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Re: Ill-typed vs. inconsistent?

From: Richard Cyganiak <richard@cyganiak.de>
Date: Mon, 12 Nov 2012 15:43:22 +0000
Cc: RDF Working Group WG <public-rdf-wg@w3.org>
Message-Id: <D72877A6-1574-46CF-88E4-CF4075C09C4B@cyganiak.de>
To: Pat Hayes <phayes@ihmc.us>
On 12 Nov 2012, at 07:58, Pat Hayes wrote:
>> What's the relevance of the distinction between “graphs containing ill-typed literals” and “inconsistent graphs” in the Semantics?
> The relevance is that it is quite possible to say sensible (and therefore consistent) things about ill-typed literals, such as that they are ill-typed. 

Do you mean, say those things in RDF? How do you say in RDF that a literal is ill-typed? Can you give a less self-referential example for something sensible that could be said about an ill-typed literal?

>> The text stresses that the presence of an ill-typed literals does not constitute an inconsistency.
>> But why does the distinction matter?
> I am not sure what you mean by "the distinction" here. Why would you expect that an ill-typed literal would produce an inconsistency?

I don't understand why a separate class of errors is introduced *just* for ill-typed literals. I don't understand what benefit is gained by classifying ill-typed literals and inconsistencies differently in the specs. I don't understand how the distinction is actionable. How does anyone benefit from knowing that a given graph is non inconsistent but contains an ill-typed literal?

> Why would the presence of an ill-typed literal make a triple false?

Because it asserts that some entity has a relationship to some non-existing thing?

>> Is there any reason anybody needs to know about this distinction who isn't interested in the arcana of the model theory?
> I'm not sure what you consider to be "arcana".

"Arcana: Highly specialized knowledge that is mysterious to the average person."

That's quite an appropriate description of the distinction between “ill-typed” and “inconsistent”, I think.

> Someone who cannot follow the model theory probably shouldn't be using RDF.

That's crazy talk.

>> From the perspective of someone who authors RDF data, or works with RDF data, they both seem like belonging to the same class of problem, and I'm a bit at a loss as to how to explain the difference.
> To me they seem quite obviously different, so apparently I am not following your intuition here.

So let's look at these three RDF graphs:

Graph A:

   :a :b "1"^^xsd:integer.

Graph B:

   :a :b "xxx"^^xsd:integer.

Graph C:

   :a :b "xxx"^^xsd:integer. :b rdfs:range rdfs:Literal.

Those graphs fall into three classes, let's call them class A, class B, and class C.

To me it's pretty clear why we would distinguish class A from class B+C. That's because it's clear what graph A is supposed to mean (assuming we know what :a and :b are), and it's rather unclear what statement graphs B and C are trying to make.

The distinction between B and C, if seen as an attempt to communicate some knowledge about the domain of interest, is not clear to me. They're both nonsense, and clearly the result of an error on the author's part. It's rather impossible to interpret the author's intent. So I don't know why we distinguish two different classes of nonsense here.

> FWIW, one should *not* think of inconsistency as a kind of error condition. (Maybe the semantics text should spend a little time explaining this point.)

Perhaps. I think of inconsistency as a data quality problem. There are other kinds of data quality problems that have nothing to do with consistency, of course.


>> (I know how both terms are defined and what conditions exactly cause them; the question is about why the spec insists that ill-typed literals do not cause a graph to be inconsistent.)
> My question, in reply, would be to ask why anyone would think it would.
> Pat
>> Best,
>> Richard
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Received on Monday, 12 November 2012 15:46:10 UTC

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