diff --git a/deps/tree-sitter-lang b/deps/tree-sitter-lang
index 1c19b47..a75ea80 160000
--- a/deps/tree-sitter-lang
+++ b/deps/tree-sitter-lang
@@ -1 +1 @@
-Subproject commit 1c19b471990acb91cc0ccc9426d988debab8cb27
+Subproject commit a75ea8012d6b104fb26efcd17e5ebc02ab49f7af
diff --git a/include/statement_nodes.hpp b/include/statement_nodes.hpp
index c86929d..7a1c661 100644
--- a/include/statement_nodes.hpp
+++ b/include/statement_nodes.hpp
@@ -256,6 +256,10 @@ public:
     return are_annotations_same_to_names_;
   }
 
+  //
+
+  bool combine(FunctionDefinition &&other_function_definition);
+
 private:
   SymbolDocs docs_;
   std::vector<Constraint> constraints_;
@@ -326,6 +330,10 @@ public:
     return &methods_.at(id);
   }
 
+  //
+
+  bool combine(TypeDefinition &&other_type_definition);
+
 private:
   SymbolDocs docs_;
   bool is_on_heap_;
@@ -378,6 +386,10 @@ public:
     return &methods_.at(id);
   }
 
+  //
+
+  bool combine(TypeclassDefinition &&other_typeclass_definition);
+
 private:
   SymbolDocs docs_;
   Identifier name_;
diff --git a/src/statement_nodes.cpp b/src/statement_nodes.cpp
new file mode 100644
index 0000000..ca028a9
--- /dev/null
+++ b/src/statement_nodes.cpp
@@ -0,0 +1,256 @@
+#include "statement_nodes.hpp"
+
+namespace nodes {
+
+// TODO: return error type instead of bool ??
+bool FunctionDefinition::combine(
+    FunctionDefinition &&other_function_definition) {
+  // should be same in all definitions of one function
+  if (*name_.get() != *other_function_definition.name_.get()) {
+    return false;
+  }
+  if (modifier_ != other_function_definition.modifier_) {
+    return false;
+  }
+  if (are_annotations_same_to_names_ !=
+      other_function_definition.are_annotations_same_to_names_) {
+    return false;
+  }
+
+  // only one definition should have constraints
+  if (!constraints_.empty() &&
+      !other_function_definition.constraints_.empty()) {
+    return false;
+  }
+
+  // only one definition should have expression
+  if (expression_.has_value() &&
+      other_function_definition.expression_.has_value()) {
+    return false;
+  }
+
+  // only one definition should have documentation
+  if (docs_.get_description().has_value() &&
+      other_function_definition.docs_.get_description().has_value()) {
+    return false;
+  }
+  if (docs_.get_annotations_info_size() > 0 &&
+      other_function_definition.docs_.get_annotations_info_size() > 0) {
+    return false;
+  }
+
+  // TODO: think about required checks
+  // check, that function definitions have same named arguments count
+  for (size_t i = 0; i < std::max(arguments_.size(),
+                                  other_function_definition.arguments_.size());
+       ++i) {
+    if (i < arguments_.size() &&
+        i < other_function_definition.arguments_.size()) {
+
+      // check that all argument annotations are the same
+      if ((!arguments_[i].get_annotation().has_value() &&
+           !other_function_definition.arguments_[i]
+                .get_annotation()
+                .has_value()) ||
+          (arguments_[i].get_annotation().has_value() &&
+           other_function_definition.arguments_[i]
+               .get_annotation()
+               .has_value() &&
+           *arguments_[i].get_annotation().value() !=
+               *other_function_definition.arguments_[i]
+                    .get_annotation()
+                    .value())) {
+        return false;
+      }
+
+      // check that all argument names are the same
+      if ((!arguments_[i].get_name().has_value() &&
+           !other_function_definition.arguments_[i].get_name().has_value()) ||
+          (arguments_[i].get_name().has_value() &&
+           other_function_definition.arguments_[i].get_name().has_value() &&
+           *arguments_[i].get_name().value()->get() !=
+               *other_function_definition.arguments_[i]
+                    .get_name()
+                    .value()
+                    ->get())) {
+        return false;
+      }
+
+      // check that types are are in zero or one set of arguments (TODO: can be
+      // same and in two sets ??)
+      if (arguments_[i].get_type().has_value() &&
+          other_function_definition.arguments_[i].get_type().has_value()) {
+        return false;
+      }
+
+      // argument modifiers should be same
+      if (arguments_[i].get_before_modifier() !=
+              other_function_definition.arguments_[i].get_before_modifier() ||
+          arguments_[i].get_after_modifier() !=
+              other_function_definition.arguments_[i].get_after_modifier()) {
+        return false;
+      }
+
+      // TODO: other checks (annotations, ...)
+
+    } else if (i < arguments_.size()) {
+      // last arguments shouldn't be annotated (because they are not annotated
+      // in other function definition statement)
+      if (arguments_[i].get_annotation().has_value()) {
+        return false;
