diff --git a/src/libexpr/eval-inline.hh b/src/libexpr/eval-inline.hh
index 0748fbd3f3e..bd003432fef 100644
--- a/src/libexpr/eval-inline.hh
+++ b/src/libexpr/eval-inline.hh
@@ -27,23 +27,15 @@ LocalNoInlineNoReturn(void throwTypeError(const char * s, const Value & v, const
 void EvalState::forceValue(Value & v, const Pos & pos)
 {
     if (v.type == tThunk) {
-        Env * env = v.thunk.env;
-        Expr * expr = v.thunk.expr;
-        try {
-            v.type = tBlackhole;
-            //checkInterrupt();
-            expr->eval(*this, *env, v);
-        } catch (Error & e) {
-            v.type = tThunk;
-            v.thunk.env = env;
-            v.thunk.expr = expr;
-            throw;
-        }
+        Value vTmp;
+        v.thunk.expr->eval(*this, *v.thunk.env, vTmp);
+        v = vTmp;
+    }
+    else if (v.type == tApp) {
+        Value vTmp;
+        callFunction(*v.app.left, *v.app.right, vTmp, noPos);
+        v = vTmp;
     }
-    else if (v.type == tApp)
-        callFunction(*v.app.left, *v.app.right, v, noPos);
-    else if (v.type == tBlackhole)
-        throwEvalError("infinite recursion encountered, at %1%", pos);
 }
 
 
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 2805f7f468f..5515c13cdc8 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -492,12 +492,13 @@ inline Value * EvalState::lookupVar(Env * env, const ExprVar & var, bool noEval)
     if (!var.fromWith) return env->values[var.displ];
 
     while (1) {
-        if (!env->haveWithAttrs) {
+        Expr * withAttrs = env->withAttrs;
+        if (withAttrs) {
             if (noEval) return 0;
             Value * v = allocValue();
-            evalAttrs(*env->up, (Expr *) env->values[0], *v);
+            evalAttrs(*env->up, withAttrs, *v);
             env->values[0] = v;
-            env->haveWithAttrs = true;
+            env->withAttrs = 0;
         }
         Bindings::iterator j = env->values[0]->attrs->find(var.name);
         if (j != env->values[0]->attrs->end()) {
@@ -906,7 +907,7 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
             if (state.countCalls && pos2) state.attrSelects[*pos2]++;
         }
 
-        state.forceValue(*vAttrs, ( pos2 != NULL ? *pos2 : this->pos ) );
+        state.forceValue(*vAttrs, pos2 ? *pos2 : this->pos);
 
     } catch (Error & e) {
         if (pos2 && pos2->file != state.sDerivationNix)
@@ -928,6 +929,7 @@ void ExprOpHasAttr::eval(EvalState & state, Env & env, Value & v)
 
     for (auto & i : attrPath) {
         state.forceValue(*vAttrs);
+        assert(vAttrs->type != tAttrs || vAttrs->attrs != 0);
         Bindings::iterator j;
         Symbol name = getName(i, state, env);
         if (vAttrs->type != tAttrs ||
@@ -1140,8 +1142,7 @@ void ExprWith::eval(EvalState & state, Env & env, Value & v)
     Env & env2(state.allocEnv(1));
     env2.up = &env;
     env2.prevWith = prevWith;
-    env2.haveWithAttrs = false;
-    env2.values[0] = (Value *) attrs;
+    env2.withAttrs = attrs;
 
     body->eval(state, env2, v);
 }
@@ -1416,12 +1417,8 @@ void EvalState::forceFunction(Value & v, const Pos & pos)
 string EvalState::forceString(Value & v, const Pos & pos)
 {
     forceValue(v, pos);
-    if (v.type != tString) {
-        if (pos)
-            throwTypeError("value is %1% while a string was expected, at %2%", v, pos);
-        else
-            throwTypeError("value is %1% while a string was expected", v);
-    }
+    if (v.type != tString)
+        throwTypeError("value is %1% while a string was expected, at %2%", v, pos);
     return string(v.string.s);
 }
 
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index ff2e3730380..28b496a9dae 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -37,8 +37,8 @@ struct Env
 {
     Env * up;
     unsigned short size; // used by ‘valueSize’
-    unsigned short prevWith:15; // nr of levels up to next `with' environment
-    unsigned short haveWithAttrs:1;
+    unsigned short prevWith; // nr of levels up to next `with' environment
+    Expr * withAttrs;
     Value * values[0];
 };
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index e52de6dd4d6..d1be817481d 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -1231,8 +1231,9 @@ static void elemAt(EvalState & state, const Pos & pos, Value & list, int n, Valu
     state.forceList(list, pos);
     if (n < 0 || (unsigned int) n >= list.listSize())
         throw Error(format("list index %1% is out of bounds, at %2%") % n % pos);
-    state.forceValue(*list.listElems()[n]);
-    v = *list.listElems()[n];
+    auto & v2 = *list.listElems()[n];
+    state.forceValue(v2);
+    v = v2;
 }
 
 
@@ -1426,7 +1427,6 @@ static void prim_sort(EvalState & state, const Pos & pos, Value * * args, Value
         v.listElems()[n] = args[1]->listElems()[n];
     }
 
-
     auto comparator = [&](Value * a, Value * b) {
         /* Optimization: if the comparator is lessThan, bypass
            callFunction. */
diff --git a/src/libexpr/value.hh b/src/libexpr/value.hh
index 048522016c6..f74087a198d 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -90,7 +90,21 @@ std::ostream & operator << (std::ostream & str, const ExternalValueBase & v);
 
 struct Value
 {
+    // FIXME: should be std::atomic, with readers using
+    // memory_order_relaxed and writers using memory_order_release.
     ValueType type;
+
+    union
+    {
+        struct {
+            Env * env;
+            Expr * expr;
+        } thunk;
+        struct {
+            Value * left, * right;
+        } app;
+    };
+
     union
     {
         NixInt integer;
@@ -128,13 +142,6 @@ struct Value
             Value * * elems;
         } bigList;
         Value * smallList[2];
-        struct {
-            Env * env;
-            Expr * expr;
-        } thunk;
-        struct {
-            Value * left, * right;
-        } app;
         struct {
             Env * env;
             ExprLambda * fun;
@@ -147,6 +154,20 @@ struct Value
         NixFloat fpoint;
     };
 
+    Value()
+    {
+    }
+
+    Value(const Value & v) = delete;
+
+    Value & operator = (const Value & v)
+    {
+        lambda.env = v.lambda.env;
+        lambda.fun = v.lambda.fun;
+        type = v.type; // FIXME: memory_order_release
+        return *this;
+    }
+
     bool isList() const
     {
         return type == tList1 || type == tList2 || type == tListN;
@@ -173,7 +194,7 @@ struct Value
    Value to ensure that the target isn't kept alive unnecessarily. */
 static inline void clearValue(Value & v)
 {
-    v.app.left = v.app.right = 0;
+    //v.app.left = v.app.right = 0;
 }