My simple-minded virtual tarot dealer now understands the Celtic cross spread.
This was tricky because it requires that one of the cards be
laid crossways, so I had to change the format of the data
which specifies the layouts. Formerly I used JavaScript
arrays to hold the positions of the cards; now
I have
switched to embedding an XML document describing the layouts in
the defs section of the SVG file.
The various flavours of RSS offer a variety of namespace requirements:
| URL | RSS version |
|---|---|
http://my.netscape.com/rdf/simple/0.9/ |
0.9 |
| (default) | 0.91, 0.92, 0.94 |
http://purl.org/rss/1.0/
|
1.0 |
http://backend.userland.com/rss2 |
2.0 |
In my opinion, it is a grave mistake to include a version number
in a namespace URI. The function of a namespace is to prevent
accidental collisions between names defined by different people
(or organizations) when two XML vocabularies are combined in one
document. The version number of the format can be specified
separately (as indeed all the RSS versions do, as an attribute
of their root element). If the 0.9 spec had only used
http://netscape.com/1999/rss as its namespace
(following the lead of
http://www.w3.org/1999/xhtml) then all
the versions could have used the same namespace.
Why do I care, you ask? Because if you use actual XML tools like XSLT to manipulate RSS feeds, then the fact that there are three or four namespaces in use for essentially the same elements makes the whole thing more complicated. Where I might have had
<xsl:template match="/rss:rss/rss:channel">...
I better be prepared for more complicated expressions like
<xsl:template match="/*/*[lname()=channel]">...
(to allow for any root element, and to ignore the namespace of
the channel element). This clutters the XSLT file
and makes it harder to maintain—and probably also less
efficient. Sigh.
This is not unique to RSS, by the way—I had all sorts
of hassle with early ve4sions of SVG tools which were caught out
by the ever-changing SVG namespace URL. They finally settled on
the quite-sane http://www.w3.org/2000/svg.
The TAG rejects HLink, not many dead scream the headlines. Is this the end for XHTML 2?
What’s XHTML 2? It is the next step in the bridge from HTML 4.01 to the futuristic world of ‘pure’ XML documents. XLink is a recommendation from the W3C for how XML documents should express links to other resources. HLink is a new proposal from the XHTML committee for how XML documents should express links to other resources. In effect, they are saying that XLink is inadequate and they need to replace it. TAG have expressed an opinion that XLink should be used instead, presumably on the grounds that we don’t want to have two W3C recommendations for one and the same thing.
Can XLink replace the special-purpose linking attributes in
HTML? I suppose we can imagine replacing img
and object
tags with something like
<object
xlink:type="simple"
xlink:show="embed"
xlink:actuate="onLoad"
xlink:href="myLogo.gif"
width="400" height="300">
My Logo
</object>
In principal the first three attributes (which would be the same
for all images) can be given default values in the DTD. This is
the approach used in SVG
and MathML.
The problem is that it
prevents the XML document in question from being stand-alone.
That is, the DTD must be downloaded and parsed before the
document can be rendered. SVG fudges this; SVG documents often do
not have DOCTYPEs, and SVG viewers
in effect use a DTD compiled in to the software. All very messy.
There’s another problem: the HTML tag img
also allows for URIs for the low-res version
(lowsrc attribute), a long description
(longdesc), and an client-side image-map
(usemap). XHTML 2 also wants to add an
href attribute to all elements (so any element in
the document can be a link). I’m not sure that XLink
defines xlink:show values for all of these. Even
if it does, we cannot have more than one simple XLink link per
element (since we can only have one xlink:href
attribute). We could possibly follow the same system as XTM, with one child element per link:
<object style="width:400px; height:300px;"> <source xlink:href="myLogo.gif"/> <usemap xlink:href="#logoMap"/> [My Logo] </object>
(Here we are assuming that the DTD is used to generate default values for the other Xlink attributes—but do we really want to rely on all XML browsers being validating browsers...?)
The question is, will this work? Yes, if we are using a
specialized XHTML-2-savvy browser (one which understands
object and source, and knows how to
interpret them). If the aim is to make XHTML 2
implementable using only XML + CSS 2 + XLink + XBase, without
making XHTML a special case, then the answer is no.
The upshot of this is that, if the
W3C want to make
XHTML 2 just another XML format, displayable in a generic
XML browser, then it looks like XLink is not quite right for the
job. It may well be that I am missing something, and the
above example can be reworked to work with XLink. It might be
that lowsrc and longdesc are dumb
features that no-one wants to carry over in to XHTML 2
anyway. But my naïve understanding of XLink and the nascent
XHTML 2 suggests that the XHTML working group might have a
point.
How is this going to end? Right now it looks to an outsider
like the question is being discussed less in terms of technical
issues like what XHTML’s goals are, and more in terms of
committees and procedure and politics and such-like. We may end
up with an XHTML 2 that requires a specialized XHTML 2
browser (requiring upgrades to existing browsers that recognize
the XML namespaces or the DOCTYPE, or any of the
other stupid heuristics in use today to distinguish different
flavours of HTML). The dawn of XML as a fully-fledged hypertext
mark-up language will delayed by a few more years...
