diff --git a/src/Value.cpp b/src/Value.cpp
index 709f133..e7b51dc 100644
--- a/src/Value.cpp
+++ b/src/Value.cpp
@@ -130,6 +130,16 @@ namespace grino
     return value;
   }
 
+    /*static*/
+  std::shared_ptr<Value> Value::make_user_data(Loc const& loc,
+                                               void* val)
+  {
+    auto value = std::make_shared<Value>(loc);
+    value->m_type = TYPE_PROGRAM;
+    value->m_user_data_val = val;
+    return value;
+  }
+
   std::shared_ptr<Program> Value::as_program() const
   {
     return m_program_val;
@@ -152,6 +162,7 @@ namespace grino
       case TYPE_FUNCTION: return "<function>";
       case TYPE_REF: return "&" + std::to_string(*m_ref_val);
       case TYPE_PROGRAM: return "<program>";
+      case TYPE_USER_DATA: return "<user data>";
       case TYPE_MODULE: return "<module " + m_module_val->name() + ">";
       case TYPE_ARRAY: {
         std::stringstream ss;
@@ -188,6 +199,7 @@ namespace grino
       case TYPE_REF: return *m_ref_val == *other.m_ref_val;
       case TYPE_PROGRAM: return false;
       case TYPE_MODULE: return false;
+      case TYPE_USER_DATA: return m_user_data_val == other.m_user_data_val;
 
       default:
         std::cerr << "cannot compare equality with value "
diff --git a/src/Value.hpp b/src/Value.hpp
index 3472f7b..4c84828 100644
--- a/src/Value.hpp
+++ b/src/Value.hpp
@@ -44,6 +44,9 @@ namespace grino
     static std::shared_ptr<Value> make_module(Loc const& loc,
                                               std::shared_ptr<Module> val);
 
+    static std::shared_ptr<Value> make_user_data(Loc const& loc,
+                                                 void* val);
+
     explicit Value(Loc const& loc);
     virtual ~Value() = default;
 
@@ -59,6 +62,7 @@ namespace grino
     val_array_t const& as_array() const { return *m_array_val; }
     std::string const& as_string() const { return *m_string_val; }
     std::shared_ptr<Module> as_module() const;
+    void* as_user_data() const { return m_user_data_val; }
 
     std::string string() const;
     bool equals(Value const& other) const;
@@ -75,6 +79,7 @@ namespace grino
     std::optional<val_array_t> m_array_val;
     std::optional<std::string> m_string_val;
     std::shared_ptr<Module> m_module_val;
+    void* m_user_data_val = nullptr;
   };
 }
 
diff --git a/src/types.hpp b/src/types.hpp
index 121d407..1113f0e 100644
--- a/src/types.hpp
+++ b/src/types.hpp
@@ -13,7 +13,8 @@
     G(TYPE_FLOAT),                              \
     G(TYPE_ARRAY),                              \
     G(TYPE_STRING),                             \
-    G(TYPE_MODULE)
+    G(TYPE_MODULE),                             \
+    G(TYPE_USER_DATA)
 
 
 namespace grino