Simple route finding.

gistfile1.hrl

% At the moment there are no loops in any route
path(a, b).
path(a, c).
path(c ,d).
path(d, e).
path(e, f).
 
%%% Calling rule
%% This is the interface the user interacts with. This just saves the
%% user from explicitly typing in the empty list. That empty list is
%% the route accumulator and it starts out empty. Every intermediate
%% node will be added to it.
route(Start, End, Route):-
     route(Start, End, [], Route).


%%% 1 - terminating
%% This clause is true when there is a direct route between two
%% nodes. When this occurs, the route accumulator, which is just a
%% list of every intermeidate node we've passed through, will unify
%% with Route. Up until this point Route has been a free
%% variable. Once this unification has taken place, the call stack
%% will unwind and Route will come back to the top of the query and
%% become the answer.
route(Start, End, Route, Route):-
     path(Start, End).
%%% 2 - recursive
%% This clause is true when there is an indirect route between the two
%% nodes. In this case, Via will be bound to each possible node coming
%% from Start. It is then added to the accumulator list and the search
%% will continue to see if there is a route between Via and the End
%% nodes.
%%
%% If this search if successful, the route will be passed all the way
%% back up to the calling procedure route/3 and back to the
%% interpreter. If it is not successful, the search will simply fail.
route(Start, End, Route_Accumulator, Route):-
     path(Start, Via),
     route(Via, End, [Via|Route_Accumulator], Route).