χrust

XPath, XQuery, and XSLT for Rust

Pronounced "crust".

The goal of this project is to implement XPath 3.1, XQuery 3.1 and XSLT 3.0 in Rust. Also included is an XML parser, using a parser combinator inspired by Nom.

Currently, the project has achieved the functional equivalent of XPath v1.0 and XSLT 1.0. That is, all of the elements and functions in v1.0 XPath and XSLT have been implemented. However, it is not compliant with v1.0. This is because it implements the v3.0 data model.

In addition to the (rudimentary) implementation of the XQuery and XPath Data Model 3.1 data model, a few other features of XPath and XSLT v2.0/v3.0 have been implemented, such as FLWR expressions, grouping (i.e. xsl:for-each-group), and user-defined functions.

Design

The library separates parsing from evaluation. The XPath and XSLT are parsed (or "compiled") into an internal representation, a "transformation", which is then evaluated.

This means it won't matter how an expression is written; it will be compiled into the same internal form. For example, the XPath expression:

if $a then "a is true" else ""

will result in the same internal format as:

<xsl:if test="$a">
  a is true
</xsl:if>

This approach means that the XPath and XSLT modules are simply mappings from their respective syntaxes to an χrust transformation.

Trees

The "Tree" is the fundamental data object for transformations. χrust provides a tree implementation, smite, that is both mutable and fully navigable. By "fully navigable" we mean that from any given node you can access its children, parent, or attributes. It achieves mutability by using the interior mutability pattern.

XML Namespaces

Support for XML Namespaces is in three parts:

1. The XML parser uses a flat mapping (NamespaceMap) of prefix to URI while it constructs the set of in-scope namespaces.
2. The smite tree implementation has a Namespace type node that represents an XML Namespace declaration.
3. Transformations can store the flat mapping (NamespaceMap) of prefix to URI so it can use that, if required, during the transformation.

There is a convenience routine that builds the flat mapping, i.e. a NamespaceMap, from the Namespace node declarations.

NamespaceNode objects are Rc-shared because they are often used but don't change much.

The Plan

1. Complete the XPath 1.0 implementation. (Done!)
2. Implement all XSLT v1.0 functionality. (Done!)
3. Improve XDM, XPath; achieve v2.0 compliance.
4. Add v2.0 features to the XSLT engine.
5. Further improve XDM, XPath; achieve v3.1 compliance.
6. Add remaining v3.0 features to the XSLT engine.

NB. We're picking the low-hanging fruit first. So major, fundamental features of the languages are being implemented to begin with. The fine detail will be added later. Although the eventual desire is to implement all of XSLT v3.0 functionality, some more advanced features will be implemented sooner rather than later.

Documentation

See the XSLT module for an example of how to evaluate an XSL stylesheet.

Examples

• Integration with Invisible XML.
• An example using XPath, but not XSLT.

Compliance

Status of standards implementation for XDM, XPath, XQuery, and XSLT.

Release Notes