diff --git a/CMakeLists.txt b/CMakeLists.txt
index c44c1f9..6ff28e9 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -19,6 +19,7 @@ include_directories(include
 add_executable(lang_interpreter src/main.cpp
                                 src/types.cpp
                                 src/values.cpp
+                                src/utils.cpp
                                 src/global_info.cpp
                                 src/typeclass_graph.cpp
                                 src/visitor.cpp
diff --git a/include/type_check_visitor.hpp b/include/type_check_visitor.hpp
index 0ff48be..c71e2b3 100644
--- a/include/type_check_visitor.hpp
+++ b/include/type_check_visitor.hpp
@@ -172,7 +172,14 @@ private:
     }
   }
 
-  //
+  void AddGraphIdLocalTypes(utils::IdType graph_id, utils::IdType type) {
+    std::unordered_set<utils::IdType> requirement_graph_ids = typeclass_graph_.GetDependenciesSet(graph_id);
+    requirement_graph_ids.insert(graph_id);
+
+    for (auto& requirement_graph_id : requirement_graph_ids) {
+      context_manager_.DefineLocalType(typeclass_graph_.GetVertex(requirement_graph_id).name, type);
+    }
+  }
 
 private:
   info::GlobalInfo::NamespaceVisitor namespace_visitor_;
diff --git a/include/types.hpp b/include/types.hpp
index 3af1975..760a128 100644
--- a/include/types.hpp
+++ b/include/types.hpp
@@ -50,14 +50,12 @@ public:
   }
 
   bool HasTypeclass(utils::IdType graph_id) {
-    error_handling::DebugPrint(name_);
-    error_handling::DebugPrint(requirement_graph_ids_.size());
-    for (auto& requirement_graph_id : requirement_graph_ids_) {
-      error_handling::DebugPrint(requirement_graph_id);
-    }
     return requirement_graph_ids_.count(graph_id) != 0;
   }
 
+  std::string ToString() {
+    return "Abstract";
+  }
 private:
   utils::AbstractTypeModifier modifier_;
   std::string name_;
@@ -93,6 +91,9 @@ public:
     return class_modifier_;
   }
 
+  std::string ToString() {
+    return "Defined";
+  }
 private:
   utils::IdType type_id_; // in defined types
   utils::IdType type_; // in types manager, created using context types (if specific type)
@@ -140,6 +141,32 @@ inline std::optional<InternalType> ToInternalType(const std::string& type) {
   return std::nullopt;
 }
 
+inline std::string ToString(InternalType type) {
+  std::string result;
+
+  switch (type) {
+    case InternalType::Float:
+      result = "Float";
+      break;
+    case InternalType::Int:
+      result = "Int";
+      break;
+    case InternalType::String:
+      result = "String";
+      break;
+    case InternalType::Char:
+      result = "Char";
+      break;
+    case InternalType::Bool:
+      result = "Bool";
+      break;
+    case InternalType::Unit:
+      result = "Unit";
+      break;
+  }
+  return result;
+}
+
 class TupleType {
 public:
   TupleType() = default;
@@ -160,6 +187,7 @@ public:
     return fields_;
   }
 
+  std::string ToString();
 private:
   std::optional<std::string> name_;
   std::vector<std::pair<std::optional<std::string>, utils::IdType>> fields_;
@@ -190,6 +218,7 @@ public:
     current_constructor_ = constructor;
   }
 
+  std::string ToString();
 private:
   std::optional<std::string> name_;
   std::vector<TupleType> constructors_;
@@ -211,6 +240,7 @@ public:
   std::optional<utils::IdType> GetFieldType(const std::string& name,
                                             const std::unordered_set<utils::IdType>& type_namespaces) const;
 
+  std::string ToString();
 private:
   utils::IdType type_;
   TypeManager* type_manager_ = nullptr;
@@ -232,6 +262,7 @@ public:
   std::optional<utils::IdType> GetFieldType(const std::string& name,
                                             const std::unordered_set<utils::IdType>& type_namespaces) const;
 
