Fix #677: Add strong typedefs via 'using Type = BaseType' syntax

Strong typedefs create distinct types backed by Dynamic_Object, so
'using Meters = int' makes Meters a type that is not interchangeable
with int or other typedefs of int. The constructor Meters(val) wraps
the base value, and function dispatch enforces the type distinction.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

include/chaiscript/language/chaiscript_common.hpp

@@ -33,7 +33,7 @@ namespace chaiscript {
     template<typename T>
     static bool is_reserved_word(const T &s) noexcept {
       const static std::unordered_set<std::uint32_t>
-          words{utility::hash("def"), utility::hash("fun"), utility::hash("while"), utility::hash("for"), utility::hash("if"), utility::hash("else"), utility::hash("&&"), utility::hash("||"), utility::hash(","), utility::hash("auto"), utility::hash("return"), utility::hash("break"), utility::hash("true"), utility::hash("false"), utility::hash("class"), utility::hash("attr"), utility::hash("var"), utility::hash("global"), utility::hash("GLOBAL"), utility::hash("_"), utility::hash("__LINE__"), utility::hash("__FILE__"), utility::hash("__FUNC__"), utility::hash("__CLASS__"), utility::hash("const")};
+          words{utility::hash("def"), utility::hash("fun"), utility::hash("while"), utility::hash("for"), utility::hash("if"), utility::hash("else"), utility::hash("&&"), utility::hash("||"), utility::hash(","), utility::hash("auto"), utility::hash("return"), utility::hash("break"), utility::hash("true"), utility::hash("false"), utility::hash("class"), utility::hash("attr"), utility::hash("var"), utility::hash("global"), utility::hash("GLOBAL"), utility::hash("_"), utility::hash("__LINE__"), utility::hash("__FILE__"), utility::hash("__FUNC__"), utility::hash("__CLASS__"), utility::hash("const"), utility::hash("using")};
 
       return words.count(utility::hash(s)) == 1;
     }
@@ -106,7 +106,8 @@ namespace chaiscript {
     Constant,
     Compiled,
     Const_Var_Decl,
-    Const_Assign_Decl
+    Const_Assign_Decl,
+    Using
   };
 
   enum class Operator_Precedence {
@@ -127,7 +128,7 @@ namespace chaiscript {
   namespace {
     /// Helper lookup to get the name of each node type
     constexpr const char *ast_node_type_to_string(AST_Node_Type ast_node_type) noexcept {
-      constexpr const char *const ast_node_types[] = {"Id", "Fun_Call", "Unused_Return_Fun_Call", "Arg_List", "Equation", "Var_Decl", "Assign_Decl", "Array_Call", "Dot_Access", "Lambda", "Block", "Scopeless_Block", "Def", "While", "If", "For", "Ranged_For", "Inline_Array", "Inline_Map", "Return", "File", "Prefix", "Break", "Continue", "Map_Pair", "Value_Range", "Inline_Range", "Try", "Catch", "Finally", "Method", "Attr_Decl", "Logical_And", "Logical_Or", "Reference", "Switch", "Case", "Default", "Noop", "Class", "Binary", "Arg", "Global_Decl", "Constant", "Compiled", "Const_Var_Decl", "Const_Assign_Decl"};
+      constexpr const char *const ast_node_types[] = {"Id", "Fun_Call", "Unused_Return_Fun_Call", "Arg_List", "Equation", "Var_Decl", "Assign_Decl", "Array_Call", "Dot_Access", "Lambda", "Block", "Scopeless_Block", "Def", "While", "If", "For", "Ranged_For", "Inline_Array", "Inline_Map", "Return", "File", "Prefix", "Break", "Continue", "Map_Pair", "Value_Range", "Inline_Range", "Try", "Catch", "Finally", "Method", "Attr_Decl", "Logical_And", "Logical_Or", "Reference", "Switch", "Case", "Default", "Noop", "Class", "Binary", "Arg", "Global_Decl", "Constant", "Compiled", "Const_Var_Decl", "Const_Assign_Decl", "Using"};
 
       return ast_node_types[static_cast<int>(ast_node_type)];
     }

include/chaiscript/language/chaiscript_eval.hpp

@@ -887,6 +887,45 @@ namespace chaiscript {
       }
     };
 
