flex Expert System Toolkit - LPIS

4. Data-Driven Programming 45
contents
constraint
check_contents
jug1
jug
update slot
contents of jug1
test
is updated contents
less than capacity
succeed
allow update
fail
fail update
actions
write 'contents not
updated'
flex toolkit
The code for this example could be written as follows:
frame jug
default contents are 0 and
default capacity is 7 .
instance jug1 is a jug .
constraint check_contents
when the contents of Jug changes to X
and Jug is some jug
then check that number( X )
and X =< Jug`s capacity
otherwise write( 'contents not updated' )
and nl .
Note the use of Jug, a local variable, which will unify with any frame or
instance which has, in this case, a contents attribute.
Attaching Demons to Slot Updates
A demon is attached to an individual slot. It is activated whenever the
current value of that slot is updated, and the activation occurs immediately
after the update.
A demon has two main parts: