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Re: rdfms-graph: Food for thought

From: Stephen Petschulat/CanWest/IBM <spetschu@ca.ibm.com>
Date: Fri, 13 Jul 2001 10:55:38 -0400
To: pat hayes <phayes@ai.uwf.edu>
Cc: w3c-rdfcore-wg@w3.org
Message-ID: <OF8DD73F86.BB4B4120-ON88256A88.005163C3@mkm.can.ibm.com>

>>>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.

Agreed. Note that it brings the Subject into the RDF graph... the lack of
arcs itself can be meaningful.

- steve

Stephen Petschulat



                                                                                                                        
                    pat hayes                                                                                           
                    <phayes@ai.uwf.edu>       To:     Graham Klyne <Graham.Klyne@Baltimore.com>                         
                    Sent by:                  cc:     w3c-rdfcore-wg@w3.org                                             
                    w3c-rdfcore-wg-requ       Subject:     Re: rdfms-graph: Food for thought                            
                    est@w3.org                                                                                          
                                                                                                                        
                                                                                                                        
                    12/07/2001 02:15 PM                                                                                 
                    Please respond to                                                                                   
                    pat hayes                                                                                           
                                                                                                                        
                                                                                                                        



>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.

>
>>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.

>
>>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.

Pat Hayes

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Received on Friday, 13 July 2001 11:13:34 EDT

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