diff --git a/src/libexpr/attr-path.cc b/src/libexpr/attr-path.cc
index 83854df49..2d37dcb7e 100644
--- a/src/libexpr/attr-path.cc
+++ b/src/libexpr/attr-path.cc
@@ -67,7 +67,7 @@ std::pair<Value *, Pos> findAlongAttrPath(EvalState & state, const string & attr
 
         if (apType == apAttr) {
 
-            if (v->type != tAttrs)
+            if (v->type() != nAttrs)
                 throw TypeError(
                     "the expression selected by the selection path '%1%' should be a set but is %2%",
                     attrPath,
diff --git a/src/libexpr/attr-set.cc b/src/libexpr/attr-set.cc
index b1d61a285..b6091c955 100644
--- a/src/libexpr/attr-set.cc
+++ b/src/libexpr/attr-set.cc
@@ -24,9 +24,7 @@ void EvalState::mkAttrs(Value & v, size_t capacity)
         v = vEmptySet;
         return;
     }
-    clearValue(v);
-    v.type = tAttrs;
-    v.attrs = allocBindings(capacity);
+    v.mkAttrs(allocBindings(capacity));
     nrAttrsets++;
     nrAttrsInAttrsets += capacity;
 }
diff --git a/src/libexpr/eval-cache.cc b/src/libexpr/eval-cache.cc
index 7b025be23..75e9af787 100644
--- a/src/libexpr/eval-cache.cc
+++ b/src/libexpr/eval-cache.cc
@@ -390,14 +390,14 @@ Value & AttrCursor::forceValue()
     }
 
     if (root->db && (!cachedValue || std::get_if<placeholder_t>(&cachedValue->second))) {
-        if (v.type == tString)
+        if (v.type() == nString)
             cachedValue = {root->db->setString(getKey(), v.string.s, v.string.context),
                            string_t{v.string.s, {}}};
-        else if (v.type == tPath)
+        else if (v.type() == nPath)
             cachedValue = {root->db->setString(getKey(), v.path), v.path};
-        else if (v.type == tBool)
+        else if (v.type() == nBool)
             cachedValue = {root->db->setBool(getKey(), v.boolean), v.boolean};
-        else if (v.type == tAttrs)
+        else if (v.type() == nAttrs)
             ; // FIXME: do something?
         else
             cachedValue = {root->db->setMisc(getKey()), misc_t()};
@@ -442,7 +442,7 @@ std::shared_ptr<AttrCursor> AttrCursor::maybeGetAttr(Symbol name, bool forceErro
 
     auto & v = forceValue();
 
-    if (v.type != tAttrs)
+    if (v.type() != nAttrs)
         return nullptr;
         //throw TypeError("'%s' is not an attribute set", getAttrPathStr());
 
@@ -512,10 +512,10 @@ std::string AttrCursor::getString()
 
     auto & v = forceValue();
 
-    if (v.type != tString && v.type != tPath)
-        throw TypeError("'%s' is not a string but %s", getAttrPathStr(), showType(v.type));
+    if (v.type() != nString && v.type() != nPath)
+        throw TypeError("'%s' is not a string but %s", getAttrPathStr(), showType(v.type()));
 
-    return v.type == tString ? v.string.s : v.path;
+    return v.type() == nString ? v.string.s : v.path;
 }
 
 string_t AttrCursor::getStringWithContext()
@@ -543,12 +543,12 @@ string_t AttrCursor::getStringWithContext()
 
     auto & v = forceValue();
 
-    if (v.type == tString)
+    if (v.type() == nString)
         return {v.string.s, v.getContext()};
-    else if (v.type == tPath)
+    else if (v.type() == nPath)
         return {v.path, {}};
     else
-        throw TypeError("'%s' is not a string but %s", getAttrPathStr(), showType(v.type));
+        throw TypeError("'%s' is not a string but %s", getAttrPathStr(), showType(v.type()));
 }
 
 bool AttrCursor::getBool()
@@ -567,7 +567,7 @@ bool AttrCursor::getBool()
 
     auto & v = forceValue();
 
-    if (v.type != tBool)
+    if (v.type() != nBool)
         throw TypeError("'%s' is not a Boolean", getAttrPathStr());
 
     return v.boolean;
@@ -589,7 +589,7 @@ std::vector<Symbol> AttrCursor::getAttrs()
 
     auto & v = forceValue();
 
-    if (v.type != tAttrs)
+    if (v.type() != nAttrs)
         throw TypeError("'%s' is not an attribute set", getAttrPathStr());
 
     std::vector<Symbol> attrs;
diff --git a/src/libexpr/eval-inline.hh b/src/libexpr/eval-inline.hh
index 30f6ec7db..f6dead6b0 100644
--- a/src/libexpr/eval-inline.hh
+++ b/src/libexpr/eval-inline.hh
@@ -32,23 +32,21 @@ LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s, const
 
 void EvalState::forceValue(Value & v, const Pos & pos)
 {
-    if (v.type == tThunk) {
+    if (v.isThunk()) {
         Env * env = v.thunk.env;
         Expr * expr = v.thunk.expr;
         try {
-            v.type = tBlackhole;
+            v.mkBlackhole();
             //checkInterrupt();
             expr->eval(*this, *env, v);
         } catch (...) {
-            v.type = tThunk;
-            v.thunk.env = env;
-            v.thunk.expr = expr;
+            v.mkThunk(env, expr);
             throw;
         }
     }
-    else if (v.type == tApp)
+    else if (v.isApp())
         callFunction(*v.app.left, *v.app.right, v, noPos);
-    else if (v.type == tBlackhole)
+    else if (v.isBlackhole())
         throwEvalError(pos, "infinite recursion encountered");
 }
 
@@ -56,7 +54,7 @@ void EvalState::forceValue(Value & v, const Pos & pos)
 inline void EvalState::forceAttrs(Value & v)
 {
     forceValue(v);
-    if (v.type != tAttrs)
+    if (v.type() != nAttrs)
         throwTypeError("value is %1% while a set was expected", v);
 }
 
@@ -64,7 +62,7 @@ inline void EvalState::forceAttrs(Value & v)
 inline void EvalState::forceAttrs(Value & v, const Pos & pos)
 {
     forceValue(v, pos);
-    if (v.type != tAttrs)
+    if (v.type() != nAttrs)
         throwTypeError(pos, "value is %1% while a set was expected", v);
 }
 
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index c6f4d1716..5a641d02c 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();
 
@@ -77,7 +77,7 @@ static void printValue(std::ostream & str, std::set<const Value *> & active, con
         return;
     }
 
-    switch (v.type) {
+    switch (v.internalType) {
     case tInt:
         str << v.integer;
         break;
@@ -158,32 +158,27 @@ std::ostream & operator << (std::ostream & str, const Value & v)
 
 const Value *getPrimOp(const Value &v) {
     const Value * primOp = &v;
-    while (primOp->type == tPrimOpApp) {
+    while (primOp->isPrimOpApp()) {
         primOp = primOp->primOpApp.left;
     }
-    assert(primOp->type == tPrimOp);
+    assert(primOp->isPrimOp());
     return primOp;
 }
 
-
 string showType(ValueType type)
 {
     switch (type) {
-        case tInt: return "an integer";
-        case tBool: return "a Boolean";
-        case tString: return "a string";
-        case tPath: return "a path";
-        case tNull: return "null";
-        case tAttrs: return "a set";
-        case tList1: case tList2: case tListN: return "a list";
-        case tThunk: return "a thunk";
-        case tApp: return "a function application";
-        case tLambda: return "a function";
-        case tBlackhole: return "a black hole";
-        case tPrimOp: return "a built-in function";
-        case tPrimOpApp: return "a partially applied built-in function";
-        case tExternal: return "an external value";
-        case tFloat: return "a float";
+        case nInt: return "an integer";
+        case nBool: return "a Boolean";
+        case nString: return "a string";
+        case nPath: return "a path";
+        case nNull: return "null";
+        case nAttrs: return "a set";
+        case nList: return "a list";
+        case nFunction: return "a function";
+        case nExternal: return "an external value";
+        case nFloat: return "a float";
+        case nThunk: return "a thunk";
     }
     abort();
 }
@@ -191,15 +186,18 @@ string showType(ValueType type)
 
 string showType(const Value & v)
 {
-    switch (v.type) {
+    switch (v.internalType) {
         case tString: return v.string.context ? "a string with context" : "a string";
         case tPrimOp:
             return fmt("the built-in function '%s'", string(v.primOp->name));
         case tPrimOpApp:
             return fmt("the partially applied built-in function '%s'", string(getPrimOp(v)->primOp->name));
         case tExternal: return v.external->showType();
+        case tThunk: return "a thunk";
+        case tApp: return "a function application";
+        case tBlackhole: return "a black hole";
     default:
-        return showType(v.type);
+        return showType(v.type());
     }
 }
 
@@ -207,9 +205,9 @@ string showType(const Value & v)
 bool Value::isTrivial() const
 {
     return
-        type != tApp
-        && type != tPrimOpApp
-        && (type != tThunk
+        internalType != tApp
+        && internalType != tPrimOpApp
+        && (internalType != tThunk
             || (dynamic_cast<ExprAttrs *>(thunk.expr)
                 && ((ExprAttrs *) thunk.expr)->dynamicAttrs.empty())
             || dynamic_cast<ExprLambda *>(thunk.expr)
@@ -432,9 +430,7 @@ EvalState::EvalState(const Strings & _searchPath, ref<Store> store)
         }
     }
 
