gold miners

Introduction

This programming exercise will illustrate some of the features of Jason, Agent Programming Language of JaCaMo, through the development and refinement of the Gold Miners implementation.

In each of the following items, we start with a general description and its objective, a description of the setting to test the exercise, followed by some brief explanation of the steps involved in achieving it and some hints on how to do it. Depending on your programming experience, you may decide to only read the introduction and objective to develop your own original solution, or also read the explanation and hints.

This tutorial was initially developed by João Leite (http://centria.di.fct.unl.pt/~jleite/wesaac14/) based on the Jason Gold Miners implementation (which is available together with Jason distribution).

The slides used at ALTISSIMO are found here.

Part I

To get started, you can install the JaCaMo eclipse plugin and download the initial JaCaMo project from here. Then, after unziping it into some directory, open Eclipse and import the downloaded project (File → Import → JaCaMo Project → Pick directory → Select Project GoldMiners). You will have a screen like

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Take a few minutes to understand the application. Read the PDF files in the doc folder and the source code miner.asl.

Run the application using environment id=1; 1 leader; 1 miner; 3 dummies (the .jcm file already encodes this configuration). The screen will be like

Note: the type of environment id=1, 2, 3, …​ is changed by changing the first parameter when creating the artifact MiningPlanet in the .jcm file:

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You can also open a browser at http://localhost:3272 and inspect the mind of the agents.

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Exercise a)

Introduction: To help understand what is going on, and what the agents are doing, it is useful to have agents printing messages on the console. This is particularly useful when agents are moving to random positions that we cannot anticipate.

Objective: modify the current miner.asl specification file in a way that the miners print a message on the console announcing the location they randomly picked to go near.

Test: environment id=1;leader;miner;dummy;dummy;dummy

Description: the objective can be accomplished by changing the plan where the next location is chosen, to include an additional action that prints the desired message.

Note
recall that there is an internal action .print(term1,term2,…​).

Solution: available here.

Exercise b)

Introduction: One of the reasons why the miner is going near a cell, and not to a cell, is because it may happen that the randomly chosen cell is not reachable, e.g. because there are walls blocking any path to it. The current implementation already deals with this by adding the belief near(X,Y) whenever the agent cannot reach the cell, or whether it is already in the vicinity. However, we are not aware of which of these two reasons actually happened.

Objective: modify the current miner.asl specification file in a way that the miners print a message on the console announcing either that they have reached the neighborhood of the target cell, or that the target cell is not reachable, depending on what is the actual cause.

Test: environment id=2; leader;miner;dummy;dummy;dummy

Description: the objective can be accomplished by changing the plans for the goal near(X,Y) so that the specific message is executed.

Tip
you might have to split one of the plans in two, so that you can differentiate the context, on which the message to be printed depends.

Solution: available here.

Part II

With the previous implementation, the miner was not really mining. It was only wondering around. You can see for yourself by trying it with environment id=3. If you now use the file miner1.asl instead, you will see that several plans have been added to deal with the mining of gold. Take a few minutes to understand the new miner1.asl file. Most parts are commented to help you understand. Try it with environment id=3;leader;miner1;dummy;dummy;dummy. You can use mouse clicks to add new pieces of gold in the scenario.

Note
if you simply copied the code of miner1.asl into your previous miner.asl agent, then the configuration should refer to miner and not miner1. From now on, we will assume that you have copied the code in miner1.asl into your miner.asl file.

Exercise c)

Introduction: Whereas so far we have only considered one miner, the miner will have to share its environment with other agents specialized in picking up gold and dropping it on the depot. To know who drops more gold pieces on the depot, we will trust agents to keep note of how many gold pieces they have dealt with.

Objective: modify miner.asl specification file in a way that the miner knows, at each point of its execution, how many gold pieces it dropped on the depot.

Test: environment id=3; leader;miner;miner;miner;miner.

Description: the objective can be accomplished by maintaining a belief whose argument is the number of gold pieces dealt with. You should start by creating an initial belief (e.g. score(0)). Then, this belief should be updated to reflect the gold pieces successfully dropped on the depot. This can be done as part of the plan to the goal handle(gold(X,Y)).

Tip
to update the existing belief score(X) with a new score score(Y) you can use -+score(Y).

Solution: available here.

Exercise d)

Introduction: As you can see, the current implementation of the miner agent has, hardwired, the location of the depot (reflected by the 0,0 in the plans for handling and dropping gold). However, with the exception of the first three scenarios, the depot is not at the location (0,0). In fact, the environment initially sends all agents the location of the depot which is kept as a belief of the form depot(_,X,Y), representing that the depot is at location X,Y. We will want the miners to be able to perform in environments where the depot is at an arbitrary location.

Objective: modify the current miner.asl specification file in a way that the miner can operate in environments where location of the depot is not known in advance.

Test: environment id=4; leader;miner;miner;miner;miner

Description: the objective can be accomplished by changing the plans to handle and drop gold so that they check the belief base for the right coordinates for the depot, instead of using (0,0).

Tip
you can include a query to the belief base in the context of the plan, or a test goal, to find out the right coordinates of the depot.

Solution: available here.

