Monday, April 10, 2023

Web Assembly Interfaces help integration of WASM libraries

 In the previous post, I showed how to run plugins generated from Rust code with wasm-bindgen, using the wasmtime crate. I then discovered Wasmer, so I rewrote the runtime to use the Wasmer API, which is very similar (see But I see Wasmer have a lot more tooling available than just a runtime, so let's see how it can help!

I followed first the tutorial at

You can first define the Web Assembly interface you want to expose in a type of IDL file - think protobuf or Corba, depending on your age :-). Easy enough in our case:

language: func() -> string

greet: func(name: string) -> string

We can put that file (greeter.wai) in our english-rs crate folder, and remove all wasm-bindgen dependencies and related code. We can then use the export! macro of the wai-bindgen-rust crate to automatically generate a trait that defines both function, and then provide an implementation:


struct Greeter;

impl crate::greeter::Greeter for Greeter {
/// The language we greet in.
fn language() -> String {

/// Greet the given name.
fn greet(name: String) -> String {
format!("Hello, {name}!")

That's it! The only change from the previous code apart from the impl block is that the name parameter is an owned String and not a &str.

Then I can publish this library to the wasmer WebAssembly libraries repositories via the cargo-wapm command. It now lives at You can download the .wasm file from there!

What about the runtime? There's not a lot of documentation yet because of lot of this is still beta (and the specs of WebAssembly Interfaces and related concepts are still in flux), but it's possible to use the import! macro of the wai-bindgen-wasmer crate to generate code to interact with the module, using the same wai file: we use the same file that defines the interface to both generate the trait we need to implement and the client struct. This is what our greeter now looks like:

use std::{env, fs};

use anyhow::{anyhow, Result};
use greeter::{Greeter, GreeterData};
use wasmer::*;
use wasmer_compiler_llvm::LLVM;


/// Greet using all the plugins.
fn main() -> Result<()> {
let args: Vec<String> = env::args().collect();
if args.len() != 2 {
return Err(anyhow!("Usage: i18n-greeter <name>"));
let compiler_config = LLVM::default();
let engine = EngineBuilder::new(compiler_config).engine();

let paths = fs::read_dir("./plugins").unwrap();

for path in paths {
let path = path?;
let mut store = Store::new(&engine);

let module = Module::from_file(&store, path.path())?;

let imports = imports! {};
let instance = Instance::new(&mut store, &module, &imports)?;
let env = FunctionEnv::new(&mut store, GreeterData {});
let greeter = Greeter::new(&mut store, &instance, env)?;

let language = greeter.language(&mut store)?;
println!("Language: {language}");
let greeting = greeter.greet(&mut store, &args[1])?;
println!("Greeting: {greeting}");

No more requirement to understand how to call WASM functions and get the return value, the Wasmer WAI generated code does that for you! The API could still be cleaner (maybe without that store parameter everywhere) but the convenience is already a clear win.

All this code can be found at

Happy WebAssembly hacking!

No comments: