--- a/interpreter.h	Thu May 23 07:41:18 2019 -0400
+++ b/interpreter.h	Thu May 23 09:27:09 2019 -0400
@@ -1,6 +1,8 @@
 #if ! defined (calc_interpreter_h)
 #define calc_interpreter_h 1
 
+#include <functional>
+
 namespace calc
 {
   class interpreter
@@ -19,11 +21,30 @@
 
     int parse_and_execute (const std::string& line);
 
+    std::function<void (const char *)>
+    set_error_handler (const std::function<void (const char *)>& error_handler)
+    {
+      std::function<void (const char *)> tmp = m_error_handler;
+      m_error_handler = error_handler;
+      return tmp;
+    }
+
+    std::function<void (double)>
+    set_result_handler (const std::function<void (double)>& result_handler)
+    {
+      std::function<void (double)> tmp = m_result_handler;
+      m_result_handler = result_handler;
+      return tmp;
+    }
+
     void emit_error (const char *msg);
 
     void emit_result (double value);
 
   private:
+
+    std::function<void (const char *)> m_error_handler;
+    std::function<void (double)> m_result_handler;
   };
 }