diff --git a/src/Ast/Expressions/ContainerInitExprs.hpp b/src/Ast/Expressions/ContainerInitExprs.hpp
index 3072434..0012c5b 100644
--- a/src/Ast/Expressions/ContainerInitExprs.hpp
+++ b/src/Ast/Expressions/ContainerInitExprs.hpp
@@ -48,14 +48,14 @@ namespace Fig::Ast
     class MapExprAst final : public ExpressionAst
     {
     public:
-        std::map<FString, Expression> val;
+        std::map<Expression, Expression> val;
 
         MapExprAst()
         {
             type = AstType::MapExpr;
         }
 
-        MapExprAst(std::map<FString, Expression> _val) :
+        MapExprAst(std::map<Expression, Expression> _val) :
             val(std::move(_val))
         {
             type = AstType::MapExpr;
diff --git a/src/Context/context.hpp b/src/Context/context.hpp
index 6ba5aa1..d72fec2 100644
--- a/src/Context/context.hpp
+++ b/src/Context/context.hpp
@@ -7,6 +7,7 @@
 #include <Context/context_forward.hpp>
 #include <Core/fig_string.hpp>
 #include <Value/value.hpp>
+#include <Value/VariableSlot.hpp>
 
 namespace Fig
 {
@@ -65,7 +66,7 @@ namespace Fig
                 return it->second;
             if (parent)
                 return parent->get(name);
-            throw RuntimeError(FStringView(std::format("Variable '{}' not defined", name.toBasicString())));
+            throw RuntimeError(FString(std::format("Variable '{}' not defined", name.toBasicString())));
         }
         AccessModifier getAccessModifier(const FString &name)
         {
@@ -79,7 +80,7 @@ namespace Fig
             }
             else
             {
-                throw RuntimeError(FStringView(std::format("Variable '{}' not defined", name.toBasicString())));
+                throw RuntimeError(FString(std::format("Variable '{}' not defined", name.toBasicString())));
             }
         }
         bool isVariableMutable(const FString &name)
@@ -98,7 +99,7 @@ namespace Fig
             {
                 if (!isVariableMutable(name))
                 {
-                    throw RuntimeError(FStringView(std::format("Variable '{}' is immutable", name.toBasicString())));
+                    throw RuntimeError(FString(std::format("Variable '{}' is immutable", name.toBasicString())));
                 }
                 variables[name]->value = value;
             }
@@ -108,7 +109,7 @@ namespace Fig
             }
             else
             {
-                throw RuntimeError(FStringView(std::format("Variable '{}' not defined", name.toBasicString())));
+                throw RuntimeError(FString(std::format("Variable '{}' not defined", name.toBasicString())));
             }
         }
         void _update(const FString &name, ObjectPtr value)
@@ -123,14 +124,14 @@ namespace Fig
             }
             else
             {
-                throw RuntimeError(FStringView(std::format("Variable '{}' not defined", name.toBasicString())));
+                throw RuntimeError(FString(std::format("Variable '{}' not defined", name.toBasicString())));
             }
         }
         void def(const FString &name, const TypeInfo &ti, AccessModifier am, const ObjectPtr &value = Object::getNullInstance())
         {
             if (containsInThisScope(name))
             {
-                throw RuntimeError(FStringView(std::format("Variable '{}' already defined in this scope", name.toBasicString())));
+                throw RuntimeError(FString(std::format("Variable '{}' already defined in this scope", name.toBasicString())));
             }
             variables[name] = std::make_shared<VariableSlot>(
                 name,
diff --git a/src/Core/fig_string.hpp b/src/Core/fig_string.hpp
index 4084e29..5391a24 100644
--- a/src/Core/fig_string.hpp
+++ b/src/Core/fig_string.hpp
@@ -15,7 +15,7 @@ namespace Fig
         
         static FStringView fromBasicStringView(std::string_view sv)
         {
-            return FStringView(reinterpret_cast<const char8_t*>(sv.data()), sv.size());
+            return FStringView(reinterpret_cast<const char8_t*>(sv.data()));
         }
 
         explicit FStringView(std::string_view sv)
@@ -28,12 +28,22 @@ namespace Fig
             *this = fromBasicStringView(std::string_view(""));
         }
 
+        std::string_view toBasicStringView() const
+        {
+            return std::string_view(reinterpret_cast<const char *>(data()), size());
+        }
     };
 
     class FString : public std::u8string
     {
     public:
         using std::u8string::u8string;
+
+        FString operator+(const FString& x)
+        {
+            return FString(toBasicString() + x.toBasicString());
+        }
+
         explicit FString(const std::u8string &str)
         {
             *this = fromU8String(str);
@@ -70,7 +80,7 @@ namespace Fig
             return FString(str.begin(), str.end());
         }
 
-        size_t length()
+        size_t length() const
         {
             // get UTF8-String real length
             size_t len = 0;
diff --git a/src/Error/error.hpp b/src/Error/error.hpp
index cace7d7..ffe2317 100644
--- a/src/Error/error.hpp
+++ b/src/Error/error.hpp
@@ -13,7 +13,7 @@ namespace Fig
     {
     public:
         explicit AddressableError() {}
-        explicit AddressableError(FStringView _msg,
+        explicit AddressableError(FString _msg,
                                   size_t _line,
                                   size_t _column,
                                   std::source_location loc = std::source_location::current()) :
@@ -35,7 +35,7 @@ namespace Fig
 
         size_t getLine() const { return line; }
         size_t getColumn() const { return column; }
-        FStringView getMessage() const { return message; }
+        FString getMessage() const { return message; }
 
         virtual FString getErrorType() const
         {
@@ -44,14 +44,14 @@ namespace Fig
 
     protected:
         size_t line, column;
-        FStringView message;
+        FString message;
     };
 
     class UnaddressableError : public std::exception
     {
     public:
         explicit UnaddressableError() {}
-        explicit UnaddressableError(FStringView _msg,
+        explicit UnaddressableError(FString _msg,
                                     std::source_location loc = std::source_location::current()) :
             src_loc(loc)
         {
@@ -59,7 +59,7 @@ namespace Fig
         }
         virtual FString toString() const
         {
-            std::string msg = std::format("[UnaddressableError] {} in [{}] {}", std::string(this->message.begin(), this->message.end()),  this->src_loc.file_name(), this->src_loc.function_name());
+            std::string msg = std::format("[UnaddressableError] {} in [{}] {}", this->message.toBasicString(),  this->src_loc.file_name(), this->src_loc.function_name());
             return FString(msg);
         }
         const char *what() const noexcept override
@@ -68,7 +68,7 @@ namespace Fig
             return msg.c_str();
         }
         std::source_location src_loc;
-        FStringView getMessage() const { return message; }
+        FString getMessage() const { return message; }
 
         virtual FString getErrorType() const
         {
@@ -76,7 +76,7 @@ namespace Fig
         }
 
     protected:
-        FStringView message;
+        FString message;
     };
 
     class SyntaxError : public AddressableError
@@ -84,7 +84,7 @@ namespace Fig
     public:
         using AddressableError::AddressableError;
 
-        explicit SyntaxError(FStringView _msg,
+        explicit SyntaxError(FString _msg,
                              size_t _line,
                              size_t _column,
                              std::source_location loc = std::source_location::current()) :
@@ -94,7 +94,7 @@ namespace Fig
 
         virtual FString toString() const override
         {
-            std::string msg = std::format("[SyntaxError] {} in [{}] {}", std::string(this->message.begin(), this->message.end()), this->src_loc.file_name(), this->src_loc.function_name());
+            std::string msg = std::format("[SyntaxError] {} in [{}] {}", this->message.toBasicString(), this->src_loc.file_name(), this->src_loc.function_name());
             return FString(msg);
         }
 
