diff --git a/src/libexpr/Makefile.am b/src/libexpr/Makefile.am
index e7228e183..6c38ecdd5 100644
--- a/src/libexpr/Makefile.am
+++ b/src/libexpr/Makefile.am
@@ -11,7 +11,7 @@ pkginclude_HEADERS = \
  names.hh symbol-table.hh
 
 libexpr_la_LIBADD = ../libutil/libutil.la ../libstore/libstore.la \
- ../boost/format/libformat.la
+ ../boost/format/libformat.la @boehmgc_lib@
 
 BUILT_SOURCES = \
  parser-tab.hh lexer-tab.hh parser-tab.cc lexer-tab.cc
diff --git a/src/libexpr/attr-path.cc b/src/libexpr/attr-path.cc
index 0660ba1c1..365b03c0b 100644
--- a/src/libexpr/attr-path.cc
+++ b/src/libexpr/attr-path.cc
@@ -5,8 +5,9 @@
 namespace nix {
 
 
+// !!! Shouldn't we return a pointer to a Value?
 void findAlongAttrPath(EvalState & state, const string & attrPath,
-    const Bindings & autoArgs, Expr * e, Value & v)
+    Bindings & autoArgs, Expr * e, Value & v)
 {
     Strings tokens = tokenizeString(attrPath, ".");
 
@@ -48,7 +49,7 @@ void findAlongAttrPath(EvalState & state, const string & attrPath,
             Bindings::iterator a = v.attrs->find(state.symbols.create(attr));
             if (a == v.attrs->end())
                 throw Error(format("attribute `%1%' in selection path `%2%' not found") % attr % curPath);
-            v = a->second.value;
+            v = *a->second.value;
         }
 
         else if (apType == apIndex) {
diff --git a/src/libexpr/attr-path.hh b/src/libexpr/attr-path.hh
index b4f5c29d2..b106da5ef 100644
--- a/src/libexpr/attr-path.hh
+++ b/src/libexpr/attr-path.hh
@@ -11,7 +11,7 @@ namespace nix {
 
     
 void findAlongAttrPath(EvalState & state, const string & attrPath,
-    const Bindings & autoArgs, Expr * e, Value & v);
+    Bindings & autoArgs, Expr * e, Value & v);
 
     
 }
diff --git a/src/libexpr/common-opts.cc b/src/libexpr/common-opts.cc
index 9131951e3..6b393c07d 100644
--- a/src/libexpr/common-opts.cc
+++ b/src/libexpr/common-opts.cc
@@ -20,12 +20,13 @@ bool parseOptionArg(const string & arg, Strings::iterator & i,
     if (i == argsEnd) throw error;
     string value = *i++;
 
-    Value & v(autoArgs[state.symbols.create(name)].value);
+    Value * v = state.allocValue();
+    autoArgs[state.symbols.create(name)].value = v;
 
     if (arg == "--arg")
-        state.mkThunk_(v, parseExprFromString(state, value, absPath(".")));
+        state.mkThunk_(*v, parseExprFromString(state, value, absPath(".")));
     else
-        mkString(v, value);
+        mkString(*v, value);
     
     return true;
 }
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index c8a031ac6..ea4894725 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -58,7 +58,7 @@ std::ostream & operator << (std::ostream & str, const Value & v)
         typedef std::map<string, Value *> Sorted;
         Sorted sorted;
         foreach (Bindings::iterator, i, *v.attrs)
-            sorted[i->first] = &i->second.value;
+            sorted[i->first] = i->second.value;
         foreach (Sorted::iterator, i, sorted)
             str << i->first << " = " << *i->second << "; ";
         str << "}";
@@ -145,24 +145,26 @@ EvalState::~EvalState()
 
 void EvalState::addConstant(const string & name, Value & v)
 {
+    Value * v2 = allocValue();
+    *v2 = v;
     staticBaseEnv.vars[symbols.create(name)] = baseEnvDispl;
     baseEnv.values[baseEnvDispl++] = v;
     string name2 = string(name, 0, 2) == "__" ? string(name, 2) : name;
-    (*baseEnv.values[0].attrs)[symbols.create(name2)].value = v;
+    (*baseEnv.values[0].attrs)[symbols.create(name2)].value = v2;
 }
 
 
 void EvalState::addPrimOp(const string & name,
     unsigned int arity, PrimOp primOp)
 {
-    Value v;
+    Value * v = allocValue();
     string name2 = string(name, 0, 2) == "__" ? string(name, 2) : name;
-    v.type = tPrimOp;
-    v.primOp.arity = arity;
-    v.primOp.fun = primOp;
-    v.primOp.name = GC_STRDUP(name2.c_str());
+    v->type = tPrimOp;
+    v->primOp.arity = arity;
+    v->primOp.fun = primOp;
+    v->primOp.name = GC_STRDUP(name2.c_str());
     staticBaseEnv.vars[symbols.create(name)] = baseEnvDispl;
-    baseEnv.values[baseEnvDispl++] = v;
+    baseEnv.values[baseEnvDispl++] = *v;
     (*baseEnv.values[0].attrs)[symbols.create(name2)].value = v;
 }
 
