class Shape2D
  def area
    raise NotImplementedError.
      new("#{self.class.name}#area is an abstract method.")
  end
end
Shape2D.new.area

NotImplementedError: Shape2D#area is an abstract method.
...

NotImplementedError: Shape2D#area is an abstract method.
	from (irb):4:in `area'
	from (irb):7

class Square < Shape2D
  def initialize(length)
    @length = length
  end
  def area
    @length ** 2
  end
end
Square.new(10).area

class Shape2D
  def initialize
    raise NotImplementedError.
      new("#{self.class.name} is an abstract class.")
  end
end
Shape2D.new

NotImplementedError: Shape2D is an abstract class.
...

NotImplementedError: Shape2D is an abstract class.
	from (irb):20:in `initialize'
	from (irb):23

class Class
  def abstract(*args)
    args.each do |method_name|
      define_method(method_name) do |*args|
        if method_name == :initialize
          msg = "#{self.class.name} is an abstract class."
        else
          msg = "#{self.class.name}##{method_name} is an abstract method."
        end
        raise NotImplementedError.new(msg)
     end
    end
  end
end
class Animal
  abstract :initialize, :move
end
Animal.new

NotImplementedError: Animal is an abstract class.
...

NotImplementedError: Animal is an abstract class.
	from (irb):33:in `initialize'
	from (irb):41

class Sponge < Animal
  def initialize
    @type = :Sponge
  end
end
sponge = Sponge.new
sponge.move

NotImplementedError: Sponge#move is an abstract method.
...

NotImplementedError: Sponge#move is an abstract method.
	from (irb):33:in `move'
	from (irb):48

class Cheetah < Animal
  def initialize
    @type = :Cheetah
  end
  def move
    "Running!"
  end
end
Cheetah.new.move

class Sponge
  def move
    "Floating on ocean currents!"
  end
end
sponge.move