RE: [TF:DbE] The easiest keys there are

Ah, I never realized this, but I think you are quite right:

"... then all bnodes are existentially bound by quantifiers "at
infinity", 
i.e. with a scope which effectively extends over the entire Web. 
Which makes them indistinguishable from names, semantically."

Thanks, Pat.

Leo 


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-----Original Message-----
From: public-owl-dev-request@w3.org
[mailto:public-owl-dev-request@w3.org] On Behalf Of Pat Hayes
Sent: Tuesday, October 02, 2007 6:03 PM
To: Bijan Parsia
Cc: Danny Ayers; Owl Dev; Dan Brickley
Subject: Re: [TF:DbE] The easiest keys there are


>>  My sense, based on my old
>>>understanding of how smushing generally works, is that it's pretty 
>>>much the same, i.e., missing keys aren't a problem, same key 
>>>causes a merge, multiple keys are ok (i.e., no functional 
>>>constraints), explicit values only, etc. The big difference is 
>>>that FOAF typically works on bnode subjects, which, under standard 
>>>BNode semantics are existential variables, thus wouldn't 
>>>(technically speaking) fall under our proposal.
>>>However, the common, deployed semantics for BNodes is that they 
>>>are local names, not existential variables.

This is a very tenuous distinction. Quantified logical syntax needn't 
even make the variable/name distinction (eg CL doesn't). Whats the 
difference between a locally quantified existential name and a local 
name?

>>>  SPARQL treats them that way. RIF shall as well, I'm pretty sure. 
>>>SPARQL/OWL probably will. So, we should work that fact into the 
>>>proposal as well.
>>
>>Could you spell out the distinction in super-simple terms, for the 
>>logically impaired? By which I mean, er, ... me.
>
>Sure.

Hey, this summary isn't fairly worded. OK, I'm a strict 
bnodes-are-existential guy, and I'll answer the questions:

>
>Do you believe that if you start from the following graph:
>
>	s p o.
>	_:x p o.
>
>and delete the second triple to get:
>	s p o.
>
>that you have lost anything?

Yes, of course: you have lost a triple. Do they MEAN THE SAME? 
(Different question) Yes, they do. Still, one has to be careful, 
because if these are subgraphs of a larger graph (as they almost 
certainly will be) then they might well not mean the same. For 
example:

s p o.
_:x p o.
_:x q r.

aint the same as

s p o.
_:x q r.

in anyone's book.

I think the 'pure' non-lean redundancy exhibited in your example here 
hardly ever occurs in practice. Maybe leanifyers aren't much deployed 
because there isn't much use for them.

>If not, then you buy and want to use the existential variable 
>semantics of BNodes. If not, then you want the local name version.
>
>Do you think it's harmless to go from
>	s p o.
>
>to
>	s p o.
>	_x p _y.
>	_z p o.
>?

Not harmless, no. The second one has a lot of useless redundancy in 
it. Do the MEAN THE SAME (different question)? Yes, they do; but 
again, if you are converting to the first from the second, be very 
careful about any other triples with those nodes in them.

>If so, you're existentially inclined (drink zee bitter coffee).

Well, I actually like bitter coffee, but wheres the bitterness here? 
Just because the first one entails the second doesn't mean that Im 
obliged to actually generate it.

>
>Are you content for the following triple:
>	s p _:x.
>
>To be replaced by:
>
>	s rdf:type [a owl:Restriction;
>		owl:onProperty p;
>		owl:someValuesFrom owl:Thing]
>
>?

No, because the first one is a lot shorter and can be understood 
without using elaborate OWL reasoning. Do they MEAN THE SAME? 
Actually, no, they don't, but they OWL-entail each other. So?

>If so, then you are pro-existential variable semantics. If not, then 
>you are sane, er, in favor of a somewhat different approach like 
>most of the world.
>
>(I'm simplifying a bunch, obviously.)

Much too much, and you are using slanted language. I don't know from 
this what you think 'local name' means. If we assume that disjoint 
graphs do not share any bnodes (which the RDF specs should have said, 
but we missed it) then I suspect that the two notions coincide, since 
then all bnodes are existentially bound by quantifiers "at infinity", 
i.e. with a scope which effectively extends over the entire Web. 
Which makes them indistinguishable from names, semantically.

Pat


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Received on Wednesday, 3 October 2007 02:17:46 UTC