diff --git a/include/types.hpp b/include/types.hpp
index 0e663df..29c837e 100644
--- a/include/types.hpp
+++ b/include/types.hpp
@@ -14,6 +14,8 @@
 
 namespace info::type {
 
+// TODO: implement DeduceContext
+
 // TODO: move in constructors
 
 class TypeManager;
@@ -29,8 +31,10 @@ public:
 
   std::optional<utils::IdType> InContext(const std::unordered_map<std::string, utils::IdType>& context);
   bool Same(const AbstractType& type) const;
-  bool operator<(const AbstractType& type) const;
-  bool operator>(const AbstractType& type) const;
+  bool Require(const AbstractType& type) const;
+
+  bool DeduceContext(const AbstractType& actual_type,
+                     std::unordered_map<std::string, std::optional<utils::IdType>>& context) const;
 
   std::optional<utils::IdType> GetFieldType(const std::string& name,
                                             const std::unordered_set<utils::IdType>& type_namespaces) const;
@@ -43,6 +47,10 @@ public:
     return graph_id == graph_id_ || typeclass_graph_.GetDependenciesSet(graph_id_).count(graph_id) != 0;
   }
 
+  std::string GetName() const {
+    return typeclass_graph_.GetVertex(graph_id_).name;
+  }
+
   std::string ToString() const {
     return "Abstract " + std::to_string(graph_id_) + " ( " + typeclass_graph_.GetVertex(graph_id_).name + " )";
   }
@@ -63,8 +71,10 @@ public:
 
   std::optional<utils::IdType> InContext(const std::unordered_map<std::string, utils::IdType>& context);
   bool Same(const DefinedType& type) const;
-  bool operator<(const DefinedType& type) const;
-  bool operator>(const DefinedType& type) const;
+  bool Require(const DefinedType& type) const;
+
+  bool DeduceContext(const DefinedType& actual_type,
+                     std::unordered_map<std::string, std::optional<utils::IdType>>& context) const;
 
   std::optional<utils::IdType> GetFieldType(const std::string& name,
                                             const std::unordered_set<utils::IdType>& type_namespaces) const;
@@ -166,8 +176,10 @@ public:
 
   std::optional<utils::IdType> InContext(const std::unordered_map<std::string, utils::IdType>& context);
   bool Same(const TupleType& type) const;
-  bool operator<(const TupleType& type) const;
-  bool operator>(const TupleType& type) const;
+  bool Require(const TupleType& type) const;
+
+  bool DeduceContext(const TupleType& actual_type,
+                     std::unordered_map<std::string, std::optional<utils::IdType>>& context) const;
 
   std::optional<utils::IdType> GetFieldType(const std::string& name,
                                             const std::unordered_set<utils::IdType>& type_namespaces) const;
@@ -193,8 +205,10 @@ public:
 
   std::optional<utils::IdType> InContext(const std::unordered_map<std::string, utils::IdType>& context);
   bool Same(const VariantType& type) const;
-  bool operator<(const VariantType& type) const;
-  bool operator>(const VariantType& type) const;
+  bool Require(const VariantType& type) const;
+
+  bool DeduceContext(const VariantType& actual_type,
+                     std::unordered_map<std::string, std::optional<utils::IdType>>& context) const;
 
   std::optional<utils::IdType> GetFieldType(const std::string& name,
                                             const std::unordered_set<utils::IdType>& type_namespaces) const;
@@ -223,8 +237,10 @@ public:
 
   std::optional<utils::IdType> InContext(const std::unordered_map<std::string, utils::IdType>& context);
   bool Same(const OptionalType& type) const;
-  bool operator<(const OptionalType& type) const;
-  bool operator>(const OptionalType& type) const;
+  bool Require(const OptionalType& type) const;
+
+  bool DeduceContext(const OptionalType& actual_type,
+                     std::unordered_map<std::string, std::optional<utils::IdType>>& context) const;
 
   std::optional<utils::IdType> GetFieldType(const std::string& name,
                                             const std::unordered_set<utils::IdType>& type_namespaces) const;
@@ -254,8 +270,10 @@ public:
 
