Re: shapes-ISSUE-22 (recursion): Treatment of recursive shape definitions [SHACL Spec]

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Past experience indicates that specifying the semantics of recursive shapes
is not easy.  The Shape Expressions W3C Submission is ill-founded on
recursive shapes.  The axiomatization for shape expressions has
counter-intuitive behaviour on recursive shapes that involve negation or
exclusive or.  Both of these problems can be demonstrated on very simple
examples.

It is not that recursive shapes with SPARQL are undecidable - I don't see
how recursive shapes with SPARQL could be undecidable - it is that coming up
with intuitions on what recursive shapes with SPARQL are supposed to mean is
difficult for starters and then determining how to formalize these
intuitions is even harder.

peter


On 04/02/2015 11:10 AM, Arthur Ryman wrote:
> Richard,
> 
> Recursive shapes are very useful.
> 
> I think there are two subcases: 1. Shapes that use the high-level
> language 2. Shapes that use arbitrary SPARQL templates
> 
> For case 1, I am confident that we can precisely specify the semantics,
> without being particularly clever. For case 2, in general this is
> probably undecidable, however we might be able to say something about
> suitably restricted templates
> 
> -- Arthur
> 
> On Sat, Mar 28, 2015 at 4:12 PM, RDF Data Shapes Working Group Issue 
> Tracker <sysbot+tracker@w3.org> wrote:
>> shapes-ISSUE-22 (recursion): Treatment of recursive shape definitions
>> [SHACL Spec]
>> 
>> http://www.w3.org/2014/data-shapes/track/issues/22
>> 
>> Raised by: Richard Cyganiak On product: SHACL Spec
>> 
>> A shape definition may refer to itself, either directly or indirectly.
>> For example, in a bug tracker, issues may depend on other issues. Such
>> shapes are problematic if there are cycles in the data. Before one can
>> determine if a node belongs to the shape, one may first have to know
>> whether it belongs to the shape.
>> 
>> Proposals for treating this include:
>> 
>> - Doing something very clever in the Semantics - Syntactic conditions
>> that outlaws recursive shape defintions - Allowing recursive shape
>> definitions, but doing something to treat them as informative
>> documentation only
>> 
>> 
>> 
> 
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Received on Thursday, 2 April 2015 19:47:27 UTC