diff --git a/lib/register/boot.rb b/lib/register/boot.rb
index 8eb5ff4b..35d21dd6 100644
--- a/lib/register/boot.rb
+++ b/lib/register/boot.rb
@@ -93,8 +93,8 @@ module Register
     # helper to create a Type, name is the parfait name, ie :Type
     def type_for( name , ivars )
       l = Parfait::Type.allocate.compile_time_init
-      l.add_instance_variable :type , name
-      ivars.each {|n,t| l.add_instance_variable( n , t) }
+      l.send(:private_add_instance_variable , :type , name)
+      ivars.each {|n,t| l.send(:private_add_instance_variable, n , t) }
       l
     end
 
diff --git a/lib/soml/compiler/class_field.rb b/lib/soml/compiler/class_field.rb
index 1b14e051..7e811c7f 100644
--- a/lib/soml/compiler/class_field.rb
+++ b/lib/soml/compiler/class_field.rb
@@ -10,7 +10,7 @@ module Soml
       index = for_class.instance_type.variable_index(statement.name)
       #raise "class field already defined:#{name} for class #{for_class.name}" if index
       #puts "Define field #{name} on class #{for_class.name}"
-      index = for_class.instance_type.add_instance_variable( statement.name , statement.type )
+      for_class.instance_type.add_instance_variable( statement.name , statement.type )
 
       return nil # statements don't reurn values, only expressions
     end
diff --git a/lib/typed/parfait/type.rb b/lib/typed/parfait/type.rb
index 35b3bd40..d9a7877c 100644
--- a/lib/typed/parfait/type.rb
+++ b/lib/typed/parfait/type.rb
@@ -40,10 +40,31 @@ module Parfait
     include Indexed
     self.offset(3)
 
-    def initialize( object_class )
+    def self.new_for_hash( object_class , hash)
+      new_type = Type.new( object_class , hash)
+      code = new_type.hash
+      Space.object_space.types[code] = new_type
+      new_type
+    end
+
+    def self.hash_code_for( dict )
+      index = 1
+      hash = 0
+      dict.each do |name , type|
+        item_hash = name.hash + type.hash
+        hash  += item_hash + (item_hash / 256 ) * index
+        index += 1
+      end
+      hash
+    end
+
+    def initialize( object_class , hash = nil)
       super()
-      add_instance_variable :type ,:Type
+      private_add_instance_variable :type ,:Type
       self.object_class = object_class
+      hash.each do |name , type|
+        private_add_instance_variable name , type
+      end if hash
     end
 
     def == other
@@ -51,17 +72,18 @@ module Parfait
     end
 
     # add the name of an instance variable
-    # The index will be returned and can subsequently be searched with index_of
-    # The index of the name is the index of the data in the object
-    #
-    # TODO , later we would need to COPY the type to keep the old constant
-    #        but now we are concerned with booting, ie getting a working structure
-    def add_instance_variable name , type
-      raise "Name shouldn't be nil" unless name
-      raise "Value Type shouldn't be nil" unless type
-      self.push(name)
-      self.push(type)
-      self.get_length
+    # Type objects are immutable, so a new object is returned
+    # As types are also unique, two same adds will result in identical results
+    def add_instance_variable( name , type )
+      hash = to_hash
+      hash[name] = type
+      code = Type.hash_code_for( hash )
+      existing = Space.object_space.types[code]
+      if existing
+        return existing
+      else
+        return Type.new_for_hash( object_class , hash)
+      end
     end
 
     def instance_names
@@ -108,8 +130,26 @@ module Parfait
       nil  # stop resolve recursing up metaclasses
     end
 
-    def hash
-      h = name.hash
+    def to_hash
+      hash = Dictionary.new
+      each_pair do |name, type |
+        hash[name] = type
+      end
+      hash
     end
+
+    def hash
+      Type.hash_code_for( to_hash )
+    end
+
+    private
+
+    def private_add_instance_variable( name , type)
+      raise "Name shouldn't be nil" unless name
+      raise "Value Type shouldn't be nil" unless type
+      self.push(name)
+      self.push(type)
+    end
+
   end
 end
diff --git a/test/typed/type/test_basic.rb b/test/typed/type/test_basic.rb
index ccca952b..cf585007 100644
--- a/test/typed/type/test_basic.rb
+++ b/test/typed/type/test_basic.rb
@@ -70,7 +70,7 @@ class BasicType < MiniTest::Test
 
   def test_add_name
     type = Parfait::Type.new Register.machine.space.get_class_by_name(:Type)
-    type.add_instance_variable :boo , :Object
+    type.send(:private_add_instance_variable, :boo , :Object)
     assert_equal 2 , type.variable_index(:boo)
     assert_equal 4 , type.get_length
     assert_equal :type , type.get(1)
diff --git a/test/typed/type/test_hash.rb b/test/typed/type/test_hash.rb
new file mode 100644
index 00000000..8c86be6f
--- /dev/null
+++ b/test/typed/type/test_hash.rb
@@ -0,0 +1,49 @@
+require_relative "../helper"
+
+class TypeHash < MiniTest::Test
+
+  def setup
+    @types = Register.machine.boot.space.types
+    @first = @types.values.first
+  end
+
+  def test_hash
+    assert_equal Parfait::Dictionary , @types.class
+  end
+
+  def test_length
+    assert_equal 16 , @types.length
+  end
+
+  def test_two_hashs_not_equal
+    assert @types.keys.last != @types.keys.first
+  end
+
+  def test_name
+    assert_equal "Word_Type" , @types.values.first.name
+  end
+
+  def test_to_hash
+    hash = @first.to_hash
+    assert_equal hash[:type] , :Word
+    assert_equal hash[:char_length] , :Integer
+    assert_equal 2 , @first.instance_length
+  end
+
+  def test_hashcode_with_hash
+    assert_equal @first.hash , Parfait::Type.hash_code_for( @first.to_hash)
+  end
+
+  def test_second_hash_different
+    hash2 = @first.to_hash
+    hash2[:random] = :Type
+    assert @first.hash != Parfait::Type.hash_code_for( hash2 )
+  end
+
+  def test_add_is_different
+    type = @first.add_instance_variable :random , :Integer
+    assert type != @first , "new: #{type.inspect} , old: #{@first.inspect}"
+    assert @first.hash != type.hash
+  end
+
+end