State

The state definitions of an entity define the structure of the information that the entity retains as its current state.

The state of an entity is defined with the state keyword within the content of an entitydefinition, like this:

entity Car {
  type Propulsion = any of { ICE, Electric, Steam, Diesel, EMDrive }
  state Static is {
    wheels: Integer
    doors: Integer
    rightHandDrive: Boolean
    propulsion: Propulsion
  }
}

One of the primary purposes of an entity is to represent the characteristics of the entity with state information. We call it state because it represents the current state of the entity at any given point in time.

The above example definition associates a state, named Static with the entity named Car having these fields in its aggregate data type:

• wheels - an Integer value providing the number of wheels on the car.
• doors - an Integer value providing the number of doors on the car.
• rightHandDrive - a Boolean value indicating if it is right hand drive.
• propulsino - a Propulsion enum value presumably representing the kind of propulsion the vehicle uses.

It is entirely possible to specify multiple named state definitions for a single entity. This is provided in the language to support finite state machines which are frequently used with entities. The state of an entity can be modelled as simply transitions of states as each handler processes a command. Multiple state values are useful when the state of the entity has different modes of operation where different sets of state values are needed.