- From: Mark Birbeck <mark.birbeck@webbackplane.com>
- Date: Mon, 26 Jan 2009 10:54:08 +0000
- To: Dan Brickley <danbri@danbri.org>
- Cc: Ben Adida <ben@adida.net>, Manu Sporny <msporny@digitalbazaar.com>, RDFa mailing list <public-rdf-in-xhtml-tf@w3.org>, Ian Hickson <ian@hixie.ch>, Henri Sivonen <hsivonen@iki.fi>, Sam Ruby <rubys@intertwingly.net>
Hi Dan, > Earlier you were sounding like > everything is finished and frozen, but as I understand it the @prefix design > is still being worked out. I don't think Ben was saying that @prefix was frozen. The earlier parts of his comments were in relation to the way that you seem to be thinking that CURIEs can simply vanish. I think he is quite rightly saying that this is a non-starter, for a number of reasons. But leaving those reasons to one side for a moment, I'd like to put the URI issue into some context for the discussion, since you seem to be thinking that the issue has only just come up, or perhaps that the alternatives were never considered properly. There were long discussions about how to express URIs in RDFa, and whilst QNames was the most obvious candidate for adoption, we didn't go that route without a struggle to find something better. (And in fact even when we went the nominally QNames route, we devised something more appropriate in the form of CURIEs.) The key issue was always how to provide extensibility for new documents, whilst preserving backwards-compatibility for old ones. If you have a document that has @rel="next" you need to preserve the fact that this is a well-know HTML relationship. Similarly, if you have @rel="foo" you don't want to simply assume that it's a relationship in the HTML namespace. But at the same time, you also need to allow authors the possibility of adding their own terms. So, with these requirements, there are many ways you could go about this. The QNames (and then CURIEs) approach is to say that "next" is using a default prefix that is bound to the XHTML namespace. I.e., "next" is short for "xhv:next". The problem though is that this makes "foo" an abbreviation for "xhv:foo". So you'll notice that CURIEs allows for the possibility of processing well-known tokens first, before cracking open the "a:b" syntax. In that model, "next" works, "a:b" works, and "foo" is simply ignored. The other model that was considered was that @rel contains a URI. In this case @rel="next" is actually a relative path, and we would hard-code the fact that it's relative to a 'base' that is the XHTML vocabulary. Unfortunately that puts @rel="foo" also into the XHTML vocabulary, and it also means that for authors to add their own values they need to express them as full URIs, which is quite laborious and error-prone. Yet a further model that was considered was a 'merge' of both approaches, and to use the safe CURIE syntax in @rel; this would give us both @rel="[dc:title]" and @rel="http://purl.org/dc/terms/title". However, it would then mean that 'next' has to be expressed as @rel="[next]", and that @rel="foo" would be a term that was local to the current document, since it's a relative path (as of course would be @rel="next"). I actually favoured this last approach, but lost the argument. :) On balance, I'm glad that I did lose, because the legacy markup issue is a big one; my approach essentially said that all uses of legacy @rel values were *local* to the document, and authors would have had to explicitly update their documents to use safe CURIEs if they wanted to use XHTML vocabulary terms in their documents. None of this is to say that we can't continue to improve the ease with which RDFa can be marked up, and the discussions around @prefix and @profile demonstrate that this is now a priority for us. But the way that discussion is moving forward, is by looking at how we can build on RDFa; we're certainly not throwing into question some of the fundamental underlying architecture. Regards, Mark -- Mark Birbeck, webBackplane mark.birbeck@webBackplane.com http://webBackplane.com/mark-birbeck webBackplane is a trading name of Backplane Ltd. (company number 05972288, registered office: 2nd Floor, 69/85 Tabernacle Street, London, EC2A 4RR)
Received on Monday, 26 January 2009 10:54:44 UTC