-    clearValue(vEmptySet);
-    vEmptySet.type = tAttrs;
-    vEmptySet.attrs = allocBindings(0);
+    vEmptySet.mkAttrs(allocBindings(0));
 
     createBaseEnv();
 }
@@ -550,16 +546,14 @@ Value * EvalState::addPrimOp(const string & name,
        the primop to a dummy value. */
     if (arity == 0) {
         auto vPrimOp = allocValue();
-        vPrimOp->type = tPrimOp;
-        vPrimOp->primOp = new PrimOp { .fun = primOp, .arity = 1, .name = sym };
+        vPrimOp->mkPrimOp(new PrimOp { .fun = primOp, .arity = 1, .name = sym });
         Value v;
         mkApp(v, *vPrimOp, *vPrimOp);
         return addConstant(name, v);
     }
 
     Value * v = allocValue();
-    v->type = tPrimOp;
-    v->primOp = new PrimOp { .fun = primOp, .arity = arity, .name = sym };
+    v->mkPrimOp(new PrimOp { .fun = primOp, .arity = arity, .name = sym });
     staticBaseEnv.vars[symbols.create(name)] = baseEnvDispl;
     baseEnv.values[baseEnvDispl++] = v;
     baseEnv.values[0]->attrs->push_back(Attr(sym, v));
@@ -574,8 +568,7 @@ Value * EvalState::addPrimOp(PrimOp && primOp)
     if (primOp.arity == 0) {
         primOp.arity = 1;
         auto vPrimOp = allocValue();
-        vPrimOp->type = tPrimOp;
-        vPrimOp->primOp = new PrimOp(std::move(primOp));
+        vPrimOp->mkPrimOp(new PrimOp(std::move(primOp)));
         Value v;
         mkApp(v, *vPrimOp, *vPrimOp);
         return addConstant(primOp.name, v);
@@ -586,8 +579,7 @@ Value * EvalState::addPrimOp(PrimOp && primOp)
         primOp.name = symbols.create(std::string(primOp.name, 2));
 
     Value * v = allocValue();
-    v->type = tPrimOp;
-    v->primOp = new PrimOp(std::move(primOp));
+    v->mkPrimOp(new PrimOp(std::move(primOp)));
     staticBaseEnv.vars[envName] = baseEnvDispl;
     baseEnv.values[baseEnvDispl++] = v;
     baseEnv.values[0]->attrs->push_back(Attr(primOp.name, v));
@@ -603,9 +595,9 @@ Value & EvalState::getBuiltin(const string & name)
 
 std::optional<EvalState::Doc> EvalState::getDoc(Value & v)
 {
-    if (v.type == tPrimOp || v.type == tPrimOpApp) {
+    if (v.isPrimOp() || v.isPrimOpApp()) {
         auto v2 = &v;
-        while (v2->type == tPrimOpApp)
+        while (v2->isPrimOpApp())
             v2 = v2->primOpApp.left;
         if (v2->primOp->doc)
             return Doc {
@@ -710,15 +702,13 @@ LocalNoInline(void addErrorTrace(Error & e, const Pos & pos, const char * s, con
 
 void mkString(Value & v, const char * s)
 {
-    mkStringNoCopy(v, dupString(s));
+    v.mkString(dupString(s));
 }
 
 
 Value & mkString(Value & v, std::string_view s, const PathSet & context)
 {
-    v.type = tString;
-    v.string.s = dupStringWithLen(s.data(), s.size());
-    v.string.context = 0;
+    v.mkString(dupStringWithLen(s.data(), s.size()));
     if (!context.empty()) {
         size_t n = 0;
         v.string.context = (const char * *)
@@ -733,7 +723,7 @@ Value & mkString(Value & v, std::string_view s, const PathSet & context)
 
 void mkPath(Value & v, const char * s)
 {
-    mkPathNoCopy(v, dupString(s));
+    v.mkPath(dupString(s));
 }
 
 
@@ -794,16 +784,9 @@ Env & EvalState::allocEnv(size_t size)
 
 void EvalState::mkList(Value & v, size_t size)
 {
-    clearValue(v);
-    if (size == 1)
-        v.type = tList1;
-    else if (size == 2)
-        v.type = tList2;
-    else {
-        v.type = tListN;
-        v.bigList.size = size;
-        v.bigList.elems = size ? (Value * *) allocBytes(size * sizeof(Value *)) : 0;
-    }
+    v.mkList(size);
+    if (size > 2)
+        v.bigList.elems = (Value * *) allocBytes(size * sizeof(Value *));
     nrListElems += size;
 }
 
@@ -812,9 +795,7 @@ unsigned long nrThunks = 0;
 
 static inline void mkThunk(Value & v, Env & env, Expr * expr)
 {
-    v.type = tThunk;
-    v.thunk.env = &env;
-    v.thunk.expr = expr;
+    v.mkThunk(&env, expr);
     nrThunks++;
 }
 
@@ -949,7 +930,7 @@ inline bool EvalState::evalBool(Env & env, Expr * e)
 {
     Value v;
     e->eval(*this, env, v);
-    if (v.type != tBool)
+    if (v.type() != nBool)
         throwTypeError("value is %1% while a Boolean was expected", v);
     return v.boolean;
 }
@@ -959,7 +940,7 @@ inline bool EvalState::evalBool(Env & env, Expr * e, const Pos & pos)
 {
     Value v;
     e->eval(*this, env, v);
-    if (v.type != tBool)
+    if (v.type() != nBool)
         throwTypeError(pos, "value is %1% while a Boolean was expected", v);
     return v.boolean;
 }
@@ -968,7 +949,7 @@ inline bool EvalState::evalBool(Env & env, Expr * e, const Pos & pos)
 inline void EvalState::evalAttrs(Env & env, Expr * e, Value & v)
 {
     e->eval(*this, env, v);
-    if (v.type != tAttrs)
+    if (v.type() != nAttrs)
         throwTypeError("value is %1% while a set was expected", v);
 }
 
@@ -1068,7 +1049,7 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
         Value nameVal;
         i.nameExpr->eval(state, *dynamicEnv, nameVal);
         state.forceValue(nameVal, i.pos);
-        if (nameVal.type == tNull)
+        if (nameVal.type() == nNull)
             continue;
         state.forceStringNoCtx(nameVal);
         Symbol nameSym = state.symbols.create(nameVal.string.s);
@@ -1153,7 +1134,7 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
             Symbol name = getName(i, state, env);
             if (def) {
                 state.forceValue(*vAttrs, pos);
-                if (vAttrs->type != tAttrs ||
+                if (vAttrs->type() != nAttrs ||
                     (j = vAttrs->attrs->find(name)) == vAttrs->attrs->end())
                 {
                     def->eval(state, env, v);
@@ -1193,7 +1174,7 @@ void ExprOpHasAttr::eval(EvalState & state, Env & env, Value & v)
         state.forceValue(*vAttrs);
         Bindings::iterator j;
         Symbol name = getName(i, state, env);
-        if (vAttrs->type != tAttrs ||
+        if (vAttrs->type() != nAttrs ||
             (j = vAttrs->attrs->find(name)) == vAttrs->attrs->end())
         {
             mkBool(v, false);
@@ -1209,9 +1190,7 @@ void ExprOpHasAttr::eval(EvalState & state, Env & env, Value & v)
 
 void ExprLambda::eval(EvalState & state, Env & env, Value & v)
 {
-    v.type = tLambda;
-    v.lambda.env = &env;
-    v.lambda.fun = this;
+    v.mkLambda(&env, this);
 }
 
 
@@ -1229,11 +1208,11 @@ void EvalState::callPrimOp(Value & fun, Value & arg, Value & v, const Pos & pos)
     /* Figure out the number of arguments still needed. */
     size_t argsDone = 0;
     Value * primOp = &fun;
-    while (primOp->type == tPrimOpApp) {
+    while (primOp->isPrimOpApp()) {
         argsDone++;
         primOp = primOp->primOpApp.left;
     }
-    assert(primOp->type == tPrimOp);
+    assert(primOp->isPrimOp());
     auto arity = primOp->primOp->arity;
     auto argsLeft = arity - argsDone;
 
@@ -1244,7 +1223,7 @@ void EvalState::callPrimOp(Value & fun, Value & arg, Value & v, const Pos & pos)
         Value * vArgs[arity];
         auto n = arity - 1;
         vArgs[n--] = &arg;
-        for (Value * arg = &fun; arg->type == tPrimOpApp; arg = arg->primOpApp.left)
+        for (Value * arg = &fun; arg->isPrimOpApp(); arg = arg->primOpApp.left)
             vArgs[n--] = arg->primOpApp.right;
 
         /* And call the primop. */
@@ -1254,9 +1233,7 @@ void EvalState::callPrimOp(Value & fun, Value & arg, Value & v, const Pos & pos)
     } else {
         Value * fun2 = allocValue();
         *fun2 = fun;
-        v.type = tPrimOpApp;
-        v.primOpApp.left = fun2;
-        v.primOpApp.right = &arg;
+        v.mkPrimOpApp(fun2, &arg);
     }
 }
 
@@ -1266,12 +1243,12 @@ void EvalState::callFunction(Value & fun, Value & arg, Value & v, const Pos & po
 
     forceValue(fun, pos);
 
-    if (fun.type == tPrimOp || fun.type == tPrimOpApp) {
+    if (fun.isPrimOp() || fun.isPrimOpApp()) {
         callPrimOp(fun, arg, v, pos);
         return;
     }
 
-    if (fun.type == tAttrs) {
+    if (fun.type() == nAttrs) {
       auto found = fun.attrs->find(sFunctor);
       if (found != fun.attrs->end()) {
         /* fun may be allocated on the stack of the calling function,
@@ -1287,7 +1264,7 @@ void EvalState::callFunction(Value & fun, Value & arg, Value & v, const Pos & po
       }
     }
 
