The Semantics of MALLET--An Agent Teamwork Encoding Language

1. The Semantics of MALLET--An Agent Teamwork Encoding Language • Xiaocong FanSchool of Information Sciences and Technology The Pennsylvania State University • July 19, 2004

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

3. Team-Oriented Programming • Language level • Tidhar: Constructs for describing team behaviors based on mutual beliefs, joint plans and social structures • Methodology level • Pynadath, et al: Mechanism (reusable team wrapper) for supporting rapid development of agent teams

4. The state of art: Encoding teamwork knowledge • Atomic team operators • Tidhar’s TOP supports atomic team operators • Joint team activities: long-term process involving multiple agents • The notion of team operator in STEAM • Shared team plans: common recipes that govern the collaboration behaviors • GRATE* has a higher-level language support • RETSINA uses the concept of shared plans, but no encoding language • JACK Teams uses Java

5. The state of art: Dynamic task assignment • Role-capability matching • STEAM, JACK Teams • Constraints reasoning • RETSINA NO language-level support for specifying task-allocations within team plans

6. The state of art: Complex teamwork process • GRATE*, JACK Teams, and TOP provided control constructs for coding complex team behaviors • Par, while, if, etc. • However, none of the existing approaches support the coding of decision points in a team plan

7. Types of information needs • Action-performing information-need • Decision-making information-need • Goal-protection information-need • Goal-escape information-need

8. 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

9. Syntax

10. Syntax

11. 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.

12. 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\'].

13. 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 gG, 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]).

14. 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

15. 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,

16. 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.

17. Semantics—termination of plans

18. Semantics—goal selection

19. Semantics—end of intention

20. Semantics—Agent selection

21. Semantics—Sequential execution

22. Semantics—individual operator

23. Semantics—team operator

24. Semantics—joint-do

25. Semantics—plan

26. Semantics—choice

27. Semantics—parallel

28. Semantics—composite plans

29. An implemented interpreter: CAST

30. 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