Elixir Abstract and Extends

base_calculator.ex

defmodule BaseCalculator do
  @moduledoc """
  Define the base calculator and provide public API. 
  Define the callback to implement specific logic in child module.
  """
  
  # it should be implemented by child module
  @callback do_calculate(num1 :: Integer.t(), num2 :: Integer.t()) :: {:ok, Integer.t()} | {:error, any()}
  
  defmacro __using__(_opts) do
    quote do
      @behaviour BaseCalculator
      
      @doc """
      Public API
      """
      def calcualte(num1, num2) do
        __MODULE__.do_calculate(num1, num2)
      end
    end
  end
end

divide_calculator.ex

defmodule DivideCalculator do
  @moduledoc """
  Implement divide calculation logic
  """
  use BaseCalculator

  @impl true
  def do_calculate(num1, num2) do
    case num2 do
      0 -> {:error, :invalid_param}
      _ -> {:ok, num1 / num2}
    end
  end
end

plus_calculator.ex

defmodule PlusCalculator do
  @moduledoc """
  Implement plus calculation logic
  """
  use BaseCalculator

  @impl true
  def do_calculate(num1, num2) do
    {:ok, num1 + num2}
  end
end

sub_calculator.ex

defmodule SubCalculator do
  @moduledoc """
  Implement substract calculation logic
  """
  use BaseCalculator

  @impl true
  def do_calculate(num1, num2) do
    {:ok, num1 * num2}
  end
end