Styler

Styler is an Elixir formatter plugin that's combination of mix format and mix credo, except instead of telling you what's wrong, it just rewrites the code for you to fit its style rules.

Installation

Add :styler as a dependency to your project's mix.exs:

def deps do
  [
    {:styler, "~> 0.8", only: [:dev, :test], runtime: false},
  ]
end

Usage

We recommend using Styler as a formatter plugin, but it comes with a task for making it easy to try styling smaller portions of your project or for installing without modifying your dependencies (via mix's archive.install feature)

As a Formatter plugin

Add Styler as a plugin to your .formatter.exs file

[
  plugins: [Styler]
]

And that's it! Now when you run mix format you'll also get the benefits of Styler's definitely-always-right style fixes.

As a Mix Task

$ mix style

The task can helpful for slowly converting a codebase directory-by-directory. It also allows you to use mix archive.install to easily test run Styler on a project without modifying its dependencies:

$ mix archive.install hex styler

mix style is designed to take the same basic options as mix format.

See mix help style for more.

Configuration

There isn't any! This is intentional.

Styler's @adobe's internal Style Guide Enforcer - allowing exceptions to the styles goes against that ethos. Happily, it's open source and thus yours to do with as you will =)

Styles

You can find the currently-enabled styles in the Styler module, inside of its @styles module attribute. Each Style's moduledoc will tell you more about what it rewrites.

Credo Rules Styler Replaces

If you're using Credo and Styler, we recommend disabling these rules in .credo.exs to save on unnecessary checks in CI.

Your first Styling

Speed: Expect the first run to take some time as Styler rewrites violations of styles.

Once styled the first time, future styling formats shouldn't take noticeably more time.

Roughly, Styler puts about a 10% slow down on mix format.

Troubleshooting: Compilation broke due to Module Directive rearrangement

Sometimes naively moving Module Directives around can break compilation.

Here's helpers on how to manually fix that and have a happy styling for the rest of your codebase's life.

Alias dependency

If you have an alias that, for example, a @behaviour relies on, compilation will break after your first run. This requires one-time manual fixing to get your repo in line with Styler's standards.

For example, if your code was this:

defmodule MyModule do
  @moduledoc "Implements MyBehaviour!"
  alias Deeply.Nested.MyBehaviour
  @behaviour MyBehaviour
  ...
end

then Styler will style the file like this, which cannot compile due to MyBehaviour not being defined.

defmodule MyModule do
  @moduledoc "Implements MyBehaviour!"
  @behaviour MyBehaviour  # <------ compilation error, MyBehaviour is not defined!

  alias Deeply.Nested.MyBehaviour

  ...
end

A simple solution is to manually expand the alias with a find-replace-all like: @behaviour MyBehaviour -> @behaviour Deeply.Nested.MyBehaviour. It's important to specify that you only want to find-replace with the @behaviour prefix or you'll unintentially expand MyBehaviour everywhere in the codebase.

Module Attribute dependency

Another common compilation break on the first run is a @moduledoc that depended on another module attribute which was moved below it.

For example, given the following broken code after an initial mix format:

defmodule MyGreatLibrary do
  @moduledoc make_pretty_docs(@library_options)

  @library_options [ ... ]
end

You can fix the code by moving the static value outside of the module into a naked variable and then reference it in the module.

Yes, this is a thing you can do in a .ex file =)

library_options = [ ... ]

defmodule MyGreatLibrary do
  @moduledoc make_pretty_docs(library_options)

  @library_options library_options
end

Thanks & Inspiration

Sourceror

This work was inspired by earlier large-scale rewrites of an internal codebase that used the fantastic tool Sourceror.

The initial implementation of Styler used Sourceror, but Sourceror's AST-embedding comment algorithm slows Styler down to the point that it's no longer an appropriate drop-in for mix format.

Still, we're grateful for the inspiration Sourceror provided and the changes to the Elixir AST APIs that it drove.

The AST-Zipper implementation in this project was forked from Sourceror's implementation.

Credo

Similarly, this project originated from one-off scripts doing large scale rewrites of an enormous codebase as part of an effort to enable particular Credo rules for that codebase. Credo's tests and implementations were referenced for implementing Styles that took the work the rest of the way. Thanks to Credo & the Elixir community at large for coalescing around many of these Elixir style credos.