@@ -265,7 +267,7 @@ Value * EvalState::lookupVar(Env * env, const VarRef & var)
         while (1) {
             Bindings::iterator j = env->values[0].attrs->find(var.name);
             if (j != env->values[0].attrs->end())
-                return &j->second.value;
+                return j->second.value;
             if (env->prevWith == 0)
                 throwEvalError("undefined variable `%1%'", var.name);
             for (unsigned int l = env->prevWith; l; --l, env = env->up) ;
@@ -275,6 +277,13 @@ Value * EvalState::lookupVar(Env * env, const VarRef & var)
 }
 
 
+Value * EvalState::allocValue()
+{
+    nrValues++;
+    return (Value *) GC_MALLOC(sizeof(Value));
+}
+
+
 Value * EvalState::allocValues(unsigned int count)
 {
     nrValues += count;
@@ -291,6 +300,14 @@ Env & EvalState::allocEnv(unsigned int size)
 }
 
 
+Value * EvalState::allocAttr(Value & vAttrs, const Symbol & name)
+{
+    Attr & a = (*vAttrs.attrs)[name];
+    a.value = allocValue();
+    return a.value;
+}
+
+    
 void EvalState::mkList(Value & v, unsigned int length)
 {
     v.type = tList;
@@ -321,11 +338,8 @@ void EvalState::mkThunk_(Value & v, Expr * expr)
 void EvalState::cloneAttrs(Value & src, Value & dst)
 {
     mkAttrs(dst);
-    foreach (Bindings::iterator, i, *src.attrs) {
-        Attr & a = (*dst.attrs)[i->first];
-        mkCopy(a.value, i->second.value);
-        a.pos = i->second.pos;
-    }
+    foreach (Bindings::iterator, i, *src.attrs)
+        (*dst.attrs)[i->first] = i->second;
 }
 
 
@@ -449,8 +463,9 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
            environment. */
         foreach (Attrs::iterator, i, attrs) {
             nix::Attr & a = (*v.attrs)[i->first];
-            mkThunk(a.value, env2, i->second.first);
-            mkCopy(env2.values[displ++], a.value);
+            a.value = state.allocValue();
+            mkThunk(*a.value, env2, i->second.first);
+            mkCopy(env2.values[displ++], *a.value);
             a.pos = &i->second.second;
         }
 
@@ -459,7 +474,7 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
         foreach (list<Inherited>::iterator, i, inherited) {
             nix::Attr & a = (*v.attrs)[i->first.name];
             Value * v2 = state.lookupVar(&env, i->first);
-            mkCopy(a.value, *v2);
+            a.value = v2;
             mkCopy(env2.values[displ++], *v2);
             a.pos = &i->second;
         }
@@ -474,10 +489,12 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
            Hence we need __overrides.) */
         Bindings::iterator overrides = v.attrs->find(state.sOverrides);
         if (overrides != v.attrs->end()) {
-            state.forceAttrs(overrides->second.value);
-            foreach (Bindings::iterator, i, *overrides->second.value.attrs) {
+            state.forceAttrs(*overrides->second.value);
+            foreach (Bindings::iterator, i, *overrides->second.value->attrs) {
                 nix::Attr & a = (*v.attrs)[i->first];
-                mkCopy(a.value, i->second.value);
+                if (a.value)
+                    mkCopy(env2.values[displs[i->first]], *i->second.value);
+                a = i->second;
             }
         }
     }
@@ -485,13 +502,14 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
     else {
         foreach (Attrs::iterator, i, attrs) {
             nix::Attr & a = (*v.attrs)[i->first];
-            mkThunk(a.value, env, i->second.first);
+            a.value = state.allocValue();
+            mkThunk(*a.value, env, i->second.first);
             a.pos = &i->second.second;
         }
 
         foreach (list<Inherited>::iterator, i, inherited) {
             nix::Attr & a = (*v.attrs)[i->first.name];
-            mkCopy(a.value, *state.lookupVar(&env, i->first));
+            a.value = state.lookupVar(&env, i->first);
             a.pos = &i->second;
         }
     }
@@ -548,13 +566,13 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
     if (i == v2.attrs->end())
         throwEvalError("attribute `%1%' missing", name);
     try {            
-        state.forceValue(i->second.value);
+        state.forceValue(*i->second.value);
     } catch (Error & e) {
         addErrorPrefix(e, "while evaluating the attribute `%1%' at %2%:\n",
             name, *i->second.pos);
         throw;
     }
-    v = i->second.value;
+    v = *i->second.value;
 }
 
