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Re: rdfms-graph: Food for thought

From: Graham Klyne <Graham.Klyne@Baltimore.com>
Date: Wed, 25 Jul 2001 14:05:43 +0100
Message-Id: <>
To: pat hayes <phayes@ai.uwf.edu>
Cc: w3c-rdfcore-wg@w3.org
At 06:17 PM 7/23/01 -0700, pat hayes wrote:
>>>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.
>Its not so much the burden on a parser that worries me, but whether saying 
>that a syntax (even an abstract syntax) consists of a *set* of expressions 
>even makes sense. RDF is a language, after all, and it is even a language 
>which is supposed to be adapted to the situation where content written in 
>the language might be found in many sources scattered (literally) around 
>the planet. So what if a triple occurs on a web page in China and the same 
>triple occurs on another page in Oklahoma? Are they the same triple or 
>not? It seems clear that they are not. So if an RDF document is a set of 
>triples, does that mean that any well-formed RDF document cannot contain 
>both of them? (Or is it just wrongheaded to even talk about a triple - as 
>opposed to a lexicalisation of a triple - being on a web page? That would 
>make sense, but then I don't see the utility of the entire RDF graph 
>model, since it isn't needed and it seems to be at odds with the basic 
>philosophy of RDF.)
>Linguistics has long used a distinction between an expression (eg a word) 
>and a token of that expression. It makes sense to say that there is a 
>single English word 'beauty', say, but it would be foolhardy to define a 
>syntax in which there could only be one *token* of that word, since as 
>soon as you wrote it twice you would be breaking the rules. It makes sense 
>to talk about sets of expressions when those are Platonic abstractions, 
>but not when they are physical tokens. I am still unsure what the RDF M&S 
>authors had in mind when they said that an RDF graph was a set of triples. 
>If the graph is a mathematical abstraction, then that makes sense; but if 
>it is anything like a syntax, then it makes a lot less sense.

I think that neatly uncovers my unstated assumptions...

I guess I was thinking of statements as an abstraction.  But if we are to 
use N-triples as the reference form then statements have both order and may 
be repeated, and we must rely on the semantics to state the equivalence of 
expressions under duplication or re-ordering of individual statements.

So, on reflection, I agree that N-triples represents a Bag of statements.


Graham Klyne                    Baltimore Technologies
Strategic Research              Content Security Group
<Graham.Klyne@Baltimore.com>    <http://www.mimesweeper.com>
Received on Wednesday, 25 July 2001 09:57:02 UTC

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