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Re: BIND, FILTERs and BGPs

From: Lee Feigenbaum <lee@thefigtrees.net>
Date: Tue, 12 Oct 2010 15:43:13 -0400
Message-ID: <4CB4BA51.6010005@thefigtrees.net>
To: Andy Seaborne <andy.seaborne@epimorphics.com>
CC: sparql Working Group <public-rdf-dawg@w3.org>
On 10/12/2010 3:36 PM, Andy Seaborne wrote:
>>  { P1 KW(Expr AS ?Var)  P2}
>> =:=
>> { { SELECT * Expr AS ?Var { P1} } P2 }
>
> Does BIND end a BGP?
> What happens to FILTERs?
>
>
> { FILTER(expr) ?x ns:p ?v . ?x ns:q ?w }
>
> is the algebra:
>
> (filter expr
> (bgp (triple ?x ns:p ?v)
> (triple ?x ns:q ?w)
> ))
>
> or written after FILTER motion:
>
> { ?x ns:p ?v . ?x ns:q ?w FILTER(expr) }
>
>
> But if we insert BIND(2*?v AS ?two) into that:
>
> { FILTER(expr) ?x ns:p ?v . BIND(2*?v AS ?two) . ?x ns:q ?w }
>
> There is a similar issue with bNode syntax label scopes.
>
> I propose that:
>
> 1/ BIND terminates the current BGP
> 2/ FILTER goes after BIND in the BGP

I understand the question & agree with this proposal.

Lee

>
> so its:
>
> (join
> (filter expr
> (extend (2*?v AS ?two)
> (bgp (triple ?x ns:p ?v))
> ))
> (bgp (triple ?x ns:q ?w))
> )
>
> or
>
> { { ?x ns:p ?v . BIND(2*?v AS ?two) . FILTER(expr)} ?x ns:q ?w }
>
> Logically it's
> BGP - BIND - FILTER
>
> Doing this keep entailment a matter of BGPs only. Without motion of
> BIND, it would need more work to come up with a design (if it exists)
> that keeps entailment on a conjuction of triple patterns and have the
> variable scoping + new variable rule.
>
> Lee's example:
>
> { ?x ns:p ?v . BIND(2*?v AS ?two) ?x ns:q ?two }
>
> will work and naturally falls out if BIND happens where it's mentioned.
>
> (join
> (extend (2*?v AS ?two)
> (bgp (triple ?x ns:p ?v)))
> (bgp (triple ?x ns:q ?two))
> )
>
> Andy
>
>
Received on Tuesday, 12 October 2010 19:49:21 GMT

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