flex Expert System Toolkit - LPIS

11. Example – Robbie Goes Shopping 158
The Shopping Question
To obtain the initial shopping list, we ask the user to choose some items
from the range of goods available.
It is convenient to group the goods under a single name which may then be
referenced by a question.
group goods
bread, butter, coffee, ice_cream, crisps,
salted_peanuts, beer, lemonade, washing_powder .
The following question may then be written.
question shopping
What is on your shopping list today ? ;
choose some of goods
because I must know your choice to continue .
The first line of the question is the actual text to be displayed when the
question is asked.
The second line indicates that the question will take the form of a multiplechoice
menu containing all the items in the group goods.
The third line contains the text to be displayed if the user enquires why the
question is being asked.
The Compatibility Rules
flex toolkit
Having obtained the initial shopping list from the user, we will use forward
chaining rules to check the compatibility of the items on the initial shopping
list to create the final shopping list.
The set of goods needs to be differentiated further in the terms of the
compatibility criteria, i.e. if there are snacks on the shopping list then there
must also be a drink.
For convenience we define the two groups for snacks and drinks.
group snacks
crisps, salted_peanuts .
group drink
lemonade, beer .
We can now write the rules for the compatibility checks.
We need a rule to ensure that if the shopping includes some snacks there is