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At 8:53 AM +0100 3/5/02, Eric van der Vlist wrote:
>Title says it all, the extensibility of XML is one of its myths...
>
Hardly. However, XML is not immune to the old OOP programmer's adage
that reuse doesn't come for free. You have to design for it and that
design costs. You won't get the cost back on the first project or the
second, but you might get it on the third.
>Technically, XML is based on trees which are not the most extensible
>structures (compared to tables or triples). If you extend a tree you
>are likely to break its structure (and existing applications). I
>would say that trees grow but are not "extended".
Trees most certainly can be extended without breaking existing
applications provided that the applications are designed to bend and
not break. I admit that many existing applications are fragile, but
that's the fault of the application designers, not XML. After all,
it's the application that breaks, not the XML document.
I've begun to push for applications to be designed around XPath and
XSLT rather than straight tree navigation because these languages are
so much better than most navigation based code at handling unexpected
changes to documents. You still need to design for reuse, and think
about how your code will react in the face of unpredictable
modifications, but it's so much easier to do in XPath/XSLT than in
pure Java/C++/Perl.
In XPath, the single biggest thing most programmers need to learn is
to treat node-sets as collections that contain 0 to N nodes and not
as single nodes. In XSLT, you need to make sure programmers don't
pick xsl:value-of where xsl:apply-templates should be used. Once
you've done that, your code will handle a lot of common extensions
and mistakes without falling apart. Of course, you can still change a
document so radically that the code will break, but this does make
your application bend a lot further before it reaches its breaking
point.
>XML is now legacy. Its users community is screaming against any
>change and its specification body seems paralysed by its structure
>and the diverging interests of its members...
>
I would say rather, that we don't want to see the elegance of XML
marred by needless syntactic extensions to provide features that can
be cleanly layered on top of the base syntax for those applications
that need them.
--
+-----------------------+------------------------+-------------------+
| Elliotte Rusty Harold | elharo@metalab.unc.edu | Writer/Programmer |
+-----------------------+------------------------+-------------------+
| The XML Bible, 2nd Edition (Hungry Minds, 2001) |
| http://www.cafeconleche.org/books/bible2/ |
| http://www.amazon.com/exec/obidos/ISBN=0764547607/cafeaulaitA/ |
+----------------------------------+---------------------------------+
| Read Cafe au Lait for Java News: http://www.cafeaulait.org/ |
| Read Cafe con Leche for XML News: http://www.cafeconleche.org/ |
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