Type system (I)

As GGL2 is a language to treat data, it is specially important to have a type system able to represent the data structutes that are going to be processed. As many other languages, GGL2 lets the user define variables that are strongly typed, this is, that can only contain values of their type (there are exceptions, though).

GGL2 variables can be of simple built-in types or a composite of one or more GGL2 types. In this post, only simple types will be explained. So, for example, it is possible to have numeric and textual variables and initialize them to integer and textual values:

int a = 2;

string text = 'hello';

but, as it is a language to process spatial data, "geometry" is also a basic type and it is possible to have geometric variables and initialize them to geometric literals. What are geometric literals? Well, kind of Well Known Text:

geometry g = POLYGON((0 0, 10 0, 10 10, 0 10, 0 0));

geometry p = POINT(0 0);

The good thing about these WKT literals is that, indeed, they are not literals. I don't want to go deeper on language terminology but, as long as you can use variables in pseudo WKT expressions, they are no longer literals:

int ten = 10;

geometry g = POLYGON((0 0, ten 0, ten ten, 0 ten, 0 0));

In a next post I'll take advantage of this feature to do some cool things with very few code. Stay tunned!

Now, enough about technical details, lets go a little thoughtful: What value do variables give to the language?

Well, consider the buffer in a previous post:

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

With variables (and loops) we can create several buffers easily:

foreach size in 0 .. 100 step 10 {

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

}

This can be performed in SQL aswell but, IMHO, GGL2 code is easier to produce, read, and therefore, maintain.

A last point on variables, although GGL2 is strongly typed it is possible to declare a variable with, apparently, no type:

a = 2;

text = 'helo';

p = POINT(0 0);

BUT, this doesn't mean that the variable has no type. Indeed, GGL2 takes the type from the right part of the assignation so it is clear that a is an int, text is a string and p is a geometry. This may seem a worthless functionality, but as types get more complicated it is VERY useful.