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Re: [xsl] When to use conditional constructions?


Subject: Re: [xsl] When to use conditional constructions?
From: Wendell Piez <wapiez@xxxxxxxxxxxxxxx>
Date: Mon, 31 Mar 2014 11:14:03 -0400

Hi,

I tend to favor XSLT logic over XPath logic -- not always but mostly
-- but I almost never use xsl:choose in favor of templates.

Most of the places where I use xsl:choose, it's because I have
grouping or something else going on that prohibits me from using the
implicit conditionality of templates to handle the contingencies.

I find this makes for code that is easier to refactor (at least as
soon as its complexity is more than trivial), and indeed easier to
understand at a glance -- once I understand that this is the way
xsl:apply-templates is being used.

Plus, start down this path and you find all kinds of amazing shortcuts
branch off from it.

For example, I don't bother to ask, when processing a cross-reference,
what "type" of cross-reference it is, if that is closely bound (as it
almost always is) to the type of its target. I just go process the
target.

I also use xsl:for-each as a quasi-conditional more often than I do as
a way to avoid a template. For example, instead of

<xsl:if test="exists(@class)">
  <xsl:attribute name="type">
    <xsl:value-of select="@class"/>
  </xsl:attribute>
</xsl:if>

say

<xsl:for-each select="@class">
  <xsl:attribute name="type">
    <xsl:value-of select="."/>
  </xsl:attribute>
</xsl:for-each>

if I don't want to bother with xsl:apply-templates select="@class",
with the template to match.

Cheers, Wendell

On Sun, Mar 30, 2014 at 8:25 AM, David Rudel <fwqhgads@xxxxxxxxx> wrote:
> On Sun, Mar 30, 2014 at 1:07 AM, Dimitre Novatchev <dnovatchev@xxxxxxxxx> wrote:
>
>> This is the full proof that XSLT conditional instructions can be
>> eliminated in any version of XSLT.
>>
>> BTW, I have quite a lot of experience writing complex transformations
>> without any XSLT conditional instructions. :)
>>
>> Cheers,
>> Dimitre Novatchev
>>
>
> Dimitre, as typical, your posts are very insightful. I'm now wondering
> how much I should consider avoiding conditionals in my own work.
>
> This provoked a general question I have for experiences XSLT
> programmers who frequently use <xsl:apply-templates> or some other
> method to avoid what would normally (in a more imperative regime) be
> done using conditional constructions.
>
> Have you found that, in general, it is best to avoid conditional
> constructs? If so, could you share what advantages you have found by
> doing so?
>
> Does this behavior extend to XPath's "if" statement as well, or do you
> see that as a different beast? (In other words, do you find that the
> advantages you obtain by avoiding XSL's conditionals do not apply as
> much when considering cases that can be addressed using XPath
> instead?)
>
> Are there specific cases where conditional constructs should be
> favored? (E.g., in <xsl:iterate> to allow Tail Call optimization?)
>
> As I have confessed in the past, for my work the thing I value most
> about XSLT is its support of XPath. I still tend to use imperative
> programming, so my scripts tend to look like the XSLT equivalent of a
> Java program that uses a single class.
>
> -David
>
> --
>
> "A false conclusion, once arrived at and widely accepted is not
> dislodged easily, and the less it is understood, the more tenaciously
> it is held." - Cantor's Law of Preservation of Ignorance.
>



-- 
Wendell Piez | http://www.wendellpiez.com
XML | XSLT | electronic publishing
Eat Your Vegetables
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