@@ -108,14 +108,14 @@ namespace Fig
     {
     public:
         using UnaddressableError::UnaddressableError;
-        explicit RuntimeError(FStringView _msg,
+        explicit RuntimeError(FString _msg,
                              std::source_location loc = std::source_location::current()) :
             UnaddressableError(_msg, loc)
         {
         }
         virtual FString toString() const override
         {
-            std::string msg = std::format("[RuntimeError] {} in [{}] {}", std::string(this->message.begin(), this->message.end()), this->src_loc.file_name(), this->src_loc.function_name());
+            std::string msg = std::format("[RuntimeError] {} in [{}] {}", this->message.toBasicString(), this->src_loc.file_name(), this->src_loc.function_name());
             return FString(msg);
         }
 
diff --git a/src/Evaluator/evaluator.cpp b/src/Evaluator/evaluator.cpp
index 1acecfc..fb0dcb0 100644
--- a/src/Evaluator/evaluator.cpp
+++ b/src/Evaluator/evaluator.cpp
@@ -22,13 +22,25 @@ namespace Fig
         LvObject base = evalLv(me->base, ctx);
         RvObject baseVal = base.get();
         const FString &member = me->member;
-        if (baseVal->getTypeInfo() != ValueType::StructInstance)
+        if (baseVal->hasMemberFunction(member))
+        {
+            return LvObject(std::make_shared<VariableSlot>(
+                member,
+                std::make_shared<Object>(
+                    Function(
+                        baseVal->getMemberFunction(member),
+                        baseVal->getMemberFunctionParaCount(member))),
+                ValueType::Function,
+                AccessModifier::PublicConst)); // fake l-value
+        }
+        if (baseVal->getTypeInfo() != ValueType::StructInstance) // and not member function found
         {
             throw EvaluatorError(
-                u8"TypeError",
+                u8"NoAttributeError",
                 std::format(
-                    "`{}` isn't a struct",
-                    base.name().toBasicString()),
+                    "`{}` has not attribute '{}'",
+                    baseVal->toString().toBasicString(),
+                    member.toBasicString()),
                 me->base);
         }
         const StructInstance &si = baseVal->as<StructInstance>();
@@ -49,11 +61,48 @@ namespace Fig
         LvObject base = evalLv(ie->base, ctx);
         RvObject index = eval(ie->index, ctx);
 
-        const TypeInfo &type = base.declaredType();
+        const TypeInfo &type = base.get()->getTypeInfo();
 
-        if (type != ValueType::List
-            && type != ValueType::Tuple
-            && type != ValueType::Map)
+        if (type == ValueType::List)
+        {
+            if (index->getTypeInfo() != ValueType::Int)
+            {
+                throw EvaluatorError(
+                    u8"TypeError",
+                    std::format(
+                        "Type `List` indices must be `Int`, got '{}'",
+                        index->getTypeInfo().toString().toBasicString()
+                    ),
+                    ie->index
+                );
+            }
+            List &list = base.get()->as<List>();
+            ValueType::IntClass indexVal = index->as<ValueType::IntClass>();
+            if (indexVal >= list.size())
+            {
+                throw EvaluatorError(
+                    u8"IndexOutOfRangeError",
+                    std::format(
+                        "Index {} out of list `{}` range",
+                        indexVal,
+                        base.get()->toString().toBasicString()
+                    ),
+                    ie->index
+                );
+            }
+            return LvObject(
+                base.get(),
+                indexVal
+            );
+        }
+        else if (type == ValueType::Map)
+        {
+            return LvObject(
+                base.get(),
+                index
+            );
+        }
+        else
         {
             throw EvaluatorError(
                 u8"NoSubscriptableError",
@@ -62,8 +111,7 @@ namespace Fig
                     base.declaredType().toString().toBasicString()),
                 ie->base);
         }
-        // TODO
-        return LvObject();
+        
     }
     LvObject Evaluator::evalLv(Ast::Expression exp, ContextPtr ctx)
     {
@@ -651,6 +699,30 @@ namespace Fig
                 return std::make_shared<Object>(StructInstance(structT.id, instanceCtx));
             }
 
+            case AstType::ListExpr: {
+                auto lstExpr = std::dynamic_pointer_cast<Ast::ListExprAst>(exp);
+                assert(lstExpr != nullptr);
+
+                List list;
+                for (auto &exp : lstExpr->val)
+                {
+                    list.push_back(eval(exp, ctx));
+                }
+                return std::make_shared<Object>(std::move(list));
+            }
+
+            case AstType::MapExpr: {
+                auto mapExpr = std::dynamic_pointer_cast<Ast::MapExprAst>(exp);
+                assert(mapExpr != nullptr);
+
+                Map map;
+                for (auto &[key, value] : mapExpr->val)
+                {
+                    map[eval(key, ctx)] = eval(value, ctx);
+                }
+                return std::make_shared<Object>(std::move(map));
+            }
+            
             default:
                 assert(false);
         }
@@ -990,7 +1062,7 @@ namespace Fig
             }
 
             default:
-                throw RuntimeError(FStringView(
+                throw RuntimeError(FString(
                     std::format("Feature stmt {} unsupported yet",
                                 magic_enum::enum_name(stmt->getType()))));
         }
diff --git a/src/Evaluator/evaluator.hpp b/src/Evaluator/evaluator.hpp
index c8565de..9e1c6ed 100644
--- a/src/Evaluator/evaluator.hpp
+++ b/src/Evaluator/evaluator.hpp
@@ -3,6 +3,7 @@
 #include <Context/context.hpp>
 #include <Error/error.hpp>
 #include <Module/builtins.hpp>
+#include <Value/LvObject.hpp>
 
 
 namespace Fig
diff --git a/src/Evaluator/evaluator_error.hpp b/src/Evaluator/evaluator_error.hpp
index 14fabca..2998c25 100644
--- a/src/Evaluator/evaluator_error.hpp
+++ b/src/Evaluator/evaluator_error.hpp
@@ -12,7 +12,7 @@ namespace Fig
         using AddressableError::AddressableError;
         EvaluatorError(FString _typeName, FString msg, Ast::AstBase ast, std::source_location loc = std::source_location::current())
         {
-            message = FStringView::fromBasicStringView(msg.toBasicString());
+            message = msg;
             line = ast->getAAI().line;
             column = ast->getAAI().column;
 
@@ -23,7 +23,7 @@ namespace Fig
         }
         EvaluatorError(FString _typeName, std::string_view msg, Ast::AstBase ast, std::source_location loc = std::source_location::current())
         {
-            message = FStringView::fromBasicStringView(msg);
+            message = FString::fromBasicString(std::string(msg.data()));
             line = ast->getAAI().line;
             column = ast->getAAI().column;
 
diff --git a/src/Lexer/lexer.cpp b/src/Lexer/lexer.cpp
index 7571b4e..b499d95 100644
--- a/src/Lexer/lexer.cpp
+++ b/src/Lexer/lexer.cpp
@@ -190,7 +190,7 @@ namespace Fig
                 }
                 else
                 {
-                    error = SyntaxError(FStringView(
+                    error = SyntaxError(FString(
                                             std::format(
                                                 "Unsupported escape character: {}",
                                                 FString(ec.getString()).toBasicString())),
@@ -307,7 +307,7 @@ namespace Fig
                 }
                 else
                 {
-                    error = SyntaxError(FStringView(
+                    error = SyntaxError(FString(
                                             std::format(
                                                 "Unsupported escape character: {}",
                                                 FString(ec.getString()).toBasicString())),
@@ -369,7 +369,7 @@ namespace Fig
         // checking legality
         if ((*numStr.end()) == u'e') // e 后面必须跟整数表示科学计数
         {
-            error = SyntaxError(FStringView(
+            error = SyntaxError(FString(
                                     std::format("Ellegal number literal: {}", numStr.toBasicString())),
                                 this->line, it.column());
             return IllegalTok;
@@ -396,7 +396,7 @@ namespace Fig
             else if (!this->symbol_map.contains(sym))
             {
                 // check legality
-                error = SyntaxError(FStringView(
+                error = SyntaxError(FString(
                                         std::format("No such a operator: {}", sym.toBasicString())),
                                     this->line, it.column());
             }
@@ -455,7 +455,7 @@ namespace Fig
 
             if (!terminated)
             {
-                error = SyntaxError(FStringView(u8"Unterminated multiline comment"), this->line, it.column());
+                error = SyntaxError(FString(u8"Unterminated multiline comment"), this->line, it.column());
                 next();
                 return IllegalTok;
             }
@@ -526,7 +526,7 @@ namespace Fig
         }
         else
         {
-            error = SyntaxError(FStringView(
+            error = SyntaxError(FString(
                                     std::format("Cannot tokenize char: '{}'", FString(ch.getString()).toBasicString())),
                                 this->line, it.column());
             if (hasNext())
diff --git a/src/Module/builtins.hpp b/src/Module/builtins.hpp
index cea9fca..6f7f1ff 100644
--- a/src/Module/builtins.hpp
+++ b/src/Module/builtins.hpp
@@ -36,16 +36,24 @@ namespace Fig
 
