Hello, world!

You can also browse this source code online and clone the wasmtime repository to run the example locally.

This example shows off how to instantiate a simple wasm module and interact with it.


  (func $hello (import "" "hello"))
  (func (export "run") (call $hello))


//! Small example of how to instantiate a wasm module that imports one function,
//! showing how you can fill in host functionality for a wasm module.

// You can execute this example with `cargo run --example hello`

use anyhow::Result;
use wasmtime::*;

fn main() -> Result<()> {
    // Configure the initial compilation environment, creating the global
    // `Store` structure. Note that you can also tweak configuration settings
    // with a `Config` and an `Engine` if desired.
    let store = Store::default();

    // Compile the wasm binary into an in-memory instance of a `Module`.
    println!("Compiling module...");
    let module = Module::from_file(store.engine(), "examples/hello.wat")?;

    // Here we handle the imports of the module, which in this case is our
    // `HelloCallback` type and its associated implementation of `Callback.
    println!("Creating callback...");
    let hello_func = Func::wrap(&store, || {
        println!("Calling back...");
        println!("> Hello World!");

    // Once we've got that all set up we can then move to the instantiation
    // phase, pairing together a compiled module as well as a set of imports.
    // Note that this is where the wasm `start` function, if any, would run.
    println!("Instantiating module...");
    let imports = [hello_func.into()];
    let instance = Instance::new(&store, &module, &imports)?;

    // Next we poke around a bit to extract the `run` function from the module.
    println!("Extracting export...");
    let run = instance
        .ok_or(anyhow::format_err!("failed to find `run` function export"))?

    // And last but not least we can call it!
    println!("Calling export...");