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Re: Weird value testing situations

From: Bijan Parsia <bparsia@cs.man.ac.uk>
Date: Mon, 14 Aug 2006 13:38:16 +0100
Message-Id: <EDD9C2CE-33D0-43E4-A03B-8C21D1B95CD0@cs.man.ac.uk>
Cc: RDF Data Access Working Group <public-rdf-dawg@w3.org>
To: Eric Prud'hommeaux <eric@w3.org>

On Aug 14, 2006, at 11:53 AM, Eric Prud'hommeaux wrote:
[snip]
> Huh, interesting problem. I would certainly expect the naive
> implementation (not supporting D-entailment) to conclude:
>   isLiteral(?x) => F
>   isBlank(?x) => T
> Having a D-entailment implementation say that isLiteral(?x) => T would
> violate the monotinicity rules we appear to have set down for  
> ourselves
> (wrongly or rightly). Thus, I think that isLiteral has to operate on
> the symbol in the graph, and not on what we may know that symbol to
> represent.

That puts a rather strong curtailment on D-entailment.

While these are, imho, corner cases, if someone ever *did* in earnest  
write down such RDF, I think they would expect different answers.

We could define a new sort of term, datatyped URI/BNode. Syntax could  
be like :x^^<type>. While, well, icky, it would differentiate at a  
syntax level in the results. (Even if you didn't report back, you  
could take results in the algebra as respecting those types). It  
would probably require some rethinking of the type testing functions.  
(e.g., isUri(:x^^<type>) should be true *and* isLiteral(:x^^<type>)  
should be true, though perhaps only under D-Entailment? Anyway, you  
get the notion.)

Also,

"""An ill-typed literal does not in itself constitute an  
inconsistency, but a graph which entails that an ill-typed literal  
has rdf:type rdfs:Literal, or that an ill-typed XML literal has  
rdf:type rdf:XMLLiteral, would be inconsistent."""

This means than any answer is sanctioned. How does SPARQL handle  
inconsistent graphs? Presumably it is an error, yes?

[snip]
> Is there a way the current wording could be improved?

*IF* we keep with the current semantics (I do believe this is new  
information, yes?), then I would expect some language explicitly   
confining the test functions to the lexical form...er...well that  
can't be right, yes? Because we don't in the case of typed literals.  
Anyway, it has to specify something along these lines that makes it  
clear what the behavior actually is. The functions are weird, yes?  
Some look at the syntax of the term, others to its meaning. That's  
going to be tricky in places.

> Do we need another paragraph on what not to expect from D-entailment?

It certainly needs clarification. But I think we need to decide this  
issue, as it seems to be a new one. I appreciate the value of  
monotonicity, but I worry that there are other traps lurking.  
Certainly, inconsistent graphs must also be dealt with.

Cheers,
Bijan.
Received on Monday, 14 August 2006 12:45:42 GMT

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