This page is a tutorial with live Datamog programs. Every code block below is
editable: click ▸ run next to a query to evaluate it (the result appears right
under the line), and click the data chip next to an extensional declaration to
inspect, edit, or reset its rows.
A directed graph is just a set of edges. We declare edge as an extensional
predicate (its rows come from data, not rules) and ask for all of them:
extensional edge(src: string, dst: string).
?- edge(X, Y).
The data for edge is pre-loaded from a CSV file. Open the chip to see the
rows, change one, and run the query again to watch the answer update.
Reachability is the transitive closure of edge. The first rule seeds the set
with everything one step from "a"; the second rule walks one more edge at a
time until nothing new appears:
extensional edge(src: string, dst: string).
reachable(X) :- edge("a", X).
reachable(X) :- edge(Y, X), reachable(Y).
?- reachable(X).
Try adding an edge from d in the data chip, then re-run: the new node shows up
in the reachable set without touching the rules.
A query with no variables asks a yes/no question. This one is satisfied exactly
when "d" is reachable from "a":
extensional edge(src: string, dst: string).
reachable(X) :- edge("a", X).
reachable(X) :- edge(Y, X), reachable(Y).
?- reachable("d").