Re: SHACL dependence on EXISTS

> On 2016-07-06, at 19:33, Andy Seaborne <andy@apache.org> wrote:
> 
> 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.

yes, there is no reason to argue with this. this passage in recommendation text is clear:

"EXISTS returns true/false depending on whether the pattern matches the dataset given the bindings in the current group graph pattern, the dataset and the active graph at this point in the query evaluation."

> 
> 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.

to reiterate, these may concern the same “thing” or they may not.
without examples, it is not possible to judge.

> 
> 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.

if the “actual situation” with SHACL were expressed, one might be able to say.

best regards, from berlin,



---
james anderson | james@dydra.com | http://dydra.com

Received on Wednesday, 6 July 2016 21:59:50 UTC