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... Cheers, E. -- Eliot Kimber http://contrext.com 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> <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]" <mailto: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]" <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 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 XSL-List info and archive <http://www.mulberrytech.com/xsl/xsl-list>EasyUnsubscribe <http://lists.mulberrytech.com/unsub/xsl-list/1278982> (by email <>)
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