Hello buffer world

As hello world is very simple with GGL2:

show 'hello world';

I'm going to show something more complicated: the omnipresent buffer.

import ggl.geom;

import ggl.shp;

read SHP '/tmp/mishape.shp' to myshape;

result = myshape select (f| the_geom=ST_Buffer(f/@the_geom, 10), id=f/@id);

write SHP result to '/tmp/test.shp';

The first two lines import the necessary libraries containing shapefile reader and writer and algorithms to process the geometries. After that, the read statement stores in myshape the contents of the shapefile. Then a select predicate is used to select the buffer of the @the_geom attribute (field) and the @id attribute as it is.

The select predicate iterates over the elements of the shapefile sequence and transforms them as specified between parenthesis. There, the current element is named f and the attributes of the resulting element are the left side of the assignments:

@the_geom=ST_Buffer(f/@the_geom, 10), @id=f/@id

Finally the result is assigned to a result variable that is written in the last statement to /tmp/test.shp.

Basically it's the same that can be accomplished with a SQL select statement, only that a different syntax is used.