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

On Tue, Jul 12, 2016 at 7:12 PM, Karen Coyle <kcoyle@kcoyle.net> wrote:

> 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.
>

Holger's proposal to rename these property scope selectors would help in
this regard
https://www.w3.org/2014/data-shapes/track/issues/169 (which I am also in
favor of)

However you are right in this, we use both predicate and property a lot in
the spec, not only in prose but also as sh:property / sh:predicate
sh:property denotes Property constraints (Constraints that are about a
particular property or path and its values for the focus node) here
property actually means predicate as property is a more general term that
can take part as subject/objecty in a triple
I think we copied this terminology from IBM Resource Shapes and for most
(technical) people the distinction is clear but I see your point of
confusion.

One way to reduce this confusion would be to rename sh:property to
something else e.g. sh:value.
This would be a very big syntax change (that I would avoid) but I wouldn't
object if there was strong support. But this should be decided as soon as
possible together with the rest of the upcoming changes.


> 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.
>

I hope the updated definitions are clear and this is no longer an issue


> 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.
>

The way validation is defined now is that it starts with an RDF node and we
validate the node itself (NodeConstraints) or it's values through a
predicate or a path (PropertyConstraints)
Scopes are one way to define the RDF nodes in-scope that in turn become
focus nodes after the pass any filtershapes
the other way to define focus nodes is through shape references e.g. with
sh:shape

so, since validation starts with a node, having ex:IRI as a focus node,
when ex:IRI exists only as predicate in the data graph,  would be something
like a silly thing to do (to reuse recent terminology :) )
depending on the constraints that are defined some would always be wrong
and some always true

Dimitris

>
> 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
>
>


-- 
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