weird.ex

defmodule Foo do
  def bar do
    :bar
  end
end

defmodule Weird do
  @moduledoc """
  Illustrates a weird behavior I noticed which doesn't seem to make sense.

  Set `MIX_ENV` to `prod` and `mix release`. Then, try these:

      $ _build/prod/rel/weird/bin/weird eval "Weird.a()"
      Function is not exported! Why? :complete amazement:
      $ _build/prod/rel/weird/bin/weird eval "Weird.b()"
      Function is not exported! Why? :complete amazement:
      $ _build/prod/rel/weird/bin/weird eval "Weird.c()"
      Function is exported.
  """

  # nope
  def a do
    if function_exported?(Foo, :bar, 0),
      do: IO.puts("Function is exported."),
      else: IO.puts("Function is not exported! Why? :complete amazement:")
  end

  # thought maybe loading the app or this might help - no dice
  def b do
    Application.ensure_all_started(:weird)
    a()
  end

  # but after simply checking the functions on the module like this,
  # `function_exported?/3` returns true! By looking, I've changed it;
  # Heisenberg principle at work here!
  def c do
    Foo.__info__(:functions)
    a()
  end
end