This is a gist that demos what behaviours and protocols look like in Elixir. Used as part of onboarding

bad_behaviours.ex

defmodule Shape do
  @callback area(map()) :: integer()
  @callback perimeter(map()) :: integer()
end

defmodule Square do
  defstruct [:side]
  @behaviour Shape

  @impl Shape
  def area(square) do
    square.side * square.side
  end

  @impl Shape
  def perimeter(square) do
    square.side * 4
  end
end

defmodule Circle do
  defstruct [:radius]
  @behaviour Shape

  @impl Shape
  def area(circle) do
    3.14 * circle.radius * circle.radius
  end

  @impl Shape
  def perimeter(circle) do
    circle.radius * 3.14 * 2
  end
end

square = %Square{side: 10}
circle = %Circle{radius: 10}

Square.area()
Circle.area()

defmodule Triangle do
  defstruct [:base, :height]
end

defmodule ShapeHelpers do
  def area(%{__struct__: shape_module} = shape) do
    shape_module.area(shape)
  end
end

ShapeHelpers.area(%Triangle{base: 1, height: 15})

defprotocol Shape do
  def area(shape)
  def perimeter(shape)
end

defimpl Shape, for: Square do
  def area(square) do
    square.side * square.side
  end

  def perimeter(square) do
    square.side * 4
  end
end

defimpl Shape, for: Cirle do
  def area(circle) do
    3.14 * circle.radius * circle.radius
  end

  def perimeter(circle) do
    circle.radius * 3.14 * 2
  end
end

defmodule Square do
  defstruct [:side]
end

Shape.area(%Square{side: 10})
Shape.perimeter(%Circle{radius: 10})