This is a Rust implementation of µKanren, a featherweight relational programming language. See the original Scheme implementation here for reference.

• Structural unification of Scheme-like cons cells.
• Streams implemented with the Iterator trait.
• State representation using a persistent vector with triangular substitutions.
• Conjunction, disjunction, and fresh based on traits (macro-free API).
• Lazy goal evaluation using inverse-η delay.
• Integer, bool, char, &str, and unit type atoms.
• Explicit ToValue trait that converts vectors and arrays into cons-lists.
• Convenience macro state! to inspect and specify state.

Here's a simple example, which defines and uses the appendo predicate.

use ukanren::*;

fn appendo(first: Value, second: Value, out: Value) -> BoxedGoal<impl Iterator<Item = State>> {
    eq(&first, &())
        .and(eq(&second, &out))
        .or(fresh(move |a: Value, d: Value, res: Value| {
            eq(&(a.clone(), d.clone()), &first)
                .and(eq(&(a, res.clone()), &out))
                .and(appendo(d, second.clone(), res))
        }))
        .boxed()
}

let iter = run(|x, y| appendo(x, y, [1, 2, 3, 4, 5].to_value()));
assert_eq!(
    iter.collect::<Vec<_>>(),
    vec![
        state![(), [1, 2, 3, 4, 5]],
        state![[1], [2, 3, 4, 5]],
        state![[1, 2], [3, 4, 5]],
        state![[1, 2, 3], [4, 5]],
        state![[1, 2, 3, 4], [5]],
        state![[1, 2, 3, 4, 5], ()],
    ],
);

More examples can be found in the tests/ folder and the API documentation.