Re: SHACL dependence on EXISTS

On 06/07/16 17:40, james anderson wrote:
>
>> On 2016-07-06, at 18:22, Peter F. Patel-Schneider
>> <pfpschneider@gmail.com <mailto:pfpschneider@gmail.com>> wrote:
...>> The most important in my view, however, is that SHACL centrally uses
>> pre-binding, i.e., evaluating SPARQL queries with variables "pre-bound" to
>> values.  In the above query the variables using $ are all pre-bound.
>
> to my understanding, as this group has been described by andy seaborne,
> that topic is not in its scope.

What SHACL ends up doing with pre-binding is obviously up to the RDF 
Shapes WG.  The WG requires, I believe, to define evaluation of a SPARQL 
query given that there are values for certain variables.

EXISTS takes a row (a number of variable/value pairs) and determines 
whether the graph pattern has zero or non-zero solutions.  But to define 
that outcome, each of the parts of the query has to have a well-defined 
evaluation; alternatively, the graph pattern could be sub-SELECT with 
ORDER BY/OFFSET/LIMIT.  Either way, needing zero or non-zero solutions 
in effect needs to define the evaluation of the pattern given the row.

They are also related conceptually - SHACL is focusing the SPARQL query 
in on on part of the graph and some context expressed as variable-value 
bindings; EXIST is focusing on evaluating a a graph pattern on one row 
of a solution.

Now SHACL may end up with a completely different mechanism but as they 
want it to be implementable it makes sense to be the same as EXISTS 
unless a good reason for different arises.  Or they may end up with 
certain addition restrictions that makes it easier in some way.

But it does look to me that it is so close to actual situation with 
EXISTS that what this CG does will be strong candidate for SHACL.

     Andy

Received on Wednesday, 6 July 2016 17:34:00 UTC