Re: [xsl] Default attribute value templates

[tcn@xxxxxxxxxxxxx (Trevor Nash)]

Thanks to Mike Kay and Dimitre Novatchev for their answers addressing
the example given.  The bottom line seems to be that using AVTs for
xsl elements which have defaults is a tricky business.

My example was over simplified: I want all the attributes of xsl:sort
(except "select" which is another problem entirely) not just 'order'.

Original example:
>> I have a template which matches a specific element, then does an
>> apply-templates with a sort.  The application is supposed to be fairly
>> generic, so it seems a nice idea to allow those attributes of sort
>> which are attribute value templates to be parameterized thus:
>> 
>> <xsl:template match="x">
>>      <xsl:apply-templates>
>>            <xsl:sort order="{@order}" ... />
Notice the '...'
>>      </xsl:apply-templates>
>> </xsl:template>
>> 
>> The problem is, given <x>...</x> the transformation fails "order must
>> be ascending or descending".
>> 

Mike Kay: 
>You can write
>order="{concat(substring('ascending', 1, 9*not(@order)), @order)}"
>
Sneaky.  I guess wiring in the default is not a problem for something
as stable as xsl:sort.  There is a problem though: for the 'lang' and
'case-order' attributes we do not know what the default is, because it
depends on the environment.

Dimitre Novatchev:
><xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform";>
><xsl:template match="x">
>  <xsl:choose>
>    <xsl:when test="@order and (@order = 'ascending'
>                                or @order = 'descending')">
Aside: I think you intended to validate that the order attribute if
supplied is a valid value: good idea in principle, but as it stands
this silently throws away invalid values.
>       <xsl:apply-templates>
>           <xsl:sort order="{@order}"/>
>       </xsl:apply-templates>
>    </xsl:when>
>    <xsl:otherwise>
>       <xsl:apply-templates>
>           <xsl:sort/>
>       </xsl:apply-templates>
>    </xsl:otherwise>
>  </xsl:choose>
></xsl:template>
>
></xsl:stylesheet>

Yes, thats the sort of thing I had in mind.  The problem is xsl:sort
has four attributes which take AVTs, which makes 16 xsl:whens and 16
copies of the xsl:apply-templates which could have all sorts of other
stuff like modes, selects, with-params etc.  Bad news for maintenance
and testing.  If we use Mike's method where it applies this comes down
to 4, but its still uglier than I like.

I think I will do two passes: the first to generate a stylesheet with
the right xsl:sort in it, then a second to apply it.  This lets me
parameterise 'select' as well, and with a change to the way 'x' is
defined allow more than one sort key.

Thanks again,
Trevor Nash

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