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Re: [ACTION-18] use case on !ASK in FILTERS to emulate negation

From: Steve Harris <steve.harris@garlik.com>
Date: Tue, 19 May 2009 14:47:10 +0100
Cc: "Seaborne, Andy" <andy.seaborne@hp.com>, "public-rdf-dawg@w3.org" <public-rdf-dawg@w3.org>
Message-Id: <51172729-4658-48AB-8C44-7C8605418F4E@garlik.com>
To: Axel Polleres <axel.polleres@deri.org>
On 19 May 2009, at 14:25, Axel Polleres wrote:

> Seaborne, Andy wrote:
>>> -----Original Message-----
>>> From: public-rdf-dawg-request@w3.org [mailto:public-rdf-dawg-
>>> request@w3.org] On Behalf Of Axel Polleres
>>> Sent: 19 May 2009 13:24
>>> To: public-rdf-dawg@w3.org
>>> Subject: [ACTION-18] use case on !ASK in FILTERS to emulate negation
>>>
>>> This completes Action-18:
>>>
>>> http://www.w3.org/2009/sparql/track/actions/18
>>>
>>> =============================================================
>>>
>>> The current spec has the following example of a query emulating
>>> NEGATION, asking for people with a name but no expressed date:
>>>
>>>
>>> PREFIX foaf: <http://xmlns.com/foaf/0.1/>
>>> PREFIX dc:   <http://purl.org/dc/elements/1.1/>
>>> SELECT ?name
>>>  WHERE { ?x foaf:givenName  ?name .
>>>          OPTIONAL { ?x dc:date ?date } .
>>>          FILTER (!bound(?date)) }
>>>
>>> If ASK queries were allowed within FILTER expressions, this could be
>>> written as:
>>>
>>>
>>> PREFIX foaf: <http://xmlns.com/foaf/0.1/>
>>> PREFIX dc:   <http://purl.org/dc/elements/1.1/>
>>> SELECT ?name
>>>  WHERE { ?x foaf:givenName  ?name .
>>>          FILTER (! {ASK { ?x dc:date ?date }})
>>>        }
>>>
>>>
>>> Note here, that the subquery in the FILTER needs to "access" the
>>> bindings from the superquery. This should not be a problem with the
>>> algebra or order dependence since per definition, FILTER expressions
>>> need to have access to the bindings of the group they appear in,  
>>> but it
>>> would mean that such ASK queries in FILTERS (similar to OPTIONAL
>>> patterns in FILTERs) are not entirely compositional in their  
>>> evaluation.
>> Because the evaluation semantics of this FILTER ASK are different  
>> [*] to plain subquery, I currently prefer to have a clearer syntax  
>> keword like EXISTS, !EXISTS.
>
> You mean in FILTERs "EXISTS" instead of "ASK", yes?
> Indeed, makes sense to me.

If we're going to have a new verb, why put it in FILTER expressions?

PREFIX foaf: <http://xmlns.com/foaf/0.1/>
PREFIX dc:   <http://purl.org/dc/elements/1.1/>
SELECT ?name
WHERE {
   ?x foaf:givenName  ?name .
   NOTEXISTS { ?x dc:date ?date }
}

or similar is clearer IMHO.

- Steve

-- 
Steve Harris
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Received on Tuesday, 19 May 2009 13:47:46 GMT

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