Prolog rules for small environment

Question

I have several Prolog facts indicating that something or someone is either a person, location or object. I have a clause go(person,location) that indicated that a person moves from where they are to the location given in the clause. However, when I ask the relevant query to find out if someone is at a certain location, Prolog responds with every person that was ever there according to the clauses. How do I go about writing a rule that says that if you are in one location you are by definition not in any of the others?

Answer 1

You probably need to add timing information to your facts. Imagine the following situtation:

go(dad, kitchen, bathroom).
go(dad, bathroom, garage).
go(dad, garage, kitchen).

Since Prolog is (more or less) declarative, in this case, the actual order of the facts in the file does not matter. So, you cannot conclude that dad is in the kitchen, he might have started from and returned to the garage. Even if you add some kind of starting predicate, say startLoc(dad, kitchen), this does not help with loops (e.g. when you add go(dad, kitchen, outside) to the above rules).

If you add timing information (and leave out the previous room, as this is clear from the timing information), this becomes:

go(dad, bathroom,1).
go(dad, garage,2).
go(dad, kitchen,3).

The actual numbers are not relevant, just their order. You can now get the latest location by ensuring that there is no later "go" command with dad:

location(X, Y) :- go(X, Y, T), \+ ( go(X, _, T2), T2 > T ).

Answer 2

It appears that you left one important aspect out when modeling the situation as Prolog facts: When did the person go to the location?

Assume you had instead facts of the form:

person_went_to_at(Person, Location, Time).

then it would be pretty easy to determine, for any point in time, where everyone was, and where they moved to last (and, therefore, are now).