Type system (II)

In the previous post we saw the different basic data types available in GGL2. In this one we'll take a look at the composite ones.

There are two posibilities to compose a type: elements and sequences.

Elements are tuples of pairs name=value and typically can be understood as "rows" in a table. For example, in GGL2, a feature read from a shapefile that has an integer attribute called id would be an element with a geometric child called the_geom and a numeric one called id. The element values can be accessed with the / (slash) operator followed by the name of the child:

Feature f = ...;

show f/the_geom;

show f/id;

On the other side, sequences are just that, a sequence of values of a concrete type (either basic or composite). So, if elements can represent shapefile features, sequences of elements can represent the whole shapefile.

There are a lot of constructions to process sequences (that may be shapefiles, postgis tables, etc.). For example, it is possible to filter them to show the elements in the shapefile that have an id lower than 10, or to sort them:

show myShape filter( f| f/id < 10);

show myShape sort( f| f/id);

Finally, note that these constructions are not limited to sequences of elements, they can be applied to any sequence, for example, sequences of ints:

sequenceof int myInts = [1, 2, 3, 5, 7, 9, 11, 13];

show myInts filter( i| i < 10);

The best part of the GGL2 type system is that elements can contain sequences of other elements and, therefore, hierarchical data structures (GPX, GML based formats) will be accessible from GGL2 without problems. I've done no test yet so... that will be another post.