+  std::string ToString();
 private:
   std::vector<utils::ReferenceModifier> references_;
   utils::IdType type_;
@@ -256,6 +287,7 @@ public:
   std::optional<utils::IdType> GetFieldType(const std::string& name,
                                             const std::unordered_set<utils::IdType>& type_namespaces) const;
 
+  std::string ToString();
 private:
   std::vector<utils::IdType> argument_types_;
   utils::IdType return_type_;
@@ -282,6 +314,7 @@ public:
     return elements_type_;
   }
 
+  std::string ToString();
 private:
   size_t size_; // = 0 for dynamic
   utils::IdType elements_type_;
@@ -316,6 +349,8 @@ public:
                OptionalType>& GetType() {
     return type_;
   }
+
+  std::string ToString();
 private:
   std::variant<AbstractType,
                DefinedType,
diff --git a/include/utils.hpp b/include/utils.hpp
index cfa30f4..175f7ec 100644
--- a/include/utils.hpp
+++ b/include/utils.hpp
@@ -31,62 +31,11 @@ enum class PartitionModifier { Exec = 0, Test = 1 };
 
 enum class ValueType { Const = 0, Var = 1, Tmp = 2 };
 
-inline ValueType IsConstModifierToValueType(IsConstModifier modifier) {
-  switch (modifier) {
-    case IsConstModifier::Const:
-      return ValueType::Const;
-    case IsConstModifier::Var:
-      return ValueType::Var;
-  }
+ValueType IsConstModifierToValueType(IsConstModifier modifier);
 
-  exit(1); // unreachable
-}
+ValueType ClassInternalsModifierToValueType(ClassInternalsModifier modifier);
 
-inline ValueType ClassInternalsModifierToValueType(ClassInternalsModifier modifier) {
-  switch (modifier) {
-    case ClassInternalsModifier::Const:
-      return ValueType::Const;
-    case ClassInternalsModifier::Var:
-      return ValueType::Var;
-    case ClassInternalsModifier::Static:
-      throw std::bad_cast(); // ??
-  }
-
-  exit(1); // unreachable
-}
-
-inline bool IsBuiltinFunction(const std::string& name) { // optimize ??
-  std::unordered_set<std::string> builtin_functions;
-
-  builtin_functions.insert("=");
-  builtin_functions.insert("<-");
-  builtin_functions.insert("==");
-  builtin_functions.insert("!=");
-  builtin_functions.insert("<");
-  builtin_functions.insert(">");
-  builtin_functions.insert("<=");
-  builtin_functions.insert(">=");
-  builtin_functions.insert("+=");
-  builtin_functions.insert("-=");
-  builtin_functions.insert("*=");
-  builtin_functions.insert("div");
-  builtin_functions.insert("mod");
-  builtin_functions.insert("/=");
-  // builtin_functions.insert("+");
-  // builtin_functions.insert("-");
-  // builtin_functions.insert("*");
-  // builtin_functions.insert("/");
-  builtin_functions.insert("&&");
-  builtin_functions.insert("||");
-  builtin_functions.insert("size");
-  builtin_functions.insert("random");
-  builtin_functions.insert("print");
-  builtin_functions.insert("scan");
-  builtin_functions.insert("zero");
-  builtin_functions.insert("one");
-
-  return builtin_functions.count(name) != 0;
-}
+bool IsBuiltinFunction(const std::string& name);
 
 template<typename T>
 class Storage {
@@ -265,34 +214,12 @@ private:
   std::vector<size_t> ranks_;
 };
 
-// move to .cpp ??
-inline void BackVisitDfs(size_t id,
-                     std::vector<size_t>& verticles,
-                     std::vector<size_t>& marks,
-                     const std::vector<std::vector<size_t>>& edges,
-                     size_t mark) {
-  if (marks[id] != 0) {
-    return;
-  }
+void BackVisitDfs(size_t id,
+                  std::vector<size_t>& verticles,
+                  std::vector<size_t>& marks,
+                  const std::vector<std::vector<size_t>>& edges,
+                  size_t mark);
 
