This repo contains an experiment with compiling Haskell to WebAssembly and runnig it on fastly’s Compute@Cloud service.

See it in action on https://haskell-on-fastly.edgecompute.app/

• Haskell's GHC compiler supports compiling to WebAssembly since GHC-9.8. This repo is using https://gitlab.haskell.org/ghc/ghc-wasm-meta to get a precompiled compiler for that target (see flake.nix)

• The generated code uses the WASI system interface (https://wasi.dev/) for generic stuff (accessing environment variables). Luckily, Fastly's platform builds on that.

• The Fastly specific stuff (access to the request, sending out the response) is exposed via additional WebAssembly functions. This “ABI” is not fully document, but one can gather how they use from

  • The low-level Rust Crate: https://docs.rs/fastly-sys
  • The high-level Rust Crate: https://docs.rs/fastly
  • A interface description in witx format: https://github.com/fastly/Viceroy/blob/main/lib/compute-at-edge-abi/compute-at-edge.witx
  • The host-side implementation in Viceroy, the local development environment: https://github.com/fastly/Viceroy/tree/05247c4addbd94b04636ff89c0a89fbaf672b2e9/lib/src/wiggle_abi

  We bind a few of function via Haskell's FFI in file Fastly.hs. Because Haskell's WebAssembly FFI currently does not support importing functions from a different WebAssembly module than env, file fastly-sys.c imports them from the right module and wraps them in a “local” C function that we can import from Haskell.

• The hello.hs file puts it all together and implements a very simple service that just prints URL and the environment.

1. Install nix
2. Enter a nix shell using nix develop. You now have tools like fastly, viceroy and especially wasm32-wasi-ghc available.
3. Compile using

   wasm32-wasi-ghc --make fastly-sys.c hello.hs
   

4. Run locally with

   viceroy -v hello.wasm
   

5. Deploy using fastly compute build && fastly compute deploy, after setting up the fastly instance.

This is just a small demo. It supports

• Reading method and URI from the request
• Adding headers and body to the response

There is still much

• Cover more of the Fastly API

  Mostly tediuos work, and in the bright future something like the WASM Component model will make that much easier.

• Be compatible with WAI. This would allow us to compile any WAI-compatible Web application to run on Fastly.

  Should not be hard, but I got stuck with the unix package not compiling on this target yet. So best to wait for the Haskell-on-WebAssembly eco system to mature a bit more.

• Optimize (mabye disable GC, maybe pre-evaluate the binary using wizer).

Contributions are welcome!

Thanks to Cheng Shao aKa TerrorJack for his work on GHC-to-WebAssembly and help with these experiments.