Free XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition by Michael Kay Page B

be software engineering benefits if a program (and a stylesheet is indeed a program) makes explicit assertions about its expected inputs and outputs. These assertions can lead to better and faster detection of errors, often enabling errors to be reported at compile time that otherwise would only be reported the first time the stylesheet was applied to some test data that happened to exercise a particular part of the code.
The more information that's available to an XSLT processor at compile time, the more potential it has to generate optimal code, giving faster execution and better use of memory.
    So why the controversy? It's mainly because XML Schema itself is less than universally popular. It's an extremely complex specification that's very hard to read, and when you discover what it says, it appears to be full of rules that seem artificial and inconsistent. It manages at the same time to be specified in very formal language, and yet to have a worryingly high number of bugs that have been fixed through published errata. Although there are good books that present XML Schema in a more readable way, they achieve this by glossing over the complications, which means that the error messages you get when you do something wrong can be extremely obscure. As a result, there has been a significant amount of support for an alternative schema language, Relax NG, which as it happens was co-developed by the designer of XSLT and XPath, James Clark, and is widely regarded as a much more elegant approach.
    The XSL and XQuery Working Groups responded to these concerns by ensuring that support for XML Schema was optional, both for implementors and for users. This has largely silenced the objections.
    The signs are that XML Schema is here to stay, whether people like it or not. It has the backing of all the major software vendors such as IBM, Oracle, and Microsoft, and it has been adopted by most of the larger user organizations and industries. And like so many things that the IT world has adopted as standards, it may be imperfect but it does actually work. Meanwhile, to simplify the situation rather cruelly, Relax NG is taking the role of the Apple Mac: the choice of the cognoscenti who judge a design by its intrinsic quality rather than by cold-blooded cost-benefit analysis.
    As I've already mentioned, W3C is not an organization that likes to let a thousand flowers bloom. It is not a loose umbrella organization in which each working group is free to do its own thing. There are strong processes that ensure the working groups cooperate and strive to reconcile their differences. There is therefore a determination to make all the specifications work properly together, and the message was that if XML Schema had its problems, then people should work together to get them fixed. XSLT and XML Schema come from the same stable, so they were expected to work together. And now that the specs are finished and products are out, I think users are starting to discover that they can work together beneficially.
    Chapter 4 provides an overview of how stylesheets and schemas are integrated in XSLT 2.0, and Chapter 19 provides a worked example of an application that uses this capability. When I first developed this application for the book (which I did at the same time as I developed the underlying support in Saxon), I was pleasantly surprised to see that I really was getting benefits from the integration. At the simplest level, I really liked the immediate feedback you get when a stylesheet generates output that does not conform to the schema for the result document, with error messages that point straight to the offending line in the stylesheet. This makes for a much faster debugging cycle than does the old approach of putting the finished output file through a schema validator as a completely separate operation.
    XSLT and XQuery
    XQuery is a separate specification from W3C, designed to allow data in XML documents to be queried. It can operate on single