Elixir の Enumerable プロトコルを利用する検証コード（Range の自作）

my_range.ex

defmodule MyRange do
  @moduledoc """
  自作の Range の実装
  
  詳しくは [Enumerable protocol のドキュメント](https://hexdocs.pm/elixir/Enumerable.html)を参照。
  """

  defstruct first: 0, last: 0

  def new(first, last) do
    %MyRange{first: first, last: last}
  end

  defimpl Enumerable do
    def count(%MyRange{first: first, last: last}), do: {:ok, last - first + 1}
    def member?(%MyRange{first: first, last: last}, value), do: {:ok, first <= value && value <= last}
    def slice(%MyRange{} = my_range), do: {:ok, Enum.count(my_range), fn first, last -> MyRange.new(first, last) end}

    def reduce(_, {:halt, acc}, _), do: {:halted, acc}
    def reduce(%MyRange{} = my_range, {:suspend, acc}, fun), do: {:suspended, acc, &reduce(my_range, &1, fun)}
    def reduce(%MyRange{first: first, last: last}, {:cont, acc}, _) when first > last, do: {:done, acc}
    def reduce(%MyRange{first: first} = my_range, {:cont, acc}, fun), do: reduce(%{my_range | first: first + 1}, fun.(first, acc), fun)
  end
end