diff --git a/deps/tree-sitter-lang b/deps/tree-sitter-lang
index 6b09331..1357783 160000
--- a/deps/tree-sitter-lang
+++ b/deps/tree-sitter-lang
@@ -1 +1 @@
-Subproject commit 6b093311dfb371cf67a218a8c97779ae5e15e234
+Subproject commit 1357783074e77c2c5ddcec16e90cf59940dabd9d
diff --git a/etc/kate_highlighting_lang.xml b/etc/kate_highlighting_lang.xml
index b076662..a93b7e4 100644
--- a/etc/kate_highlighting_lang.xml
+++ b/etc/kate_highlighting_lang.xml
@@ -64,7 +64,7 @@
             <RegExpr String="\b[0-9]+[il]?\b" attribute="Decimal" context="#stay"/>
             <RegExpr String="&quot;([^\\&quot;]|(\\.))*&quot;u?" attribute="String" context="#stay" />
 
-            <RegExpr String="((?&lt;![\+\\\-\*/%\^\!\?\|&amp;,&lt;>=\.])((\.+)|([\+\\\-\*/%\^\!\?\|&amp;,&lt;>=]+\.?\.?\.?)))|(\|)" attribute="Operator" context="#stay"/>
+            <RegExpr String="((?&lt;![\+\\\-\*/%\^\!\?\|&amp;,&lt;>=\.])((\.+)|([\+\\\-\*/%\^\!\?\|&amp;,&lt;>=]+\.?\.?\.?)))|(\|)|(\[\[)|(\]\])|(\(\()|(\)\))" attribute="Operator" context="#stay"/>
 
             <RegExpr String="([a-z_][a-z0-9_]*\.)*(([a-z_][a-z0-9_]*)|_)(?![a-z0-9_])" attribute="Name" context="#stay"/>
 
diff --git a/include/builders/type_builders.hpp b/include/builders/type_builders.hpp
index c39607c..1afce88 100644
--- a/include/builders/type_builders.hpp
+++ b/include/builders/type_builders.hpp
@@ -14,6 +14,9 @@ nodes::TypeProxy build_variant_type(parser::ParseTree::Node parse_node,
 nodes::TypeProxy build_tuple_type(parser::ParseTree::Node parse_node,
                                   nodes::TypeStorage &type_storage);
 
+nodes::TypeProxy build_function_type(parser::ParseTree::Node parse_node,
+                                     nodes::TypeStorage &type_storage);
+
 nodes::TypeProxy build_array_type(parser::ParseTree::Node parse_node,
                                   nodes::TypeStorage &type_storage);
 
diff --git a/include/builtin_types.hpp b/include/builtin_types.hpp
index d9f0eb5..854a222 100644
--- a/include/builtin_types.hpp
+++ b/include/builtin_types.hpp
@@ -13,6 +13,8 @@ const static std::string TUPLE_IDENTIFIER = "Tuple";
 
 const static std::string VARIANT_IDENTIFIER = "Variant";
 
+const static std::string FUNCTION_IDENTIFIER = "Function";
+
 const static std::string ARRAY_IDENTIFIER = "Array";
 
 const static std::string OPTIONAL_IDENTIFIER = "Optional";
@@ -50,6 +52,7 @@ enum class Type {
   // -- containers
   TUPLE,
   VARIANT,
+  FUNCTION,
   ARRAY,
   OPTIONAL,
   RESULT,
@@ -75,6 +78,8 @@ inline std::string to_string(Type type) {
     return TUPLE_IDENTIFIER;
   case Type::VARIANT:
     return VARIANT_IDENTIFIER;
+  case Type::FUNCTION:
+    return FUNCTION_IDENTIFIER;
   case Type::ARRAY:
     return ARRAY_IDENTIFIER;
   case Type::OPTIONAL:
@@ -118,6 +123,7 @@ inline Type to_type(const std::string &str) {
     // -- containers
     builtin_types[TUPLE_IDENTIFIER] = Type::TUPLE;
     builtin_types[VARIANT_IDENTIFIER] = Type::VARIANT;
+    builtin_types[FUNCTION_IDENTIFIER] = Type::FUNCTION;
     builtin_types[ARRAY_IDENTIFIER] = Type::ARRAY;
     builtin_types[OPTIONAL_IDENTIFIER] = Type::OPTIONAL;
     builtin_types[RESULT_IDENTIFIER] = Type::RESULT;
@@ -161,6 +167,7 @@ inline std::optional<size_t> get_parameters_count(Type type) {
     // -- containers
   case Type::TUPLE:
   case Type::VARIANT:
+  case Type::FUNCTION:
     return std::nullopt;
   case Type::ARRAY:
   case Type::OPTIONAL:
diff --git a/include/expression_type_check.hpp b/include/expression_type_check.hpp
index 5c3f266..caa6ceb 100644
--- a/include/expression_type_check.hpp
+++ b/include/expression_type_check.hpp
@@ -3,6 +3,7 @@
 #include "expression_nodes.hpp"
 #include "sources_manager.hpp"
 #include "type_nodes.hpp"
+#include "utils.hpp"
 