-    if (fun.type != tLambda)
+    if (!fun.isLambda())
         throwTypeError(pos, "attempt to call something which is not a function but %1%", fun);
 
     ExprLambda & lambda(*fun.lambda.fun);
@@ -1370,7 +1347,7 @@ void EvalState::autoCallFunction(Bindings & args, Value & fun, Value & res)
 {
     forceValue(fun);
 
-    if (fun.type == tAttrs) {
+    if (fun.type() == nAttrs) {
         auto found = fun.attrs->find(sFunctor);
         if (found != fun.attrs->end()) {
             Value * v = allocValue();
@@ -1380,7 +1357,7 @@ void EvalState::autoCallFunction(Bindings & args, Value & fun, Value & res)
         }
     }
 
-    if (fun.type != tLambda || !fun.lambda.fun->matchAttrs) {
+    if (!fun.isLambda() || !fun.lambda.fun->matchAttrs) {
         res = fun;
         return;
     }
@@ -1564,7 +1541,7 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
     NixFloat nf = 0;
 
     bool first = !forceString;
-    ValueType firstType = tString;
+    ValueType firstType = nString;
 
     for (auto & i : *es) {
         Value vTmp;
@@ -1575,36 +1552,36 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
            since paths are copied when they are used in a derivation),
            and none of the strings are allowed to have contexts. */
         if (first) {
-            firstType = vTmp.type;
+            firstType = vTmp.type();
             first = false;
         }
 
-        if (firstType == tInt) {
-            if (vTmp.type == tInt) {
+        if (firstType == nInt) {
+            if (vTmp.type() == nInt) {
                 n += vTmp.integer;
-            } else if (vTmp.type == tFloat) {
+            } else if (vTmp.type() == nFloat) {
                 // Upgrade the type from int to float;
-                firstType = tFloat;
+                firstType = nFloat;
                 nf = n;
                 nf += vTmp.fpoint;
             } else
                 throwEvalError(pos, "cannot add %1% to an integer", showType(vTmp));
-        } else if (firstType == tFloat) {
-            if (vTmp.type == tInt) {
+        } else if (firstType == nFloat) {
+            if (vTmp.type() == nInt) {
                 nf += vTmp.integer;
-            } else if (vTmp.type == tFloat) {
+            } else if (vTmp.type() == nFloat) {
                 nf += vTmp.fpoint;
             } else
                 throwEvalError(pos, "cannot add %1% to a float", showType(vTmp));
         } else
-            s << state.coerceToString(pos, vTmp, context, false, firstType == tString);
+            s << state.coerceToString(pos, vTmp, context, false, firstType == nString);
     }
 
-    if (firstType == tInt)
+    if (firstType == nInt)
         mkInt(v, n);
-    else if (firstType == tFloat)
+    else if (firstType == nFloat)
         mkFloat(v, nf);
-    else if (firstType == tPath) {
+    else if (firstType == nPath) {
         if (!context.empty())
             throwEvalError(pos, "a string that refers to a store path cannot be appended to a path");
         auto path = canonPath(s.str());
@@ -1631,7 +1608,7 @@ void EvalState::forceValueDeep(Value & v)
 
         forceValue(v);
 
-        if (v.type == tAttrs) {
+        if (v.type() == nAttrs) {
             for (auto & i : *v.attrs)
                 try {
                     recurse(*i.value);
@@ -1654,7 +1631,7 @@ void EvalState::forceValueDeep(Value & v)
 NixInt EvalState::forceInt(Value & v, const Pos & pos)
 {
     forceValue(v, pos);
-    if (v.type != tInt)
+    if (v.type() != nInt)
         throwTypeError(pos, "value is %1% while an integer was expected", v);
     return v.integer;
 }
@@ -1663,9 +1640,9 @@ NixInt EvalState::forceInt(Value & v, const Pos & pos)
 NixFloat EvalState::forceFloat(Value & v, const Pos & pos)
 {
     forceValue(v, pos);
-    if (v.type == tInt)
+    if (v.type() == nInt)
         return v.integer;
-    else if (v.type != tFloat)
+    else if (v.type() != nFloat)
         throwTypeError(pos, "value is %1% while a float was expected", v);
     return v.fpoint;
 }
@@ -1674,7 +1651,7 @@ NixFloat EvalState::forceFloat(Value & v, const Pos & pos)
 bool EvalState::forceBool(Value & v, const Pos & pos)
 {
     forceValue(v, pos);
-    if (v.type != tBool)
+    if (v.type() != nBool)
         throwTypeError(pos, "value is %1% while a Boolean was expected", v);
     return v.boolean;
 }
@@ -1682,14 +1659,14 @@ bool EvalState::forceBool(Value & v, const Pos & pos)
 
 bool EvalState::isFunctor(Value & fun)
 {
-    return fun.type == tAttrs && fun.attrs->find(sFunctor) != fun.attrs->end();
+    return fun.type() == nAttrs && fun.attrs->find(sFunctor) != fun.attrs->end();
 }
 
 
 void EvalState::forceFunction(Value & v, const Pos & pos)
 {
     forceValue(v, pos);
-    if (v.type != tLambda && v.type != tPrimOp && v.type != tPrimOpApp && !isFunctor(v))
+    if (v.type() != nFunction && !isFunctor(v))
         throwTypeError(pos, "value is %1% while a function was expected", v);
 }
 
@@ -1697,7 +1674,7 @@ void EvalState::forceFunction(Value & v, const Pos & pos)
 string EvalState::forceString(Value & v, const Pos & pos)
 {
     forceValue(v, pos);
-    if (v.type != tString) {
+    if (v.type() != nString) {
         if (pos)
             throwTypeError(pos, "value is %1% while a string was expected", v);
         else
@@ -1730,7 +1707,7 @@ void copyContext(const Value & v, PathSet & context)
 std::vector<std::pair<Path, std::string>> Value::getContext()
 {
     std::vector<std::pair<Path, std::string>> res;
-    assert(type == tString);
+    assert(internalType == tString);
     if (string.context)
         for (const char * * p = string.context; *p; ++p)
             res.push_back(decodeContext(*p));
@@ -1763,11 +1740,11 @@ string EvalState::forceStringNoCtx(Value & v, const Pos & pos)
 
 bool EvalState::isDerivation(Value & v)
 {
-    if (v.type != tAttrs) return false;
+    if (v.type() != nAttrs) return false;
     Bindings::iterator i = v.attrs->find(sType);
     if (i == v.attrs->end()) return false;
     forceValue(*i->value);
-    if (i->value->type != tString) return false;
+    if (i->value->type() != nString) return false;
     return strcmp(i->value->string.s, "derivation") == 0;
 }
 
@@ -1792,17 +1769,17 @@ string EvalState::coerceToString(const Pos & pos, Value & v, PathSet & context,
 
     string s;
 
-    if (v.type == tString) {
+    if (v.type() == nString) {
         copyContext(v, context);
         return v.string.s;
     }
 
-    if (v.type == tPath) {
+    if (v.type() == nPath) {
         Path path(canonPath(v.path));
         return copyToStore ? copyPathToStore(context, path) : path;
     }
 
-    if (v.type == tAttrs) {
+    if (v.type() == nAttrs) {
         auto maybeString = tryAttrsToString(pos, v, context, coerceMore, copyToStore);
         if (maybeString) {
             return *maybeString;
@@ -1812,18 +1789,18 @@ string EvalState::coerceToString(const Pos & pos, Value & v, PathSet & context,
         return coerceToString(pos, *i->value, context, coerceMore, copyToStore);
     }
 
-    if (v.type == tExternal)
+    if (v.type() == nExternal)
         return v.external->coerceToString(pos, context, coerceMore, copyToStore);
 
     if (coerceMore) {
 
         /* Note that `false' is represented as an empty string for
            shell scripting convenience, just like `null'. */
-        if (v.type == tBool && v.boolean) return "1";
-        if (v.type == tBool && !v.boolean) return "";
-        if (v.type == tInt) return std::to_string(v.integer);
-        if (v.type == tFloat) return std::to_string(v.fpoint);
-        if (v.type == tNull) return "";
+        if (v.type() == nBool && v.boolean) return "1";
+        if (v.type() == nBool && !v.boolean) return "";
+        if (v.type() == nInt) return std::to_string(v.integer);
+        if (v.type() == nFloat) return std::to_string(v.fpoint);
+        if (v.type() == nNull) return "";
 
         if (v.isList()) {
             string result;
@@ -1886,40 +1863,38 @@ bool EvalState::eqValues(Value & v1, Value & v2)
     if (&v1 == &v2) return true;
 
     // Special case type-compatibility between float and int
-    if (v1.type == tInt && v2.type == tFloat)
+    if (v1.type() == nInt && v2.type() == nFloat)
         return v1.integer == v2.fpoint;
-    if (v1.type == tFloat && v2.type == tInt)
+    if (v1.type() == nFloat && v2.type() == nInt)
         return v1.fpoint == v2.integer;
 
     // All other types are not compatible with each other.
-    if (v1.type != v2.type) return false;
+    if (v1.type() != v2.type()) return false;
 
-    switch (v1.type) {
+    switch (v1.type()) {
 
-        case tInt:
+        case nInt:
             return v1.integer == v2.integer;
 
-        case tBool:
+        case nBool:
             return v1.boolean == v2.boolean;
 
-        case tString:
+        case nString:
             return strcmp(v1.string.s, v2.string.s) == 0;
 
-        case tPath:
+        case nPath:
             return strcmp(v1.path, v2.path) == 0;
 
-        case tNull:
+        case nNull:
             return true;
 
