diff --git a/src/common.cpp b/src/common.cpp
index e1c3b3e..ad0d399 100644
--- a/src/common.cpp
+++ b/src/common.cpp
@@ -40,6 +40,7 @@ Result<Symbol> Symbol::create(Arena& arena, String& rhs) {
   uint64_t res_size = rhs_size;
 
   auto pod = TRY(arena.alloc<PodSymbol>(res_size * sizeof(char32_t)));
+  pod->header.tag = Tag::Symbol;
   pod->size = res_size;
   memcpy(pod->data, rhs._value->data, sizeof(char32_t) * rhs_size);
 
@@ -52,6 +53,7 @@ Result<ByteArray> ByteArray::create(Arena& arena, String& str) {
     size += utf8_codepoint_size(TRY(str[i]));
   }
   auto pod = TRY(arena.alloc<PodByteArray>(size * sizeof(char)));
+  pod->header.tag = Tag::ByteArray;
   pod->size = size;
 
   char* res = pod->data;
@@ -106,6 +108,7 @@ Result<Value> Bool::copy(Arena& arena) {
 
 Result<Pair> Pair::create(Arena& arena, Value& first, Value& rest) {
   auto pod = TRY(arena.alloc<PodPair>());
+  pod->header.tag = Tag::Pair;
   pod->first = OffPtr<PodObject>(pod, first.pod());
   pod->rest = OffPtr<PodObject>(pod, rest.pod());
 
diff --git a/src/common.hpp b/src/common.hpp
index 40c0df4..0f86dce 100644
--- a/src/common.hpp
+++ b/src/common.hpp
@@ -39,6 +39,7 @@ class Nil : public Object {
 
   static Result<Nil> create(Arena& arena) {
     auto pod = TRY(arena.alloc<PodNil>());
+    pod->header.tag = Tag::Nil;
 
     return Nil(TRY(MkGcRoot(pod, arena)));
   }
@@ -68,6 +69,7 @@ class ByteArray : public Object {
   }
   static Result<ByteArray> create(Arena& arena, char* chars, int64_t size) {
     auto pod = TRY(arena.alloc<PodByteArray>(size * sizeof(char)));
+    pod->header.tag = Tag::ByteArray;
 
     memcpy(pod->data, chars, size * sizeof(char32_t));
     pod->size = size;
@@ -78,6 +80,7 @@ class ByteArray : public Object {
   static Result<ByteArray> create(Arena& arena, const char* str) {
     uint64_t size = strlen(str);
     auto pod = TRY(arena.alloc<PodByteArray>(size * sizeof(char32_t)));
+    pod->header.tag = Tag::ByteArray;
 
     memcpy(pod->data, str, size);
     pod->size = size;
@@ -165,22 +168,24 @@ class String : public Object {
     return String(TRY(MkGcRoot(obj, arena)));
   }
   static Result<String> create(Arena& arena, char32_t* chars, int64_t size) {
-    auto pod_string = TRY(arena.alloc<PodString>(size * sizeof(char32_t)));
+    auto pod = TRY(arena.alloc<PodString>(size * sizeof(char32_t)));
+    pod->header.tag = Tag::String;
 
-    memcpy(pod_string->data, chars, size * sizeof(char32_t));
-    pod_string->size = size;
+    memcpy(pod->data, chars, size * sizeof(char32_t));
+    pod->size = size;
 
-    return String(TRY(MkGcRoot(pod_string, arena)));
+    return String(TRY(MkGcRoot(pod, arena)));
   }
 
   static Result<String> create(Arena& arena, const char* str) {
     uint64_t size = strlen(str);
-    auto pod_string = TRY(arena.alloc<PodString>(size * sizeof(char32_t)));
+    auto pod = TRY(arena.alloc<PodString>(size * sizeof(char32_t)));
+    pod->header.tag = Tag::String;
 
-    for (uint64_t i = 0; i < size; i++) pod_string->data[i] = str[i];
-    pod_string->size = size;
+    for (uint64_t i = 0; i < size; i++) pod->data[i] = str[i];
+    pod->size = size;
 
-    return String(TRY(MkGcRoot(pod_string, arena)));
+    return String(TRY(MkGcRoot(pod, arena)));
   }
 
   uint64_t size() { return _value->size; }
@@ -197,6 +202,7 @@ class String : public Object {
     uint64_t res_size = lhs_size + rhs_size;
 
     auto pod = TRY(arena.alloc<PodString>(res_size * sizeof(char32_t)));
+    pod->header.tag = Tag::String;
     pod->size = res_size;
     memcpy(pod->data, _value->data, sizeof(char32_t) * lhs_size);
     for (uint64_t i = 0; i < rhs_size; i++) pod->data[lhs_size + i] = rhs[i];
@@ -209,6 +215,7 @@ class String : public Object {
     uint64_t res_size = lhs_size + rhs_size;
 
     auto pod = TRY(arena.alloc<PodString>(res_size * sizeof(char32_t)));
+    pod->header.tag = Tag::String;
     pod->size = res_size;
     memcpy(pod->data, _value->data, sizeof(char32_t) * lhs_size);
     for (uint64_t i = 0; i < rhs_size; i++) pod->data[lhs_size + i] = rhs[i];
@@ -222,6 +229,7 @@ class String : public Object {
     uint64_t res_size = lhs_size + rhs_size;
 
