custom_enum.exs

defmodule CustomEnum do
  defstruct items: []
end

defimpl Enumerable, for: CustomEnum do
  @doc """
    Return the amount of items

    ##Examples
    iex> Enum.count %CustomEnum{items: []}
    0

    iex> Enum.count %CustomEnum{items: [1, 2, 3]}
    3
  """
  def count(enum) do
    {:ok, count_size(enum)}
  end

  defp count_size(%CustomEnum{items: []}), do: 0
  defp count_size(enum = %CustomEnum{items: [_|tail]}) do
    %{enum | items: tail}
    |> count_size
    |> Kernel.+(1)
  end

  @doc """
    Return whether an element is a member of items

    ##Examples
    iex> Enum.member? %CustomEnum{items: []}, 1
    false

    iex> Enum.member? %CustomEnum(items: [1, 2, 3]), 1
    true
  """
  def member?(enum, elm) do
    {:ok, contains?(enum, elm)}
  end

  defp contains?(%CustomEnum{items: []}, _), do: false
  defp contains?(%CustomEnum{items: [elm|_]}, elm), do: true
  defp contains?(enum = %CustomEnum{items: [_|tail]}, elm) do
    %{enum | items: tail}
    |> contains? elm
  end
end