an example

require File.dirname(__FILE__) + '/../spec_helper'

class Foo
  def self.per_page
    10
  end
end

describe Foo do
  should_have_per_page 10
end

spec_helper.rb

def define_macros
  Spec::Example::ExampleGroupMethods.extend Module.new { yield }
end

the macro file.rb

define_macros do
   def should_have_per_page(count) 
     klass = self.described_type
     describe "#{klass}" do 
       it "should respond to per_page" do 
         klass.should respond_to(:per_page)
       end 

       it "should have #{count} per page" do 
         klass.per_page.should == count
       end 
     end 
  end
end