Defining constraints

We can define our own constraints. A constraint declaration takes the following form:

       constraint Name(FormalParameterList) ConstraintBody

Constraint definitions do not reside in any class, and the constraints defined can be used by any classes. This is a source for debate because it does not compliant with the pure object-oriented design principle. Nevertheless, as a relation is not directional, putting it into any one of the involved classes would make description unnatural.

The following defines the symbolic constraint inside(O1,O2) for two rectangles:

      constraint inside(Rectangle O1, Rectangle O2) {
           O1.x <= O2.x;
           O1.y <= O2.y;
           O1.x + O1.width <= O2.x + O2.width;
           O1.y + O2.height <= O2.y + O2.height;
      }