XSLT Powers a New Wave of Web Applications
We still have a lot to learn about XSLT as a profession. As fast as its use is expanding, there remain fewer programmers competent in XSLT than, say, Object Pascal.
Another hurdle in XSLT's diffusion, along with its unconventional XML-based syntax and confusing deployment, is its functional or applicative semantics. Most computing languages that appear in Linux Journal are more or less procedural: Java and C programs instruct a processor to perform one operation, then another, then another. Proceduralism is wrapped up in the how of computation.
XSLT is related to Lisp in its functionality. Good XSLT programs express the “what” of a desired result. Instead of a focus on sequence-in-time, XSLT operates on whole XML documents or well-formed fragments to yield results. This is called functional (to evoke the model of mathematics), where functions turn inputs into outputs without side effects or variation in time. Moreover, mathematical functions can be composed (stacked) in combination. Typical XSLT semantics express several different transformations without specification of their sequence-in-time. Stylesheets are applied simultaneously.
XSLT has variables, but they are immutable. They can receive only one value and cannot, in particular, be looped, as in
for (i = 0; i < 10; i++);
Parametrized or repetitive operations are done through explicit recursion and iteration. The syntax of XSLT variable use is rather ugly, as it must fit within XML's constraints. In C or Java we might write
if (level > 20) code = 3; else code = 5;To approximate that in XSLT, we need
<xsl:variable name = "code"> <xsl:choose> <xsl:when test = "$level > 20"> <xsl:text>3</xsl:text> </xsl:when> <xsl:otherwise> <xsl:text>5</xsl:text> </xsl:otherwise> </xsl:choose </xsl:variable>Such exercises illustrate that, while XSLT has enough abstract power to handle general problems by itself, it's often best used in a dual-programming mode. XSLT's strengths generally lie in template processing, pattern matching and sorting and grouping XML elements. Steve Ball, a principal for consultancy Zveno Pty. Ltd., does as much as practical with XSLT, then embeds it in an application with another language to handle interfaces to external systems, including filesystems and user view.
Most popular among the developers I've encountered during the last six months are Java and Tcl, although Python, C, Perl and other partner languages also manage XSLT engines more or less adequately. Moreover, XSLT also defines extensibility mechanisms that allow developers to provide new semantics within XSLT: “extension elements”, “extension functions” and “fallback processing”.
XSLT's ultimate destiny remains unclear. In this, also, it's a bit like Java. Five years ago, Java's purpose appeared to be to construct cute visual applets. As we've discovered since then, heavy-duty enterprise servers actually make better hosts for the best Java programming. We're still at an early stage in deciding when to use XSLT. ReportLab, Inc., for example, is an enterprise vendor that delivers products and services having to do with high-quality report generation to some of the largest organizations in the world, including Fidelity Investments and American Insurance Group. ReportLab founder Andy Robinson explained to me the deep experience his development team has in projects that transform XML. Each project his company has fulfilled has involved coding in a lighter-weight scripting language rather than relying on XSLT. Even though XSLT is specialized for XML transformation, his consulting teams have found it easier to use Python as a more general-purpose, but powerful language.
My special thanks to Rolf Ade, who contributes both to tDOM software and especially to my understanding of it.
Cameron Laird is a full-time developer and vice president of Phaseit, Inc. He also writes frequently on programming topics and has published several articles during the last year on XSLT. He's currently preparing a training course on the language.
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