Expressions that are used more than once or twice should be separated into functions and shared. 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.

In addition, function declarations encourage reuse. When reused, an expression needs to be 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 in one place only.

Functions can be made even more reusable by separating them into library modules. XQuery libraries can also be used to create standardized sets of functions for specific XML vocabularies. These libraries can serve as an API to an XML vocabulary, shielding query authors from some of the complexity of the vocabulary. They can then be distributed to a variety of query writers, allowing reuse among an entire community of users.