         const std::unordered_map<FString, BuiltinFunction> builtinFunctions{
             {u8"__fstdout_print", [](const std::vector<ObjectPtr> &args) -> ObjectPtr {
+                 bool first_flag = true;
                  for (auto arg : args)
                  {
-                     std::print("{}", arg->toString().toBasicString());
+                     if (!first_flag)
+                         std::print(" ");
+                     std::print("{}", arg->toStringIO().toBasicString());
+                     first_flag = false;
                  }
                  return std::make_shared<Object>(ValueType::IntClass(args.size()));
              }},
             {u8"__fstdout_println", [](const std::vector<ObjectPtr> &args) -> ObjectPtr {
+                 bool first_flag = true;
                  for (auto arg : args)
                  {
-                     std::print("{}", arg->toString().toBasicString());
+                     if (!first_flag)
+                         std::print(" ");
+                     std::print("{}", arg->toStringIO().toBasicString());
+                     first_flag = false;
                  }
                  std::print("\n");
                  return std::make_shared<Object>(ValueType::IntClass(args.size()));
@@ -72,7 +80,7 @@ namespace Fig
                  }
                  catch (...)
                  {
-                     throw RuntimeError(FStringView(std::format("Invalid int string for parsing", str.toBasicString())));
+                     throw RuntimeError(FString(std::format("Invalid int string for parsing", str.toBasicString())));
                  }
              }},
             {u8"__fvalue_int_from", [](const std::vector<ObjectPtr> &args) -> ObjectPtr {
@@ -87,7 +95,7 @@ namespace Fig
                  }
                  else
                  {
-                     throw RuntimeError(FStringView(std::format("Type '{}' cannot be converted to int", val->getTypeInfo().toString().toBasicString())));
+                     throw RuntimeError(FString(std::format("Type '{}' cannot be converted to int", val->getTypeInfo().toString().toBasicString())));
                  }
              }},
             {u8"__fvalue_double_parse", [](const std::vector<ObjectPtr> &args) -> ObjectPtr {
@@ -99,7 +107,7 @@ namespace Fig
                  }
                  catch (...)
                  {
-                     throw RuntimeError(FStringView(std::format("Invalid double string for parsing", str.toBasicString())));
+                     throw RuntimeError(FString(std::format("Invalid double string for parsing", str.toBasicString())));
                  }
              }},
             {u8"__fvalue_double_from", [](const std::vector<ObjectPtr> &args) -> ObjectPtr {
@@ -114,7 +122,7 @@ namespace Fig
                  }
                  else
                  {
-                     throw RuntimeError(FStringView(std::format("Type '{}' cannot be converted to double", val->getTypeInfo().toString().toBasicString())));
+                     throw RuntimeError(FString(std::format("Type '{}' cannot be converted to double", val->getTypeInfo().toString().toBasicString())));
                  }
              }},
             {u8"__fvalue_string_from", [](const std::vector<ObjectPtr> &args) -> ObjectPtr {
@@ -134,7 +142,7 @@ namespace Fig
             auto it = builtinFunctions.find(name);
             if (it == builtinFunctions.end())
             {
-                throw RuntimeError(FStringView(std::format("Builtin function '{}' not found", name.toBasicString())));
+                throw RuntimeError(FString(std::format("Builtin function '{}' not found", name.toBasicString())));
             }
             return it->second;
         }
@@ -144,7 +152,7 @@ namespace Fig
             auto it = builtinFunctionArgCounts.find(name);
             if (it == builtinFunctionArgCounts.end())
             {
-                throw RuntimeError(FStringView(std::format("Builtin function '{}' not found", name.toBasicString())));
+                throw RuntimeError(FString(std::format("Builtin function '{}' not found", name.toBasicString())));
             }
             return it->second;
         }
diff --git a/src/Parser/parser.cpp b/src/Parser/parser.cpp
index 4c3f56a..de4f15f 100644
--- a/src/Parser/parser.cpp
+++ b/src/Parser/parser.cpp
@@ -69,7 +69,7 @@ namespace Fig
         if (!isThis(TokenType::Assign) and !isThis(TokenType::Walrus)) expect(TokenType::Assign, u8"assign or walrus");
         if (isThis(TokenType::Walrus))
         {
-            if (hasSpecificType) throwAddressableError<SyntaxError>(FStringView(u8""));
+            if (hasSpecificType) throwAddressableError<SyntaxError>(FString(u8""));
             tiName = Parser::varDefTypeFollowed;
         }
         next();
@@ -93,7 +93,7 @@ namespace Fig
                 }
                 catch (...)
                 {
-                    throwAddressableError<SyntaxError>(FStringView(u8"Illegal number literal"));
+                    throwAddressableError<SyntaxError>(FString(u8"Illegal number literal"));
                 }
                 return std::make_shared<Object>(d);
             }
@@ -107,7 +107,7 @@ namespace Fig
                 }
                 catch (...)
                 {
-                    throwAddressableError<SyntaxError>(FStringView(u8"Illegal number literal"));
+                    throwAddressableError<SyntaxError>(FString(u8"Illegal number literal"));
                 }
                 return std::make_shared<Object>(i);
             }
@@ -253,7 +253,7 @@ namespace Fig
             }
             else
             {
-                throwAddressableError<SyntaxError>(FStringView(std::format("expect field name or field attribute")));
+                throwAddressableError<SyntaxError>(FString(std::format("expect field name or field attribute")));
             }
             FString tiName = ValueType::Any.name;
             if (isThis(TokenType::Colon))
@@ -267,7 +267,7 @@ namespace Fig
             if (isThis(TokenType::Assign))
             {
                 next();
-                if (isEOF()) throwAddressableError<SyntaxError>(FStringView(u8"expect an expression"));
+                if (isEOF()) throwAddressableError<SyntaxError>(FString(u8"expect an expression"));
                 initExpr = parseExpression(0);
             }
             expectSemicolon();
@@ -314,7 +314,7 @@ namespace Fig
                 }
                 else
                 {
-                    throwAddressableError<SyntaxError>(FStringView("Invalid syntax"));
+                    throwAddressableError<SyntaxError>(FString("Invalid syntax"));
                 }
             }
             else if (isThis(TokenType::Function))
@@ -333,16 +333,16 @@ namespace Fig
             }
             else if (isThis(TokenType::Variable))
             {
-                throwAddressableError<SyntaxError>(FStringView("Variables are not allowed to be defined within a structure."));
+                throwAddressableError<SyntaxError>(FString("Variables are not allowed to be defined within a structure."));
             }
             else
             {
-                throwAddressableError<SyntaxError>(FStringView("Invalid syntax"));
+                throwAddressableError<SyntaxError>(FString("Invalid syntax"));
             }
         }
         if (!braceClosed)
         {
-            throwAddressableError<SyntaxError>(FStringView("braces are not closed"));
+            throwAddressableError<SyntaxError>(FString("braces are not closed"));
         }
         return makeAst<Ast::StructDefSt>(isPublic, structName, fields, makeAst<Ast::BlockStatementAst>(stmts));
     }
@@ -368,7 +368,7 @@ namespace Fig
             }
             else
             {
-                throwAddressableError<SyntaxError>(FStringView(u8"Expected `var`, `const`, `function` or `struct` after `public`"));
+                throwAddressableError<SyntaxError>(FString(u8"Expected `var`, `const`, `function` or `struct` after `public`"));
             }
         }
         else if (isThis(TokenType::Variable) || isThis(TokenType::Const))
@@ -392,7 +392,7 @@ namespace Fig
         }
         else if (isThis(TokenType::Else))
         {
-            throwAddressableError<SyntaxError>(FStringView(u8"`else` without matching `if`"));
+            throwAddressableError<SyntaxError>(FString(u8"`else` without matching `if`"));
         }
         else if (isThis(TokenType::LeftBrace))
         {
@@ -625,15 +625,10 @@ namespace Fig
     {
         // entry: current is `{`
         next(); // consume `{`
-        std::map<FString, Ast::Expression> val;
+        std::map<Ast::Expression, Ast::Expression> val;
         while (!isThis(TokenType::RightBrace))
         {
-            expect(TokenType::Identifier, FString(u8"key (identifier)"));
-            FString key = currentToken().getValue();
-            if (val.contains(key)) throwAddressableError<SyntaxError>(FStringView(std::format(
-                "Redefinition of immutable key {} in mapping literal",
-                key.toBasicString())));
-            next(); // consume key
+            Ast::Expression key = parseExpression(0, TokenType::Colon);
             expect(TokenType::Colon);
             next(); // consume `:`
             val[key] = parseExpression(0, TokenType::RightBrace, TokenType::Comma);
@@ -711,7 +706,7 @@ namespace Fig
             }
             else if (!isThis(TokenType::RightBrace))
             {
-                throwAddressableError<SyntaxError>(FStringView(
+                throwAddressableError<SyntaxError>(FString(
                     std::format("Expect `,` or `}}` in struct initialization expression, got {}",
                                 currentToken().toString().toBasicString())
                 ));
@@ -762,7 +757,7 @@ namespace Fig
         }
         else
         {
-            throwAddressableError<SyntaxError>(FStringView(u8"Expect ')' or ',' after expression in parentheses"));
+            throwAddressableError<SyntaxError>(FString(u8"Expect ')' or ',' after expression in parentheses"));
         }
         return nullptr; // to suppress compiler warning
     }
@@ -794,10 +789,10 @@ namespace Fig
 