 
@@ -578,9 +596,9 @@ void ExprApp::eval(EvalState & state, Env & env, Value & v)
 {
     Value vFun;
     state.eval(env, e1, vFun);
-    Value vArg;
-    mkThunk(vArg, env, e2); // !!! should this be on the heap?
-    state.callFunction(vFun, vArg, v);
+    Value * vArg = state.allocValue();
+    mkThunk(*vArg, env, e2);
+    state.callFunction(vFun, *vArg, v);
 }
 
 
@@ -656,7 +674,7 @@ void EvalState::callFunction(Value & fun, Value & arg, Value & v)
                 mkThunk(env2.values[displ++], env2, i->def);
             } else {
                 attrsUsed++;
-                mkCopy(env2.values[displ++], j->second.value);
+                mkCopy(env2.values[displ++], *j->second.value);
             }
         }
 
@@ -677,7 +695,7 @@ void EvalState::callFunction(Value & fun, Value & arg, Value & v)
 }
 
 
-void EvalState::autoCallFunction(const Bindings & args, Value & fun, Value & res)
+void EvalState::autoCallFunction(Bindings & args, Value & fun, Value & res)
 {
     forceValue(fun);
 
@@ -690,7 +708,7 @@ void EvalState::autoCallFunction(const Bindings & args, Value & fun, Value & res
     mkAttrs(actualArgs);
 
     foreach (Formals::Formals_::iterator, i, fun.lambda.fun->formals->formals) {
-        Bindings::const_iterator j = args.find(i->name);
+        Bindings::iterator j = args.find(i->name);
         if (j != args.end())
             (*actualArgs.attrs)[i->name] = j->second;
         else if (!i->def)
@@ -780,11 +798,8 @@ void ExprOpUpdate::eval(EvalState & state, Env & env, Value & v)
 
     state.cloneAttrs(v1, v);
 
-    foreach (Bindings::iterator, i, *v2.attrs) {
-        Attr & a = (*v.attrs)[i->first];
-        mkCopy(a.value, i->second.value);
-        a.pos = i->second.pos;
-    }
+    foreach (Bindings::iterator, i, *v2.attrs)
+        (*v.attrs)[i->first] = i->second;
 
     state.nrOpUpdateValuesCopied += v.attrs->size();
 }
@@ -866,7 +881,7 @@ void EvalState::strictForceValue(Value & v)
     
     if (v.type == tAttrs) {
         foreach (Bindings::iterator, i, *v.attrs)
-            strictForceValue(i->second.value);
+            strictForceValue(*i->second.value);
     }
     
     else if (v.type == tList) {
@@ -957,7 +972,7 @@ bool EvalState::isDerivation(Value & v)
 {
     if (v.type != tAttrs) return false;
     Bindings::iterator i = v.attrs->find(sType);
-    return i != v.attrs->end() && forceStringNoCtx(i->second.value) == "derivation";
+    return i != v.attrs->end() && forceStringNoCtx(*i->second.value) == "derivation";
 }
 
 
@@ -1001,7 +1016,7 @@ string EvalState::coerceToString(Value & v, PathSet & context,
         Bindings::iterator i = v.attrs->find(sOutPath);
         if (i == v.attrs->end())
             throwTypeError("cannot coerce an attribute set (except a derivation) to a string");
-        return coerceToString(i->second.value, context, coerceMore, copyToStore);
+        return coerceToString(*i->second.value, context, coerceMore, copyToStore);
     }
 
     if (coerceMore) {
@@ -1087,9 +1102,9 @@ bool EvalState::eqValues(Value & v1, Value & v2)
 
         case tAttrs: {
             if (v1.attrs->size() != v2.attrs->size()) return false;
-            Bindings::iterator i, j;
-            for (i = v1.attrs->begin(), j = v2.attrs->begin(); i != v1.attrs->end(); ++i, ++j)
-                if (i->first != j->first || !eqValues(i->second.value, j->second.value))
+            Bindings::iterator i = v1.attrs->begin(), j = v2.attrs->begin();
+            for ( ; i != v1.attrs->end(); ++i, ++j)
+                if (i->first != j->first || !eqValues(*i->second.value, *j->second.value))
                     return false;
             return true;
         }
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 738ca9439..ee7db91a0 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -122,7 +122,7 @@ struct Env
 
 struct Attr
 {
-    Value value;
+    Value * value;
     Pos * pos;
     Attr() : pos(&noPos) { };
 };
@@ -294,12 +294,15 @@ public:
 
