RE: firts and rdf:rest as functional property

>-----Original Message-----
>From: semantic-web-request@w3.org [mailto:semantic-web-request@w3.org]
>On Behalf Of Reto Bachmann-Gmür
>Sent: Wednesday, February 18, 2009 12:28 PM
>To: Semantic Web
>Subject: rdf:firts and rdf:rest as functional property
>
>>From the description of RDF collections in the primer I would consider
>rdf:first and rdf:rest as functional properties. However,
>
>http://www.w3.org/TR/rdf-schema/#ch_list says:
>>
>> Note: RDFS does not require that there be only one first element of a
>> list-like structure, or even that a list-like structure have a first
>> element.
>
>
>I understand that RDFS doesn't have the notion of functional properties,
>but it could nevertheless be specified specifically for rdf:first and
>rdf:rest.
>
>Especially since RDFS also states that:
>
>> A triple of the form:
>>
>> L rdf:rest rdf:nil
>>
>> states that L is an instance of rdf:List that has one item; that item
>> can be indicated using the rdf:first property
>
>I'm not sure if this means that the last element has exactly or at least
>one item. In any case to me this seems to be a rather weird
>special-casing of the last rdf:List in a structure.
>
>My questions:
>- Are there useful usages where an rdf:list has several distinct
>rdf:first and rdf:rest value?
>- 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?

Let's assume you have:

  _:x rdf:first <u>
  _:x rdf:first <w>

What do you expect to receive from functionality here? There is no
owl:sameAs in RDF(S). 

It would need OWL for "completion" of the list semantics. But in OWL DL, RDF
lists are only used as part of the syntax to represent argument lists; you
cannot define axioms about lists. And OWL Full, the flavor of OWL that
actually extends the RDFS semantics, would probably not add semantics for a
feature that has no semantics in OWL DL. So far for the "layering".

The RDFS semantics for lists could perhaps be arranged to produce results in
the special case that both <w> and <u> are /classes/, by inferring:
 
  <u> rdfs:subClassOf <w>
  <w> rdfs:subClassOf <u>

Because this is what you would get in OWL Full for two classes that happen
to be equal. Analog for properties.
 
But would these restricted usage scenarios be worth the additional
complexity of RDFS reasoners?

But why does anybody want semantic constraints for lists at all? One
certainly does not want to reason about duplicate rdf:first occurrences,
right? What one probably rather wants is to make sure that lists are
structurally ok. And, I guess, for all realistic scenarios, a syntax checker
will simply do the job.
 
>Cheers,
>reto

Cheers,
Michael

--
Dipl.-Inform. Michael Schneider
Research Scientist, Dept. Information Process Engineering (IPE)
Tel  : +49-721-9654-726
Fax  : +49-721-9654-727
Email: schneid@fzi.de
WWW  : http://www.fzi.de/ipe/eng/mitarbeiter.php?id=555

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