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defmodule Square do
  defstruct [:side]
end

defmodule Circle do
  defstruct [:radius]
end

defprotocol Area do

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defprotocol Perimeter do
  def calculate(shape)
end

defimpl Perimeter, for: Square do
  def calculate(square) do
    square.side * 4
  end
end

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# Make some shapes
defmodule Square do
  defstruct [:side]
end

defmodule Circle do
  defstruct [:radius]
end

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

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defmodule Perimeter do
  def calculate(shape) do
    case shape do
      %Square{side: side} -> side * 4
      %Circle{radius: radius} -> radius * 2 * 3.14
    end
  end
end

Perimeter.calculate(%Square{side: 10}) #=> 40

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defmodule Square do
  defstruct [:side]
end

defmodule Circle do
  defstruct [:radius]
end

defmodule Area do
  def calculate(shape) do

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

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

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