Term for "predicate" (was Re: [xdi] golden triangle)

[Drummond Reed <drummond.reed@xdi.org>]

I renamed this thread so we can keep it just about the term for "predicate", and have a separate thread on the golden triangle.

For my part, +1 on sticking with the term "predicate", simply because RDF uses the same term for the same second-slot-of-a-triple.

To address Joseph's concern that XDI allows predicates without objects, I think we just need to state that in the specs, and note that expressing partial triples is a key difference between XDI and RDF.

On a side note, I've been wondering whether our "partial statements" can't be expressed as complete triples where either the predicate and object are both null or just the object is null.

From an XDI addressing standpoint, this would mean the XDI statement =drummond (just a subject) could also be expressed as =drummond// , or =drummond/+friend could be expressed as =drummond/+friend/

I have no strong opinion on it at all, other than if we did this, it would be important for the above to be considered equivalent, so that we didn't introduce interop problem.

The benefit would be that we could say all XDI statements are complete triples.

Thoughts?

=Drummond

On Thu, Jul 1, 2010 at 10:26 AM, Markus Sabadello <markus.sabadello@xdi.org> wrote:

I think I also like "predicate" better, but don't really have a good reason for it, nor do I have strong feelings about the golden triangle..

Markus

On Thu, Jul 1, 2010 at 5:10 PM, Giovanni Bartolomeo <giovanni.bartolomeo@uniroma2.it> wrote:

-1. I'd like "predicate" better, to maintain backward compatibility with RDF.

BTW there are currently many issues with the "golden triangle" which are still very obscure (at least to me), including: $is$a as $word relating predicates with objects, $is and self referencing arc definition, $has$a definition as traversal of subject and predicate, +x/+y/+y and +x/+y/+x+y reintroduced, after we agreed that they were not needed, etc...

Note that the current specs are totally based on the golden triangle: http://wiki.oasis-open.org/xdi/XdiOne/AddressingAndGraphModel

This way it becomes very difficult - or sometimes even impossible - to think at XDI productions in terms of description logic.

I think it would be better to rediscuss together the golden triangle, and probably revise the current specs page accordingly.

Kind Regards,
Giovanni

Def. Quota "Drummond Reed" <drummond.reed@xdi.org>:

I don't think it's any more confusing than "predicate". "verb" is just a
role - the same XRI could be a subject, verb, and object (especially in an
XDI dictionary).

But that's just one person's view. What do others think?

=Drummond

On Thu, Jun 10, 2010 at 2:28 AM, Markus Sabadello
<markus.sabadello@xdi.org>wrote:

Oh nooo I'll have to rename lots of stuff in XDI4j :)

But seriously, isn't "verb" a bit confusing? +name, +address etc. don't
look like verbs to me.

Markus


On Thu, Jun 10, 2010 at 10:49 AM, Drummond Reed <drummond.reed@xdi.org>wrote:

On Thu, Jun 10, 2010 at 12:51 AM, Joseph Boyle <boyle.joseph@gmail.com>wrote:

On Jun 9, 2010, at 11:22 PM, Drummond Reed wrote:

Joseph, first, my apologies for not replying earlier - I had another trip
this week so my email is way behind.

But we have another XDI TC telecon coming up tomorrow so I wanted to move
discussion forward on the individual issues/questions about the example
PDX document <http://wiki.oasis-open.org/xdi/PdxExample>. Here are my
answers to your two questions about $is (copied from below to keep the
thread clean):

> 1) How do the two roles of $is form a single coherent concept? Right
now the modifier role (as a passive voice marker modifying the following
verb) and the standalone role (as the copulative verb) seem like distinct
definitions to me. I realize this is analogous to the English verb "to be"
that also serves in both these roles, but is there a philosophical /
semantic / formal (take your pick) argument that this should logically be
the case in XDI?

You phrase that question very well. I have been thinking that in the
spec, we need to define the semantics for each of the metagraph predicates
for each of the following uses:

1) Standalone, e.g., $is

2) As a restriction on another predicate (i.e., preceeding it, e.g.,
$is+foo)

3) As an extension on another predicate (i.e., following it, e.g.,
+foo$is)


Agreed, we must do this, and explain what the 3 usages for a given
predicate have in common. (Are these the only 3, or are there even more
possible uses?)