-  marks[id] = mark;
-  verticles.push_back(id);
-
-  for (size_t i = 0; i < edges[id].size(); ++i) {
-    BackVisitDfs(id, verticles, marks, edges, mark);
-  }
-}
-
-// move to .cpp ??
-inline std::vector<size_t> BackTopSort(const std::vector<std::vector<size_t>>& edges_) {
-  std::vector<size_t> sorted_verticles;
-  std::vector<size_t> marks(edges_.size(), 0);
-
-  for (size_t i = 0; i < marks.size(); ++i) {
-    BackVisitDfs(i, sorted_verticles, marks, edges_, 1);
-  }
-
-  return sorted_verticles;
-}
+std::vector<size_t> BackTopSort(const std::vector<std::vector<size_t>>& edges_);
 
 } // namespace utils
diff --git a/src/type_check_visitor.cpp b/src/type_check_visitor.cpp
index 9f37969..5a3bee1 100644
--- a/src/type_check_visitor.cpp
+++ b/src/type_check_visitor.cpp
@@ -52,11 +52,9 @@ void TypeCheckVisitor::Visit(Namespace* node) {
       context_manager_.EnterContext();
     }
 
-    // including namespace typeclass
-    // define typeclasses local types as namespace typeclass
-    for (auto& requirement_graph_id : requirement_graph_ids) {
-      context_manager_.DefineLocalType(typeclass_graph_.GetVertex(requirement_graph_id).name, abstract_type);
-    }
+    abstract_type = context_manager_.ToModifiedValue(abstract_type, utils::ValueType::Tmp); // ??
+
+    AddGraphIdLocalTypes(graph_id, abstract_type);
   } else if (node->link_type_id_.has_value()) {
     auto maybe_type_info = global_info_.GetTypeInfo<info::definition::AnyType>(node->link_type_id_.value());
 
@@ -80,12 +78,7 @@ void TypeCheckVisitor::Visit(Namespace* node) {
       context_manager_.EnterContext();
     }
 
-    // better decision ??
-    // define typeclasses local types as namespace type
-    auto typeclass_graph_ids = typeclass_graph_.GetDependenciesVector(type_info->type.node->graph_id_);
-    for (auto& typeclass_graph_id : typeclass_graph_ids) {
-      context_manager_.DefineLocalType(typeclass_graph_.GetVertex(typeclass_graph_id).name, type);
-    }
+    AddGraphIdLocalTypes(type_info->type.node->graph_id_, type);
 
     if (type_namespaces_.count(node->link_type_id_.value()) != 0) {
         error_handling::HandleTypecheckError("Namespaces of one type are one in another", node->base);
@@ -269,6 +262,8 @@ void TypeCheckVisitor::Visit(FunctionDefinitionStatement* node) {
     returned_type_ = current_type_;
   }
   if (!context_manager_.EqualValues(returned_type, returned_type_.value())) {
+    error_handling::DebugPrint(context_manager_.GetAnyValue(returned_type)->ToString());
+    error_handling::DebugPrint(context_manager_.GetAnyValue(returned_type_.value())->ToString());
     error_handling::HandleTypecheckError("Wrong function return type", node->base);
   }
   returned_type_ = std::nullopt;
@@ -319,7 +314,7 @@ void TypeCheckVisitor::Visit(AbstractTypeDefinitionStatement* node) {
   current_type_ = context_manager_.AddValue(info::type::AbstractType(node->modifier, node->type->type, requirements),
                                            utils::ValueType::Tmp);
   if (!context_manager_.DefineLocalType(node->type->type, current_type_)) {
-    error_handling::HandleTypecheckError("Can't define basic / astract type: abstract type redefinition", node->base);
+    error_handling::HandleTypecheckError("Can't define basic / abstract type: abstract type redefinition", node->base);
   }
 
   current_type_ = context_manager_.AddValue(info::type::InternalType::Unit, utils::ValueType::Tmp);
@@ -731,6 +726,8 @@ void TypeCheckVisitor::Visit(AccessExpression* node) {
 