   std::optional<utils::IdType> InContext(const std::unordered_map<std::string, utils::IdType>& context);
   bool Same(const ReferenceToType& type) const;
-  bool operator<(const ReferenceToType& type) const;
-  bool operator>(const ReferenceToType& type) const;
+  bool Require(const ReferenceToType& type) const;
+
+  bool DeduceContext(const ReferenceToType& actual_type,
+                     std::unordered_map<std::string, std::optional<utils::IdType>>& context) const;
 
   std::optional<utils::IdType> GetFieldType(const std::string& name,
                                             const std::unordered_set<utils::IdType>& type_namespaces) const;
@@ -279,8 +297,10 @@ public:
 
   std::optional<utils::IdType> InContext(const std::unordered_map<std::string, utils::IdType>& context);
   bool Same(const FunctionType& type) const;
-  bool operator<(const FunctionType& type) const;
-  bool operator>(const FunctionType& type) const;
+  bool Require(const FunctionType& type) const;
+
+  bool DeduceContext(const FunctionType& actual_type,
+                     std::unordered_map<std::string, std::optional<utils::IdType>>& context) const;
 
   std::optional<utils::IdType> GetFieldType(const std::string& name,
                                             const std::unordered_set<utils::IdType>& type_namespaces) const;
@@ -302,8 +322,10 @@ public:
 
   std::optional<utils::IdType> InContext(const std::unordered_map<std::string, utils::IdType>& context);
   bool Same(const ArrayType& type) const;
-  bool operator<(const ArrayType& type) const;
-  bool operator>(const ArrayType& type) const;
+  bool Require(const ArrayType& type) const;
+
+  bool DeduceContext(const ArrayType& actual_type,
+                     std::unordered_map<std::string, std::optional<utils::IdType>>& context) const;
 
   std::optional<utils::IdType> GetFieldType(const std::string& name,
                                             const std::unordered_set<utils::IdType>& type_namespaces) const;
@@ -326,8 +348,10 @@ public:
 
   std::optional<utils::IdType> InContext(const std::unordered_map<std::string, utils::IdType>& context);
   bool Same(const Type& type) const;
-  bool operator<(const Type& type) const; // TODO: rule exceptions
-  bool operator>(const Type& type) const;
+  bool Require(const Type& type) const; // TODO: check abstract type requirements for not abstract types
+
+  bool DeduceContext(const Type& actual_type,
+                     std::unordered_map<std::string, std::optional<utils::IdType>>& context) const;
 
   std::optional<utils::IdType> GetFieldType(const std::string& name,
                                             const std::unordered_set<utils::IdType>& type_namespaces) const;
@@ -397,7 +421,7 @@ public:
   }
 
   bool AddValueRequirement(utils::IdType type, utils::IdType requrement) {
-    return *GetAnyValue(requrement) < *GetAnyValue(type);
+    return GetAnyValue(requrement)->Require(*GetAnyValue(type));
   }
 
 private:
diff --git a/src/type_check_visitor.cpp b/src/type_check_visitor.cpp
index 2bc96ea..d9fae1b 100644
--- a/src/type_check_visitor.cpp
+++ b/src/type_check_visitor.cpp
@@ -764,10 +764,6 @@ void TypeCheckVisitor::Visit(FunctionCallExpression* node) {
                                         &node->base);
   }
 
-  if (node->name[0] == '_') {
-    error_handling::HandleTypecheckError("Only builtin functions can start with _. This function is not found", node->base);
-  }
-
   // try to find function declaration
   if (node->prefix.has_value()) {
     if (std::holds_alternative<std::unique_ptr<SubExpressionToken>>(node->prefix.value())) {
@@ -794,9 +790,14 @@ void TypeCheckVisitor::Visit(FunctionCallExpression* node) {
   }
   FunctionDeclaration* function_declaration = maybe_function_declaration.value();
 
+  bool deduce_parameters = false;
+
   // check & collect parmeters
   if (function_declaration->parameters.size() != node->parameters.size()) {
-    error_handling::HandleTypecheckError("Mismatched parameter count in function call expression", node->base);
+    if (node->parameters.size() != 0) {
+      error_handling::HandleTypecheckError("Mismatched parameter count in function call expression", node->base);
+    }
+    deduce_parameters = true;
   }
   for (size_t i = 0; i < node->parameters.size(); ++i) {
     Visit(node->parameters[i].get());
@@ -1706,7 +1707,7 @@ std::optional<FunctionDeclaration*>
     error_handling::HandleTypecheckError("Function not present in typeclass (abstract type)", node->base);
   }
 
