diff --git a/src/common.cpp b/src/common.cpp
index bded8c3..15c5425 100644
--- a/src/common.cpp
+++ b/src/common.cpp
@@ -228,6 +228,29 @@ Result<Value> String::get(const Value& key) const {
   return res;
 }
 
+Result<Value> String::set_value(const Value& key, const Value& value) const {
+  if (!key.is<Int64>()) {
+    return ERROR(TypeMismatch);
+  }
+
+  if (!value.is<String>()) {
+    return ERROR(TypeMismatch);
+  }
+
+  uint64_t idx = (uint64_t)key.to<Int64>()->value();
+  const String& value_str = *value.to<String>();
+
+  auto str_size = TRY(size());
+  if (idx >= str_size + 1) return ERROR(IndexOutOfRange);
+
+  auto res = TRY(slice(0, idx));
+  res = TRY(res.concat(value_str));
+  auto rem = TRY(slice(std::min(str_size, idx + 1), str_size));
+  res = TRY(res.concat(rem));
+
+  return Value(std::move(res));
+}
+
 Result<Value> Symbol::copy_value() const {
   return Value(Symbol(TRY(_value.copy())));
 }
diff --git a/src/common.hpp b/src/common.hpp
index a99445b..422e70a 100644
--- a/src/common.hpp
+++ b/src/common.hpp
@@ -541,8 +541,9 @@ class String : public Object {
   }
 
   Result<Value> get(const Value& key) const;
+  Result<Value> set_value(const Value& key, const Value& value) const;
 
-  Result<String> concat(const char* rhs, uint64_t rhs_size) {
+  Result<String> concat(const char* rhs, uint64_t rhs_size) const {
     uint64_t lhs_size = TRY(size());
     uint64_t res_size = lhs_size + rhs_size;
 
@@ -557,7 +558,7 @@ class String : public Object {
 
   Result<String> concat(const char* rhs) { return concat(rhs, strlen(rhs)); }
 
-  Result<String> concat(const char32_t* rhs, uint64_t rhs_size) {
+  Result<String> concat(const char32_t* rhs, uint64_t rhs_size) const {
     uint64_t lhs_size = TRY(size());
     uint64_t res_size = lhs_size + rhs_size;
 
@@ -570,7 +571,7 @@ class String : public Object {
     return String(TRY(MkGcRoot(pod)));
   }
 
-  Result<String> concat(String& rhs) {
+  Result<String> concat(const String& rhs) const {
     uint64_t rhs_size = TRY(rhs.size());
     uint64_t lhs_size = TRY(size());
     uint64_t res_size = lhs_size + rhs_size;