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 of popular Haskell apps, running in your browser:
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.
See the Building guide in the documentation for details.
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.
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
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Note that they may be slightly out-of-date as the project evolves.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 :)
- Almost all GHC language features (TH support is partial, cross-splice state persistence doesn't work yet).
- The pure parts in standard libraries and other packages. IO is achieved via rts primitives or user-defined JavaScript imports.
- Importing JavaScript expressions via the
foreign import javascriptsyntax. First-class garbage collectedJSValtype in Haskell land. - Preliminary copying GC, managing both Haskell heap objects and JavaScript references.
- Preliminary Cabal support.
- Marshaling between Haskell/JavaScript types based on
aeson. - Calling Haskell functions from JavaScript via the
foreign export javascriptsyntax. Haskell closures can be passed between the Haskell/JavaScript boundary viaStablePtr. - Invoking RTS API on the JavaScript side to manipulate Haskell closures and trigger evaluation.
- A linker which performs aggressive dead-code elimination, producing as small WebAssembly binary as possible.
- A debugger which checks invalid memory access and outputs memory loads/stores and control flow transfers.
- Complete
binaryen/wabtraw bindings, plus a monadic EDSL to construct WebAssembly code directly in Haskell. - A Haskell library to handle WebAssembly code, which already powers binary code generation.
- Besides WebAssembly MVP and
BigInt, no special requirements on the underlying JavaScript engine at the moment.
Asterius is maintained by Tweag I/O.
Have questions? Need help? Tweet at @tweagio.