Say we have a Datalog file example.dl, whose contents are as shown.
.decl edge(x:number, y:number)
.input edge
.decl path(x:number, y:number)
.output path
path(x, y) :- edge(x, y).
path(x, y) :- path(x, z), edge(z, y).
We see that edge is a .input relation, and so will be read from disk. Also, path is a .output relation, and so will be written to disk.
The last two lines say that 1) “there is a path from x to y if there is an edge from x to y”, and 2) “there is a path from x to y if there is a path from x to some z, and there is an edge from that z to y”.
So if the input edge relation is pairs of vertices in a graph, by these two rules the output path relation will give us all pairs of vertices x and y for which a path exists in that graph from x to y.
Last modified 25 May 2023