Description

This repository contains an example of the following pipeline:

• Compile Rust code exporting a function into WebAssembly linkable object
• Compile Haskell code containing FFI to import the function from above into WebAssembly
• Link the two object files together
• Use the WebAssembly file in browser

Compiling Rust to WASM

You'll need cargo and rustup

You'll also need to install plugin for targeting wasm:

rustup target add wasm32-unknown-unknown

After that, compilation can be performed with

RUSTFLAGS="--emit=obj" cargo build --release --target=wasm32-unknown-unknown

This will produce the linkable object file in target/wasm32-unknown-unknown/release/deps/hello_wasm.o

Compiling Haskell to WASM

This library can be compiled into Web assembly and used in a browser.

You'll need to install GHC wasm backend first. See full article for more details.

The GHCup approach was tested and confirmed to work with some extra steps:

export CONFIGURE_ARGS="--host=x86_64-linux --with-intree-gmp --with-system-libffi"
curl https://gitlab.haskell.org/haskell-wasm/ghc-wasm-meta/-/raw/master/bootstrap.sh | SKIP_GHC=1 sh
source ~/.ghc-wasm/env
ghcup config add-release-channel https://gitlab.haskell.org/haskell-wasm/ghc-wasm-meta/-/raw/master/ghcup-wasm-0.0.9.yaml
ghcup install ghc wasm32-wasi-9.12 -- $CONFIGURE_ARGS

First, copy the compiled Rust object file into libs/librust.a.

After that, the library can be compiled with

cabal --with-compiler=wasm32-wasi-ghc-9.12 --with-hc-pkg=wasm32-wasi-ghc-pkg-9.12 --with-hsc2hs=wasm32-wasi-hsc2hs-9.12 build

The compiler will output a file a.out in the root directory. This is the WASM module that can be imported and used in a browser. It currently exports two functions, mkProofBytesWasm and mkProofBytesMockWasm, which produce Plonkup proofs that the user possesses a valid JSON Web Token issued by Google.

The library uses JS FFI, so one last step is required to generate a helper file:

$(wasm32-wasi-ghc-9.12 --print-libdir)/post-link.mjs -i a.out -o ghc_wasm_jsffi.js

Running JS

You'll need Deno to run the example

Copy the compiled Haskell code (a.out and ghc_wasm_jsffi.js) into the js directory. Rename a.out into linked.wasm.

Then, run

deno index.mjs

Grant access to the file.

This should print 89 as the 10th Fibonacci number

Testing

The whole pipeline can be run with

./build.sh

This should print 89 as the 10th Fibonacci number

Further reading:

https://ghc.gitlab.haskell.org/ghc/doc/users_guide/wasm.html

https://finley.dev/blog/2024-08-24-ghc-wasm.html