Re: A question about referential opacity (again)

> On 21. Oct 2023, at 14:22, Peter F. Patel-Schneider <pfpschneider@gmail.com> wrote:
> 
> It's important to be clear as to what is formal and what is informal in discussions of this sort.
> 
> Formally in the current semantics for RDF, all IRIs are mapped (via the map IS) to resources by interpretations in RDF.  One generally says that an IRI E denotes or refers to IS(E).
> 
> A formal semantics that provides for referential opacity of IRIs generally provides a different mapping (let's call it IS') for IRIs that occur in opaque contexts, i.e., inside triple terms.   The details may differ, but the targets of this mapping are usually either left unspecified or are to some synthetic resources (such as copies of IRIs).
> 
> So if one was to construct an interpretation in this sort of formal semantics that actually included real cities in the world as resources and whose IS mapping mapped IRIs that are generally accepted as names of cities to the actual cities one would say that in transparent contexts, i.e., subjects, objects, and properties of asserted triples, dbr:Linköping refers to the city of Linköping but in opaque contexts, i.e., in triple terms, refers to something else.  (It may be possible that […in…, TL] some interpretations dbr:Linköping in an opaque context does refer to the city, but the important point is that there are interpretations where dbr:Linköping in an opaque context does not refer to the city and that absent special constructs to force transparency on opaque contexts there is no way to force an RDF graph to be false in all these interpretations.)

Thank you for the thorough explanation! I think I have now, at last, understood it.

So I take it that I wasn’t all wrong, as the CG report doesn’t exclude my interpretation of its meaning, but I was plain wrong in that the CG report doesn’t enforce my interpretation either. Bummer.

> peter
> 
> PS: The coordination group semantics uses a different mechanism entirely, instead syntactically transforming graphs that contain triple terms to regular RDF graphs.  Among other things, the opacity mechanism in this treatment transforms IRIs in triple terms to literal strings.  So part of this semantics is a relationship (similar to but not exactly denotes) from IRIs in opaque contexts to sequences of Unicode code points.
> 
> PPS:  There are other ways of obtaining opacity.  If the working group switches to graph terms the kind of semantics described above might not be adequate and some other treatment might have to be used.

I was meaning to ask you about that, as you voiced such concerns in the WG meeting last week. I just can’t see which profound changes to the semantics a switch from Triple Terms to Graph Terms would incur or require, so I assume those are technicalities. The only "issue" I can see is the need for transparent blank nodes going away, but that should make things easier, not harder. 

Anyway, I think the increase in usability far outweighs any additional effort in adjusting the semantics (although, of course, I’m not the one who would have to adjust them ;-), but that topic probably should be discussed in a separate thread.

Thomas

> 
> 
> 
> On 10/21/23 07:00, Thomas Lörtsch wrote:
>> Hi,
>> Enrico was kind enough to guide me through the work of the Semantics TF in a one-on-one TelCo a week ago. However, when I now look at my notes, I’m again confused.
>> If I understood Enrico correctly then a referentially opaque IRI doesn’t refer to anything. However, it was my understanding of the CG report semantics that IRIs in quoted triples are interpreted, but strictly following the syntactic form. My reading of the unstar-mapping supports that intuition [1].
>> To give an example, I understood referential opacity as meaning that "dbr:Linköping" and "DBR:LINKÖPING" both refer to the city of Linköping, and yet are not equal (and can not infered to be equal) because their lexical representation differs.
>> But according to how I understood Enrico they don’t refer to anything.
>> Was I wrong all along? Am I just not getting it and does there exist a world in which both interpretations are true? Or has the TF diverged from the CG? Or is there no consensus in the TF?
>> Best,
>> Thomas
>> [0] https://github.com/w3c/rdf-star-wg/wiki/Semantics%3A-Behaviour-catalogue
>> [1] https://w3c.github.io/rdf-star/cg-spec/2021-12-17.html#mapping
>

Received on Saturday, 21 October 2023 13:15:28 UTC