-  // TODO: check that const functions not called on var expressions
+  // TODO: check that var functions not called on const expressions
   if ((maybe_typeclass_function_info.value()->modifier != utils::ClassInternalsModifier::Static) != is_method) {
     error_handling::HandleTypecheckError("Typeclass function has wrong modifier (abstract type)", node->base);
   }
@@ -1735,7 +1736,7 @@ std::optional<FunctionDeclaration*>
     // error_handling::HandleTypecheckError("Function is not declared in type", node->base);
   }
 
-  // TODO: check that const functions not called on var expressions
+  // TODO: check that var functions not called on const expressions
   if ((maybe_type_function_info.value()->modifier != utils::ClassInternalsModifier::Static) != is_method) {
     error_handling::HandleTypecheckError("Typeclass function has wrong modifier (abstract type)", node->base);
   }
diff --git a/src/types.cpp b/src/types.cpp
index 061bfac..bc8827a 100644
--- a/src/types.cpp
+++ b/src/types.cpp
@@ -18,12 +18,13 @@ bool AbstractType::Same(const AbstractType& type) const {
   return graph_id_ == type.graph_id_;
 }
 
-bool AbstractType::operator<(const AbstractType& type) const { // TODO: cache DependenciesSet
+bool AbstractType::Require(const AbstractType& type) const { // TODO: cache DependenciesSet
   return typeclass_graph_.GetDependenciesSet(graph_id_).count(type.graph_id_) != 0 || graph_id_ == type.graph_id_;
 }
 
-bool AbstractType::operator>(const AbstractType& type) const {
-  return type < *this;
+bool AbstractType::DeduceContext(const AbstractType& actual_type,
+                                 std::unordered_map<std::string, std::optional<utils::IdType>>& context) const {
+  return typeclass_graph_.GetDependenciesSet(graph_id_).count(actual_type.graph_id_) != 0 || graph_id_ == actual_type.graph_id_;
 }
 
 std::optional<utils::IdType> AbstractType::GetFieldType(const std::string&,
@@ -48,13 +49,15 @@ bool DefinedType::Same(const DefinedType& type) const {
     && type_manager_->GetAnyValue(type_)->Same(*type_manager_->GetAnyValue(type.type_));
 }
 
-bool DefinedType::operator<(const DefinedType& type) const {
+bool DefinedType::Require(const DefinedType& type) const {
   return type_id_ == type.type_id_
-    && *type_manager_->GetAnyValue(type_) < *type_manager_->GetAnyValue(type.type_);
+    && type_manager_->GetAnyValue(type_)->Require(*type_manager_->GetAnyValue(type.type_));
 }
 
-bool DefinedType::operator>(const DefinedType& type) const {
-  return type < *this;
+bool DefinedType::DeduceContext(const DefinedType& actual_type,
+                                std::unordered_map<std::string, std::optional<utils::IdType>>& context) const {
+  return type_id_ == actual_type.type_id_
+    && type_manager_->GetAnyValue(type_)->DeduceContext(*type_manager_->GetAnyValue(actual_type.type_), context);
 }
 
 std::optional<utils::IdType> DefinedType::GetFieldType(const std::string& name,
@@ -97,13 +100,13 @@ bool TupleType::Same(const TupleType& type) const {
   return true;
 }
 
-bool TupleType::operator<(const TupleType& type) const {
+bool TupleType::Require(const TupleType& type) const {
   if (fields_.size() != type.fields_.size()) {
     return false;
   }
 
   for (size_t i = 0; i < fields_.size(); ++i) {
-    if (!(*type_manager_->GetAnyValue(fields_[i].second) < *type_manager_->GetAnyValue(type.fields_[i].second))) {
+    if (!type_manager_->GetAnyValue(fields_[i].second)->Require(*type_manager_->GetAnyValue(type.fields_[i].second))) {
       return false;
     }
   }
@@ -111,8 +114,19 @@ bool TupleType::operator<(const TupleType& type) const {
   return true;
 }
 