-        case tList1:
-        case tList2:
-        case tListN:
+        case nList:
             if (v1.listSize() != v2.listSize()) return false;
             for (size_t n = 0; n < v1.listSize(); ++n)
                 if (!eqValues(*v1.listElems()[n], *v2.listElems()[n])) return false;
             return true;
 
-        case tAttrs: {
+        case nAttrs: {
             /* If both sets denote a derivation (type = "derivation"),
                then compare their outPaths. */
             if (isDerivation(v1) && isDerivation(v2)) {
@@ -1941,15 +1916,13 @@ bool EvalState::eqValues(Value & v1, Value & v2)
         }
 
         /* Functions are incomparable. */
-        case tLambda:
-        case tPrimOp:
-        case tPrimOpApp:
+        case nFunction:
             return false;
 
-        case tExternal:
+        case nExternal:
             return *v1.external == *v2.external;
 
-        case tFloat:
+        case nFloat:
             return v1.fpoint == v2.fpoint;
 
         default:
diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index 3e866e1f9..4f021570c 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -73,7 +73,7 @@ static std::tuple<fetchers::Tree, FlakeRef, FlakeRef> fetchOrSubstituteTree(
 
 static void forceTrivialValue(EvalState & state, Value & value, const Pos & pos)
 {
-    if (value.type == tThunk && value.isTrivial())
+    if (value.isThunk() && value.isTrivial())
         state.forceValue(value, pos);
 }
 
@@ -82,9 +82,9 @@ static void expectType(EvalState & state, ValueType type,
     Value & value, const Pos & pos)
 {
     forceTrivialValue(state, value, pos);
-    if (value.type != type)
+    if (value.type() != type)
         throw Error("expected %s but got %s at %s",
-            showType(type), showType(value.type), pos);
+            showType(type), showType(value.type()), pos);
 }
 
 static std::map<FlakeId, FlakeInput> parseFlakeInputs(
@@ -93,7 +93,7 @@ static std::map<FlakeId, FlakeInput> parseFlakeInputs(
 static FlakeInput parseFlakeInput(EvalState & state,
     const std::string & inputName, Value * value, const Pos & pos)
 {
-    expectType(state, tAttrs, *value, pos);
+    expectType(state, nAttrs, *value, pos);
 
     FlakeInput input;
 
@@ -108,19 +108,19 @@ static FlakeInput parseFlakeInput(EvalState & state,
     for (nix::Attr attr : *(value->attrs)) {
         try {
             if (attr.name == sUrl) {
-                expectType(state, tString, *attr.value, *attr.pos);
+                expectType(state, nString, *attr.value, *attr.pos);
                 url = attr.value->string.s;
                 attrs.emplace("url", *url);
             } else if (attr.name == sFlake) {
-                expectType(state, tBool, *attr.value, *attr.pos);
+                expectType(state, nBool, *attr.value, *attr.pos);
                 input.isFlake = attr.value->boolean;
             } else if (attr.name == sInputs) {
                 input.overrides = parseFlakeInputs(state, attr.value, *attr.pos);
             } else if (attr.name == sFollows) {
-                expectType(state, tString, *attr.value, *attr.pos);
+                expectType(state, nString, *attr.value, *attr.pos);
                 input.follows = parseInputPath(attr.value->string.s);
             } else {
-                if (attr.value->type == tString)
+                if (attr.value->type() == nString)
                     attrs.emplace(attr.name, attr.value->string.s);
                 else
                     throw TypeError("flake input attribute '%s' is %s while a string is expected",
@@ -158,7 +158,7 @@ static std::map<FlakeId, FlakeInput> parseFlakeInputs(
 {
     std::map<FlakeId, FlakeInput> inputs;
 
-    expectType(state, tAttrs, *value, pos);
+    expectType(state, nAttrs, *value, pos);
 
     for (nix::Attr & inputAttr : *(*value).attrs) {
         inputs.emplace(inputAttr.name,
@@ -199,10 +199,10 @@ static Flake getFlake(
     Value vInfo;
     state.evalFile(flakeFile, vInfo, true); // FIXME: symlink attack
 
-    expectType(state, tAttrs, vInfo, Pos(foFile, state.symbols.create(flakeFile), 0, 0));
+    expectType(state, nAttrs, vInfo, Pos(foFile, state.symbols.create(flakeFile), 0, 0));
 
     if (auto description = vInfo.attrs->get(state.sDescription)) {
-        expectType(state, tString, *description->value, *description->pos);
+        expectType(state, nString, *description->value, *description->pos);
         flake.description = description->value->string.s;
     }
 
@@ -214,9 +214,9 @@ static Flake getFlake(
     auto sOutputs = state.symbols.create("outputs");
 
     if (auto outputs = vInfo.attrs->get(sOutputs)) {
-        expectType(state, tLambda, *outputs->value, *outputs->pos);
+        expectType(state, nFunction, *outputs->value, *outputs->pos);
 
-        if (outputs->value->lambda.fun->matchAttrs) {
+        if (outputs->value->isLambda() && outputs->value->lambda.fun->matchAttrs) {
             for (auto & formal : outputs->value->lambda.fun->formals->formals) {
                 if (formal.name != state.sSelf)
                     flake.inputs.emplace(formal.name, FlakeInput {
@@ -231,21 +231,21 @@ static Flake getFlake(
     auto sNixConfig = state.symbols.create("nixConfig");
 
     if (auto nixConfig = vInfo.attrs->get(sNixConfig)) {
-        expectType(state, tAttrs, *nixConfig->value, *nixConfig->pos);
+        expectType(state, nAttrs, *nixConfig->value, *nixConfig->pos);
 
         for (auto & setting : *nixConfig->value->attrs) {
             forceTrivialValue(state, *setting.value, *setting.pos);
-            if (setting.value->type == tString)
+            if (setting.value->type() == nString)
                 flake.config.settings.insert({setting.name, state.forceStringNoCtx(*setting.value, *setting.pos)});
-            else if (setting.value->type == tInt)
+            else if (setting.value->type() == nInt)
                 flake.config.settings.insert({setting.name, state.forceInt(*setting.value, *setting.pos)});
-            else if (setting.value->type == tBool)
+            else if (setting.value->type() == nBool)
                 flake.config.settings.insert({setting.name, state.forceBool(*setting.value, *setting.pos)});
-            else if (setting.value->isList()) {
+            else if (setting.value->type() == nList) {
                 std::vector<std::string> ss;
                 for (unsigned int n = 0; n < setting.value->listSize(); ++n) {
                     auto elem = setting.value->listElems()[n];
-                    if (elem->type != tString)
+                    if (elem->type() != nString)
                         throw TypeError("list element in flake configuration setting '%s' is %s while a string is expected",
                             setting.name, showType(*setting.value));
                     ss.push_back(state.forceStringNoCtx(*elem, *setting.pos));
diff --git a/src/libexpr/get-drvs.cc b/src/libexpr/get-drvs.cc
index 91916e8bf..32c115c12 100644
--- a/src/libexpr/get-drvs.cc
+++ b/src/libexpr/get-drvs.cc
@@ -128,7 +128,7 @@ DrvInfo::Outputs DrvInfo::queryOutputs(bool onlyOutputsToInstall)
     if (!outTI->isList()) throw errMsg;
     Outputs result;
     for (auto i = outTI->listElems(); i != outTI->listElems() + outTI->listSize(); ++i) {
-        if ((*i)->type != tString) throw errMsg;
+        if ((*i)->type() != nString) throw errMsg;
         auto out = outputs.find((*i)->string.s);
         if (out == outputs.end()) throw errMsg;
         result.insert(*out);
@@ -172,20 +172,20 @@ StringSet DrvInfo::queryMetaNames()
 bool DrvInfo::checkMeta(Value & v)
 {
     state->forceValue(v);
-    if (v.isList()) {
+    if (v.type() == nList) {
         for (unsigned int n = 0; n < v.listSize(); ++n)
             if (!checkMeta(*v.listElems()[n])) return false;
         return true;
     }
-    else if (v.type == tAttrs) {
+    else if (v.type() == nAttrs) {
         Bindings::iterator i = v.attrs->find(state->sOutPath);
         if (i != v.attrs->end()) return false;
         for (auto & i : *v.attrs)
             if (!checkMeta(*i.value)) return false;
         return true;
     }
-    else return v.type == tInt || v.type == tBool || v.type == tString ||
-                v.type == tFloat;
+    else return v.type() == nInt || v.type() == nBool || v.type() == nString ||
+                v.type() == nFloat;
 }
 
 
@@ -201,7 +201,7 @@ Value * DrvInfo::queryMeta(const string & name)
 string DrvInfo::queryMetaString(const string & name)
 {
     Value * v = queryMeta(name);
-    if (!v || v->type != tString) return "";
+    if (!v || v->type() != nString) return "";
     return v->string.s;
 }
 
@@ -210,8 +210,8 @@ NixInt DrvInfo::queryMetaInt(const string & name, NixInt def)
 {
     Value * v = queryMeta(name);
     if (!v) return def;
-    if (v->type == tInt) return v->integer;
-    if (v->type == tString) {
+    if (v->type() == nInt) return v->integer;
+    if (v->type() == nString) {
         /* Backwards compatibility with before we had support for
            integer meta fields. */
         NixInt n;
@@ -224,8 +224,8 @@ NixFloat DrvInfo::queryMetaFloat(const string & name, NixFloat def)
 {
     Value * v = queryMeta(name);
     if (!v) return def;
-    if (v->type == tFloat) return v->fpoint;
-    if (v->type == tString) {
+    if (v->type() == nFloat) return v->fpoint;
+    if (v->type() == nString) {
         /* Backwards compatibility with before we had support for
            float meta fields. */
         NixFloat n;
@@ -239,8 +239,8 @@ bool DrvInfo::queryMetaBool(const string & name, bool def)
 {
     Value * v = queryMeta(name);
     if (!v) return def;
-    if (v->type == tBool) return v->boolean;
-    if (v->type == tString) {
+    if (v->type() == nBool) return v->boolean;
+    if (v->type() == nString) {
         /* Backwards compatibility with before we had support for
            Boolean meta fields. */
         if (strcmp(v->string.s, "true") == 0) return true;
@@ -331,7 +331,7 @@ static void getDerivations(EvalState & state, Value & vIn,
     /* Process the expression. */
     if (!getDerivation(state, v, pathPrefix, drvs, done, ignoreAssertionFailures)) ;
 
