A comparative analysis of partial order planning and task reduction planning

Abstract

Although task reduction (HTN) planning historically preceded partial order (PO) planning, and is believed to be more general than the latter, very little comparative analysis of the two planning formalisms has been done. Part of the reason for this has been the lack of systematic understanding of the functionalities provided by HTN planning over and above that of partial order planning. In this paper I will describe a generalized algorithm template for partial order planning based on refinement search, and extend it to cover HTN planning. I will use this framework as a basis to ( i ) discuss the similarities and differences between the HTN and the partial order planning methods, ( ii ) critically examine the claims regarding the efficiency and expressiveness of HTN planning, and ( iii ) shed light on several of the less understood features of HTN planning.