         Token tok = currentToken();
         if (tok == EOFTok)
-            throwAddressableError<SyntaxError>(FStringView(u8"Unexpected end of expression"));
+            throwAddressableError<SyntaxError>(FString(u8"Unexpected end of expression"));
         if (tok.getType() == stop || tok.getType() == stop2)
         {
-            if (lhs == nullptr) throwAddressableError<SyntaxError>(FStringView(u8"Expected expression"));
+            if (lhs == nullptr) throwAddressableError<SyntaxError>(FString(u8"Expected expression"));
             return lhs;
         }
         if (tok.getType() == TokenType::LeftBracket)
@@ -818,7 +813,7 @@ namespace Fig
             if (currentToken().getType() == TokenType::Identifier)
             {
                 // err
-                throwAddressableError<SyntaxError>(FStringView(u8"Function literal should not have a name"));
+                throwAddressableError<SyntaxError>(FString(u8"Function literal should not have a name"));
             }
             expect(TokenType::LeftParen);
             lhs = __parseFunctionLiteralExpr();
@@ -849,7 +844,7 @@ namespace Fig
         }
         else
         {
-            throwAddressableError<SyntaxError>(FStringView(u8"Unexpected token in expression"));
+            throwAddressableError<SyntaxError>(FString(u8"Unexpected token in expression"));
         }
 
         // infix / (postfix) ?
@@ -872,7 +867,7 @@ namespace Fig
                 next(); // consume '.'
                 Token idTok = currentToken();
                 if (!idTok.isIdentifier())
-                    throwAddressableError<SyntaxError>(FStringView(u8"Expected identifier after '.'"));
+                    throwAddressableError<SyntaxError>(FString(u8"Expected identifier after '.'"));
 
                 FString member = idTok.getValue();
                 next(); // consume identifier
diff --git a/src/Parser/parser.hpp b/src/Parser/parser.hpp
index a0e49fa..146f3dd 100644
--- a/src/Parser/parser.hpp
+++ b/src/Parser/parser.hpp
@@ -92,7 +92,7 @@ namespace Fig
         }
 
         template <class _ErrT, typename = AddressableError>
-        void throwAddressableError(FStringView msg, size_t line, size_t column, std::source_location loc = std::source_location::current())
+        void throwAddressableError(FString msg, size_t line, size_t column, std::source_location loc = std::source_location::current())
         {
             static_assert(std::is_base_of_v<AddressableError, _ErrT>,
                           "_ErrT must derive from AddressableError");
@@ -101,7 +101,7 @@ namespace Fig
             throw spError;
         }
         template <class _ErrT, typename = AddressableError>
-        void throwAddressableError(FStringView msg, std::source_location loc = std::source_location::current())
+        void throwAddressableError(FString msg, std::source_location loc = std::source_location::current())
         {
             static_assert(std::is_base_of_v<AddressableError, _ErrT>,
                           "_ErrT must derive from AddressableError");
@@ -112,7 +112,7 @@ namespace Fig
         }
 
         template <class _ErrT, typename = UnaddressableError>
-        void throwUnaddressableError(FStringView msg, std::source_location loc = std::source_location::current())
+        void throwUnaddressableError(FString msg, std::source_location loc = std::source_location::current())
         {
             static_assert(std::is_base_of_v<AddressableError, _ErrT>,
                           "_ErrT must derive from AddressableError");
@@ -215,7 +215,7 @@ namespace Fig
         {
             if (peekToken().getType() != type)
             {
-                throwAddressableError<SyntaxError>(FStringView(std::format("Expected `{}`, but got `{}`",
+                throwAddressableError<SyntaxError>(FString(std::format("Expected `{}`, but got `{}`",
                                                                            magic_enum::enum_name(type),
                                                                            magic_enum::enum_name(peekToken().getType()))));
             }
@@ -225,7 +225,7 @@ namespace Fig
         {
             if (currentToken().getType() != type)
             {
-                throwAddressableError<SyntaxError>(FStringView(std::format("Expected `{}`, but got `{}`",
+                throwAddressableError<SyntaxError>(FString(std::format("Expected `{}`, but got `{}`",
                                                                            magic_enum::enum_name(type),
                                                                            magic_enum::enum_name(currentToken().getType()))));
             }
@@ -235,7 +235,7 @@ namespace Fig
         {
             if (peekToken().getType() != type)
             {
-                throwAddressableError<SyntaxError>(FStringView(std::format("Expected `{}`, but got `{}`",
+                throwAddressableError<SyntaxError>(FString(std::format("Expected `{}`, but got `{}`",
                                                                            expected.toBasicString(),
                                                                            magic_enum::enum_name(peekToken().getType()))));
             }
@@ -245,7 +245,7 @@ namespace Fig
         {
             if (currentToken().getType() != type)
             {
-                throwAddressableError<SyntaxError>(FStringView(std::format("Expected `{}`, but got `{}`",
+                throwAddressableError<SyntaxError>(FString(std::format("Expected `{}`, but got `{}`",
                                                                            expected.toBasicString(),
                                                                            magic_enum::enum_name(currentToken().getType()))));
             }
@@ -319,8 +319,8 @@ namespace Fig
         Ast::Expression __parseCall(Ast::Expression);
         
         Ast::ListExpr __parseListExpr(); // entry: current is `[`
-
         Ast::MapExpr __parseMapExpr(); // entry: current is `{`
+
         Ast::InitExpr __parseInitExpr(FString);  // entry: current is `{`, ahead is struct name. arg (struct name : FString)
         Ast::Expression __parseTupleOrParenExpr();             // entry: current is `(`
 
