Typing violation because of `Application.compile_env/2`

Apparently, at the time being this shuts the type checker up:

defmodule TypeTest do
  foo = Application.compile_env(:my_app, :param, true)

  defp fun(arg) do
    case [unquote(foo), not unquote(foo)] do
      [false, true] -> {:error, arg}
      [true, false] -> {:ok, arg}
    end
  end
end

This happens, because the list in the question becomes non_empty_list(boolean()). Just matching [foo] won’t work because the latter has a type non_empty_list(true).

You can also not generate the unreachable branches to begin with.

defmodule TypeTest do
  foo = Application.compile_env(:my_app, :param, true)

  if foo do
    defp fun(arg), do: {:error, arg}
  else
    defp fun(arg), do: {:ok, arg}
  end
end

Indeed, thanks!

A bit late to the conversation - but this solution will create the same complaint from the compiler →
typing violation found at: …

Is there any way to make the compiler aware of other possibilities?

I ran into this today, and the way I solved it was to move the logic from a module attribute using compile_env to an inner-function call using Application.get_env.

# Old way
defmodule MyApp.SomeModule do
  @debug Application.compile_env(:my_app, [__MODULE__, :debug], false)

  def debug?, do: @debug
end

# New way
defmodule MyApp.SomeModule do
  def debug? do
    Application.get_env(:my_app, [__MODULE__, :debug], false)
  end
end

Yeah, but that’s not the isomorphic approach, because in the not-so-contrived example the code might rely on @debug value during compile time all the turtles down.