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Re: link/@rel=profile, was: HTML5+RDFa first Editors Draft published

From: Steven Pemberton <Steven.Pemberton@cwi.nl>
Date: Thu, 16 Jul 2009 18:02:51 +0200
To: "Mark Birbeck" <mark.birbeck@webbackplane.com>, "Julian Reschke" <julian.reschke@gmx.de>
Cc: "Manu Sporny" <msporny@digitalbazaar.com>, "RDFa Developers" <public-rdf-in-xhtml-tf@w3.org>, "HTMLWG WG" <public-html@w3.org>
Message-ID: <op.uw5361rbsmjzpq@steven-750g>
Thanks for describing this so carefully Mark. However, I have to say I  
disagree.

If a document has a statement

	<link rel="profile" href="http://example.com/profile"/>

and a RDFa processor uses this information in the processing, it doesn't  
mean that the resulting graph can't or shouldn't contain

	<> <http://www.w3.org/1999/xhtml#profile> <http://example.com/profile>

By the time you have the graph, you have done the RDFa processing. The  
statement "This document uses a profile of http://example.com/profile" is  
true, whether or not the profile has been used in the generation of the  
graph. There is no contradiction, Gödelian or other.

Best wishes,

Steven

On Tue, 14 Jul 2009 11:06:59 +0200, Mark Birbeck  
<mark.birbeck@webbackplane.com> wrote:


> Hi Julian,
>
>> I have one question with respect to
>> <http://dev.w3.org/html5/rdfa/rdfa-module.html#document-conformance>:
>>
>> "There has also been strong support from the RDFa Task Force that the
>> profile attribute should be retained in HTML5, as it provides an
>> "out-of-band" mechanism for signaling that the document contains RDFa.  
>> The
>> profile attribute may also be used extensively to provide [RDFa  
>> Profiles]
>> support. Adding profile to the list of rel values and using it to signal
>> that the document contains RDFa places document processing instructions  
>> into
>> the RDF graph, which is problematic."
>>
>> I'm with you in that I'd like to see head/@profile be carried over from
>> HTML4, but I have trouble understanding the last sentence:
>>
>> "Adding profile to the list of rel values and using it to signal that  
>> the
>> document contains RDFa places document processing instructions into the  
>> RDF
>> graph, which is problematic."
>>
>> How is that different from other link relations, such as "stylesheet",
>> "nofollow", whatnot?
>
> I'm struggling to think of a good metaphor here...many apologies.
>
> The core issue is that if you need some information to guide the
> processing of a graph, then that information shouldn't be in the graph
> itself, but should be in some kind of 'meta' place.
>
> So say we wanted to create the following triple (a graph is a
> collection of triples, so this is a graph as well):
>
>   <#me> <http://xmlns.com/foaf/0.1/name> "Julian" .
>
> This graph means that:
>
>   There is something with the name of '#me'...
>
>   ...and it has a property of 'http://xmlns.com/foaf/0.1/name'...
>
>   ...and the value of that property is 'Julian'.
>
> Now, for convenience, we might map the URI
> 'http://xmlns.com/foaf/0.1/' to a token of 'foaf', so that we can use
> it in places. By convention, a mapping like this would take the
> following form:
>
>   <#me> foaf:name "Julian" .
>
> But note that what this 'means' is still the same as the first triple:
>
>   <#me> <http://xmlns.com/foaf/0.1/name> "Julian" .
>
> It's merely a convenience that we have 'foaf' as a token, and doesn't
> change the underlying meaning.
>
> Anyway, the question is, where do we put the information that tells us
> about this mapping? Could we put it into the graph itself?:
>
>   <http://xmlns.com/foaf/0.1/> <http://example.com/mapsto> "foaf" .
>   <#me> foaf:name "Julian" .
>
> (Remember, a graph is a collection of triples, so this is now a graph
> with two triples.)
>
> The problem with putting information like this, that guides
> processing, into the graph, is that we create a lot of potential for
> confusion. We now have values in our graph that are supposed to be
> plucked out of the graph before the graph can be understood; a
> processor would have to scan this graph for values of 'mapsto' before
> it could understand the graph...making it inherently brittle.
>
> In other words, to process the graph, you have to first process the  
> graph!
>
> (I'm not a mathematician, but I wouldn't be surprised if Godel would
> have something to say about this on a theoretical level.)
>
> So, by convention, we take these mappings out of the graph:
>
>   @prefix foaf: <http://xmlns.com/foaf/0.1/>
>
>   <#me> foaf:name "Julian" .
>
> Now it's clear that the information about the mapping of 'foaf' is
> distinct from the graph itself; now a processor would know that it had
> to load these mappings first (anything beginning with @prefix), before
> using those mappings to help understand the graph.
>
> I realise this probably seems like we're in
> counting-angels-on-the-head-of-a-pin territory, but the key thing is
> that when you have a collection of data -- be it a relational
> database, a CSV file, or some RDF -- you invariably need something
> outside of that data that tells you how to process it. And if you mix
> this information in with the data, then you have the problem that in
> order to process the data, you first need to process it.
>
> I've used prefix mappings to make the case, but @profile also falls
> into this category.
>
> A value of @profile on <head> has always been defined in HTML as a way
> to instruct a processor about how to interpret tokens that are used in
> a document. The mechanism itself has never been defined before, but
> the fact remains that the attribute is there for that express purpose.
>
> In RDFa we'd like to use this attribute to be very precise about the
> meanings of tokens used in your graphs, and it's ideal, because it's
> 'out-of-band', in the sense that the information about processing is
> not in the resulting graph.
>
> A value of @rel="profile" on the other hand may _seem_ to amount to
> the same, but it would fall into the same category as our 'foaf'
> example above, in that it would require special pre-processing of the
> graph before you can know how to interpret the graph.
>
> Of course, you could say that an RDFa processor shouldn't put triple's
> that use @rel="profile" into the graph, but then you no longer have a
> generic parser. And you can also be sure that in a few weeks time,
> someone will be proposing another value to go into the list of 'values
> to do special processing with', and before you know it, we have a
> right mess on our hands.
>
> One last thing; the proposal is to use @profile to point to another
> document, which might in turn contain further RDFa, telling you how to
> map the tokens. That may seem at first sight to fall foul of the same
> issue I'm describing, but in fact it's ok -- the data loaded from
> subsequent graphs is still out-of-band to the 'primary' graph.
>
> So the key point is not 'don't use RDFa to describe how to interpret
> RDFa' -- the key message here is that 'instructions on how to process
> a graph cannot go into the graph itself'.
>
> Regards,
>
> Mark
>
Received on Thursday, 16 July 2009 16:03:54 GMT

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