Practical Temporal Projection

Steve Hanks

Temporal projection-predicting future states of a changing world-has been studied mainly as a formal problem. Researchers have been concerned with getting the concepts of causality and change right, and have ignored the practical issues surrounding projection. In planning, for example, when the effects of a plan’s actions depend on the prevailing state of the world and that state of the world is not known with certainty, projecting the plan may generate an exponential number of possible outcomes. This problem has traditionally been eliminated by (1) restricting the domain so the world state is always known, and (2) by restricting the action representation so that either the action’s intended effect is realized or the action cannot be projected at all. We argue against these restrictions and instead present a system that (1) represents and reasons about an uncertain world, (2) supports a representation that allows context-sensitive action effects, and (3) generates projections that reflect only the significant or relevant outcomes of the plans, where relevance is determined by the planner’s queries about the resulting world state.

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