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, num2 :: integer) :: {:ok, integer} | {:error, any()}
  
  def calculate(module, num1, num2) do
    module.do_calculate(num1, num2)
  end
end

divide_calculator.ex

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

  @impl BaseCalculator
  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
  """
  @behaviour BaseCalculator

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

sub_calculator.ex

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

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