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Re: Blank nodes semantics - existential variables?

From: Holger Knublauch <holger@topquadrant.com>
Date: Wed, 1 Jul 2020 09:48:56 +1000
To: semantic-web@w3.org
Message-ID: <ed7c8996-9b61-e8ea-2f6f-e47cbd83d93a@topquadrant.com>
On 30/06/2020 23:45, Antoine Zimmermann wrote:

> In passing, I'd like to add a use case for existential variables that 
> has not been mentioned and is absolutely crucial: in the RDF-based 
> interpretation of OWL, bnodes being existential is essential. The 
> RDF-based semantics of OWL would be completely disconnected from its 
> direct semantics if bnodes were not interpreted as existential variables.

Pardon me, but RDF is a standard by itself. OWL is just one other 
vocabulary with additional semantics, but it shouldn't dictate what 
happens in RDF. There is for example also SPARQL and SHACL, where bnodes 
are *not* existential variables but simply resources where the creator 
didn't care about a URI. That's BTW exactly how objects in any 
mainstream programming language work - they are essentially anonymous 
resources which you can only access if you have a pointer to them.

Eric is spot on w.r.t. the mismatch between the official specs and what 
is being implemented in practice. It would be good if the official specs 
would be adjusted to that reality. The logicians have dominated this 
space for way too long.

As a practical example, several years ago I spoke to a couple of 
database vendors because if you uploaded an RDF file with blank nodes, 
their implementations were returning new blank nodes for the same data 
with each query. As a result of this, it basically wasn't possible to do 
incremental graph traversals or even delete nodes that you had fetched 
in previous queries. Imagine delivering such useless semantics in any 
other object store or relational database...

Received on Tuesday, 30 June 2020 23:49:14 UTC

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