example_group.rb

      def self.define_shared_group_method(new_name, report_label=nil)
        report_label = "it should behave like" unless report_label
        module_eval(<<-END_RUBY, __FILE__, __LINE__)
          def self.#{new_name}(name, &customization_block)
            shared_block = world.shared_example_groups[name]
            raise "Could not find shared example group named \#{name.inspect}" unless shared_block

            providings = extract_providings(customization_block)

            describe "#{report_label} \#{name}" do
              # setup instance and class helper methods with this value...
              providings.each do |key, value|
                define_method(key); { value }; end
                class << self; define_method(key); { value }; end; end
              end

              module_eval &shared_block
              
              # run the customization block last so that it can override helper methods.
              module_eval &customization_block if customization_block 
            end 
          end
        END_RUBY
      end

      # Note: any other class-level DSL methods that you want to be available
      #       before the shared block runs can be extracted here in a similar fashion.
      #       We could also extract the class helper method definitions using method missing
      #       here so they can be evaluated first.
      def self.extract_providings(customization_block)
        class = Class.new do
          @providings = {}
          def self.providings; @providings; end
          def self.providing(key, value); @providings[key] = value; end
 
          # swallow any other missing class methods...
          def self.method_missing(*a); end

          class_eval(&customization_block)
        end

        class.providings
      end
             

some_spec.rb

shared_examples_for "a measurable object" do
  it "should return #{measurement} from ##{measurement_method}" do
    subject.send(measurement_method).should == measurement
  end
end

describe Array do
  it_should_behave_like "a measurable object" do
    subject { [2, 13] }
    providing :measurement, 2
    providing :measurement_method, :size
  end
end

describe String do
  it_should_behave_like "a measurable object" do
    subject { "FooBar" }
    providing :measurement, 6
    providing :measurement_method, :length
  end
end