     /* Automatically call a function for which each argument has a
        default value or has a binding in the `args' map. */
-    void autoCallFunction(const Bindings & args, Value & fun, Value & res);
+    void autoCallFunction(Bindings & args, Value & fun, Value & res);
     
     /* Allocation primitives. */
+    Value * allocValue();
     Value * allocValues(unsigned int count);
     Env & allocEnv(unsigned int size);
 
+    Value * allocAttr(Value & vAttrs, const Symbol & name);
+
     void mkList(Value & v, unsigned int length);
     void mkAttrs(Value & v);
     void mkThunk_(Value & v, Expr * expr);
diff --git a/src/libexpr/get-drvs.cc b/src/libexpr/get-drvs.cc
index 82a92416b..63106b87b 100644
--- a/src/libexpr/get-drvs.cc
+++ b/src/libexpr/get-drvs.cc
@@ -10,7 +10,7 @@ string DrvInfo::queryDrvPath(EvalState & state) const
     if (drvPath == "" && attrs) {
         Bindings::iterator i = attrs->find(state.sDrvPath);
         PathSet context;
-        (string &) drvPath = i != attrs->end() ? state.coerceToPath(i->second.value, context) : "";
+        (string &) drvPath = i != attrs->end() ? state.coerceToPath(*i->second.value, context) : "";
     }
     return drvPath;
 }
@@ -21,7 +21,7 @@ string DrvInfo::queryOutPath(EvalState & state) const
     if (outPath == "" && attrs) {
         Bindings::iterator i = attrs->find(state.sOutPath);
         PathSet context;
-        (string &) outPath = i != attrs->end() ? state.coerceToPath(i->second.value, context) : "";
+        (string &) outPath = i != attrs->end() ? state.coerceToPath(*i->second.value, context) : "";
     }
     return outPath;
 }
@@ -36,21 +36,21 @@ MetaInfo DrvInfo::queryMetaInfo(EvalState & state) const
     Bindings::iterator a = attrs->find(state.sMeta);
     if (a == attrs->end()) return meta; /* fine, empty meta information */
 
-    state.forceAttrs(a->second.value);
+    state.forceAttrs(*a->second.value);
 
-    foreach (Bindings::iterator, i, *a->second.value.attrs) {
+    foreach (Bindings::iterator, i, *a->second.value->attrs) {
         MetaValue value;
-        state.forceValue(i->second.value);
-        if (i->second.value.type == tString) {
+        state.forceValue(*i->second.value);
+        if (i->second.value->type == tString) {
             value.type = MetaValue::tpString;
-            value.stringValue = i->second.value.string.s;
-        } else if (i->second.value.type == tInt) {
+            value.stringValue = i->second.value->string.s;
+        } else if (i->second.value->type == tInt) {
             value.type = MetaValue::tpInt;
-            value.intValue = i->second.value.integer;
-        } else if (i->second.value.type == tList) {
+            value.intValue = i->second.value->integer;
+        } else if (i->second.value->type == tList) {
             value.type = MetaValue::tpStrings;
-            for (unsigned int j = 0; j < i->second.value.list.length; ++j)
-                value.stringValues.push_back(state.forceStringNoCtx(*i->second.value.list.elems[j]));
+            for (unsigned int j = 0; j < i->second.value->list.length; ++j)
+                value.stringValues.push_back(state.forceStringNoCtx(*i->second.value->list.elems[j]));
         } else continue;
         ((MetaInfo &) meta)[i->first] = value;
     }
@@ -99,13 +99,13 @@ static bool getDerivation(EvalState & state, Value & v,
         Bindings::iterator i = v.attrs->find(state.sName);
         /* !!! We really would like to have a decent back trace here. */
         if (i == v.attrs->end()) throw TypeError("derivation name missing");
-        drv.name = state.forceStringNoCtx(i->second.value);
+        drv.name = state.forceStringNoCtx(*i->second.value);
 
-        i = v.attrs->find(state.sSystem);
-        if (i == v.attrs->end())
+        Bindings::iterator i2 = v.attrs->find(state.sSystem);
+        if (i2 == v.attrs->end())
             drv.system = "unknown";
         else
-            drv.system = state.forceStringNoCtx(i->second.value);
+            drv.system = state.forceStringNoCtx(*i2->second.value);
 
         drv.attrs = v.attrs;
 