-    else if (v.type == tAttrs) {
+    else if (v.type() == nAttrs) {
 
         /* !!! undocumented hackery to support combining channels in
            nix-env.cc. */
@@ -353,7 +353,7 @@ static void getDerivations(EvalState & state, Value & vIn,
                 /* If the value of this attribute is itself a set,
                    should we recurse into it?  => Only if it has a
                    `recurseForDerivations = true' attribute. */
-                if (i->value->type == tAttrs) {
+                if (i->value->type() == nAttrs) {
                     Bindings::iterator j = i->value->attrs->find(state.sRecurseForDerivations);
                     if (j != i->value->attrs->end() && state.forceBool(*j->value, *j->pos))
                         getDerivations(state, *i->value, pathPrefix2, autoArgs, drvs, done, ignoreAssertionFailures);
@@ -362,7 +362,7 @@ static void getDerivations(EvalState & state, Value & vIn,
         }
     }
 
-    else if (v.isList()) {
+    else if (v.type() == nList) {
         for (unsigned int n = 0; n < v.listSize(); ++n) {
             string pathPrefix2 = addToPath(pathPrefix, (format("%1%") % n).str());
             if (getDerivation(state, *v.listElems()[n], pathPrefix2, drvs, done, ignoreAssertionFailures))
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index e4cbc660f..b80a7de4e 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -129,7 +129,7 @@ struct ExprPath : Expr
 {
     string s;
     Value v;
-    ExprPath(const string & s) : s(s) { mkPathNoCopy(v, this->s.c_str()); };
+    ExprPath(const string & s) : s(s) { v.mkPath(this->s.c_str()); };
     COMMON_METHODS
     Value * maybeThunk(EvalState & state, Env & env);
 };
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index e9c1f3b43..18438f681 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -356,24 +356,20 @@ static void prim_typeOf(EvalState & state, const Pos & pos, Value * * args, Valu
 {
     state.forceValue(*args[0], pos);
     string t;
-    switch (args[0]->type) {
-        case tInt: t = "int"; break;
-        case tBool: t = "bool"; break;
-        case tString: t = "string"; break;
-        case tPath: t = "path"; break;
-        case tNull: t = "null"; break;
-        case tAttrs: t = "set"; break;
-        case tList1: case tList2: case tListN: t = "list"; break;
-        case tLambda:
-        case tPrimOp:
-        case tPrimOpApp:
-            t = "lambda";
-            break;
-        case tExternal:
+    switch (args[0]->type()) {
+        case nInt: t = "int"; break;
+        case nBool: t = "bool"; break;
+        case nString: t = "string"; break;
+        case nPath: t = "path"; break;
+        case nNull: t = "null"; break;
+        case nAttrs: t = "set"; break;
+        case nList: t = "list"; break;
+        case nFunction: t = "lambda"; break;
+        case nExternal:
             t = args[0]->external->typeOf();
             break;
-        case tFloat: t = "float"; break;
-        default: abort();
+        case nFloat: t = "float"; break;
+        case nThunk: abort();
     }
     mkString(v, state.symbols.create(t));
 }
@@ -393,7 +389,7 @@ static RegisterPrimOp primop_typeOf({
 static void prim_isNull(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     state.forceValue(*args[0], pos);
-    mkBool(v, args[0]->type == tNull);
+    mkBool(v, args[0]->type() == nNull);
 }
 
 static RegisterPrimOp primop_isNull({
@@ -413,18 +409,7 @@ static RegisterPrimOp primop_isNull({
 static void prim_isFunction(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     state.forceValue(*args[0], pos);
-    bool res;
-    switch (args[0]->type) {
-        case tLambda:
-        case tPrimOp:
-        case tPrimOpApp:
-            res = true;
-            break;
-        default:
-            res = false;
-            break;
-    }
-    mkBool(v, res);
+    mkBool(v, args[0]->type() == nFunction);
 }
 
 static RegisterPrimOp primop_isFunction({
@@ -440,7 +425,7 @@ static RegisterPrimOp primop_isFunction({
 static void prim_isInt(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     state.forceValue(*args[0], pos);
-    mkBool(v, args[0]->type == tInt);
+    mkBool(v, args[0]->type() == nInt);
 }
 
 static RegisterPrimOp primop_isInt({
@@ -456,7 +441,7 @@ static RegisterPrimOp primop_isInt({
 static void prim_isFloat(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     state.forceValue(*args[0], pos);
-    mkBool(v, args[0]->type == tFloat);
+    mkBool(v, args[0]->type() == nFloat);
 }
 
 static RegisterPrimOp primop_isFloat({
@@ -472,7 +457,7 @@ static RegisterPrimOp primop_isFloat({
 static void prim_isString(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     state.forceValue(*args[0], pos);
-    mkBool(v, args[0]->type == tString);
+    mkBool(v, args[0]->type() == nString);
 }
 
 static RegisterPrimOp primop_isString({
@@ -488,7 +473,7 @@ static RegisterPrimOp primop_isString({
 static void prim_isBool(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     state.forceValue(*args[0], pos);
-    mkBool(v, args[0]->type == tBool);
+    mkBool(v, args[0]->type() == nBool);
 }
 
 static RegisterPrimOp primop_isBool({
@@ -504,7 +489,7 @@ static RegisterPrimOp primop_isBool({
 static void prim_isPath(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     state.forceValue(*args[0], pos);
-    mkBool(v, args[0]->type == tPath);
+    mkBool(v, args[0]->type() == nPath);
 }
 
 static RegisterPrimOp primop_isPath({
@@ -520,20 +505,20 @@ struct CompareValues
 {
     bool operator () (const Value * v1, const Value * v2) const
     {
-        if (v1->type == tFloat && v2->type == tInt)
+        if (v1->type() == nFloat && v2->type() == nInt)
             return v1->fpoint < v2->integer;
-        if (v1->type == tInt && v2->type == tFloat)
+        if (v1->type() == nInt && v2->type() == nFloat)
             return v1->integer < v2->fpoint;
-        if (v1->type != v2->type)
+        if (v1->type() != v2->type())
             throw EvalError("cannot compare %1% with %2%", showType(*v1), showType(*v2));
-        switch (v1->type) {
-            case tInt:
+        switch (v1->type()) {
+            case nInt:
                 return v1->integer < v2->integer;
-            case tFloat:
+            case nFloat:
                 return v1->fpoint < v2->fpoint;
-            case tString:
+            case nString:
                 return strcmp(v1->string.s, v2->string.s) < 0;
-            case tPath:
+            case nPath:
                 return strcmp(v1->path, v2->path) < 0;
             default:
                 throw EvalError("cannot compare %1% with %2%", showType(*v1), showType(*v2));
@@ -777,7 +762,7 @@ static RegisterPrimOp primop_deepSeq({
 static void prim_trace(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     state.forceValue(*args[0], pos);
-    if (args[0]->type == tString)
+    if (args[0]->type() == nString)
         printError("trace: %1%", args[0]->string.s);
     else
         printError("trace: %1%", *args[0]);
@@ -902,7 +887,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
 
             if (ignoreNulls) {
                 state.forceValue(*i->value, pos);
-                if (i->value->type == tNull) continue;
+                if (i->value->type() == nNull) continue;
             }
 
             if (i->name == state.sContentAddressed) {
@@ -1308,7 +1293,7 @@ static void prim_dirOf(EvalState & state, const Pos & pos, Value * * args, Value
 {
     PathSet context;
     Path dir = dirOf(state.coerceToString(pos, *args[0], context, false, false));
-    if (args[0]->type == tPath) mkPath(v, dir.c_str()); else mkString(v, dir, context);
+    if (args[0]->type() == nPath) mkPath(v, dir.c_str()); else mkString(v, dir, context);
 }
 
 static RegisterPrimOp primop_dirOf({
@@ -1449,7 +1434,7 @@ static void prim_readDir(EvalState & state, const Pos & pos, Value * * args, Val
         Value * ent_val = state.allocAttr(v, state.symbols.create(ent.name));
         if (ent.type == DT_UNKNOWN)
             ent.type = getFileType(path + "/" + ent.name);
-        mkStringNoCopy(*ent_val,
+        ent_val->mkString(
             ent.type == DT_REG ? "regular" :
             ent.type == DT_DIR ? "directory" :
             ent.type == DT_LNK ? "symlink" :
@@ -1813,7 +1798,7 @@ static void prim_filterSource(EvalState & state, const Pos & pos, Value * * args
         });
 
     state.forceValue(*args[0], pos);
-    if (args[0]->type != tLambda)
+    if (args[0]->type() != nFunction)
         throw TypeError({
             .hint = hintfmt(
                 "first argument in call to 'filterSource' is not a function but %1%",
@@ -2079,7 +2064,7 @@ static RegisterPrimOp primop_hasAttr({
 static void prim_isAttrs(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     state.forceValue(*args[0], pos);
-    mkBool(v, args[0]->type == tAttrs);
+    mkBool(v, args[0]->type() == nAttrs);
 }
 
