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5 Designing a Reactive Implementation
wall (left) and follow_wall(right). Each instance of follow wall
would receive the sonar polar plot and extract the relevant wall. The associ-
ated class diagram is shown on the right in Fig. 5.2.
In both implementations, the motor schema schemas ran continuously and
the vectors were summed internally in order to produce a single output
vector. Since there were multiple motor schemas, the coordinated control
program for follow-corridor is not null as it was for move-to-goal. The vec-
tor summation and the concurrency form the conceptual coordinated control
program in this case.

5.2.3 Where do releasers go in OOP?
The previous examples showed how behaviors can be implemented using
OOP constructs, such as classes. Another important part of a behavior is how
TWO PURPOSES OF it is activated. As was discussed in Ch. 3, perception serves two purposes:
PERCEPTION to release a behavior and to guide it. Perceptual schemas are clearly used for
guiding the behavior, either moving toward a distinctively colored goal or
following a wall. But what object or construct contains the releaser and how
is it “attached” to the behavior?
The answer to the first part of the question is that the releaser is itself a per-
ceptual schema. It can execute independently of whatever else is happening
with the robot; it is a perceptual schema not bound to a motor schema. For
example, the robot is looking for red Coke cans with the extract_color
perceptual schema. One way to implement this is when the schema sees
red, it can signal the main program that there is red. The main program can
determine that that is the releaser for the move to goal behavior has been
satisfied, and instantiate move_to_goal with goal=red. move_to_goal
can instantiate a new instance of extract_color or the main program
can pass a pointer to the currently active extract_color. Regardless,
move_to_goal has to instantiate pfield.attraction, since the attrac-
tion motor schema wouldn’t be running. In this approach, the main program
is responsible for calling the right objects at the right time; the releaser is at-
tached to the behavior by the designer with little formal mechanisms to make
sure it is correct. This is awkward to program.
Another, more common programming approach is to have the releaser be
part of the behavior: the single perceptual schema does double duty. This
programming style requires a coordinated control program. The behavior
is always active, but if the releaser isn’t satisfied, the coordinated control
program short-circuits processing. The behavior returns an identity function,