How to get Prolog to explain your result beyond the true statement

Question

I have the following facts and rules:

  flight(sea,msp).
  flight(msp,jfk).

 route(A,B) :- flight(A,B). 
 route(B,A) :- flight(A,B). 
 route(A,C) :- flight(A,B) , flight(B,C). 

when query for route(sea,jfk) I get a result true but what I wish to get is the explination:

sea-->msp-->jfk this way I can tell not only that it's true but also how it's true.

Answer 1

So you want to get from A to B, but not only that, you also want to know the list of stations of your itinerary.

Be sure to carefully look at the following two related questions and the answers proposed to the question:

• Definition of path/trail/walk
• Definition of Reflexive Transitive Closure

The meta-predicates presented in above links allow you to delegate the handling of recursion to a solid, tested, reusable component. More time to focus on other parts of the problem solving!

Answer 2

You keep track of what nodes in your graph that you've already visited. You need to do this anyway, as you need to detect cycles in your graph lest you fall into the rabbit hole of infinite recursion.

And in Prolog, we use helper methods that carry state around as 1 or more extra arguments. A frequently used convention is to have a "public" predicate — say route/3 that invokes a "private" worker predicate having the same name with a higher arity, say route/4. Something like this ought to do you:

route( A , B , R  ) :- % find a route R from A to B
  route(A,B,[],R)      % - by invoking a worker, seeding its list of visited nodes with the empty list
  .                    % Easy!

route(B,B,V,R) :-    % we've arrived at the destination (B) when the origination node is the same as the destination node.
  reverse([B|V],R)   % - just reverse the list of visited nodes to get the routing.
  .                  %
route(A,B,V,R) :-    % otherwise...
  flight(A,T) ,      % - if there's an edge out of the current node (A) ,
  \+ member(T,V) ,   % - to an as-yet unvisited node...
  route(T,B,[A|V],R) % - go visit that node, marking the current node as visited.
  .                  % Easy!