4 releases

0.1.3 May 4, 2021
0.1.2 Apr 28, 2021
0.1.1 Apr 14, 2021
0.1.0 Apr 14, 2021

#300 in WebAssembly

Used in hotdrink-wasm




HotDrink implemented in Rust.

HotDrink lets you describe relations between values declaratively and how to enforce them, and can then automatically do so when the value of a variable changes.

Crates.io docs.rs


Before getting started, here is a quick introduction to the terminology and how it works. A Component is a set of variables with a set of Constraints between them. A Constraint consists of a set of Methods that are essentially functions that enforce the constraint by reading from some subset of the variables of the Component and writing to another. Components can be gathered in a ConstraintSystem, which provides an API for interacting with multiple Components at once, such as update.


A component is a collection of variables and constraints between them that should be enforced. One can easily be created by using the [component!] macro, as shown in the example below.


A constraint represents a relation between variables we want to maintain. It contains a collection of constraint satisfaction methods that describe the different ways to do so. In the example, we want the relation a + b = c to hold at all times. One way to enforce it is to re-compute a + b and set c to that value.


A constraint satisfaction method describes one way to enforce a constraint. It reads the values of some variables, and write to others.


Add the following to your Cargo.toml:

hotdrink-rs = "0.1.1"

Then you are ready to begin!

use hotdrink_rs::{component, model::ConstraintSystem, ret, Event};

// Define a set of variables and relations between them
let mut component = component! {
    // Define a component `Component`.
    component Component {
        // Define variables and their default values.
        // The value can be omitted for any type that implements `Default`.
        let a: i32 = 0, b: i32, c: i32 = 3;
        // Define a constraint `Sum` that must hold between variables.
        constraint Sum {
            // Provide three ways to enforce the constraint.
            // Only one will be selected, so each one *MUST* enforce the constraint.
            abc(a: &i32, b: &i32) -> [c] = ret![*a + *b];
            acb(a: &i32, c: &i32) -> [b] = ret![*c - *a];
            bca(b: &i32, c: &i32) -> [a] = ret![*c - *b];

// Describe what should happen when `a` changes.
component.subscribe("a", |event| match event {
    Event::Pending => println!("A new value for `a` is being computed"),
    Event::Ready(value) => println!("New value for `a`: {}", value),
    Event::Error(errors) => println!("Computation for `a` failed: {:?}", errors),

// Change the value of `a`
component.set_variable("a", 3);

// Enforce all the constraints by selecting a method for each one,
// and then executing the methods in topological order.

// Add the component to a constraint system.
// One constraint system can contain many components.
let mut cs = ConstraintSystem::new();

// Update every component in the constraint system.


The project uses multiple nightly features, and must be built using nightly Rust. I recommend using rustup, which can be downloaded here.

The examples in ./examples can then be run with cargo run --example <name>.


Licensed under either of

at your option.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.


~53K SLoC