Probabilistic Effects

Probabilistic effects are a feature in PDDLSIM that problems can use via the :probabilistic-effects[^1] requirement. Probabilistic effects allow actions to affect the state of the problem in unpredictable ways, by having some part of an action's effects be chosen at random.

Syntax and semantics

A probabilistic effect may be used in any part where an effect is expected, including as sub-effects of other effects (mainly and-effect, i.e., conjunctive effects). They are written as follows:

(probabilistic [<PROBABILITY> <POSSIBLE-EFFECT>]*)

Where [...]* denotes zero or more repetitions of the contents in the square brackets. Each repetition specifies a probability-possible-effect pair, denoting the probability of that possible effect being chosen, where <PROBABILITY> is a decimal number representing the probability of the effects being chosen, and <POSSIBLE-EFFECT> being the relevant effect. Naturally, and-effects and probabilistic effects may be nested.

Note that the total probabilities of having any possible effect occur need not be 1.0, but rather only at most 1.0. For cases where no possible effect is chosen, the probabilistic effect has no effect on the state.

[^1]: This requirement originates from PPDDL, which extends PDDL to model MDPs. See this page for more information.