Wasm Builders

Moksh Pathak
Moksh Pathak

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SwiftWasm and WASI

Concise, expressive, fast, safe are some of the adjectives used by Swift developers to describe their choice of language. Introduced in 2014 to the world and made open source in December 2015, Swift, although young, overcame the popularity of Objective C in the first quarter of 2018.

In this article, I would be briefing about SwiftWasm, a toolchain which helps set wasm32-wasi as a compilation target for Swift code and then running the generated .wasm file in a WebAssembly runtime, wasmtime.

  1. Setting up the environment

    To install Docker on your local system, refer the official guide.

    If you don’t want to install Docker locally on your system, you can use Docker Playground.

    First, we need to pull the docker image and run it in an interactive mode; we would be accessing the container from its bash. To do the same, run the following command.

    docker run --rm -it ghcr.io/swiftwasm/swift:latest /bin/bash

    Docker installation

    But what is Docker? Citing the official documentation,

    Docker is an open platform for developing, shipping,
    and running applications.
    Docker enables you to separate your applications from
    your infrastructure so you can deliver software quickly.
    With Docker, you can manage your infrastructure
    in the same ways you manage your applications.
    By taking advantage of Docker’s methodologies
    for shipping, testing, and deploying code quickly,
    you can significantly reduce the delay between
    writing code and running it in production.

    With that clear, lets understand what's a Docker container. According to the documentation,

    Docker provides the ability to package and run an
    application in a loosely isolated environment
    called a container. The isolation and security allows
    you to run many containers simultaneously on a
    given host. Containers are lightweight and contain
    everything needed to run the application, so you do
    not need to rely on what is currently installed on
    the host. You can easily share containers while you
    work, and be sure that everyone you share with gets
    the same container that works in the same way.

    Or simply, a Docker container is nothing but a Docker image brought to action. Docker image acts as a set of instructions to build a Docker container and is made of Dockerfile and any necessary dependencies.

    The above line of Docker code pulls the latest Docker image of SwiftWasm from Docker hub, extracts it, and runs the container. The --rm flag causes Docker to automatically remove the container when we exit from it. We are using -i and -t together as -it command. -i is short form for --interactive, this keeps the STDIN of the Docker container open. Without --interactive, our container won’t take any input. -t is short form for --tty. This command gives us a pseudo terminal to our container, it connects the STDIN and STDOUT, or input and output of the container to our terminal.

  2. Code

    To write the Fibonacci code, we need to have a text editor in the container. You can use any preferred text editor, be it Vim (Vi IMproved) or Emacs. For this demo, we would be using Nano text editor, considering its ease of use and minimal learning curve. To install the same, follow the below instructions :

    1. First, we need to run the apt update command in our container.
    2. To install Nano text editor using apt, we need to run apt install nano

    With our text editor now installed in the container, lets make a swift file and write the fibonacci code in it.

    To make a new file, we would be using the touch command.

        touch fibonacci.swift

    To open the newly created file in nano text editor, we use this simple command.

        nano fibonacci.swift

    The fibonacci code in Swift is as follows :

        func fibonacci(n: Int) -> Int {
            var a = 0
            var b = 1
            for _ in 0..<n {
                let temp = a
                a = b
                b = temp + b
            return a
  3. Compiling the code

    To compile your Swift code to wasm, simply run:

    swiftc -target wasm32-unknown-wasi fibonacci.swift -o fibonacci.wasm

    This would generate a fibonacci.wasm file.

  4. Running .wasm file in Wasmtime

    We would be using Wasmtime as our WebAssembly runtime to run our fibonacci.wasm file. Before we install wasmtime in our container, we would need to install curl command, which is a command-line tool to transfer data to or from a server. To install the curl command, run the following command :

    apt install curl

    With that done, lets install our WebAssembly runtime, Wasmtime with the following command.

    curl https://wasmtime.dev/install.sh -sSf | bash

    We need to open a new terminal to start using Wasmtime, but the same can be achieved simply by the following command.

    exec bash

    To run our fibonacci.wasm , run the following command :

    wasmtime fibonacci.wasm


    To stop and exit from the container, type the exit command and hit enter, or press ctrl+c and then ctrl+d

To recap briefly, you now know :

  • What is Docker, Docker container and Docker Image
  • How to access a Docker container through bash
  • docker run command and the commonly used flags and their function.
  • How to install utilities such as nano and curl in a container
  • What is SwiftWasm, a tool-chain used to convert Swift code to WebAssembly.
  • How to set wasm32-wasi as a compilation target and generate a .wasm file from Swift code.
  • How to run the generated .wasm file in a WebAssembly runtime, wasmtime.

I hope that this article was able to add value to your time.

Any suggestions and feedback to help make this article even better are more than welcome.

I will be seeing you in the next one, till then keep exploring and learning 😃

Moksh Pathak

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