Using WebAssembly from Ruby

Wasmtime is available on RubyGems and can be used programmatically to interact with Wasm modules. This guide will go over installing the Wasmtime gem and running a simple Wasm module from Ruby.

Make sure you've got Ruby 3.0 or newer installed locally, and we can get started!

Getting started and simple example

First, copy this example WebAssembly text module into your project. It exports a function for calculating the greatest common denominator of two numbers.

  (func $gcd (param i32 i32) (result i32)
    (local i32)
    block  ;; label = @1
      block  ;; label = @2
        local.get 0
        br_if 0 (;@2;)
        local.get 1
        local.set 2
        br 1 (;@1;)
      loop  ;; label = @2
        local.get 1
        local.get 0
        local.tee 2
        local.set 0
        local.get 2
        local.set 1
        local.get 0
        br_if 0 (;@2;)
    local.get 2
  (export "gcd" (func $gcd))

Next, install the Wasmtime Ruby gems by either adding it your project's Gemfile:

bundle add wasmtime

Or by using the gem command directly:

gem install wasmtime

The gem has a Rust-based native extension, but thanks to precompiled gems, you should not have to compile anything. It'll just work!

Now that you have the Wasmtime gem installed, let's create a Ruby script to execute the gcd module from before.

require "wasmtime"

engine =
mod = Wasmtime::Module.from_file(engine, "gcd.wat")
store =
instance =, mod)

puts "gcd(27, 6) = #{instance.invoke("gcd", 27, 6)}"

This script should output

gcd(27, 6) = 3

If this is the output you see, congrats! You've successfully ran your first WebAssembly code in Ruby!

More examples and contributing

To learn more, check out the more advanced examples and the API documentation. If you have any questions, do not hesitate to open an issue on the GitHub repository.