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Planning

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Planning is another search problem, but it is slightly different than 8-puzzle, chess, etc. In a planning problem, we typically do not completely describe a state. Rather, we give just partial descriptions, like on(monkey, floor), at(monkey, 5, 2). There might be lots of other things in the room besides the monkey, but we don't want to describe all those other facts. Breadth-first search, depth-first search, A*, etc. would have trouble with these kinds of states because they are incomplete.

But we can handle these states by treating it as a planning problem. The way the planning algorithm works, and the way we describe planning problems, is slightly different because we want to know how different actions add or remove facts from the state; the action does not describe all the facts of the state that will result.

Here is an example (from your book) to illustrate the problem BFS/DFS/A* would face. Suppose we have 10 airports, 50 planes, and 200 pieces of cargo. All the cargo is at SFO and needs to be at JFK. Planes are assumed to have infinite capacity. There are three actions: load cargo, unload cargo, and fly the plane from one airport to another. Suppose each of the 50 planes can fly to 9 other airports, and that each of the 200 packages can be loaded on any plane. Then we have 9*50*50*200 possible actions at the start; that's 4.5 million possible actions.

Definition of a classical planning problem

Initial state
a conjunction of positive "literals" (no variables), e.g., on(monkey, floor), at(monkey, 5, 2), at(box, 3, 0), on(bananas, box)
Actions
a set of possible actions to take. Each action has an action name, relevant variables, preconditions, and effects. Think of an action like a function with arguments. The preconditions say under what conditions the action is valid; they can refer to the arguments. The effects say what the result of the action is; they can also refer to the arguments, and can include positive and negative effects, sometimes separated as "add" effects and "delete" effects.
Goal state
another conjunction of literals. Both positive and negative literals are allowed. E.g., on(monkey, box), not(on(bananas, box)).

Typical assumptions

  • Atomic time: Each action is indivisible
  • No concurrent actions are allowed (though actions do not need to be ordered with respect to each other in the plan)
  • Deterministic actions: The result of actions are completely determined—there is no uncertainty in their effects
  • Agent is the sole cause of change in the world
  • Agent is omniscient: Has complete knowledge of the state of the world
  • Closed World Assumption: everything known to be true in the world is included in the state description. Anything not listed is false.

(From: http://people.cs.pitt.edu/~litman/courses/cs2710/lectures/ch10RNa.pdf)

Generic planning algorithm

Forward search, like breadth-first or depth-first search, would try any action that met the preconditions, and see what resulted. Eventually, it would find a state that met the goal criteria. This is a bit slow, however, since BFS and DFS are not considering relevant actions.

It is better to work backwards from the goal. Find out first which action would put you in the goal state, then figure out which action is required to meet its preconditions. Go backwards like this until the preconditions for some action are already met by the starting state.

Suppose you want to find actions relevant to the state \(g\). Then a relevant action is one where some of the effects of the action are elements of \(g\). For example, suppose we want on(monkey, box) and some action like climb(X, Y) has as the effect we are looking for if X=monkey and Y=box. Then this action would be relevant. We would look at its preconditions, and meeting those would be the new goal. Thus, we build the plan backwards, so the final plan needs to be reversed before we can actually follow it in the real world. (Note, we also have to be careful that no part of the effects of an action cancel out some part of our goal.)

function find-plan(state, goal):

  # goal not found
  if(initial-state does not meet goal):

    relevant-actions <- Find actions that include part
                        of the goal in their effects

    foreach(relevant-action):

      if(state meets preconditions-of-action):

        # our plan is simply this action itself
        return action

      else:

        # try to find a plan to achieve this action's preconditions
        partial-plan <- find-plan(state, preconditions-of-action)

        if(partial-plan is found):

          # we can achieve this action's preconditions; so the new plan
          # is the partial-plan plus this action itself
          return (action + partial-plan)

  # no action was relevant or its preconditions could not be met,
  # or we are already at the goal, so return no-plan
  return no-plan

The frame problem

Our actions indicate what effects the actions have, e.g., the "climb" action in which the monkey climbs on the box results in the monkey being on the box. But our actions normally do not state all the facts of the world that don't change. But then how can be sure what exactly changed and what didn't? Did the box move as the monkey tried to climb it? How can we be sure? And if we wanted to, how could we list all the things that do and don't change? This might not matter in a toy problem, but in the real world it matters a lot.

The analogy of the stage magician is particularly apt. One is not likely to make much progress in figuring out how the tricks are done by simply sitting attentively in the audience and watching like a hawk. Too much is going on out of sight. Better to face the fact that one must either rummage around backstage or in the wings, hoping to disrupt the performance in telling ways; or, from one's armchair, think aphoristically about how the tricks must be done, given whatever is manifest about the constraints. The frame problem is then rather like the unsettling but familiar 'discovery' that so far as armchair thought can determine, a certain trick we have just observed is flat impossible. — "Cognitive Wheels: The frame problem of AI," Daniel C. Dennett

The frame problem is not the problem of induction in disguise. For suppose the problem of induction were solved. Suppose—perhaps miraculously—that our agent has solved all its induction problems or had them solved by fiat; it believes, then, all the right generalizations from its evidence, and associates with all of them the appropriate probabilities and conditional probabilities. This agent, ex hypothesi, believes just what it ought to believe about all empirical matters in its ken, including the probabilities of future events. It might still have a bad case of the frame problem, for that problem concerns how to represent (so it can be used) all that hard-won empirical information—a problem that arises independently of the truth value, probability, warranted assertability, or subjective certainty of any of it. Even if you have excellent knowledge (and not mere belief) about the changing world, how can this knowledge be represented so that it can be efficaciously brought to bear? — "Cognitive Wheels: The frame problem of AI," Daniel C. Dennett

Limitations of classical planning

Classical planning considers what to do in what order, but not…

  • When actions happen and how long they take.
  • Limited resources that actions may need.
  • Semi-observable environments.
  • Dynamic environments.
  • Uncertain environments.
  • Stochastic (partially random) environments.
Intro to AI material by Joshua Eckroth is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License. Source code for this website available at GitHub.