## Enumerations

Enumerations represent collections of related entities, such as types of products, types of available resources, available machine types, different countries, different colors, different genders, and so on. It is possible to use numbers to represent the different entities, for instance `0` for red, `1` for orange, and `2` for green, to represent the different colors of a traffic light. However, these numbers are rather arbitrary. Furthermore, they don’t actually represent numbers, but rather they represent one of the entities (red, orange, green). Enumerations allow giving each entity a name, and to use those names instead of numbers. This usually makes the model easier to read and understand. For instance, consider the following:

    enum TrafficColor = RED, ORANGE, GREEN;

The `enum` keyword is used to declare an enumeration. The `TrafficColor` enumeration has three possible values or literals. The literals are named `RED`, `ORANGE`, and `GREEN`. An enumeration can be used as data type, and the enumeration literals can be used as values:

    disc TrafficColor light = RED;

The `TrafficColor` enumeration is used as type of the `light` variable. The `light` variable is given value `RED` as its initial value. The default value of an enumeration type is its first literal (`RED` in this case). However, it is usually preferred to explicitly initialize variables with enumeration types, for readability.

    edge change_color when light = RED do light := GREEN;

This edge has a guard that compares the value of the `light` variable to enumeration literal `RED`. Only if the `light` is currently `RED`, may this edge be taken. The edge further assigns enumeration literal `GREEN` as the new value of variable `light`. The edge as a whole models that if the `light` is currently `RED`, it may change color (event `change_color`) and become `GREEN`.