bagof or setof

Consider the following database :

age(peter,7).
age(anne,5).
age(pat,8).
age(tom,5).

Q: bagof(Child, age(Child,5), List)
A: List = [anne,tom]

• The bagof primitive is used like findall to obtain multiple solutions (and has the same arguments as findall)
• In the above example if the age is also left unspecified, then we obtain lists according to the ages involved

Q: bagof(Child, age(Child,Age), List)
A: Age = 7
   List = [peter];

   Age = 5
   List = [anne, tom];

   Age = 8
   List = [pat];

Q: bagof(Child, Age ^ age(Child,Age), List)
A: List = [peter,anne,pat,tom]

where ^ : is a predefined infix operator

• Essentially saying that we do not care about the value of age, as long as a value exists
• bagof fails if goal to satisfy fails - in this case this is age(Child,Age)