seed 0.1.11

A Rust framework for creating web apps, using WebAssembly


A Rust framework for creating web apps

API Documentation on

The best place to learn is the guide - this readme is an excerpt from it.



This framework requires you to install Rust - This will enable the CLI commands below:

You'll need a recent version of Rust: rustup update

The wasm32-unknown-unknown target: rustup target add wasm32-unknown-unknown

And wasm-bindgen: cargo install wasm-bindgen-cli

The theoretical minimum

To start, clone This quickstart repo, run or build.ps1 in a terminal, then start a dev server that supports WASM. For example, with Python installed, run python (Linux users may need to run python3 Once you change your package name, you'll need to tweak the html file and build script, as described below.

A little deeper

Or, create a new lib with Cargo: cargo new --lib appname. Here and everywhere it appears in this guide, appname should be replaced with the name of your app.

If not using the quickstart repo, create an Html file that loads your app's compiled module, and provides an element with id to load the framework into. It also needs the following code to load your WASM module - Ie, the body should contain this:

<section id="main"></section>

<script src='./pkg/appname.js'></script>

   const { render } = wasm_bindgen;
   function run() {

The quickstart repo includes this file, but you will need to rename the two occurances of appname. (If your project name has a hyphen, use an underscore instead here) You will eventually need to modify this file to change the page's title, add a description, favicon, stylesheet etc.

Cargo.toml, which is a file created by Cargo that describes your app, needs wasm-bindgen, web-sys, and seed added as depdendencies, and crate-type of "cdylib". (The version in the quickstart repo has these set up already) Example:

name = "appname"
version = "0.1.0"
authors = ["Your Name <>"]
edition = "2018"

crate-type = ["cdylib"]

seed = "^0.1.6"
wasm-bindgen = "^0.2.29"
web-sys = "^0.3.6"

# For serialization, eg sending requests to a server. Otherwise, not required.
serde = "^1.0.80"
serde_derive = "^1.0.80"
serde_json = "1.0.33"

A short example

Here's an example demonstrating structure and syntax; it can be found in working form under examples/counter. Descriptions of its parts are in the Guide section below. Its structure follows The Elm Architecture.

extern crate seed;
use seed::prelude::*;
use wasm_bindgen::prelude::*;

// Model

struct Model {
    count: i32,
    what_we_count: String

// Setup a default here, for initialization later.
impl Default for Model {
    fn default() -> Self {
        Self {
            count: 0,
            what_we_count: "click".into()

// Update

enum Msg {

/// The sole source of updating the model; returns a fresh one.
fn update(msg: Msg, model: Model) -> Model {
    match msg {
        Msg::Increment => Model {count: model.count + 1, ..model},
        Msg::Decrement => Model {count: model.count - 1, ..model},
        Msg::ChangeWWC(what_we_count) => Model {what_we_count, ..model }

// View

/// A simple component.
fn success_level(clicks: i32) -> El<Msg> {
    let descrip = match clicks {
        0 ... 3 => "Not very many 🙁",
        4 ... 7 => "An OK amount 😐",
        8 ... 999 => "Good job! 🙂",
        _ => "You broke it 🙃"
    p![ descrip ]

/// The top-level component we pass to the virtual dom. Must accept a ref to the model as its
/// only argument, and output a single El.
fn view(model: Model) -> El<Msg> {
    let plural = if model.count == 1 {""} else {"s"};

    // Attrs, Style, Events, and children may be defined separately.
    let outer_style = style!{
            "display" => "flex";
            "flex-direction" => "column";
            "text-align" => "center"

     div![ outer_style,
        h1![ "The Grand Total" ],
                // Example of conditional logic in a style.
                "color" => if model.count > 4 {"purple"} else {"gray"};
                // When passing numerical values to style!, "px" is implied.
                "border" => "2px solid #004422"; "padding" => 20
            // We can use normal Rust code and comments in the view.
            h3![ format!("{} {}{} so far", model.count, model.what_we_count, plural) ],
            button![ simple_ev("click", Msg::Increment), "+" ],
            button![ simple_ev("click", Msg::Decrement), "-" ],

            // Optionally-displaying an element
            if model.count >= 10 { h2![ style!{"padding" => 50}, "Nice!" ] } else { seed::empty() }

        success_level(model.count),  // Incorporating a separate component

        h3![ "What precisely is it we're counting?" ],
        input![ attrs!{"value" => model.what_we_count}, input_ev("input", Msg::ChangeWWC) ]

pub fn render() {
    // The final parameter is an optional routing map.
    seed::run(Model::default(), update, view, "main", None);

For truly minimimal example, see in the quickstart repo

Building and running

To build your app, create a pkg subdirectory, and run the following two commands:

cargo build --target wasm32-unknown-unknown


wasm-bindgen target/wasm32-unknown-unknown/debug/appname.wasm --no modules --out-dir ./pkg

where appname is replaced with your app's name. This compiles your code in the target folder, and populates the pkg folder with your WASM module, a Typescript definitions file, and a Javascript file used to link your module from HTML.