 static RegisterPrimOp primop_isAttrs({
@@ -2259,11 +2244,11 @@ static RegisterPrimOp primop_catAttrs({
 static void prim_functionArgs(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     state.forceValue(*args[0], pos);
-    if (args[0]->type == tPrimOpApp || args[0]->type == tPrimOp) {
+    if (args[0]->isPrimOpApp() || args[0]->isPrimOp()) {
         state.mkAttrs(v, 0);
         return;
     }
-    if (args[0]->type != tLambda)
+    if (!args[0]->isLambda())
         throw TypeError({
             .hint = hintfmt("'functionArgs' requires a function"),
             .errPos = pos
@@ -2342,7 +2327,7 @@ static RegisterPrimOp primop_mapAttrs({
 static void prim_isList(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     state.forceValue(*args[0], pos);
-    mkBool(v, args[0]->isList());
+    mkBool(v, args[0]->type() == nList);
 }
 
 static RegisterPrimOp primop_isList({
@@ -2694,7 +2679,7 @@ static void prim_sort(EvalState & state, const Pos & pos, Value * * args, Value
     auto comparator = [&](Value * a, Value * b) {
         /* Optimization: if the comparator is lessThan, bypass
            callFunction. */
-        if (args[0]->type == tPrimOp && args[0]->primOp->fun == prim_lessThan)
+        if (args[0]->isPrimOp() && args[0]->primOp->fun == prim_lessThan)
             return CompareValues()(a, b);
 
         Value vTmp1, vTmp2;
@@ -2836,7 +2821,7 @@ static void prim_add(EvalState & state, const Pos & pos, Value * * args, Value &
 {
     state.forceValue(*args[0], pos);
     state.forceValue(*args[1], pos);
-    if (args[0]->type == tFloat || args[1]->type == tFloat)
+    if (args[0]->type() == nFloat || args[1]->type() == nFloat)
         mkFloat(v, state.forceFloat(*args[0], pos) + state.forceFloat(*args[1], pos));
     else
         mkInt(v, state.forceInt(*args[0], pos) + state.forceInt(*args[1], pos));
@@ -2855,7 +2840,7 @@ static void prim_sub(EvalState & state, const Pos & pos, Value * * args, Value &
 {
     state.forceValue(*args[0], pos);
     state.forceValue(*args[1], pos);
-    if (args[0]->type == tFloat || args[1]->type == tFloat)
+    if (args[0]->type() == nFloat || args[1]->type() == nFloat)
         mkFloat(v, state.forceFloat(*args[0], pos) - state.forceFloat(*args[1], pos));
     else
         mkInt(v, state.forceInt(*args[0], pos) - state.forceInt(*args[1], pos));
@@ -2874,7 +2859,7 @@ static void prim_mul(EvalState & state, const Pos & pos, Value * * args, Value &
 {
     state.forceValue(*args[0], pos);
     state.forceValue(*args[1], pos);
-    if (args[0]->type == tFloat || args[1]->type == tFloat)
+    if (args[0]->type() == nFloat || args[1]->type() == nFloat)
         mkFloat(v, state.forceFloat(*args[0], pos) * state.forceFloat(*args[1], pos));
     else
         mkInt(v, state.forceInt(*args[0], pos) * state.forceInt(*args[1], pos));
@@ -2901,7 +2886,7 @@ static void prim_div(EvalState & state, const Pos & pos, Value * * args, Value &
             .errPos = pos
         });
 
-    if (args[0]->type == tFloat || args[1]->type == tFloat) {
+    if (args[0]->type() == nFloat || args[1]->type() == nFloat) {
         mkFloat(v, state.forceFloat(*args[0], pos) / state.forceFloat(*args[1], pos));
     } else {
         NixInt i1 = state.forceInt(*args[0], pos);
diff --git a/src/libexpr/primops/fetchMercurial.cc b/src/libexpr/primops/fetchMercurial.cc
index a77035c16..845a1ed1b 100644
--- a/src/libexpr/primops/fetchMercurial.cc
+++ b/src/libexpr/primops/fetchMercurial.cc
@@ -17,7 +17,7 @@ static void prim_fetchMercurial(EvalState & state, const Pos & pos, Value * * ar
 
     state.forceValue(*args[0]);
 
-    if (args[0]->type == tAttrs) {
+    if (args[0]->type() == nAttrs) {
 
         state.forceAttrs(*args[0], pos);
 
diff --git a/src/libexpr/primops/fetchTree.cc b/src/libexpr/primops/fetchTree.cc
index d094edf92..6e7ddde8e 100644
--- a/src/libexpr/primops/fetchTree.cc
+++ b/src/libexpr/primops/fetchTree.cc
@@ -85,25 +85,25 @@ static void fetchTree(
 
     state.forceValue(*args[0]);
 
-    if (args[0]->type == tAttrs) {
+    if (args[0]->type() == nAttrs) {
         state.forceAttrs(*args[0], pos);
 
         fetchers::Attrs attrs;
 
         for (auto & attr : *args[0]->attrs) {
             state.forceValue(*attr.value);
-            if (attr.value->type == tPath || attr.value->type == tString)
+            if (attr.value->type() == nPath || attr.value->type() == nString)
                 addURI(
                     state,
                     attrs,
                     attr.name,
                     state.coerceToString(*attr.pos, *attr.value, context, false, false)
                 );
-            else if (attr.value->type == tString)
+            else if (attr.value->type() == nString)
                 addURI(state, attrs, attr.name, attr.value->string.s);
-            else if (attr.value->type == tBool)
+            else if (attr.value->type() == nBool)
                 attrs.emplace(attr.name, Explicit<bool>{attr.value->boolean});
-            else if (attr.value->type == tInt)
+            else if (attr.value->type() == nInt)
                 attrs.emplace(attr.name, attr.value->integer);
             else
                 throw TypeError("fetchTree argument '%s' is %s while a string, Boolean or integer is expected",
@@ -163,7 +163,7 @@ static void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
 
     state.forceValue(*args[0]);
 
-    if (args[0]->type == tAttrs) {
+    if (args[0]->type() == nAttrs) {
 
         state.forceAttrs(*args[0], pos);
 
diff --git a/src/libexpr/value-to-json.cc b/src/libexpr/value-to-json.cc
index 6ec8315ba..bfea24d40 100644
--- a/src/libexpr/value-to-json.cc
+++ b/src/libexpr/value-to-json.cc
@@ -16,30 +16,30 @@ void printValueAsJSON(EvalState & state, bool strict,
 
     if (strict) state.forceValue(v);
 
-    switch (v.type) {
+    switch (v.type()) {
 
-        case tInt:
+        case nInt:
             out.write(v.integer);
             break;
 
-        case tBool:
+        case nBool:
             out.write(v.boolean);
             break;
 
-        case tString:
+        case nString:
             copyContext(v, context);
             out.write(v.string.s);
             break;
 
-        case tPath:
+        case nPath:
             out.write(state.copyPathToStore(context, v.path));
             break;
 
-        case tNull:
+        case nNull:
             out.write(nullptr);
             break;
 
-        case tAttrs: {
+        case nAttrs: {
             auto maybeString = state.tryAttrsToString(noPos, v, context, false, false);
             if (maybeString) {
                 out.write(*maybeString);
@@ -61,7 +61,7 @@ void printValueAsJSON(EvalState & state, bool strict,
             break;
         }
 
-        case tList1: case tList2: case tListN: {
+        case nList: {
             auto list(out.list());
             for (unsigned int n = 0; n < v.listSize(); ++n) {
                 auto placeholder(list.placeholder());
@@ -70,15 +70,18 @@ void printValueAsJSON(EvalState & state, bool strict,
             break;
         }
 
-        case tExternal:
+        case nExternal:
             v.external->printValueAsJSON(state, strict, out, context);
             break;
 
-        case tFloat:
+        case nFloat:
             out.write(v.fpoint);
             break;
 
-        default:
+        case nThunk:
+            throw TypeError("cannot convert %1% to JSON", showType(v));
+
+        case nFunction:
             throw TypeError("cannot convert %1% to JSON", showType(v));
     }
 }
diff --git a/src/libexpr/value-to-xml.cc b/src/libexpr/value-to-xml.cc
index 1f0b1541d..7464455d8 100644
--- a/src/libexpr/value-to-xml.cc
+++ b/src/libexpr/value-to-xml.cc
@@ -58,31 +58,31 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
 
     if (strict) state.forceValue(v);
 
-    switch (v.type) {
+    switch (v.type()) {
 
-        case tInt:
+        case nInt:
             doc.writeEmptyElement("int", singletonAttrs("value", (format("%1%") % v.integer).str()));
             break;
 
-        case tBool:
+        case nBool:
             doc.writeEmptyElement("bool", singletonAttrs("value", v.boolean ? "true" : "false"));
             break;
 
-        case tString:
+        case nString:
             /* !!! show the context? */
             copyContext(v, context);
             doc.writeEmptyElement("string", singletonAttrs("value", v.string.s));
             break;
 
-        case tPath:
+        case nPath:
             doc.writeEmptyElement("path", singletonAttrs("value", v.path));
             break;
 
-        case tNull:
+        case nNull:
             doc.writeEmptyElement("null");
             break;
 
-        case tAttrs:
+        case nAttrs:
             if (state.isDerivation(v)) {
                 XMLAttrs xmlAttrs;
 
