attributes.exs

defmodule Foo do
  Module.register_attribute __MODULE__, :a, [persist: true]
  Module.register_attribute __MODULE__, :a_list, [persist: true]
  Module.register_attribute __MODULE__, :b, [persist: true, accumulate: true]
  Module.register_attribute __MODULE__, :b_list, [persist: true, accumulate: true]

  @a 1
  @a 2

  @a_list [1]
  @a_list [2]

  @b 1
  @b 2

  @b_list [1]
  @b_list [2]

  def a, do: @a
  def a_list, do: @a_list

  def b, do: @b
  def b_list, do: @b_list
end

# Unexpected: why does [2] get persisted instead of 2?
IO.inspect Foo.a
#=> 2
IO.inspect Foo.__info__(:attributes) |> Keyword.get_values(:a)
#=> [[2]]

# OK
IO.inspect Foo.a_list
#=> [2]
IO.inspect Foo.__info__(:attributes) |> Keyword.get_values(:a_list)
#=> [[2]]

# Unexpected: why do lists get persisted when each @b is an integer?
IO.inspect Foo.b
#=> [2, 1]
IO.inspect Foo.__info__(:attributes) |> Keyword.get_values(:b)
#=> [[1], [2]]

# OK
IO.inspect Foo.b_list
#=> [[2], [1]]
IO.inspect Foo.__info__(:attributes) |> Keyword.get_values(:b_list)
#=> [[1], [2]]