Re: [xsl] best practices for using XSLT modes

Subject: Re: [xsl] best practices for using XSLT modes
From: "Dimitre Novatchev dnovatchev@xxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>
Date: Thu, 5 Dec 2019 18:05:07 -0000
It is well-known that code inside a conditional operator *increases *the
cyclomatic complexity.

Thus we see many sources that recommend to avoid or refactor conditional
statements -- in many different languages.

I believe XSLT is no exception.

Just for example, here is a recommendation how to refactor Javascript code:

and inside it:
       Refactoring a switch-like logic

Here is another example -- the Pluralsight course "Refactoring for C#
In the expanded table of contents, under "Code Smells"  we see:

   Smell: Switch Statements

In the "method Code Smells module we see:

  Smell: Conditional complexity

I saw a phrase on a candy cover: "The magic is in the mess" ...

Let us not create "magic" code, please


On Thu, Dec 5, 2019 at 8:34 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>
>      <xsl:template match="@val[. ge 0]">
>         <xsl:value-of select="@val || ': positive'"/>
>     </xsl:template>
>      <xsl:template match="@val[. lt  0]">
>         <xsl:value-of select="@val || ': negative"/>
>     </xsl:template>
> 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">
>       <val>
>         <xsl:choose>
>         <xsl:when test="@val ge 0">{@val}: positive</xsl:when>
>         <xsl:when test="@val lt 0">{@val}: negative</xsl:when>
>       </xsl:choose>
>     </val>
> </xsl:template>
> (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]" <a/@val[.mod2eq0]>
> 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 contained:
> <xsl:template match="a/@val[. eq 0]" <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 templates.
> 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 program?
>    - 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
> john@xxxxxxxxxxxx
> on behalf of Saxonica Ltd
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