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Re: RDF List and Collection questions

From: Ivan Herman <ivan@w3.org>
Date: Tue, 12 Oct 2010 09:48:08 +0200
Cc: Semantic Web <semantic-web@w3.org>, Antoine Zimmermann <antoine.zimmermann@deri.org>
Message-Id: <B6FD4326-53B0-468D-A032-BF70AC74508F@w3.org>
To: Nathan Rixham <nathan@webr3.org>
Just some more minor comments

On Oct 12, 2010, at 08:11 , Antoine Zimmermann wrote:


>> rdf:Alt, does anybody use this? (I'm struggling to see why it's defined
>> in rdf to be honest)

I think it falls under the consideration of 'it sounded like a good idea at the time':-)

> I don't know but the only times I saw it used was in the RDF specs and RDF tutorials. I'm sure there are a few marginal data publishers that use it.

Well... If you annotate a photograph with Photoshop, then the metadata is stored in RDF using Adobe's XMP toolkit. The RDF/XML is embedded into the JPEG or Photoshop file as a text and can be retrieved by various tools. That XMP stores data using, among others, both Seq and Alt.

I am _not_ saying that the way Adobe is using Alt (or Seq) in this data is really good or necessary; my first impression is that it is neither of the two. But the data exist i.e., in fact, there is therefore a huge number of RDF data out there in the wild using these...

>> Do we actually need unordered and ordered lists in rdf? (something
>> niggles that we only need say List and consideration of whether to treat
>> it like an ordered or unordered list happens when processing, based on
>> context)
> I a list is unordered, how can one know what is the first element? It seems that what you suggest is rather "make everything ordered then ignore the order if you don't need it". In principle, this would be possible using rdf:Seq because you can order the element explicitly (using rdf:_1, rdf:_2, etc) or ignore the order with the generic rdfs:member. However, due to the Open World Assumption, you can never be sure that all elements of a rdf:Seq, e.g.,
> ex:s a rdf:Seq ;
>     rdf:_3 ex:firstElement ;
>     rdf:_17 ex:seventeenthElement ;
>     rdf:_42 ex:fourtysecondElement .
> is a valid sequence in RDF and it does not say that 42 is the last element. With rdf:List, assuming it's used "normally" (no fork, no loop, etc) you can close the list with rdf:nil.

There is also a historical issue here. The first, 1999 version of RDF had only containers. The need for collection a.k.a. list came around for the 2004 version and was, afaik, strongly motivated by the needs of OWL's RDF encoding, too. But the group did not want to remove containers.

>> Are any of Bag, Seq, Collection, Alt likely to be deprecated in the
>> future? are they used anywhere important (I think I saw mention of OWL
>> using them?)
> rdf:List won't be deprecated for sure. They are used a lot. I don't know about the others but there are prominent Sem Web figures that are supporting deprecation.

There are indeed discussions around that, although there is no final decision yet.



Ivan Herman, W3C Semantic Web Activity Lead
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Received on Tuesday, 12 October 2010 07:48:14 UTC

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