Re: shapes-ISSUE-174 (Scopenode): Scopenode does not use RDF node definition [SHACL - Core]

Thanks, Dimitris. As I said in my response to myself, Yes, property 
scopes do what I was looking for. It is confusing that SHACL refers to 
predicates in many of its explanations of scopes, but calls the scope 
that takes a predicate identifier for scope selection a "property 
scope". I can understand why, to some extent, but there is a moving back 
and forth between "predicate" and "property" that causes me confusion. I 
would prefer that we use the terms of the RDF abstract syntax 
consistently, and always use the same term for the same thing.

I understand that all scopes are nodes. (it might be good to say that in 
the spec).  Defining a scope in terms of a predicate IRI results in a 
scope that is a node (either the subject or object of the predicate 
IRI). My original question was, if my data graph has this triple:

<a> <ex:IRI> <b>

and sh:scopenode = ex:IRI, is this triple in scope? I am assuming that 
it is not, because ex:IRI is not a node, but I haven't been able to get 
a clear answer.

The definition "A node scope with value $scopeNode, defines $scopeNode 
as the node in-scope in the data graph" reads as being circular (a node 
scope is a node in scope), so I'd like to see a definition that is more 
along the lines of:

When the value of sh:scopeNode matches the value of any node (subject or 
object of a triple) in the data graph, that node is the focus node that 
is in scope for matching against the defined constraints.

That may be too long, but I can't think of anything shorter at the 
moment. Basically, the definition needs to say both what the action is 
but also what the result is. The current definition says that, but the 
tautology of "scope node is node in scope" makes it hard to read.

Also, the meaning of "in scope" has not been defined in the document 
before that point, AFAIK, so "in scope" isn't meaningful. And again, 
there seem to be two parallel terms (node in scope, and focus node) that 
aren't clearly differentiated. Are they different? They both appear to 
apply to the data graph, although in the definitions "scope" is an 
aspect of the SHACL graph - however "in scope in the data graph" seems 
to belie that.

kc



On 7/12/16 6:55 AM, Dimitris Kontokostas wrote:
> Hi Karen
>
> On Mon, Jul 11, 2016 at 11:02 PM, RDF Data Shapes Working Group Issue
> Tracker <sysbot+tracker@w3.org <mailto:sysbot+tracker@w3.org>> wrote:
>
>     shapes-ISSUE-174 (Scopenode): Scopenode does not use RDF node
>     definition [SHACL - Core]
>
>     http://www.w3.org/2014/data-shapes/track/issues/174
>
>     Raised by: Karen Coyle
>     On product: SHACL - Core
>
>     After chatting with various folks I am convinced that the use of
>     "node" in the definition of "node scope" in SHACL is not correct.
>     Node in RDF (which definition SHACL claims to follow) is defined thus:
>
>     "The set of nodes of an RDF graph is the set of subjects and objects
>     of triples in the graph. It is possible for a predicate IRI to also
>     occur as a node in the same graph."[1]
>
>     When we look at that:
>
>     "The set of nodes of an RDF graph is the set of subjects and objects
>     of triples in a graph."
>
>     -> Only subjects and objects are "nodes."
>
>     "It is possible for a predicate IRI to also occur as a node in the
>     same graph."
>
>     This refers to the IRI that is a predicate in a graph, not the
>     predicate position in the triple, which is always an "arc" and not a
>     node. The IRI can be a node (a subject or an object) if it is the
>     subject or object of a triple. This means that if you "say
>     something" about the predicate (for example adding a provenance
>     statement about the predicate) then it has become a node in a graph
>     as well as being a predicate in the graph.
>
>     In fact, most predicates are not also subjects or objects. This
>     means that at the moment SHACL does not have a way to define a scope
>     on predicates.
>
>
> At the moment, SHACL and all existing parallel proposals (correct me if
> I am wrong) including ShEx & Peter's proposal work on RDF nodes.
> No proposal has any special treatment for predicates, when the
> predicates do not exists as subjects / objects on the graph to be tested
>
>
>     If the above holds true, then two things must happen: 1) SHACL must
>     not use "node" where it means all components of a triple and
>
>
> We do not currently have any discussion on how a predicate can work as
> scope nor any use-cases. e.g. what does it mean to to have e.g.
> rdfs:label as scope?
>
> however, we do have sh:scopeProperty and sh:scopeInverse property
>
> e.g. sh:scopeProperty rdfs:label selects all subjects in the data graph
> that appear in a triple with rdfs:label as predicate
> e.g. sh:scopeInverseProperty rdfs:label selects all objects in the data
> graph that appear in a triple with rdfs:label as predicate
>
> in that sense I think the term node here is correct
>
>
>     2a) there must either be a SHACL vocabulary element that allows
>     scoping based on *any* component of a triple, or 2b) there must be a
>     separate SHACL vocabulary element that allows scoping based on
>     matching a predicate.
>
>
> as stated above, we first need to define what does it mean to have a
> predicate as a scope
> e.g. sh:scopePredicate rdfs:label
>
> to me something like this would intuitively translate to the following
> SHACL scopes
> sh:scopeNode rdfs:label
> sh:scopeProperty rdfs:label
> sh:scopeInverseProperty rdfs:label
> i.e. the actual predicate as a node as well as all the subjects /
> objects in the data graph that appear in a triple with rdfs:label as
> predicate
> is this what you have in mind as well? in that case we can discuss if we
> can create such a syntax shortcut
>
> Best,
> Dimitris
>
>
>
>     [1]
>     https://www.w3.org/TR/2014/REC-rdf11-concepts-20140225/#section-rdf-graph
>
>
>
>
>
>
> --
> Dimitris Kontokostas
> Department of Computer Science, University of Leipzig & DBpedia Association
> Projects: http://dbpedia.org, http://rdfunit.aksw.org,
> http://aligned-project.eu
> Homepage: http://aksw.org/DimitrisKontokostas
> Research Group: AKSW/KILT http://aksw.org/Groups/KILT
>

-- 
Karen Coyle
kcoyle@kcoyle.net http://kcoyle.net
m: 1-510-435-8234
skype: kcoylenet/+1-510-984-3600

Received on Tuesday, 12 July 2016 16:13:07 UTC