restrict list index to integer

lib/parfait/list.rb

@@ -53,6 +53,7 @@ module Parfait
     # set the value at index.
     # Lists start from index 0
     def get( index )
+      raise "Only integers, #{index.class}" unless index.is_a?(::Integer)
       raise "Only positive indexes, #{index}" if index < 0
       if index >= data_length
         return nil unless @next_list

test/parfait/test_list1.rb

@@ -32,6 +32,9 @@ module Parfait
       assert_equal 2 , @list.index_of( :three )
       assert_nil  @list.index_of( :four )
     end
+    def test_get_fail
+      assert_raises{ @list.get(:me) }
+    end
     def test_inspect
       assert @list.inspect.include?("one") , @list.inspect
       assert @list.inspect.include?("three") , @list.inspect