diff --git a/gtsam/base/types.h b/gtsam/base/types.h
index 2d593409c..5559518b3 100644
--- a/gtsam/base/types.h
+++ b/gtsam/base/types.h
@@ -21,11 +21,17 @@
 
 #include <cstddef>
 
+#include <boost/function/function1.hpp>
+
 namespace gtsam {
 
 	/// Integer variable index type
 	typedef size_t Index;
 
+  /** A function to convert indices to strings, for example by translating back
+   * to a nonlinear key and then to a Symbol. */
+  typedef boost::function<std::string(Index)> IndexFormatter;
+
 	/**
 	 * Helper class that uses templates to select between two types based on
 	 * whether TEST_TYPE is const or not.