     auto pod = TRY(arena.alloc<PodString>(res_size * sizeof(char32_t)));
+    pod->header.tag = Tag::String;
     pod->size = res_size;
     memcpy(pod->data, _value->data, sizeof(char32_t) * lhs_size);
     memcpy(pod->data + lhs_size, rhs._value->data, sizeof(char32_t) * rhs_size);
@@ -233,6 +241,7 @@ class String : public Object {
     if (start > end) return ErrorCode::IndexOutOfRange;
     uint64_t res_size = end - start;
     auto pod = TRY(arena.alloc<PodString>(res_size * sizeof(char32_t)));
+    pod->header.tag = Tag::String;
     pod->size = res_size;
     memcpy(pod->data, _value->data + start, sizeof(char32_t) * res_size);
 
@@ -258,11 +267,12 @@ class Symbol : public Object {
   }
 
   static Result<Symbol> create(Arena& arena, char32_t* chars, int64_t size) {
-    auto pod_symbol = TRY(arena.alloc<PodSymbol>(size * sizeof(char32_t)));
+    auto pod = TRY(arena.alloc<PodSymbol>(size * sizeof(char32_t)));
+    pod->header.tag = Tag::Symbol;
 
-    memcpy(pod_symbol->data, chars, size * sizeof(char32_t));
+    memcpy(pod->data, chars, size * sizeof(char32_t));
 
-    return Symbol(TRY(MkGcRoot(pod_symbol, arena)));
+    return Symbol(TRY(MkGcRoot(pod, arena)));
   }
 
   static Result<Symbol> create(Arena& arena, String& rhs);
@@ -336,6 +346,7 @@ class Int64 : public Object {
 
   static Result<Int64> create(Arena& arena, double val) {
     auto pod = TRY(arena.alloc<PodInt64>());
+    pod->header.tag = Tag::Int64;
     pod->value = val;
 
     return Int64(TRY(MkGcRoot(pod, arena)));
@@ -363,6 +374,7 @@ class Float : public Object {
 
   static Result<Float> create(Arena& arena, double val) {
     auto pod = TRY(arena.alloc<PodFloat>());
+    pod->header.tag = Tag::Float;
     pod->value = val;
 
     return Float(TRY(MkGcRoot(pod, arena)));
@@ -391,6 +403,7 @@ class Bool : public Object {
 
   static Result<Bool> create(Arena& arena, bool val) {
     auto pod = TRY(arena.alloc<PodBool>());
+    pod->header.tag = Tag::Bool;
     pod->value = val;
 
     return Bool(TRY(MkGcRoot(pod, arena)));
diff --git a/src/vli.cpp b/src/vli.cpp
index 662bcde..3685ebe 100644
--- a/src/vli.cpp
+++ b/src/vli.cpp
@@ -20,6 +20,7 @@ int main() {
   std::cout << int(((Int64&)*val).value()) << "\n";
   */
 
+  /*
   {
     auto s = DIEIF(String::create(arena, "foo"));
 
@@ -27,6 +28,7 @@ int main() {
     std::cout << "char: " << (char)DIEIF(s[2]) << "\n";
   }
   std::cout << "root count: " << arena.root_count() << "\n";
+  */
 
   auto s = DIEIF(String::create(arena, "(1 2 3 \"foo\")"));
   auto reader = Reader(arena, s);
@@ -36,8 +38,17 @@ int main() {
   auto writer = Writer(arena);
   auto s2 = DIEIF(writer.write_one(r));
 
-  std::cout << "size: " << s.size() << "\n";
-  std::cout << "char: " << (char)DIEIF(s[0]) << "\n";
   DIEIF(debug_print(arena, s2));
+
+  /*
+  auto one = Value(DIEIF(Int64::create(arena, 1)));
+  auto nil = Value(DIEIF(Nil::create(arena)));
+  auto pair = Value(DIEIF(Pair::create(arena, one, nil)));
+
+  auto writer = Writer(arena);
+  auto s2 = DIEIF(writer.write_one(pair));
+  DIEIF(debug_print(arena, s2));
+  */
+
   return 0;
 }
diff --git a/src/writer.cpp b/src/writer.cpp
index 2c91ca8..33a3e50 100644
--- a/src/writer.cpp
+++ b/src/writer.cpp
@@ -26,7 +26,7 @@ Result<String> Writer::write_one(Value& obj) {
 
 Result<String> Writer::write_int64(Int64& val) {
   char tmp[32];
-  sprintf(tmp, "%luu64", val.value());
+  sprintf(tmp, "%lu", val.value());
   size_t len = strlen(tmp);
 
   return String::create(_arena, tmp);
@@ -159,6 +159,7 @@ Result<String> Writer::write_pair(Pair& val) {
     res = TRY(res.concat(_arena, first_str));
 
     cur = TRY(pair.rest(_arena));
+    is_first = false;
   }
 
   res = TRY(res.concat(_arena, ")"));