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Re: ISSUE-68: Updated definition

From: Peter F. Patel-Schneider <pfpschneider@gmail.com>
Date: Thu, 10 Mar 2016 22:20:04 -0800
To: Holger Knublauch <holger@topquadrant.com>, "public-data-shapes-wg@w3.org" <public-data-shapes-wg@w3.org>
Message-ID: <56E26394.60805@gmail.com>


On 03/10/2016 10:15 PM, Holger Knublauch wrote:
> On 11/03/2016 15:27, Peter F. Patel-Schneider wrote:
>>
>> On 03/10/2016 08:37 PM, Holger Knublauch wrote:
>>> On 11/03/2016 13:22, Peter F. Patel-Schneider wrote:
>>>> On 03/10/2016 06:04 PM, Holger Knublauch wrote:
>>>>> On 10/03/2016 1:17, Peter F. Patel-Schneider wrote:
>>>>>> On 03/09/2016 12:46 AM, Holger Knublauch wrote:
>>>>>>> On 9/03/2016 18:17, Peter F. Patel-Schneider wrote:
>>>>>>>> I'm pretty sure that this fails in a number of places.
>> [...]
>>>>>>>> The substitution can modify variables from different scopes, which will
>>>>>>>> change
>>>>>>>> results.
>>>>>>> Do you have an example for this?
>>>>>> SELECT ?this ?that
>>>>>> WHERE { ?this ex:a ex:b
>>>>>>      SELECT ?that WHERE { ?this ex:a ?that } }
>>>>> The definition states that substitution also happens in nested SELECTs. I
>>>>> believe this meets user expectations, and would be needed for cases like
>>>>> sh:minCount that use a nested SELECT. I don't quite see a problem with the
>>>>> above. Do you have data to illustrate why this would cause problems?
>>>> Because the ?this in the inner SELECT is a different variable.  Before
>>>> substitution it would return any ?that that is the object of any ex:a triple.
>>>>    After substitution it returns only those that have the substituted
>>>> value as a
>>>> subject.
>>> Yes, but that is exactly the desired outcome.
>>>
>>> Holger
>> I'm not a SPARQL expert, but I don't think so.
>>
>>
>> Let's pre-bind  ?this to ex:c (this is a very easy case of pre-binding, so
>> what to do is pretty obvious) in
>>
>> SELECT ?this ?that
>> WHERE { ?this ex:a ex:b .
>>         SELECT ?that WHERE { ?this ex:a ?that } }
>>
>> against graph
>>
>> ex:c ex:a ex:b .
>> ex:d ex:a ex:f .
>>
>> The result set is
>> ?this = ex:c, ?that = ex:b
>> ?this = ex:c, ?that = ex:f
> 
> Yes that's without binding the inner ?this.
> 
>>
>> Let's substitute ?this by ex:c to get (roughly)
>>
>> SELECT (ex:c AS ?this) ?that
>> WHERE { ex:c ex:a ex:b .
>> SELECT ?that WHERE { ex:c ex:a ?that } }
>>
>> which results in only one solution
>> ?this = ex:c, ?that = ex:b
> 
> Yes, that's the intended result. I don't see the problem (yet?)
> 
> Holger

They are different.  You proposed that substitution into embedded queries
would implement pre-binding.  It doesn't, at least as I understand this simple
case of pre-binding.

If pre-binding doesn't work the way I think it should, then the old intuition
(values statements) was wrong.

peter
Received on Friday, 11 March 2016 06:20:35 UTC

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