gistfile1.rb

module TemporarilyExtend
  def self.track_method(klass, method)
    @methods ||= {}
    @methods[klass] ||= []
    @methods[klass] << method
  end
  
  def self.tracked_methods(klass)
    @methods ||= {}
    @methods[klass]
  end
  
  def self.clear_tracked_methods(klass)
    @methods ||= {}
    if @methods[klass]
      @methods[klass].each do |meth|
        klass.send(:remove_method, meth)
      end
    end
  end
  
  def method_added(meth)
    TemporarilyExtend.track_method(self, meth)
  end
  
  def singleton_method_added(meth)
    return if meth == :singleton_method_added
    TemporarilyExtend.track_method(self, meth)
  end
end

def extending(klass)
  klass.class_eval do
    extend TemporarilyExtend
  end
  yield
  TemporarilyExtend.clear_tracked_methods(klass)
end

a = "foo"
extending(String) do
  class String
    def bar
      puts "bar"
    end
  end
  
  a.bar
end
a.bar