Re: Blank nodes semantics - existential variables?

On 2020-06-30 19:48, Holger Knublauch wrote:
> 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...

We don't have to imagine when we have nulls in relational databases:

https://www.wired.com/story/null-license-plate-landed-one-hacker-ticket-hell/

(... though this is what happens *without* semantics.)

Received on Wednesday, 1 July 2020 01:02:20 UTC