@@ -138,7 +138,7 @@ static string addToPath(const string & s1, const string & s2)
 
 
 static void getDerivations(EvalState & state, Value & vIn,
-    const string & pathPrefix, const Bindings & autoArgs,
+    const string & pathPrefix, Bindings & autoArgs,
     DrvInfos & drvs, Done & done)
 {
     Value v;
@@ -168,7 +168,7 @@ static void getDerivations(EvalState & state, Value & vIn,
         foreach (SortedSymbols::iterator, i, attrs) {
             startNest(nest, lvlDebug, format("evaluating attribute `%1%'") % i->first);
             string pathPrefix2 = addToPath(pathPrefix, i->first);
-            Value & v2((*v.attrs)[i->second].value);
+            Value & v2(*(*v.attrs)[i->second].value);
             if (combineChannels)
                 getDerivations(state, v2, pathPrefix2, autoArgs, drvs, done);
             else if (getDerivation(state, v2, pathPrefix2, drvs, done)) {
@@ -178,7 +178,7 @@ static void getDerivations(EvalState & state, Value & vIn,
                    attribute. */
                 if (v2.type == tAttrs) {
                     Bindings::iterator j = v2.attrs->find(state.symbols.create("recurseForDerivations"));
-                    if (j != v2.attrs->end() && state.forceBool(j->second.value))
+                    if (j != v2.attrs->end() && state.forceBool(*j->second.value))
                         getDerivations(state, v2, pathPrefix2, autoArgs, drvs, done);
                 }
             }
@@ -200,7 +200,7 @@ static void getDerivations(EvalState & state, Value & vIn,
 
 
 void getDerivations(EvalState & state, Value & v, const string & pathPrefix,
-    const Bindings & autoArgs, DrvInfos & drvs)
+    Bindings & autoArgs, DrvInfos & drvs)
 {
     Done done;
     getDerivations(state, v, pathPrefix, autoArgs, drvs, done);
diff --git a/src/libexpr/get-drvs.hh b/src/libexpr/get-drvs.hh
index 7b96decf4..7c014b7e4 100644
--- a/src/libexpr/get-drvs.hh
+++ b/src/libexpr/get-drvs.hh
@@ -70,7 +70,7 @@ typedef list<DrvInfo> DrvInfos;
 bool getDerivation(EvalState & state, Value & v, DrvInfo & drv);
 
 void getDerivations(EvalState & state, Value & v, const string & pathPrefix,
-    const Bindings & autoArgs, DrvInfos & drvs);
+    Bindings & autoArgs, DrvInfos & drvs);
 
  
 }
diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index 898fdb609..9f2ea7883 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -213,10 +213,10 @@ void ExprAttrs::bindVars(const StaticEnv & env)
         unsigned int displ = 0;
 
         foreach (ExprAttrs::Attrs::iterator, i, attrs)
-            newEnv.vars[i->first] = displ++;
+            displs[i->first] = newEnv.vars[i->first] = displ++;
 
         foreach (list<Inherited>::iterator, i, inherited) {
-            newEnv.vars[i->first.name] = displ++;
+            displs[i->first.name] = newEnv.vars[i->first.name] = displ++;
             i->first.bind(env);
         }
 
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 1d03220f6..5c0071dca 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -137,6 +137,7 @@ struct ExprAttrs : Expr
     Attrs attrs;
     list<Inherited> inherited;
     std::map<Symbol, Pos> attrNames; // used during parsing
+    std::map<Symbol, unsigned int> displs;
     ExprAttrs() : recursive(false) { };
     COMMON_METHODS
 };
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 973670e68..c7709ca9e 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -119,24 +119,24 @@ static void prim_genericClosure(EvalState & state, Value * * args, Value & v)
         args[0]->attrs->find(state.symbols.create("startSet"));
     if (startSet == args[0]->attrs->end())
         throw EvalError("attribute `startSet' required");
-    state.forceList(startSet->second.value);
+    state.forceList(*startSet->second.value);
 
     list<Value *> workSet;
-    for (unsigned int n = 0; n < startSet->second.value.list.length; ++n)
-        workSet.push_back(startSet->second.value.list.elems[n]);
+    for (unsigned int n = 0; n < startSet->second.value->list.length; ++n)
+        workSet.push_back(startSet->second.value->list.elems[n]);
 
     /* Get the operator. */
     Bindings::iterator op =
         args[0]->attrs->find(state.symbols.create("operator"));
     if (op == args[0]->attrs->end())
         throw EvalError("attribute `operator' required");
-    state.forceValue(op->second.value);
+    state.forceValue(*op->second.value);
 
