W3C home > Mailing lists > Public > w3c-rdfcore-wg@w3.org > July 2001

Re: rdfms-graph: Food for thought

From: Graham Klyne <Graham.Klyne@baltimore.com>
Date: Mon, 16 Jul 2001 13:01:17 +0100
Message-Id: <5.0.2.1.2.20010716123456.035b68b0@joy.songbird.com>
To: pat hayes <phayes@ai.uwf.edu>
Cc: w3c-rdfcore-wg@w3.org
At 02:15 PM 7/12/01 -0700, pat hayes wrote:
>>At 01:01 PM 7/11/01 -0500, Aaron Swartz wrote:
>>>Here's some questions (with proposed answers) to get us thinking about 
>>>rdfms-graph. I'm also curious whether there are other questions that 
>>>should be considered part of the resolution -- the issue description 
>>>isn't really enough for me to tell.
>>>
>>>1) Does an RDF graph have a URI?
>>>
>>>It is a Resource, and it can. M&S does not define a specific one.
>>
>>I agree.  I'm not sure anything more needs to be said.  Any such URI is 
>>not part of the abstract syntax/model.
>
>I agree except with the last sentence. The abstract syntax does not 
>exclude URIs that may happen to denote an RDF graph, so we need to say 
>that it means when they do occur. I think we can simply be agnostic about it.

I think I was a bit sloppy there -- I meant to say they don't have a 
distinguished or required place in the abstract syntax -- which I think is 
consistent with what you say.

>>>2) Is an RDF graph a set or a bag?
>>>
>>>A set, as it has conjunctive assertion semantics, or whatever they're 
>>>called:
>>>       (A && A) => (A)
>>
>>Again, I agree.  I'm not sure anything more needs to be said.
>
>The fact that it has this semantics doesn't mean that it is a set. Logical 
>expressions are not sets, but (and A A ) still implies A and vice versa. 
>The issue is whether we want to say that an RDF graph cannot contain two 
>copies of the same triple, not what we interpret those triples to be 
>saying. I would urge that it would be harmless to let them be bags, and 
>insisting that they are sets places an unnecessary burden on a parser 
>(which would need to remove all duplications whenever it merged two 
>graphs), so let them be bags.

Hmmm... I think this is a design choice that could go either way, each of 
which puts some kind of constraint on the parser:

Bag - requires that the parser accurately create a separate copy of each 
triple defined by the input;  this might be a burden for some 
implementations -- e.g. a relational database that uses the 3 triple 
components as the primary key in a table.

Set - requires that the parser detects duplicates, as you say.

In defining the abstract syntax, I think we are trying to capture something 
of the *intent* (read "meaning") of the RDF language used, rather than to 
prescribe a particular implementation.  Calling an RDF graph a "set" seems 
to more closely reflect the intent.

One might say it doesn't matter which it is (set or bag), and in a sense 
that would be true.  But then we lose a small practical benefit of the 
approach this group has been followings, viz. to prescribe a collection of 
triples that correspond to any RDF graph.

In defining and formalizing N-triples, I think the Bag approach of not 
forbidding duplicates is right, because that means the semantics is 
explicitly defined by the formalism.  In defining the mapping from RDF/XML 
to N-triples, the choice is less clear (to me).

So I'll offer a test case:

<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
          xmlns:ex="http//example.org/#">
   <rdf:Description about="http://example.org/#Subject" >
     <ex:property>Value</ex:property>
   </rdf:Description>
   <rdf:Description about="http://example.org/#Subject" >
     <ex:property>Value</ex:property>
   </rdf:Description>
</rdf:RDF>

For which, SiRPAC says the triples are:

<http://example.org/#Subject> <http//example.org/#property> "Value" .
<http://example.org/#Subject> <http//example.org/#property> "Value" .

Similarly, the graph displayed has two identical arcs.

This seems to be at odds with RDFM&S, section 5, (P162) which says "There 
is a set called Statements, each element of which is a triple of the form ...":

http://lists.w3.org/Archives/Public/www-archive/2001Jun/att-0021/00-part#162


>>>3) Can a node exist in a graph without any properties?
>>>
>>>Yes. This is indicated in the current XML syntax with an empty 
>>>Description element.
>>
>>Here, I disagree:  there is no obvious way to represent an isolated node 
>>in an abstract syntax/model based on triples.  I think an empty 
>><Description> adds nothing to the semantics so should not appear in the 
>>abstract syntax/model.
>
>As a matter of general methodology, the question to ask is whether 
>allowing it would cause any harm. I can't see any harm, so would opt for 
>not forbidding it.

The "harm" I see is that (a) not forbidding would seem to suggest a 
requirement to represent it, and (b) it's not clear to me how one would 
represent (in N-triples) a node on its own.  Also, I think a node not part 
of a property has no "meaning", so allowing this would seem to introduce 
spurious alternatives for the same semantics.  It seems simpler to just 
ignore isolated nodes at the abstract syntax level.

#g


------------------------------------------------------------
Graham Klyne                    Baltimore Technologies
Strategic Research              Content Security Group
<Graham.Klyne@Baltimore.com>    <http://www.mimesweeper.com>
                                 <http://www.baltimore.com>
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Received on Monday, 16 July 2001 10:51:43 EDT

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