diff --git a/furvm/include/furvm/context.hpp b/furvm/include/furvm/context.hpp
index 230bfda..ba0f6b8 100644
--- a/furvm/include/furvm/context.hpp
+++ b/furvm/include/furvm/context.hpp
@@ -104,9 +104,9 @@ private:
     std::vector<thing_p>    m_things;
     std::vector<executor_p> m_executors;
 
-    std::queue<thing_handle> m_deadThings;
-    std::vector<void*>       m_deadThingData;
-    furlang::arena           m_thingArena;
+    std::queue<thing_id> m_deadThings;
+    std::vector<void*>   m_deadThingData;
+    furlang::arena       m_thingArena;
 };
 
 } // namespace furvm
diff --git a/furvm/include/furvm/executor.hpp b/furvm/include/furvm/executor.hpp
index 858f48f..88f69be 100644
--- a/furvm/include/furvm/executor.hpp
+++ b/furvm/include/furvm/executor.hpp
@@ -76,7 +76,7 @@ public:
      * @param mod Module.
      * @param function Function handle.
      */
-    void push_frame(const mod_p& mod, function_handle function);
+    void push_frame(const mod_p& mod, function_id function);
 
     /**
      * @brief Pops the top frame.
diff --git a/furvm/include/furvm/function.hpp b/furvm/include/furvm/function.hpp
index 4356a05..ca19c01 100644
--- a/furvm/include/furvm/function.hpp
+++ b/furvm/include/furvm/function.hpp
@@ -35,13 +35,13 @@ public:
       : m_name(name), m_type(function_t::Native), m_value(native) {}
 
     template <typename Name, typename ModuleName>
-    function(Name&& name, ModuleName&& moduleName, function_handle function)
+    function(Name&& name, ModuleName&& moduleName, function_id function)
       : m_name(std::forward<Name>(name)),
         m_type(function_t::Import),
         m_value(std::forward<ModuleName>(moduleName), function) {}
 
     template <typename ModuleName>
-    function(const char* name, ModuleName&& moduleName, function_handle function)
+    function(const char* name, ModuleName&& moduleName, function_id function)
       : m_name(name), m_type(function_t::Import), m_value(std::forward<ModuleName>(moduleName), function) {}
 
     ~function();
@@ -108,7 +108,7 @@ public:
      *
      * @return A handle to the module.
      */
-    function_handle imported_function() const {
+    function_id imported_function() const {
         if (m_type != function_t::Import) throw std::runtime_error("function type mismatch");
         return m_value.imp.function;
     }
@@ -120,8 +120,8 @@ private:
         std::size_t     position = 0;
         native_function native;
         struct {
-            std::string     moduleName;
-            function_handle function;
+            std::string moduleName;
+            function_id function;
         } imp;
 
         value() = default;
@@ -133,7 +133,7 @@ private:
           : native(native) {}
 
         template <typename Name>
-        value(Name&& moduleName, function_handle function)
+        value(Name&& moduleName, function_id function)
           : imp({ std::forward<Name>(moduleName), function }) {}
 
         ~value() {}
diff --git a/furvm/include/furvm/fwd.hpp b/furvm/include/furvm/fwd.hpp
index ef8e3f2..8098d78 100644
--- a/furvm/include/furvm/fwd.hpp
+++ b/furvm/include/furvm/fwd.hpp
@@ -81,7 +81,7 @@ using function_p = std::shared_ptr<function>;
 /**
  * @brief Furvm function's index.
  */
-using function_handle = std::uint16_t;
+using function_id = std::uint16_t;
 
 // module.hpp
 
@@ -128,7 +128,7 @@ using thing_p = std::shared_ptr<thing>;
 /**
  * @brief Furvm thing's index.
  */
-using thing_handle = std::uint32_t;
+using thing_id = std::uint32_t;
 
 // executor.hpp
 
@@ -159,7 +159,7 @@ using executor_p = std::shared_ptr<executor>;
 /**
  * @brief Furvm executor's index.
  */
-using executor_handle = std::uint32_t;
+using executor_id = std::uint32_t;
 
