XML processing

The new 2.0M2 release will be out soon. One of the main reasons to start the development of the second version of the GGL language was the difficulties to access XML data (and any hierarchical data in general) with GGL1 "flat table" model.

This task will be much more easy with GGL2 so let's take a look.

In previous posts we've seen how to access element(feature) attributes using the '/' operator like this:

show myshape/the_geom

The big difference with GGL1 is that elements attributes may be also complex elements so that the slash operator can be used several times in the same expression, provided that the data structure being accessed has those levels of hierarchy:

show car/engine/brand

As, typically, this hierarchical data will be stored in XML files, GGL2 provides a generic XML reader which means that... yes: GGL2 can read any* XML file. For example, in the following code we show the "lat" and "lon" attributes of every point in every route in a GPX file:

import ggl.xml;

read XML '/tmp/fells_loop.gpx' to gpx;

points = gpx/rte/rtept;

foreach p in points {
show p/@lat + ', ' + p/@lon;
}

Another example of the use of XML in GGL2 is the storage of a Shapefile as XML:

import ggl.xml;
import ggl.shp;

read SHP '/tmp/lineas.shp' to lineas;

elem = {linea=lineas};

write XML elem to '/tmp/out.xml';

The previous code produces the following XML file:

<?xml version="1.0" encoding="utf-8"?>
<root xmlns="http://www.gearscape.org/xmlio" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:gml="http://www.opengis.net/gml/3.2" xsi:schemaLocation="http://www.gearscape.org/xmlio out.xsd">
<linea>
<the_geom>
<gml:MultiCurve gml:id="_0">
<gml:curveMember>
<gml:LineString gml:id="_1">
<gml:pos>29.749046840958705 68.23897058823553</gml:pos>
<gml:pos>32.352498638344336 70.79517505787064</gml:pos>
<gml:pos>33.847724332788786 68.72824542143272</gml:pos>
</gml:LineString>
</gml:curveMember>
</gml:MultiCurve>
</the_geom>
<ID>-5.0</ID>
</linea>
<linea>
<the_geom>
<gml:MultiCurve gml:id="_2">
<gml:curveMember>
<gml:LineString gml:id="_3">
<gml:pos>32.352498638344336 70.79517505787064</gml:pos>
<gml:pos>34.61292807053388 68.45558661832814</gml:pos>
<gml:pos>36.78540305010905 67.91421568627474</gml:pos>
</gml:LineString>
</gml:curveMember>
</gml:MultiCurve>
</the_geom>
<ID>-1.0</ID>
</linea>
<linea>
<the_geom>
<gml:MultiCurve gml:id="_4">
<gml:curveMember>
<gml:LineString gml:id="_5">
<gml:pos>30.25038722086067 70.4233419713374</gml:pos>
<gml:pos>30.742932155501194 71.04781858489949</gml:pos>
<gml:pos>31.10354541122015 70.34418296398445</gml:pos>
<gml:pos>30.63738681236394 69.7988653577753</gml:pos>
<gml:pos>31.886340039488125 69.27993408735045</gml:pos>
<gml:pos>32.16779428785414 69.99236515352693</gml:pos>
<gml:pos>32.352498638344336 70.79517505787064</gml:pos>
</gml:LineString>
</gml:curveMember>
</gml:MultiCurve>
</the_geom>
<ID>3.0</ID>
</linea>
</root>

which, you'll guess, is standard GML.

With this functionality we've accomplished the last of the aims that motivated the remake of the language. Some usability and performance issues remain to be solved but we're almost at the end of the road. Soon we'll have a new release and then: courses, workshops, etc.

* Any XML file which schema is understood by the GGL2 XML reader. It's a work in progress and there is no support for some rare (and not so rare) XSD constructions.