diff --git a/src/common.cpp b/src/common.cpp
index e143760..af70f67 100644
--- a/src/common.cpp
+++ b/src/common.cpp
@@ -156,18 +156,18 @@ Result<void> debug_print(Value& val) {
   return debug_print(s);
 }
 
-Result<Value> Array::get(uint64_t idx) {
+Result<Value> Array::get(uint64_t idx) const {
   if (idx >= _value->size) return ERROR(IndexOutOfRange);
   auto val = _value->data[idx].get();
   return Value::create(val);
 }
 
-Result<Value> Array::get(Value& key) {
+Result<Value> Array::get(const Value& key) const {
   if (!key.is<Int64>()) return ERROR(TypeMismatch);
   return get(key.to<Int64>()->value());
 }
 
-Result<Array> Array::append(Value& rhs) {
+Result<Array> Array::append(const Value& rhs) const {
   uint64_t res_size = size() + 1;
 
   auto pod = TRY(arena_alloc<PodArray>(res_size * sizeof(OffPtr<PodObject>)));
@@ -187,9 +187,9 @@ short cmp_tag(Tag lhs, Tag rhs) {
   return 0;
 }
 
-Result<Value> Dict::get(Value& key) {
+Result<Value> Dict::get(const Value& key) const {
   auto pos = TRY(find(key));
-  if (pos > size()) return ERROR(KeyError);
+  if (pos >= size()) return ERROR(KeyError);
   auto k = TRY(Value::create(_value->data[pos * 2].get()));
 
   if (TRY(k.cmp(key)) != 0) {
@@ -232,7 +232,7 @@ Result<Dict> Dict::insert(const Value& key, const Value& value) {
   return Dict(TRY(MkGcRoot(pod)));
 }
 
-Result<uint64_t> Dict::find(const Value& key) {
+Result<uint64_t> Dict::find(const Value& key) const {
   uint64_t left = 0;
   uint64_t right = size();
   uint64_t pos = (left + right) / 2;
@@ -284,4 +284,6 @@ Result<short> Dict::cmp(const Dict& rhs) const {
   return 0;
 }
 
-Result<Value> Object::get(Value& key) { return ERROR(TypeMismatch); }
+Result<Value> Object::get(const Value& key) const {
+  return ERROR(TypeMismatch);
+}
diff --git a/src/common.hpp b/src/common.hpp
index 7e02f79..092be5a 100644
--- a/src/common.hpp
+++ b/src/common.hpp
@@ -68,7 +68,7 @@ class Object {
   virtual Result<short> cmp(const ByteArray&) const {
     return cmp_tag(tag(), Tag::ByteArray);
   }
-  virtual Result<Value> get(Value& key);
+  virtual Result<Value> get(const Value& key) const;
 
   Object() = default;
   Object(const Object&) = delete;
@@ -138,15 +138,15 @@ class Array : public Object {
     return Array(TRY(MkGcRoot(pod)));
   }
 
-  uint64_t size() { return _value->size; }
+  uint64_t size() const { return _value->size; }
   virtual Result<Value> copy() const final;
 
-  Result<Value> get(uint64_t idx);
-  virtual Result<Value> get(Value& key) final;
+  Result<Value> get(uint64_t idx) const;
+  virtual Result<Value> get(const Value& key) const final;
 
-  Result<Array> append(Value& rhs);
+  Result<Array> append(const Value& rhs) const;
   template <class V>
-  Result<Array> append(const V& value);
+  Result<Array> append(const V& value) const;
 
   Result<Array> concat(Array& rhs) {
     uint64_t rhs_size = rhs.size();
@@ -323,7 +323,7 @@ class Dict : public Object {
 
   virtual Result<Value> copy() const final;
 
-  virtual Result<Value> get(Value& key) final;
+  virtual Result<Value> get(const Value& key) const final;
 
   Result<Dict> insert(const Value& key, const Value& value);
 
@@ -349,7 +349,7 @@ class Dict : public Object {
   }
 
  private:
-  Result<uint64_t> find(const Value& key);
+  Result<uint64_t> find(const Value& key) const;
 
   uint64_t size() const { return _value->size; }
   GcRoot<PodDict> _value;
@@ -820,12 +820,12 @@ class Value {
   }
 
   template <class T>
-  bool is() {
+  bool is() const {
     return dynamic_cast<T*>((Object*)buf) != nullptr;
   }
 
   template <class T>
-  T* to() {
+  T* to() const {
     return dynamic_cast<T*>((Object*)buf);
   }
 
@@ -871,7 +871,7 @@ Result<Dict> Dict::insert(const K& key, const V& value) {
 }
 
 template <class V>
-Result<Array> Array::append(const V& value) {
+Result<Array> Array::append(const V& value) const {
   Value v = TRY(Value::create(value));
   return append(v);
 }
diff --git a/src/compiler.cpp b/src/compiler.cpp
index f0555d4..9abc77a 100644
--- a/src/compiler.cpp
+++ b/src/compiler.cpp
@@ -23,7 +23,7 @@ struct Context {
     return reg;
   }
 
-  Result<int64_t> add_const(Value& val) {
+  Result<int64_t> add_const(const Value& val) {
     auto idx = constants_dict.get(val);
 
     if (idx.has_value()) {
@@ -64,6 +64,7 @@ Result<Value> Compiler::compile(Value& expr) {
   TRY(compile_expr(context, expr));
   return ERROR(NotImplemented);
 }
+
 Result<Expression> Compiler::compile_expr(Context& context, Value& expr) {
   switch (expr.tag()) {
     case Tag::Pair:
@@ -91,6 +92,7 @@ Result<Expression> Compiler::compile_primop(Context& context, Symbol& op,
   Array code = TRY(Array::create());
 
   uint64_t reg = context.alloc_reg();
+  int64_t zero = TRY(context.add_const(TRY(Value::create((int64_t)0))));
   std::cout << "load-immediate r" << reg << ", " << 0 << "\n";
   code = TRY(add_opcode(code, Oc::LoadImmediate, {0, (int64_t)reg}, {0, 0}));