codepoints.exs

defmodule Codepoints do
  defstruct str: ""

  def new(str) when is_list(str) do
    new(to_string(str))
  end

  def new(str) when is_binary(str) do
    %__MODULE__{str: str}
  end

  defimpl Enumerable do
    def reduce(_, {:halt, acc}, _fun) do
      {:halted, acc}
    end
    def reduce(%Codepoints{str: str}, {:suspend, acc}, fun) do
      {:suspended, acc, &reduce(str, &1, fun)}
    end
    def reduce(%Codepoints{str: ""}, {:cont, acc}, _fun) do
      {:done, acc}
    end
    def reduce(%Codepoints{str: <<h::utf8, t::binary>>}, {:cont, acc}, fun) do
      reduce(%Codepoints{str: t}, fun.(<<h::utf8>>, acc), fun)
    end

    def member?(_str, _value) do
      {:error, __MODULE__}
    end

    def count(_str) do
      {:error, __MODULE__}
    end
  end
end

IO.inspect "abc" |> Codepoints.new |> Enum.map(&{&1, &1}) |> Enum.into(%{})