XPath Data Model for encoding XML as JSON

There is a model for mapping arbitrary XML to a JSON-like data model. Consider the following document:

<document lang="de">
    <list>
        <item tag="X">A</item>
        <item tag="Y">B</item>
    </list>
    <field>Hello World</field>
</document>

The JSON equivalent for that looks something like:

{
    "@lang": "de",
    "list": {
        "item": [
            { "@tag": "X", "$value": "A" },
            { "@tag": "Y", "$value": "B" }
        ],
    },
    "field": "Hello World"
}

The attribute syntax in particular (e.g. @lang) seems to be derived from XPath, where it was already in use to differentiate attributes from elements.

One example where this model is used in the serde support for quick-xml.

Edit: 2024-05-10

If we look at the encoding for qualified names (QName c.f. Namespaces in XML) in XPath, we can see that a URIQualifiedName was introduced in Version 3.0, used in the EQName production. That one looks like:

Q{http://example.com/ns}invoice

This syntax could be a good candidate to encode a namespace within a data model that only supports simple field names.

Note that the spec defines equality comparisions on expanded QNames (Namespace, Prefix, Local Part) as only considering the namespace and the local part.

Normal QNames (e.g. example:invoice) are also allowed as EQNames but require the namespace to be defined before used as a Terminal in the XPath expression.

See also: QName-to-URI mapping