Only fetch deps compatible for a specific version of Elixir

I do fear you have to do it like this:

{:poison, cond do           
  Version.match?(System.version, "~> 1.6") -> ">= 3.1.0 and < 5.0.0"
  Version.match?(System.version, "~> 1.3") -> ">= 3.1.0 and < 4.0.0"
end, optional: true}

Any idea how hex.pm would handle reporting this?

Plug uses a different lock file to test with older dependencies: plug/mix.exs at main · elixir-plug/plug · GitHub

defmodule Plug.MixProject do
  use Mix.Project

  # ...
  def project do
    [
      # ...
      lockfile: lockfile(),
      # ...
    ]

  defp lockfile() do
    case System.get_env("COWBOY_VERSION") do
      "1" <> _ -> "mix-cowboy1.lock"
      _ -> "mix.lock"
    end
  end
end

Are you able to build a different lockfile using mix or will I have to handle it manually?

I would update mix.exs and then run mix deps.get to generate the lock file: {:poison, ">= 3.1.0 and < 4.0.0", optional: true}

Then I can rename the lock file, and revert changes on mix.lock and mix.exs.

Got it figured out! Thanks @NobbZ and @danschultzer for your help! Went with a combination of your two approaches.

For reference:

https://github.com/sticksnleaves/accent/blob/9f5011677827d98e2cedf976f4120132b719004c/mix.exs#L41-L50

Hmmm, I wonder if we could add this as tip for library maintainers. I’m not sure what the best place for that would be. Does anyone know?

This is where I’ve seen library guidelines before:
https://hexdocs.pm/elixir/master/library-guidelines.html