diff --git a/src/Value/LvObject.hpp b/src/Value/LvObject.hpp
new file mode 100644
index 0000000..54e5cbe
--- /dev/null
+++ b/src/Value/LvObject.hpp
@@ -0,0 +1,114 @@
+#pragma once
+
+#include <Value/VariableSlot.hpp>
+#include <Value/value.hpp>
+
+namespace Fig
+{
+
+    struct LvObject
+    {
+        enum class Kind
+        {
+            Variable,
+            ListElement,
+            MapElement
+        } kind;
+        std::shared_ptr<VariableSlot> slot;
+
+        ObjectPtr listOrMap = nullptr;
+        size_t listIndex;
+
+        ObjectPtr mapIndex;
+
+        LvObject(std::shared_ptr<VariableSlot> _slot) :
+            slot(std::move(_slot))
+        {
+            kind = Kind::Variable;
+        }
+        LvObject(ObjectPtr _v, size_t _index) :
+            listOrMap(_v), listIndex(_index)
+        {
+            assert(_v->getTypeInfo() == ValueType::List);
+            kind = Kind::ListElement;
+        }
+        LvObject(ObjectPtr _v, ObjectPtr _index) :
+            listOrMap(_v), mapIndex(_index)
+        {
+            assert(_v->getTypeInfo() == ValueType::Map);
+            kind = Kind::MapElement;
+        }
+
+        const ObjectPtr &get() const
+        {
+            if (kind == Kind::Variable)
+            {
+                auto s = resolve(slot);
+                return s->value;
+            }
+            else if (kind == Kind::ListElement)
+            {
+                List &list = listOrMap->as<List>();
+                if (listIndex >= list.size())
+                    throw RuntimeError(FString(
+                        std::format("Index {} out of range", listIndex)));
+                return list.at(listIndex);
+            }
+            else // map
+            {
+                Map &map = listOrMap->as<Map>();
+                if (!map.contains(mapIndex))
+                    throw RuntimeError(FString(
+                        std::format("Key {} not found", mapIndex->toString().toBasicString())));
+                return map.at(mapIndex);
+            }
+        }
+
+        void set(const ObjectPtr &v)
+        {
+            if (kind == Kind::Variable)
+            {
+                auto s = resolve(slot);
+                if (s->declaredType != ValueType::Any && s->declaredType != v->getTypeInfo())
+                {
+                    throw RuntimeError(
+                        FString(
+                            std::format("Variable `{}` expects type `{}`, but got '{}'",
+                                        s->name.toBasicString(),
+                                        s->declaredType.toString().toBasicString(),
+                                        v->getTypeInfo().toString().toBasicString())));
+                }
+                if (isAccessConst(s->am))
+                {
+                    throw RuntimeError(FString(
+                        std::format("Variable `{}` is immutable", s->name.toBasicString())));
+                }
+                s->value = v;
+            }
+            else if (kind == Kind::ListElement)
+            {
+                List &list = listOrMap->as<List>();
+                if (listIndex >= list.size())
+                    throw RuntimeError(FString(
+                        std::format("Index {} out of range", listIndex)));
+                list[listIndex] = v;
+            }
+            else // map
+            {
+                Map &map = listOrMap->as<Map>();
+                map[mapIndex] = v;
+            }
+        }
+
+        FString name() const { return resolve(slot)->name; }
+        TypeInfo declaredType() const { return resolve(slot)->declaredType; }
+        AccessModifier access() const { return resolve(slot)->am; }
+
+    private:
+        std::shared_ptr<VariableSlot> resolve(std::shared_ptr<VariableSlot> s) const
+        {
+            while (s->isRef) s = s->refTarget;
+            return s;
+        }
+    };
+}
\ No newline at end of file
diff --git a/src/Value/Type.hpp b/src/Value/Type.hpp
index 3b3ac82..adfa3c0 100644
--- a/src/Value/Type.hpp
+++ b/src/Value/Type.hpp
@@ -14,6 +14,8 @@ namespace Fig
         size_t id;
 
     public:
+        friend class TypeInfoHash;
+
         FString name;
 
         FString toString() const
@@ -27,7 +29,7 @@ namespace Fig
         {
             return typeMap.at(_name);
         }
-        size_t getInstanceID(FString _name) const
+        size_t getInstanceID() const
         {
             return id;
         }
@@ -42,6 +44,15 @@ namespace Fig
         }
     };
 
+    class TypeInfoHash
+    {
+    public:
+        std::size_t operator()(const TypeInfo &ti) const
+        {
+            return ti.id;
+        }
+    };
+
     // class Value;
     namespace ValueType
     {
@@ -56,7 +67,7 @@ namespace Fig
         extern const TypeInfo StructInstance;
         extern const TypeInfo List;
         extern const TypeInfo Map;
-        extern const TypeInfo Tuple;
+        // extern const TypeInfo Tuple;
 
         using IntClass = int64_t;
         using DoubleClass = double;
diff --git a/src/Value/VariableSlot.hpp b/src/Value/VariableSlot.hpp
new file mode 100644
index 0000000..984c40c
--- /dev/null
+++ b/src/Value/VariableSlot.hpp
@@ -0,0 +1,22 @@
+#pragma once
+
+
+#include <Ast/AccessModifier.hpp>
+#include <Core/fig_string.hpp>
+#include <Value/Type.hpp>
+#include <memory>
+namespace Fig
+{
+    class Object;
+    using ObjectPtr = std::shared_ptr<Object>;
+    struct VariableSlot
+    {
+        FString name;
+        ObjectPtr value;
+        TypeInfo declaredType;
+        AccessModifier am;
+
+        bool isRef = false;
+        std::shared_ptr<VariableSlot> refTarget;
+    };
+}
\ No newline at end of file
diff --git a/src/Value/containers.hpp b/src/Value/containers.hpp
deleted file mode 100644
index 71ba15c..0000000
--- a/src/Value/containers.hpp
+++ /dev/null
@@ -1,6 +0,0 @@
-#pragma once
-
-namespace Fig
-{
-    
-};
\ No newline at end of file
diff --git a/src/Value/structType.hpp b/src/Value/structType.hpp
index 78b1ff8..fe5b79c 100644
--- a/src/Value/structType.hpp
+++ b/src/Value/structType.hpp
@@ -29,7 +29,6 @@ namespace Fig
         {
             return am == AccessModifier::Const || am == AccessModifier::PublicConst;
         }
-
     };
 
     struct StructType
@@ -65,3 +64,15 @@ namespace Fig
         }
     };
 } // namespace Fig
+
+namespace std
+{
+    template <>
+    struct hash<Fig::Field>
+    {
+        size_t operator()(const Fig::Field &f)
+        {
+            return std::hash<Fig::FString>{}(f.name);
+        }
+    };
+}; // namespace std
\ No newline at end of file
diff --git a/src/Value/value.cpp b/src/Value/value.cpp
index c0b9f43..f53e14b 100644
--- a/src/Value/value.cpp
+++ b/src/Value/value.cpp
@@ -25,6 +25,50 @@ namespace Fig
         }
     }
     
+    size_t ValueKeyHash::operator()(const ValueKey &key) const
+    {
+        {
+            ObjectPtr value = key.value;
+            const TypeInfo &type = value->getTypeInfo();
+
+            if (type == ValueType::Int)
+            {
+                return std::hash<ValueType::IntClass>{}(value->as<ValueType::IntClass>());
+            }
+            if (type == ValueType::Double)
+            {
+                return std::hash<ValueType::DoubleClass>{}(value->as<ValueType::DoubleClass>());
+            }
+            if (type == ValueType::String)
+            {
+                return std::hash<ValueType::StringClass>{}(value->as<ValueType::StringClass>());
+            }
+            if (type == ValueType::Bool)
+            {
+                return std::hash<ValueType::BoolClass>{}(value->as<ValueType::BoolClass>());
+            }
+            if (type == ValueType::StructType)
+            {
+                auto HashFields = [](std::vector<Field> fields) {
+                    size_t r = 0;
+                    for (auto &f : fields)
+                    {
+                        r += std::hash<Field>{}(f);
+                    }
+                    return r;
+                };
+                const StructType &st = value->as<StructType>();
+                return std::hash<std::size_t>{}(st.id) + HashFields(st.fields);
+            }
+            if (type == ValueType::StructInstance)
+            {
+                const StructInstance &si = value->as<StructInstance>();
+                return std::hash<std::size_t>{}(si.parentId) + std::hash<uint64_t>{}(reinterpret_cast<uint64_t>(std::addressof(*si.localContext)));
+            }
+            assert(false);
+        }
+    }
+
     const TypeInfo ValueType::Any(FString(u8"Any"), true);           // id: 1
     const TypeInfo ValueType::Null(FString(u8"Null"), true);         // id: 2
     const TypeInfo ValueType::Int(FString(u8"Int"), true);           // id: 3
@@ -36,5 +80,5 @@ namespace Fig
     const TypeInfo ValueType::StructInstance(FString(u8"StructInstance"), true); // id: 9
     const TypeInfo ValueType::List(FString(u8"List"), true); // id: 10
     const TypeInfo ValueType::Map(FString(u8"Map"), true); // id: 11
-    const TypeInfo ValueType::Tuple(FString(u8"Tuple"), true); // id: 12
+    // const TypeInfo ValueType::Tuple(FString(u8"Tuple"), true); // id: 12
 } // namespace Fig
\ No newline at end of file
diff --git a/src/Value/value.hpp b/src/Value/value.hpp
index 73fb8d3..064d7e9 100644
--- a/src/Value/value.hpp
+++ b/src/Value/value.hpp
@@ -9,6 +9,8 @@
 #include <cmath>
 #include <string>
 #include <format>
+#include <functional>
+#include <unordered_map>
 
