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Beliefs in JaKtA

Beliefs are a fundamental concept in the Belief-Desire-Intention (BDI) model and play a key role in defining an agent’s knowledge about the world. In JaKtA, beliefs represent facts and rules that an agent holds, and they can dynamically change as the agent interacts with its environment.

Understanding Beliefs​

JaKtA directly leverages (and exposes as an API) the logic programming toolkit for Kotlin 2P-Kt and its internal DSL for Prolog

Beliefs influence an agent’s decision-making by triggering goals or activating plans when conditions change.

In JaKtA, Beliefs expose the source of their information, which can be:

  • self source: If the information comes from the agent's internal knowledge
  • percept source: If the information is obtained after perceiving changes that happen in the environment
  • other source: Occurs in the case agents are exchanging message information, the source contains the sender of the message.

Defining Beliefs in JaKtA DSL​

In JaKtA, beliefs can be defined within an agent’s configuration.

For instance, in the following, we define a belief base containing information about paths in a graph, by means of logic facts, as well as logic rules for computing whether some location X is reachable from another location Y:

mas {
    agent("moon walker") {
    beliefs {
        fact { "path"("location1", "location2") }
        fact { "path"("location2", "location3") }                
        rule { "reachable"(X, Y) impliedBy "path"(X, Y) }
        rule { "reachable"(X, Z) impliedBy "path"(X, Y) and "reachable"(Y, Z) }
    }
}

Under the assumption that the graph represents some sort of map from the real world, the above belief base can be exploited by the agent to reason about reachability among any two locations in the map; in fact, through 2P-Kt, JaKtA fully supports Prolog’s unification and resolution mechanisms.

If you need other info about the library, visit its documentation.

Updating and Using Beliefs​

Beliefs are not static; they can be updated during execution. Agents modify beliefs based on changes in the environment or after executing specific actions.

Modifying Beliefs​

mas {
    agent("Drinker") {
        beliefs {
            fact { "holding"("beer") }
        }

        goals { achieve("eatSomething") }

        plans {
            +achieve("eatSomething") onlyIf { "holding"("beer").fromSelf } then {
                update("holding"("chips").fromSelf)
                achieve("eatSomething")
            }
            +achieve("eatSomething") onlyIf { "holding"("chips").fromSelf } then {
                execute("print"("I can finally eat!"))
                execute("stop")
            }
        }
    }
}.start()

Adding Beliefs​

mas {
    agent("Drinker") {
        goals { achieve("drink") }

        plans {
            +achieve("drink") then {
                + "holding"("beer")
            }
        }
    }
}.start()

Removing Beliefs​

mas {
    agent("Drinker") {
        beliefs {
            fact { "holding"("beer") }
        }
        goals { achieve("dropBeer") }
        plans {
            +achieve("dropBeer") then {
                - "holding"("beer").fromSelf
            }
        }
    }
}.start()

Reacting to Belief Changes​

Agents can define event-driven behaviors that trigger actions when a belief changes. The events are triggered by the addition or the removal of a belief.

Example: Reacting to a Belief Addition from percept source​

fun main() {
     mas {
        environment(
            // Custom implementation of the environment which models temperature
            TemperatureEnvironment()
        )
        agent("reactiveAgent") {
            plans {
                +"temperature"(X).fromPercept onlyIf { X greaterThan 30 } then {
                    execute("print"("Warning: High temperature detected!"))
                }

                +"temperature"(X).fromPercept then {
                    execute("print"("Temperature under the threshold"))
                }
            }
        }
    }.start()
}

Example: Reacting to a Belief Addition from self source​

mas {
    agent("Drinker") {
        goals { achieve("drink") }

        plans {
            +achieve("drink") then {
                + "holding"("beer")
            }
            + "holding"("beer").fromSelf then {
                execute("print"("Finally I can have a beer!"))
            }
        }
    }
}.start()

Example: Reacting to a Belief Removal from self source​

mas {
    agent("Drinker") {
        beliefs {
            fact { "holding"("beer") }
        }
        goals { achieve("dropBeer") }
        plans {
            +achieve("dropBeer") then {
                - "holding"("beer").fromSelf
            }
            - "holding"("beer").fromSelf then {
                execute("print"("Ops! I dropped my beer!"))
            }
        }
    }
}.start()