diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 5f9d19b8d..1d11039ad 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -68,7 +68,7 @@ RootValue allocRootValue(Value * v)
 }
 
 
-static void printValue(std::ostream & str, std::set<const Value *> & active, const Value & v)
+void printValue(std::ostream & str, std::set<const Value *> & active, const Value & v)
 {
     checkInterrupt();
 
diff --git a/src/libexpr/value.hh b/src/libexpr/value.hh
index e743da9c3..4050d7e4b 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -106,8 +106,14 @@ std::ostream & operator << (std::ostream & str, const ExternalValueBase & v);
 
 struct Value
 {
+private:
     ValueType type;
 
+friend std::string showType(const Value & v);
+friend void printValue(std::ostream & str, std::set<const Value *> & active, const Value & v);
+
+public:
+
     inline void setInt() { type = tInt; };
     inline void setBool() { type = tBool; };
     inline void setString() { type = tString; };