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Re: Pragmatics of Blank Nodes Re: Toward easier RDF: a proposal

From: Patrick J Hayes <phayes@ihmc.us>
Date: Tue, 4 Dec 2018 18:31:31 -0600
CC: semantic-web <semantic-web@w3.org>, Aiden Hogan <aidhog@gmail.com>, Henry Story <henry.story@bblfish.net>
Message-ID: <A9F31019-3AAD-47A6-9ADF-A1F0EC17CBC2@ihmc.us>
To: David Booth <david@dbooth.org>


> On Dec 4, 2018, at 2:30 PM, David Booth <david@dbooth.org> wrote:
> 
> On 12/3/18 8:29 AM, Henry Story wrote:
> > . . .  So what are the advantages of blank nodes
> > pragmatically? They make a description local to the graph
> > in which they appear and this locality is maintained
> > across merges. The meaning of URI referenced resources can
> > be completed by external information of course but the
> > description ensures that no further links need to be taken
> > into account when understanding the bnode's meaning. So it
> > looks like it's ideal for things that need to be entirely
> > defined by description.

OR that cannot be *defined* at all, which is closer to the original idea. Henry, why would you assume that everything that can be mentioned, can also be /defined/?
> 
> Interesting point!   That means that blank nodes enjoy a
> form of closed world assumption (CWA),

No. That is exactly the kind of mistake that one gets into by being too loose with words like ‘define’. 

> in that there *cannot*
> be any other triples asserted (directly) about a blank node,
> other than the ones already in the document/graph/dataset
> at hand.  (Inference could add some though.)

Yes, it certainly could, if one has access to something like OWL.
> 
> Of course, if we are dealing with implicit blank nodes -- the ones generated by [] or () notation in Turtle -- then it's even more obvious that the only property connections to/from that blank node are the ones provided right there.

Inference can add extra trioles to those also. Suppose for example you know that the property rdf:rest is funcitonal and you know that x:A rdf:rest _:x ., and someone tells you that 

x:A rdf:rest _:y .
_:y x:Q x:C .

then you know know that  _:x owl:sameAs _:y ., and hence that _:x x:Q x:C .

Now, someone might argue that such cases are vanishingly rare, or even that they shouldn’t be allowed or encouraged, but that would be a different argument. 

> 
> This brings me to an interesting question.  To rephrase, the "identity" of a blank node object is determined entirely by the identities of its connected nodes, because it is guaranteed to not have any other connections.

It isn't, if we allow inferences.

>  Therefore, a blank node labeling algorithm (or standard Skolemization algorithm) only needs to take into account the subgraph of that blank node's tightly connected neighbors.  By "tightly connected" I mean the subgraph that is connected only through consecutive blank nodes.  (I think this may be slightly different from the Concise Bounded Description (CBD), because the CBD starts only with the *subject* of a triple.)
> https://www.w3.org/Submission/CBD/
> 
> Aiden (or someone else), is this correct?  If so, this would be very beneficial, because the labeling algorithm could then be guaranteed to generate the *same* label (or Skolem URI) for the blank nodes in that subgraph, regardless of any larger graph in which that subgraph appears.  This is very pertinent to n-ary relations, because it means that blank nodes for the same n-ary relation, appearing in different RDF graphs, could be automatically given the *same* label (or Skolem URI) -- even without knowing a key for that object.

That would be a wildly invalid conclusion. The coding of an n-ary atomic sentence into binary RDF basically says that an ‘event’ (or a ‘fact’, or ‘situation’, or…)  exists which represents the fact of the relation holding between the participants. So my hitting a wall with a hammer (a three-place relation) might be encoded as a bnode of type hitting with an agent being me and an object being the wall and the means being the hammer. But there might be a whole lot of hits of that wall with that hammer by me. You can’t infer that the many bnodes which encode various assertions of this kind are all the same single entity with a single global identifier: for one thing, that would imply that I only hit the wall once. 

>  Aiden, is this what such canonicalization algorithms already do?

I REALLY hope they don’t.

Pat


> 
> David Booth
> 
> 
Received on Wednesday, 5 December 2018 00:32:02 UTC

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