 // TODO: more builtin functions, better handling (??)
 void TypeCheckVisitor::Visit(FunctionCallExpression* node) {
+  context_manager_.EnterContext();
+
   std::optional<FunctionDeclaration*> maybe_function_declaration;
 
   std::unordered_map<std::string, utils::IdType> context;
@@ -749,7 +746,8 @@ void TypeCheckVisitor::Visit(FunctionCallExpression* node) {
   if (node->prefix.has_value()) {
     if (std::holds_alternative<std::unique_ptr<SubExpressionToken>>(node->prefix.value())) {
       Visitor::Visit(*std::get<std::unique_ptr<SubExpressionToken>>(node->prefix.value()));
-      maybe_function_declaration = FindExpressionMethodAndUpdate(node, current_type_);
+      utils::IdType expression_type = current_type_;
+      maybe_function_declaration = FindExpressionMethodAndUpdate(node, expression_type);
     } else if (std::holds_alternative<std::unique_ptr<TypeExpression>>(node->prefix.value())) {
       maybe_function_declaration = FindTypeFunctionAndUpdate(
         node,
@@ -843,6 +841,8 @@ void TypeCheckVisitor::Visit(TupleExpression* node) {
     info::type::TupleType(std::nullopt, fields, context_manager_.GetValueManager()),
     utils::ValueType::Tmp);
 
+  context_manager_.ExitContext();
+
   node->base.type_ = current_type_;
 }
 
@@ -988,7 +988,10 @@ void TypeCheckVisitor::Visit(TypeConstructor* node) {
   current_type_ = TypeInContext(current_type_, context);
 
   current_type_ = context_manager_.AddValue(
-    info::type::DefinedType(type_id, current_type_, type_info.modifier, context_manager_.GetValueManager()),
+    info::type::DefinedType(type_id,
+                            current_type_,
+                            type_info.modifier,
+                            context_manager_.GetValueManager()),
     utils::ValueType::Tmp);
 
   node->base.type_ = current_type_;
@@ -1279,7 +1282,7 @@ void TypeCheckVisitor::Visit(TypeExpression* node) {
 
   auto maybe_internal_type = info::type::ToInternalType(node->type.type);
 
-  if (maybe_internal_type.has_value()) {
+  if (maybe_internal_type.has_value()) { // TODO: ???
     // checks made in link_symbols_visitor
     current_type_ =
       context_manager_.AddValue<info::type::InternalType>(
@@ -1471,6 +1474,18 @@ std::optional<FunctionDeclaration*>
                                                     utils::IdType expression_type) {
   std::optional<FunctionDeclaration*> maybe_function_declaration;
 
+  auto maybe_internal_type_info = context_manager_.GetValue<info::type::InternalType>(expression_type);
+  if (maybe_internal_type_info.has_value()) {
+    auto maybe_abstract_type_id = context_manager_.FindLocalType(info::type::ToString(*maybe_internal_type_info.value()));
+    if (maybe_abstract_type_id.has_value()) {
+      expression_type = maybe_abstract_type_id.value();
+    } else {
+      error_handling::HandleInternalError("InternalType local abstract type not found",
+                                          "TypeCheckVisitor.FindExpressionMethodAndUpdate",
+                                          &node->base);
+    }
+  }
+
   auto maybe_abstract_type_info = context_manager_.GetValue<info::type::AbstractType>(expression_type);
 
