I've finally had some time to think about this. Another potential use case
for this would be some future state of the W3C Data Cube ontology. Data
Structure Definitions could be changed to be represented as Node Shapes,
and the focus nodes of one of these would be the Observations that are part
of the DataSet(s) that have that Data Structure Definition. This would
require the use of a path instead of a single predicate.
I've thought of two possible implementations. The first is the creation of
a new Target Type. Here's a (non-functional) prototype:
sh:PathFromShapeTarget
a sh:TargetType ;
rdfs:subClassOf sh:Target ;
sh:parameter [
sh:path sh:path ;
sh:description "The path connecting one or more shapes to their
focus nodes" ;
] ;
sh:select """
SELECT ?this ?currentShape
WHERE {
?shape $PATH ?this
FILTER EXISTS {?shape a/rdfs:subClassOf sh:NodeShape} # This
may or may not be necessary
}
""" ;
.
Targets then take a SHACL path to identify the path desired and injects it
in the SPARQL query like the path in a sh:SPARQLSelectValidator(if
implemented in SPARQL). So for the example function ontology:
fno:FunctionTarget
a sh:PathFromShapeTarget ;
sh:path [
sh:inversePath fno:executes ;
] ;
.
Or for the Data Cube example:
qb:DataStructureDefinitionTarget
a sh:PathFromShapeTarget ;
sh:path [
sh:inversePath (
qb:dataSet
qb:structure
) ;
] ;
.
Now, I realize that following the current behavior of Node Shapes and
Targets, each and every fno:Function or qb:DataStructureDefinition would
require an extra triple connecting it to the above example Target via
sh:target. While that could work, it feels like that the Target itself
really captures the meaning that the specified path connects shapes to
focus nodes independently of the specific shape and that requiring that
extra triple to exist every time feels redundant.
Therefore, it would be nice if it were possible to enable that
functionality, which is why I added the ?currentShape variable in the
query; either a shape could be bound to ?currentShape get focus nodes, or a
(potential) focus node could be bound to ?this to obtain the shapes that
apply to that node via the path.
A second possible implementation would be to create a new Constraint
Component that functions like sh:node, using a property perhaps called
sh:nodePath. However, instead of specifying the URI of a Node Shape that
focus nodes must also conform to, it specifies a SHACL path pointing to
Node Shape(s) that focus nodes must also conform to.
This would enable the following additions for the function ontology:
fno:Execution
sh:nodePath fno:executes ;
.
Or these additions for the Data Cube ontology:
qb:Observation
sh:nodePath (
qb:dataSet
qb:structure
) ;
.
This approach seems a bit cleaner, more practical, and it has the benefit
of applying to all instances of qb:Observation regardless of the specific
Data Structure Definition relevant to a given Observation. Note that this
doesn't apply a targeting rule based on a path independently; if there were
multiple classes/shapes that would also conform to a shape at the same
path, that would have to be expressed multiple times unlike the first
approach (potentially). That's not a dealbreaker though, just a comment.
My main reservation with this approach is that I'm not a huge fan of how if
sh:node fails validation, the error message states just that validation
failed and not *why* it failed (like how the original SHACL Playground
<https://shacl.org/playground/> example says "Value does not have shape
schema:AddressShape" instead of the actual error message "Value is not >=
10000"). The sh:node error messaging is not super helpful, and I would hope
that the error messaging for this feature would show the expected helpful
messages (and that in the future the error messages from sh:node could be
propagated through to the final report as well).
I haven't had the time to dig into SHACL validator implementation details
yet, so I'm not sure how feasible either of these options are to actually
implement in a current SHACL validator. I'm curious to see what you think.
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On Wed, Jul 21, 2021 at 11:38 PM Holger Knublauch <hol...@topquadrant.com>
wrote:
> Hi Matt,
>
> sounds interesting. Do you have a draft spec for this someplace, or a
> worked out example?
>
> One problem or design constraint with SHACL targets is that they should be
> executable in two modes:
>
> 1) for a given shape, find all target nodes
>
> 2) for a given node, find all shapes that target it
>
> If a constraint types makes computing 2) hard then tools will not be
> efficiently able to validate a given instance, e.g. after a user has made
> changes on a form.
>
> I am not saying this applies here but wanted to put this out as a thought
> to consider.
>
> Holger
>
>
> On 2021-07-22 1:11 am, Matt Goldberg wrote:
>
> Hello-
>
> I'm continuing this thread to propose a potential feature to add to some
> future version of SHACL related to this discussion. This feature would be a
> special type of target that would enable additional focus nodes for a shape
> to be specified by a property in a domain ontology that connects the focus
> node to the shape. This could be configured such that the subject would be
> a focus node and the object would be the shape, or such that the subject is
> the shape and the object is a focus node, or perhaps using a SHACL path
> which would enable more complex behavior than just a single predicate. I
> realize that the second case is like sh:targetNode, except the idea is to
> enable an existing domain property to have similar functionality to
> sh:targetNode. This would be useful for domains in which data may contain
> the specification of data requirements. As discussed earlier, there is
> currently no easy way to do this.
