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.

You can learn more about the history, purpose and implementation of Styler from our talk: Styler: Elixir Style-Guide Enforcer @ GigCity Elixir 2023

Features

• auto-fixes many credo rules, meaning you can turn them off to speed credo up
• keeps a strict module layout
  • alphabetizes module directives
• extracts repeated aliases
• makes your pipe chains pretty as can be
  • pipes and unpipes function calls based on the number of calls
  • optimizes standard library calls (a |> Enum.map(m) |> Enum.into(Map.new) => Map.new(a, m))
• replaces strings with sigils when the string has many escaped quotes
• ... and so much more

See our Rewrites documentation on hexdocs

Who is Styler for?

Styler was designed for a **large team (40+ engineers) working in a single codebase. It helps remove fiddly code review comments and removes failed linter CI slowdowns, helping teams get things done faster. Teams in similar situations might appreciate Styler.

Its automations are also extremely valuable for taming legacy elixir codebases or just refactoring in general. Some of its rewrites have inspired code actions in elixir language servers.

Conversely, Styler probably isn't a good match for:

• experimental, macro-heavy codebases
• teams that don't care about code standards

Installation

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

def deps do
  [
    {:styler, "~> 1.0.0-rc.1", only: [:dev, :test], runtime: false},
  ]
end

Then 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 Stylish Stylings.

Speed: Expect the first run to take some time as Styler rewrites violations of styles and bottlenecks on disk I/O. Subsequent formats formats won't take noticeably more time.

Configuration

Styler can be configured in your .formatter.exs file

[
  plugins: [Styler],
  styler: [
    alias_lifting_exclude: [...]
  ]
]

Styler's only current configuration option is :alias_lifting_exclude, which accepts a list of atoms to not lift. See the Module Directive documentation for more.

No Credo-Style Enable/Disable

Styler will not add configuration for ad-hoc enabling/disabling of rewrites. Sorry! Its implementation simply does not support that approach. There are however many forks out there that have attempted this; please explore the Github forks tab to see if there's a project that suits your needs or that you can draw inspiration from.

Ultimately Styler is @adobe's internal tool that we're happy to share with the world. We're delighted if you like it as is, and just as excited if it's a starting point for you to make something even better for yourself.

WARNING: Styler can change the behaviour of your program!

In some cases, this can introduce bugs. It goes without saying, but look over your changes before committing to main :)

A simple example of a way Styler changes the behaviour of code is the following rewrite:

# Before: this case statement...
case foo do
  true -> :ok
  false -> :error
end

# After: ... is rewritten by Styler to be an if statement!.
if foo do
  :ok
else
  :error
end

These programs are not semantically equivalent. The former would raise if foo returned any value other than true or false, while the latter blissfully completes.

However, Styler is about style, and the if statement is (in our opinion) of much better style. If the exception behaviour was intentional on the code author's part, they should have written the program like this:

case foo do
  true -> :ok
  false -> :error
  other -> raise "expected `true` or `false`, got: #{inspect other}"
end

Also good style! But Styler assumes that most of the time people just meant the if equivalent of the code, and so makes that change. If issues like this bother you, Styler probably isn't the tool you're looking for.

Other ways Styler can change your program:

• with statement rewrites
• config file sorting
• and likely other ways. stay safe out there!

Thanks & Inspiration

Sourceror

Styler's first incarnation was as one-off scripts to rewrite an internal codebase to allow Credo rules to be turned on.

These rewrites were entirely powered by the terrific Sourceror library.

While Styler no longer relies on Sourceror, we're grateful for its author's help with those scripts, the inspiration Sourceror provided in showing us what was possible, and the changes to the Elixir AST APIs that it drove.

Styler's AST-Zipper implementation in this project was forked from Sourceror. Zipper has been a crucial part of our ability to ergonomically zip around (heh) Elixir AST.

Credo

We never would've bothered trying to rewrite our codebase if we didn't have Credo rules we wanted to apply.

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.