Specialize List methods for EmptyList.

lib/apricot/list.rb

@@ -15,18 +15,14 @@ def self.[](*args)
 
     def initialize(head, tail)
       @head = head
-      @tail = tail
+      @tail = tail || EMPTY_LIST
       @count = tail ? tail.count + 1 : 1
     end
 
     def cons(x)
       List.new(x, self)
     end
 
-    def empty?
-      !@tail
-    end
-
     def initialize_copy(other)
       super
       @tail = other.tail.dup if other.tail && !other.tail.empty?
@@ -39,15 +35,15 @@ def to_list
     end
 
     def first
-      empty? ? nil : @head
+      @head
     end
 
     def next
       @tail.empty? ? nil : @tail
     end
 
     def to_seq
-      empty? ? nil : self
+      self
     end
 
     def inspect
@@ -65,6 +61,21 @@ class EmptyList < List
       def initialize
         @count = 0
       end
+
+      def each
+      end
+
+      def empty?
+        true
+      end
+
+      def first
+        nil
+      end
+
+      def next
+        nil
+      end
     end
 
     EMPTY_LIST = EmptyList.new