array_inquirer.rb

module ArrayInquirer
  def method_missing method, *args
    begin
      select { |elem| elem.send :"#{method}?", *args }
    rescue NoMethodError
      super method, *args
    end
  end
end

if __FILE__ == $0
  # Test cases should suffice as documentation
  require 'test/unit'
  
  class TestArrayInquirer < Test::Unit::TestCase
    def test_it
      assert_equal [nil,true].extend(ArrayInquirer).nil, [nil]
      assert_equal [0,1,0].extend(ArrayInquirer).zero, [0,0]
      assert_equal (0..10).extend(ArrayInquirer).even, [0,2,4,6,8,10]
    end
    def test_fail
      assert_raise(NoMethodError) { [nil,true].extend(ArrayInquirer).foobar }
    end
  end
end

object_inquirer.rb

module ObjectInquirer
  def method_missing method, *args
    begin
      case method.to_s
      when /^(.*)_is\?$/
        name = Regexp.last_match[1]
        send(name) == args.first
      else
        super method, *args
      end
    rescue NoMethodError
      super method, *args
    end
  end
end

if __FILE__ == $0
  # Test cases should suffice as documentation
  require 'test/unit'
  class Foo
    include ObjectInquirer
    attr_accessor :bar
  end
  
  class TestObjectInquirer < Test::Unit::TestCase
    def setup
      @foo = Foo.new
      @foo.bar = 'baz'
    end
    def test_it
      assert @foo.bar_is?('baz')
      assert !@foo.bar_is?('cuux')
    end
    def test_fail
      assert_raise(NoMethodError) { @foo.cuux_is?('the shizzle') }
    end
  end
end