* The << operator on values should be const.

src/libexpr/eval.cc

@@ -16,7 +16,7 @@
 namespace nix {
     
 
-std::ostream & operator << (std::ostream & str, Value & v)
+std::ostream & operator << (std::ostream & str, const Value & v)
 {
     switch (v.type) {
     case tInt:

src/libexpr/eval.hh

@@ -171,7 +171,7 @@ typedef std::map<Path, Path> SrcToStore;
 struct EvalState;
 
 
-std::ostream & operator << (std::ostream & str, Value & v);
+std::ostream & operator << (std::ostream & str, const Value & v);
 
 
 class EvalState