GOLOG David Mui EEL6938. Outline Introduction Situational Calculus GOLOG Personal Banking Assistant Using GOLOG ConGOLOG – GOLOG variant Conclusion.

GOLOGDavid Mui EEL6938

Outline

Introduction Situational Calculus GOLOG Personal Banking Assistant Using GOLOG ConGOLOG – GOLOG variant Conclusion

Introduction

Computers System Embedded in complex environments Software for such systems does not maintain explicit

model of the world Users and designers of the system have a general mental

model of the environment Designers/Programmers

Problematic because they need to reconstruct the model

Difficult to extend because of high level abstraction Solution: GOLOG

GOLOG

What is GOLOG? Logic Programming Language for Dynamic Domains Maintains explicit model of environment domain Can be queried, reasoned at runtime Based on theory of actions and preconditions

An Extension of situational calculus First,Second order logic

Applications of GOLOG? Robotics Artificial Intelligence Mechanical Devices Modeling and Simulation

Situational Calculus

Logic Formalism designed for representing dynamic domains First Order/Second Order logic formulae Actions performed in the world Fluent describe the world state

Can be thought of as properties of the world Situations

Finite sequence of actions Changes to the environment result in Actions. Actions can be parametrized Sequence of actions is described as a situation S0 defined as initial situation constant (no action or

situation)

Situational Calculus Cont.

Binary function do: do(a,s), denotes successor situation based on “a”

(action) on “s” (situation), (i.e. the new situation) Example:

pickup(A ,S0) do(putdown(A) ,do(walk(L), do(pickup(A) ,S0)))

Situational Calculus Cont.

Properties of the environment or world can be seen as fluents

Relational Fluents Truth values that may change

is_carrying(robot, item, s) Functional Fluents

Functions that take the situation as their final argument

Returns a situation dependent valueloc(robot, s)

Creating Axioms from Actions

Actions and effects of the actions are axiomatized Actions have preconditions. World Dynamics are specified by effect axioms

Frame Problem

To define a dynamic world it requires more than just action preconditions and effect axioms

Frame Axioms Defines action invariants of the domain Could be a vast number of frame axiom in a domain Fluents unaffected by the action

Example: If robot picks up an object location does not change.

Solution to the Frame Problem

Generate Successor state Axiom Collect all effect axioms from fluent and make a

completeness assumption Assume it specifies all possibilities the fluent may

change Transform effect axioms to generate successor state

axiom of given fluent

Situational Calculus, Cont.

A domain is defined by the following theory: Axioms defining the world in different situations

Action preconditions

Successor state axioms

Foundational axioms

Complex Actions in GOLOG

Situational Calculus methods described in previous slides can not handle complex actions and reasoning Procedures Loops Nondeterministic actions

Need to define complex actions with additional symbols

Complex Actions, cont.

Define Complex Actions using extralogical symbols (e.g., while, if, etc.)

Extralogical expressions are macros that expand into formulas

Do(δ, s, s`) is the basic abbreviation in the GOLOG language,

where δ is a complex action expression, for complex

operations

Do(δ, s, s`) means that executing δ (complex action) in

situation “s” has s` as a terminating situation

Complex Actions, cont.Complex Actions, cont.Complex Actions, cont.

1.Primitive Actions

Complex Actions, cont.

2. Test Actions

3. Sequence

Complex Actions, cont.

4. Nondeterministic choice of two actions

5. Nondeterministic choice of two arguments

6. Nondeterministic Iterations

Complex Actions, cont.

Conditional and loops definition in GOLOG

Procedures difficult to define in GOLOG No easy way of macro expansion on recursive

procedure calls to itself

Complex Actions, cont.

Create auxiliary macro definition: For any predicate symbol P of arity n+2 taking a pair of situation arguments

Define a semantic for procedures utilizing recursive calls

GOLOG in a Nutshell

GOLOG programs are executed uses a theorem prover User supplies, axioms, successor state axioms, initial situation

condition of domain, and GOLOG program describing agent behaviour

Execution of program gives:

Example GOLOG

Elevator Controller Example Primitive Actions

Up(n): move the elevator to a floor n Down(n): move the elevator down to a floor n Turnoff: turn off call button n Open: open elevator door Close: close the elevator door

Fluents CurrentFloor(s) = n, in situation s, the elevator is at

floor n On(n,s), in situation s call button n is on NextFloor(n,s) = in situation s the next floor (n)

Example, cont.

Primitive Action Preconditions

Successor State Axiom

Example, cont.

