Soar Tutorial - the Cognitive Systems Group

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3.1 Hello World Operator: English Version
To demonstrate a very simple operator, you will now write an operator instead of a single rule to print
“Hello World”. Operators allow you to have an action considered in multiple situations (rules that propose
the operator), allow multiple reasons for selecting an action (rules for selecting operators), and allow
multiple ways for doing it (rules that apply the operator). For printing “Hello World”, an operator is not
necessary because there are no alternatives, but as soon as other actions are possible and choices must
be made, operators become necessary.
To use an operator, we need two rules: one to propose the operator and one to apply it:
Propose*hello­world:
If I exist, propose the hello­world operator.
Apply*hello­world:
If the hello­world operator is selected, write “Hello World” and halt.
The first rule proposes the hello-world operator, and the second performs its actions after the operator
has been selected. Notice that the first rule only proposes the hello-world operator. As described earlier,
an operator is selected by Soar’s decision procedure, which collects together proposed operators and
selects one.
To see this operator run, you can either create a new agent and then destroy your previous agent (this is
done on the same pull down menu that was used to create an agent), or you can remove all of the
existing rules in your agent by using the “excise ­­all” command, which removes all of the rules
from memory and initializes Soar. Next, load in the hello-world-operator file using source. To see the
individual productions fire, you can modify the “watch” level. There are many different watch levels: and
watch level 3 (and higher) includes traces of production firings. Go ahead and set the highest level:
watch 5. You can use the “Watch 5” button at the bottom of the interaction window or use the Debug
Level entry in the menu bar at the top. Then click the run button.
The interaction window allows you to trace the execution of Soar at many different levels (decisions,
phases, production firings, working memory changes), but selectively view the trace by expanding and
collapsing the trace (using the “+” and “-” items on the left of the trace) and setting filters. Make sure you
have the “tree view,” which is set under the layout tab in the top tool bar. A standard practice is to do a full
trace (Watch 5), but only view decisions by collapsing the trace. If an error arises, or you want to look at
the behavior in more detail, you can expand the trace. The button at the lower right corner of the
interaction window allows you to toggle between “collapse” and “expand” easily. You can then control the
level of the trace by using the “filters” button that is next to the collapse/expand button. This allows you to
view the trace at a high level and only dive into the details when necessary.
After typing run, expand the “run” and “1: O: O1 (hello-world)” lines by clicking on the “+” signs next to
them. The trace shows that propose*hello-world fires first to propose the operator. The operator helloworld
(O1) is then selected (by the decision procedure). After hello-world is selected, apply*hello-world
fires and performs the actions of the operator by printing out “Hello World” and halting.
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