Actions

Overview

Actions are defined through the keyword action. An action is a user-defined operator which specifies how a model can evolve over time.

Each specification must define at least two actions:

1. an initialization entry (usually named init) which is executed exactly once as the entrypoint of the model.
2. a stepping entry (usually named step) which is executed in a infinite loop, and drives forward the model evolution over time.

Example

The example below is called actions.qnt and contains a valid model that does nothing, forever.

module actions {
    action init: bool = {
        true
    }

    action step: bool = {
        true
    }
}

Further notes

Actions are typed, but type-signatures are optional and can be automatically inferred. That is, action init: bool and action init are equivalent syntax.

Both init and step actions return a bool, which has the following meaning:

• true: the action can be selected and executed by the model checker (i.e. it is "enabled").
• false: the action cannot be selected and executed by the model checker (i.e. it is not "enabled").

Changing one (or both) of the actions in example above to false results in a deadlock: the model checker will reach a state where it cannot progress further, as it has no valid action to select anymore.