リスコフの置換原則

class Rectangle
  attr_accessor :width, :height

  def area
    @width * @height
  end
end

class Square < Rectangle
  def width=(value)
    @width = @height = value
  end

  def height=(value)
    @width = @height = value
  end
end

def resize_and_calculate_area(rectangle)
  rectangle.width = 10
  rectangle.height = 5
  rectangle.area
end

rectangle = Rectangle.new
square = Square.new

puts resize_and_calculate_area(rectangle) # => 50 (正しい)
puts resize_and_calculate_area(square)    # => 25 (予期しない結果)

class Shape
  def area
    raise NotImplementedError
  end
end

class Rectangle < Shape
  attr_accessor :width, :height

  def initialize(width, height)
    @width = width
    @height = height
  end

  def area
    @width * @height
  end
end

class Square < Shape
  attr_accessor :side

  def initialize(side)
    @side = side
  end

  def area
    @side * @side
  end
end

def calculate_area(shape)
  shape.area
end

rectangle = Rectangle.new(10, 5)
square = Square.new(5)

puts calculate_area(rectangle) # => 50 (正しい)
puts calculate_area(square)    # => 25 (正しい)

class Bird
  def fly
    "飛んでいます"
  end
end

class Penguin < Bird
  def fly
    raise "ペンギンは飛べません！"
  end
end

def let_bird_fly(bird)
  bird.fly
end

bird = Bird.new
penguin = Penguin.new

puts let_bird_fly(bird)    # => 飛んでいます
puts let_bird_fly(penguin) # => 例外発生

class Bird
  def move
    "移動しています"
  end
end

class FlyingBird < Bird
  def move
    "飛んでいます"
  end
end

class Penguin < Bird
  def move
    "泳いでいます"
  end
end

def let_bird_move(bird)
  bird.move
end

bird = FlyingBird.new
penguin = Penguin.new

puts let_bird_move(bird)    # => 飛んでいます
puts let_bird_move(penguin) # => 泳いでいます

class Payment
  def pay(amount)
    "¥#{amount}を支払いました"
  end
end

class FreePayment < Payment
  def pay(amount)
    raise "無料ユーザーは支払えません！"
  end
end

class User
  def available_payment_method?
    false
  end
end

class PaidUser < User
  def available_payment_method?
    true
  end

  def pay(amount)
    "¥#{amount}を支払いました"
  end
end

def process_payment(user, amount)
  if user.available_payment_method?
    user.pay(amount)
  else
    "無料ユーザーは支払えません"
  end
end

free_user = User.new
paid_user = PaidUser.new

puts process_payment(free_user, 1000) # => 無料ユーザーは支払えません
puts process_payment(paid_user, 1000) # => ¥1000を支払いました