Elixir 1.6 introduced the formatter: mix format. This plugin makes it easy to run the formatter asynchronously from within Vim 8 and Neovim.

Use your favorite plugin manager, e.g. vim-plug:

Plug 'mhinz/vim-mix-format'

• To format the current file, use :MixFormat. Use :verb MixFormat to see the exact shell command used.

• The formatter is not perfect yet, so :MixFormatDiff will open a diff window that can be used for previewing the changes or picking only those that seem reasonable.

  dp pushes changes from the diff window to the source file. q closes the diff window. ]c and [c jump between the changes.

  If you're not used to Vim's diff mode, watch this screencast.

• Automatically format on saving.

  let g:mix_format_on_save = 1

• Set options for the formatter. See mix help format in the shell.

  let g:mix_format_options = '--check-equivalent'

• By default this plugin opens a window containing the stacktrace on errors. With this option enabled, there will be just a short message in the command-line bar. The stacktrace can still be looked up via :messages.

  let g:mix_format_silent_errors = 1

• If you're not using Elixir 1.6 in your project, but want to use the formatter anyway, you can specify the bin directory of an alternative Elixir installation:

  let g:mix_format_elixir_bin_path = '~/repo/elixir/bin'

When using :MixFormatDiff, a new diff window will be opened and an user event is emitted. It can be used to set different settings or switch back to the source window:

autocmd User MixFormatDiff wincmd p

If you like this plugin, star it! It helps me deciding which projects to spend more time on.