     /* Construct the closure by applying the operator to element of
        `workSet', adding the result to `workSet', continuing until
        no new elements are found. */
     list<Value> res;
-    set<Value, CompareValues> doneKeys;
+    set<Value, CompareValues> doneKeys; // !!! use Value *?
     while (!workSet.empty()) {
 	Value * e = *(workSet.begin());
 	workSet.pop_front();
@@ -147,15 +147,15 @@ static void prim_genericClosure(EvalState & state, Value * * args, Value & v)
             e->attrs->find(state.symbols.create("key"));
         if (key == e->attrs->end())
             throw EvalError("attribute `key' required");
-        state.forceValue(key->second.value);
+        state.forceValue(*key->second.value);
 
-        if (doneKeys.find(key->second.value) != doneKeys.end()) continue;
-        doneKeys.insert(key->second.value);
+        if (doneKeys.find(*key->second.value) != doneKeys.end()) continue;
+        doneKeys.insert(*key->second.value);
         res.push_back(*e);
         
         /* Call the `operator' function with `e' as argument. */
         Value call;
-        mkApp(call, op->second.value, *e);
+        mkApp(call, *op->second.value, *e);
         state.forceList(call);
 
         /* Add the values returned by the operator to the work set. */
@@ -213,11 +213,13 @@ static void prim_tryEval(EvalState & state, Value * * args, Value & v)
     state.mkAttrs(v);
     try {
         state.forceValue(*args[0]);
-        (*v.attrs)[state.symbols.create("value")].value = *args[0];
-        mkBool((*v.attrs)[state.symbols.create("success")].value, true);
+        printMsg(lvlError, format("%1%") % *args[0]);
+        (*v.attrs)[state.symbols.create("value")].value = args[0];
+        mkBool(*state.allocAttr(v, state.symbols.create("success")), true);
+        printMsg(lvlError, format("%1%") % v);
     } catch (AssertionError & e) {
-        mkBool((*v.attrs)[state.symbols.create("value")].value, false);
-        mkBool((*v.attrs)[state.symbols.create("success")].value, false);
+        mkBool(*state.allocAttr(v, state.symbols.create("value")), false);
+        mkBool(*state.allocAttr(v, state.symbols.create("success")), false);
     }
 }
 
@@ -326,7 +328,7 @@ static void prim_derivationStrict(EvalState & state, Value * * args, Value & v)
     string drvName;
     Pos & posDrvName(*attr->second.pos);
     try {        
-        drvName = state.forceStringNoCtx(attr->second.value);
+        drvName = state.forceStringNoCtx(*attr->second.value);
     } catch (Error & e) {
         e.addPrefix(format("while evaluating the derivation attribute `name' at %1%:\n") % posDrvName);
         throw;
@@ -349,9 +351,9 @@ static void prim_derivationStrict(EvalState & state, Value * * args, Value & v)
             /* The `args' attribute is special: it supplies the
                command-line arguments to the builder. */
             if (key == "args") {
-                state.forceList(i->second.value);
-                for (unsigned int n = 0; n < i->second.value.list.length; ++n) {
-                    string s = state.coerceToString(*i->second.value.list.elems[n], context, true);
+                state.forceList(*i->second.value);
+                for (unsigned int n = 0; n < i->second.value->list.length; ++n) {
+                    string s = state.coerceToString(*i->second.value->list.elems[n], context, true);
                     drv.args.push_back(s);
                 }
             }
@@ -359,7 +361,7 @@ static void prim_derivationStrict(EvalState & state, Value * * args, Value & v)
             /* All other attributes are passed to the builder through
                the environment. */
             else {
-                string s = state.coerceToString(i->second.value, context, true);
+                string s = state.coerceToString(*i->second.value, context, true);
                 drv.env[key] = s;
                 if (key == "builder") drv.builder = s;
                 else if (i->first == state.sSystem) drv.platform = s;
@@ -488,8 +490,8 @@ static void prim_derivationStrict(EvalState & state, Value * * args, Value & v)
 
     /* !!! assumes a single output */
     state.mkAttrs(v);
-    mkString((*v.attrs)[state.sOutPath].value, outPath, singleton<PathSet>(drvPath));
-    mkString((*v.attrs)[state.sDrvPath].value, drvPath, singleton<PathSet>("=" + drvPath));
+    mkString(*state.allocAttr(v, state.sOutPath), outPath, singleton<PathSet>(drvPath));
+    mkString(*state.allocAttr(v, state.sDrvPath), drvPath, singleton<PathSet>("=" + drvPath));
 }
 
 
@@ -713,8 +715,8 @@ static void prim_getAttr(EvalState & state, Value * * args, Value & v)
     if (i == args[1]->attrs->end())
         throw EvalError(format("attribute `%1%' missing") % attr);
     // !!! add to stack trace?
-    state.forceValue(i->second.value);
-    v = i->second.value;
+    state.forceValue(*i->second.value);
+    v = *i->second.value;
 }
 