I really should not be writing about this—I have plenty of other things to work on. I just find it difficult to tune out all these arguments about XHTML when that’s what I work with every day...
Found some time to continue work on the Picky Picky Game. I have something which, given a graphics file, writes it in to the correct place in the directory structure. Tonight’s task was a routine for generating the index page, based on the pictures stored so far. In the eventual web application, this routine will be invoked in CGI scripts whenever a new picture is added or vote recorded. For the present I can just run the Python script (one of the ways in which creating web apps in Python is less hassle than, say, ASP .Net or webclasses).
The index page format is mainly controlled through a
‘skin’ file index.skin. This has most
of the HTML, with special XML tags for interpolating the dynamic
content. This way hopefully Jeremy will be able to hack the
HTML without touching any of the application code. (The
immediate inspiration for the term skin comes from the
Helma Object Publisher system,
which does something similar, but using JavaScript.)
The picture metadata is written in XML which is straightforward
enough except that Python’s native
SAX support is broken:
it does not support XML namespaces! I have fixed this
with my own SAX filter
dubbed SaxLifter: it processes
startElement events by scanning the attributes for
namespace prefixes, maintaining a stack of namespace mappings,
and generating startElementNS events. Presumably
if I were using the XML-SIG or 4Thought enhancements to
Python things would work better. Sigh.
The overall strategy is to generate as much static HTML as possible—that is, instead of creating the HTML for the list of pictures afresh each time someone visits the site (which is what PHP and ASP, etc., do), I intend to generate it only when a new picture is added to the list. Since adding pictures will happen much more rarely than viewing the list, this reduces the overall load on the web server. The aim is to use CGI only in the pages that make a change (adding a picture or voting).
I’ve been amusing myself by concocting an RSS reader using XSLT to do the processing. XSLT can even handle the downloading of the RSS files, but this does not allow for caching or aggregating—so I thought I would knock something together in Python.
I will describe here the solution to a problem that taxed me at work this week, in the faint hope that it will prove useful to someone else who needs to do the same strange thing. Read more
Good news, everyone! Mozilla now has a working SVG implementation thanks to Alex Fritze of croczilla.org. It even has suppport for plugging in platform-specific back-ends so that in principle the Mac OS X version might be able to exploit Quartz Extreme. Cool! This could mean a working SVG-enabled release of Mozilla sometime before I die of old age. Read more
I'm awake at 05:00 unable to sleep and instead working on migrating the working copy of my web site to my laptop so that I can eventually retire my old desktop. One of the importunate thoughts bouncing about in my sleep-deprived brain is something someone at work said about some weirdos he'd heard of who actually (ab)use XML as some sort of (standard) generalized mark-up language. This bizarre (to him) concept involves one choosing a set of XML tags to express the structure of the text (as if text could have structure!), which is ludicrous because how would anyone be able to read it? One would have to use XSLT to transform it in to HTML, so why not use HTML in the first place, eh? Read more
This is a continuation of my pointless musing about a hypothetical alternative to XML called MU. Read more
Apologies to people reading this via LiveJournal's syndicated feed; a combination of my software converting every header in to an RSS item and LiveJournal duplicating each item every time I edited the title has created a flurry of links to essays that I expect no-one but me has any interest in anyway. Read more
There has been all sorts of trouble with web developers being unable to
cause their web servers to issue the correct Content-type headers.
Most recent fallout was Mark Pilgrim's essay on XML.com.
Read more
I have been outlining a hypothetical alternative to XML that I am calling MU. In this note I compare MU to some other mark-up notations. Read more
A lot of people use XML to serialize data structures; with the XML parsers bundled with many programming environments it is easier than writing one's own parser. But XML was not designed with this in mind and contains too many traps causzed by the mismatch between the XML object model and that of your application. A text format designed expressly for for the purpose (my favourite is YAML) would be more convenient and safer. Read more
I've started a gradual redesgin of my personal webspace. Anyone who actually visits the page will have noticed I added a background pattern taken from Squidfingers.com. I am in the process of revamping the links to other stuff I do on-line. Read more
My website is maintained by a rather complex amalgamation of software,
accreted over generations. Having migrated it from my old desktop
lickity to my new(ish) PowerBook Ariel, I now want to migrate it
again to my new server Tranq (a Tranquil PC T2); this will allow me to use cron to keep
some parts up-to-date automatically.
Read more
CAP is the OASIS Common Alerting Protocol, which is a specification of an XNL format for disseminating warnings of hurricanes, earthquakes, and suchlike. The CAP v1.1 format is mandated by the European R&D project I am working on. This is an inconvenience, because CAP is badly flawed XML standard. I am going to discuss here some of the problems I have had with message identity as defined by CAP. Read more