Re: DOCBOOK-APPS: conditionalization of XML

[Elliotte Rusty Harold <elharo@metalab.unc.edu>]

At 2:42 PM -0400 10/10/02, Eric S. Raymond wrote:

>Here's my tutorial example:
>
>Preamble
><!-- this is the test on the manual page -->
>Always issue this text.
><?if condition='html'>
>Issue this text if 'condition=html' is given on the command line.
><?elif condition='pdf|ps'>
>Issue this text if 'condition=pdf' or 'condition=ps'
>is given on the command line.
><?else>
>Otherwise issue this text.
><?fi>
>Always issue this text.
><!-- this ends the test on the manual page -->
><?if condition='foo'>
>Should display only if condition is foo
><?elif condition='baz'>
>Should display only if condition is baz
><?else>
>Should display only if condition is not foo and not baz
><?fi>
><?if not condition='bar'>
>Should display only if condition is not bar
><?if cond2='on'>
>This should be displayed only when cond2 is on.
><?fi>
><?else>
>Should display only if condition is bar
><?fi>
>Postamble.

I knew something bothered me about this, and the problem just smacked 
me upside the head:

    An XSLT processor operates a on a tree of nodes, but
    processing instructions cross tree boundaries.

In other words, a single processing instruction in one place is OK. 
However, a begin-end pair is not. That needs to be an element. 
Otherwise, we're stuck with the possibility of this:

<section>
   <para>
     <?if cond2='on'>
       This should be displayed only when cond2 is on.
    </para>
     But wait! We just eliminated the end-tag!
<?fi>
<?else>
    We could sneak an end-tag in here, but do we really want to
    rely on this?
<?fi>
</section>

Is this fixable? Not with PIs. I think if you really need overlapping 
markup, you need to look at something other than XML. However, maybe 
it could be fixed by using attributes rather than PIs. For example,

<section xmlns:cnd="http://www.example.org";>
   <para cnd:if='on'>
       This should be displayed only when cond2 is on.
    </para>
</section>

I can't work an else into this scheme though. If you really want to 
do this, I think it needs to be done with elements, not processing 
instructions. It would take a preprocessor step to convert it to 
valid, transformable XML.
-- 

+-----------------------+------------------------+-------------------+
| Elliotte Rusty Harold | elharo@metalab.unc.edu | Writer/Programmer |
+-----------------------+------------------------+-------------------+
|           Processing XML with Java (Addison-Wesley, 2002)          |
|              http://www.cafeconleche.org/books/xmljava             |
| http://www.amazon.com/exec/obidos/ISBN%3D0201771861/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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