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hello — 90 seconds

A minimal Haskell-to-WebAssembly "hello world" you can run under Node.js or in a browser, built with the stable-haskell wasm cross-compiler + dual-compiler cabal-install.

Download the template Browse files

Prerequisites

If you haven't already done the install steps, do them first. For convenience:

ghcup config add-release-channel \
  https://stable-haskell.github.io/ghc/ghcup-wasm.yaml
ghcup install ghc   wasm32-wasi-9.14.0.stable.1
ghcup install cabal 3.17.0.0.stable.0
ghcup set    cabal 3.17.0.0.stable.0

Plus Node.js 22+ on $PATH.

Build and run

curl -L -o hello.tar.gz https://stable-haskell.github.io/ghc/examples/stable-haskell-wasm-hello.tar.gz
tar xf hello.tar.gz && cd stable-haskell-wasm-hello
export PATH="$HOME/.ghcup/ghc/wasm32-wasi-9.14.0.stable.1/bin:$PATH"

make build      # cabal build via the wasm cross-compiler
make run-node   # post-link + node run.mjs       → prints "Hello from the WASM reactor!"
make run-web    # post-link + serve public/      → http://localhost:8000
make clean      # remove build artifacts

make help (the default target) shows all targets.

What's interesting in here

  • myapp.cabal — the WASM-only ghc-options block sets up the WASI reactor pattern: -no-hs-main + -optl-mexec-model=reactor + -optl-Wl,--export=hs_start. The host JS calls hs_start() instead of the wasm running its own _start.
  • cabal.project — declares the dual-compiler form: with-build-compiler: ghc (native) for Setup.hs and TH host; with-compiler: wasm32-unknown-wasi-ghc for the actual package code.
  • app/Main.hs — foreign export javascript "hs_start" main :: IO () is the surface that the JS host invokes.
  • run.mjs / public/index.js — the reactor bring-up sequence: wasi.initialize → __ghc_wasm_jsffi_init → hs_start. All three steps are mandatory; skipping __ghc_wasm_jsffi_init yields "RTS is not initialised; call hs_init() first".
  • Makefile — wires post-link.mjs (ships in ~/.ghcup/ghc/wasm32-wasi-9.14.0.stable.1/lib/targets/wasm32-unknown-wasi/lib/) into the build so you don't have to remember the path. Override WASM_VERSION if you need a different one.

Want more?

Step up to the miso-counter template — same recipe, but with 50+ Template-Haskell-heavy deps (aeson, lens, jsaddle, miso) exercising the full dyld + JSFFI link chain at real-world scale.

License

Apache-2.0.