find_index.ex

defmodule FindIndex do

  def find_index(list, pred, index \\ 0)

  def find_index([], _pred, _index), do: :notfound
  def find_index([el | rest], pred, index) do
    case pred.(el) do
      {:ok, value} -> {:ok, value, index}
      :notfound -> find_index(rest, pred, index + 1)
    end
  end

  def contains?(el, pattern) do
    if String.contains?(el, pattern) do
      {:ok, el}
    else
      :notfound
    end
  end

  def find_matrix_index(matrix, pattern) do
    found_in_matrix? = find_index(matrix, fn(row) ->

      found_in_row? = find_index(row, &contains?(&1, pattern))

      case found_in_row? do
        {:ok, _el, col_index} -> {:ok, col_index}
        :notfound -> :notfound
      end
    end)

    case found_in_matrix? do
      {:ok, col_index, row_index} -> {row_index, col_index}
      :notfound -> nil
    end
  end

end