Re: New public version of RDF Model and Syntax Draft Specification

.. message starts after citation ..

> "Ralph R. Swick" <swick@w3.org> on 07/24/98 10:18:28 AM
> To:   Bob Schloss/Watson/IBM
> cc:   w3c-rdf-syntax-wg@w3.org
> Subject:  Re: New public version of RDF Model and Syntax Draft Specifi
> 
> At 08:16 PM 7/23/98 -0400, Bob Schloss wrote:
> >"Pasqualino \"Titto\" Assini" <assini@kamus.it> on 07/23/98 03:45:57 AM
> >To: meta <meta2@mrrl.lboro.ac.uk>
> >I also have a question. By reading the standard I couldn't understand if
> >the following description:
> >
> > <rdf:Description about="http://mycollege.edu/courses/6.001">
> >      <s:students resource="http://mycollege.edu/students/Amy"/>
> >      <s:students resource="http://mycollege.edu/students/Tim"/>
> >  </rdf:Description>
> >
> >is equivalent to:
> >
> > <rdf:Description about="http://mycollege.edu/courses/6.001">
> >      <s:students>
> >        <rdf:Bag>
> >          <rdf:li resource="http://mycollege.edu/students/Amy"/>
> >          <rdf:li resource="http://mycollege.edu/students/Tim"/>
> >        </rdf:Bag>
> >      </s:students>
> >    </rdf:Description>
> >
> >or to:
> >
> > <rdf:Description about="http://mycollege.edu/courses/6.001">
> >      <s:students>
> >        <rdf:Seq>
> >          <rdf:li resource="http://mycollege.edu/students/Amy"/>
> >          <rdf:li resource="http://mycollege.edu/students/Tim"/>
> >        </rdf:Bag>
> >      </s:students>
> >    </rdf:Description>
> >
> >
> >or if its meaning is application dependent and not specified by the
> >standard.
> 
> I guess we need a forward reference to 3.4. in 3.2.1.  Of course,
> "meaning" is schema-dependent not application dependent.  But
> the three RDF graphs above are quite distinct.
> -Ralph

Why are they distinct ?

According to section 3.1 the meaning of (1) is "The students in course
6.001 are Amy, Tim and their order isn't significant"

(1)
<rdf:Description about="http://mycollege.edu/courses/6.001">
      <s:students>
        <rdf:Bag>
          <rdf:li resource="http://mycollege.edu/students/Amy"/>
          <rdf:li resource="http://mycollege.edu/students/Tim"/>
        </rdf:Bag>
      </s:students>
</rdf:Description>

while the meaning of (2) is "The students in course 6.001 are Amy, Tim
and their order is significant"

(2)
<rdf:Description about="http://mycollege.edu/courses/6.001">
      <s:students>
        <rdf:Seq>
          <rdf:li resource="http://mycollege.edu/students/Amy"/>
          <rdf:li resource="http://mycollege.edu/students/Tim"/>
        </rdf:Seq>
      </s:students>
</rdf:Description>

Now consider the meaning of (3): it's clearly that "The students in
course 6.001 are Amy, Tim".

(3)
<rdf:Description about="http://mycollege.edu/courses/6.001">
      <s:students resource="http://mycollege.edu/students/Amy"/>
      <s:students resource="http://mycollege.edu/students/Tim"/>
</rdf:Description>

There are now only two possibilities, either the order of the students
is significant and then (3) is logically equivalent to (2) or their
order isn't significant and then (3) is equivalent to (1).

As the reification of a Description is a Bag this implies that the order
of properties is not significant so I would say that (3) should be
considered equivalent to (1).

Section 3.4 states that these 3 RDF graphs are distinct but I think that
the adduced reasons are wrong.

The assertion: 

"In this example there is no stated relationship between the
publications other than that they were written by the same person."

is right but it's wrong to imply that, had we used a Bag instead of a
the simple repetion of the property, this would have implied the
existence of some additional relation between the Bag elements. What
would such relation be ?

Also the assertion that:

"It would be incorrect to model this sentence as three separate
approvedBy properties,
one for each committee member, as this would state the vote of each
individual member."

is right but it's wrong to add that:

" Rather, it is better to model this as a
single approvedBy property whose value is a Bag containing the committee
members' identities:
"

The proper modelling in this case would be to define a Description
instanceOf Committee and refer to is as the value of the property
approvedBy.

A committee is not a Bag, it's a logical entity that will likely have a
name, some powers and that probably will be composed by different people
at different times.

In conclusion I don't think that section 3.4 provide any good reason for
differentiating between (1) and (3).

Naturally it would be possible to argue that the real meaning of (3) is
that "The students in course 6.001 are Amy, Tim and it's not specified
if their order is significant or not"

But choosing such an alternative might lead to undesiderable logical
consequences.

Consider the description:

(4)
<rdf:Description about="http://mycollege.edu/courses/6.001">
      <s:students resource="http://mycollege.edu/students/Tim"/>
      <s:students resource="http://mycollege.edu/students/Amy"/>
</rdf:Description>

that's to say (3) but with the order of students inverted.

Is (3) equivalent to (4) ?

The question can not be answered as we are unaware if order is
significant. 
This means that the relation of equality between two RDF graphs can
assume 3 values: true, false and unknown.

Do we really want to get in such a mess ?

If for some reason it's deemed inappropriate to state in the RDF
standard if the order of the repetition of a particular property is
significant or not it should be made mandatory to specify it in the RDF
schema.
But this would also mean making mandatory the presence of an explicit
schema, at least if equivalence between RDF descriptions needs to be
calculable.


I have another question.

Would you consider defining a normal form for RDF, that's to say a form
such that if two RDF graphs have the same normal form they are
equivalent ? (a nice consequence of that would be that equivalence might
be determined by a simple string comparison between the XML
serializations of the normalized graphs).

It would be better to specify it in the standard, otherwise any
application will have to define its own normalized form.


-- 
Pasqualino "Titto" Assini  ---  assini@kamus.it  
Kamus Internet Consulting  ---  http://www.kamus.it/

Received on Sunday, 26 July 1998 18:10:02 UTC