Config.exs and System.get_env/2: are values actually read at runtime?

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4

@v0idpwn I am surprised actually, I too was believing that this was compile-time only but indeed the values defined here must be available at runtime and are not stored into a file like in a release.

So the runtime execution erases the value set before compilation.

What is confusing is that if you wanted to have this config:

import Config
config :demo, foo: System.get_env("FOO")

And this code:

defmodule Demo do
  @foo Application.get_env(:demo, :foo)

  def foo do
    @foo
  end
end

You cannot call FOO=bar mix compile and then just iex -S mix and get the expected value, because iex -S mix will always trigger a recompilation. The code above will emit a warning as you should use Application.compile_env. Very different behaviour than calling @foo System.get_env("FOO") directly.

Personnally, I just put everything possible in runtime.exs.

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