-bool TupleType::operator>(const TupleType& type) const {
-  return type < *this;
+bool TupleType::DeduceContext(const TupleType& actual_type,
+                              std::unordered_map<std::string, std::optional<utils::IdType>>& context) const {
+  if (fields_.size() != actual_type.fields_.size()) {
+    return false;
+  }
+
+  for (size_t i = 0; i < fields_.size(); ++i) {
+    if (!type_manager_->GetAnyValue(fields_[i].second)->DeduceContext(*type_manager_->GetAnyValue(actual_type.fields_[i].second), context)) {
+      return false;
+    }
+  }
+
+  return true;
 }
 
 std::optional<utils::IdType> TupleType::GetFieldType(const std::string& name,
@@ -158,7 +172,6 @@ bool VariantType::Same(const VariantType& type) const {
   }
 
   for (size_t i = 0; i < constructors_.size(); ++i) {
-
     if (constructors_[i].first != type.constructors_[i].first) { // TODO: decide
       return false;
     }
@@ -177,22 +190,53 @@ bool VariantType::Same(const VariantType& type) const {
   return true;
 }
 
-bool VariantType::operator<(const VariantType& type) const {
+bool VariantType::Require(const VariantType& type) const {
   if (constructors_.size() != type.constructors_.size()) {
     return false;
   }
 
   for (size_t i = 0; i < constructors_.size(); ++i) {
-    if (!(constructors_[i] < constructors_[i])) {
+    if (constructors_[i].first != type.constructors_[i].first) { // TODO: decide
       return false;
     }
+
+    if (constructors_[i].second.has_value() != type.constructors_[i].second.has_value()) {
+      return false;
+    }
+
+    if (constructors_[i].second.has_value()) {
+      if (!constructors_[i].second.value().Require(type.constructors_[i].second.value())) {
+        return false;
+      }
+    }
   }
 
   return true;
 }
 
-bool VariantType::operator>(const VariantType& type) const {
-  return type < *this;
+bool VariantType::DeduceContext(const VariantType& actual_type,
+                                std::unordered_map<std::string, std::optional<utils::IdType>>& context) const {
+  if (constructors_.size() != actual_type.constructors_.size()) {
+    return false;
+  }
+
+  for (size_t i = 0; i < constructors_.size(); ++i) {
+    if (constructors_[i].first != actual_type.constructors_[i].first) { // TODO: decide
+      return false;
+    }
+
+    if (constructors_[i].second.has_value() != actual_type.constructors_[i].second.has_value()) {
+      return false;
+    }
+
+    if (constructors_[i].second.has_value()) {
+      if (!constructors_[i].second.value().DeduceContext(actual_type.constructors_[i].second.value(), context)) {
+        return false;
+      }
+    }
+  }
+
+  return true;
 }
 
 std::optional<utils::IdType> VariantType::GetFieldType(const std::string& name,
@@ -237,12 +281,13 @@ bool OptionalType::Same(const OptionalType& type) const {
   return type_manager_->GetAnyValue(type_)->Same(*type_manager_->GetAnyValue(type.type_));
 }
 
-bool OptionalType::operator<(const OptionalType& type) const {
-  return *type_manager_->GetAnyValue(type_) < *type_manager_->GetAnyValue(type.type_);
+bool OptionalType::Require(const OptionalType& type) const {
+  return type_manager_->GetAnyValue(type_)->Require(*type_manager_->GetAnyValue(type.type_));
 }
 
-bool OptionalType::operator>(const OptionalType& type) const {
-  return type < *this;
+bool OptionalType::DeduceContext(const OptionalType& actual_type,
+                                 std::unordered_map<std::string, std::optional<utils::IdType>>& context) const {
+  return type_manager_->GetAnyValue(type_)->DeduceContext(*type_manager_->GetAnyValue(actual_type.type_), context);
 }
 
 std::optional<utils::IdType> OptionalType::GetFieldType(const std::string&,
@@ -270,12 +315,13 @@ bool ReferenceToType::Same(const ReferenceToType& type) const {
   return references_ == type.references_ && type_manager_->GetAnyValue(type_)->Same(*type_manager_->GetAnyValue(type.type_));
 }
 
-bool ReferenceToType::operator<(const ReferenceToType& type) const {
-  return references_ == type.references_ && *type_manager_->GetAnyValue(type_) < *type_manager_->GetAnyValue(type.type_);
+bool ReferenceToType::Require(const ReferenceToType& type) const {
+  return references_ == type.references_ && type_manager_->GetAnyValue(type_)->Require(*type_manager_->GetAnyValue(type.type_));
 }
 
