; especially if
>  >we'll standardize inverse predicates, we >
>  >should allow a subject to be a literal, as well
>  >as a XDI document or a >
>  >xri-reference. > > > > > > > >  RE whether a
>  >predicate should be able to be an XDI document,
>  >my immediate > answer is no ­ RDF predicates are
>
> >strictly URIs; XDI preedicates should be >
>
> >strictly XRIs. > > > > Yes, I agree with this.
>  >To summarize, I would be in favour of having
>  >the > same definition for subjects and
>  >objects: > >  sub = [ comment ] (xri-reference /
>  >literal / X3) [ comment ] > > > pred = [ comment
>  >] xri [ comment ] >  obj = [ comment ] (
>  >xri-reference / literal / X3 ) [ comment ] > >
>  >what do you
>  >think? > >  Thanks, >
>  >Giovanni > > > > > > > > >  Talk to you
>  >shortly, > >  =Drummond > >
>  >________________________________ > > > From:
>  >Giovanni Bartolomeo [ >
>  >mailto:giovanni.bartolomeo@uniroma2.it] >  Sent:
>  >Thursday, March 13, 2008 3:11 AM >  To: Drummond
>  >Reed; 'Markus Sabadello' >  Cc:
>  >barnhill_william@bah.com;
>  >xdi@lists.oasis-open.org >  Subject: RE: [xdi]
>  >Groups - XDI RDF v8 Comments-Barnhill (pdf) (XDI
>  >RDF v8 > Comments-Barnhill.pdf)
>  >uploaded > >  Hello Drummond, > >  Thanks for
>  >this clarification; however, if my understanding
>  >is correct, > this means that an XDI document
>  >can also be subject of another XDI document, >
>  >other than object. >  Doesn't this implies that
>  >we should update ABNF syntax making the >
>  >definition of subject somehow similar to the one
>  >for object? > >  X3 = *( "[" sub *( "[" pred *(
>  >"[" obj "]" ) "]" ) "]" ) >  sub = [ comment ]
>  >xri-reference [ comment ] >  sub = [ comment ]
>  >(xri-reference / X3) [ comment ] >  pred = [
>  >comment ] xri [ comment ] >  obj = [ comment ] (
>  >xri-reference / literal / X3 ) [ comment
>  >] >  literal = """ *char """ >  comment = "<--"
>  >*c-char "-->" > >  Further questions
>  >are: > >          should allow literals as a
>  >subject? >          should be predicate
>  >definition similar to subject and object one >
>  >update, e.g. may a predicate contain an XDI
>  >document? > >  however I'm not quite convinced
>  >with these last two ideas... comments >
>  >welcome! > >  Giovanni > > >  At 22.38
>  >12/03/2008, Drummond Reed wrote: > >  Just to be
>  >clear, the "n-segment" syntax was deprecated in
>  >the V9 XDI RDF > Model document ( >
>  >http://wiki.oasis-open.org/xdi/XdiRdfModel) due
>  >to the > problems identified in this thread. To
>  >be precise, it was unclear
>  >whether > >  s1 >              p1 >
>  >             o1 >
>  >       p2 >
>  >             o2 > >  meant that s1/p1/o1 was the
>  >subject of p2/o2 or not. It was also unclear >
>  >how cross-reference syntax would be
>  >applied. > >  We solved both problems by
>  >eliminating "n-segment" syntax in the V9 >
>  >document. Now it should be unambiguous that if
>  >you want to express that > s1/p1/o1 is the
>  >subject of p2/o2, you
>  >say: > >              (s1/p1/o1)/p2/o2 > >  What
>  >did remain is the // syntax for subcontexts,
>  >which allows you to solve > the RDF "blank node
>  >problem" by providing an address for a blank
>  >node. That > address is simply // (which fits
>  >very nicely from a conceptual standpoint > since
>  >the identifier for that segment is
>  >"blank"). > >  So if I want to say that the
>  >object of s1/p1 is a blank node, I can write >
>  >it as s1/p1// . This creates a new XDI context
>  >in which I can express > another set of XDI
>  >statements whose XRIs are unique in this
