Re: RDF document?

----- Original Message ----- 
From: "Dan Brickley" <danbri@w3.org>
To: "Jeremy Wong" <jeremy@miko.hk>
Cc: "Frank Manola" <fmanola@acm.org>; "Chris Purcell" <cjp39@cam.ac.uk>; 
<semantic-web@w3.org>
Sent: Thursday, June 09, 2005 8:54 PM
Subject: Re: RDF document?


>
> On Thu, 2005-06-09 at 09:30 +0800, Jeremy Wong wrote:
>> Thanks for the correction of the term "document". It is an important 
>> concept
>> of RDF.
>>
>> Consider the property foaf:workplaceHomepage [1], the domain of this
>> property is the class foaf:Document. Let's see the example..
>>
>> <foaf:Person>
>>  <foaf:name>Dan Brickley</foaf:name>
>>  <foaf:workplaceHomepage rdf:resource="http://www.w3.org/"/>
>> </foaf:Person>
>>
>> I always think that this example is misleading. I would prefer to put the
>> above example in the following way..
>>
>> <foaf:Person>
>> <foaf:name>Dan Brickley</foaf:name>
>> <foaf:workplaceHomepage
>> rdf:datatype="http://www.w3.org/2001/XMLSchema#anyURI">http://www.w3..org/</foaf:workplaceHomepage>
>> </foaf:Person>
>
> Thanks for the feedback. I don't see it as misleading at all:
> foaf:workplaceHomepage is a relationship between a foaf:Person and
> a foaf:Document that is a homepage of a workplace of that person.
> This is explained in prose and in RDFS in the FOAF
> specification. I agree that you could, instead, have defined a similar
> relationship that holds between a person and a datatype-represented
> URI that identifies such a homepage. These are different modeling
> styles, each with their own benefits. For FOAF, we focus on the
> document,
> and not its identifier, since it is the document that is of interest,
> and we
> can mention the document even if we don't know it's URI.

It is a design issue. I would say that Literal is a medium to communicate 
with the end-user. Obviously the property foaf:workplaceHomepage is intended 
to show the information, http://www.w3.org/ in the example, to the end-user. 
Therefore I prefer to use the typed literal, 
"http:///www.w3.org"^^xsd:anyURI. It is to present a literal to the 
end-user, rather than to present the URI of a resoruce to the end-user.

Both "http://www.w3.org/"^^xsd:anyURI and <http://www.w3.org/> map to some 
nodes, hence the typed literal can be manipulated as if it is a resource in 
the view of nodes. The problem is that the typed literal cannot be 
represented as the subject of a triple.

I don't know if it is useful to link http://www.w3.org/ as in the following 
example..

[] foaf:homepage <http://www.w3.org/> .
<http://www.w3.org/> foaf:sha1 
"1480931ce0e2f1e89f05cf763390fbc57254724a"^^xsd:hexBinary .

If I treat the node <http://www.w3.org/> as if it is a blank node, it would 
become..

[] foaf:homepage [ foaf:sha1 
"1480931ce0e2f1e89f05cf763390fbc57254724a"^^xsd:hexBinary . ] .

Someone has a homepage that has a SHA1 sum 
"1480931ce0e2f1e89f05cf763390fbc57254724a".

>
> There is a larger issue here, which is that of the relationship between
> RDF's built-in notion that resource nodes can have URIs, and its
> property-based
> mechanism for datatyping some literals as being URIs. I think there is
> a
> reasonable case that some future (hypothetical!) version of RDF could
> do
> things differently, eg. by representing URIs as properties. I expect
> that
> would create a lot of work at the model theory layer, and might not be
> worthwhile.
> But right now, RDF doesn't have a very natural idiom for describing more
> than
> one URI for the same resource. You either have to use OWL, and describe
> two resources as being 'sameAs', or you have to use properties such as
> dc:identifier, with URI values (as strings? as resource nodes?). The
> former is a little awkward, especially in RDF/XML notation. The latter
> lacks well-established
> conventions, eg. regarding use of datatyping, and could result in
> information
> being hidden from RDF-based data merging tools.
>
> To be clear, there really is nothing wrong with that FOAF example. It is
> compatible
> with the FOAF spec, the RDF semantics, RDFS and RDF/XML syntax.
> Using...
> <foaf:workplaceHomepage
>> rdf:datatype="http://www.w3.org/2001/XMLSchema#anyURI">http://www.w3..org/</foaf:workplaceHomepage>
> ...in the present day would hide information from RDF tools, since it
> would
> fail to merge with other information that attached properties to a
> resource node with URI http://www.w3.org/

In this sense, the RDF tools treat the nodes as some resources that can be 
dereferenced...

>
> Try it and see:
>
> <link rel="alternate" type="application/rss+xml" title="RSS"
> href="http://www.w3.org/2000/08/w3c-synd/home.rss" />
>
> http://www.w3.org//Overview-about.rdf
>
> Actually if you look at those 2 files, you'll see in the RSS feed that
> the W3C homepage URL is hidden as a string literal in an rss:link
> property.

RSS 1.0 or even RSS 1.1 is just a direct translation from RSS 0.9x... and 
non-RDF RSS people have already given up RSS 1.x versions.. they think that 
RDF/XML is too complex to understand.. they would prefer the newer version 
RSS 2.0 that is Really Simple Syndication.

Anyway, rss:link is better than foaf:homepage in my own view, though you 
have your design rationale with foaf:homepage.