I left out the other six options: Using the metagraph predicate as a 1)
standalone subject, 2) subject restriction, or 3) subject extension, as well
as a 4) standalone object, 5) object restriction, 6) object extension.

For many of those, the answer may be "undefined", but for some there are
very good answers. For example, $ as a standalone subject is the XDI context
self-descriptor; and $has and $a are both used as the proposed subject
extensions to create link contracts as shown in the lower part of the example
PDX document <http://wiki.oasis-open.org/xdi/PdxExample>.

 I believe the definitions in each of these three roles must be logically
consistent. For example, the definition of $is as a standalone predicate is
synonymity between the subject XRI and object XRI (they both identify the
same logical resource). This is as shown as a reflexive<http://en.wikipedia.org/wiki/Reflexive_relation>arc (self-referential -- originating and terminating in the same node) as
illustrated in the golden triangle<http://www.oasis-open.org/committees/download.php/37568/xdi-golden-triangle.png>.



The Golden Triangle diagram (I'm tempted to say the XDI Holy Trinity but
should probably refrain) itself shows S and O as separate nodes though
connected by arrows labeled $is. For the arc to be reflexive, S and O would
have to merge and become a single node. Sorry if this sounds too literal. We
do understand "$is" as making S and O equivalent - but this is something
that we have to explain with text external to the diagram. Looking at the
diagram alone naively, it is not obvious that the $is arcs merge S and O but
that the other $a, $has arcs do not make S and P equivalent or P and O
equivalent.

I agree that the Golden Triangle diagram by itself does not make it clear
that the $is arc is reflexive. It needs some text with it to explain that. I
have a separate intermediate diagram that explains the origins of the Golden
Triangle diagram that makes that much clearer. I propose we use both in the
final spec.

The definition of $is as a restriction on another predicate is that it
expresses the inverse of that predicate, e.g., the inverse of +b is $is+b
(example: +a/+b/+c <=> +c/$is+b/+a). The logical connection with $is as a
standalone verb is that $is, being reflexive arc, is being used to describe
the verb it is restricting. As a reflexive arc, it is literally "reversing"
the restricted verb. So $is+foo is the reverse (inverse) of +foo.

This is one simplest yet most powerful examples of the utility of
semantic (non-opaque) identifiers in XDI.


> 2) One difference I notice between XDI terminology and linguistics
terminology is that in the latter, "predicate" means verb together with
object, not simply the verb.

Ahhh, I didn't know that. As you know, I have no formal background in
either linguistics or formal logic, so I am constantly learning nuances like
this. What's the solution: are you suggesting we use the term "verb" instead
of "predicate"? As in: XDI subject, XDI verb, XDI object?


http://en.wikipedia.org/wiki/Predicate lists the differing meanings in
grammar, logic, etc.

 XDI gets the term "predicate" from RDF which gets it from mathematical
logic, where it specifically means a boolean-valued function. In this case
both S and O would be considered arguments to the predicate, and the boolean
result True from P(S,O) is expressed by the fact that the predicate is being
stated / graphed at all while the boolean result False from P(S,O) would be
expressed by not stating / graphing anything. This makes sense in one sense,
but may not be the most intuitively obvious meaning, in addition to the
conflict with the natural-language grammar that most people are familiar
with.

I would vote for "verb" not "predicate" in line with the trend towards
using simple everyday natural-language-like terms in XDI, which has included
using "$is", "$has", "$a" to replace more technical terms. This would be
another break with RDF terminology, which may be good or bad depending on
your viewpoint. I think some other knowledge representation systems have
used "verb" in some way, but don't remember specifically. However, the only
programming language I can think of offhand where "verb" is part of normal
terminology is COBOL. :/

I agree with your logic, and with using "subject, verb, object" instead of
"subject, predicate, object". If anyone on the TC disagrees, please post,
else I will start using that in all the XDI-related text I'm writing.

Thanks,

=Drummond

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