Asterius: A Haskell to WebAssembly compiler

Asterius is a Haskell to WebAssembly compiler based on GHC. It compiles Haskell source files or Cabal executable targets to WebAssembly+JavaScript code which can be run in Node.js or browsers. It features seamless JavaScript interop (lightweight Async FFI with Promise support) and small output code (~600KB hello.wasm for a Hello World). A lot of common Haskell packages like lens are already supported. The project is actively maintained by Tweag I/O.

Demos

Demos of popular Haskell apps, running in your browser:

• diagrams
• ormolu
• pandoc
• todomvc

Quickstart using the prebuilt container image

We host a prebuilt container image on Docker Hub. The image also ships ~2k prebuilt packages from a recent Stackage snapshot for convenience of testing simple programs without needing to set up a Cabal project.

To use the image, mount the working directory containing the source code as a shared volume, then use the ahc-link program:

terrorjack@hostname:/project$ podman run -it --rm -v $(pwd):/workspace -w /workspace terrorjack/asterius
root@hostname:/workspace#

There are a lot of link-time options available to ahc-link, e.g. targeting the browser platform instead of node, adding extra GHC options or setting runtime parameters. Check the documentation for further details.

It's also possible to use ahc-cabal as a drop-in replacement of cabal to build a Cabal project. Use ahc-dist with --input-exe on the output "executable" file to generate actual WebAssembly and JavaScript artifacts. See the diagrams blog post for an example.

Check the documentation section about the prebuilt image for more information, e.g. versioning policy, how to use with podman/docker, etc.

Building and using asterius locally

See the Building guide in the documentation for details.

Hacking on Asterius

We recommend using VSCode Remote Containers to reproduce the very same dev environment used by our core team members. See the Hacking guide in the documentation for details.

Documentation

We have documentation and blog posts:

• Fibonacci compiles end-to-end: Haskell to WebAssembly via GHC
• Haskell WebAssembly calling JavaScript and back again
• Asterius GHC WebAssembly backend reaches TodoMVC
• Haskell art in your browser with Asterius
• Edge computing with Servant on Cloudflare

Also checkout the HIW 2018 lightning talk, and the slides of an introductory talk in 2020 here.

Note that they may be slightly out-of-date as the project evolves. Whenever you find something in the docs of blog posts which doesn't reflect the status quo, it's a bug and don't hesitate to open a ticket :)

Project status & roadmap

See the roadmap section in the documentation for details.

Contributors

Asterius is maintained by Tweag I/O.

Have questions? Need help? Tweet at @tweagio.