>
> http://www.w3.org/2002/01/tr-automation/ has some more RDF about W3C,
> though I forget if it mentions the homepage URL anywhere.
>
> Dan
>
>
>
>
>
> Dan
>
>
>>
>> Regards,
>> Jeremy Wong 黃泓量
>>
>> [1] http://xmlns.com/foaf/0.1/#term_workplaceHomepage
>>
>> ----- Original Message ----- 
>> From: "Frank Manola" <fmanola@acm.org>
>> To: "Jeremy Wong 黃泓量" <jeremy@miko.hk>
>> Cc: "Chris Purcell" <cjp39@cam.ac.uk>; <semantic-web@w3.org>
>> Sent: Wednesday, June 08, 2005 1:41 AM
>> Subject: Re: URI ends with #
>>
>>
>> > Section 0.1 of RDF Semantics says:  "There are several aspects of
>> > meaning in RDF which are ignored by this semantics; in particular, it
>> > treats URI references as simple names, ignoring aspects of meaning
>> > encoded in particular URI forms [RFC 2396] and does not provide any
>> > analysis of time-varying data or of changes to URI references. It does
>> > not provide any analysis of indexical uses of URI references, for
>> > example to mean 'this document'."
>> >
>> > Section 1.2 goes on to say: "The semantics does not assume any
>> > particular relationship between the denotation of a URI reference and a
>> > document or Web resource which can be retrieved by using that URI
>> > reference in an HTTP transfer protocol, or any entity which is
>> > considered to be the source of such documents. Such a requirement could
>> > be added as a semantic extension, but the formal semantics described
>> > here makes no assumptions about any connection between the denotations
>> > of URI references and the uses of those URI references in other
>> > protocols."
>> >
>> > IOW, RDF doesn't associate any special meanings with various URI forms,
>> > including those with empty fragids.  They are simply names, to be
>> > compared as strings (as noted already). RDF, strictly speaking, doesn't
>> > really include the concept of a "document" per se;  the closest thing 
>> > is
>> > a "graph".  I think phrases like "RDF document" in various specs should
>> > be understood as meaning "some document that contains RDF statements"
>> > (such as an RDF/XML document;  since RDF/XML is XML, it's legitimate to
>> > talk about "documents" in this case, since XML formally defines what a
>> > "document" is).  The RDF Concepts material referred to here doesn't say
>> > that there *are* "RDF documents" corresponding to the URIrefs it's
>> > talking about;  it says you can reconcile the RDF and RFC 2396 uses of
>> > fragids by imagining that there such documents.
>> >
>> > --Frank
>> >
>> > Jeremy Wong 黃泓量 wrote:
>> >>
>> >> "Two RDF URI references are equal if and only if they compare as 
>> >> equal,
>> >> character by character, as Unicode strings" [1]
>> >>
>> >> It is actually a Simple String Comparison [2].
>> >>
>> >> Another point is that, the meaning of a fragment is scheme dependent.
>> >>
>> >> I don't know whether an empty fragid reference the entire document
>> >> either.
>> >>
>> >>
>> >>
>> >> Regards,
>> >> Jeremy Wong 黃泓量
>> >>
>> >>
>> >> -- 
>> >> [1]
>> >> http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/#dfn-URI-reference
>> >> [2] RFC3986, Section 6.2.1.  Simple String Comparison
>> >>
>> >>
>> >> ----- Original Message ----- From: "Chris Purcell" <cjp39@cam.ac.uk>
>> >> To: "Jeremy Wong 黃泓量" <jeremy@miko.hk>
>> >> Cc: <semantic-web@w3.org>
>> >> Sent: Tuesday, June 07, 2005 8:05 PM
>> >> Subject: Re: URI ends with #
>> >>
>> >>
>> >>> Does an empty fragid reference the entire document? That would mean
>> >>> <http://www.example.org/#> is the same URI as 
>> >>> <http://www.example.org/>.
>> >>>
>> >>> Chris
>> >>>
>> >>>> Thanks Arjohn,
>> >>>>
>> >>>> I am actually writing a serializer that put me into the question. I
>> >>>> am sure that I should throw an exception for this case and those
>> >>>> cases mentioned in the Serialising section.
>> >>>>
>> >>>>
>> >>>> Regards,
>> >>>> Jeremy Wong 黃泓量
>> >>>>
>> >>>>>> Consider the rdf:Property http://example.org/#
>> >>>>>>  in Notation 3, I can write a triple as
>> >>>>>> <http://example.org/A> <http://example.org/#> 
>> >>>>>> <http://example.org/B>
>> >>>>>> .
>> >>>>>>  in RDF/XML, how can I write the triple? Let me try...
>> >>>>>>  <Description xmlns:ex="http://example.org/#"
>> >>>>>> rdf:about="http://example.org/A">
>> >>>>>>   <ex: rdf:resource="http://example.org/B" />
>> >>>>>> </Description>
>> >>>>>>  However, "ex:" does not match the definition of Qualified Names
>> >>>>>> [1] and the character "#" is not an NCNameChar [2]. I don't have
>> >>>>>> any idea to express the predicate which its URI ends with #. Any
>> >>>>>> suggestion?
>> >>>>>
>> >>>>>
>> >>>>> This triple cannot be serialized in RDF/XML. See [1] for a short
>> >>>>> discussion on the serialization of RDF in XML. The only way to
>> >>>>> serialize
>> >>>>> such a triple is to use one of the other formats (N3, Turtle, ....).
>> >>>>>
>> >>>>> Arjohn
>> >>>>>
>> >>>>> [1] http://www.w3.org/TR/rdf-syntax-grammar/#section-Serialising
>> >>>
>> >>>
>> >>>
>> >>
>> >>
>> >>
>> >>
>> >
>>
>>
> -- 
> Dan Brickley <danbri@w3.org>
> W3C
>
>
>

Received on Friday, 10 June 2005 02:29:14 UTC