   if (maybe_abstract_type_info.has_value()) {
@@ -1478,8 +1493,7 @@ std::optional<FunctionDeclaration*>
                                                                             maybe_abstract_type_info.value(),
                                                                             true);
   } else {
-    auto maybe_defined_type_info =
-      context_manager_.GetValue<info::type::DefinedType>(expression_type);
+    auto maybe_defined_type_info = context_manager_.GetValue<info::type::DefinedType>(expression_type);
     if (!maybe_defined_type_info.has_value()) {
       error_handling::HandleTypecheckError("There is no non-builtin methods for not defined type", node->base);
     }
@@ -1488,7 +1502,7 @@ std::optional<FunctionDeclaration*>
     auto maybe_type_info = global_info_.GetTypeInfo<info::definition::AnyType>(type_id);
     if (!maybe_type_info.has_value()) {
       error_handling::HandleInternalError("Functions / Methods implemented only for AnyType",
-                                          "TypeCheckVisitor.FunctionCallExpresssion",
+                                          "TypeCheckVisitor.FindExpressionMethodAndUpdate",
                                           &node->base);
     }
     info::definition::AnyType* type_info = maybe_type_info.value();
@@ -1645,6 +1659,7 @@ std::optional<FunctionDeclaration*>
   }
 
   node->graph_id_ = maybe_typeclass_graph_id;
+
   node->abstract_type_name_ = abstract_type_info->GetName();
   maybe_function_declaration = maybe_typeclass_function_info.value()->declaration;
 
@@ -1671,6 +1686,7 @@ std::optional<FunctionDeclaration*>
   }
 
   node->graph_id_ = defined_type->type.node->graph_id_;
+
   // type defined -> abstract type name not needed
   maybe_function_declaration = maybe_type_function_info.value()->declaration;
 
diff --git a/src/typeclass_graph.cpp b/src/typeclass_graph.cpp
index 0fa996c..3dd8e93 100644
--- a/src/typeclass_graph.cpp
+++ b/src/typeclass_graph.cpp
@@ -128,7 +128,7 @@ void TypeclassGraph::CalculateGraph() {
   }
 
   std::vector<size_t> sorted_verticles = utils::BackTopSort(edges);
-  std::reverse(sorted_verticles.begin(), sorted_verticles.end());
+  // std::reverse(sorted_verticles.begin(), sorted_verticles.end());
 
   for (auto& id : sorted_verticles) {
     for (auto& dependency : verticles_[id].dependencies) {
@@ -146,7 +146,7 @@ void TypeclassGraph::CalculateGraph() {
           }
         }
       }
-      for (auto& inherited_dependency : verticles_[method_to_vertex_[dependency]].dependencies) {
+      for (auto& inherited_dependency : verticles_[name_to_typeclass_[dependency]].dependencies) {
         verticles_[id].dependencies.insert(inherited_dependency);
       }
     }
diff --git a/src/types.cpp b/src/types.cpp
index 6a36158..0d9d9eb 100644
--- a/src/types.cpp
+++ b/src/types.cpp
@@ -124,6 +124,21 @@ std::optional<utils::IdType> TupleType::GetFieldType(const std::string& name,
   return std::nullopt;
 }
 
+std::string TupleType::ToString() {
+  std::string result;
+
+  result += "(";
+
+  for (auto& field : fields_) {
+    result += "& ";
+    result += type_manager_->GetAnyValue(field.second)->ToString();
+  }
+
+  result += ")";
+
+  return result;
+}
+
 //
 
 std::optional<utils::IdType> VariantType::InContext(const std::unordered_map<std::string, utils::IdType>& context) {
@@ -174,6 +189,21 @@ std::optional<utils::IdType> VariantType::GetFieldType(const std::string& name,
   return std::nullopt;
 }
 
+std::string VariantType::ToString() {
+  std::string result;
+
+  result += "(";
+
+  for (auto& constructor : constructors_) {
+    result += "& ";
+    result += constructor.ToString();
+  }
+
+  result += ")";
+
+  return result;
+}
+
 //
 
 std::optional<utils::IdType> OptionalType::InContext(const std::unordered_map<std::string, utils::IdType>& context) {
@@ -203,6 +233,10 @@ std::optional<utils::IdType> OptionalType::GetFieldType(const std::string&,
   return std::nullopt;
 }
 
