Re: [xsl] best practices for using XSLT modes

Subject: Re: [xsl] best practices for using XSLT modes
From: "Eliot Kimber ekimber@xxxxxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>
Date: Thu, 5 Dec 2019 17:39:04 -0000
John's points are well taken--it certainly points up the art of XSLT
implementation design.

At least in the kind of stuff I do, maybe amplified by the editing tools I use
(Oxygen XML), I've moved to prefer templates over inline conditionals where
the logic is potentially variable or overridable or applies to multiple
contexts (although I often create local functions to encapsulate the sort of
atomic logic John shows).

The example that spawned this is too trivial to really be illustrative--we
don't know any of the larger requirements that might be in play.

I can certainly attest to the frustration with both templates that don't match
when you think they should and finding some errant template where you didn't
expect it. Those experiences have led me to value neatness and consistency of
coding patterns to a high degree, not something you always have a lot of
control over.

Lately I've been updating code that was written using XSLT 1 patterns and
antipatterns, including a heavy use of inappropriate xsl:choose, so maybe I'm
more sensitive to that than I might otherwise be...



Eliot Kimber

o;?On 12/5/19, 10:34 AM, "John Lumley john@xxxxxxxxxxxx"
<xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:

        On 05/12/2019 14:51, Eliot Kimber
          ekimber@xxxxxxxxxxxx wrote:

          Not really about modes, but I would replace the choice that acts on
different @val values with templates applied to the @val attribute, i.e.:

            <xsl:template match="a">
              <val><xsl:apply-templates select="@val"/></val>

         <xsl:template match="@val[. ge 0]">
            <xsl:value-of select="@val || ': positive'"/>

         <xsl:template match="@val[. lt  0]">
            <xsl:value-of select="@val || ': negative"/>

    Note that I handle the bug in the original in that it would produce no
result when @val is "0" (zero).

    The use of templates rather than xsl:choose makes the code cleaner, I
think, puts the focus at the template level on the @val attribute, which is
the focus of the business logic, and enables extension and override. For
example, if you want a value of exactly zero to have a different result, you
could do that by adding an override template. The original use of xsl:choose
would require overriding the entire template for the <a> element.

        I'm of a different opinion, somewhat in line with some remarks
          Mike made earlier. (Note that this topic can sometimes get a
          little 'heated' in a gentle XSLT manner.)
        The design issues in part depend on the 'scope' and 'size' of the
          operation you're invoking and the likelihood that you'll need to
          'add additional semantics' at a later date or by overriding with
          'library' importation. Lets look at a slight rewriting of the
          'choose' method in XSLT3:

           <xsl:template match="a" expand-text="true">
                    <xsl:when test="@val ge 0">{@val}:
                    <xsl:when test="@val lt 0">{@val}:

        (Yes it could be terser or it could be conditional XPath, but
          bear with me.) The point of the choose over the template is that
          the entirety of the operation choice semantics
          (identifying the signum of a/@val) is contained in a single place,
          in fact inside a single XML element, and won't be effected by any
          other additional code outside this segment. You're pretty
          safe in the knowledge that if something's going wrong (e.g. the
          zero case Eliot pointed out and another possible fault described
          later), then the fault should only be within this code
            section, nowhere else. And choose also has a totally
          defined order of checking - each when is tested in turn, so you
          can control accurately the sequence of checks.

        If that is what you wanted (contained scope), then if you used
          templates, even moded ones, an additional matching template,
          almost anywhere else in your stylesheets, might confound your nice
          solution. For example:

          <xsl:template match="a/@val[. mod 2 eq 0]"
            priority="2">{@val}: even</xsl:template>

        would alter the result. And similarly the 'default priority
          order' between template rules is sometimes not as straightforward
          as it seems. Believe me, with large stylesheets spread over many
          files it's extremely easy to find that an errant template
          somewhere in the back of beyond is trumping the code sections
          you're tearing your hair out debugging. And with stylesheet
          importation this becomes even more prevalent as importation
          precedence wins out over priority every time. This means that for
          example if your stylesheet was imported into another, which

           <xsl:template match="a/@val[. eq
            0]" <mailto:a/@val[.eq0]>><zero/></xsl:template>

        then your design will have been 'broken' (perhaps
          unintentionally) by someone else - the designer of the importing
          stylesheet. (Currently there are no mechanisms, outside use of
          packages, to make templates local, even as children of a mode, so
          all are effectively global in scope.)

        But if you do expect to have a large and variable set of
          conditions that in different cases will supercede each other, such
          as for example in the design of DITA-OT, then templates are
          certainly the way. And if you're using next-match to
          pre/post-process special cases, then you will need to use

          Note that in this particular example lurks a perhaps more
            insidious design issue for which a choose may have advantage.
            What if a/@val="abc", assuming no schema-awareness checking for
            integer values. With choose an error would be thrown during
            evaluation of the first when/@test ('abc' cannot be cast to an
            integer); in the template case nothing would happen - errors in
            patterns mean 'no match' and the usual default behaviour for @*
            is never matching: hence the result would be just <val/>
            and you might be left scratching your head.

        For me the choices really come down to:

    * How local is this operation? How many separable sections? How
            large are they?

    * Are some parts likely to be used from another section of the

    * Do I expect to modify it, or update it, or override it?

        etc... For some (most?) templates are the answer, for others choose
        wins out. The most interesting are of course a toss-up!

          John Lumley MA PhD CEng FIEE

          on behalf of Saxonica Ltd

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