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The Semantics of MALLET--An Agent Teamwork Encoding Language

Xiaocong FanSchool of Information Sciences and Technology

The Pennsylvania State University

July 19, 2004

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Outline

Background on TOP Objectives of MALLET Syntax Semantics An implemented interpreter What’s the distinguishing features?

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Team-Oriented Programming

Language levelTidhar: Constructs for describing team behaviors

based on mutual beliefs, joint plans and social structures

Methodology levelPynadath, et al: Mechanism (reusable team

wrapper) for supporting rapid development of agent teams

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The state of art: Encoding teamwork knowledge

Atomic team operatorsTidhar’s TOP supports atomic team operators

Joint team activities: long-term process involving multiple agentsThe notion of team operator in STEAM

Shared team plans: common recipes that govern the collaboration behaviorsGRATE* has a higher-level language supportRETSINA uses the concept of shared plans, but no

encoding language JACK Teams uses Java

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The state of art: Dynamic task assignment

Role-capability matchingSTEAM, JACK Teams

Constraints reasoningRETSINA

NO language-level support for specifying task-allocations within team plans

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The state of art: Complex teamwork process

GRATE*, JACK Teams, and TOP provided control constructs for coding complex team behaviorsPar, while, if, etc.

However, none of the existing approaches support the coding of decision points in a team plan

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Types of information needs

Action-performing information-needDecision-making information-needGoal-protection information-needGoal-escape information-need

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Objectives of MALLET

A language suitable for encoding agent teamwork knowledge, especially for composing complex teamwork behaviors Expressivity: three-level support, choice points Understandability: high-level Reusability: representation be independent of the context in which

the knowledge is used Encourage inferred team intelligence

support the specification of collaboration constraints, such as action preconditions and plan termination conditions

Allow the specification of adaptive team structure and team process

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SyntaxCompilationUnit

::= ( AgentDef | TeamDef | MemberOf | GoalDef | Start | CapabilityDef | RoleDef | PlaysRole | FulfilledBy | IOperDef | TOperDef | PlanDef | RuleDecl)*

AgentDef ::= "(" <AGENT> AgentName ")"

TeamDef ::= "(" <TEAM> TeamName ( "(" ( AgentName )+ ")" )? ")"

MemberOf ::= "(" <MEMBEROF> AgentName ( TeamName | "(" ( TeamName )+ ")" ) ")"

GoalDef ::= "(" <GOAL> AgentOrTeamName ( Cond )+ ")"

Start ::= "(" <START> AgentOrTeamName Invocation ")"

CapabilityDef ::= "(" <CAPABILITY> ( AgentName | "(" ( AgentName )+ ")" ) ( Invocation | "(" ( Invocation )+ ")" ) ")"

RoleDef ::= "(" <ROLE> RoleName ( Invocation | "(" ( Invocation )+ ")" ) ")"

PlaysRole ::= "(" <PLAYSROLE> AgentName ( RoleName | "(" ( RoleName )+ ")" ) ")"

FulfilledBy ::= "(" <FULFILLEDBY> RoleName ( AgentName | "(" ( AgentName )+ ")" ) ")"

IOperDef ::= "(" <IOPER> OperName “(” (<Variable>)* “)” ( PreConditionList )* (EffectsList )? ")"

TOperDef ::= "(" <TOPER> OperName “(” (<Variable>)* “)” ( PreConditionList )* ( EffectsList )? ( NumSpec )? ")"

PlanDef ::= "(" <PLAN> PlanName “(” (<Variable>)* “)” ( PreConditionList | EffectsList | TermConditionList )* "(" <PROCESS> MalletProcess ")" ")"

RuleDecl ::= "(" ( Pred )+ ")"

Cond ::= Pred | "(" <NOT> Cond ")"

Pred ::= "(" <IDENTIFIER> ( <IDENTIFIER> | <VARIABLE>)* ")"

Invocation ::= "(" PlanOrOperName ( <IDENTIFIER> | <VARIABLE> )* ")"

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SyntaxPreConditionList ::= "(" <PRECOND> ( Cond )+ ( ":IF-FALSE" ( <FAIL> | <WAIT> ( ( <DIGIT> )+ )? | <ACHIEVE> ) )? ")"

EffectsList ::= "(" <EFFECTS> ( Cond )+ ")"

TermConditionList ::= "(" <TERMCOND> ( <SUCCESS> | <FAILURE> )? ( Cond )+ ")"

NumSpec ::= "(" <NUM> ( <EQ> | <LT> | <GT> | <LE> | <GE> ) ( <DIGIT > )+ ")"

PrefCondList ::= "(" <PREFCOND> ( Cond )+ ( ":IF-FALSE" ( <FAIL> | <WAIT> ( ( <DIGIT> )+ )? | <ACHIEVE> ) )? ")"

