Plans

We regard a plan as a sequence of operators assembled prior to their execution, and designed to satisfy an explicit goal. Operators take as their input a state and return a state: visually, they are the arcs joinging the nodes (states). Typically the representation of the operators requires that they incorporate some representation of the goal to which they contribute, preconditions which are known to be necessary, some effects and the actions that achieve these effects.

Here is an operator table that might have been used by SHAKEY to move objects around a room.

The operator column indicates the operator name. However, the entries are more than just a name - go(X,Y) indicates that the go operator requires two inputs. Note also that it is important that the operators are parameterised. Some other terms:

Preconditions:

things that must be true in order that the operator can be applied

Delete List:

things that cease to be true after the application of the operaotr

Add List:

things that become true after operator application

Operator tables are used by STRIPS in three different ways:

1. Operator Selection: we have to choose some operator so we take the current goal and see which of the operators might be effective in satisfying the goal. The implication is that the current goal should be in the Add List of a possible operator.
2. Operator Application: if the preconditions are satisfied then we have to delete the items in the Delete List and add the items in the Add List.
3. Subgoaling: if, in trying to satisfy the preconditions of a selected operator, a precondition fails then introduce the failing precondition(s) as a new subgoal(s).