📦 Watcher, bundler, and test runner for your SwiftWasm apps

What's New



This is a bugfix release that fixes parsing of Package.swift manifests that contain dependencies on system targets. It also adds support for Chrome and Safari stack traces. Many thanks to @j-f1 for the contribution!

Merged pull requests:

carton 📦

Watcher, bundler, and test runner for your SwiftWasm apps

The main goal of carton is to provide a smooth zero-config experience when developing for WebAssembly. It currently supports these features with separate commands:

  • Creating basic package boilerplate for apps built with SwiftWasm with carton init.
  • Watching the app for source code changes and reloading it in your browser with carton dev.
  • Running your XCTest suite in the full JavaScript/DOM environment with carton test.
  • Optimizing and packaging the app for distribution with carton bundle.
  • Managing SwiftWasm toolchain and SDK installations with carton sdk.


The main motivation for carton came after having enough struggles with webpack.js, trying to make its config file work, looking for appropriate plugins. At some point the maintainers became convinced that the required use of webpack in SwiftWasm projects could limit the wider adoption of SwiftWasm itself. Hopefully, with carton you can avoid using webpack altogether. carton also simplifies a few other things in your SwiftWasm development workflow such as toolchain and SDK installations.

Getting started


  • macOS 10.15 and Xcode 11.4 or later.
  • Swift 5.2 or later and Ubuntu 18.04 or 20.04 for Linux users.


On macOS carton can be installed with Homebrew. Make sure you have Homebrew installed and then run:

brew install swiftwasm/tap/carton

carton is also available as a Docker image for Linux. You can pull it with this command:

docker pull

If Docker images are not suitable for you, you'll have to build carton from sources on Ubuntu, and unfortunately other Linux distributions are currently not supported. Clone the repository and run swift build -c release, the carton binary will be located in the .build/release directory after that.


The carton init command initializes a new SwiftWasm project for you (similarly to swift package init) with multiple templates available at your choice. carton init --template tokamak creates a new Tokamak project, while carton init --template basic (equivalent to carton init) creates an empty SwiftWasm project with no dependencies. Also, carton init list-templates provides a complete list of templates (with only basic and tokamak available currently).

The carton dev command builds your project with the SwiftWasm toolchain and starts an HTTP server that hosts your WebAssembly executable and a corresponding JavaScript entrypoint that loads it. The app, reachable at, will automatically open in your default web browser. The port that the development server uses can also be controlled with the --port option (or -p for short). You can edit the app source code in your favorite editor and save it, carton will immediately rebuild the app and reload all browser tabs that have the app open. You can also pass a --verbose flag to keep the build process output available, otherwise stale output is cleaned up from your terminal screen by default. If you have a custom index.html page you'd like to use when serving, pass a path to it with a --custom-index-page option.

The carton test command runs your test suite in the wasmer environment, or in the browser environment. You can switch between these with the --environment option, passing either wasmer or defaultBrowser values to it respectively.

The carton sdk command and its subcommands allow you to manage installed SwiftWasm toolchains, but is rarely needed, as carton dev installs the recommended version of SwiftWasm automatically. carton sdk versions lists all installed versions, and carton sdk local prints the version specified for the current project in the .swift-version file. You can however install SwiftWasm separately if needed, either by passing an archive URL to carton sdk install directly, or just specifying the snapshot version, like carton sdk install wasm-5.3-SNAPSHOT-2020-09-25-a.

carton dev can also detect existing installations of swiftenv, so if you already have SwiftWasm installed via swiftenv, you don't have to do anything on top of that to start using carton.

The carton bundle command builds your project using the release configuration (although you can pass the --debug flag to it to change that), and copies all required assets to the Bundle directory. You can then use a static file hosting (e.g. GitHub Pages) or any other server with support for static files to deploy your application. All resulting bundle files except index.html are named by their content hashes to enable cache busting. As with carton dev, a custom index.html page can be provided through the --custom-index-page option.

The carton package command proxies its subcommands to swift package invocations on the currently-installed toolchain. This may be useful in situations where you'd like to generate an Xcode project file for your app with something like carton package generate-xcodeproj. It would be equivalent to swift package generate-xcodeproj, but invoked with the SwiftWasm toolchain instead of the toolchain supplied by Xcode.

All of these commands and subcommands can be passed a --help flag that prints usage info and information about all available options.

How does it work?

carton bundles a WASI polyfill, which is currently required to run any SwiftWasm code, and the JavaScriptKit runtime for convenience. carton also embeds an HTTP server for previewing your SwiftWasm app directly in a browser. The development version of the polyfill establishes a helper WebSocket connection to the server, so that it can reload development browser tabs when rebuilt binary is available. This brings the development experience closer to Xcode live previews, which you may have previously used when developing SwiftUI apps.

carton does not require any config files for these basic development scenarios, while some configuration may be supported in the future, for example for complex asset pipelines if needed. The only requirement is that your Package.swift contains at least a single executable product, which then will be compiled for WebAssembly and served when you start carton dev in the directory where Package.swift is located.

carton is built with Vapor, SwiftNIO, swift-tools-support-core, and OpenCombine, and supports both macOS and Linux. (Many thanks to everyone supporting and maintaining those projects!)

Running carton dev with the release configuration

By default carton dev will compile in the debug configuration. Add the --release flag to compile in the release configuration.


Since a subset of Foundation and XCTest already work and are supplied in the latest snapshots of SwiftWasm SDK, the next top priority for carton is to allow running your XCTest suites directly in browsers and receiving test results back to the HTTP server, so that test results can be reported in CLI. This was blocked by XCTest not allowing customized test report formats, which is now partially resolved with a new argument available on XCTMain and a custom JSON test reporter.

As cross-compiling to WebAssembly and running apps and tests remotely is not too dissimilar to Android development, or even development on macOS for Linux through Docker, carton could potentially become a generic tool for cross-platform Swift developers. I'm not developing any Android apps currently, but if there are interested Swift for Android developers, I'd be very happy to review and merge their contributions enabling that.



If this tool saved you any amount of time or money, please consider sponsoring the work of its maintainers on their sponsorship pages: @carson-katri, @kateinoigakukun, and @MaxDesiatov. While some of the sponsorship tiers give you priority support or even consulting time, any amount is appreciated and helps in maintaining the project.

Coding Style

This project uses SwiftFormat and SwiftLint to enforce formatting and coding style. We encourage you to run SwiftFormat within a local clone of the repository in whatever way works best for you either manually or automatically via an Xcode extension, build phase or git pre-commit hook etc.

To guarantee that these tools run before you commit your changes on macOS, you're encouraged to run this once to set up the pre-commit hook:

brew bundle # installs SwiftLint, SwiftFormat and pre-commit
pre-commit install # installs pre-commit hook to run checks before you commit

Refer to the pre-commit documentation page for more details and installation instructions for other platforms.

SwiftFormat and SwiftLint also run on CI for every PR and thus a CI build can fail with incosistent formatting or style. We require CI builds to pass for all PRs before merging.

Code of Conduct

This project adheres to the Contributor Covenant Code of Conduct. By participating, you are expected to uphold this code. Please report unacceptable behavior to


  • Swift Tools 5.3.0
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Last updated: Thu Feb 02 2023 12:15:01 GMT-0500 (GMT-05:00)