Testing with comments

There's another cool feature of the @moduledoc and @doc attributes and that is what's called doctesting. That is, the lines in our comments that look similar to iex sessions can be used as tests where the output return value is the expected result.

Let's return to our flatten project from earlier.

We can add some @doc comments to the function, and add what would look similar to an iex session of us manually testing the function, but in fact, would be tests run by the mix test.

Open the flatten.ex file from earlier and add the @doc attribute:

defmodule Flatten do
  @doc """
  Flatten an arbitrarily nested lists

  ## Examples

      iex> Flatten.flatten [[1, 2], [3], [4, 5]]
      [1, 2, 3, 4, 5]
      iex> Flatten.flatten [1, 2, 3, 4, 5]
      [1, 2, 3, 4, 5]
  """
  def flatten([]), do: []
  def flatten([h|t]) when is_list(h), do: h ++ flatten(t)
  def flatten([h|t]), do: [h] ++ flatten(t)
end

Next, open the flatten_test.exs file as well and add the following line...