Preface

Extensible Stylesheet Language Transformations (XSLT) is a powerful technology for transforming XML documents into other useful forms, but it is sometimes considered difficult to learn. Its template-based approach makes it a prime candidate for learning by example, and XSLT examples are often easily repurposed. XSLT 2.0 greatly increases the power and elegance of XSLT but also increases its complexity.

When I first began working with XSLT (and again when learning XSLT 2.0), I longed for a cookbook that would accelerate my productivity by providing ready-made solutions to the challenges I faced. My first experience with such a book was O'Reilly's Perl Cookbook. This book was more influential to my reluctant learning and ultimate appreciation of Perl than the original camel book (Programming Perl) by Larry Wall. I believe cookbooks are important because most software developers are not satisfied with simply figuring out how to make something work: they are interested in mastering the technology and using the best-known techniques, and they want answers fast. There is no better way to master a subject than by borrowing from those who have already discovered better ways to do things.

Longing for a cookbook soon turned into a desire to write one, especially since I collected several useful recipessome that were developed by others and some that I created. However, I did not want to write an XSLT book simply packaged in an alternate form; I wanted to provide a useful resource that also highlighted some less-obvious ways to apply XSLT. In the process, I hoped to attract XML developers who have not yet been motivated to learn XSLT and who, in my opinion, are missing out on one of XML's best productivity tools. If you are one of these folks who has not yet experienced XSLT, please bear with me for a few more paragraphs while I pitch the value of XSLT and the role of this book in helping you realize its potential.

XSLT is a language that lives simultaneously on the fringes and in the mainstream of current software-development technology. While working on the first edition of this project, I often found myself explaining to friends what XSLT was and why it was important enough to spend time writing a whole book about it. These same friends had heard of Java, Perl, and even XML, but not XSLT. I also observed an increasing number of requests for XSLT assistance on XSLT mailing lists and more industry attention in the form of books, articles, and sophisticated XSLT development tools. The XSLT user base is clearly growing daily; however, many software professionals and technology enthusiasts do not understand what it is and why it is important. With the release of new XSLT 2.0 implementations, I hope adoption of XSLT will accelerate, but this is not certain, partly due to competition from XQuery 1.0 and other XML manipulation methodologies. One thing is certain: mastering XSLT 2.0 is a worthwhile endeavor because its use will certainly increase, even if it never explodes. Further, learning XSLT will give you a deeper insight into XML processing even if you favor an alternative solution.

Although XSLT 1.0 is a mature language and XSLT 2.0 is not far behind, I would still guess that more that half of all companies and individuals working with XML do not use XSLT. Not so long ago, a colleague who is otherwise well-versed in all the latest technologies described XSLT as just another styling language. This misunderstanding is forgivable because XSLT advertises itself through the first three words in its name (Extended Stylesheet Language) and with the keyword that begins most XSLT programs (xsl:stylesheet). However, the last word in the XSLT acronym, Transformations, is what makes XSLT so important and is what drew me to the language in the first place. One of my goals in writing this book is to show how XSLT is relevant to a wide variety of problems. I also want to provide both novice and intermediate users of XSLT a one-stop shopping place for some of the most commonly requested XSLT techniques. Finally, I want to push the envelope of what one can do with XSLT so current users can go even further and the unconvinced can join the fold of highly productive XML transformers.

Over the years, I have heard many sweeping statements about computer science. Opinions like, "All computation is simply fancy bit manipulation," "Computers are really just sophisticated number crunchers," or "Everything a computer does can be understood in terms of symbol manipulation" are true to some extent. However, I would like to make a sweeping generalization of my own: "Every problem we solve with software can be understood in terms of transformations." Mastery of computer science is mastery of transformation. Transformation is what CPUs do, it is what algorithms do, and it is what software developers do. And transformation is what XSLT does, at least when the input is XML (and sometimes when it is not). Of course, XSLT is not the only transformational game in town, and as with the thousands of languages that came before it, it is unclear whether it will evolve as an independent language or be absorbed into the next "big thing." What is clear is that the ideas behind XSLT will not go away because many of these ideas are as old as computer science itself. This book helps the reader master and apply these ideas to specific problems.