# Extensibility

The Saxon XQuery implementation allows you to call Java methods as external functions. The function does not need to be declared. Use a namespace declaration such as `declare namespace math="java:java.lang.Math"`, and invoke the method as `math:sqrt(2)`.

More details of this mechanism are found in Writing Extension Functions; note however that for XQuery the only form of namespace URI accepted is `java:full.class.Name`.

Saxon recognizes the XQuery pragma syntax, but it currently defines only one pragma of its own, the `saxon:validate-type` pragma (see Extensions). Saxon will adopt the correct fallback behavior if presented with a query that uses another vendor's extensions, provided these are designed in conformance with the W3C pragma specification.

Saxon also recognizes the XQuery option declaration syntax. Several specific option declarations are provided: `declare option saxon:default` declares a default value for external variables (query parameters); `declare option saxon:output` declares a serialization parameter; and `declare option saxon:memo-function` defines whether the following function declaration is to be implemented as a memo function. These are described under Extensions. Any other option declaration in the Saxon namespace is ignored with a warning; an option declaration in any other namespace is ignored silently.

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