Ruby: Assigning instance variables in a constructor

Every time I assign an instance variable in a constructor I remember that I've been meaning to write something that takes care of it for me.

class DomainObject
  attr_reader :arg1, :arg2

  def initialize(arg1, arg2)
    @arg1, @arg2 = arg1, arg2
  end
end

So, without further ado.

class Module
  def initializer(*args, &block)
    define_method :initialize do |*ctor_args|
      ctor_named_args = (ctor_args.last.is_a?(Hash) ? ctor_args.pop : {})
      (0..args.size).each do |index|
        instance_variable_set("@#{args[index]}", ctor_args[index])
      end
      ctor_named_args.each_pair do |param_name, param_value|
        instance_variable_set("@#{param_name}", param_value) 
      end
      initialize_behavior
    end
    
    define_method :initialize_behavior, &block
  end
end

The above code allows you to create a constructor that takes arguments in order or from a hash. Here's the tests that demonstrate the behavior of the initializer method.

class ModuleExtensionTest < Test::Unit::TestCase
  def test_1st_argument
    klass = Class.new do
      attr_reader :foo

      initializer :foo
    end
    foo = klass.new('foo')
    assert_equal 'foo', foo.foo
  end

  def test_block_executed
    klass = Class.new do
      attr_reader :bar

      initializer do
        @bar = 1
      end
    end
    foo = klass.new
    assert_equal 1, foo.bar
  end

  def test_2nd_argument
    klass = Class.new do
      attr_reader :foo, :baz

      initializer :foo, :baz
    end
    foo = klass.new('foo', 'baz')
    assert_equal 'baz', foo.baz
  end

  def test_used_hash_to_initialize_attrs
    klass = Class.new do
      attr_reader :foo, :baz, :cat

      initializer :foo, :baz, :cat
    end
    foo = klass.new(:cat => 'cat', :baz => 2, :foo => 'foo')
    assert_equal 'foo', foo.foo
    assert_equal 2, foo.baz
    assert_equal 'cat', foo.cat
  end
end

There are limitations, such as not being able to use default values. But, for 80% of the time, this is exactly what I need.