flex Expert System Toolkit - LPIS

7. Knowledge Specification Language 96
Constraints
Demons
flex toolkit
A constraint is a validity check which can be attached to an attribute of a
frame. It is automatically invoked whenever the value for that slot changes.
The checks associated with a constraint are executed immediately before the
value of the slot is to be changed, and the value only changes if this check
succeeds. If the check fails then the slot is not updated, and the update itself
will fail.
Note that a demon may be used to perform checks after a slot value has
changed.
The general format of a constraint is as follows.
constraint constraint_name
when Attribute changes
from Expression1
to Expression2
and condition1(s)
then check that condition2(s)
otherwise directive(s) .
The from, to, the conditions1 and the otherwise are all optional.
Example
This example checks that a vessel is never filled to beyond its capacity. If
the proposed value is invalid, an error message is written (the otherwise
part of the constraint) and the slot update does not take place.
constraint maximum_contents_of_vessel
when the contents of Vessel changes to X
and Vessel is some vessel
then check that number( X )
and X =< the Vessel `s capacity
otherwise write( 'Illegal contents of vessel ' )
and write( Vessel )
and write( ' Contents ' )
and write( X )
and nl .
This next example makes sure that the number of widgets in a
container never falls below zero.
constraint minimum_widgets
when the widgets of Container changes to N
and Container is some container
then check that N > 0
otherwise Container`s widgets becomes 0.
A demon is a procedure which can be attached to an attribute of a frame. It