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Re: First order logic and SPARQL

From: Bijan Parsia <bparsia@cs.man.ac.uk>
Date: Sun, 5 Sep 2010 15:21:16 +0100
Cc: Juan Sequeda <juanfederico@gmail.com>, Jitao Yang <jitao.yang@gmail.com>, semantic-web@w3.org, public-sparql-dev@w3.org
Message-Id: <979AF82B-AE40-4FE4-BC7B-8434C9482A20@cs.man.ac.uk>
To: Bob MacGregor <bob.macgregor@gmail.com>
On 5 Sep 2010, at 02:29, Bob MacGregor wrote:
[snip]
>
> Yes, really.  It sounds very much like you have defined/referenced a  
> cleaned-up version of SPARQL which
> unfortunately does not reflect the real-world semantics.

http://www.w3.org/TR/rdf-sparql-query/#sparqlDefinition

The semantics of (a good chunk) of the algebra is in terms of the  
relational algebra.

The formalization is based on this paper:
	http://arxiv.org/pdf/cs/0605124v1

I wouldn't conflated declarative (or formal) semantics with model  
theoretic.

> The problem with SPARQL stems from the OPTIONAL operator.  A mantra  
> of RDF has been that it
> has open world semantics.  The OPTIONAL operator is inherently non- 
> monotonic.

?? I don't think so. I'd be interested in a reference.

Note that non-communitivity doesn't imply non-monotinicity. After all,  
implication is non-communitive. Optional is defined in terms of left- 
outer join.

> A few of us devised
> a closed-world semantics for OPTIONAL, but the open-world advocates  
> rejected the notion, favoring instead
> a procedural semantics.

The meaning is the meaning, regardless of the presentation of that  
semantics.

> Not only are arguments to OPTIONAL defined to be order-dependent  
> (analogous to a series
> of if-then-else clauses),

Like implications in first order logic.

> but the SPARQL AND operator became polluted as well -- changing the  
> order of conjuncts
> that contain OPTIONALs can change the semantics of a SPARQL query.   
> I don't have examples available
> on the tip of my tongue, but a talk I gave a year ago at SEMTECH had  
> an example, and there are many
> others out there who should be able to furnish examples.

Can we dig this out?

> It would be a great service to the RDF community if you or someone  
> would propose a semantically
> well-founded variant of SPARQL (call it SPARQLL for "logical  
> SPARQL", or whatever).

I think that's called SPARQL/1.0.

> It would necessarily
> have closed-world semantics (as does Datalog).

Well, unbound requires epistemic reflection, but I don't think  
OPTIONAL does per se.

There's a lot of tricky parts of any query language because of e.g.,  
the need to report and control answers. It's perfectly reasonable to  
quarrel with choices you don't like, but I think we should be a bit  
more careful about the source of the problems. SPARQL/1.0 has a pretty  
reasonable and standard formalization.

Cheers,
Bijan.
Received on Sunday, 5 September 2010 14:21:55 GMT

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