diff --git a/td/telegram/files/FileGenerateManager.cpp b/td/telegram/files/FileGenerateManager.cpp
index 7a75496b..e94933aa 100644
--- a/td/telegram/files/FileGenerateManager.cpp
+++ b/td/telegram/files/FileGenerateManager.cpp
@@ -375,7 +375,7 @@ static Status check_mtime(std::string &conversion, CSlice original_path) {
   if (original_path.empty()) {
     return Status::OK();
   }
-  Parser parser(conversion);
+  ConstParser parser(conversion);
   if (!parser.skip_start_with("#mtime#")) {
     return Status::OK();
   }
diff --git a/tdutils/td/utils/HttpUrl.cpp b/tdutils/td/utils/HttpUrl.cpp
index 1a71549c..b21314bf 100644
--- a/tdutils/td/utils/HttpUrl.cpp
+++ b/tdutils/td/utils/HttpUrl.cpp
@@ -45,9 +45,9 @@ string HttpUrl::get_url() const {
   return result;
 }
 
-Result<HttpUrl> parse_url(MutableSlice url, HttpUrl::Protocol default_protocol) {
+Result<HttpUrl> parse_url(Slice url, HttpUrl::Protocol default_protocol) {
   // url == [https?://][userinfo@]host[:port]
-  Parser parser(url);
+  ConstParser parser(url);
   string protocol_str = to_lower(parser.read_till_nofail(':'));
 
   HttpUrl::Protocol protocol;
@@ -61,7 +61,7 @@ Result<HttpUrl> parse_url(MutableSlice url, HttpUrl::Protocol default_protocol)
       return Status::Error("Unsupported URL protocol");
     }
   } else {
-    parser = Parser(url);
+    parser = ConstParser(url);
     protocol = default_protocol;
   }
   Slice userinfo_host_port = parser.read_till_nofail("/?#");
diff --git a/tdutils/td/utils/HttpUrl.h b/tdutils/td/utils/HttpUrl.h
index 4cff1e6b..f265f95b 100644
--- a/tdutils/td/utils/HttpUrl.h
+++ b/tdutils/td/utils/HttpUrl.h
@@ -36,8 +36,7 @@ class HttpUrl {
   }
 };
 
-// TODO Slice instead of MutableSlice
-Result<HttpUrl> parse_url(MutableSlice url,
+Result<HttpUrl> parse_url(Slice url,
                           HttpUrl::Protocol default_protocol = HttpUrl::Protocol::HTTP) TD_WARN_UNUSED_RESULT;
 
 StringBuilder &operator<<(StringBuilder &sb, const HttpUrl &url);
diff --git a/tdutils/td/utils/Parser.h b/tdutils/td/utils/Parser.h
index 5d10849b..d783b22d 100644
--- a/tdutils/td/utils/Parser.h
+++ b/tdutils/td/utils/Parser.h
@@ -17,14 +17,17 @@
 
 namespace td {
 
-class Parser {
+namespace detail {
+
+template <class SliceT>
+class ParserImpl {
  public:
-  explicit Parser(MutableSlice data) : ptr_(data.begin()), end_(data.end()), status_() {
+  explicit ParserImpl(SliceT data) : ptr_(data.begin()), end_(data.end()), status_() {
   }
-  Parser(Parser &&other) : ptr_(other.ptr_), end_(other.end_), status_(std::move(other.status_)) {
+  ParserImpl(ParserImpl &&other) : ptr_(other.ptr_), end_(other.end_), status_(std::move(other.status_)) {
     other.clear();
   }
-  Parser &operator=(Parser &&other) {
+  ParserImpl &operator=(ParserImpl &&other) {
     if (&other == this) {
       return *this;
     }
@@ -34,9 +37,9 @@ class Parser {
     other.clear();
     return *this;
   }
-  Parser(const Parser &) = delete;
-  Parser &operator=(const Parser &) = delete;
-  ~Parser() = default;
+  ParserImpl(const ParserImpl &) = delete;
+  ParserImpl &operator=(const ParserImpl &) = delete;
+  ~ParserImpl() = default;
 
   bool empty() const {
     return ptr_ == end_;
@@ -47,57 +50,57 @@ class Parser {
     status_ = Status::OK();
   }
 