 
@@ -766,15 +768,13 @@ static void prim_listToAttrs(EvalState & state, Value * * args, Value & v)
         Bindings::iterator j = v2.attrs->find(state.sName);
         if (j == v2.attrs->end())
             throw TypeError("`name' attribute missing in a call to `listToAttrs'");
-        string name = state.forceStringNoCtx(j->second.value);
+        string name = state.forceStringNoCtx(*j->second.value);
         
-        j = v2.attrs->find(state.symbols.create("value"));
-        if (j == v2.attrs->end())
+        Bindings::iterator j2 = v2.attrs->find(state.symbols.create("value"));
+        if (j2 == v2.attrs->end())
             throw TypeError("`value' attribute missing in a call to `listToAttrs'");
 
-        Attr & a = (*v.attrs)[state.symbols.create(name)];
-        mkCopy(a.value, j->second.value);
-        a.pos = j->second.pos;
+        (*v.attrs)[state.symbols.create(name)] = j2->second;
     }
 }
 
@@ -791,11 +791,8 @@ static void prim_intersectAttrs(EvalState & state, Value * * args, Value & v)
 
     foreach (Bindings::iterator, i, *args[0]->attrs) {
         Bindings::iterator j = args[1]->attrs->find(i->first);
-        if (j != args[1]->attrs->end()) {
-            Attr & a = (*v.attrs)[j->first];
-            mkCopy(a.value, j->second.value);
-            a.pos = j->second.pos;
-        }
+        if (j != args[1]->attrs->end())
+            (*v.attrs)[j->first] = j->second;
     }
 }
 
@@ -824,7 +821,8 @@ static void prim_functionArgs(EvalState & state, Value * * args, Value & v)
     if (!args[0]->lambda.fun->matchAttrs) return;
 
     foreach (Formals::Formals_::iterator, i, args[0]->lambda.fun->formals->formals)
-        mkBool((*v.attrs)[i->name].value, i->def);
+        // !!! should optimise booleans (allocate only once)
+        mkBool(*state.allocAttr(v, i->name), i->def);
 }
 
 
@@ -1007,8 +1005,8 @@ static void prim_parseDrvName(EvalState & state, Value * * args, Value & v)
     string name = state.forceStringNoCtx(*args[0]);
     DrvName parsed(name);
     state.mkAttrs(v);
-    mkString((*v.attrs)[state.sName].value, parsed.name);
-    mkString((*v.attrs)[state.symbols.create("version")].value, parsed.version);
+    mkString(*state.allocAttr(v, state.sName), parsed.name);
+    mkString(*state.allocAttr(v, state.symbols.create("version")), parsed.version);
 }
 
 
diff --git a/src/libexpr/value-to-xml.cc b/src/libexpr/value-to-xml.cc
index 8955a8a33..1bb200fb8 100644
--- a/src/libexpr/value-to-xml.cc
+++ b/src/libexpr/value-to-xml.cc
@@ -45,7 +45,7 @@ static void showAttrs(EvalState & state, bool strict, bool location,
         
         XMLOpenElement _(doc, "attr", xmlAttrs);
         printValueAsXML(state, strict, location,
-            a.value, doc, context, drvsSeen);
+            *a.value, doc, context, drvsSeen);
     }
 }
 
@@ -90,16 +90,16 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
                 Path drvPath;
                 a = v.attrs->find(state.sDrvPath);
                 if (a != v.attrs->end()) {
-                    if (strict) state.forceValue(a->second.value);
-                    if (a->second.value.type == tString)
-                        xmlAttrs["drvPath"] = drvPath = a->second.value.string.s;
+                    if (strict) state.forceValue(*a->second.value);
+                    if (a->second.value->type == tString)
+                        xmlAttrs["drvPath"] = drvPath = a->second.value->string.s;
                 }
         
                 a = v.attrs->find(state.sOutPath);
                 if (a != v.attrs->end()) {
-                    if (strict) state.forceValue(a->second.value);
-                    if (a->second.value.type == tString)
-                        xmlAttrs["outPath"] = a->second.value.string.s;
+                    if (strict) state.forceValue(*a->second.value);
+                    if (a->second.value->type == tString)
+                        xmlAttrs["outPath"] = a->second.value->string.s;
                 }
 
