Constants

Remarks#

So this is a summary analysis I’ve done based on the methods listed at https://stackoverflow.com/questions/33851536/how-do-you-define-constants-in-elixir-modules. I’m posting it for a couple reasons:

Most Elixir documentation is quite thorough, but I found this key architectural decision lacking guidance - so I would have requested it as a topic.
I wanted to get a little visibility and comments from others about the topic.
I also wanted to test out the new SO Documentation workflow. ;)

I’ve also uploaded the entire code to the GitHub repo elixir-constants-concept.

Module-scoped constants

defmodule MyModule do
  @my_favorite_number 13
  @use_snake_case "This is a string (use double-quotes)"
end

These are only accessible from within this module.

Constants as functions

Declare:

defmodule MyApp.ViaFunctions.Constants do
  def app_version, do: "0.0.1"
  def app_author, do: "Felix Orr"
  def app_info, do: [app_version, app_author]
  def bar, do: "barrific constant in function"
end

Consume with require:

defmodule MyApp.ViaFunctions.ConsumeWithRequire do
  require MyApp.ViaFunctions.Constants

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

  # This generates a compiler error, cannot invoke `bar/0` inside a guard.
  # def foo(_bar) when is_bitstring(bar) do
  #   IO.puts "We just used bar in a guard: #{bar}"
  # end
end

Consume with import:

defmodule MyApp.ViaFunctions.ConsumeWithImport do
  import MyApp.ViaFunctions.Constants

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

This method allows for reuse of constants across projects, but they will not be usable within guard functions that require compile-time constants.

Constants via macros

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.