I do this in the demo for one of my projects, in “mix.exs”:

defp deps do
  [
    {:phoenix, "~> 1.7.14"},
    ...,
    my_lib_dep()
  ]
end

defp my_lib_dep() do
    if File.exists?(Path.join(__DIR__, "../my_lib/mix.exs")) do
      {:my_lib, path: "../my_lib"}
    else
      {:my_lib, "~> 0.1.0"}
    end
end

Or you can append an entire list to the end if you have multiple deps. The nice thing about “mix.exs” is that it’s code, and is executed every time you run mix.

Current implementation is like this. I did commit an .envrc file which sets the path. It seems checking path existence is the only ‘fully native’ solution. Combining it with env override and a warning might hit the sweet spot.

defp routex_dep() do
    if path = System.get_env("ROUTEX_PATH") do
      IO.puts(">> !! USING LOCAL ROUTEX PATH !! <<")
      {:routex, path: path}
    else
      {:routex, ">= 0.0.0"}
    end
  end

Something that might be useful is that Mix.env() returns :prod when your package is compiled for use in a user’s application.

So

@dependency if Mix.env() == :prod do
  {:package, "~> ..."}
else
  {:package, path: "..."}
end