+    template<typename T>
+    struct Using_AST_Node final : AST_Node_Impl<T> {
+      Using_AST_Node(std::string t_ast_node_text, Parse_Location t_loc, std::vector<AST_Node_Impl_Ptr<T>> t_children)
+          : AST_Node_Impl<T>(std::move(t_ast_node_text), AST_Node_Type::Using, std::move(t_loc), std::move(t_children)) {
+        assert(this->children.size() == 2);
+      }
+
+      Boxed_Value eval_internal(const chaiscript::detail::Dispatch_State &t_ss) const override {
+        const auto &new_type_name = this->children[0]->text;
+        const auto &base_type_name = this->children[1]->text;
+
+        const auto base_type = t_ss->get_type(base_type_name, true);
+
+        t_ss->add(user_type<dispatch::Dynamic_Object>(), new_type_name);
+
+        dispatch::Param_Types param_types(std::vector<std::pair<std::string, Type_Info>>{
+            {new_type_name, Type_Info()},
+            {base_type_name, base_type}});
+
+        auto ctor_body = dispatch::make_dynamic_proxy_function(
+            [](const Function_Params &t_params) -> Boxed_Value {
+              auto *obj = static_cast<dispatch::Dynamic_Object *>(t_params[0].get_ptr());
+              obj->get_attr("__value") = t_params[1];
+              return void_var();
+            },
+            2,
+            std::shared_ptr<AST_Node>(),
+            param_types);
+
+        try {
+          t_ss->add(std::make_shared<dispatch::detail::Dynamic_Object_Constructor>(new_type_name, ctor_body), new_type_name);
+        } catch (const exception::name_conflict_error &e) {
+          throw exception::eval_error("Type alias redefined '" + e.name() + "'");
+        }
+
+        return void_var();
+      }
+    };
+
     template<typename T>
     struct If_AST_Node final : AST_Node_Impl<T> {
       If_AST_Node(std::string t_ast_node_text, Parse_Location t_loc, std::vector<AST_Node_Impl_Ptr<T>> t_children)

include/chaiscript/language/chaiscript_parser.hpp

@@ -2031,6 +2031,43 @@ namespace chaiscript {
         return retval;
       }
 
+      bool Using(const bool t_class_allowed) {
+        Depth_Counter dc{this};
+
+        const auto prev_stack_top = m_match_stack.size();
+
+        if (Keyword("using")) {
+          if (!t_class_allowed) {
+            throw exception::eval_error("Type alias definitions only allowed at top scope",
+                                        File_Position(m_position.line, m_position.col),
+                                        *m_filename);
+          }
+
+          if (!Id(true)) {
+            throw exception::eval_error("Missing type name in 'using' declaration",
+                                        File_Position(m_position.line, m_position.col),
+                                        *m_filename);
+          }
+
+          if (!Symbol("=", true)) {
+            throw exception::eval_error("Missing '=' in 'using' declaration",
+                                        File_Position(m_position.line, m_position.col),
+                                        *m_filename);
+          }
+
+          if (!Id(true)) {
+            throw exception::eval_error("Missing base type name in 'using' declaration",
+                                        File_Position(m_position.line, m_position.col),
+                                        *m_filename);
+          }
+
+          build_match<eval::Using_AST_Node<Tracer>>(prev_stack_top);
+          return true;
+        }
+
+        return false;
+      }
+
       /// Reads a while block from input
       bool While() {
         Depth_Counter dc{this};
@@ -2776,7 +2813,7 @@ namespace chaiscript {
 
         while (has_more) {
           const auto start = m_position;
-          if (Def() || Try() || If() || While() || Class(t_class_allowed) || For() || Switch()) {
+          if (Def() || Try() || If() || While() || Class(t_class_allowed) || Using(t_class_allowed) || For() || Switch()) {
             if (!saw_eol) {
               throw exception::eval_error("Two function definitions missing line separator",
                                           File_Position(start.line, start.col),

unittests/strong_typedef.chai

@@ -0,0 +1,45 @@
+// Strong typedef: using Type = int creates a distinct type
+using Meters = int
+
+def measure(Meters m) {
+  return m
+}
+
+// Constructing a strong typedef value should work
+var m = Meters(42)
+
+// Calling with the typedef'd value should succeed
+measure(m)
+
+// Calling with a plain int should fail (strong typedef)
+try {
+  measure(42)
+  assert_equal(true, false)
+} catch(e) {
+  // Expected: type mismatch because int is not Meters
+}
+
+// Multiple strong typedefs from the same base type should be distinct
+using Seconds = int
+
+def wait(Seconds s) {
+  return s
+}
+
+var s = Seconds(10)
+wait(s)
+
+// Meters and Seconds should not be interchangeable
+try {
+  wait(m)
+  assert_equal(true, false)
+} catch(e) {
+  // Expected: Meters is not Seconds
+}
+
+try {
+  measure(s)
+  assert_equal(true, false)
+} catch(e) {
+  // Expected: Seconds is not Meters
+}