Re: [xsl] Perfomance: 'conditional instruction' vs. 'multi template'

[Dimitre Novatchev <dnovatchev@xxxxxxxxx>]

Great explanation, Ken.

To make this clear to "classical" programmers, Approach 1 fully
follows the "Open-Closed" principle [1] of OOP (which is the "O" in
SOLID [2]) -- open for extension and closed for modification.

On the other side, Approach 2 cannot be extended without modification.


Cheers,

Dimitre

[1]. Wikipedia: Open/Closed Principle , at:
http://en.wikipedia.org/wiki/Open/closed_principle

[2]. Wikipedia: SOLID (object-oriented design), at:
http://en.wikipedia.org/wiki/SOLID_(object-oriented_design)



On Sat, Nov 3, 2012 at 10:58 AM, G. Ken Holman
<gkholman@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> At 2012-11-03 16:24 +0100, Norbert Heidbrink wrote:
>>
>> if a node is to be processed differently, depending on a certain
>> condition, there are two approaches.
>>
>> Let's say, the condition is "existance of child x".
>>
>>
>> Approach 1: two templates with different matching XPaths
>>
>> <template match="thenode[child::x]">
>>   b& do_this b&
>> B&
>> </template>
>>
>> <template match="thenode[not(child::x)]">
>>    b& do_that b&
>> B&
>> </template>
>>
>>
>> Approach 2: one template with a conditional instruction
>>
>> <template match="thenode">
>>   <xsl:choose>
>>   <xsl:when test="x">
>>     b& do_this b&
>> B&
>>   </xsl:when>
>>   <xsl:otherwise>
>>     b& do_that b&
>> B&
>>   </xsl:otherwise>
>> </xsl:choose>
>> </template>
>>
>>
>> I wonder, if one of these approaches is to be favoured?
>> Are there any severe implications on performance?
>> Any other advantages / disadvantages that speak in favour of approach 1
>> or approach 2?
>>
>> I am curious to read your estimations,
>
>
> I am not an implementer, just a user, but this is how I see it and teach
> it.
>
> Approach 1 allows the processor to optimize the method by which it decides
> between the two template rules each with a template to build the result
> tree.
>
> Approach 2 burdens the processor to execute your own determination of how
> to choose between the two templates to build the result tree.
>
> Approach 1 is declarative, approach 2 is imperative.
>
> Approach 2 may be more intuitive to classical programmers, approach 1 may
> be a bit harder for classical programmers to wrap their heads around.
>
> Approach 1 can be exploited by an importing stylesheet in order to effect
> nuanced differences in the stylesheet for one of the two template rules,
> even when the imported stylesheet is read-only.  Approach 2 prevents
nuanced
> overriding of only one of the two templates and imposes the burden on the
> importing stylesheet to replicate the behaviours of the other template
> already in the imported stylesheet that don't need to change.
>
> For these reasons, especially the last one, I tell my students approach 1
> is "better" and approach 2 has drawbacks.  I always try to write my
> stylesheets so that in the future I can better exploit the stylesheet
> without changing it, even if I don't yet know what those future
requirements
> might be.  That allows me to continue using the unchanged stylesheet
> wherever I am using it, and yet leverage it for other reasons in other
> contexts by the importing stylesheet selectively changing behaviours.
> Top-level constructs can be overridden in their entirety, while their
> contents cannot be selectively overridden.  The more template rules I have,
> the more nuanced I can change the behaviours.  The fewer template rules I
> have, the less flexibility I have to override smaller portions of the
> imported stylesheet.
>
> I hope this helps.
>
> . . . . . . . . . . Ken
>
>
> --
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>



--
Cheers,
Dimitre Novatchev
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