The Metric-FF Planning System: Translating "Ignoring Delete Lists" to Numeric State Variables

In particular, modeling context dependent eects, concurrent execution of actions with dierent duration, and continuous resources are all awkward, or impossible, within the STRIPS language. To overcome the rst of these limitations, Pednault (1989) dened the (nowadays widely accepted) ADL language, which amongst other things allows for conditional eects (eects that only occur when their condition holds true in the state of execution). To overcome (one or both of) the latter two limitations, various proposals have been made (e.g., Ghallab & Laruelle, 1994; Koehler, 1998; Smith & Weld, 1999). The most recent eort in this direction is the PDDL2.1 language dened by Fox and Long (2002) as the input language for the 3rd International Planning Competition (IPC-3). The IPC series is a biennial challenge for the planning community, inviting planning systems to participate in a large scale publicly accessible evaluation. IPC-3 was hosted at AIPS-2002, and stressed planning beyond the STRIPS formalism, featuring tracks for temporal and numeric planners. This article describes the approach behind one of the planners that participated in IPC-3, Metric-FF. Metric-FF is an extension of the FF system (that can handle ADL) to numeric constructs.