Re: Consolidating triple/edges

I created a Draft PR [1] with some Turtle Grammar changes based on my interpretation of Andy’s concept. You can see the rendered version of the EBNF via GitHack [2].

As I noted, the change to the “annotation” production makes it context-sensitive, as an LL(1) parser would get confused when seeing the IRI/BlankNode that could either identify the triple occurence or be a predicate annotating the annotation, which requires the parser be able to backtrack. Not really a problem for more modern parsers, but a notable divergence. Other alternatives in the grammar could eliminate this at the cost of being less intuitive.

Gregg Kellogg
gregg@greggkellogg.net

[1] https://github.com/w3c/rdf-turtle/pull/51
[2] https://raw.githack.com/w3c/rdf-turtle/triple-term-occurance/spec/turtle-bnf.html

> On Dec 20, 2023, at 2:51 AM, Pierre-Antoine Champin <pierre-antoine@w3.org> wrote:
> 
> Just to concur 100% with Olaf's interpretation of Andy's email.
> 
> On 20/12/2023 10:26, Olaf Hartig wrote:
>> On Tue, 2023-12-19 at 16:39 -0800, Gregg Kellogg wrote:
>>>> On Dec 18, 2023, at 12:47 PM, Andy Seaborne <andy@apache.org>
>>>> wrote:
>>>> 
>>>> [...]
>>>> 
>>>> So we have:
>>>> 
>>>> Occurrence:
>>>>   << :s :p :o >>
>>>>   <<| N | :s :p :o >>
>>>> 
>>>> Triple term:
>>>>  <<( :s :p :o )>>
>>> To be clear, a Triple term would be a type, while an occurrence is a
>>> token?
>> That's my reading as well. However, maybe someone with a more intimate
>> understanding of the subtleties* of the notions of a token and an
>> occurrence should look at this question.
>> 
>> *https://plato.stanford.edu/entries/types-tokens/#Occ
>> 
>>> Are these fundamental in the abstract syntax? Or is the token
>>> considered syntactic sugar for something like [] rdfx:occurrenceOf
>>> <<( :s :p :o >>?
>> When I read Andy's email, I was assuming the latter, and that's also
>> what my immediate reaction would be, now that you ask this question
>> explicitly.
>> 
>> The options that I can currently think of to make tokens/occurrences an
>> explicit concept in the abstract syntax, would mean that we have to add
>> another new type of term or introduce some additional mathematical
>> structure that the notion of an RDF graph would have to be accompanied
>> with. I don't think these are very attractive options. Yet, if it
>> appears that there is a use for treating tokens/occurrences in a
>> special way in SPARQL (e.g., dedicated operators or build-in
>> functions), then we may have to capture them explicitly in some way
>> (but I don't see a need for that at the moment).
>> 
>>> Can a term contain an occurrence, or visa-versa? E.g. <<( << :s :p :o
>>>>> :o1 :o2 )>> or << <<( :s :p :o )>> :o1 :o2 >>?
>> The latter is probably not particularly controversial, in particular if
>> we understand expressions of the form
>> 
>>   << :s :p :o >>
>> 
>> as syntactic sugar as suggested in Andy's email. Then, the shorthand
>> 
>>   << <<( :s :p :o )>> :o1 :o2 >>
>> 
>> expands to
>> 
>>   [] rdfx:occurrenceOf <<( <<( :s :p :o )>> :o1 :o2 )>> .
>> 
>> (plus, the blank node in the subject of this triple would then also be
>> in the subject / the object of the triple in which the shorthand is
>> used).
>> 
>> Regarding the former, i.e.,
>> 
>>   <<( << :s :p :o >> :o1 :o2 )>>
>> 
>> perhaps this can also be considered (and, thus, defined) as a shorthand
>> notation for
>> 
>>   <<( _:b :o1 :o2 )>>
>> 
>> together with the addition of
>> 
>>   _:b rdfx:occurrenceOf <<( :s :p :o )>> .
>> 
>> into the same graph in which the shorthand is used as subject or object
>> of a triple. (Note that _:b is meant to be a fresh blank node
>> identifier that is not yet used in the document in which these things
>> are written).
>> 
>>> Would N-Triples contain both variations, or just the triple term?
>> I can see how supporting both variations in N-Triples maybe appreciated
>> for some use cases, but it may also be confusing because it would
>> diverge from the current principle that every line in an N-Triples file
>> is a serialization of a single triple only.
>> 
>> (Note that my assumption here is, again, that an expression of the form
>> 
>>   << :s :p :o >>
>> 
>> is really just syntactic sugar.)
>> 
>>> And, to James’s point, can you say << :s :p :o >> a <<( :s1 :p1 :o1
>>> )>>; if so, would this be the same as rdfx:occurrenceOf?
>> Well, by resolving the syntactic sugar as suggested in Andy's email,
>> this would expand to
>> 
>>   _:b rdfx:occurrenceOf <<( :s :p :o )>> .
>>   _:b rdf:type <<( :s1 :p1 :o1 )>> .
>> 
>> where, again, _:b is a fresh blank node identifier. So, the predicate
>> "a" (or, rdf:type) in James' triple is not necessarily the same as rdfx
>> :occurrenceOf.
>> 
>>>> Annotation:
>>>>  :s :p :o {| :p :z |}
>>>>  :s :p :o {| N | :p :z |}
>>>> (the last one is fiddly in the grammar because simply writing in
>>>> ABNF is ambiguous for some parsers)
>>> Presumably, an annotation is on an occurrence and not on a triple
>>> term/type?
>> I assume that's what Andy is suggesting here.
>> 
>> Best,
>> Olaf
>> 
>> 
>>> Gregg
>>>>     Andy
>>>> 
>>> 
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