-bool ReferenceToType::operator>(const ReferenceToType& type) const {
-  return type < *this;
+bool ReferenceToType::DeduceContext(const ReferenceToType& actual_type,
+                                    std::unordered_map<std::string, std::optional<utils::IdType>>& context) const {
+  return references_ == actual_type.references_ && type_manager_->GetAnyValue(type_)->DeduceContext(*type_manager_->GetAnyValue(actual_type.type_), context);
 }
 
 std::optional<utils::IdType> ReferenceToType::GetFieldType(const std::string& name,
@@ -335,13 +381,13 @@ bool FunctionType::Same(const FunctionType& type) const {
   return true;
 }
 
-bool FunctionType::operator<(const FunctionType& type) const {
+bool FunctionType::Require(const FunctionType& type) const {
   if (argument_types_.size() != type.argument_types_.size()) {
     return false;
   }
 
   for (size_t i = 0; i < argument_types_.size(); ++i) {
-    if (!(*type_manager_->GetAnyValue(argument_types_[i]) < *type_manager_->GetAnyValue(type.argument_types_[i]))) {
+    if (!type_manager_->GetAnyValue(argument_types_[i])->Require(*type_manager_->GetAnyValue(type.argument_types_[i]))) {
       return false;
     }
   }
@@ -349,8 +395,19 @@ bool FunctionType::operator<(const FunctionType& type) const {
   return true;
 }
 
-bool FunctionType::operator>(const FunctionType& type) const {
-  return type < *this;
+bool FunctionType::DeduceContext(const FunctionType& actual_type,
+                                 std::unordered_map<std::string, std::optional<utils::IdType>>& context) const {
+  if (argument_types_.size() != actual_type.argument_types_.size()) {
+    return false;
+  }
+
+  for (size_t i = 0; i < argument_types_.size(); ++i) {
+    if (!type_manager_->GetAnyValue(argument_types_[i])->DeduceContext(*type_manager_->GetAnyValue(actual_type.argument_types_[i]), context)) {
+      return false;
+    }
+  }
+
+  return true;
 }
 
 std::optional<utils::IdType> FunctionType::GetFieldType(const std::string&,
@@ -395,12 +452,13 @@ bool ArrayType::Same(const ArrayType& type) const {
   return size_ == type.size_ && type_manager_->GetAnyValue(elements_type_)->Same(*type_manager_->GetAnyValue(type.elements_type_));
 }
 
-bool ArrayType::operator<(const ArrayType& type) const {
-  return size_ == type.size_ && *type_manager_->GetAnyValue(elements_type_) < *type_manager_->GetAnyValue(type.elements_type_);
+bool ArrayType::Require(const ArrayType& type) const {
+  return size_ == type.size_ && type_manager_->GetAnyValue(elements_type_)->Require(*type_manager_->GetAnyValue(type.elements_type_));
 }
 
-bool ArrayType::operator>(const ArrayType& type) const {
-  return type < *this;
+bool ArrayType::DeduceContext(const ArrayType& actual_type,
+                              std::unordered_map<std::string, std::optional<utils::IdType>>& context) const {
+  return size_ == actual_type.size_ && type_manager_->GetAnyValue(elements_type_)->DeduceContext(*type_manager_->GetAnyValue(actual_type.elements_type_), context);
 }
 
 std::optional<utils::IdType> ArrayType::GetFieldType(const std::string&,
@@ -477,30 +535,30 @@ bool Type::Same(const Type& type) const {
   return false;
 }
 
-bool Type::operator<(const Type& type) const {
+bool Type::Require(const Type& type) const { // TODO: check abstract type requirements for not abstract types
   size_t this_index = type_.index();
   size_t type_index = type.type_.index();
 
