RDF star and LPGs

I mentioned in the call today that I had concerns about the "RDF star and LPGs” document[1] that was linked on IRC. I didn’t find any previous mention of that document in the mailing list archives, so unsure if it has been discussed previously (perhaps on a previous call?). Overall, I'm skeptical of the mapping presented in the document, and especially as the basis for making arguments that the current baseline proposal is compatible with LPG use-cases that have been previously discussed.

(I may be making assumptions about the mapping here, but since it is presented in the form of a single, simple example, I guess that’s to be expected. Similarly, I may be reading a lot into the "<—>” mapping indicator used between the example LPG and RDF data; please let me know if I’ve misunderstood the intention.)



I think on its face the RDF-LPG mapping presented does not preserve the intended semantics. To me, the intent of the LPG syntax is pretty clearly to assert that there are two transactions that occurred. They exist!

```
CREATE (a1)-[:TRANSACTION {amount: 1000, currency: "gbp", date: "2002-09-24Z"}]->(a2)
CREATE (a2)-[:TRANSACTION {amount: 900, currency: "gbp", date: "2002-10-03Z"}]->(a3)
```

But the mapped RDF does *not* define any transactions. There are no `:TRANSACTION` triples in the graph – that predicate only exists inside triple terms. So maybe they are hypothetical transactions?

```
<< :e1 | :a1 :TRANSACTION :a2 >> ;
  :amount 1000 ;
  :currency "gbp" ;
  :date "2002-09-24Z"^^xsd:date .
  
<< :e2 | :a2 :TRANSACTION :a3 >> ;
  :amount 900 ;
  :currency "gbp" ;
  :date "2002-10-03Z"^^xsd:date .
```

Furthermore, the mapping does not appear to be a bijection, and cannot be round-tripped. What happens to those triple terms if you try to map back to LPG? Do they disappear, as they are not asserted in the RDF graph? If, instead, they are mapped back into actual `:TRANSACTION` edges, how would that be different from mapping similar RDF that also asserted the `:TRANSACTION` triples in the graph (in addition to the triple terms)? I think this starts to touch on the point that Thomas has been making about asserted vs. unasserted assertions and the difficulty of modeling that difference in our current formalization.

Under “OBSERVATIONS":

> Reifiers denote edge identifiers.

I think this suggests that you'd end up with an empty graph if you tried to map RDF without any reifiers back to LPG (or at least a graph with no edges). That doesn't seem viable, as all existing RDF data prior to rdf-star would be entirely inaccessible based on this mapping. I am nervous about any one-way mapping, as it has implications for the sorts of systems you can build and the use-cases that they can address. And I’m similarly nervous about basing support for the baseline proposal on this document as an assurance that we are now able to capture LPG use cases. The mapping is just too casual, and leaves out far too many specifics to be useful in understanding if it can practically address LPG use-cases.

thanks,
.greg

[1] https://github.com/w3c/rdf-star-wg/wiki/RDF-star-and-LPGs