Summary

Extension functions are useful to extend the capabilities of XSLT stylesheets. They allow stylesheets to access external system services, and to perform calculations that are difficult or inefficient to achieve in "pure" XSLT and XPath.

There are other extensibility mechanisms in XSLT, including extension instructions, extension declarations, and extension attributes, but extension functions are by far the most widely used, so that's what we concentrated on in this chapter.

In the next chapter, we move away from detailed specifications of interfaces, and look at using the facilities of XSLT to create well-designed stylesheets.