HTN Planning in Answer Set Programming

Abstract

In this paper we introduce a formalism for solving Hierarchical Task NEtwork (HTN) Planning using Answer Set Programming (ASP). The ASP paradigm evolved out of the stable semantics for logic programs in recent years and is strongly related to nonmonotonic logics. We consider the formulation of HTM planning as described in the SHOP planning system and define a systematic translation method from SHOP's representation of the planning problems into logic programs with negation. We show that our translation is sound and complete: answer sets of the logic programs obtained by our translation correspond exactly to the solutions of the planning problems. Our approach does not rly on a particular system for computing answer sets. It can therefore serve as a means to evaluate ASP systems by using well-established benchmarks from the planning community. We tested our method on various such benchmarks and used smodels and DLV for computing answer sets. We compared our method to (1) similar approaches based on non-HTN planning and (2) SHOP, a dedicated planning system. We show that our approach outperforms non-HTN methods and that its performance is closer to that of SHOP, when we are using ASP systems which allow for nonground programs. (Also UMIACS-TR-2002-18)