Addendum to "HTN Acting: A Formalism and an Algorithm"

TL;DR

This paper provides a formal framework for HTN acting, enabling interleaved planning, acting, and failure recovery, and extends traditional HTN planning to better integrate with agent systems.

Contribution

It introduces a formal account of HTN acting that supports interleaved deliberation, acting, and failure recovery, building on the most general HTN formalism.

Findings

Formal semantics for HTN acting established

Algorithm for integrating exogenous events developed

Analysis of properties and relation to HTN planning conducted

Abstract

Hierarchical Task Network (HTN) planning is a practical and efficient approach to planning when the 'standard operating procedures' for a domain are available. Like Belief-Desire-Intention (BDI) agent reasoning, HTN planning performs hierarchical and context-based refinement of goals into subgoals and basic actions. However, while HTN planners 'lookahead' over the consequences of choosing one refinement over another, BDI agents interleave refinement with acting. There has been renewed interest in making HTN planners behave more like BDI agent systems, e.g. to have a unified representation for acting and planning. However, past work on the subject has remained informal or implementation-focused. This paper is a formal account of 'HTN acting', which supports interleaved deliberation, acting, and failure recovery. We use the syntax of the most general HTN planning formalism and build on its core semantics, and we provide an algorithm which combines our new formalism with the processing of exogenous events. We also study the properties of HTN acting and its relation to HTN planning.