@@ -92,14 +92,14 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
                 a = v.attrs->find(state.sDrvPath);
                 if (a != v.attrs->end()) {
                     if (strict) state.forceValue(*a->value);
-                    if (a->value->type == tString)
+                    if (a->value->type() == nString)
                         xmlAttrs["drvPath"] = drvPath = a->value->string.s;
                 }
 
                 a = v.attrs->find(state.sOutPath);
                 if (a != v.attrs->end()) {
                     if (strict) state.forceValue(*a->value);
-                    if (a->value->type == tString)
+                    if (a->value->type() == nString)
                         xmlAttrs["outPath"] = a->value->string.s;
                 }
 
@@ -118,14 +118,19 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
 
             break;
 
-        case tList1: case tList2: case tListN: {
+        case nList: {
             XMLOpenElement _(doc, "list");
             for (unsigned int n = 0; n < v.listSize(); ++n)
                 printValueAsXML(state, strict, location, *v.listElems()[n], doc, context, drvsSeen);
             break;
         }
 
-        case tLambda: {
+        case nFunction: {
+            if (!v.isLambda()) {
+                // FIXME: Serialize primops and primopapps
+                doc.writeEmptyElement("unevaluated");
+                break;
+            }
             XMLAttrs xmlAttrs;
             if (location) posToXML(xmlAttrs, v.lambda.fun->pos);
             XMLOpenElement _(doc, "function", xmlAttrs);
@@ -143,15 +148,15 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
             break;
         }
 
-        case tExternal:
+        case nExternal:
             v.external->printValueAsXML(state, strict, location, doc, context, drvsSeen);
             break;
 
-        case tFloat:
+        case nFloat:
             doc.writeEmptyElement("float", singletonAttrs("value", (format("%1%") % v.fpoint).str()));
             break;
 
-        default:
+        case nThunk:
             doc.writeEmptyElement("unevaluated");
     }
 }
diff --git a/src/libexpr/value.hh b/src/libexpr/value.hh
index fe11bb2ed..b317c1898 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -27,8 +27,24 @@ typedef enum {
     tPrimOpApp,
     tExternal,
     tFloat
-} ValueType;
+} InternalType;
 
+// This type abstracts over all actual value types in the language,
+// grouping together implementation details like tList*, different function
+// types, and types in non-normal form (so thunks and co.)
+typedef enum {
+    nThunk,
+    nInt,
+    nFloat,
+    nBool,
+    nString,
+    nPath,
+    nNull,
+    nAttrs,
+    nList,
+    nFunction,
+    nExternal
+} ValueType;
 
 class Bindings;
 struct Env;
@@ -90,7 +106,28 @@ std::ostream & operator << (std::ostream & str, const ExternalValueBase & v);
 
 struct Value
 {
-    ValueType type;
+private:
+    InternalType internalType;
+
+friend std::string showType(const Value & v);
+friend void printValue(std::ostream & str, std::set<const Value *> & active, const Value & v);
+
+public:
+
+    // Functions needed to distinguish the type
+    // These should be removed eventually, by putting the functionality that's
+    // needed by callers into methods of this type
+
+    // type() == nThunk
+    inline bool isThunk() const { return internalType == tThunk; };
+    inline bool isApp() const { return internalType == tApp; };
+    inline bool isBlackhole() const { return internalType == tBlackhole; };
+
+    // type() == nFunction
+    inline bool isLambda() const { return internalType == tLambda; };
+    inline bool isPrimOp() const { return internalType == tPrimOp; };
+    inline bool isPrimOpApp() const { return internalType == tPrimOpApp; };
+
     union
     {
         NixInt integer;
@@ -147,24 +184,161 @@ struct Value
         NixFloat fpoint;
     };
 
+    // Returns the normal type of a Value. This only returns nThunk if the
+    // Value hasn't been forceValue'd
+    inline ValueType type() const
+    {
+        switch (internalType) {
+            case tInt: return nInt;
+            case tBool: return nBool;
+            case tString: return nString;
+            case tPath: return nPath;
+            case tNull: return nNull;
+            case tAttrs: return nAttrs;
+            case tList1: case tList2: case tListN: return nList;
+            case tLambda: case tPrimOp: case tPrimOpApp: return nFunction;
+            case tExternal: return nExternal;
+            case tFloat: return nFloat;
+            case tThunk: case tApp: case tBlackhole: return nThunk;
+        }
+        abort();
+    }
+
+    /* After overwriting an app node, be sure to clear pointers in the
+       Value to ensure that the target isn't kept alive unnecessarily. */
+    inline void clearValue()
+    {
+        app.left = app.right = 0;
+    }
+
+    inline void mkInt(NixInt n)
+    {
+        clearValue();
+        internalType = tInt;
+        integer = n;
+    }
+
+    inline void mkBool(bool b)
+    {
+        clearValue();
+        internalType = tBool;
+        boolean = b;
+    }
+
+    inline void mkString(const char * s, const char * * context = 0)
+    {
+        internalType = tString;
+        string.s = s;
+        string.context = context;
+    }
+
+    inline void mkPath(const char * s)
+    {
+        clearValue();
+        internalType = tPath;
+        path = s;
+    }
+
+    inline void mkNull()
+    {
+        clearValue();
+        internalType = tNull;
+    }
+
+    inline void mkAttrs(Bindings * a)
+    {
+        clearValue();
+        internalType = tAttrs;
+        attrs = a;
+    }
+
+    inline void mkList(size_t size)
+    {
+        clearValue();
+        if (size == 1)
+            internalType = tList1;
+        else if (size == 2)
+            internalType = tList2;
+        else {
+            internalType = tListN;
+            bigList.size = size;
+        }
+    }
+
+    inline void mkThunk(Env * e, Expr * ex)
+    {
+        internalType = tThunk;
+        thunk.env = e;
+        thunk.expr = ex;
+    }
+
+    inline void mkApp(Value * l, Value * r)
+    {
+        internalType = tApp;
+        app.left = l;
+        app.right = r;
+    }
+
+    inline void mkLambda(Env * e, ExprLambda * f)
+    {
+        internalType = tLambda;
+        lambda.env = e;
+        lambda.fun = f;
+    }
+
+    inline void mkBlackhole()
+    {
+        internalType = tBlackhole;
+        // Value will be overridden anyways
+    }
+
+    inline void mkPrimOp(PrimOp * p)
+    {
+        clearValue();
+        internalType = tPrimOp;
+        primOp = p;
+    }
+
+
+    inline void mkPrimOpApp(Value * l, Value * r)
+    {
+        internalType = tPrimOpApp;
+        app.left = l;
+        app.right = r;
+    }
+
+    inline void mkExternal(ExternalValueBase * e)
+    {
+        clearValue();
+        internalType = tExternal;
+        external = e;
+    }
+
+    inline void mkFloat(NixFloat n)
+    {
+        clearValue();
+        internalType = tFloat;
+        fpoint = n;
+    }
+
     bool isList() const
     {
-        return type == tList1 || type == tList2 || type == tListN;
+        return internalType == tList1 || internalType == tList2 || internalType == tListN;
     }
 
     Value * * listElems()
     {
-        return type == tList1 || type == tList2 ? smallList : bigList.elems;
+        return internalType == tList1 || internalType == tList2 ? smallList : bigList.elems;
     }
 
     const Value * const * listElems() const
     {
-        return type == tList1 || type == tList2 ? smallList : bigList.elems;
+        return internalType == tList1 || internalType == tList2 ? smallList : bigList.elems;
     }
 
     size_t listSize() const
     {
-        return type == tList1 ? 1 : type == tList2 ? 2 : bigList.size;
+        return internalType == tList1 ? 1 : internalType == tList2 ? 2 : bigList.size;
     }
 
     /* Check whether forcing this value requires a trivial amount of
@@ -176,86 +350,42 @@ struct Value
 };
 
 
-/* After overwriting an app node, be sure to clear pointers in the
-   Value to ensure that the target isn't kept alive unnecessarily. */
-static inline void clearValue(Value & v)
-{
-    v.app.left = v.app.right = 0;
-}
-
 
+// TODO: Remove these static functions, replace call sites with v.mk* instead
 static inline void mkInt(Value & v, NixInt n)
 {
-    clearValue(v);
-    v.type = tInt;
-    v.integer = n;
+    v.mkInt(n);
 }
 
-
 static inline void mkFloat(Value & v, NixFloat n)
 {
-    clearValue(v);
-    v.type = tFloat;
-    v.fpoint = n;
+    v.mkFloat(n);
 }
 
-
 static inline void mkBool(Value & v, bool b)
 {
-    clearValue(v);
-    v.type = tBool;
-    v.boolean = b;
+    v.mkBool(b);
 }
 
-
 static inline void mkNull(Value & v)
 {
-    clearValue(v);
-    v.type = tNull;
+    v.mkNull();
 }
 
-
 static inline void mkApp(Value & v, Value & left, Value & right)
 {
-    v.type = tApp;
-    v.app.left = &left;
-    v.app.right = &right;
+    v.mkApp(&left, &right);
 }
 
-
-static inline void mkPrimOpApp(Value & v, Value & left, Value & right)
-{
-    v.type = tPrimOpApp;
-    v.app.left = &left;
-    v.app.right = &right;
-}
-
-
-static inline void mkStringNoCopy(Value & v, const char * s)
-{
-    v.type = tString;
-    v.string.s = s;
-    v.string.context = 0;
-}
-
-
 static inline void mkString(Value & v, const Symbol & s)
 {
-    mkStringNoCopy(v, ((const string &) s).c_str());
+    v.mkString(((const string &) s).c_str());
 }
 
 
 void mkString(Value & v, const char * s);
 
