]> Witch of Git - mukan/summary
descriptionA Rust implementation of MicroKanren.
last changeMon, 17 Feb 2020 16:58:58 +0000 (17:58 +0100)


mukan = { git = "https://git.witchoflight.com/mukan" }

(Micro Kanaya :3)

This is a Rust implementation of MicroKanren, based on the very readable original paper at: http://webyrd.net/scheme-2013/papers/HemannMuKanren2013.pdf.

There's a nice Clojure implementation that I also found very informative, for getting another perspective on the unification and an idea of where the natural extension points are: https://mullr.github.io/micrologic/literate.html.


You build up goals with goal constructors, and then execute them with the solve method of Goal, running that on an initial state. One of the simplest possible programs you could write is:

use mukan::{Goal, call_fresh, State, eq};

fn main() {
    let goal = call_fresh(|x| eq(413, x));
    for soln in goal.solve(State::new()) {
        println!("{}", soln);

This will create a goal that wants to solve 413 = X, and then show you all of the solutions starting with an empty state. You can get slightly fancier, with problems that have multiple solutions:

let goal = call_fresh(|x| or(eq(612, x), eq(1025, x)));

This will produce two solutions, assigning x to 612 and 1025 in them, respectively. There are macros that make several of the features easier to use. Here, we can use fresh! to create multiple variables, and all! to and together an arbitrary number of goals. You can also work with pairs of variables, letting you produce some more interesting cases.

let goal = fresh!((a, b, c) => all! {
    eq((a, b), c),
    eq((b, 8), c),

Here this should solve to a = 8, b = 8, and c = (8, 8). Of course, the solution that gets printed is messier, so you might want to ask for a solution in terms of the raw values. You can get the entire state evaluated like this with the reify function, but if you just want to get a few particular variables, then you can use reify_vars with variables that you own.

One way to get those variables, is make your variables in a particular state:

let (vars, state) = State::with_vars(2);
let (a, b) = (vars[0], vars[1]);
let goal = fresh!((c) => all! {
    eq((a, b), c),
    eq((b, 8), c),
for soln in goal.solve(state) {
    println!("{:?}", reify_vars(&vars, soln));

You can write recursive goals as rust functions, as well!

fn peano(n: impl Into<Term>) -> impl Goal {
    let n = n.into();
        eq(n.clone(), 0),
        fresh!((x) => and(
            eq(n.clone(), (1, x)),
            delay(move || peano(x)),

We need the delay() in order to prevent the goal from being constructed forever without being searched at all. If you try to solve peano with a variable, then you'd get an infinite stream of assignments that satisfy it. Here we just take the first 10:

let (vars, state) = State::with_vars(1);
let goal = peano(vars[0]);
for soln in goal.solve(state).take(10) {
    println!("{:?}", reify_vars(&vars, soln));
2020-02-17 Cassie JonesMake the Bind iterator for AND goals fair develop
2020-02-17 Cassie JonesFormatting fix
2020-02-17 Cassie JonesAllow goals to yield to other goals while working
2020-02-17 Cassie JonesDerive Eq for Term for performance
2020-02-17 Cassie JonesEvaluate larger peano for performance evaluation
2020-02-17 Cassie JonesChange Subst to store a single mapping for vars
2020-02-17 Cassie JonesMove the terms out into a separate file
2020-02-17 Cassie JonesAdd a README
2020-02-16 Cassie JonesWhat a horrible night to have an initial commit
4 years ago develop