Exporting function for host environment

You can expose a Swift function for host environment using special attribute and linker option.

@_cdecl("add")
func add(_ lhs: Int, _ rhs: Int) -> Int {
    return lhs + rhs
}

You need to compile the Swift code with linker option --export.

To call the exported function as a library multiple times, you need to:

1. Compile it as a WASI reactor execution model. The default execution model is command, so you need to pass -mexec-model=reactor to linker.
2. Call _initialize function before interacting with the instance.

If your code has any top-level code, you need to export main function as well, and call it after _initialize function.

$ swiftc \
    -target wasm32-unknown-wasi \
    lib.swift -o lib.wasm \
    -Xlinker --export=add \
    -Xclang-linker -mexec-model=reactor \
    -Xlinker --export=main # Optional

Then, you can use the exported function from host environment.

const WASI = require("@wasmer/wasi").WASI;
const WasmFs = require("@wasmer/wasmfs").WasmFs;

const promisify = require("util").promisify;
const fs = require("fs");
const readFile = promisify(fs.readFile);

const main = async () => {
  // Instantiate a new WASI Instance
  const wasmFs = new WasmFs();
  let wasi = new WASI({
    args: [],
    env: {},
    bindings: {
      ...WASI.defaultBindings,
      fs: wasmFs.fs,
    },
  });

  const wasmBinary = await readFile("lib.wasm");

  // Instantiate the WebAssembly file
  const { instance } = await WebAssembly.instantiate(wasmBinary, {
    wasi_snapshot_preview1: wasi.wasiImport,
  });
  // Initialize the instance by following WASI reactor ABI
  instance.exports._initialize();
  // (Optional) Run the top-level code
  instance.exports.main();
  // Get the exported function
  const addFn = instance.exports.add;
  console.log("2 + 3 = " + addFn(2, 3))

};

main()

If you use SwiftPM package, you can omit linker flag using clang's __atribute__. Please see swiftwasm/JavaScriptKit#91 for more detail info