What’s code formatting and why do we care?

On my past job, I had the pleasure of writing quite a bit of Elm (which has the most outstanding developer experience that I’ve ever seen and is going to take over the frontend eventually, I hope).

One feature about the Elm workflow that really stood out to me, is that you:

  • Write some code that you think should work
  • Hit Save
  • Suddenly your reedit-problem-solving-jumbled code looks perfectly fine!

This does not sound like a big deal at first, but I invite you to try it for a day and see if you ever want to go back. And in my understanding, it’s not even about saved keystrokes, but about the mental context switches you’re not making.

OK, that works, time to make it looks nice, and then I can move on with the next step

Just imagine deleting this step from the equation. If you are a developer, I don’t think I have to argue to you that suspending your current train of thought does not help with solving the problem at hand. Therefore, I believe it won’t be hard for you to agree that reindenting lines is not what makes you happy or valuable to the world.

Code formatting in Elixir

Fortunately, José Valim and the amazing Elixir core team seem to agree with me and made code formatting a first-class citizen in the upcoming release 1.6 of Elixir.

At the time of writing this post, version 1.6 is not out yet. If you want to try it today, I recommend just running the master branch of Elixir.


For all Linux and MacOS users, I recommend using the ASDF version manager and it’s excellent Elixir plugin.

From there, you can run:

# Install the current master version of Elixir, to update just reinstall it
asdf install elixir master-otp-20

# Use edge Elixir as the global default
asdf global elixir master-otp-20

# Or just for the project that you're currently in
asdf local elixir master-otp-20

And have access to mix format, which runs the code formatter.


In my book, this is the most important use case and the setup on your editor of choice is going to vary, but any self-respecting editor has some way of running commands on save or on a keyboard shortcut.

There’s even some plugins that take that configuration from you:

Side note If you happen to set this up by yourself, most formatting frameworks support piping the file contents to the formatter via STDIN and reading the formatted output from STDOUT, which makes for snappier formatting. This is supported by mix format -.

Whole-project formatting

To do this, you can either pass a pattern like:

mix format "lib/**/*.{ex,exs}"

However, I’d recommend adding a .formatter.exs file to your project root (that’s where mix format is looking for it by default, but you can put it wherever you want. That’s what she said.):

  inputs: [

This enables you to just run mix format with no arguments to reformat all the code in the specified paths.

That’s all I got for now, hopefully my little essay persuaded you to give automated code formatting a try, if not in Elixir, then maybe in your language of choice. Until next time.