Re: rdfms-graph: Food for thought

>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 Thursday, 12 July 2001 17:15:56 UTC