This proposal defines a packaging system for various core XML technologies: XSLT, XQuery, and XProc. The goal is to define it in a way enough generic so to adapt it to other technologies in the future (such as XML Schema, XForms, etc.) using the same framework. Besides enabling the delivery of libraries written in standard XSLT, XQuery and XProc, it provides support for extensions specific to some processors, as well as enabling new processors to be supported by using the same framework.
Must be ignored, but is required by the schema...
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XSLT, XQuery and XProc are amazing programming languages. But they lack a large choice of libraries, and when such libraries do exist, this is a challenge to install. There is no automatic install process, the rules are different for each processor, library authors do not follow the same rules regarding the info they provide, the cataloging, the way they reference third-party libraries, etc.
All those problems (well, most of them) can be addressed by a packaging system that would be broadly adopted by processor vendors and library authors. The cornerstone of such a system is the packaging format: a description of the information to be provided by the library authors and how to provide and structure them.
A library is a set of files fulfilling a common purpose. An XSLT library can for
instance provide a set of template rules and functions to help formating a particular
XML document type. A package is a way to bundle those files into a single ZIP file,
following a defined structure and providing more information within the expath-pkg.xml at the root of the ZIP file, and containing information
about the library (like its name and its version number) and about the files it provides
and how to reference them (for instance stylesheets and query modules.)
The ZIP file structure (aka the package structure) must have exactly two entries at the
top level: the package descriptor and one directory entry. This directory contains all
the library files, and all file references in the package descriptor are relative to
this directory. This directory is called the
All the elements in the package descriptor are defined in the namespace
http://expath.org/mod/expath-pkg. All the elements defined in this
specification or used in samples and in text are in this namespace, even if no prefix is
used. The root element is package, and contains exactly one child element
module:
name is the library name. The top-level directory in the package structure
must have the exact same name. The module has also a version number, and a
human-readable title. It then provides information about one or several files. Those
files are called the
The components are the files
The components are accessed from the outside of the package by using a URI. This URI is
the file:.
Its exact usage depends on the kind of component (for instance, with XSLT it is aimed at
be used in xsl:import, and in XQuery this is the target namespace of an XQuery library
module.) Each kind of component defines its own
Here is the description of the standard component kinds supported by this specification, and how they contribute to the package descriptor document type.
An XSLT file is associated a xsl:import) to import the XSLT file
provided in the package. This file is configured with the element
xslt.
The element file contains the path to the file within the package
structure, relative to the library directory. Both elements import-uri
and file are of type anyURI.
An XQuery library module is referenced by its namespace URI. Thus the
xquery element associates a namespace URI to an XQuery file. An
importing module just need to use an import statement of the form import module
namespace xx = "<namespace-uri>";.
Note that there is no way to set any location hint (as the at clause in
the import statement.) To use this packaging system, an XQuery library module must be
referenced by its target namespace.
An XProc pipeline, like an XSLT stylesheet, is associated a p:import statement.
An XML schema can be imported using its target namespace. Like for XQuery, there is
no way to use any schemaLocation instead. There is neither the ability
to set several files as several sources for the schema. If the schema is spread over
multiple files, there must be one top-level file that includes the other files.
(TODO: Should we support schemas with empty target namespace? I am sure this is a
good idea in a packaging system...) (TODO: This does not support
xs:redefine, as it requires a hint, not a TNS)
A RelaxNG schema, like an XSLT stylesheet, is associated a public import URI, aimed
to be used in an
A Schematron schema is associated a public URI.
An NVDL script is associated a public URI.
Documentation (like result of XSLStyle or xqDoc) is not taken into account in the packaging format, though that could be used by IDEs for instance to provide documentation for functions in an editor with a live completion feature. Some support for documentation can of course be added as a product-specific feature to the package descriptor.
A
The installed package list is implementation-defined. Each implementation (for a
specific processor) can define its own way to install and remove packages, as long as it
properly documents it. A processor should use, when appropriate, the
When a reference to a file of a specific kind is done via an absolute URI, a processor must look up for this URI in the corresponding URI space in the repository. How the repository is set to the processor is implementation-defined (a processor can also use a list of repositories, and enable or disable some libraries in any implementation-defined way.)
The URI space to use is defined by the nature of the reference. An XSLT
href attribute on xsl:import will use the xslt
URI space, while it will use the xsd space for
xsl:import-schema.
An XProc processor in particular has to pay great attention to the space it use
regarding the step that is beng evaluated. Any xsl:import instruction
encountered on the stylesheet port of the step p:xslt has
to be looked for in the xslt space (regardless if the stylesheet
document is inlined in the pipeline, computed, loaded from the file system or
retrieved from the Internet, or if the containing stylesheet has been imported
itself.)
The XProc elements p:document and p:data, as well as the
step p:load are handled specially. They can be used to access any kind
of resource, including but not limited to components in a repository. The user has to
tell explicitely the processor what kind of component is looked for by using the
pkg:kind extension attribute. For instance, a stylesheet can be
loaded from a repository as input to the step xslt as following:
This section defines a standard structure for on-disk repositories. An implementation can choose to not support this kind of repository and to define its own one (or even to not define it publicly, just to provide the ability to install and remove packages, in a clearly documented way.) However, there are several advantages to support this structure, the most obvious one is to be able to benefit from existing tools to manage such repositories as well as existing libraries to access those repositories.
The resolving machinery is based on OASIS XML Catalogs .expath-pkg/ which is dedicated to store working information about the
installed packages, among which the catalogs (aka the
The [space]-catalog.xml where [space] is either xslt,
xquery, rnc, etc. Those catalogs are called
[ ... TODO ... ]
[... TODO ...] (package structures, package descriptors, directory layout, processor behaviour...)
[ ... TODO ... ]
Should the package system define a set of XPath functions? Instead of just defining the package format and letting everything else as implementation-defined, should it in addition define a module of functions to install, delete, and more generally manage packages from within a processor?
Drawback: potential problems if the processor requires to be stopped?
Advantages: enables writing tools on top of the system (one single graphical package manager for one system, simply using the XPath functions, as well as easy integration within IDEs; or even other systems could be more easily be built on top of it, like a packaging system for XRX applications for instance.)
Should we add a generic "xml" URI space, for any XML document?
Add a restriction (actual add a definition of such URIs at the beginning) on public URIs: require them to be absolute, and maybe only either HTTP or URN (or at least prohibit the FILE scheme.)
Interesting use case with XProc and NVDL: How to configure the XSLT processor in Calabash with the NVDL URI space for an NVDL implementation for XProc written using XSLT as a plain library step?
[ ... TODO ... ]