                 XMLOpenElement _(doc, "derivation", xmlAttrs);
diff --git a/src/nix-env/nix-env.cc b/src/nix-env/nix-env.cc
index df32b9f60..4895c68e9 100644
--- a/src/nix-env/nix-env.cc
+++ b/src/nix-env/nix-env.cc
@@ -161,7 +161,7 @@ static Expr * loadSourceExpr(EvalState & state, const Path & path)
 
 
 static void loadDerivations(EvalState & state, Path nixExprPath,
-    string systemFilter, const Bindings & autoArgs,
+    string systemFilter, Bindings & autoArgs,
     const string & pathPrefix, DrvInfos & elems)
 {
     Value v;
@@ -321,7 +321,7 @@ static bool isPath(const string & s)
 
 
 static void queryInstSources(EvalState & state,
-    const InstallSourceInfo & instSource, const Strings & args,
+    InstallSourceInfo & instSource, const Strings & args,
     DrvInfos & elems, bool newestOnly)
 {
     InstallSourceType type = instSource.type;
diff --git a/src/nix-env/user-env.cc b/src/nix-env/user-env.cc
index 0a619f698..07c94fe4d 100644
--- a/src/nix-env/user-env.cc
+++ b/src/nix-env/user-env.cc
@@ -25,7 +25,8 @@ DrvInfos queryInstalled(EvalState & state, const Path & userEnv)
     if (pathExists(manifestFile)) {
         Value v;
         state.eval(parseExprFromFile(state, manifestFile), v);
-        getDerivations(state, v, "", Bindings(), elems);
+        Bindings bindings;
+        getDerivations(state, v, "", bindings, elems);
     } else if (pathExists(oldManifestFile))
         readLegacyManifest(oldManifestFile, elems);
 
@@ -62,19 +63,19 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
         manifest.list.elems[n++] = &v;
         state.mkAttrs(v);
 
-        mkString((*v.attrs)[state.sType].value, "derivation");
-        mkString((*v.attrs)[state.sName].value, i->name);
-        mkString((*v.attrs)[state.sSystem].value, i->system);
-        mkString((*v.attrs)[state.sOutPath].value, i->queryOutPath(state));
+        mkString(*state.allocAttr(v, state.sType), "derivation");
+        mkString(*state.allocAttr(v, state.sName), i->name);
+        mkString(*state.allocAttr(v, state.sSystem), i->system);
+        mkString(*state.allocAttr(v, state.sOutPath), i->queryOutPath(state));
         if (drvPath != "")
-            mkString((*v.attrs)[state.sDrvPath].value, i->queryDrvPath(state));
+            mkString(*state.allocAttr(v, state.sDrvPath), i->queryDrvPath(state));
         
-        state.mkAttrs((*v.attrs)[state.sMeta].value);
+        state.mkAttrs(*state.allocAttr(v, state.sMeta));
         
         MetaInfo meta = i->queryMetaInfo(state);
 
         foreach (MetaInfo::const_iterator, j, meta) {
-            Value & v2((*(*v.attrs)[state.sMeta].value.attrs)[state.symbols.create(j->first)].value);
+            Value & v2(*state.allocAttr(*(*v.attrs)[state.sMeta].value, state.symbols.create(j->first)));
             switch (j->second.type) {
                 case MetaValue::tpInt: mkInt(v2, j->second.intValue); break;
                 case MetaValue::tpString: mkString(v2, j->second.stringValue); break;
@@ -114,10 +115,10 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
        builder with the manifest as argument. */
     Value args, topLevel;
     state.mkAttrs(args);
-    mkString((*args.attrs)[state.sSystem].value, thisSystem);
-    mkString((*args.attrs)[state.symbols.create("manifest")].value,
+    mkString(*state.allocAttr(args, state.sSystem), thisSystem);
+    mkString(*state.allocAttr(args, state.symbols.create("manifest")),
         manifestFile, singleton<PathSet>(manifestFile));
-    (*args.attrs)[state.symbols.create("derivations")].value = manifest;
+    (*args.attrs)[state.symbols.create("derivations")].value = &manifest;
     mkApp(topLevel, envBuilder, args);
         
     /* Evaluate it. */
diff --git a/src/nix-instantiate/nix-instantiate.cc b/src/nix-instantiate/nix-instantiate.cc
index 2925f9c5e..b330f0cf1 100644
--- a/src/nix-instantiate/nix-instantiate.cc
+++ b/src/nix-instantiate/nix-instantiate.cc
@@ -38,7 +38,7 @@ static bool indirectRoot = false;
 
 
 void processExpr(EvalState & state, const Strings & attrPaths,
-    bool parseOnly, bool strict, const Bindings & autoArgs,
+    bool parseOnly, bool strict, Bindings & autoArgs,
     bool evalOnly, bool xmlOutput, bool location, Expr * e)
 {
     if (parseOnly)