+      }
+
+      // last arguments can only be unnamed arguments (returned values)
+      if (arguments_[i].get_name().has_value()) {
+        return false;
+      }
+    } else { // i < other_function_definition.size()
+      // last arguments shouldn't be annotated (because they are not annotated
+      // in other function definition statement)
+      if (other_function_definition.arguments_[i]
+              .get_annotation()
+              .has_value()) {
+        return false;
+      }
+
+      // last arguments can only be unnamed arguments (returned values)
+      if (other_function_definition.arguments_[i].get_name().has_value()) {
+        return false;
+      }
+    }
+  }
+
+  // combine docs
+  // all docs should be in one definition
+  if (other_function_definition.docs_.get_description().has_value() ||
+      other_function_definition.docs_.get_annotations_info_size() > 0) {
+    if (docs_.get_annotations_info_size() > 0 ||
+        docs_.get_description().has_value()) {
+      return false;
+    }
+
+    docs_ = std::move(other_function_definition.docs_);
+  }
+
+  if (!other_function_definition.constraints_.empty()) {
+    constraints_ = std::move(other_function_definition.constraints_);
+  }
+
+  if (other_function_definition.expression_.has_value()) {
+    expression_ = other_function_definition.expression_;
+  }
+
+  // TODO: arguments: check and merge
+
+  return true;
+}
+
+bool TypeDefinition::combine(TypeDefinition &&other_type_definition) {
+  // should be same in all definitions of one type
+  if (*name_.get() != *other_type_definition.name_.get()) {
+    return false;
+  }
+  if (is_on_heap_ != other_type_definition.is_on_heap_) {
+    return false;
+  }
+
+  // only one definition should have documentation
+  if (docs_.get_description().has_value() &&
+      other_type_definition.docs_.get_description().has_value()) {
+    return false;
+  }
+  if (docs_.get_annotations_info_size() > 0 &&
+      other_type_definition.docs_.get_annotations_info_size() > 0) {
+    return false;
+  }
+
+  // only one definition should define type and arguments (they should be
+  // defined in one place)
+  if ((other_type_definition.type_.has_value() ||
+       !other_type_definition.arguments_.empty()) &&
+      (type_.has_value() || !arguments_.empty())) {
+    return false;
+  }
+
+  // combine docs
+  // all docs should be in one definition
+  if (other_type_definition.docs_.get_description().has_value() ||
+      other_type_definition.docs_.get_annotations_info_size() > 0) {
+    if (docs_.get_annotations_info_size() > 0 ||
+        docs_.get_description().has_value()) {
+      return false;
+    }
+
+    docs_ = std::move(other_type_definition.docs_);
+  }
+
+  if (!other_type_definition.arguments_.empty()) {
+    arguments_ = std::move(other_type_definition.arguments_);
+  }
+
+  if (other_type_definition.type_.has_value()) {
+    type_ = std::move(other_type_definition.type_);
+  }
+
+  methods_.reserve(methods_.size() + other_type_definition.methods_.size());
+
+  for (size_t i = 0; i < other_type_definition.methods_.size(); ++i) {
+    methods_.push_back(std::move(other_type_definition.methods_[i]));
+  }
+
+  // TODO: combine methods ??
+
+  return true;
+}
+
+bool TypeclassDefinition::combine(
+    TypeclassDefinition &&other_typeclass_definition) {
+  // should be same in all definitions of one typeclass
+  if (*name_.get() != *other_typeclass_definition.name_.get()) {
+    return false;
+  }
+
+  // only one definition should have documentation
+  if (docs_.get_description().has_value() &&
+      other_typeclass_definition.docs_.get_description().has_value()) {
+    return false;
+  }
+  if (docs_.get_annotations_info_size() > 0 &&
+      other_typeclass_definition.docs_.get_annotations_info_size() > 0) {
+    return false;
+  }
+
+  // only one definition should define base typeclasses
+  if (!base_typeclasses_.empty() &&
+      !other_typeclass_definition.base_typeclasses_.empty()) {
+    return false;
+  }
+
+  // combine docs
+  // all docs should be in one definition
+  if (other_typeclass_definition.docs_.get_description().has_value() ||
+      other_typeclass_definition.docs_.get_annotations_info_size() > 0) {
+    if (docs_.get_annotations_info_size() > 0 ||
+        docs_.get_description().has_value()) {
+      return false;
+    }
+
+    docs_ = std::move(other_typeclass_definition.docs_);
+  }
+
+  if (!other_typeclass_definition.base_typeclasses_.empty()) {
+    base_typeclasses_ = std::move(other_typeclass_definition.base_typeclasses_);
+  }
+
+  methods_.reserve(methods_.size() +
+                   other_typeclass_definition.methods_.size());
+  for (size_t i = 0; i < other_typeclass_definition.methods_.size(); ++i) {
+    methods_.push_back(std::move(other_typeclass_definition.methods_[i]));
+  }
+
+  // TODO: combine methods ??
+
+  return true;
+}
+
+} // namespace nodes