diff --git a/src/sl/connectivity.hpp b/src/sl/connectivity.hpp
index 1942c6c..1b66b9d 100644
--- a/src/sl/connectivity.hpp
+++ b/src/sl/connectivity.hpp
@@ -26,6 +26,7 @@
 #include <sl/operators.hpp>
 #include <sl/hash.hpp>
 #include <cassert>
+#include <functional>
 
 namespace sl {
 
diff --git a/src/sl/hash.hpp b/src/sl/hash.hpp
index 3093a13..5f5b144 100644
--- a/src/sl/hash.hpp
+++ b/src/sl/hash.hpp
@@ -24,6 +24,7 @@
 #define SL_HASH_HPP
 
 #include <sl/utility.hpp>
+#include <functional>
 
 namespace sl {