Rewrite the SHACL Rules introduction, incorporating all the topics we have discussed so far and that we want the spec to cover

[liviorobaldo]

• See this document rendered online here

[liviorobaldo]

My first PR ☺️

Some comments:

• I wrote “for deriving new content” rather than “for deriving new RDF triples” because rules can also add new RDF resources and RDF literals.

• I removed “beyond validation” because, as discussed in the last PR for SHACL Core, we should avoid over-emphasising validation and instead present it as one task among others. The same applies to the phrase “While SHACL Core focuses on constraint validation”.

• Regarding “SHACL Rules only add content to the knowledge graph”, rules do not remove content. See the discussion here: allowing rules to remove content, although potentially useful, could introduce several issues, particularly with respect to guaranteeing termination of rule execution. For this reason, I think we should avoid this.

• For the discussion on infer() and query(), see discussion here.

• Concerning the statement “Well-formedness conditions in SHACL Rules prevent infinite loops”, see discussion here . However, I am not sure whether stratification also plays a role in preventing infinite loops; if so, we could consider stating this explicitly in the abstract as well.

[afs]

“Well-formedness conditions in SHACL Rules prevent infinite loops"

The well-formedness conditions are not about loops. They are "set variable before use" where use is needing the variable setting - a filter or a head - or for stopping re-assignment.
https://raw.githack.com/w3c/data-shapes/SHACL-rules-introduction/shacl12-rules/index.html#wellformed

Infinite loops: There is an open issue here related to term introduction where stratification may be part of the solution.

[liviorobaldo]

Hi Andy, okay. My understanding is that this should still be implemented as part of the Rule Set Evaluation (§5). Perhaps the abstract should not go into these details, but I think it is important to at least state that correct implementations of SHACL Rules prevent infinite loops and non-deterministic behaviour.

By the way, I have just noticed that what I called the "initial knowledge graph" in the current version of the abstract is referred to as the "base graph" in §5. I am going to update index.html in this branch to keep the terminology consistent with §5.