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9.4.3 Why is this representation useful?
Representing the monkeys and bananas world using situations and fluents might
seem like a lot of effort for very little benefit. There was already a less verbose and
quite workable characterization of this world for planning (figure 9.6). So why bother?
The answer is that, in general, a representation in terms of situations and fluents has
one significant advantage:
A representation in terms of situations and fluents allows characterizing part of a
world without having to know in advance all the other actions and fluents.
Consider again the fluent on_box , for example. Once the actions affecting this fluent
are identified, one can write the successor state clauses:
on_box([climb_on|_]).
on_box([A|S]) :- \+ A=climb_off, on_box(S).
Even if it later becomes important to keep track of a new aspect, say, the color of the
box, with a new action paint_box that changes the color, the successor state clause
for the on_box predicate will not be affected. It continues to work as is.
So a representation in terms of situations and fluents has the advantage of making
a very complex state with a large number of properties and actions more manageable.
9.4.4 Other kinds of actions
It is worth noting some of the complications that arise in planning when other kinds
of actions are included, such as the following.
Durative actions
So far the planning has been done as if the actions (or moves) under consideration
were instantaneous , like turning on a light. But some actions, like filling a bathtub,
take time. This means that plans with a smaller number of actions may take longer to
complete. Among other things, such actions compel us to consider plans where more
than one action is performed simultaneously.
Exogenous actions
In the real world, the actions that one person takes are not the only things that change
the state of the world. Other agents can be performing actions completely outside
of an individual's control, and natural processes like the weather can unfold with-
out anybody doing anything. Finding a plan to achieve a goal becomes much more
challenging in the presence of such exogenous actions by other agents.
 
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