Calypso Module 6: Conflict Resolution
Learning Goal: In this module you will learn what happens when
rules tell Cozmo to do two different things, but he can only do one of
and the Third Law
Consider this sample program. What do you think it will do?
You could run this program either on Cozmo with a real cube, or in
simulator mode. But before you run it, try to guess what it will do.
Here are some possibilities:
Take your best guess, then run the program and see what happens.
- It could leave the nearest cube glowing orange.
- It could leave the nearest cube glowing green.
- It could make the nearest cube switch back and forth between orange and green.
- It could color one cube orange and another cube green.
- It could refuse to do anything.
Why Did It Do That?
Look at the rules while the program is running. Both WHEN parts are
highlighted, which means both rules can run. But only the first
rule's DO part is highlighted. The second rule's DO part is dimmed
because its action is blocked:
Now switch the order of the rules. You can do this by putting the
pencil on the "WHEN" block, pressing the green "A" button to pick up
the rule, then using the left stick to move the rule up or own, and
pressing the "A" button again to drop the rule in its new location.
Put Cozmo and the cubes back in their original positions and run the
program again now that you've reordered the rules. What happens this
Note: when you have picked up a rule, do not move the stick to the
right, which would indent the rule. If you indent it accidentally,
pick it up and move the stick to the left to un-indent it.
The Third Law of Calypso
By now you've figured out that whichever rule comes first is the one
whose action will be taken. The other rule's action will be blocked.
This is the Third Law of Calypso: "When actions conflict, the earliest
Let's look a bit more closely at this law. First, what does it mean
for actions to "conflict"? In Calypso it means there is no way to do
both actions at the same time. A cube cannot be "glowed" orange and
"glowed" green simultaneously. Cozmo has to choose one of these
actions. Which action should he choose?
The Third Law says that the earliest action wins. "Earliest"
simply means whichever rule appears first on the page. That's why
changing the order of the rules changes the result. If the "glow it
green" rule is earliest, and both rules can run, then "glow it green"
will win and "glow it orange" will be blocked.
Two Pursue Rules
Let's look at another example of the Third Law. In the graphic below,
Cozmo cannot move toward a red cube and move toward and a blue cube at
the same time, especially when they're in opposite directions!
Notice that both rules have the same action: "move toward it". But in
the first rule "it" refers to a red cube, while in the second rule
"it" refers to a blue cube. Cozmo sees both cubes, so both rules can
run. But the first rule's action will override the second rule's
action. That's what the Third Law says: the earliest action wins,
and any conflicting actions are blocked.
sample program can be used to try this out:
You might be surprised to see that Cozmo always goes to the red cube,
even if the blue one is closer. Why do you think he behaves this
- If you're running Calypso, you can load the program by
left-clicking on the link above. Otherwise, right-click on the link, choose
"Save Link As", and save the program to your Documents/Calypso folder,
then run Calypso and load it from there.
- Enter the two pursue rules shown on the Third Law graphic above.
- Arrange the cubes so they are well separated, but Cozmo can still
see them, then run the program.
- Which cube does Cozmo go to? What happens when he gets there?
- Stop the program and rearrange the cubes so that the blue cube
(the "deli slicer") is much closer to Cozmo than the red cube ("the
paperclip"). Make sure he can still see all the cubes.
- Run the program again. Which cube does Cozmo go to?
The First Law vs. The Third Law
To understand why Cozmo always goes to the red cube, not the closest
cube, let's remember what the First Law says: "Each rule picks
the closest matching object." So the first rule will pick the closest
red cube, and the second rule will pick the closest blue cube. Both
rules can run: you can see an LOS indicator for each one showing which
cube it picked, and both WHEN parts are highlighted.
The First Law determines which objects Cozmo pays attention to. It
doesn't say anything about which actions he takes. That's governed by
the Third Law. It's the Third Law that causes Cozmo to always go to
the red cube, because that rule comes first.
Suspending Pursuit So Cozmo Can Recognize Cubes
If the cubes are laid out so that Cozmo cannot see all of them when
the program starts, he will pursue whatever cubes appear on his world
map. You can use the game controller to turn him toward a new cube;
it can take him a couple of seconds to recognize it. But as soon as
you let go of the left stick he will try to pursue one of the cubes
he's already recognized, so you need a way to get him to hold still.
You can do this by holding the right trigger to freeze Cozmo for a few
seconds. You can also do it by pressing down on the left stick
without moving it.
Double Pursue and Consume Program
Can the second pursue rule ever actually pursue a cube? It can, but
only if the first pursue rule can't run -- which could happen if there
were no red cubes. Let's use the Pursue and Consume idiom twice so we
can extinguish both red and blue cubes (but leave the green ones
Run this program and notice that after Cozmo grabs the red cube, "see
red cube" is no longer true; the first rule is dimmed. This allows
the third rule to pursue the blue cube since its action is no longer
Let's try one more case: suppose we have two red cubes and one
blue cube. The Pursue3.calypso sample program can be used
to try this out. If Calypso is running, just click on the link to load
the same program. Then:
After Cozmo grabs the first red cube, he shifts his attention to the
second red cube and begins pursuing that. He drives right past the
blue cube. This is because of the Third Law: since the pursue rule
for red cubes comes first, it blocks any action that conflicts with
But why did Cozmo go to the closer of the two red cubes first? Why
didn't he go to the farther one first? This is due to the First Law:
each pursue rule picks the closest matching object. For the
red pursue rule this would be the closest red cube.
- Cubes 1 and 2 will be red; cube 3 will be blue. You can verify
this by running the program even though Cozmo doesn't have any rules
- Enter the two Pursue and Consume rule pairs as shown above. Also
add a Let Me Drive rule.
- Position the cubes to form a large triangle with Cozmo in the
center. You will have to use the left stick to turn Cozmo so he sees
all the cubes when you run the program.
- In what order do you think the cubes will be grabbed? Will it be 1-2-3?
Will it be 1-3-2? Or something else?
- Run the program and see if you were right. If Cozmo gets stuck
at a cube, use the left stick to nudge him a little.
Rules That Don't Conflict
The Third Law only applies when two rules have conflicting
actions. It's perfectly fine for Cozmo to take several actions at
once, as long as they don't disagree with each other. For example, he
could play a sound and change a cube's color at the same time. The
Third Law does nothing to prevent this.
What are some other examples of things Cozmo could do at the same time
without causing a conflict?
What are some other examples of actions that do cause a conflict?
- Can Cozmo say two things at the same time?
- Can he say something and play a sound at the same time?
- Can he say something and express an emotion at the same time?
Review and Discovery
Do the Module 6 Review and Discovery
activity to review what you've learned.
In the next module you'll learn how to
make Cozmo act one way in unusual situations and another way when
things are normal.
Back to Calypso Curriculum overview.
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