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Re: first and rdf:rest as functional property

From: Pat Hayes <phayes@ihmc.us>
Date: Mon, 23 Mar 2009 10:05:15 -0500
Cc: Jeremy Carroll <jeremy@topquadrant.com>, 'Michael Schneider' <schneid@fzi.de>, 'Bijan Parsia' <bparsia@cs.man.ac.uk>, Story Henry <henry.story@bblfish.net>, 'Semantic Web' <semantic-web@w3.org>
Message-Id: <3B8C5EEC-95FD-4FAA-9163-EDE68A9F8CAB@ihmc.us>
To: Reto Bachmann-Gmür <reto.bachmann@trialox.org>

On Mar 23, 2009, at 6:43 AM, Reto Bachmann-Gmür wrote:

> Thank you all for your contributions to these thread. I'll try to
> summarize how I understood things with regards to the original  
> questions:
>> My questions:
>> - Are there useful usages where an rdf:list has several distinct
>> rdf:first and rdf:rest value?
> RDF Semantics speaks about "well-formed" list to refer to lists for
> which the above (and possibly other) conditions are true. RDF however
> does not impose "well-formedness" so it would be possible write
> 'surprising', 'elegant', 'creative', 'unconventional', 'imaginative'  
> or
> 'beautiful' lists that do not match the condition. However it  
> explicitly
> mentions that extensions may exclude lists that violate the convention
> so that creating beautiful non-regular lists is an interoperability  
> bug.
>> - Is it just not written that rdf:first and rdf:rest are functional
>> (maybe due to some spec layering reasons) or is false to consider
>> rdf:first and rdf:next as functional?
> There's currently no specification asserting that they are, but as RDF
> semantics explicitly allows well-formedness limitations in semantic
> extension there statements are not false (as this would contradict   
> the
> semantic extensions).
> To me this situation this situation is not really satisfactory.   
> Asserting:
> |
> _:666 rdf:first <ex:aaa> .
> _:666 rdf:first <ex:bbb> .
> _:666 rdf:rest rdf:nil .
> <ex:aaa> ||owl:differentFrom <ex:bbb>
> |would be an interoperability bug as it would be a contradiction with
> possible semantic extensions

It is impossible to guarantee compatibility with all semantic  
extensions, as they will be inconsistent with one another. Different  
versions of OWL are already inconsistent with one another. Indeed,  
this is one reason why the RDF specs take a minimalist approach to  
specifying anything at all controversial (such as axioms for lists.)

> while on the other hand we do not currently
> have spec legitimation to draw the following conclusion:
> |_:666 rdf:first <ex:aaa> .
> _:666 rdf:first <ex:bbb> .
> _:666 rdf:rest rdf:nil .
> =>
> <ex:aaa> ||owl:sameAs <ex:bbb>|

But there is nothing to stop you declaring that you will make this  
inference, and making it. You should not seek to find spec  
legitimation: no specification can determine all your ontological  
decisions. Bear in mind that we are here talking about an RDF  
_ontology_ of "lists", not about list data structures themselves.

Pat Hayes

> Even if asserting functionality of rdf:first and rdf:rest doens't rule
> out all usage of the properties that result in non standard lists, it
> would in my opinion be a benefit for the interoperability of different
> system if these statements would be asserted by semantic web  
> standards.
> Any chance that a semantic extension as foreseen by RDF Semantics
> becomes part of the semantic web specifications?
> Cheers,
> Reto

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Received on Monday, 23 March 2009 15:07:01 UTC

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