flex Expert System Toolkit - LPIS

7. Knowledge Specification Language 97
is automatically invoked whenever the value for that slot changes.
The directives associated with a demon are executed immediately after the
slot value changes.
A demon can be tailored such that it fires only for given values and/or only
under certain circumstances.
Note that a constraint may be used to perform checks before a slot
value is changed.
The general format of a demon is as follows.
demon demon_name
when Attribute changes
from Expression1
to Expression2
and condition(s)
then directive(s) .
The from and to and the conditions are all optional.
Examples
The first example displays information about the contents of any jug
when its value changes.
demon spy_the_contents
when the contents of any jug changes from X to Y
then write( 'jug change ... ' )
and write( Y - X )
and nl .
The second example checks the changes in temperature value (in any
frame), and takes necessary actions when it rises above boiling point.
demon check_for_melt_down_of_core
when the temperature changes to T
and T is above boiling_point
then remember that danger_level( red )
and shut_down .
Watchdogs
A watchdog checks the access rights to an attribute of a frame. It is
automatically invoked whenever there is a request for the current value (not
the default value) of that slot.
The checks associated with a watchdog are executed immediately before the
value is accessed. If the check fails then the access call also fails.
The general format of a watchdog is as follows.
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