focus

defprotocol Lensable do
  @fallback_to_any true

  @doc "A function to get a value out of a data structure"
  def getter(structure, view)

  @doc "A function to set a value out of a data structure"
  def setter(structure, view, func)
end

defimpl Lensable, for: Map do
  def getter(s, x), do: Access.get(s, x, {:error, {:lens, :bad_path}})
  def setter({:error, {:lens, :bad_path}} = e), do: e

  def setter(s, x, f) do
    if Map.has_key?(s, x) do
      Map.put(s, x, f)
    else
      s
    end
  end
end

defimpl Lensable, for: Tuple do
  def getter({:error, _e} = error, _x), do: error
  def getter(s, x), do: elem(s, x)

  def setter(s, x, f) do
    s
    |> Tuple.delete_at(x)
    |> Tuple.insert_at(x, f)
  end
end

defimpl Lensable, for: List do
  def getter(s, x) do
    if Keyword.keyword?(s) && !Enum.empty?(s) do
      Keyword.get(s, x)
    else
      if is_number(x) && !Enum.empty?(s) do
        get_in(s, [Access.at(x)])
      else
        {:error, {:lens, :bad_path}}
      end
    end
  end

  def setter([] = s, x, f), do: List.replace_at(s, x, f)

  def setter(s, x, f) do
    if Keyword.keyword?(s) do
      Keyword.put(s, x, f)
    else
      List.replace_at(s, x, f)
    end
  end
end

defimpl Lensable, for: Any do
  @bad_data_structure_error {:error, {:lens, :bad_data_structure}}
  def getter(s, x) do
    case s do
      %_type{} -> Lensable.getter(Map.from_struct(s), x)
      _ -> @bad_data_structure_error
    end
  end

  def setter(s, x, f) do
    case s do
      %type{} ->
        struct(type, Lensable.setter(Map.from_struct(s), x, f))

      _ ->
        @bad_data_structure_error
    end
  end
end