On Mar 18, 2009, at 09:35 , Andrew Welch wrote:
Are you sure changing the namespace is preferable to a version
attribute? Seems very drastic, and usually it's best to avoid doing
it as it makes all tools that process existing markup redundant.
I didn't say that changing the namespace should be the versioning
mechanism, I said "If at some point we make breaking changes, then we
just change the namespace." That's very different!
The idea is that in version 1.1, 1.2, perhaps even 2.0, 3.0 we just
add elements and attributes, and when they are not understood they are
simply skipped. We only ever get to change the namespace if and only
if at some point, in a future version, we realise that we have got
everything wrong (perhaps because the usage of widgets has changed
radically) and need to start from scratch. I don't see that happening
within a reasonable time frame, and if it does we'll have a bunch of
other problems to solve anyway.
Also, you mention most users "just copy-paste the root element" -
surely you'd still have the problem if you change the namespace?
Yes, but presumably if we were brought to be in a situation where we
would need to change the namespace, we would be making a whole lot of
other changes that would make copy-pasting impossible anyway. In other
words, we'd be defining a new language.
You describe further applications of a version attribute, notably the
ability to give a "proper error" (you have a v1 processor, this is a
v2 document) and the ability for an implementation to switch code paths.
For one, errors that the end-user can't do much about should be
proscribed as much as possible. If your phone tells you you need a v2
user agent, there's not much that the average user can do.
You also don't address the case of a user stating that a document is
v1 but using v2 features. That's going to happen a lot, not just
because of copy-pasting but simply because people don't know which
version added what feature. Do you know which version of HTML added
sup, blockquote, listing, tab, font, math, em, or the script attribute
on form?
At the moment, I don't think there is any establised "right way" when
versioning xml, but changing the namespace is a bad idea.
There's no fully established right way, but there's a body of work and
experience to guide us. See:
http://www.w3.org/2001/tag/issues.html#XMLVersioning-41
for some stuff, or more precisely (if you have the time):
http://lists.w3.org/Archives/Public/www-tag/2004Nov/att-0071/versioning-part1
http://lists.w3.org/Archives/Public/www-tag/2004Nov/att-0071/versioning-part2
Seeing as these are fairly long write-ups, you might wish to focus on
part 1 (which concerns us) and more specifically on section 7. The
approach I'm proposing (and which is also that of the draft) is based
on experience with languages that are to be used by web developers and
deployed to a large and varied set of distributed clients over which
there is very little control (i.e. HTML and SVG, mostly). It can be
summed up as:
- ignore what you don't know
- in a new version, add new features in such a way that ignoring
them doesn't break anything
- if and only if you need to make a new version that can't be made
compatible, then change the namespace
--
Robin Berjon - http://berjon.com/
Feel like hiring me? Go to http://robineko.com/