   if (this_index == type_index) {
     switch (this_index) {
       case 0:
-        return std::get<AbstractType>(type_) < std::get<AbstractType>(type.type_);
+        return std::get<AbstractType>(type_).Require(std::get<AbstractType>(type.type_));
       case 1:
-        return std::get<DefinedType>(type_) < std::get<DefinedType>(type.type_);
+        return std::get<DefinedType>(type_).Require(std::get<DefinedType>(type.type_));
       case 2:
         return std::get<InternalType>(type_) == std::get<InternalType>(type.type_);
       case 3:
-        return std::get<TupleType>(type_) < std::get<TupleType>(type.type_);
+        return std::get<TupleType>(type_).Require(std::get<TupleType>(type.type_));
       case 4:
-        return std::get<VariantType>(type_) < std::get<VariantType>(type.type_);
+        return std::get<VariantType>(type_).Require(std::get<VariantType>(type.type_));
       case 5:
-        return std::get<ReferenceToType>(type_) < std::get<ReferenceToType>(type.type_);
+        return std::get<ReferenceToType>(type_).Require(std::get<ReferenceToType>(type.type_));
       case 6:
-        return std::get<FunctionType>(type_) < std::get<FunctionType>(type.type_);
+        return std::get<FunctionType>(type_).Require(std::get<FunctionType>(type.type_));
       case 7:
-        return std::get<ArrayType>(type_) < std::get<ArrayType>(type.type_);
+        return std::get<ArrayType>(type_).Require(std::get<ArrayType>(type.type_));
       case 8:
-        return std::get<OptionalType>(type_) < std::get<OptionalType>(type.type_);
+        return std::get<OptionalType>(type_).Require(std::get<OptionalType>(type.type_));
       default:
         // error
         break;
@@ -510,8 +568,57 @@ bool Type::operator<(const Type& type) const {
   return false;
 }
 
-bool Type::operator>(const Type& type) const {
-  return type < *this;
+// TODO: check abstract type requirements for not abstract types
+bool Type::DeduceContext(const Type& actual_type,
+                         std::unordered_map<std::string, std::optional<utils::IdType>>& context) const {
+  size_t this_index = type_.index();
+  size_t type_index = actual_type.type_.index();
+
+  if (this_index == 0) {
+    std::string type_name = std::get<AbstractType>(type_).GetName();
+    if (context.count(type_name) != 0) { // is abstract type
+      // context[type_name] = // TODO: actual_type.id_; ??
+      // TODO: fixes
+    }
+  }
+
+  error_handling::HandleGeneralError("Type::DeduceContext: not implemented");
+
+  if (this_index == type_index) {
+    switch (this_index) {
+      case 0:
+        return std::get<AbstractType>(type_).DeduceContext(std::get<AbstractType>(actual_type.type_),
+                                                           context);
+      case 1:
+        return std::get<DefinedType>(type_).DeduceContext(std::get<DefinedType>(actual_type.type_),
+                                                          context);
+      case 2:
+        return std::get<InternalType>(type_) == std::get<InternalType>(actual_type.type_);
+      case 3:
+        return std::get<TupleType>(type_).DeduceContext(std::get<TupleType>(actual_type.type_),
+                                                        context);
+      case 4:
+        return std::get<VariantType>(type_).DeduceContext(std::get<VariantType>(actual_type.type_),
+                                                          context);
+      case 5:
+        return std::get<ReferenceToType>(type_).DeduceContext(std::get<ReferenceToType>(actual_type.type_),
+                                                       context);
+      case 6:
+        return std::get<FunctionType>(type_).DeduceContext(std::get<FunctionType>(actual_type.type_),
+                                                     context);
+      case 7:
+        return std::get<ArrayType>(type_).DeduceContext(std::get<ArrayType>(actual_type.type_),
+                                                        context);
+      case 8:
+        return std::get<OptionalType>(type_).DeduceContext(std::get<OptionalType>(actual_type.type_),
+                                                           context);
+      default:
+        // error
+        break;
+    }
+  }
+
+  return false;
 }
 
 std::optional<utils::IdType> Type::GetFieldType(const std::string& name,
diff --git a/tests/test_code.lang b/tests/test_code.lang
index 99810a0..2dcd6dd 100644
--- a/tests/test_code.lang
+++ b/tests/test_code.lang
@@ -280,8 +280,8 @@ exec main {
   var x = (for _ in 0--n do scan-int:) // $array[int] & 0
 
   var k? = if n < 2 then n * 2 +. 3 in
-  , print-anything:[string] "n < 2"
-  , print-anything:[int] k
+    , print-anything:[string] "n < 2"
+    , print-anything:[int] k
 
   ; print-anything:[int] n