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

From: Sandro Hawke <sandro@w3.org>
Date: Fri, 04 May 2012 21:06:24 -0400
To: Thomas Baker <tom@tombaker.org>
Cc: Richard Cyganiak <richard@cyganiak.de>, RDF Working Group WG <public-rdf-wg@w3.org>
Message-ID: <1336179984.27977.254.camel@waldron>
On Fri, 2012-05-04 at 17:04 -0400, Thomas Baker wrote:
> On Fri, May 04, 2012 at 07:45:12PM +0100, Richard Cyganiak wrote:
> > On 4 May 2012, at 15:05, Thomas Baker wrote:
> > >> [[
> > >> An RDF vocabulary is a collection of IRIs with clearly established referents intended for use in RDF graphs.
> > >> ]]
> > >>
> > >> Based on that, I'd say:
> > >>
> > >> In the RDF context, any set of IRIs that start with the same substring are a
> > >> namespace. The shared substring is the namespace's namespace IRI. This says
> > >> nothing about what these IRIs identify or what they dereference to; it's
> > >> purely a matter of IRI syntax.
> > >
> > > Okay, but I read this as strongly implying that a namespace is type of RDF
> > > vocabulary
> >
> > I would disagree.
> >
> > 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"?
> 
> > > Also, I wonder if this gives me license to speak of the
> > > "http://www.w3.org/1999/" namespace, "http://purl.org/dc/" namespce,
> >
> > Of course. People do this all the time ? ?anywhere in W3C's namespace? and so on.
> 
> Fine.
> 
> > > 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.
> 
> 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.  At any rate, that is how I read your
>     earlier point:
> 
>     > 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.
> 
> and
> 
>     "The RDF namespace is also used as an XML namespace [XML-NAMES]..."
> 
>     ...where "RDF namespace" means (I think) "rdf: namespace IRI".
> 
> 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?
> 
> [1] http://dvcs.w3.org/hg/rdf/raw-file/default/rdf-concepts/index.html#section-URIspaces
> [2] http://www.w3.org/TR/skos-reference/#namespace-documents
> 
> > >> It is very common for all IRIs in an RDF vocabulary to be in the same
> > >> namespace. In this case, it makes sense to speak of the vocabulary's
> > >> namespace IRI.
> > >
> > > 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.
> 
> > >    In this case, it makes sense to speak of the
> > >    vocabulary's namespace IRI.  An RDF vocabulary that consists of IRIs
> > >    starting with a common namespace IRI is, by convention, referred to
> > >    informally as a "namespace".
> >
> > Not an appropriate use of the word ?namespace?, IMO, see above.
> 
> I think we are agreeing that usage needs to be tightened.
> 
> > > But if a namespace is a type of RDF vocabulary, as implied above,
> >
> > In my mind it's not.
> >
> > > would one not perhaps want to refer to it as an RDF namespace?
> >
> > Doesn't make sense to me. 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...
> 
> > >> The wording used in RDF Schema ? ?class X is defined in the Y namespace? ?
> > >> is sloppy language, IMO. Classes are defined in documents, not in
> > >> namespaces.
> > >
> > > Okay - and rdfs:isDefinedBy points to documents?
> >
> > Formally it can point to anything. I think pointing it to a document
> > containing the description of the subject is good practice because it's
> > useful ? I can look up the definition.
> 
> I'd love to see this clarified right in the section on rdfs:isDefinedBy in [1],
> though I'd half-expect that a lack of consensus about whether an OWL ontology
> is a document (see below) might also apply here to the notion of an RDF
> vocabulary.
> 
> [1] https://dvcs.w3.org/hg/rdf/raw-file/default/rdf-schema/index.html#ch_isdefinedby
> 
> > >> You can argue about whether skos:Concept is defined in the SKOS specification
> > >> or in the SKOS namespace document, but saying that ?it is defined in the SKOS
> > >> namespace? doesn't really make sense. The IRI of the class X may be in the Y
> > >> namespace.
> > >>
> > >> What is identified by a namespace IRI? It is up to the IRI owner to tell us.
> > >
> > > I think it is very important to say this clearly somewhere.
> >
> > 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.  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.
> 
> > >> It is a useful convention to do so in the namespace document. In the case of
> > >> slash namespace IRIs, the namespace IRI coincides with the IRI of the
> > >> namespace document, so that's what it identifies. In a hash namespace IRI,
> > >> the IRI of the namespace document is the namespace IRI minus the trailing
> > >> hash, so the namespace IRI might potentially identify something other than
> > >> the namespace document. My preferred setup (which differs from early W3C
> > >> practice) is to have it identify *nothing*, and just talk about the hash-less
> > >> IRI of the namespace document.
> > >
> > > Fine.  Maybe the RDF specs could leave it at: "it is up to the IRI owner to
> > > tell us", and anything beyond that could be in primers and best practice
> > > notes.
> >
> > Well, the terms ?namespace?, ?namespace prefix? and ?namespace IRI? don't
> > really have proper definitions anywhere in the RDF documents, as far as I
> > know. That's because they are sort of just syntax, so there's little
> > justification for explaining them in the Concepts or Schema or Semantics doc.
> > So every syntax document (RDF/XML, Turtle, etc.) defines them again, and only
> > says the minimum needed for the specific syntax. Perhaps the Primer would be
> > a good place, but people don't tend to read it as a reference document. A
> > difficult situation!
> >
> > What's the first place where people would look for these terms, in your opinion?
> >
> > Do we need a ?Semantic Web Glossary? document?
> 
> If the term "namespace" is being used in normative specifications for RDF, I
> should think it would need also to be defined there, but I guess a separate
> Glossary would do the trick.  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"?  Is it really a document I can print out and staple to the wall?
> Then point to the Glossary from the RDF documents...

It seems to me that "RDF Concepts" is where I'd look for this kind of
thing, not some separate Glossary document.   Maybe Glossary appendix to
Concepts.     But it's a very slippery slope -- I know I'd be very
tempted to throw some Linked Data advice in there.   A namespace IRI
doesn't necessarily denote anything in particular, but the Linked Data
principles could be read as saying dereferencing the namespace IRI
SHOULD give you some useful information about the namespace and the
names in it.   I don't think I would read them that way, but it will
occur to some people and they could use some guidance.  I'm just not
sure we're the right people to give that guidance right now.

    -- Sandro
Received on Saturday, 5 May 2012 01:06:35 GMT

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