foo.rb

module X
  def type(code, name)
    instance_eval do
      define_method "#{name}?" do |type|
        type == code
      end
    end
  end
end

class Foo
  extend X

  type 0, :integer
  type 1, :string
end

describe Foo do
  it "knows if it is an integer" do
    subject.should be_integer(0)
  end

  it "knows if it is not an integer" do
    subject.should_not be_integer(1)
  end

  it "knows if it is a string" do
    subject.should be_string(1)
  end
end

describe X do
  class Bar
    extend X

    type 99, :cat
  end

  it "associates a name with a type code" do
    Bar.new.should be_cat(99)
  end
end