+std::string OptionalType::ToString() {
+  return "Optional " + type_manager_->GetAnyValue(type_)->ToString();
+}
+
 //
 
 std::optional<utils::IdType> ReferenceToType::InContext(const std::unordered_map<std::string, utils::IdType>& context) {
@@ -232,6 +266,29 @@ std::optional<utils::IdType> ReferenceToType::GetFieldType(const std::string& na
   return type_manager_->GetAnyValue(type_)->GetFieldType(name, type_namespaces);
 }
 
+
+std::string ReferenceToType::ToString() {
+  std::string result;
+
+  for (auto& reference : references_) {
+    switch (reference) {
+      case utils::ReferenceModifier::Dereference:
+        result += '~';
+        break;
+      case utils::ReferenceModifier::Reference:
+        result += '^';
+        break;
+      case utils::ReferenceModifier::UniqueReference:
+        result += '@';
+        break;
+    }
+  }
+
+  result += type_manager_->GetAnyValue(type_)->ToString();
+
+  return result;
+}
+
 //
 
 std::optional<utils::IdType> FunctionType::InContext(const std::unordered_map<std::string, utils::IdType>& context) {
@@ -284,6 +341,26 @@ std::optional<utils::IdType> FunctionType::GetFieldType(const std::string&,
   return std::nullopt;
 }
 
+std::string FunctionType::ToString() {
+  std::string result;
+
+  result += "(";
+
+  bool is_first_argument = true;
+  for (auto& argument_type : argument_types_) {
+    if (is_first_argument) {
+      is_first_argument = false;
+    } else {
+      result += " -> ";
+    }
+    result += type_manager_->GetAnyValue(argument_type)->ToString();
+  }
+
+  result += " -> " + type_manager_->GetAnyValue(return_type_)->ToString() + ")";
+
+  return result;
+}
+
 //
 
 std::optional<utils::IdType> ArrayType::InContext(const std::unordered_map<std::string, utils::IdType>& context) {
@@ -314,6 +391,10 @@ std::optional<utils::IdType> ArrayType::GetFieldType(const std::string&,
   return std::nullopt;
 }
 
+std::string ArrayType::ToString() {
+  return "Array (" + std::to_string(size_) + ") " + type_manager_->GetAnyValue(elements_type_)->ToString();
+}
+
 //
 
 std::optional<utils::IdType> Type::InContext(const std::unordered_map<std::string, utils::IdType>& context) {
@@ -477,5 +558,35 @@ std::string Type::GetTypeName() const {
   return ""; // ??
 }
 
+std::string Type::ToString() {
+  size_t index = type_.index();
+
+  switch (index) {
+    case 0:
+      return std::get<AbstractType>(type_).ToString();
+    case 1:
+      return std::get<DefinedType>(type_).ToString();
+    case 2:
+      return ::info::type::ToString(std::get<InternalType>(type_));
+    case 3:
+      return std::get<TupleType>(type_).ToString();
+    case 4:
+      return std::get<VariantType>(type_).ToString();
+    case 5:
+      return std::get<ReferenceToType>(type_).ToString();
+    case 6:
+      return std::get<FunctionType>(type_).ToString();
+    case 7:
+      return std::get<ArrayType>(type_).ToString();
+    case 8:
+      return std::get<OptionalType>(type_).ToString();
+    default:
+      // error
+      break;
+  }
+
+  return "";
+}
+
 } // namespace info::type
 
