Value Constraint Checking

Many FRSs provide some sort of run-time slot-value constraint checking. Each time a slot value is changed (either locally, or through a change to inherited values), the FRS evaluates constraints that have been defined by the user to specify what values are allowable for a given slot.

Constraint checking is described by two behaviors: :constraint-checking-time controls when constraint checking should be performed, and :constraint-report-time controls when constraint violations should be reported to the user, as follows.

Allowable values of :constraint-checking-time are:

• :immediate -- Constraints are checked wherever slot values change
• :deferred -- Constraints are checked at some user-specified time that may follow long after slot values change
• :never -- Constraint checking is not performed

Acceptable values of :constraint-report-time are:

• :immediate -- Constraint violations are signalled as soon as they are detected
• :deferred -- Constraint violations are recorded for later perusal by the user