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Re: [xsl] When to use conditional constructions?
Subject: Re: [xsl] When to use conditional constructions? From: Dimitre Novatchev <dnovatchev@xxxxxxxxx> Date: Sun, 30 Mar 2014 09:11:25 -0700 |
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 --------------------------------------- Truly great madness cannot be achieved without significant intelligence. --------------------------------------- To invent, you need a good imagination and a pile of junk ------------------------------------- Never fight an inanimate object ------------------------------------- To avoid situations in which you might make mistakes may be the biggest mistake of all ------------------------------------ Quality means doing it right when no one is looking. ------------------------------------- You've achieved success in your field when you don't know whether what you're doing is work or play ------------------------------------- To achieve the impossible dream, try going to sleep. ------------------------------------- Facts do not cease to exist because they are ignored. ------------------------------------- Typing monkeys will write all Shakespeare's works in 200yrs.Will they write all patents, too? :) ------------------------------------- I finally figured out the only reason to be alive is to enjoy it.
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