One of the possible fluents

Elevator GOLOG Procedures

Example, cont.

Theorem proving task

Successful Execution of GOLOG program

Returns the following to elevator hardware control system

Personal Banking Assistant Using GOLOG

Personal Banking Assistant (PBA) Assists users in personal banking over computer

networks Perform transactions based on certain actions,

preconditions, and situations Collection of GOLOG agents that interact Over 2000 lines of GOLOG Code Currently implemented in simulated financial

environment

System Components

Personal Banking Assistant Agents User interface, performs actions directed by user, and

monitors for certain situations Bank Agents

Perform backend bank operations on accounts Transfer Facilitator Agents

Conducts fund transfers between different bank institutions

Router Agents Performs network operations/maintenance

Automated Teller Agents Provides ATM interface to bank agents

System Diagram

PBA Fluents

Fluents used by the PBA to model the world: USERACCOUNT(type, bank, account, balance,

lastUpdate, rateOfReturn, moveFunds, minBalance, penalty, refreshRate, s)

Monitor(type, bank, account, limit, lowerOrHigher, priority, response, monID, s)

ALERT(alertMessage, maxPriority, monID, s) ALERTACKNOLWEDGED(monID,s) WAITINGUPDATE(bank, account, s)

PBA Primitive Actions

SENDMESSAGE(method, recipient, message)

STARTWAITINGUPDT(bank, account)

STOPWAITINGUPDT(bank,account)

CREATEALERT(message, maxPriority, monID)

SENDALERT(priority, message, medium,

monID)

PBA, cont.

ControlPBA Requests balance updates for accounts Process messages Send out alert messages to users

PBA, cont.

RefreshMonitoredAccts Request balance updates for accounts Process new messages Send out new messages to users

PBA, cont.

HandleCommunications Procedure Main message handling loop Reads message from port and dispatches to appropriate

action GenerateAlerts Procedure

Directs agent to monitor triggers defined by user Alerts the user

PBA Results

Pros: GOLOG capable of building useful applications Provides structure for the programmer

Preconditions, successor state axioms Encourages a layered design

Cons: Certain operations are tricky to accomplish

Performing arithmetic Assigning a value to a variable

Limited debugging tools Lack of standard libraries Lack of event driven reactive behaviors

ConGOLOG

ConGOLOG

Extended version of GOLOG that incorporates concurrency Concurrent processes with different priorities High level interrupts Arbitrary actions

ConGolog differs from other formal models of concurrency Allows incomplete information about the environment Allows primitive actions to affect the environment in a complex

way and such changes to the environment can affect the execution of the remainder of the program

New Semantic for Concurrency

ConGOLOG adopts a transition semantic Trans Predicate

Defines a transition relation between two processes Final Predicate

Final process Determines when process is completed

Trans Axioms

1. Empty Program

2. Primitive Action

3. Wait/Test Actions

New Concurrency Constructs

Constructs to handle concurrent programming in ConGOLOG

Other GOLOG Variants

CcGOLOG Incorporates continous change and event driven

behavior GOLEX

Execution and monitoring system, distributed control software

Autonomous mobile robots, sensing and interaction IndiGOLOG

Incremental Interpreter for high level programs involving nondeterminisim and sensing actions

Conclusion

Logic programming for dynamic domains such as robotics, intelligent software agents, and modeling and simulations

GOLOG is based on situational calculus, utilizing first/second order logic and formal theory of actions

Variants (ccGOLOG, ConGOLOG…etc.) To solve weakness such as concurrency, event driven,

sensing

References

Hector J. Levesque, Raymond Reiter, Yves Lesperance, Fngzhen Lin, and Richard B. Scherl. GOLOG: A logic programming language for dynamic domains. To appear in the Journal of Logic Programming, special issue on Reasoning about Action and Change, 1996.

Yves Lesperance, Hector J. Levesque, and Shane J. Ruman. An Experiment in Using GOLOG to Build a Personal Banking Assistant. To Appear in Intelligent Agent Systems: Theoretical and Practical Issues, 1997.

References, cont.

Giuseppe De Giacomo, Yves Lespérance, and Hector Levesque. ConGolog, a concurrent programming language based on the situation calculus. Artificial Intelligence, 121(1-2):109-169, 2000.

Yves Lespérance, Todd G. Kelly, John Mylopoulos, and Eric S.K. Yu. Modeling dynamic domains with ConGolog. In Proceedings of CAiSE-99, Heidelberg, Germany, June 1999.