diff --git a/src/utils.cpp b/src/utils.cpp
new file mode 100644
index 0000000..5d80484
--- /dev/null
+++ b/src/utils.cpp
@@ -0,0 +1,99 @@
+// for clangd
+#include "../include/utils.hpp"
+#include"../include/error_handling.hpp"
+
+namespace utils {
+
+using std::size_t;
+
+using IdType = size_t;
+
+ValueType IsConstModifierToValueType(IsConstModifier modifier) {
+  switch (modifier) {
+    case IsConstModifier::Const:
+      return ValueType::Const;
+    case IsConstModifier::Var:
+      return ValueType::Var;
+  }
+
+  exit(1); // unreachable
+}
+
+ValueType ClassInternalsModifierToValueType(ClassInternalsModifier modifier) {
+  switch (modifier) {
+    case ClassInternalsModifier::Const:
+      return ValueType::Const;
+    case ClassInternalsModifier::Var:
+      return ValueType::Var;
+    case ClassInternalsModifier::Static:
+      throw std::bad_cast(); // ??
+  }
+
+  exit(1); // unreachable
+}
+
+bool IsBuiltinFunction(const std::string& name) { // optimize ??
+  std::unordered_set<std::string> builtin_functions;
+
+  builtin_functions.insert("=");
+  builtin_functions.insert("<-");
+  builtin_functions.insert("==");
+  // builtin_functions.insert("!=");
+  builtin_functions.insert("<");
+  // builtin_functions.insert(">");
+  // builtin_functions.insert("<=");
+  // builtin_functions.insert(">=");
+  builtin_functions.insert("+=");
+  builtin_functions.insert("-=");
+  builtin_functions.insert("*=");
+  builtin_functions.insert("div");
+  builtin_functions.insert("mod");
+  builtin_functions.insert("/=");
+  // builtin_functions.insert("+");
+  // builtin_functions.insert("-");
+  // builtin_functions.insert("*");
+  // builtin_functions.insert("/");
+  builtin_functions.insert("&&");
+  builtin_functions.insert("||");
+  builtin_functions.insert("size");
+  builtin_functions.insert("random");
+  builtin_functions.insert("print");
+  builtin_functions.insert("scan");
+  builtin_functions.insert("zero");
+  builtin_functions.insert("one");
+
+  return builtin_functions.count(name) != 0;
+}
+
+void BackVisitDfs(size_t id,
+                  std::vector<size_t>& verticles,
+                  std::vector<size_t>& marks,
+                  const std::vector<std::vector<size_t>>& edges,
+                  size_t mark) {
+  if (marks[id] != 0) {
+    return;
+  }
+
+  marks[id] = mark;
+
+  for (size_t i = 0; i < edges[id].size(); ++i) {
+    BackVisitDfs(edges[id][i], verticles, marks, edges, mark);
+  }
+
+  verticles.push_back(id);
+}
+
+std::vector<size_t> BackTopSort(const std::vector<std::vector<size_t>>& edges_) {
+  std::vector<size_t> sorted_verticles;
+  std::vector<size_t> marks(edges_.size(), 0);
+
+  for (size_t i = 0; i < edges_.size(); ++i) {
+    if (marks[i] == 0) {
+      BackVisitDfs(i, sorted_verticles, marks, edges_, 1);
+    }
+  }
+
+  return sorted_verticles;
+}
+
+} // namespace utils
diff --git a/tests/test_code.lang b/tests/test_code.lang
index eac5aea..559ecd8 100644
--- a/tests/test_code.lang
+++ b/tests/test_code.lang
@@ -77,8 +77,8 @@ namespace var Mult {
 }
 
 typeclass (IDiv : #Mult) =
-  & var div : IDiv -> Unit
-  & var mod : IDiv -> Unit
+  & var div : IDiv -> IDiv
+  & var mod : IDiv -> IDiv
 
 namespace var IDiv {
   def mod : x = self -. x * self.div: x
@@ -190,7 +190,7 @@ def ( -- ) : begin end = {
   var current = begin
   return (while current < end do {
     ; current += 1
-    return current - 1
+    bring current - 1
   })
 }