 namespace Fig
 {
@@ -26,6 +28,22 @@ namespace Fig
             static_cast<ValueType::DoubleClass>(std::numeric_limits<ValueType::IntClass>::min());
         return d > intMaxAsDouble || d < intMinAsDouble;
     }
+    class Object;
+    using ObjectPtr = std::shared_ptr<Object>;
+    using List = std::vector<ObjectPtr>;
+
+    struct ValueKey
+    {
+        ObjectPtr value;
+        ValueKey(ObjectPtr _value) :
+            value(_value) {}
+    };
+
+    struct ValueKeyHash
+    {
+        size_t operator()(const ValueKey &key) const;
+    };
+    using Map = std::unordered_map<ValueKey, ObjectPtr, ValueKeyHash>;
 
     class Object
     {
@@ -38,7 +56,104 @@ namespace Fig
             ValueType::BoolClass,
             Function,
             StructType,
-            StructInstance>;
+            StructInstance,
+            List,
+            Map>;
+
+        std::unordered_map<TypeInfo,
+                           std::unordered_map<FString,
+                                              std::function<ObjectPtr(std::vector<ObjectPtr>)>>,
+                           TypeInfoHash>
+            memberTypeFunctions{
+                {ValueType::Null, {}},
+                {ValueType::Int, {}},
+                {ValueType::Double, {}},
+                {ValueType::String, {
+                                        {u8"length", [this](std::vector<ObjectPtr> args) -> ObjectPtr {
+                                             if (args.size() != 0)
+                                                 throw RuntimeError(FString(
+                                                     std::format("`length` expects 0 arguments, {} got", args.size())));
+                                             const FString &str = as<ValueType::StringClass>();
+                                             return std::make_shared<Object>(static_cast<ValueType::IntClass>(str.length()));
+                                         }},
+
+                                    }},
+                {ValueType::Function, {}},
+                {ValueType::StructType, {}},
+                {ValueType::StructInstance, {}},
+                {ValueType::List, {{u8"length", [this](std::vector<ObjectPtr> args) -> ObjectPtr {
+                                        if (args.size() != 0)
+                                            throw RuntimeError(FString(
+                                                std::format("`length` expects 0 arguments, {} got", args.size())));
+                                        const List &list = as<List>();
+                                        return std::make_shared<Object>(static_cast<ValueType::IntClass>(list.size()));
+                                    }},
+                                   {u8"get", [this](std::vector<ObjectPtr> args) -> ObjectPtr {
+                                        if (args.size() != 1)
+                                            throw RuntimeError(FString(
+                                                std::format("`get` expects 1 arguments, {} got", args.size())));
+                                        ObjectPtr arg = args[0];
+                                        if (arg->getTypeInfo() != ValueType::Int)
+                                            throw RuntimeError(FString(
+                                                std::format("`get` argument 1 expects Int, {} got", arg->getTypeInfo().toString().toBasicString())));
+                                        ValueType::IntClass i = arg->as<ValueType::IntClass>();
+                                        const List &list = as<List>();
+                                        if (i >= list.size())
+                                            return Object::getNullInstance();
+                                        return list[i];
+                                    }}}},
+                {ValueType::Map, {
+                                     {u8"get", [this](std::vector<ObjectPtr> args) -> ObjectPtr {
+                                          if (args.size() != 1)
+                                              throw RuntimeError(FString(
+                                                  std::format("`get` expects 1 arguments, {} got", args.size())));
+                                          ObjectPtr index = args[0];
+                                          const Map &map = as<Map>();
+                                          if (!map.contains(index))
+                                              return Object::getNullInstance();
+                                          return map.at(index);
+                                      }},
+                                     {u8"contains", [this](std::vector<ObjectPtr> args) -> ObjectPtr {
+                                          if (args.size() != 1)
+                                              throw RuntimeError(FString(
+                                                  std::format("`contains` expects 1 arguments, {} got", args.size())));
+                                          ObjectPtr index = args[0];
+                                          const Map &map = as<Map>();
+                                          return std::make_shared<Object>(
+                                              map.contains(index));
+                                      }},
+                                 }},
+            };
+        std::unordered_map<TypeInfo, std::unordered_map<FString, int>, TypeInfoHash> memberTypeFunctionsParas{
+            {ValueType::Null, {}},
+            {ValueType::Int, {}},
+            {ValueType::Double, {}},
+            {ValueType::String, {
+                                    {u8"length", 0},
+
+                                }},
+            {ValueType::Function, {}},
+            {ValueType::StructType, {}},
+            {ValueType::StructInstance, {}},
+            {ValueType::List, {{u8"length", 0}, {u8"get", 1}}},
+            {ValueType::Map, {
+                                 {u8"get", 1},
+                                 {u8"contains", 1},
+                             }},
+
+        };
+        bool hasMemberFunction(const FString &name) const
+        {
+            return memberTypeFunctions.at(getTypeInfo()).contains(name);
+        }
+        std::function<ObjectPtr(std::vector<ObjectPtr>)> getMemberFunction(const FString &name) const
+        {
+            return memberTypeFunctions.at(getTypeInfo()).at(name);
+        }
+        int getMemberFunctionParaCount(const FString &name) const
+        {
+            return memberTypeFunctionsParas.at(getTypeInfo()).at(name);
+        }
 
         VariantType data;
 
@@ -49,9 +164,7 @@ namespace Fig
         Object(const ValueType::IntClass &i) :
             data(i) {}
         explicit Object(const ValueType::DoubleClass &d) :
-            data(d)
-        {
-        }
+            data(d) {}
         Object(const ValueType::StringClass &s) :
             data(s) {}
         Object(const ValueType::BoolClass &b) :
@@ -62,6 +175,10 @@ namespace Fig
             data(s) {}
         Object(const StructInstance &s) :
             data(s) {}
+        Object(const List &l) :
+            data(l) {}
+        Object(const Map &m) :
+            data(m) {}
 
         Object(const Object &) = default;
         Object(Object &&) noexcept = default;
@@ -78,6 +195,10 @@ namespace Fig
                 return Object(ValueType::StringClass(u8""));
             else if (ti == ValueType::Bool)
                 return Object(ValueType::BoolClass(false));
+            else if (ti == ValueType::List)
+                return Object(List{});
+            else if (ti == ValueType::Map)
+                return Object(Map{});
             else
                 return *getNullInstance();
         }
@@ -120,22 +241,37 @@ namespace Fig
         {
             return std::visit([](auto &&val) -> TypeInfo {
                 using T = std::decay_t<decltype(val)>;
+
                 if constexpr (std::is_same_v<T, ValueType::NullClass>)
                     return ValueType::Null;
+
                 else if constexpr (std::is_same_v<T, ValueType::IntClass>)
                     return ValueType::Int;
+
                 else if constexpr (std::is_same_v<T, ValueType::DoubleClass>)
                     return ValueType::Double;
+
                 else if constexpr (std::is_same_v<T, ValueType::StringClass>)
                     return ValueType::String;
+
                 else if constexpr (std::is_same_v<T, ValueType::BoolClass>)
                     return ValueType::Bool;
+
                 else if constexpr (std::is_same_v<T, Function>)
                     return ValueType::Function;
+
                 else if constexpr (std::is_same_v<T, StructType>)
                     return ValueType::StructType;
+
                 else if constexpr (std::is_same_v<T, StructInstance>)
                     return ValueType::StructInstance;
+
+                else if constexpr (std::is_same_v<T, List>)
+                    return ValueType::List;
+
+                else if constexpr (std::is_same_v<T, Map>)
+                    return ValueType::Map;
+
                 else
                     return ValueType::Any;
             },
@@ -155,12 +291,18 @@ namespace Fig
                 throw RuntimeError(u8"getNumericValue: Not a numeric value");
         }
 
