diff --git a/slp.hpp b/slp.hpp
index 432cfb6..d0df089 100644
--- a/slp.hpp
+++ b/slp.hpp
@@ -3,6 +3,7 @@
 #include <stdio.h>
 
 #include <array>
+#include <functional>
 #include <list>
 #include <map>
 #include <memory>