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%Square{side: 10}
|> Shape.area() #=> 100

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defmodule EquilateralTriangle do
  defstruct [:side]
end

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defimpl Shape, for: EquilateralTriangle do
  def area(shape) do
    # some basic high school math that I've forgotten
  end
end

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defmodule Project do
  def total_cost(shape, cost_per_square_meter) do
    Shape.area(shape) * cost_per_square_meter
  end
end

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defmodule Square do
  defstruct [:side]
end

defmodule Circle do
  defstruct [:radius]
end

defmodule EquilateralTriangle do
  defstruct [:side]

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defmodule Square do
  defstruct [:side]

  def area(%Square{side: side}) do
    side * side
  end
end

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defmodule Circle do
  defstruct [:radius]

  def area(%Circle{radius: radius}) do
    radius * radius * 3.14
  end
end

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class Square
  def initialize(side)
     @side = side
  end

  def area()
    @side * @side
  end
end

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class Circle
  def initialize(radius)
    @radius = radius
  end
  
  def area()
    3.14 * @radius * @radius
  end
end

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Shape.area(%Square{side: 10}) #=> 100