Re: on recursion in SHACL

Hi Peter,

I am not sure to which comment you refer to here.

Thanks,
Dimitris

On Sat, Oct 1, 2016 at 6:32 PM, Peter F. Patel-Schneider <
pfpschneider@gmail.com> wrote:

> Could your provide a pointer to the message from Holger so that the full
> record of this comment is retained?
>
> Thanks,
>
> peter
>
>
> 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
> >
>
>


-- 
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