Implement experimentally singleton

lib/rekkyo/type.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 require "set"
+require "singleton"
+require "forwardable"
 
 module Rekkyo
   module Type
@@ -12,6 +14,32 @@ def included(klass)
         klass.extend(DSLMethods)
         klass.extend(EnumMethods)
       end
+
+      def create(member_names)
+        ::Class.new do
+          include ::Singleton
+          extend ::SingleForwardable
+
+          include EnumMethods
+          def_single_delegators :instance, :all, :members, :freeze
+
+          const_set(:Member, ::Class.new(Member))
+
+          define_method :initialize do
+            member_class = self.class.const_get(:Member, false)
+            @members = ::Set.new
+            member_names.each do |member_name|
+              member = member_class.new(member_name.upcase.to_sym, member_name).freeze
+              @members << member
+              self.class.const_set(member.key, member)
+            end
+            @members.freeze
+          end
+
+          instance # define a class and instantiate it
+          freeze
+        end
+      end
     end
   end
 end