 #include <optional>
 
@@ -10,9 +11,46 @@
 
 namespace type_check {
 
-struct State {
+class State {
+public:
+  bool insert_variable(const std::string &name, nodes::TypeProxy type) {
+    if (contexts_.size() == 0) {
+      error_handling::handle_general_error(
+          "Insert variable into contexts_ with zero elements in State");
+    }
+
+    return contexts_.back().insert({name, type}).second;
+  }
+
+  std::optional<nodes::TypeProxy> find_variable(const std::string &name) {
+    for (ssize_t i = contexts_.size(); i >= 0; --i) {
+      auto iter = contexts_[i].find(name);
+      if (iter != contexts_[i].end()) {
+        return iter->second;
+      }
+    }
+
+    return std::nullopt;
+  }
+
+  void enter_context() { contexts_.emplace_back(); }
+
+  void exit_context() {
+    if (contexts_.size() == 0) {
+      error_handling::handle_general_error(
+          "Pop from contexts_ with zero elements in State");
+    }
+
+    contexts_.pop_back();
+  }
+
+public:
   std::optional<nodes::TypeProxy> brought_type;
   std::optional<nodes::TypeProxy> returned_type;
+
+private:
+  std::vector<std::unordered_map<std::string, nodes::TypeProxy>> contexts_ = {
+      {}};
 };
 
 nodes::TypeCheckResult
diff --git a/include/nodes/type_nodes.hpp b/include/nodes/type_nodes.hpp
index 26b46fd..c661c87 100644
--- a/include/nodes/type_nodes.hpp
+++ b/include/nodes/type_nodes.hpp
@@ -9,6 +9,7 @@
 #include <optional>
 #include <string>
 #include <unordered_map>
+#include <unordered_set>
 #include <variant>
 #include <vector>
 
@@ -207,10 +208,11 @@ class TypeStorage {
 
 public:
   TypeProxy primitive_type(builtin::types::Type type) {
-    if (unit_type_id.has_value()) {
-      return TypeProxy(*this, unit_type_id.value());
+    auto iter = primitive_type_ids_.find(type);
+    if (iter != primitive_type_ids_.end()) {
+      return TypeProxy(*this, iter->second);
     } else {
-      unit_type_id = storage_.size();
+      primitive_type_ids_[type] = storage_.size();
       return add_type(Type(Identifier(Node(), Identifier::SIMPLE_TYPE,
                                       builtin::types::to_string(type))));
     }
@@ -251,72 +253,27 @@ public:
     return TypeProxy(*this, storage_.size() - 1);
   }
 
+  // TODO:
+  // bool try_resolve_all_generic_names
+  // bool add_generic_type
+  // bool add_local_type
+  // // add local type requirement
+  // // deduce generic type
+  // void clear_local_types
+  // ...
+
 private:
   Type *get_type(size_t id) { return &storage_.at(id); }
 
   const Type *get_type(size_t id) const { return &storage_.at(id); }
 
 private:
-  std::optional<size_t> unit_type_id;
-
+  // TODO
+  // std::unordered_map<std::string, size_t> named_local_types_;
+  std::unordered_map<builtin::types::Type, size_t> primitive_type_ids_;
   std::vector<Type> storage_;
 };
 
-// class ModifiedTypeProxy {
-// public:
-//   ModifiedTypeProxy(nodes::TypeProxy type,
-//                     nodes::Modifier modifier = nodes::Modifier::NONE)
-//       : type_(type), modifier_(modifier) {}
-//
-//   //
-//
-//   nodes::Modifier get_modifier() const { return modifier_; }
-//
-//   void set_modifier(nodes::Modifier modifier) { modifier_ = modifier; }
-//
-//   //
-//
-//   nodes::Type *get_type() { return type_.get(); }
-//
-//   const nodes::Type *get_type() const { return type_.get(); }
-//
-//   const nodes::TypeProxy get_type_proxy() const { return type_; }
-//
-//   //
-//
-//   bool operator==(const ModifiedTypeProxy &other) const {
-//     return *type_.get() == *other.type_.get() && modifier_ ==
-//     other.modifier_;
-//   }
-//
-//   bool operator!=(const ModifiedTypeProxy &other) const {
-//     return !(*this == other);
-//   }
-//
-//   //
-//
-//   bool operator<(const ModifiedTypeProxy &other) const {
-//     return *type_.get() < *other.type_.get() ||
-//            (*type_.get() == *other.type_.get() && modifier_ <
-//            other.modifier_);
-//   }
-//
-//   bool operator>(const ModifiedTypeProxy &other) const { return other <
-//   *this; }
-//
-//   bool operator<=(const ModifiedTypeProxy &other) const {
-//     return !operator>(other);
-//   }
-//
-//   bool operator>=(const ModifiedTypeProxy &other) const {
-//     return !operator<(other);
-//   }
-//
-// private:
-//   nodes::TypeProxy type_;
-//   nodes::Modifier modifier_;
-// };
-
 class TypeCheckResult {
 public:
   // for invalid type
diff --git a/include/tokens.hpp b/include/tokens.hpp
index 52c0fce..d64480b 100644
--- a/include/tokens.hpp
+++ b/include/tokens.hpp
@@ -54,6 +54,7 @@ enum class Type {
 
