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Re: Booleans as the degenerate case of variable binding results

From: Pat Hayes <phayes@ihmc.us>
Date: Tue, 22 Jun 2004 09:51:26 -0500
Message-Id: <p06001f02bcfdf746a13a@[10.0.100.20]>
To: Simon Raboczi <raboczi@tucanatech.com>
Cc: RDF Data Access Working Group <public-rdf-dawg@w3.org>

>On 18/06/2004, at 7:39, Kendall Clark wrote:
>
>>I can think of several use cases where what I care about isn't the
>>particular answers -- the actual string comprising someone's email
>>address -- but *that* some graph has "an email address".
>
>If we have anything resembling a SELECT clause that can be used to 
>project away variables, you get this effect by projecting away ALL 
>the variables.  Once the projection operation finishes removing 
>duplicates, you end up with either true or false depending whether 
>there were any solutions or not.
>
>>Or, for example, I don't care about the values of foaf:knows predicates, I
>>just want to know whether some FOAF resource contains more than 8 of
>>them.
>
>I think this is an entirely different kind of query.  Rather than 
>"Can this query be satisfied? I don't care about the particular 
>variable bindings required to satisfy it.", it's "Calculate a 
>particular variable binding ?x = (count > 8) whose value just 
>happens to be a boolean".  In this case you certainly will get an 
>explicit literal value back, presumably datatyped using XSD. 
>However, it's just a normal variable binding expression.

Oh dear. I think this is a very bad idea. If we allow variables 
binding to booleans, the logical framework suddenly gets wildly 
different. This would take the language well outside the 
description-logic subset, for example: in fact it takes it outside 
normal logic altogether.

I'd strongly suggest that we do not have variables ranging over 
boolean values, or if we do then we loudly ask for comments from 
other WGs about what the consequences would be.

Pat Hayes
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Received on Tuesday, 22 June 2004 11:10:06 GMT

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