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

Spec::Runner.configure do |config|
  # ... other config stuff
  config.include WillPaginateMacros
end

the macro file.rb

module WillPaginateMacros 
  def self.included(klass)
    klass.extend ClassMethods
  end
  
  module ClassMethods
    def should_have_per_page(count) 
      klass = self.described_type
      context "#{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
end