diff --git a/include/input/PrintTools.h b/include/input/PrintTools.h
index 0a75278..55f730b 100644
--- a/include/input/PrintTools.h
+++ b/include/input/PrintTools.h
@@ -17,6 +17,7 @@
 #pragma once
 
 #include <map>
+#include <optional>
 #include <set>
 #include <string>
 
@@ -28,6 +29,15 @@
 }
 
 /**
+ * Convert an optional type to string.
+ */
+template <typename T>
+std::string toString(const std::optional<T>& optional,
+                     std::string (*toString)(const T&) = constToString) {
+    return optional ? toString(*optional) : "<not set>";
+}
+
+/**
  * Convert a set of integral types to string.
  */
 template <typename T>