[Proposal] Add auto-formatting of alias/import/use statements to mix format

[Tim Gent]

The Elixir style guide has guidance on how to organise module attributes, directives, and macros - https://github.com/christopheradams/elixir_style_guide#module-attribute-ordering 

What do people think about applying these conventions as part of mix format?

Background - we have linting rules using credo which check these conventions, but no way to automatically apply fixes at the moment, which means it can be a bit painful to do that manually. Feels like mix format might be a good place for this to live.

[Wojtek Mach]

I don't think the proposal is going to be accepted. A very important design goal of `mix format` is never to change the semantics of the code and changing the order of statements is changing semantics.

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[Andrea Leopardi]

Hey Tim, thanks for the proposal.

As Wojtek mentioned, a strict goal of "mix format" is to never change the AST. This means that, in principle, we should not reorder statements.

In this particular case though, we have a much bigger issue: these statements might depend on each other. Take this snippet:

defmodule MyModule do
  @some_mod GenServer
  use @some_mod
end

You would not be able to reorder these statements without breaking things.

These kinds of checks belong in a linter, not a formatter, such as Credo (as you already mentioned). The fact that these changes are not applied automatically right now doesn't mean that they couldn't in the future 🙃. It might be useful for Credo to be able to apply some automatic fixes if it doesn't already have this ability.

Thanks for the proposal though!

Andrea

[Tim Gent]

Many thanks for the responses guys! Makes total sense :)