Part I

Let’s first focus on the commonality among the different traffic light systems around the world. Every system uses the basic three-color lamps: Red, Yellow, and Green to control the right of way. In Clafer, we first define a concrete, prototypical TrafficLightSystem and its properties as follows:

In a Clafer model, every line declares a new clafer, which represents a concept or a property. The clafers are nested using indentation (red is a child clafer of Lamps). Clafers are followed by clafer cardinality, which indicates the allowed number of their instances: in general, the cardinality is an interval n..m, where m can be * to indicate infinity. A clafer instance can exist only when its parent clafer’s instance exists (e.g., instance of red cannot exist without an instance of Lamps). In our model, we use 1..1 and a shorthand 1 to indicate mandatory clafers, that is, clafers which must exist in every instance of the model as long as their parent exists. For green, we do not specify any cardinality since mandatory is the default cardinality and therefore it can be omitted.

What are the model instances? In our case, we can interpret our model as a description of variability across all jurisdictions, whereas the instances of our model will correspond to a particular variant of the system for a given jurisdiction. Since all clafers in our model are mandatory, there is only a single instance of our model (i.e., there is actually no variability yet).

Let’s use the tool ClaferIDE to generate the instance.

Press the button to open the model from this page in the IDE and press the button Run in the Instance Generator window. Then press the button Next to generate another instance and see what happens.

Close the IDE and move to the page adding variability.