# code we don't own
class User
  def initialize(first_name, last_name)
    @first_name, @last_name = first_name, last_name
  end

  def full_name
    @first_name + " " + @last_name
  end
end

# code we do own
module FixUserFullName
  refine User do
    def full_name
      name = @first_name
      name += " #@last_name" if @last_name
      name
    end
  end
end

class MyMailer
  using FixUserFullName

  def send_invitation(user)
    "Welcome, #{user.full_name}"
  end
end


josh = User.new 'Josh', nil
josh.full_name rescue $! # => #<TypeError: no implicit conversion of nil into String>
MyMailer.new.send_invitation(josh) # => "Welcome, Josh"