diff --git a/README.md b/README.md
index f5c5c0e..3fabe9b 100644
--- a/README.md
+++ b/README.md
@@ -1,8 +1,10 @@
 # Nasal Script Language
 
+## Introduction
+
 [Nasal](http://wiki.flightgear.org/Nasal_scripting_language) is a script language that used in [FlightGear](https://www.flightgear.org/).
 
-The interpreter is totally rewritten by ValKmjolnir using C++(standard c++11) without reusing the code in Andy Ross's nasal interpreter(https://github.com/andyross/nasal). But we really appreciate that Andy created this amazing programming language and his interpreter project.
+The interpreter is totally rewritten by ValKmjolnir using C++(standard c++11) without reusing the code in Andy Ross's nasal interpreter(<https://github.com/andyross/nasal>). But we really appreciate that Andy created this amazing programming language and his interpreter project.
 
 The interpreter is still in development(now it works well --2021/2/15). We really need your support!
 
@@ -10,9 +12,9 @@ Also,i am a member of [FGPRC](https://www.fgprc.org/), welcome to join us!
 
 (2021/5/4) Now this project uses MIT license.Edit it if you want, use this project to learn or create more interesting things(But don't forget me XD).
 
-# Why Writing Nasal Interpreter
+## Why Writing Nasal Interpreter
 
-Nasal is a script language first used in Flightgear, created by Andy Ross(https://github.com/andyross).
+Nasal is a script language first used in Flightgear, created by Andy Ross(<https://github.com/andyross>).
 
 But in last summer holiday, members in FGPRC told me that it is hard to debug with nasal-console in Flightgear, especially when checking syntax errors.
 
@@ -26,7 +28,7 @@ Also, you could use this language to write some interesting programs and run the
 
 You could add your own built-in functions to change this interpreter to a useful tool in your own projects(such as a script in your own game).
 
-# How to Compile
+## How to Compile
 
 Better choose the latest update of the interpreter.
 
@@ -34,7 +36,7 @@ MUST USE -O2 ! pragma gcc optimize(2) seems useless when using g++
 
 > g++ -std=c++11 -O2 main.cpp -o main.exe
 
-# Parser
+## Parser
 
 LL(k) parser.
 
@@ -47,7 +49,7 @@ These two expressions have the same first set,so LL(1) is useless for this langu
 
 Maybe in the future i can refactor it to LL(1) with special checks.
 
-## version 1.0(last update 2019/10/14)
+### version 1.0(last update 2019/10/14)
 
 First fully functional version of nasal_parser.
 
@@ -59,17 +61,17 @@ In version 0.17(2019/9/15) 0.18(2019/9/18) 0.19(2019/10/1)i was playing the pars
 
 This project began at 2019/8/31.
 
-# Abstract Syntax Tree
+## Abstract Syntax Tree
 
-## Version 1.2(last update 2019/10/31)
+### Version 1.2(last update 2019/10/31)
 
 The ast has been completed in this version.
 
-## Version 2.0(last update 2020/8/31)
+### Version 2.0(last update 2020/8/31)
 
 A completed ast-interpreter with unfinished lib functions.
 
-## Version 3.0(last update 2020/10/23)
+### Version 3.0(last update 2020/10/23)
 
 The ast is refactored and is now easier to read and maintain.
 
@@ -79,15 +81,15 @@ Now you can add your own functions as builtin-functions in this interpreter!
 
 I decide to save the ast interpreter after releasing v4.0. Because it took me a long time to think and write...
 
-## Version 5.0(last update 2021/3/7)
+### Version 5.0(last update 2021/3/7)
 
 I change my mind.AST interpreter leaves me too much things to do.
 
 If i continue saving this interpreter,it will be harder for me to make the bytecode vm become more efficient.
 
-# Byte Code Interpreter
+## Byte Code Interpreter
 
-## Version 4.0(last update 2020/12/17)
+### Version 4.0(last update 2020/12/17)
 
 I have just finished the first version of byte-code-interpreter.
 
@@ -120,7 +122,7 @@ for(var i=0;i<4000000;i+=1);
 0x0000000b: nop    0x00000000
 ```
 
-## Version 5.0(last update 2021/3/7)
+### Version 5.0(last update 2021/3/7)
 
 I decide to optimize bytecode vm in this version.
 
@@ -128,7 +130,7 @@ Because it takes more than 1.5s to count i from 0 to 4000000-1.This is not effic
 
 2021/1/23 update: Now it can count from 0 to 4000000-1 in 1.5s.
 
-## Version 6.0(last update 2021/6/1)
+### Version 6.0(last update 2021/6/1)
 
 Use loadg loadl callg calll mcallg mcalll to avoid branches.
 
@@ -169,7 +171,7 @@ for(var i=0;i<4000000;i+=1);
 0x0000000c: nop    0x00000000
 ```
 
-## Version 6.5(latest)
+### Version 6.5(latest)
 
 2021/5/31 update: Now gc can collect garbage correctly without re-collecting,which will cause fatal error.
 
