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Re: Expressing conditional properties

From: Holger Knublauch <holger@topquadrant.com>
Date: Wed, 12 Sep 2018 09:02:33 +1000
To: public-shacl@w3.org
Message-ID: <94e1daf6-c7ba-010d-83fc-1aa209fc9e99@topquadrant.com>
As a bit of history, the SHACL WG had included a feature called 
sh:filterShape for a long time, but we got bogged into all kinds of 
technical discussions and in the end had to drop that feature. It was 
one of the optional things, and it was difficult enough to agree on 
anything in that WG. I think it was a quite elegant solution to the 
scenarios you describe, yet it would also have caused complications to 
many algorithms. The proposed work-around was to use a pattern along the 
lines of "either a BroadcastEvent or have minCount 1" using sh:or. Of 
course such complex expressions are harder to maintain and hard to 
understand by algorithms such as form builders too, but I cannot think 
of a much better solution now in SHACL Core.

One approach would be to re-instantiate sh:filterShape as a de-facto 
standard, if enough people agree that it should be brought back. SHACL 
1.0 isn't the last word, and this Community Group would be the right 
place to discuss that.

And yes, as Irene says SPARQL-based targets are able to express what you 
need, although this isn't supported by the SHACL playground. 
SPARQL-based target types would even allow you to encapsulate recurring 
design patterns into a high-level vocabulary. Then, as these target 
types get used and reused, tools may start to support them more "natively".


On 12/09/2018 7:13 AM, Gary Murphy wrote:
> Another novice question in my quest to translate my spin ;) I know I 
> can do these things with sh:sparql but thought I'd ask if there is a 
> more elegant strategy for defining shapes that apply only 
> conditionally to a property based on the particular subclass of $this.
> So for example, if I have a schema:Event, I would want to ensure that 
> it has the schema:location but not if it happens to be a 
> BroadcastEvent or OnDemandEvent -- normally I'd express this in spin 
> rules with a filter on the ?type of ?this.  Is there a shacl equivalent?
> A similar pattern happens where the rule applies only if some other 
> property is or is not in a list of classes, for example, an 
> ItemListElement only needs a position specified if its sibling 
> property itemListOrder is ascending or descending, so in one 
> sh:property I need to test the value/class of another property of the 
> current focus.
> by the way, the shacl playground is wonderful :)
> -- 
> *Gary Murphy*
> Developer, Schema App
> e: gary@schemaapp.com <mailto:martha@schemaapp.com>
> w: https://www.schemaapp.com <https://www.schemaapp.com/>
Received on Tuesday, 11 September 2018 23:03:00 UTC

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