flex Expert System Toolkit - LPIS

4. Data-Driven Programming 44
Constraining the Values of Slots
A constraint is attached to an individual slot, and is designed to constrain
the contents of a slot to valid values. It is activated whenever the current
value of that slot is updated, and the activation occurs immediately before
the update.
A constraint has three main parts:
• context
• check
• error
A test to see whether certain conditions hold.
A test to see whether the update is valid.
A series of commands to be performed for an invalid update.
The check will only be made if the context holds. If the check is successful
then the update is allowed, otherwise the error commands are invoked and
the update is not allowed.
Example
If we had a frame system representing instances of water containers, we
could put a constraint on the contents slot of any jug, such that when
the value for the particular slot is being updated, a test is performed making
sure that the new value is less than the value of the jug`s capacity, thus
ensuring the jug does not overflow!
In the example the constraint is activated if the contents attribute of any
jug changes. The prospective new value for the slot is then tested to see if it
is less than that jug's capacity. If the test succeeds the update is allowed. If
the test fails the update is not allowed and a message is displayed.
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