flex Expert System Toolkit - LPIS

11. Example – Robbie Goes Shopping 163
required. The following frame is used as a mould for generating new carrier
bags.
frame carrier
default contents are empty .
default contents_count is 0 .
demon contents_counter
when the contents of Bag changes
and Bag is an instance of carrier
then add 1 to the contents_count of Bag .
This demon will be activated whenever we change the contents of any
carrier.
Packing an item can be thought of as follows.
To pack an item, first choose a carrier bag for the item, then remove the
item from the shopping list and add the item to the contents of the bag.
action pack_item( Item );
if choose_bag( Bag, Item )
and remove the Item from the shopping
and include the Item in the contents of the Bag
and write( 'Packing ' ) and write( Item )
and write( ' into ' ) and write( Bag ) and nl .
We now need to establish how to choose a carrier bag for an item. This may
be done using a backward chaining relation, which can be thought of as
follows.
To choose a bag for an item, get an existing carrier bag, and check that
the number of items already in the bag is less than the maximum
allowed for the item about to be packed.
The first clause of the choose_bag relation is:
relation choose_bag( Bag, Item )
if the Bag is some carrier
whose contents_count is less than
the maximum_allowed of Item .
Whenever a carrier bag is needed it is always better to check existing bags
(indicated by the keywords Bag is some carrier) if at all possible.
The second clause of the choose_bag relation is
relation choose_bag( Bag, Item )
if gensym( bag_number_ , Bag )
and Bag is a new carrier .
Note that gensym/2 is a built-in predicate of the underlying Prolog system
for generating unique names from a given root symbol (bag_number_ in
flex toolkit