 
-static inline void mkPathNoCopy(Value & v, const char * s)
-{
-    clearValue(v);
-    v.type = tPath;
-    v.path = s;
-}
-
-
 void mkPath(Value & v, const char * s);
 
 
diff --git a/src/nix-env/nix-env.cc b/src/nix-env/nix-env.cc
index a4b5c9e2c..6c2e075ed 100644
--- a/src/nix-env/nix-env.cc
+++ b/src/nix-env/nix-env.cc
@@ -1138,38 +1138,38 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
                                         i.queryName(), j)
                                 });
                             else {
-                                if (v->type == tString) {
+                                if (v->type() == nString) {
                                     attrs2["type"] = "string";
                                     attrs2["value"] = v->string.s;
                                     xml.writeEmptyElement("meta", attrs2);
-                                } else if (v->type == tInt) {
+                                } else if (v->type() == nInt) {
                                     attrs2["type"] = "int";
                                     attrs2["value"] = (format("%1%") % v->integer).str();
                                     xml.writeEmptyElement("meta", attrs2);
-                                } else if (v->type == tFloat) {
+                                } else if (v->type() == nFloat) {
                                     attrs2["type"] = "float";
                                     attrs2["value"] = (format("%1%") % v->fpoint).str();
                                     xml.writeEmptyElement("meta", attrs2);
-                                } else if (v->type == tBool) {
+                                } else if (v->type() == nBool) {
                                     attrs2["type"] = "bool";
                                     attrs2["value"] = v->boolean ? "true" : "false";
                                     xml.writeEmptyElement("meta", attrs2);
-                                } else if (v->isList()) {
+                                } else if (v->type() == nList) {
                                     attrs2["type"] = "strings";
                                     XMLOpenElement m(xml, "meta", attrs2);
                                     for (unsigned int j = 0; j < v->listSize(); ++j) {
-                                        if (v->listElems()[j]->type != tString) continue;
+                                        if (v->listElems()[j]->type() != nString) continue;
                                         XMLAttrs attrs3;
                                         attrs3["value"] = v->listElems()[j]->string.s;
                                         xml.writeEmptyElement("string", attrs3);
                                     }
-                              } else if (v->type == tAttrs) {
+                              } else if (v->type() == nAttrs) {
                                   attrs2["type"] = "strings";
                                   XMLOpenElement m(xml, "meta", attrs2);
                                   Bindings & attrs = *v->attrs;
                                   for (auto &i : attrs) {
                                       Attr & a(*attrs.find(i.name));
-                                      if(a.value->type != tString) continue;
+                                      if(a.value->type() != nString) continue;
                                       XMLAttrs attrs3;
                                       attrs3["type"] = i.name;
                                       attrs3["value"] = a.value->string.s;
diff --git a/src/nix/eval.cc b/src/nix/eval.cc
index 0f02919de..ea82e5300 100644
--- a/src/nix/eval.cc
+++ b/src/nix/eval.cc
@@ -97,10 +97,10 @@ struct CmdEval : MixJSON, InstallableCommand
             recurse = [&](Value & v, const Pos & pos, const Path & path)
             {
                 state->forceValue(v);
-                if (v.type == tString)
+                if (v.type() == nString)
                     // FIXME: disallow strings with contexts?
                     writeFile(path, v.string.s);
-                else if (v.type == tAttrs) {
+                else if (v.type() == nAttrs) {
                     if (mkdir(path.c_str(), 0777) == -1)
                         throw SysError("creating directory '%s'", path);
                     for (auto & attr : *v.attrs)
diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 7a7c71676..066430c5d 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -260,7 +260,7 @@ struct CmdFlakeCheck : FlakeCommand
         auto checkOverlay = [&](const std::string & attrPath, Value & v, const Pos & pos) {
             try {
                 state->forceValue(v, pos);
-                if (v.type != tLambda || v.lambda.fun->matchAttrs || std::string(v.lambda.fun->arg) != "final")
+                if (!v.isLambda() || v.lambda.fun->matchAttrs || std::string(v.lambda.fun->arg) != "final")
                     throw Error("overlay does not take an argument named 'final'");
                 auto body = dynamic_cast<ExprLambda *>(v.lambda.fun->body);
                 if (!body || body->matchAttrs || std::string(body->arg) != "prev")
@@ -276,10 +276,10 @@ struct CmdFlakeCheck : FlakeCommand
         auto checkModule = [&](const std::string & attrPath, Value & v, const Pos & pos) {
             try {
                 state->forceValue(v, pos);
-                if (v.type == tLambda) {
+                if (v.isLambda()) {
                     if (!v.lambda.fun->matchAttrs || !v.lambda.fun->formals->ellipsis)
                         throw Error("module must match an open attribute set ('{ config, ... }')");
-                } else if (v.type == tAttrs) {
+                } else if (v.type() == nAttrs) {
                     for (auto & attr : *v.attrs)
                         try {
                             state->forceValue(*attr.value, *attr.pos);
@@ -371,7 +371,7 @@ struct CmdFlakeCheck : FlakeCommand
         auto checkBundler = [&](const std::string & attrPath, Value & v, const Pos & pos) {
             try {
                 state->forceValue(v, pos);
-                if (v.type != tLambda)
+                if (!v.isLambda())
                     throw Error("bundler must be a function");
                 if (!v.lambda.fun->formals ||
                     v.lambda.fun->formals->argNames.find(state->symbols.create("program")) == v.lambda.fun->formals->argNames.end() ||
diff --git a/src/nix/main.cc b/src/nix/main.cc
index 27b1d7257..e7a15dec9 100644
--- a/src/nix/main.cc
+++ b/src/nix/main.cc
@@ -272,7 +272,7 @@ void mainWrapped(int argc, char * * argv)
         auto builtins = state.baseEnv.values[0]->attrs;
         for (auto & builtin : *builtins) {
             auto b = nlohmann::json::object();
-            if (builtin.value->type != tPrimOp) continue;
+            if (!builtin.value->isPrimOp()) continue;
             auto primOp = builtin.value->primOp;
             if (!primOp->doc) continue;
             b["arity"] = primOp->arity;
diff --git a/src/nix/repl.cc b/src/nix/repl.cc
index 71794a309..a992d8732 100644
--- a/src/nix/repl.cc
+++ b/src/nix/repl.cc
@@ -446,11 +446,11 @@ bool NixRepl::processLine(string line)
 
         Pos pos;
 
-        if (v.type == tPath || v.type == tString) {
+        if (v.type() == nPath || v.type() == nString) {
             PathSet context;
             auto filename = state->coerceToString(noPos, v, context);
             pos.file = state->symbols.create(filename);
-        } else if (v.type == tLambda) {
+        } else if (v.isLambda()) {
             pos = v.lambda.fun->pos;
         } else {
             // assume it's a derivation
@@ -551,9 +551,7 @@ bool NixRepl::processLine(string line)
         {
             Expr * e = parseString(string(line, p + 1));
             Value & v(*state->allocValue());
-            v.type = tThunk;
-            v.thunk.env = env;
-            v.thunk.expr = e;
+            v.mkThunk(env, e);
             addVarToScope(state->symbols.create(name), v);
         } else {
             Value v;
@@ -669,31 +667,31 @@ std::ostream & NixRepl::printValue(std::ostream & str, Value & v, unsigned int m
 
     state->forceValue(v);
 
-    switch (v.type) {
+    switch (v.type()) {
 
-    case tInt:
+    case nInt:
         str << ANSI_CYAN << v.integer << ANSI_NORMAL;
         break;
 
-    case tBool:
+    case nBool:
         str << ANSI_CYAN << (v.boolean ? "true" : "false") << ANSI_NORMAL;
         break;
 
-    case tString:
+    case nString:
         str << ANSI_YELLOW;
         printStringValue(str, v.string.s);
         str << ANSI_NORMAL;
         break;
 
-    case tPath:
+    case nPath:
         str << ANSI_GREEN << v.path << ANSI_NORMAL; // !!! escaping?
         break;
 
-    case tNull:
+    case nNull:
         str << ANSI_CYAN "null" ANSI_NORMAL;
         break;
 
-    case tAttrs: {
+    case nAttrs: {
         seen.insert(&v);
 
         bool isDrv = state->isDerivation(v);
@@ -738,9 +736,7 @@ std::ostream & NixRepl::printValue(std::ostream & str, Value & v, unsigned int m
         break;
     }
 
-    case tList1:
-    case tList2:
-    case tListN:
+    case nList:
         seen.insert(&v);
 
         str << "[ ";
@@ -761,22 +757,21 @@ std::ostream & NixRepl::printValue(std::ostream & str, Value & v, unsigned int m
         str << "]";
         break;
 
-    case tLambda: {
-        std::ostringstream s;
-        s << v.lambda.fun->pos;
-        str << ANSI_BLUE "«lambda @ " << filterANSIEscapes(s.str()) << "»" ANSI_NORMAL;
-        break;
-    }
-
-    case tPrimOp:
-        str << ANSI_MAGENTA "«primop»" ANSI_NORMAL;
+    case nFunction:
+        if (v.isLambda()) {
+            std::ostringstream s;
+            s << v.lambda.fun->pos;
+            str << ANSI_BLUE "«lambda @ " << filterANSIEscapes(s.str()) << "»" ANSI_NORMAL;
+        } else if (v.isPrimOp()) {
+            str << ANSI_MAGENTA "«primop»" ANSI_NORMAL;
+        } else if (v.isPrimOpApp()) {
+            str << ANSI_BLUE "«primop-app»" ANSI_NORMAL;
+        } else {
+            abort();
+        }
         break;
 
-    case tPrimOpApp:
-        str << ANSI_BLUE "«primop-app»" ANSI_NORMAL;
-        break;
-
-    case tFloat:
+    case nFloat:
         str << v.fpoint;
         break;