PDDL2.1 – The Art of the Possible

PDDL2.1 was designed to push the envelope of what planning algorithms can do, and it has succeeded. It adds two important features: durative actions, which take time (and may have continuous effects); and objective functions for measuring the quality of plans. The concept of durative actions is flawed; and the treatment of their semantics reveals too strong an attachment to the way many contemporary planners work. Future PDDL innovators should focus on producing a clean semantics for additions to the language, and let planner implementers worry about coupling their algorithms to problems expressed in the latest version of the language. All things considered, Fox and Long have done a terrific job producing PDDL2.1. I know from experience that getting a committee to agree on a language requires a delicate com-bination of diplomacy and decree. The language extensions that emerged from the 2002 competition are not exactly what anyone wanted, but apparently everyone can live with them. PDDL2.1 is serving as a sturdy basis for evaluating and comparing planning algo-rithms, which is the prime purpose of the language in the first place. It appears that fo