Priority ::= "("<PRIORITY> ( <DIGIT> )+ ") "

ByWhom ::= AgentOrTeamName | <VARIABLE> | MixedList

MixedList ::= "(" ( <IDENTIFIER> | <VARIABLE> )+ ")"

Branch ::= "(" (PrefConditionsList )? (Priority)? "(" <DO> ByWhomSpec Invocation ")" ")"

MalletProcess ::= Invocation

| "(" <DO> ByWhomSpec MalletProcess ")"

| "(" <AGENTBIND> VariableList "(" <CONSTRAINTS> ( Cond )+ ")" ")"

| "(" <JOINTDO> ( <AND> | <OR> | <XOR> )? ( "(" ByWhomSpec MalletProcess ")" )+ ")"

| "(" <SEQ> ( MalletProcess )+ ")"

| "(" <PAR> ( MalletProcess )+ ")"

| "(" <IF> "(" <COND> ( Cond )+ ")" MalletProcess ( MalletProcess )? ")"

| "(" <WHILE> "(" <COND> ( Cond )+ ")" MalletProcess ")"

| "(" <FOREACH> "(" <COND> ( Cond )+ ")" MalletProcess ")"

| "(" <FORALL> "(" <COND> ( Cond )+ ")" MalletProcess ")"

| "(" <CHOICE> ( Branch)+ ")"

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Preparations—Control messages A control message is a tuple < type, aid, gid, pid,olist >,

type {sync, ctell, cask, unachievable}, aid is the sending agent, gid is a well-formed first-order formula, denoting a goal. pid is a plan id, olist is a list of extra parameters varying from message type to message type.

A message of type sync is used to synchronize with a recipient with respect to the committed goal gid and current activity pid;

A message of type ctell is used to tell a recipient about the status of pid; A message of type cask is used to request a recipient to perform pid; A message of type unachievable is used to inform a recipient of the unachievability of pid.

MALLET has a built-in domain-independent operator send (receivers, msg) Assume pre(send)=true, and the execution of send always succeeds. If <typ,a, …> is a control message, the effect of send (b,<typ,a,…>) will assert

(typ\ a …>) as a fact into agent a's belief base. The recipient of a control message will also assert an appropriate fact into its belief

base. For instance, when agent b receives message <sync, a, g, p>, predicate (sync a g p)

will be asserted into b's belief base.

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Preparations—Goals and Intentions

A goal g is a pair <,A>.

An intention slice is of form (,A) s.

An intention is : [0\...\ k],

where i (0 i k) is of form (i,Ai) si.

The empty intention is denoted by T.If h =[0\...\ k], [h\ '] = [0\...\ k\'].

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Preparations—Configuration

Def. 1 (MALLET configuration): A MALLET configuration of an agent is a tuple < B, G, H, >, where (1) B is consistent,(2) for any goal gG, g is not derivable from B and g is consistent

by itself.

Def. 2 (achievement goal):Function agls is defined recursively as:

agls(T) = {}, for any intention h=[0\...\ k-1\ (k, Ak) sk],

agls(h) = {k} agls([0\...\ k-1]).

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Preparations—Assumptions

A belief update function BU(B,p), which revises the belief base B with a new fact p.

Two domain-independent operators working on B: unsync(,), and untell(,s). Their effects are to remove all the predicates that can be unified with

sync(?a, ,) and ctell(?a, ,s,?id),

respectively, from B. B allows explicit negation. For any b(t), its explicit negation is

denoted by The execution of atomic operators always succeeds

( )b t

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Semantics—beliefs, goals, intentions

Def. 3 :

Given a Mallet configuration M= <B, G, H, >, for any wff , any belief or goal formula ,’, any agent a,

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Semantics—termination of plans

The execution of a plan invocation is terminated (i.e., isTermed() is derivable from the current configuration), iff: On plan entering, the plan precondition does not hold, In plan execution, the termination conditions become true, On plan completion, the expected postcondition is not true.

Note: When isTermed ) is evaluated as true, a predicate of form (termed t) will be asserted into B, so that in later transitions the termination can be propagated upwards to a higher plan level.

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Semantics—termination of plans

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Semantics—goal selection

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Semantics—end of intention

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Semantics—Agent selection

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Semantics—Sequential execution

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Semantics—individual operator

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Semantics—team operator

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Semantics—joint-do

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Semantics—plan

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Semantics—choice

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Semantics—parallel

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Semantics—composite plans

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An implemented interpreter: CAST

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What’s the distinguishing features? MALLET vs. OWL-S

MALLET is designed for encoding team intelligence that requires collaboration

MALLET vs. planning language (e.g., PDDL) Guiding collaboration vs. guiding planning

MALLET vs. JACK Teams High-level vs. lower-level