>  >context. > >  We'll go over the practical uses
>
> >for this on the call tomorrow ­ aagenda > coming
>
> >out
>  >shortly. > >  =Drummond > >
>  >________________________________ > > > From:
>  >markus.sabadello@gmail.com [ >
>  >mailto:markus.sabadello@gmail.com] On Behalf Of
>  >Markus > Sabadello >  Sent: Wednesday, March 12,
>  >2008 10:44 AM >  To: Giovanni Bartolomeo >  Cc:
>  >barnhill_william@bah.com;
>  >xdi@lists.oasis-open.org >  Subject: Re: [xdi]
>  >Groups - XDI RDF v8 Comments-Barnhill (pdf) (XDI
>  >RDF v8 > Comments-Barnhill.pdf)
>  >uploaded > > >  To be honest, I don't really
>  >understand the N-Segment syntax anyway. > >  Why
>  >is > >  s1 >          p1 >
>  >o1 >                            p2 >
>  >                         o2 > >  better
>  >than > >  s1 >          p1 >
>  >o1 >  o1 >          p2 >
>  >o2 > >  ? > >  Markus >  On Tue, Mar 11, 2008 at
>  >4:29 PM, Giovanni Bartolomeo < >
>  >giovanni.bartolomeo@uniroma2.it> wrote: >  Dear
>  >Bill, All, > >  reading your comments about XDI
>  >RDF v8 ( >
>  >http://www.oasis-open.org/committees/download.php/27112/XDI%20RDF%20v8%20Comments-Barnhill.pdf
>  > > ) I've found this sentence: > >  One problem
>  >with the 3-Segment syntax is that the N-Segment
>  >syntax uses > cross-references
>  >for >  reification. This means 3-Segment syntax
>  >has to have a different notation > for a subject
>  >which is the >  statement itself rather than the
>  >object of the statement. The 3-Segment >
>  >notation for this is a crossreference >  within
>  >a cross-reference: (()). So a subject of (s/p/o)
>  >asserts s/p/o and > starts a new
>  >statement >  whose subject is o, while a subject
>  >of ((s/p/o)) asserts s/p/o and starts a > new
>  >statement whose subject is >  the statement
>  >s/p/o. For example to say that =Drummond is
>  >author of the > statement =Bill.Barnhill is
>  >a >  contributor to the resource represented by
>  >@example we would use the
>  >XRI: >
>  >((@example/+dc+contributor/=Bill.Barnhill))/+dc+author/=Drummond.
>  > > >  Well, I'm wondering how N3 syntax (and
>  >consequently X3 simple) addresses > this
>  >problem: > >  If my understanding is correct,
>  >the N3 syntaxt >  <s1> <p1> <o1> >  <o1> <p2>
>  ><o2> >  is equivalent to X3
>  >simple >  s1 >          p1 >
>  >o1 >                            p2 >
>  >                         o2 >  what if I want to
>  >express that the whole statement <s1> <p1> <o1>
>  >is the > subject of <p2> <o2>? How this can be
>  >represented with X3 Simple? >  Breaking into a
>  >new subcontext doesn't seem to help, as this
>  >explicitly > introduce a new
>  >subject! >  s1 >          p1 >
>  >/ >                            s2 >
>  >                        p2 >
>  >                          o2 > >  Whereas I just
>  >want that the whole statement (s1/p1/o1) is the
>  >subject of > p2! > >  What do you think? Am I
>  >missing
>  >something? > >  Thanks, >  Giovanni > > >  At
>  >11.06 07/02/2008, barnhill_william@bah.com
>  >wrote: > >  The document named XDI RDF v8
>  >Comments-Barnhill (pdf) (XDI RDF v8 >
>  >Comments-Barnhill.pdf) has been submitted by Mr.
>  >William Barnhill to the > OASIS XRI Data
>  >Interchange (XDI) TC document repository.
>  >Document > Description: View Document Details: >
>  >http://www.oasis-open.org/apps/org/workgroup/xdi/document.php?document_id=27112
>  > > Download Document: >
>  >http://www.oasis-open.org/apps/org/workgroup/xdi/download.php/27112/XDI%20RDF%20v8%20Comments-Barnhill.pdf
>  > > PLEASE NOTE:  If the above links do not work
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