Case Study: Planning Courses of Action - Thinking as Computation - page 198

of the bananas, the color of the bananas, any other objects in the room, whether or
not doors are open, the temperature in the room, the weather outside, the students
enrolled at a nearby university, the price of tea in China, and so on.
What is needed is a representation of the state of the world that is more explicit
about what aspects of the world are affected by what actions. The dynamic pred-
icates considered in section 8.3.2 had this desirable property. One could start
with single(john) in the knowledge base, for example, then use the predicate
get_married(john) in a query, and end up with married(john) , quite independently
of any other predicates. Unfortunately, dynamic predicates cannot be used as is for
planning. They allow changing the state by changing the knowledge base, but they
do not yield sentences that talk about the before and after states simultaneously , which
is what is needed for legal_move .
This section considers a new way to represent the state of the world that is suitable
for planning but makes it possible to talk explicitly about changing aspects of the
world, like the marital status of John or the location of the box.
9.4.1 Situations and fluents
The purpose of a state in planning is to keep track of the relevant aspects of the world
and how these are affected by actions. Instead of representing these aspects in a big
list, another approach is to make a list of all the actions that have been performed, and
then to calculate from this list any relevant aspect.
For example, suppose there is a list of all the actions performed in the monkey and
bananas world, and one wants to calculate the location of the box from this list. The
thinking can go as follows:
If there is no push action in the list, then the box will be located wherever it was
in the initial state.
If there are push actions in the list, and the most recent one performed is
push(loc6) , then the box will be located at loc6 .
So the location of the box is fully determined by a history of all the actions performed,
which is called a situation.
A situation is an ordered list of actions used to represent a state of the world. The
actions will appear in this list most recent ones first. For example,
[] is the initial situation , before any action has been performed;
[ a ] is the situation after the single action a has been performed;