XQuery

User-Defined Functions

XQuery allows you to create your own functions. This allows query fragments to be reused, and allows code libraries to be developed and reused by other parties. User-defined functions can also make a query more readable by separating out expressions and naming them. For a starter set of user-defined function examples, see http://www.xqueryfunctions.com.

Why Define Your Own Functions?

There are many good reasons for user-defined functions, such as:

Reuse

If you are evaluating the same expression repeatedly, it makes sense to define it as a separate function, and then call it from multiple places. This has the advantage of being written (and maintained) only once. If you want to change the algorithm later—for example, to accept the empty sequence or to fix a bug—you can do it only in one place.

Clarity

Functions make it clearer to the query reader what is going on. Having a function clearly named, with a set of named, typed parameters, serves as a form of documentation. It also physically separates it from the rest of the query, which makes it easier to decipher complex queries with many nested expressions.

Recursion

It is virtually impossible to implement some algorithms without recursion. For example, if you want to generate a table of contents based on section headers, you can write a recursive function that processes section elements, their children, their grandchildren, and so on.

Managing change

By encapsulating functionality such as "get ...