   VARIANT_TYPE,
   TUPLE_TYPE,
+  FUNCTION_TYPE,
   ARRAY_TYPE,
   REFERENCE_TYPE,
   MODIFIED_TYPE,
@@ -149,6 +150,7 @@ const static std::string LAMBDA = "lambda";
 
 const static std::string VARIANT_TYPE = "variant_type";
 const static std::string TUPLE_TYPE = "tuple_type";
+const static std::string FUNCTION_TYPE = "function_type";
 const static std::string ARRAY_TYPE = "array_type";
 const static std::string REFERENCE_TYPE = "reference_type";
 const static std::string MODIFIED_TYPE = "modified_type";
@@ -242,6 +244,8 @@ inline Type string_to_type(const std::string &str) {
     return Type::VARIANT_TYPE;
   } else if (str == TUPLE_TYPE) {
     return Type::TUPLE_TYPE;
+  } else if (str == FUNCTION_TYPE) {
+    return Type::FUNCTION_TYPE;
   } else if (str == ARRAY_TYPE) {
     return Type::ARRAY_TYPE;
   } else if (str == REFERENCE_TYPE) {
diff --git a/src/builders/statement_builders.cpp b/src/builders/statement_builders.cpp
index e8f6e69..b6a5178 100644
--- a/src/builders/statement_builders.cpp
+++ b/src/builders/statement_builders.cpp
@@ -382,6 +382,7 @@ nodes::FunctionDefinition build_function_definition(
       break;
     case tokens::Type::VARIANT_TYPE:
     case tokens::Type::TUPLE_TYPE:
+    case tokens::Type::FUNCTION_TYPE:
     case tokens::Type::ARRAY_TYPE:
     case tokens::Type::REFERENCE_TYPE:
     case tokens::Type::MODIFIED_TYPE:
diff --git a/src/builders/type_builders.cpp b/src/builders/type_builders.cpp
index 64aae9a..1ac9fc7 100644
--- a/src/builders/type_builders.cpp
+++ b/src/builders/type_builders.cpp
@@ -23,6 +23,8 @@ nodes::TypeProxy build_type(parser::ParseTree::Node parser_node,
     return build_variant_type(parser_node, type_storage);
   case tokens::Type::TUPLE_TYPE:
     return build_tuple_type(parser_node, type_storage);
+  case tokens::Type::FUNCTION_TYPE:
+    return build_function_type(parser_node, type_storage);
   case tokens::Type::ARRAY_TYPE:
     return build_array_type(parser_node, type_storage);
   case tokens::Type::REFERENCE_TYPE:
@@ -78,6 +80,13 @@ nodes::TypeProxy build_tuple_type(parser::ParseTree::Node parser_node,
                               builtin::types::Type::TUPLE);
 }
 
+// '((' (annotation? type)+ '))'
+nodes::TypeProxy build_function_type(parser::ParseTree::Node parser_node,
+                                     nodes::TypeStorage &type_storage) {
+  return build_container_type(parser_node, type_storage,
+                              builtin::types::Type::FUNCTION);
+}
+
 // '[[' type ']]'
 nodes::TypeProxy build_array_type(parser::ParseTree::Node parser_node,
                                   nodes::TypeStorage &type_storage) {
diff --git a/tests/test.langexp b/tests/test.langexp
index e5c2e39..49ba767 100644
--- a/tests/test.langexp
+++ b/tests/test.langexp
@@ -226,7 +226,7 @@ result_func! 'a 'b -> 'c -> 'd = ?? 'a == 0 => error "some error" !!=> ('c := 'a
 tuple_argument_test 'x : (A & B & C) = do_something;
 
 // ((A1 & A2) | B | C) same to Variant[Tuple[A1 A2] B C]
-variant_argument_test 'x : ((A1 & A2) | B | C) = do_something;
+variant_argument_test 'x : ( (A1 & A2) | B | C) = do_something;
 
 literals_test = {
   %float_number_literal := 1.0f;
@@ -242,3 +242,7 @@ literals_test = {
   %unit_literal := ();
   %null_literal := null;
 }
+
+array_argument_test 'x : [[ Int ]] = do_something;
+
+functional_arguments_test 'x 'y : (( Int -> Int )) (( Float <- Float -> Float )) = do_something;