JSONXPointer

A demonstration of the use of XPointer URIs containing XPath 3.1 expressions to refer to JSON documents

The example demonstrates how a TEI XML file could use external JSON data files to specify the geometric properties of <zone> elements (which specify geometric regions on a writing surface). Normally, a TEI <zone> specifies the vertices of the region directly as attributes. One method is to use the points attribute e.g.

<zone points="103.71093749999999,-43.83452678223682
  103.71093749999999,-32.54681317351514
  131.484375,-32.54681317351514
  131.484375,-43.83452678223682
  103.71093749999999,-43.83452678223682"/>

In this example, the TEI XML file json-zone-test.xml instead uses a custom attribute pointsRef on a <zone> element to point, using an XPointer URI, to geometric data contained with an external geoJSON data file zones.json, as shown:

{
  "type": "FeatureCollection",
  "features": [
    {
      "type": "Feature",
      "properties": {},
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [103.71093749999999, -43.83452678223682],
            [103.71093749999999, -32.54681317351514],
            [131.484375, -32.54681317351514],
            [131.484375, -43.83452678223682],
            [103.71093749999999, -43.83452678223682]
          ]
        ]
      }
    },
    {
      "type": "Feature",
      "properties": {},
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [121.640625, -20.632784250388013],
            [121.640625, -11.178401873711772],
            [138.8671875, -11.178401873711772],
            [138.8671875, -20.632784250388013],
            [121.640625, -20.632784250388013]
          ]
        ]
      }
    }
  ]
}

The pointsRef URI resolves to a list of points in the same format as that required by the standard TEI points attribute, meaning that a document using this mechanism can be converted to standard TEI by replacing each pointsRef attribute with a points attribute whose value is set to the result of resolving the pointsRef URI. This demonstration performs this transformation.

The test TEI file includes two <zone> elements; one of which has a pointsRef which literally specifies an XPointer URI in full, and the other of which specifies an "abbreviated" pointsRef by using the TEI <prefixDef> mechanism to define a custom geojson: URI scheme.

<zone pointsRef="#xpath(json-doc(%27zones.json%27)%3Ffeatures%3F1%3Fgeometry%3Fcoordinates%3F1%3F*!string-join(string-join(%3F*%2C%27%2C%27)%2C%27%20%27))"/>
<zone pointsRef="geojson:zones.json#2"/>

The data flow is controlled by the XProc pipeline resolve-json-zones.xpl, and consists of these steps:

1. The json-zone-test.xml is first transformed by the resolve-prefixes.xsl stylesheet, which replaces the custom URIs with XPointer fragment URIs. This replacement is a standard TEI processing behaviour, specified by a TEI <prefixDef> element. The resulting document contains two <zone> elements, both with pointsRef attributes containing XPointer fragment URIs.
2. The document is then transformed by the stylesheet resolve-json-zones.xsl (and subsidiary stylesheet url-decode.xsl), to convert the TEI into a simplified XSLT stylesheet which simply "quotes" the original TEI file, but in which the XPointer URIs are URI-decoded to yield the XPath expressions, and used to specify the value of points elements using Attribute Value Templates in the XSLT. The stylesheet produced in this step is only an intermediate file, but for the sake of demonstration, it is saved as json-zone-test-output.xsl.
3. The resulting XSLT is then executed to produce a TEI document in which the points attribute values are set to the result of evaluating the XPath expressions contained in those Attribute Value Templates. Finally the resulting document is stored as json-zone-test-output.xml.