Part III

You may have noticed that the multi-agent system has an agent named leader, which has yet to have an active participation. Whereas in a full-fledged implementation of a team of agents it could serve as the coordinator, here we will simply use it to illustrate communication between agents.

Exercise e)

Introduction: If you inspect the file leader.asl, you will realize that the leader has a plan to handle messages from the agents informing it that they have dropped gold, keeping every agent’s score. According to the plan, every time the leader receives a message from the miners telling it that they dropped some gold at the depot, the leader updates the score of the agent and prints a message. However, right now, the miners are not sending the leader any messages!

Objective: modify the current miner.asl specification file in a way that miners send the leader a message informing it whenever they dropped some gold.

Description: the objective can be accomplished through the modification of the plan to handle gold, adding the action to send the desired message.

Note
recall the existence of the action .send(Receiver, Type, Content) where ToAgent is the name of the destination agent, Type is the kind of message and Content its content (e.g. if Sender sends such a message with Type=tell, the effect is the addition of Content[source(Sender)] to the belief base of the Receiver, thus generating the corresponding event). You can use the mind inspector (http://localhost:3272) to see whether the leader is receiving the messages.

Solution: available here.

Exercise f)

Introduction: Despite the fact that the leader keeps the scores, we do not know who is winning.

Objective: modify the current leader.asl specification file in a way that every time there is a new agent leading in number of gold pieces dropped, it prints a message informing who the agent is and how many gold pieces it has dealt with.

Description: the objective can be accomplished through the modification of the leader plan to handle the new belief dropped, considering the case where there is a new winning miner and the case where the winning miner did not change.

Tip
keeping a belief of the form winning(agent_name,score) might make it easy to determine when a new message informing of a newly dropped piece of gold causes a change in the winning agent.

Solution: available here.

Exercise g)

Introduction: We know (through the console) who is winning, but the agents still do not know.

Objective: modify the current leader.asl specification file in a way that every time there is a new winning agent, it broadcasts a message to every agent.

Description: the objective can be accomplished through the modification of the leader plan to handle the new belief dropped.

Note
recall the existence of the action .broadcast(Type, Content) which acts as the action .send, except that the message is sent to every agent.

Solution: available here.

Exercise h)

Introduction: Now we want the winning miner to brag about it.

Objective: modify the current miner.asl specification file in a way that every time a miner receives a message telling it that it is the now winning, it prints a message bragging about it.

Description: the objective can be accomplished through the addition of a suitable plan to handle the new belief resulting from the message broadcasted by the leader.

Tip
the internal action .my_name(X) instantiates X with the agent’s name.

Solution: available here.

Part IV

[Bonus]

Exercise i)

Introduction: The way the miner.asl specification was implemented is not very efficient because it is too committed to picking up some gold piece, neglecting any gold pieces it finds on its way. It is often the case that a miner walks by a gold piece and doesn’t pick it up, simply because it is set to go and pickup another one.

Objective: modify the current miner.asl specification file in a way that miners pick up a gold piece if it is located in its cell, even if it is on its way to picking up another one.

Description: the objective can be accomplished through the addition of a suitable plan to handle the new event of the form +cell(X,Y,gold) when the agent is not carrying gold, but it is also not free.

Note
this is a more advanced problem. The miner will have to determine its current desire (the internal action .desire(handle(gold(X, Y))) to query the current desires can help) and determine whether the new gold is closer than the one in the current desire. Then, if it is, the miner should drop the current desire (.drop_desire(handle(gold(X, Y)))) and adopt a new goal.

Solution: available here.

In the Jason distribution file you will find a more detailed (and efficient) implementation of the miners team.

Exercise j)

Alternative solution for exercise f): to post a message at Twitter announcing the current winner.

The following steps will prepare the environment for the leader to access the twitter.

  1. Go to your Twitter account and create an application (it is a simple form): https://apps.twitter.com/app/new.

  2. Go to the "permission tab" and change it to "read and write". The API tab will show important parameters for the next step.

  3. Add an artifact responsible for interacting with Twitter in the .jcm file (replace the four parameters by the values given by Twitter in the "API tab" above):

workspace web {
   artifact tweetTool : cartago.tools.twitter.Twitter("your API key","your API secret", "your Access token", "Access token secret") {
      focused-by: leader
   }
}

Now you can replace the print message in the leader (solution of exercise f) by a tweet! The above artifact provides the action tweet(M), where M is a string to be posted at Twitter. The .concat internal action is quite useful to compose the string, see documentation here.

Part V

This part of the tutorial revisit the agent concepts and their relation with the programming language.

Agent definition

Considering the usual definition of agent as

An intelligent agent is a computer system capable of flexible autonomous action in some environment.
Flexible = reactive, pro-active, social
— Wooldridge

How do you evaluate the behaviour of the miner agents regarding this definition? Are the agents reactive, pro-active, social, autonomous?

BDI agents

  1. Where in the code of the agent we can see the BDI concepts?

  2. Can a Jason agent handle its own intentions? Where in the source code of the miners it can be noticed?

  3. How the practical reasoning (deliberation, means-ends, …​) is implemented in Jason? And the theoretical reasoning?