% graph.dl -- Graph reachability in Datalog
%
% Demonstrates transitive closure computation, a classic
% Datalog application.

% --- Facts: directed edges ---

edge(a, b).
edge(b, c).
edge(c, d).
edge(d, e).
edge(b, d).
edge(a, e).

% --- Rules ---

% Reachability: transitive closure of edge relation
reachable(X, Y) :- edge(X, Y).
reachable(X, Y) :- edge(X, Z), reachable(Z, Y).

% Path of length exactly 2
path2(X, Y) :- edge(X, Z), edge(Z, Y).

% Nodes with no outgoing edges (sinks)
has_out(X) :- edge(X, _).

% --- Queries ---

?- reachable(a, X).
?- reachable(X, e).
?- path2(a, X).