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Re: Ambiguity of "RDF namespace" - was: Re: Contradicting definitions of "property"

From: Richard Cyganiak <richard@cyganiak.de>
Date: Wed, 9 May 2012 00:45:47 +0100
Cc: RDF Working Group WG <public-rdf-wg@w3.org>
Message-Id: <B357009D-A6BC-42E2-802F-46308E726A8B@cyganiak.de>
To: Thomas Baker <tom@tombaker.org>
On 4 May 2012, at 22:04, Thomas Baker wrote:
>> RDF vocabulary: IRIs with clearly established referents intended for use in RDF graphs.
>> 
>> Namespace: IRIs that syntactically start with the same sub-IRI.
> 
> I'll buy that.  Hmm, not "set of IRIs"?

Pedant! ;-)

>>> If this really is purely a matter of IRI syntax, slipping into saying that
>>> there is a conceptual resource called a "namespace" denoted by the
>>> "namespace URI" seems to muddy the waters.
>> 
>> Right, that's why I didn't say it. Namespace IRIs are just the shared
>> substring. Says nothing about what they denote.
> 
> I'm with you re: namespace IRIs being just the shared substring.  However, you
> do refer to "a namespace" when you say: "In the RDF context, any set of IRIs
> that start with the same substring are a namespace" (above).  The distinction
> between "collection of IRIs" (RDF vocabulary) and "set of IRIs" (namespace)
> somehow needs to be brought out more clearly.

I used “collection” and “set” synonymously. There's no meaningful distinction between those words here!

Does this make it clear?

>> RDF vocabulary: A set of IRIs with clearly established referents intended for use in RDF graphs.
>> 
>> Namespace: A set of IRIs that syntactically start with the same sub-IRI.

To pick an analogy with natural language: “Alice, Bob, Charlie” is a “vocabulary” (assuming the names have clearly established referents in your universe). “All sequences of letters starting with A” is a “namespace”. Most of the sequences in the namespace don't mean anything (they don't have referents), but that's not required for being a member of a namespace.

> Also, the current draft of RDF Concepts 1.1 [1] still says:
> 
>    "Vocabulary terms in the rdf: namespace are listed and described in
>    detail..."
> 
>    This suggests that the "rdf: namespace" holds not just any IRIs, but
>    "vocabulary terms" denoted by IRIs.  

That's not a valid inference. It says that *some* of the things in the rdf: namespace are vocabulary terms. It doesn't say that all of the things in the rdf: namespace are vocabulary terms.

>> The analogy with classes shows IMO that RDF Concepts is wrong and RDF
>> Semantics is right. The IRI <http://xmlns.com/foaf/0.1/Person> *is* not a
>> class, but it *denotes* a class.

That's correct, but has nothing to do with namespaces.

>    "The RDF namespace is also used as an XML namespace [XML-NAMES]..."
> 
>    ...where "RDF namespace" means (I think) "rdf: namespace IRI".

[[
Definition: An XML namespace is identified by a URI reference [RFC3986]; element and attribute names may be placed in an XML namespace using the mechanisms described in this specification.
]]
http://www.w3.org/TR/xml-names/#dt-namespace

So yeah, the sentence you quoted should read: “The rdf: namespace IRI is also used to identify an XML namespace [XML-NAMES] containing a number of additional element and attribute names…”

> I note in passing that SKOS Reference [2] says:
> 
>    "The SKOS vocabulary is a conceptual resource identified by the namespace
>    URI http://www.w3.org/2004/02/skos/core#..."
> 
> Would you disagree?

I wouldn't. That's consistent with everything above. The owners of the SKOS namespace IRI tell us that they decided that the IRI denotes the SKOS vocabulary.

>>> How about:
>>> 
>>>   It is very common for all IRIs in an RDF vocabulary to start with a common
>>>   substring, or base URI.
>> 
>> Base URI is an inappropriate technical term here. A base URI is a URI used to
>> resolve relative URIs to absolute URIs in documents. (?resolve? in the URI
>> syntax sense, not in the HTTP sense.)
> 
> Fine - "shared substring" seems to avoid the misleading association.

Yes.

>> If namespaces are sets of IRIs, then there's
>> nothing that makes a namespace ?RDF-specific?. Sets of IRIs are not specific
>> to a particular data model or representation format.
> 
> Fine - I can live with this, but only if it is clearly stated somewhere...

We'll have to carefully dig through a few more specs to make sure that none of them define “namespace” in a way that contradicts the “story” I've been trying to tell here.

>> How is this?
>> http://dvcs.w3.org/hg/rdf/raw-file/default/rdf-concepts/index.html#referents
> 
> It looks pretty good to me!  I'm not sure what makes a referent "clearly
> established", but maybe the vagueness is appropriate.  

Unfortunately, no one is quite sure about that, so vagueness is the only option.

> If a place is found to
> document the social use of the term "namespace", such as in a glossary, I'd
> perhaps take pity on the reader and at least point to it from here.

:-)

I'll see what I can do.

> It could provide a place to clarify other
> troublesome concepts in an informal, readable way, especially where there is a
> *ahem* "range of opinions" among reasonable experts.  For starters, how about
> "ontology"?  

Let me try.

[[
An OWL ontology is a formal description of a domain of interest, and can be used to describe the terms in an RDF vocabulary and their relationships. OWL ontologies can themselves be expressed as RDF graphs, using terms in the owl: namespace. OWL is more expressive, but also more complex, than RDF Schema. In fact, OWL contains RDF Schema as a simple subset.
]]

> Is it really a document I can print out and staple to the wall?

Well, the first step to unraveling this would be an excursion into the Work/Expression/Manifestation/Item distinctions of FRBR. I'd say, an ontology is a kind of Work. The ontology formalized in OWL 2 and written in RDF/XML syntax is an Expression or maybe a Manifestation of that Work. My printout is an Item of the Work.

Best,
Richard


[1] http://archive.ifla.org/VII/s13/frbr/frbr_current_toc.htm


> Then point to the Glossary from the RDF documents...
> 
> Tom
> 
> --
> Tom Baker <tom@tombaker.org>
> 
Received on Tuesday, 8 May 2012 23:46:18 GMT

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