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Re: RDF* vs RDF vs named graphs

From: thomas lörtsch <tl@rat.io>
Date: Wed, 2 Dec 2020 23:06:13 +0100
Message-Id: <340858E0-D425-4623-B7BE-54DECCF5726E@rat.io>
Cc: public-rdf-star@w3.org
To: Pierre-Antoine Champin <pierre-antoine.champin@ercim.eu>


> On 2. Dec 2020, at 22:02, Pierre-Antoine Champin <pierre-antoine.champin@ercim.eu> wrote:
> 
> 
> 
> On 02/12/2020 01:33, thomas lörtsch wrote:
>>> Features that have been stable from the very beginning (e.g. "abstract" triples rather than triple occurrences)
>>> 
>> How do you support this claim?
>> 
> from Olaf Hartig and Bryan Thompson: Foundations of an Alternative Approach to Reification in RDF. In CoRR abs/1406.3399, Jun. 2014 (https://arxiv.org/pdf/1406.3399.pdf)
> 
> Definition 1:
> 
>   Assume pairwise disjoint sets I (all IRIs), B (blank nodes), and L (literals).
>   Let T* be an (infinite) set of tuples that is defined recursively as follows:
> 
>     1.T* includes all RDF triples, i.e.,T* ⊇ (I ∪ B)× I ×(I ∪ B ∪ L); and
>     2. if t ∈ T* and t' ∈ T*, then (t, p, o) ∈ T*, (s, p, t) ∈ T* and (t, p, t') ∈ T*
>         for all s ∈ (I ∪ B), p ∈ I, and o ∈(I ∪ B ∪ L).
>   Any tuple (s, p, o)∈ T* is an RDF* triple. A set of RDF* triples is called an RDF* graph.
> According to this definition, there is no way to distinguish two triples with the same subject, predicate and object... They are one and the same. So they can not represent triple occurrences, they can only represent "abstract" triples.

Maybe there are two definitions of "abstract" triples going around: up to now I understood the term as referring to a triple on the syntactic level, not interpreted, not part of the world the interpretation of the syntax describes. I think you call this the intension of a triple in your proposed semantics. The definition above as you explain it (and honestly, I’m just taking your word for it) describes the extension of a triple type: all occurrences of a triple. If that is your "abstarct" triple, then what do you call what I called "abstract"? A, right, the "triple (type)". Well, I’m not convinced that your use of the term "abstract" reflects common usage, but let’s for the moment just asume it does.

> That being said, I grant you that the example in that same paper, is poorly chosen:
> 
>     :bob foaf:name "Bob" .
>     <<:bob foaf:age 23>>
>         dct:creator <http://example.com/crawlers#c1>;
>         dct:source <http://example.net/listing.html>.
> 
> This modelling is brittle, because it breaks as soon as you have two occurrences of the same triple. That's unfortunate.

It reflects common expectations on and usage of reification. Therefor it is not unfortunate, but maybe it is misleading. Or the definition doesn’t get it right. Or the definition doesn’t want to go the extra mile to properly describe an occurrence (the graph issue that Olaf doesn’t want to tackle to keep things simple) but the example wants to evoke that it did all the same. Maybe this is just sloppy, maybe it is false advertising, in any case: it is wrong.

> From this, you may conclude that the example rules, and that the definition is broken. Or, that the definition rules, and that the example is broken (or at least, brittle). Traditionally (in scientific litterature or standards), definitions win over examples...

On the web practice wins and if your standard claims to cover practice but doesn’t then the standard is broken, and looses.

But you who designs a formal semantics have a responsibility to capture what people mean, not the legalese of a definition in a 6 years old paper. And if the definition doesn’t match the example you have to raise an issue and not just go with the fineprint in the definition.


> Furthermore, it is not too hard to fix the example to match the definition (add an intermediate node to represent the occurrence).

Which is exactly what nobody wants to do. You can model everything with extra triples. Adding extra triples to get it right is not the value propositin of RDF*. Fail.

> The opposite would be trickier -- and would break all existing implementations.

Instead you seem to think it’s easier to break all existing usage.
And besides: the claim w.r.t. all existing implementations is dubious but I’ll let the practitioneers in this round speak to that.

> With that, I rest my case ;)

There’s a german saying that on open sea and before a court we are all in god's hands. But apart from that I’d say: you most certainly loose. Because in the interpretation you give of the definition in Olaf’s paper (and again I assume you are right) the tail wags with the dog. And the semantics you design may be a nice and finessed car, but it goes into the wrong direction.


As I said before:
>> RDF* is about annotating statements that actually have been stated. It’s poster child usecase, capturing provenance, only makes sense for occurrences. It positions itself as an alternative to RDF reifcation, singleton properties and singleton graphs - all approaches that target occurrences.
>> Everything here screams 'occurrence'.

There are cases where one wants to speak about all occurrences of a triple type, like
<< :world :is :flat >> :is :nonsense
but that’s hardly a common case.

And the special case of unasserted assertions:
>> There is one exception: the case when in SA mode an embedded statement doesn’t reflect an actual occurrence. But this is a corner case, a niche usecase - although definitely an important one IMO.

This referred to what I understand as 'abstract' but what you probably call "triple (type)".

This makes 3 different cases, your semantics covers 2 but the 1 that everyone expects is not covered. This is broken by design and I don’t care if Olaf’s paper or your semantics is to blame. This just sucks and all the apologetic references to teh many implementations suck as well because I bet that implementations implement what they think is expeceted by their users and not what the definition says that probably not only I find hard to comprehend. Or they don’t care so much as long as it works on the surface and that’s just how it is, and it is pretty excusable if the example encourages them.

Thomas


Received on Wednesday, 2 December 2020 22:06:47 UTC

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