+        FString toStringIO() const
+        {
+            if (is<ValueType::StringClass>()) return as<ValueType::StringClass>();
+            return toString();
+        }
+
         FString toString() const
         {
             if (is<ValueType::NullClass>()) return FString(u8"null");
             if (is<ValueType::IntClass>()) return FString(std::to_string(as<ValueType::IntClass>()));
             if (is<ValueType::DoubleClass>()) return FString(std::format("{}", as<ValueType::DoubleClass>()));
-            if (is<ValueType::StringClass>()) return as<ValueType::StringClass>();
+            if (is<ValueType::StringClass>()) return FString(u8"<String \"") + as<ValueType::StringClass>() + FString(u8"\" >");
             if (is<ValueType::BoolClass>()) return as<ValueType::BoolClass>() ? FString(u8"true") : FString(u8"false");
             if (is<Function>())
                 return FString(std::format("<Function {} at {:p}>",
@@ -174,6 +316,36 @@ namespace Fig
                 return FString(std::format("<StructInstance '{}' at {:p}>",
                                            as<StructInstance>().parentId,
                                            static_cast<const void *>(&as<StructInstance>())));
+            if (is<List>())
+            {
+                FString output(u8"[");
+                const List &list = as<List>();
+                bool first_flag = true;
+                for (auto &ele : list)
+                {
+                    if (!first_flag)
+                        output += u8", ";
+                    output += ele->toString();
+                    first_flag = false;
+                }
+                output += u8"]";
+                return output;
+            }
+            if (is<Map>())
+            {
+                FString output(u8"{");
+                const Map &map = as<Map>();
+                bool first_flag = true;
+                for (auto &[key, value] : map)
+                {
+                    if (!first_flag)
+                        output += u8", ";
+                    output += key.value->toString() + FString(u8" : ") + value->toString();
+                    first_flag = false;
+                }
+                output += u8"}";
+                return output;
+            }
             return FString(u8"<error>");
         }
 
@@ -191,7 +363,7 @@ namespace Fig
         friend Object operator+(const Object &lhs, const Object &rhs)
         {
             if (lhs.isNull() || rhs.isNull())
-                throw ValueError(FStringView(makeTypeErrorMessage("Cannot add", "+", lhs, rhs)));
+                throw ValueError(FString(makeTypeErrorMessage("Cannot add", "+", lhs, rhs)));
             if (lhs.isNumeric() && rhs.isNumeric())
             {
                 bool bothInt = lhs.is<ValueType::IntClass>() && rhs.is<ValueType::IntClass>();
@@ -202,13 +374,13 @@ namespace Fig
             }
             if (lhs.is<ValueType::StringClass>() && rhs.is<ValueType::StringClass>())
                 return Object(FString(lhs.as<ValueType::StringClass>() + rhs.as<ValueType::StringClass>()));
-            throw ValueError(FStringView(makeTypeErrorMessage("Unsupported operation", "+", lhs, rhs)));
+            throw ValueError(FString(makeTypeErrorMessage("Unsupported operation", "+", lhs, rhs)));
         }
 
         friend Object operator-(const Object &lhs, const Object &rhs)
         {
             if (lhs.isNull() || rhs.isNull())
-                throw ValueError(FStringView(makeTypeErrorMessage("Cannot subtract", "-", lhs, rhs)));
+                throw ValueError(FString(makeTypeErrorMessage("Cannot subtract", "-", lhs, rhs)));
             if (lhs.isNumeric() && rhs.isNumeric())
             {
                 bool bothInt = lhs.is<ValueType::IntClass>() && rhs.is<ValueType::IntClass>();
@@ -217,13 +389,13 @@ namespace Fig
                     return Object(static_cast<ValueType::IntClass>(result));
                 return Object(result);
             }
-            throw ValueError(FStringView(makeTypeErrorMessage("Unsupported operation", "-", lhs, rhs)));
+            throw ValueError(FString(makeTypeErrorMessage("Unsupported operation", "-", lhs, rhs)));
         }
 
         friend Object operator*(const Object &lhs, const Object &rhs)
         {
             if (lhs.isNull() || rhs.isNull())
-                throw ValueError(FStringView(makeTypeErrorMessage("Cannot multiply", "*", lhs, rhs)));
+                throw ValueError(FString(makeTypeErrorMessage("Cannot multiply", "*", lhs, rhs)));
             if (lhs.isNumeric() && rhs.isNumeric())
             {
                 bool bothInt = lhs.is<ValueType::IntClass>() && rhs.is<ValueType::IntClass>();
@@ -232,82 +404,82 @@ namespace Fig
                     return Object(static_cast<ValueType::IntClass>(result));
                 return Object(result);
             }
-            throw ValueError(FStringView(makeTypeErrorMessage("Unsupported operation", "*", lhs, rhs)));
+            throw ValueError(FString(makeTypeErrorMessage("Unsupported operation", "*", lhs, rhs)));
         }
 
         friend Object operator/(const Object &lhs, const Object &rhs)
         {
             if (lhs.isNull() || rhs.isNull())
-                throw ValueError(FStringView(makeTypeErrorMessage("Cannot divide", "/", lhs, rhs)));
+                throw ValueError(FString(makeTypeErrorMessage("Cannot divide", "/", lhs, rhs)));
             if (lhs.isNumeric() && rhs.isNumeric())
             {
                 auto rnv = rhs.getNumericValue();
                 if (rnv == 0)
-                    throw ValueError(FStringView(makeTypeErrorMessage("Division by zero", "/", lhs, rhs)));
+                    throw ValueError(FString(makeTypeErrorMessage("Division by zero", "/", lhs, rhs)));
                 bool bothInt = lhs.is<ValueType::IntClass>() && rhs.is<ValueType::IntClass>();
                 auto result = lhs.getNumericValue() / rnv;
                 if (bothInt && !isNumberExceededIntLimit(result))
                     return Object(static_cast<ValueType::IntClass>(result));
                 return Object(result);
             }
-            throw ValueError(FStringView(makeTypeErrorMessage("Unsupported operation", "/", lhs, rhs)));
+            throw ValueError(FString(makeTypeErrorMessage("Unsupported operation", "/", lhs, rhs)));
         }
 
         friend Object operator%(const Object &lhs, const Object &rhs)
         {
             if (lhs.isNull() || rhs.isNull())
-                throw ValueError(FStringView(makeTypeErrorMessage("Cannot modulo", "%", lhs, rhs)));
+                throw ValueError(FString(makeTypeErrorMessage("Cannot modulo", "%", lhs, rhs)));
             if (lhs.isNumeric() && rhs.isNumeric())
             {
                 auto rnv = rhs.getNumericValue();
                 if (rnv == 0)
-                    throw ValueError(FStringView(makeTypeErrorMessage("Modulo by zero", "/", lhs, rhs)));
+                    throw ValueError(FString(makeTypeErrorMessage("Modulo by zero", "/", lhs, rhs)));
                 bool bothInt = lhs.is<ValueType::IntClass>() && rhs.is<ValueType::IntClass>();
                 auto result = std::fmod(lhs.getNumericValue(), rnv);
                 if (bothInt && !isNumberExceededIntLimit(result))
                     return Object(static_cast<ValueType::IntClass>(result));
                 return Object(result);
             }
-            throw ValueError(FStringView(makeTypeErrorMessage("Unsupported operation", "%", lhs, rhs)));
+            throw ValueError(FString(makeTypeErrorMessage("Unsupported operation", "%", lhs, rhs)));
         }
 
         // logic
         friend Object operator&&(const Object &lhs, const Object &rhs)
         {
             if (!lhs.is<ValueType::BoolClass>() || !rhs.is<ValueType::BoolClass>())
-                throw ValueError(FStringView(makeTypeErrorMessage("Logical AND requires bool", "&&", lhs, rhs)));
+                throw ValueError(FString(makeTypeErrorMessage("Logical AND requires bool", "&&", lhs, rhs)));
             return Object(lhs.as<ValueType::BoolClass>() && rhs.as<ValueType::BoolClass>());
         }
 
         friend Object operator||(const Object &lhs, const Object &rhs)
         {
             if (!lhs.is<ValueType::BoolClass>() || !rhs.is<ValueType::BoolClass>())
-                throw ValueError(FStringView(makeTypeErrorMessage("Logical OR requires bool", "||", lhs, rhs)));
+                throw ValueError(FString(makeTypeErrorMessage("Logical OR requires bool", "||", lhs, rhs)));
             return Object(lhs.as<ValueType::BoolClass>() || rhs.as<ValueType::BoolClass>());
         }
 
         friend Object operator!(const Object &v)
         {
             if (!v.is<ValueType::BoolClass>())
-                throw ValueError(FStringView(std::format("Logical NOT requires bool: '{}'", v.getTypeInfo().name.toBasicString())));
+                throw ValueError(FString(std::format("Logical NOT requires bool: '{}'", v.getTypeInfo().name.toBasicString())));
             return Object(!v.as<ValueType::BoolClass>());
         }
 
         friend Object operator-(const Object &v)
         {
             if (v.isNull())
-                throw ValueError(FStringView(u8"Unary minus cannot be applied to null"));
+                throw ValueError(FString(u8"Unary minus cannot be applied to null"));
             if (v.is<ValueType::IntClass>())
                 return Object(-v.as<ValueType::IntClass>());
             if (v.is<ValueType::DoubleClass>())
                 return Object(-v.as<ValueType::DoubleClass>());
-            throw ValueError(FStringView(std::format("Unary minus requires int or double: '{}'", v.getTypeInfo().name.toBasicString())));
+            throw ValueError(FString(std::format("Unary minus requires int or double: '{}'", v.getTypeInfo().name.toBasicString())));
         }
 
         friend Object operator~(const Object &v)
         {
             if (!v.is<ValueType::IntClass>())
-                throw ValueError(FStringView(std::format("Bitwise NOT requires int: '{}'", v.getTypeInfo().name.toBasicString())));
+                throw ValueError(FString(std::format("Bitwise NOT requires int: '{}'", v.getTypeInfo().name.toBasicString())));
             return Object(~v.as<ValueType::IntClass>());
         }
 
