 
     char - ?a + 1
   end
+  defp letter_to_col(_), do: {:error, :invalid_coord}
 
   # Reveal all the squares on a `board` for debugging.
   @doc false

    |> do_place_bombs(pid)
  end

  @spec parse_coord(pid, coord) :: {:ok, Grid.coord} | {:error, :invalid_coord}
  defp parse_coord(pid, coord) when is_binary(coord) do
    {row_string, letter} = coord |> String.split_at(-1)

    try do

    place_bombs(pid, [coord])
  end

  @doc """
  Places a bomb on each of the board squares at the given coordinates.

  Returns `:ok` on success, or `{:error, :invalid_coord}` on failure.
  """
  @spec place_bombs(pid, [coord]) :: :ok | {:error, :invalid_coord}
  def place_bombs(pid, coords) do
    parsed_coords = coords |> Enum.map(&parse_coord(pid, &1))
