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

@BartOtten already mentioned the env variable approach which I do like better. I’m certainly aware mix.exs is just code which I thought was implied by my saying I already considered @Nicodemus’s approach (well, same goes for checking an env var, really).

So far my top pick: Demo code in same repo (excluded from Hex package)

Pre:

• use demo app for integration test
• (major) 1 clone and easy hacking in both example as lib
• no split commits; no sync issues

——-

Also on the table: dotfile + env var

• Presence check a dotfile which is .gitignored. (Default). If available, use path: …/my_lib
• If env set and path exists, use path of env var. (Override)
• If env set and false: use Hex (Override)

Pre over only env variable:

• no need for an env-setter.
• no clobbering ~.profile etc.
• shell and terminal agnostic.
• Easy (un)set: touch .local / rm .local.

Pre over only path check:

• can set set non-default path.
• can force Hex.

no env, no file == no possible path execution