@@ -319,7 +491,7 @@ namespace Fig
             if (lhs.isNumeric() && rhs.isNumeric()) return lhs.getNumericValue() < rhs.getNumericValue();
             if (lhs.is<ValueType::StringClass>() && rhs.is<ValueType::StringClass>())
                 return lhs.as<ValueType::StringClass>() < rhs.as<ValueType::StringClass>();
-            throw ValueError(FStringView(makeTypeErrorMessage("Unsupported comparison", "<", lhs, rhs)));
+            throw ValueError(FString(makeTypeErrorMessage("Unsupported comparison", "<", lhs, rhs)));
         }
         friend bool operator<=(const Object &lhs, const Object &rhs) { return lhs == rhs || lhs < rhs; }
         friend bool operator>(const Object &lhs, const Object &rhs)
@@ -327,7 +499,7 @@ namespace Fig
             if (lhs.isNumeric() && rhs.isNumeric()) return lhs.getNumericValue() > rhs.getNumericValue();
             if (lhs.is<ValueType::StringClass>() && rhs.is<ValueType::StringClass>())
                 return lhs.as<ValueType::StringClass>() > rhs.as<ValueType::StringClass>();
-            throw ValueError(FStringView(makeTypeErrorMessage("Unsupported comparison", ">", lhs, rhs)));
+            throw ValueError(FString(makeTypeErrorMessage("Unsupported comparison", ">", lhs, rhs)));
         }
         friend bool operator>=(const Object &lhs, const Object &rhs) { return lhs == rhs || lhs > rhs; }
 
@@ -335,49 +507,49 @@ namespace Fig
         friend Object bit_and(const Object &lhs, const Object &rhs)
         {
             if (!lhs.is<ValueType::IntClass>() || !rhs.is<ValueType::IntClass>())
-                throw ValueError(FStringView(makeTypeErrorMessage("Bitwise AND requires int", "&", lhs, rhs)));
+                throw ValueError(FString(makeTypeErrorMessage("Bitwise AND requires int", "&", lhs, rhs)));
             return Object(lhs.as<ValueType::IntClass>() & rhs.as<ValueType::IntClass>());
         }
 
         friend Object bit_or(const Object &lhs, const Object &rhs)
         {
             if (!lhs.is<ValueType::IntClass>() || !rhs.is<ValueType::IntClass>())
-                throw ValueError(FStringView(makeTypeErrorMessage("Bitwise OR requires int", "|", lhs, rhs)));
+                throw ValueError(FString(makeTypeErrorMessage("Bitwise OR requires int", "|", lhs, rhs)));
             return Object(lhs.as<ValueType::IntClass>() | rhs.as<ValueType::IntClass>());
         }
 
         friend Object bit_xor(const Object &lhs, const Object &rhs)
         {
             if (!lhs.is<ValueType::IntClass>() || !rhs.is<ValueType::IntClass>())
-                throw ValueError(FStringView(makeTypeErrorMessage("Bitwise XOR requires int", "^", lhs, rhs)));
+                throw ValueError(FString(makeTypeErrorMessage("Bitwise XOR requires int", "^", lhs, rhs)));
             return Object(lhs.as<ValueType::IntClass>() ^ rhs.as<ValueType::IntClass>());
         }
 
         friend Object bit_not(const Object &v)
         {
             if (!v.is<ValueType::IntClass>())
-                throw ValueError(FStringView(std::format("Bitwise NOT requires int: '{}'", v.getTypeInfo().name.toBasicString())));
+                throw ValueError(FString(std::format("Bitwise NOT requires int: '{}'", v.getTypeInfo().name.toBasicString())));
             return Object(~v.as<ValueType::IntClass>());
         }
 
         friend Object shift_left(const Object &lhs, const Object &rhs)
         {
             if (!lhs.is<ValueType::IntClass>() || !rhs.is<ValueType::IntClass>())
-                throw ValueError(FStringView(makeTypeErrorMessage("Shift left requires int", "<<", lhs, rhs)));
+                throw ValueError(FString(makeTypeErrorMessage("Shift left requires int", "<<", lhs, rhs)));
             return Object(lhs.as<ValueType::IntClass>() << rhs.as<ValueType::IntClass>());
         }
 
         friend Object shift_right(const Object &lhs, const Object &rhs)
         {
             if (!lhs.is<ValueType::IntClass>() || !rhs.is<ValueType::IntClass>())
-                throw ValueError(FStringView(makeTypeErrorMessage("Shift right requires int", ">>", lhs, rhs)));
+                throw ValueError(FString(makeTypeErrorMessage("Shift right requires int", ">>", lhs, rhs)));
             return Object(lhs.as<ValueType::IntClass>() >> rhs.as<ValueType::IntClass>());
         }
 
         friend Object power(const Object &base, const Object &exp)
         {
             if (base.isNull() || exp.isNull())
-                throw ValueError(FStringView(makeTypeErrorMessage("Cannot exponentiate", "**", base, exp)));
+                throw ValueError(FString(makeTypeErrorMessage("Cannot exponentiate", "**", base, exp)));
             if (base.isNumeric() && exp.isNumeric())
             {
                 bool bothInt = base.is<ValueType::IntClass>() && exp.is<ValueType::IntClass>();
@@ -386,68 +558,17 @@ namespace Fig
                     return Object(static_cast<ValueType::IntClass>(result));
                 return Object(result);
             }
-            throw ValueError(FStringView(makeTypeErrorMessage("Unsupported operation", "**", base, exp)));
+            throw ValueError(FString(makeTypeErrorMessage("Unsupported operation", "**", base, exp)));
         }
     };
 
     using ObjectPtr = std::shared_ptr<Object>;
     using RvObject = ObjectPtr;
 
-    struct VariableSlot
+
+    inline bool operator==(const ValueKey &l, const ValueKey &r)
     {
-        FString name;
-        ObjectPtr value;
-        TypeInfo declaredType;
-        AccessModifier am;
-
-        bool isRef = false;
-        std::shared_ptr<VariableSlot> refTarget;
-    };
-
-    struct LvObject
-    {
-        std::shared_ptr<VariableSlot> slot;
-
-        const ObjectPtr& get() const
-        {
-            auto s = resolve(slot);
-            return s->value;
-        }
-
-        void set(const ObjectPtr& v)
-        {
-            auto s = resolve(slot);
-            if (s->declaredType != ValueType::Any && s->declaredType != v->getTypeInfo())
-            {
-                throw RuntimeError(
-                    FStringView(
-                        std::format("Variable `{}` expects type `{}`, but got '{}'",
-                            s->name.toBasicString(),
-                            s->declaredType.toString().toBasicString(),
-                            v->getTypeInfo().toString().toBasicString())
-                    )
-                );
-            }
-            if (isAccessConst(s->am))
-            {
-                throw RuntimeError(FStringView(
-                    std::format("Variable `{}` is immutable", s->name.toBasicString())
-                ));
-            }
-            s->value = v;
-        }
-
-        FString name() const { return resolve(slot)->name; }
-        TypeInfo declaredType() const { return resolve(slot)->declaredType; }
-        AccessModifier access() const { return resolve(slot)->am; }
-
-    private:
-        std::shared_ptr<VariableSlot> resolve(std::shared_ptr<VariableSlot> s) const
-        {
-            while (s->isRef) s = s->refTarget;
-            return s;
-        }
-    };
-
+        return *l.value == *r.value;
+    }
 
 } // namespace Fig