Impasses and Subgoals
Impasses and Subgoals
When the
preferences generated by the
elaboration phase of Soar
decision cycle do not specify a unique preference (or indifferent
preferences), the
decision procedure reaches an
impasse. The four possible impasses may be characterized as follows:
-
Tie Impasse
A collection of possible alternatives that can not be discriminated.
-
No-Change Impasse
There are no acceptable preferences.
-
Reject Impasse
The only preference is to reject a decision previously made.
-
Conflict Impasse
Of the decisions that can be made, there is contradictory information
about which of the choices is preferred.
Soar's response to an impasse is to automatically create a subgoal to
resolve it. The new goal is then added to the bottom of the
context stack
and in the following decision cycles, productions will fire that will propose
a problem space, state, and operator to resolve the impasse. If, at any point,
these decisions can not be made, another impasse is reached, and a new subgoal
proposed to resolve it. Creating subgoals to resolve
inadequacies in the knowledge about how to proceed toward a goal is known as
universal subgoaling.
This process provides the structure
of Soar's learning mechanism,
chunking. Another characteristic of the
subgoaling process is that subgoals are proposed only when an impasse
occurs. This is different from many AI systems in which subgoals are
proposed by several different mechanisms. Thus, Soar's behavior is completely
impasse-driven.
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