The Logos Programming Language

package graph_reach;
use logos.std.wql.wql;           // the deem query engine
use logos.mem.collections.vec;

struct Edge { pub src: i64, pub dst: i64 }

// A recursive Datalog query as a language item, prepared at
// compile time: transitive reachability over a graph of edges.
pub deem reach(edges: &[Edge], start: i64) {
    rel path(a: i64, b: i64) {
        from edges e select (e.src, e.dst);
        from path p join edges e on p.b == e.src select (p.a, e.dst);
    }
    from path p where p.a == start select p.b order by p.b
}

fn main() -> i32 {
    let edges: [Edge; 4] = [
        Edge { src: 1i64, dst: 2i64 },
        Edge { src: 2i64, dst: 3i64 },
        Edge { src: 2i64, dst: 4i64 },
        Edge { src: 2i64, dst: 3i64 },   // duplicate — set semantics dedup it
    ];
    // Everything reachable from node 1, transitively:
    let hits: Vec<i64> = reach(&edges[..], 1i64).unwrap();   // → [2, 3, 4]
    return hits.len() as i32;   // 3
}