Decision Making - Learning Game AI Programming with Lua - page 354

Creating evaluators
Creating evaluators relies on simple functions that perform isolated operations on the
agent's userData table. Typically, most evaluators will only perform calculations based
on userData instead of modifying the data itself, although there is no limitation on doing
this. As the same evaluator might appear within a decision structure, care must be taken to
create consistent decision choices.
Tip
Evaluators that modify userData can easily become a source of bugs; try to avoid this if
at all possible. Actions should modify the agent's state, when evaluators begin to modify
the agent's state as a decision structure is processed; it becomes very difficult to tell how
and why any eventual action was chosen, as the order of operations will affect the outcome.