>
> For example, consider this function ontology
> <https://fno.io/spec/#ontology-abstract>. If you look at the descriptions
> for fno:Parameter <https://fno.io/spec/#fn-parameter> and fno:Output they
> look very similar to sh:PropertyShape (or perhaps sh:Parameter) in spirit,
> and the class fno:Function is therefore like sh:NodeShape (or perhaps
> sh:Function). If these classes were additionally modeled as subclasses of
> the corresponding SHACL class, then the focus nodes of an instance of
> fno:Function (and sh:NodeShape) would be the instances of fno:Execution
> connected to it via fno:executes. Therefore, it would be convenient to be
> able to create a target that effectively says "for all triples with
> fno:executes as predicate, the subject is a focus node of the object".
>
> I realize that this is targeting behavior is different than the other
> targets; shapes effectively say what the focus nodes for that shape are
> with existing targets while this is a target that could apply to multiple
> shapes. However, I think it could enable SHACL to be a lot more flexible,
> help enable simpler integration with existing ontologies (especially those
> in which data requirements are part of the domain), and help reduce the
> amount of metamodeling required to get extra SHACL functionality added to
> existing models.
>
> Matt Goldberg
> On Tuesday, November 3, 2020 at 2:06:55 AM UTC-5 Holger Knublauch wrote:
>
>> Just to add I don't see a solution to your specific issue either. Custom
>> target types can only access their own parameters, not the context shape or
>> the properties of that.
>>
>> Holger
>>
>>
>> On 11/3/2020 2:01 PM, Matt Goldberg wrote:
>>
>> Hello-
>>
>> Thanks for the reply. I have considered doing exactly this, and may end
>> up trying this further. My reservations with this method are that this
>> requires Advanced Features which may not be supported by all SHACL engines,
>> and with every usage of this Target Type the shape must specify itself as a
>> parameter and I was hoping there would be a way to make it a bit smarter to
>> avoid doing that. However, if that's the best option, I'll experiment with
>> it.
>>
>> On Monday, November 2, 2020 at 10:44:27 PM UTC-5 Irene Polikoff wrote:
>>
>>> Hi Matt,
>>>
>>> There are pre-defined target types in SHACL. You can not change their
>>> behavior.
>>>
>>> There is also a way to create SPARQL based targets using a SPARQL query.
>>> If a query is a pattern that may be re-used in different contexts, you
>>> could declare a custom type by parametrizing the query as described here
>>> *https://w3c.github.io/shacl/shacl-af/#SPARQLTargetType
>>> <https://w3c.github.io/shacl/shacl-af/#SPARQLTargetType>*
>>>
>>> This would be your own type of target, using your namespace, not sh:.
>>>
>>> The query, for example, could have predicate and object as parameters:
>>>
>>> ex:MyTarget
>>> a sh:SPARQLTargetType ;
>>> rdfs:subClassOf sh:Target ;
>>> sh:parameter [
>>> sh:path ex:predicate ;
>>> sh:nodeKind sh:IRI ;
>>> ] ;
>>> sh:parameter [
>>> sh:path ex:object ;
>>> sh:nodeKind sh:IRI ;
>>> ] ;
>>> sh:prefixes ex: ;
>>> sh:select """
>>> SELECT ?this
>>> WHERE {
>>> ?this $predicate $object .
>>> }
>>> """ .
>>>
>>> Then, when you assign this custom target type, you would provide values
>>> for the predicate and object. I have assumed above that objects are
>>> resources.
>>>
>>> This is just a quickly sketched example. I have not tried it. Read the
>>> spec for more details.
>>>
>>> On Nov 2, 2020, at 8:20 PM, Matt Goldberg <mgbe...@gmail.com> wrote:
>>>
>>> Left out a detail- an ideal solution would also avoid sh:node such that
>>> errors in validation reports would be informative instead of sh:node's
>>> uninformative "does not conform to shape" message.
>>>
>>> Thanks for any help!
>>>
>>> On Monday, November 2, 2020 at 8:07:44 PM UTC-5 Matt Goldberg wrote:
>>>
>>>> Hello-
>>>>
>>>> I've experimented with the Custom Targets and Custom Target Types and I
>>>> can't seem to find a reasonable way to define targets in a particular way.
>>>> What I'd like to do is have the focus nodes specified by resources
>>>> themselves via a property that points to the shape to use, effectively how
>>>> rdf:type does for implicit shapes, but by using a specified term in the
>>>> domain ontology instead of rdf:type. In other words, I'd like to specify
>>>> that the focus nodes for some node shape S are the subjects of triples with
>>>> predicate P and object S, where P is specified and is not necessarily
>>>> rdf:type.
>>>>
>>>> sh:targetSubjectsOf and sh:targetObjectsOf do not provide this
>>>> functionality, as they only look at the predicate. I don't think
>>>> $currentShape is pre-bound for custom target types like for constraint
>>>> components (at least my experiments didn't seem to work), but even if it
>>>> is, it would likely be an optional feature as is stated in the
>>>> specification. An ideal solution would be something that would not use any
>>>> TopBraid specific features, as we are using SHACL in other systems as well.
>>>>
>>>> Thanks for any help!
>>>>
>>>
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