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Action description language (ADL) is a planning system in particular for robots. It is considered an advancement of STRIPS.

The sense of a planning language is to represent certain conditions in the environment, and, based on these, automatically generate a chain of actions which lead to a desired goal. A goal is a certain partially specified condition. Before an action can be executed, its preconditions must be fulfilled; after the execution, the action yields effects, by which the environment changes. The environment is described by means of certain predicates, which are either fulfilled or not.

Contrary to STRIPS, the principle of the "open world" applies with ADL: everything not occurring in the conditions is unknown (Instead of being assumed false). In addition, while in STRIPS only positive literals and conjunctions are permitted, ADL allows negative literals and disjunctions as well.

Example

Consider the problem of air freight transport, where certain goods must be transported from an airport to another airport by plane and where airplanes need to be loaded and unloaded.

The necessary actions would be loading, unloading and flying; over the descriptors one could express In(c, p) and At(x, a) whether a freight C is in an airplane p and whether an object x is at an airport A.

The actions could be defined then as follows:

Action (
  Load (c: Freight, p: Airplane, A: Airport)
  Precondition: At(c, A) ^ At(p, A)
  Effect: ¬At(c, A) ^ In(c, p)
)

Action (
  Unload (c: Freight, p: Airplane, A: Airport)
  Precondition: In(c, p) ^ At(p, A) 
  Effect: At(c, A) ^ ¬In(c, p)
)

Action (
  Fly (p: Airplane, from: Airport, to: Airport)
  Precondition: At(p, from)
  Effect: ¬At(p, from) ^ At(p, to)
)