1 files changed, 19 insertions, 9 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 0833603b2a9e..64f3874db614 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -293,6 +293,8 @@ EvalState::EvalState(const Strings & _searchPath, ref<Store> store)
     , sColumn(symbols.create("column"))
     , sFunctor(symbols.create("__functor"))
     , sToString(symbols.create("__toString"))
+    , sRight(symbols.create("right"))
+    , sWrong(symbols.create("wrong"))
     , store(store)
     , baseEnv(allocEnv(128))
     , staticBaseEnv(false, 0)
@@ -379,9 +381,9 @@ void EvalState::addPrimOp(const string & name,
 }
 
 
-void EvalState::getBuiltin(const string & name, Value & v)
+Value & EvalState::getBuiltin(const string & name)
 {
-    v = *baseEnv.values[0]->attrs->find(symbols.create(name))->value;
+    return *baseEnv.values[0]->attrs->find(symbols.create(name))->value;
 }
 
 
@@ -462,7 +464,7 @@ void mkString(Value & v, const char * s)
 }
 
 
-void mkString(Value & v, const string & s, const PathSet & context)
+Value & mkString(Value & v, const string & s, const PathSet & context)
 {
     mkString(v, s.c_str());
     if (!context.empty()) {
@@ -473,6 +475,7 @@ void mkString(Value & v, const string & s, const PathSet & context)
             v.string.context[n++] = dupString(i.c_str());
         v.string.context[n] = 0;
     }
+    return v;
 }
 
 
@@ -993,11 +996,18 @@ void EvalState::callFunction(Value & fun, Value & arg, Value & v, const Pos & po
     if (fun.type == tAttrs) {
       auto found = fun.attrs->find(sFunctor);
       if (found != fun.attrs->end()) {
+        /* fun may be allocated on the stack of the calling function,
+         * but for functors we may keep a reference, so heap-allocate
+         * a copy and use that instead.
+         */
+        auto & fun2 = *allocValue();
+        fun2 = fun;
+        /* !!! Should we use the attr pos here? */
         forceValue(*found->value, pos);
-        Value * v2 = allocValue();
-        callFunction(*found->value, fun, *v2, pos);
-        forceValue(*v2, pos);
-        return callFunction(*v2, arg, v, pos);
+        Value v2;
+        callFunction(*found->value, fun2, v2, pos);
+        forceValue(v2, pos);
+        return callFunction(v2, arg, v, pos);
       }
     }
 
@@ -1368,11 +1378,11 @@ NixFloat EvalState::forceFloat(Value & v, const Pos & pos)
 }
 
 
-bool EvalState::forceBool(Value & v)
+bool EvalState::forceBool(Value & v, const Pos & pos)
 {
     forceValue(v);
     if (v.type != tBool)
-        throwTypeError("value is %1% while a Boolean was expected", v);
+        throwTypeError("value is %1% while a Boolean was expected, at %2%", v, pos);
     return v.boolean;
 }