 // context.hpp
 
diff --git a/furvm/include/furvm/module.hpp b/furvm/include/furvm/module.hpp
index d39e85d..4c394e1 100644
--- a/furvm/include/furvm/module.hpp
+++ b/furvm/include/furvm/module.hpp
@@ -103,25 +103,13 @@ public:
      * @param id Id of the function.
      * @return The function.
      */
-    constexpr const function_p& function_at(function_handle id) const { return m_functions.at(id); }
-
-    /**
-     * @brief Returns an id of a function.
-     *
-     * @param function Function to get the id of.
-     * @return The function's id.
-     */
-    function_handle function_id(const function_p& function) const {
-        if (auto it = std::find(m_functions.begin(), m_functions.end(), function); it != m_functions.end())
-            return it - m_functions.begin();
-        throw std::runtime_error("function not in the module");
-    }
+    constexpr const function_p& function_at(function_id id) const { return m_functions.at(id); }
 
     template <typename... Args>
-    function_handle emplace_function(Args&&... args) {
+    function_id emplace_function(Args&&... args) {
         function_p function = std::make_shared<class function>(std::forward<Args>(args)...);
         m_functionMap.emplace(function->name(), function);
-        function_handle id = m_functions.size();
+        function_id id = m_functions.size();
         m_functions.emplace_back(std::move(function));
         return id;
     }
diff --git a/furvm/include/furvm/thing.hpp b/furvm/include/furvm/thing.hpp
index 7c0ce11..d85c5fc 100644
--- a/furvm/include/furvm/thing.hpp
+++ b/furvm/include/furvm/thing.hpp
@@ -56,7 +56,7 @@ class thing final {
 public:
     using nref_t = std::size_t; /**< Type of reference count. */
 
-    static constexpr thing_handle GENERATION_SIZE = 12; /**< Bit size of generation part in thing_handle. */
+    static constexpr thing_id GENERATION_SIZE = 12; /**< Bit size of generation part in thing_handle. */
 public:
     thing(const context_p& context, thing_t type);
 
diff --git a/furvm/src/executor.cpp b/furvm/src/executor.cpp
index 9479e6c..3514bcd 100644
--- a/furvm/src/executor.cpp
+++ b/furvm/src/executor.cpp
@@ -13,7 +13,7 @@
 
 namespace furvm {
 
-void executor::push_frame(const mod_p& mod, function_handle function) {
+void executor::push_frame(const mod_p& mod, function_id function) {
     const auto& func = mod->function_at(function);
     if (func->type() != function_t::Normal) throw std::runtime_error("unexpected function type");
     m_frames.emplace((struct executor::frame){ mod, func->position(), m_stack.size() });
@@ -168,9 +168,9 @@ void executor::step() {
         store_thing(variable, std::move(pop_thing()));
     } break;
     case instruction_t::Call: {
-        function_handle funcId = static_cast<std::uint16_t>(frame.mod->byte(frame.position)) |
-                                 (static_cast<std::uint16_t>(frame.mod->byte(frame.position + 1)) << 8);
-        frame.position        += 2;
+        function_id funcId = static_cast<std::uint16_t>(frame.mod->byte(frame.position)) |
+                             (static_cast<std::uint16_t>(frame.mod->byte(frame.position + 1)) << 8);
+        frame.position    += 2;
 
         const function_p& function = frame.mod->function_at(funcId);
         switch (function->type()) {
diff --git a/furvm/src/serializer.cpp b/furvm/src/serializer.cpp
index dc76b58..bac4019 100644
--- a/furvm/src/serializer.cpp
+++ b/furvm/src/serializer.cpp
@@ -75,7 +75,7 @@ bool serializer::serialize_module(std::ostream& os, const mod_p& mod) {
     write_u32(os, VERSION);
 
     write_u32(os, mod->m_functions.size());
-    for (function_handle id = 0; id < static_cast<function_handle>(mod->m_functions.size()); ++id) {
+    for (function_id id = 0; id < static_cast<function_id>(mod->m_functions.size()); ++id) {
         const auto& func = mod->m_functions[id];
         write_u16(os, id);
         write_u8(os, static_cast<std::uint8_t>(func->type()));
diff --git a/furvm/src/thing.cpp b/furvm/src/thing.cpp
index 4289e02..75cae45 100644
--- a/furvm/src/thing.cpp
+++ b/furvm/src/thing.cpp
@@ -201,13 +201,13 @@ thing_p operator>=(const thing_p& lhs, const thing_p& rhs) {
 }
 
 thing_p thing::clone(const thing_p& thing) {
-    thing_handle id = thing->m_context->m_things.size();
+    thing_id id = thing->m_context->m_things.size();
     if (!thing->m_context->m_deadThings.empty()) {
         id = thing->m_context->m_deadThings.front();
         thing->m_context->m_deadThings.pop();
         id += 1 << GENERATION_SIZE;
     }
-    thing_handle idx = id & ((1ULL << ((sizeof(id) * 8) - GENERATION_SIZE)) - 1);
+    thing_id idx = id & ((1ULL << ((sizeof(id) * 8) - GENERATION_SIZE)) - 1);
 
     auto th = std::make_shared<class thing>(thing->m_context, thing->m_type);
     switch (thing->m_type) {