3.5 Provide a mechanism for importing user-defined functions

[ndw]

Experience with user-defined functions in XQuery and XSLT reveals that they can be a powerful addition to the language. Providing some feature that allowed users to extend the vocabulary of functions available in, for example, the test expressions on p:when elements would greatly simplify some pipelines.

Such a mechanism might take the form of the ability to load extension functions defined in, for example, XQuery, or it might include adding the ability to define functions in XProc.

[ndw]

Proposal for p:import-functions instruction

It would be useful to be able to add user-defined functions to XProc. XProc steps themselves don't immediately lend themselves to this task because there's not a completely obvious mapping between step signatures and function signatures. However, most XProc engines are going to have access to XSLT and/or XQuery engines and both of those languages have a built-in mechanism for creating functions.

This proposal is for a p:import-functions element that can import externally-defined functions from languages such as XQuery or XSLT.

<p:import-functions
    href = xs:anyURI
    namespace? = xs:anyURI
    type? = xs:string
    />

This element instruct the XProc processor to load external functions from the specified library. It is a static error if the processor cannot access the URI or if the URI points to a format that the implementation does not recognize.

The namespace attribute specifies the namespace in which the imported functions must reside. It is a static error if the specified URI does not define functions in that namespace.

The type attribute specifies the expected type of the content. Implementations should support application/xslt+xml and application/xquery for XSLT and XQuery implementations. Support for other formats is impelementation-defined.