lib/option.rb in option-0.1.0 vs lib/option.rb in option-0.1.1

@@ -50,21 +50,25 @@
 
   def flat_map(&blk)
     result = blk.call(get)
     case result
       when OptionClass then return result
-      else raise TypeError, "Must be Option"
+      else raise TypeError, "Must be an Option"
     end
   end
 
   def fold(if_empty, &blk)
     blk.call(get)
   end
 
   def exists?(&blk)
     !! blk.call(get)
   end
+
+  def filter(&blk)
+    exists?(&blk) ? self : None
+  end
 end
 
 class NoneClass < OptionClass
 
   def to_a
@@ -107,9 +111,13 @@
     if_empty.call
   end
 
   def exists?(&blk)
     false
+  end
+
+  def filter(&blk)
+    self
   end
 end
 
 None = NoneClass.new
 Some = SomeClass