Mix v1.6.0 mix format View Source

Formats the given files and patterns.

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

If any of the files is -, then the output is read from stdin and written to stdout.

Formatting options

Formatting is done with the Code.format_string!/2 function. For complete list of formatting options please refer to its description. A .formatter.exs file can also be defined for customizing input files and the formatter itself.

Task-specific options

  • --check-formatted - check that the file is already formatted. This is useful in pre-commit hooks and CI scripts if you want to reject contributions with unformatted code. However, keep in mind, that the formatting output may differ between Elixir versions as improvements and fixes are applied to the formatter.

  • --check-equivalent - check if the file after formatting has the same AST. If the ASTs are not equivalent, it is a bug in the code formatter. This option is recommended if you are automatically formatting files.

  • --dry-run - do not save files after formatting.

  • --dot-formatter - the file with formatter configuration. Defaults to .formatter.exs if one is available, see next section.

If any of the --check-* flags are given and a check fails, the formatted contents won’t be written to disk nor printed to stdout.


The formatter will read a .formatter.exs in the current directory for formatter configuration. It should return a keyword list with any of the options supported by Code.format_string!/2.

The .formatter.exs also supports other options:

  • :inputs (a list of paths and patterns) - specifies the default inputs to be used by this task. For example, ["mix.exs", "{config,lib,test}/**/*.{ex,exs}"].

  • :import_deps (a list of dependencies as atoms) - specifies a list of dependencies whose formatter configuration will be imported. See the “Importing dependencies configuration” section below for more information.

  • :export (a keyword list) - specifies formatter configuration to be exported. See the “Importing dependencies configuration” section below.

When to format code

We recommend developers to format code directly in their editors. Either automatically on save or via an explicit command/key binding. If such option is not yet available in your editor of choice, adding the required integration is relatively simple as it is a matter of invoking

cd $project && mix format $file

where $file refers to the current file and $project is the root of your project.

It is also possible to format code across the whole project by passing a list of patterns and files to mix format, as showed at the top of this task documentation. This list can also be set in the .formatter.exs under the :inputs key.

Importing dependencies configuration

This task supports importing formatter configuration from dependencies.

A dependency that wants to export formatter configuration needs to have a .formatter.exs file at the root of the project. In this file, the dependency can export a :export option with configuration to export. For now, only one option is supported under :export: :locals_without_parens (whose value has the same shape as the value of the :locals_without_parens in Code.format_string!/2).

The functions listed under :locals_without_parens in the :export option of a dependency can be imported in a project by listing that dependency in the :import_deps option of the formatter configuration file of the project.

For example, consider I have a project my_app that depends on my_dep. my_dep wants to export some configuration, so my_dep/.formatter.exs would look like this:

# my_dep/.formatter.exs
  # Regular formatter configuration for my_dep
  # ...

  export: [
    locals_without_parens: [some_dsl_call: 2, some_dsl_call: 3]

In order to import configuration, my_app’s .formatter.exs would look like this:

# my_app/.formatter.exs
  import_deps: [:my_dep]

Link to this section Summary


A task needs to implement run which receives a list of command line args

Link to this section Functions

A task needs to implement run which receives a list of command line args.

Callback implementation for Mix.Task.run/1.