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Re: on recursion in SHACL

From: Peter F. Patel-Schneider <pfpschneider@gmail.com>
Date: Mon, 26 Sep 2016 09:07:15 -0700
To: Dimitris Kontokostas <kontokostas@informatik.uni-leipzig.de>
Cc: "public-rdf-sha." <public-rdf-shapes@w3.org>
Message-ID: <340e30e6-bbb6-c130-1be2-8653464518c5@gmail.com>
That's slightly better, but now there is a "disallow", which is not defined.


On 09/26/2016 02:17 AM, Dimitris Kontokostas wrote:
> Thanks for your feedback Peter,
> I moved
> <https://github.com/w3c/data-shapes/commit/f80725b162937683cc2f8aaecf497f2e0a3339c8>
> the recursion definition to section 3.1.1 and adjusted the definitions a bit.
> I also made a request to revisit issue-22 since property paths already provide
> an easy way to define recursion in SHACL
> https://lists.w3.org/Archives/Public/public-data-shapes-wg/2016Sep/0054.html
> Your comments for recursion on section 9.4 were addressed by Holger in a
> separate email
> Let me know if this resolves your issue
> Dimitris
> On Fri, Sep 23, 2016 at 8:00 PM, Peter F. Patel-Schneider
> <pfpschneider@gmail.com <mailto:pfpschneider@gmail.com>> wrote:
>     Following up one of the recent responses to my comments on Shapes Constraint
>     Language (SHACL) lead me to look at how recursion works in Shapes Constraint
>     Language (SHACL), W3C Editor's Draft 22 September 2016.
>     Here is the initial discussion of recursion, from Section 4.8.1.
>     "A shape may refer to itself directly or indirectly via sh:shape,
>     sh:filterShape, etc. Such a shape is said to be recursive. The meaning of
>     non-recursive shapes is always well-founded. In contrast, the meaning of a
>     recursive shape may not be well-founded. The handling of recursive shapes in
>     SHACL is left to implementations. Some implementations MAY reject shapes
>     graphs containing recursive shape definitions. Some implementations MAY
>     report a failure if a recursion has been detected at validation time."
>     This is the wrong place for the initial discussion of recursion.  First,
>     sh:filterShape was discussed much earlier, in Section 2.2.  Second, the
>     discussion of recursion deserves not to be buried within the discussion of
>     sh:shape.
>     The definition of recursive shapes is much too sloppy.  What is covered by
>     the "etc."?  Is
>       s:s1 rdf:type sh:Shape ;
>         rdfs:comment s:s1 .
>     a recursive shape?
>     What is the process for rejecting a shape graph containing recursive shape
>     definitions?  The term "reject" occurs only in this one place.
>     What does well-founded mean here?
>     The meaning of "a recursion has been detected at validation time" has
>     several problems.  Validation time is not defined.  What counts as detecting
>     a recursion is not defined.
>     Here is another aspect of recursion in SHACL, from Section 9.4.
>     "Recursive use of functions is undefined: If a SPARQL-based function
>     contains calls to other functions so that the same function with the same
>     combination of parameters would be visited twice then the result of the
>     function call is undefined. An implementation may either return no result
>     (unbound) or terminate the surrounding SPARQL query with an error."
>     It is not that all recursive use of functions is undefined.  What is
>     undefined here by the more detailed description is a call to the same
>     function and with the same parameters within another call.  It appears that
>     this is an attempt to prevent infinite recursion.  Such calls, however, need
>     not lead to infinite recursion if uncaught, even in a limited language like
>     SPARQL.  Nor is it that all cases of infinite recursion involve calls of
>     this sort.  As detecting such calls is neither necessary nor sufficient to
>     prevent infinite recursion it is puzzling as to why a complex and
>     potentially expensive mechanism is being described, and maybe even mandated.
>     Peter F. Patel-Schneider
>     Nuance Communications
> -- 
> 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
Received on Monday, 26 September 2016 16:07:49 UTC

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