class Commander
 def initialize(&block)
  # Initialize me!

  if block.arity > 0
    yield self
  else
    instance_eval &block
  end
 end

 def welcome; puts "hello" end
 def launch; puts "whee" end
 def goodbye; puts "bye" end
end

Commander.new do
  welcome
  launch
  goodbye
end

Commander.new do |c|
  c.welcome
  c.launch
  c.goodbye
end