Re: [xsl] When to use conditional constructions?

[Dimitre Novatchev <dnovatchev@xxxxxxxxx>]

David,

> I'm now wondering
> how much I should consider avoiding conditionals in my own work.
 .   .   .   .   .   .   .   .   .   .
> 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?

This is for you to decide, but I personally am trying to
systematically avoid XSLT conditionals and have never been sorry. The
code gets shorter and in most cases more readable. Equally significant
is the added power by using the polymorphism of <xsl:apply-templates>.
This gives us incredibly more flexibility and extensibility.

If the stylesheet module that contains the <xsl:apply-templates>
instruction is imported by different stylesheet modules, each of these
can define their own version of the templates to be selected for
execution -- in the terms of the "calculator" example, one can have
different implementations of the calculator.

> 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?)

I believe it is impossible to eliminate "if-expressions" in XPath
because we don't have templates there. It's a pity.
If this were possible, we could have the same advantages when
eliminating if-expressions in XPath. One could try using a HO function
to implement conditionals -- maybe I will try this.


> Are there specific cases where conditional constructs should be
> favored? (E.g., in <xsl:iterate> to allow Tail Call optimization?)
>

I haven't yet used <xsl:iterate> but from what I remember about it, I
would probably replace it, if possible, by using HOFs.

One doesn't need <xsl:if> in recursive processing. Here I recommend
not to use <xsl:call-template> for the recursive call (this *does*
require a conditional), but to use <xsl:apply-templates> with the
condition specified as predicate in the "select" attribute. This
doesn't affect any tail-recursiveness properties of the code.


> 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.

Based on my experience, the imperative programming style is like
following a cookbook recipe step by step -- rather boring and full of
risks.

On the other side, the declarative/functional style is more like
casting magic spells and enjoying most of the wanted work getting
magically done for you -- just because it was named.


Cheers,
Dimitre


On Sun, Mar 30, 2014 at 5:25 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.
>



-- 
Cheers,
Dimitre Novatchev
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