MarkLogic Server 11.0 Product Documentation

XQuery and XSLT Reference Guide — Chapter 8

Creating Reusable and Modular Code

Functions provide a convenient way to modularize or componentize your XQuery code. When you move some functionality into a function in a library module, it allows you to call that library module and use any of its functions from any other XQuery module, allowing maximum code reuse. You can separate the library modules any way that makes sense for your development environment. For example, you can use a model-view-controller (MVC) approach where you have a set of functions that are used to access the content, a set of functions used to display the content in a user-interface, and a set of functions used to control the business logic of the application (for example, workflow logic based on various events).

Recursive Functions

Using functions recursively (creating functions that call themselves) is a useful design pattern in XQuery. Recursive functions are very convenient for iterating through an XML tree structure to perform XML transformations from one structure to another.

Note that MarkLogic will apply tail call optimization to a recursive XQuery function if and only if the function return type is untyped. For example:

(: Can be tail call optimized - no explicit return type :)
declare function my:func(
  $param as xs:string
) {
  ...
};

(: Cannot be tail call optimized - explicitly returns node() :)
declare function my:func(
  $param as xs:string
) as node() {
  ...
};

Recursive functions that are not tail call optimized create a new stack frame for each call and can eventually cause a stack overflow if the call stack gets too deep. By contrast, tail call optimized recursive functions use constant stack space.

For details on performing recursive transformations, see the Transforming XML Structures With a Recursive typeswitch Expression chapter of the Application Developer's Guide.

You can also use XSLT to perform transformations. For more information about XSLT, see XSLT in MarkLogic Server.

MarkLogic Server Documentation

In addition to this document, which describes the XQuery language implemented in MarkLogic Server, the MarkLogic Server documentation also includes XQuery API documentation for all of the XQuery-standard functions as well as the MarkLogic-defined XQuery functions. Included in the API documentation are many useful XQuery code samples.

The other documents in the MarkLogic Server library describe various other aspects of the product. In particular, the Application Developer's Guide includes many useful XQuery design patterns that work well with MarkLogic Server. For a description of MarkLogic Server documentation, see the product documentation section of the MarkLogic Developer site (http://developer.marklogic.com/).

XQuery Use Cases

MarkLogic Server includes an application that shows the XQuery Use Cases. The Use Cases have been developed by the W3C XQuery Working Group and demonstrates how a significant number of core tasks can be implemented using the XQuery language. The W3C describes the use cases in the following document:

http://www.w3.org/TR/xquery-use-cases/

The Use Cases have a default XQuery dialect of 1.0, so if you want to run code in 1.0-ml, use an XQuery version declaration in the prolog, as described in Specifying the XQuery Dialect in the Prolog. The Getting Started with MarkLogic Server walks you through this process of using the Use Cases application some detail.

Other Publications

In addition to the MarkLogic Server documentation, there are many excellent third-party books on XQuery. See the MarkLogic developer site for some recommendations (http://developer.marklogic.com).

You can also look directly at the XQuery specification, although much of the specification is geared more toward people who are implementing an XQuery processor rather than for people who are writing applications in XQuery. Nevertheless, it is very useful to at least get some familiarity with the following specifications:

• The current XQuery language recommendation (http://www.w3.org/TR/xquery/).
• The current recommendation for XQuery Functions and Operators (http://www.w3.org/TR/xquery-operators/).
• The XML Schema standard--useful for both type definitions and to understand the schema definitions that can be used in MarkLogic Server.