-  MutableSlice read_till_nofail(char c) {
+  SliceT read_till_nofail(char c) {
     if (status_.is_error()) {
-      return MutableSlice();
+      return SliceT();
     }
-    char *till = static_cast<char *>(std::memchr(ptr_, c, end_ - ptr_));
+    auto till = static_cast<decltype(ptr_)>(std::memchr(ptr_, c, end_ - ptr_));
     if (till == nullptr) {
       till = end_;
     }
-    MutableSlice result(ptr_, till);
+    SliceT result(ptr_, till);
     ptr_ = till;
     return result;
   }
 
-  MutableSlice read_till_nofail(Slice str) {
+  SliceT read_till_nofail(Slice str) {
     if (status_.is_error()) {
-      return MutableSlice();
+      return SliceT();
     }
-    char *best_till = end_;
+    auto best_till = end_;
     for (auto c : str) {
-      char *till = static_cast<char *>(std::memchr(ptr_, c, end_ - ptr_));
+      auto till = static_cast<decltype(ptr_)>(std::memchr(ptr_, c, end_ - ptr_));
       if (till != nullptr && till < best_till) {
         best_till = till;
       }
     }
-    MutableSlice result(ptr_, best_till);
+    SliceT result(ptr_, best_till);
     ptr_ = best_till;
     return result;
   }
 
   template <class F>
-  MutableSlice read_while(const F &f) {
+  SliceT read_while(const F &f) {
     auto save_ptr = ptr_;
     while (ptr_ != end_ && f(*ptr_)) {
       ptr_++;
     }
-    return MutableSlice(save_ptr, ptr_);
+    return SliceT(save_ptr, ptr_);
   }
-  MutableSlice read_all() {
+  SliceT read_all() {
     auto save_ptr = ptr_;
     ptr_ = end_;
-    return MutableSlice(save_ptr, ptr_);
+    return SliceT(save_ptr, ptr_);
   }
 
-  MutableSlice read_till(char c) {
+  SliceT read_till(char c) {
     if (status_.is_error()) {
-      return MutableSlice();
+      return SliceT();
     }
-    MutableSlice res = read_till_nofail(c);
+    SliceT res = read_till_nofail(c);
     if (ptr_ == end_ || ptr_[0] != c) {
       status_ = Status::Error(PSLICE() << "Read till " << tag("char", c) << " failed");
-      return MutableSlice();
+      return SliceT();
     }
     return res;
   }
@@ -147,13 +150,13 @@ class Parser {
   void skip_whitespaces() {
     skip_till_not(" \t\r\n");
   }
-  MutableSlice read_word() {
+  SliceT read_word() {
     skip_whitespaces();
     return read_till_nofail(" \t\r\n");
   }
 
-  MutableSlice data() const {
-    return MutableSlice(ptr_, end_);
+  SliceT data() const {
+    return SliceT(ptr_, end_);
   }
 
   Status &status() {
@@ -181,9 +184,14 @@ class Parser {
   }
 
  private:
-  char *ptr_;
-  char *end_;
+  decltype(std::declval<SliceT>().begin()) ptr_;
+  decltype(std::declval<SliceT>().end()) end_;
   Status status_;
 };
 
+}  // namespace detail
+
+using Parser = detail::ParserImpl<MutableSlice>;
+using ConstParser = detail::ParserImpl<Slice>;
+
 }  // namespace td
diff --git a/tdutils/td/utils/tests.cpp b/tdutils/td/utils/tests.cpp
index 7980f07c..31accc81 100644
--- a/tdutils/td/utils/tests.cpp
+++ b/tdutils/td/utils/tests.cpp
@@ -100,7 +100,7 @@ class RegressionTesterImpl : public RegressionTester {
 
   Status load_db(CSlice path) {
     TRY_RESULT(data, read_file(path));
-    Parser parser(data.as_slice());
+    ConstParser parser(data.as_slice());
     auto db_magic = parser.read_word();
     if (db_magic != magic()) {
       return Status::Error(PSLICE() << "Wrong magic " << db_magic);