Doctest of a functions that have side effects

humanized_weather.ex

defmodule MyApp.WeatherAPI do
  @callback temp(MyApp.LatLong.t()) :: {:ok, integer()}
  @callback humidity(MyApp.LatLong.t()) :: {:ok, integer()}

  def temp(lat_long), do: impl().temp(lat_long)
  def humidity(lat_long), do: impl().humidity(lat_long)
  defp impl, do: Application.get_env(:my_app, :weather, MyApp.ExternalWeatherAPI)
end

defmodule MyApp.HumanizedWeather do
  @doc """
  Displays current temperature at the given coordinates

  ## Examples

      iex> HumanizedWeather.display_temp({50.06, 19.94})
      "Current temperature is 30 degrees"

      iex> HumanizedWeather.display_temp({102.06, 19.94})
      ** (RuntimeError) latitude must be in the range of -90 to 90 degrees
  """
  def display_temp({lat, long}) do
    case MyApp.WeatherAPI.temp({lat, long}) do
      {:ok, temp} -> "Current temperature is #{temp} degrees"
      {:error, reason} -> raise reason
    end
  end

  @doc """
  Displays current humidity at the given coordinates

  ## Examples

      iex> HumanizedWeather.display_humidity({50.06, 19.94})
      "Current humidity is 60%"
  """
  def display_humidity({lat, long}) do
    {:ok, humidity} = MyApp.WeatherAPI.humidity({lat, long})
    "Current humidity is #{humidity}%"
  end
end

humanized_weather_test.exs

defmodule MyApp.HumanizedWeatherTest do
  use ExUnit.Case, async: true
  import Mox
  alias MyApp.HumanizedWeather

  setup :verify_on_exit!

  describe "display_temp/1" do
    doctest MyApp.HumanizedWeather, only: [display_temp: 1]

    setup do
      expect(MyApp.MockWeatherAPI, :temp, fn
        {lat, _long} when lat < -90 or lat > 90 ->
          {:error, "latitude must be in the range of -90 to 90 degrees"}

        {_lat, _long} ->
          {:ok, 30}
      end)

      :ok
    end
  end

  describe "display_humidity/1" do
    doctest MyApp.HumanizedWeather, only: [display_humidity: 1]

    setup do
      expect(MyApp.MockWeatherAPI, :humidity, fn {_lat, _long} -> {:ok, 60} end)

      :ok
    end
  end
end