mix format in vim from anywhere (or just in umbrella apps)

Spoiler alert: This post is about setting up vim so that you run mix format automatically when you save a file and have it detect the nearest .formatter.exs

I have this weird issue with mix format and umbrella apps.

The issue is that mix format assumes you have a .formatter.exs file in the current directory. If you don’t, it doesn’t look upwards in the file tree. It simply assumes you want to run it with the default config. You can change this behaviour by using the --dot-formatter flag to explicitly point to the formatter file you want to use.

Now, in vim you can also use ale to run mix format on save. If you don’t know ale, take the time to do so. I set up ale to do this precisely by adding the following line to my (n)vim config:

let g:ale_fixers['elixir'] = ['mix_format']

Most of the time you will be good to go with this and you won’t find any more issues.

But when I’m working with Elixir umbrella apps, I sometimes cd to the apps/<app> directly so that my Ctrl+P doesn’t get all cluttered by similarly named files from different applications. If you have a .formatter.exs file with custom rules and ale set up to run mix format on file save, things get tricky. mix format won’t detect your config unless you explicitly set the --dot-formatter flag.

As of a few weeks ago, my PR to allow custom mix format options in ale has been accepted.

With this, we can attempt to find the nearest .formatter.exs and dynamically pass the location to ale. Here’s a bit of vimscript to do so (ignore my n00bness writing vimscript):

function! LoadNearestFormatter()
  let l:formatters = []
  let l:directory = fnameescape(expand("%:p:h"))

  for l:fmt in findfile(".formatter.exs", l:directory . ";", -1)
     call insert(l:formatters, l:fmt)

  call reverse(l:formatters)

  let g:ale_fixers['elixir'] = ['mix_format']

  if len(l:formatters) > 0
    let g:ale_elixir_mix_format_options = "--dot-formatter " . l:formatters[0]

call LoadNearestFormatter()

If you have a better version of this, please let me know.

That bit of code will look for .formatter.exs files along the file tree and if any is found, it passes that along with the correct option.

You can also define a .formatter.exs in your home and use it as fallback for when no other .formatter.exs is present, but I’d advise against this.

@naps62 has the following vim code instead:

let l:git_root = system("git rev-parse --show-toplevel")[:-2]
let l:fmt = findfile(".formatter.exs", l:git_root)
let g:ale_elixir_mix_format_options = "--dot-formatter " . l:fmt

It’s a lot less verbose and it won’t search for .formatter.exs files in your home directory.

And that’s it! Now you can use ale to automatically run mix format inside umbrella apps.

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