You may wish to create a build script with these two lines. ( for Linux; build.ps1 for Windows). The Quickstart repo includes these, but you'll still need to do the rename. You can then use ./ or .\build.ps1 If you run into permission errors on, try this command to allow executing the file:chmod +x

For development, you can view your app using a shimmed Python dev server described above. (Set up this mime-type shim from the quickstart repo, and run python

For details, reference the wasm-bindgen documention. In the future, I'd like the build script and commands above to be replaced by wasm-pack.

Running included examples

To run an example located in the examples folder, navigate to that folder in a terminal, run the build script for your system ( or build.ps1), then start a dev server as described above. Note that if you copy an example to a separate folder, you'll need to edit its Cargo.toml to point to the package on instead of locally: Ie replace seed = { path = "../../" with seed = "^0.1.0", and in the build script, remove the leading ../../ on the second line.



  • Learning the syntax, creating a project, and building it should be easy - regardless of your familiarity with Rust.

  • Complete documentation that always matches the current version. Getting examples working, and starting a project should be painless, and require nothing beyond this guide.

  • Expressive, flexible vew syntax that's easy to read and write.

A note on view syntax

This project takes a different approach to describing how to display DOM elements than others. It neither uses completely natural (ie macro-free) Rust code, nor an HTML-like abstraction (eg JSX or templates). My intent is to make the code close to natural Rust, while streamlining the syntax in a way suited for creating a visual layout with minimal repetition. The macros used here are thin wrappers for constructors, and don't conceal much. Specifically, the element-creation macros allow for accepting a variable number of arguments, and the attrs/style marcros are essentially HashMap literals, with wrappers that let el macros know how to distinguish them.

The lack of resemblance to HTML be offputting, but the learning curve is shallow, and I think the macro syntax used to create elements, attributes etc is close-enough to normal Rust syntax that it's easy to reason about how the code should come together, without compartmentalizing it into logic code and display code. This lack of separation in particlar is a subjective, controversial decision, but I think the benefits are worth it.

Where to start if you're familiar with existing frontend frameworks

The todomvc example is an implementation of the TodoMVC project, which has example code in my frameworks that do the same thing. Compare the example in this project to one on that page that uses a framework you're familiar with.

Suggestions? Critique? Submit an issue or pull request on Github


This project is strongly influenced by Elm, React, and Redux. The overall layout of Seed apps mimicks that of The Elm Architecture.

Why another entry in a saturated field?

There are already several Rust/WASM frameworks; why add another?

My goal is for this to be easy to pick up from looking at a tutorial or documentation, regardless of your level of experience with Rust. I'm distinguising this package through clear examples and documentation (see goals above), and using wasm-bindgen internally. I started this project after being unable to get existing frameworks to work due to lack of documented examples, and inconsistency between documentation and published versions. My intent is for anyone who's proficient in a frontend framework to get a standalone app working in the browser within a few minutes, using just the quickstart guide.

Seed approaches HTML-display syntax differently from existing packages: rather than use an HTML-like markup similar to JSX, it uses Rust builtin types, thinly-wrapped by a macro for each DOM element. This decision may not appeal to everyone, but I think it integrates more naturally with the language.

Why build a frontend in Rust over Elm or Javascript-based frameworks?

You may prefer writing in Rust, and using packages from Cargo vis npm. Getting started with this framework will, in most cases be faster, and require less config and setup overhead than with JS frameworks. You like the advantages of compile-time error-checking.

You may choose this approach over Elm if you're already comfortable with Rust, want the performance benefits, or don't want to code business logic in a purely-functional langauge.

Compared with React, you may appreciate the consistency of how to write apps: There's no distinction between logic and display code; no restrictions on comments; no distinction between components and normal functions. The API is flexible, and avoids OOP boilerplate.

I also hope that config, building, and dependency-management is cleaner with Cargo and wasm-bindgen than with npm.


  • The WASM-Bindgen team: For building the tools this project relies on
  • Alex Chrichton, for being extraodinarily helpful in the Rust / WASM community
  • The Elm team: For creating and standardizing the Elm architecture
  • Denis Kolodin: for creating the inspirational Yew framework
  • Utkarsh Kukreti, for through his Draco repo, helping me understand how wasm-bindgen's closure system can be used to update state.
  • Tim Robinson, for being very helpful on the Rust Gitter.

Features to add

  • High-level fetch API
  • Lifecycle hooks
  • SVG support
  • More flexible routing
  • Virtual DOM optimization
  • High-level CSS-grid/Flexbox API ?

Bugs to fix

  • Text renders above children instead of below