Elixir code quality tools and checks

28 April, 2018

Elixir programming language has its great, huge community and ecosystem. As for now, we can easily do static code analysis and code quality checks by using plenty of standard or external tools. This allows us to write robust solid Elixir code in a uniform way according to the style guide .

Let’s start with the most popular tools and solutions:

mix compile –warnings-as-errors

The first and the simplest check that could possibly exist. Elixir compiler is smart enough to detect easily harsh mistakes like unused variables or mismatched module names. At the same time it is pretty friendly, because compiler just warns you about these problems, but does not stop compilation. For some reasons, especially if we are running the CI, we want to make it more obvious and stop any further checks. This can be achieved by running mix compile task with related option:

mix compile --warnings-as-errors

mix format –check-formatted

Elixir 1.6 introduced yet another useful tool - the formatter. After that we can keep our codebase consistent in one uniform code style without any contradictions. However, in the real life, not everyone uses the formatter and we need to force this option by running mix format task with the --check-formatted option during CI.

mix format --check-formatted


Credo is a static analysis code tool for Elixir. It’s more than just a usual code checker - it can teach you how to write your code better, show refactoring possibilities and inconsistencies in naming.

In order to start using Credo you need to add this line to your mix.exs deps:

{:credo, "~> 0.9.1", only: ~w(dev test)a, runtime: false}

You can enforce your own code style for your team by using Credo configuration file. For example, you can create config/.credo.exs file with this content:

  # You can have as many configs as you like in the `configs:` field.
  configs: [
      # Run any exec using `mix credo -C <name>`. If no exec name is given
      # "default" is used.
      name: "default",
      # These are the files included in the analysis:
      files: %{
        # You can give explicit globs or simply directories.
        # In the latter case `**/*.{ex,exs}` will be used.
        excluded: [~r"/_build/", ~r"/deps/", ~r"/priv/"]
      # If you create your own checks, you must specify the source files for
      # them here, so they can be loaded by Credo before running the analysis.
      requires: [],
      # Credo automatically checks for updates, like e.g. Hex does.
      # You can disable this behaviour below:
      check_for_updates: true,
      # If you want to enforce a style guide and need a more traditional linting
      # experience, you can change `strict` to `true` below:
      strict: true,
      # If you want to use uncolored output by default, you can change `color`
      # to `false` below:
      color: true,
      # You can customize the parameters of any check by adding a second element
      # to the tuple.
      # To disable a check put `false` as second element:
      #     {Credo.Check.Design.DuplicatedCode, false}
      checks: [
        {Credo.Check.Readability.Specs, priority: :low},
        {Credo.Check.Design.TagTODO, exit_status: 0},
        {Credo.Check.Design.TagFIXME, exit_status: 0},
        {Credo.Check.Readability.MaxLineLength, priority: :low, max_length: 100}

After that, it would be nice to force these settings by running Credo mix task with --strict option:

mix credo --strict


Elixir has a mix xref task that performs cross-reference checks between modules. This check can print all unavailable or deprecated references, create a dependencies graph and show callers of the given function. During the CI we want to check if we have any unavailable or deprecated functions/modules:

mix xref unavailable
mix xref deprecated

Don’t forget to include --include-siblings option if you are using this in umbrella application.


Sobelow is a security-based static analysis tool. Unfortunately, it comes just for the Phoenix framework, so you can use it only in your web applications. Sobelow can detect the following types of security issues:

To install Sobelow you can use the next command:

mix archive.install hex sobelow

To run Sobelow just start the related mix task:

mix sobelow


Dialyzer is the most powerful and yet complex analysis tool for the BEAM platform. Dialyzer means DIscrepancy AnaLYZer for ERlang programs, but it could be used in Elixir too. It identifies software discrepancies like definite type errors, dead or unreachable code.

To use Dialyzer in your Elixir application you may want to use Dialyxir. Just add this line to your mix.exs file:

defp deps do
  [{:dialyxir, "~> 0.5", only: [:dev], runtime: false}]

You can also configure warnings, dependencies and paths in mix.exs:

def project do
  dialyzer: [plt_add_deps: :apps_direct, plt_add_apps: [:wx]]
  # flags: ["-Wunmatched_returns", :error_handling, :race_conditions, :underspecs]
  # paths: ["_build/dev/lib/my_app/ebin", "_build/dev/lib/foo/ebin"]

You can ignore any unwanted warnings by providing ignore_warnings option:

def project do
  [dialyzer: [ignore_warnings: "dialyzer.ignore-warnings"]]

To run dialyzer on the CI add the next option to make sure that the build fails in case of any errors:

mix dialyzer --halt-exit-status


As you can see, Elixir by itself and by its ecosystem has many useful checks and tools that allow you to keep your code nice, simple, robust and consistent. These checks are also highly configurable and extensible. You can easily use them for any CI platforms to keep your development workflow bright and shiny.

Happy hacking!