Aggregate solution over multiple facts

Question

Trying to create a predicate (timePeriod/2) that calculates the time period between two dates for a specific fact. I\'ve managed to do this by myself, but face issu

Answer 1

SWI-Prolog has a nice library to handle aggregation: it builds upon standard 'all solutions' predicates like findall/3,setof/3,bagof/3, so you should first grasp the basic of these (as Boris explained in his answer). With the library, a single query solves your problem:

timePeriod(PS,X) :-
    aggregate(sum(P), B^E^(popStar(PS,B,E),P is E-B+1), X).