transformable.rb

require 'rubygems'
require 'spec'

module Transformable

  private

  def transform(&block)
    copy = self.clone
    copy.instance_eval(&block)
    copy.freeze
  end

end

class Immutable

  include Transformable

  attr_reader :state, :delta

  def initialize(state)
    @state = state
    @delta = nil
  end

  def modify(delta)
    transform { @delta = delta }
  end

end

describe 'Transformable#transform' do

  subject { instance.modify(delta) }

  let(:instance) { Immutable.new(state) }
  let(:state)    { 'foo'                }
  let(:delta)    { 'baz'                }

  it { should_not equal(instance)      }
  it { should be_instance_of(Immutable) }
  it { should be_frozen                 }

  its(:state) { should == state }
  its(:delta) { should == delta }

end

__END__

ree-1.8.7-2010.02 mungo:playground snusnu$ spec transform_spec.rb 
Profiling enabled.
.....
Finished in 0.003661 seconds

5 examples, 0 failures
ree-1.8.7-2010.02 mungo:playground snusnu$ spec transform_spec.rb --heckle="Immutable#modify" 
**********************************************************************
***  Immutable#modify loaded with 2 possible mutations
**********************************************************************

2 mutations remaining...
1 mutations remaining...
No mutants survived. Cool!