* Direction's parent is Object now, not String.

direction.rb

@@ -1,20 +1,32 @@
 class Robot
-  class Direction < String
+  class Direction
     ALL_STR = %w(NORTH EAST SOUTH WEST)
 
     def self.Direction direction
+      return direction if direction.is_a? Direction
+
       raise ArgumentError, "#{direction}: unknown direction" \
 	unless ALL_STR.include? direction
 
       Direction.new direction
     end
 
+    attr_reader :name
+
+    def initialize name
+      @name = name
+    end
+
     NORTH = Direction 'NORTH'
     EAST = Direction 'EAST'
     SOUTH = Direction 'SOUTH'
     WEST = Direction 'WEST'
     ALL = [NORTH, EAST, SOUTH, WEST].map! { |d| d.freeze }
 
+    def == other
+      @name == other.name
+    end
+
     def next
       case self
       when NORTH then EAST
@@ -25,7 +37,8 @@ def next
     end
 
     def next!
-      replace self.next
+      @name = self.next.name
+      self
     end
 
     def prev
@@ -38,7 +51,12 @@ def prev
     end
 
     def prev!
-      replace self.prev
+      @name = prev.name
+      self
+    end
+
+    def to_s
+      @name
     end
   end
 end

state.rb

@@ -1,5 +1,10 @@
 class Robot
   State = Struct.new(:x, :y, :direction) do
+    def initialize x, y, direction
+      self.x, self.y, self.direction =
+	Integer(x), Integer(y), Direction(direction)
+    end
+
     def to_s
       [x, y, direction].join ','
     end