gistfile1.rbx

class Foo
  
  # Both setter and accessor
  def on_paste
    if block_given?
      @on_paste = Proc.new # Implicit &block.to_proc
    else
      @on_paste
    end
  end
  
  def initialize
    yield self if block_given?
  end
end

f = Foo.new do |f|
  on_paste do
    p 'yay'
  end
end