Elixir Language Constants via macros


Example

Declare:

defmodule MyApp.ViaMacros.Constants do
  @moduledoc """
  Apply with `use MyApp.ViaMacros.Constants, :app` or `import MyApp.ViaMacros.Constants, :app`.

  Each constant is private to avoid ambiguity when importing multiple modules
  that each have their own copies of these constants.
  """

  def app do
    quote do
      # This method allows sharing module constants which can be used in guards.
      @bar "barrific module constant"
      defp app_version, do: "0.0.1"
      defp app_author, do: "Felix Orr"
      defp app_info, do: [app_version, app_author]
    end
  end

  defmacro __using__(which) when is_atom(which) do
    apply(__MODULE__, which, [])
  end
end

Consume with use:

defmodule MyApp.ViaMacros.ConsumeWithUse do
  use MyApp.ViaMacros.Constants, :app

  def foo() do
    IO.puts app_version
    IO.puts app_author
    IO.puts inspect app_info
  end

  def foo(_bar) when is_bitstring(@bar) do
    IO.puts "We just used bar in a guard: #{@bar}"
  end
end

This method allows you to use the @some_constant inside guards. I'm not even sure that the functions would be strictly necessary.