@@ -213,9 +215,9 @@ f(1024,2048);
 0x00000011: nop    0x00000000
 ```
 
-# How to Use Nasal to Program
+## How to Use Nasal to Program
 
-## basic value type
+### basic value type
 
 Nasal has 6 value types.Number,string,vector,hash,function,nil.
 
@@ -281,7 +283,7 @@ var f=func(x,y,z,dynamic_para...)
 }
 ```
 
-## operators
+### operators
 
 ```javascript
 1+2;
@@ -311,7 +313,7 @@ a/=1;
 a~='string';
 ```
 
-## definition
+### definition
 
 ```javascript
 var a=1;
@@ -321,14 +323,14 @@ var (a,b,c)=(0,1,2);
 (var a,b,c)=(0,1,2);
 ```
 
-## multi-assignment
+### multi-assignment
 
 ```javascript
 (a,b[0],c.d)=[0,1,2];
 (a,b[1],c.e)=(0,1,2);
 ```
 
-## conditional expression
+### conditional expression
 
 ```javascript
 if(1)
@@ -349,7 +351,7 @@ else
 }
 ```
 
-## loop
+### loop
 
 ```javascript
 while(condition)
@@ -365,7 +367,7 @@ foreach(var i;elem)
     print(i);
 ```
 
-## subvec
+### subvec
 
 Use index to search one element in the string will get the ascii number of this character.If you want to get the character,use built-in function chr().
 
@@ -374,7 +376,7 @@ a[-1,1,0:2,0:,:3,:,nil:8,3:nil,nil:nil];
 "hello world"[0];
 ```
 
-## special function call
+### special function call
 
 This is of great use but is not very efficient(because hashmap use string as the key to compare).
 
@@ -382,7 +384,7 @@ This is of great use but is not very efficient(because hashmap use string as the
 a(x:0,y:1,z:2);
 ```
 
-## lambda
+### lambda
 
 Also functions have this kind of use:
 
@@ -391,7 +393,7 @@ func(x,y){return x+y}(0,1);
 func(x){return 1/(1+math.exp(-x));}(0.5);
 ```
 
-## closure
+### closure
 
 Use closure to OOP.
 
@@ -419,7 +421,7 @@ var student=func(name,age)
 }
 ```
 
-## built-in functions
+### built-in functions
 
 Must import lib.nas or has these functions' definitions inside your code.
 
@@ -522,11 +524,3 @@ nasal_val* builtin_getkeys(std::vector<nasal_val*>& local_scope,nasal_gc& gc)
     return ret_addr;
 }
 ```
-
-# 写在最后
-
-我要怎么才能告诉gitee的逆天管理员gitee上的ValKmjolnir和github的ValKmjolnir是一个人？
-
-他居然告诉我，我从github同步的仓库是他人项目？
-
-我证明我是我自己？:sweat_smile:
\ No newline at end of file
diff --git a/main.cpp b/main.cpp
index e0c7f9d..67a85d6 100644
--- a/main.cpp
+++ b/main.cpp
@@ -28,7 +28,7 @@ void logo()
     return;
 }
 
-void die(std::string stage,std::string filename)
+void die(const char* stage,std::string& filename)
 {
 	std::cout<<">> ["<<stage<<"] in <\""<<filename<<"\">: error(s) occurred,stop.\n";
 	return;
diff --git a/nasal_ast.h b/nasal_ast.h
index c43528f..90fbbce 100644
--- a/nasal_ast.h
+++ b/nasal_ast.h
@@ -4,8 +4,7 @@
 enum ast_node
 {
     ast_null=0,
-    ast_root,
-    ast_block,
+    ast_root,ast_block,
     ast_nil,ast_num,ast_str,ast_id,ast_func,ast_hash,ast_vec,
     ast_hashmember,ast_call,ast_callh,ast_callv,ast_callf,ast_subvec,
     ast_args,ast_default_arg,ast_dynamic_id,
@@ -20,74 +19,31 @@ enum ast_node
     ast_for,ast_forindex,ast_foreach,ast_while,ast_new_iter,
     ast_conditional,ast_if,ast_elsif,ast_else,
     ast_multi_id,ast_multi_scalar,
-    ast_definition,ast_multi_assign,
-    ast_continue,ast_break,ast_return
+    ast_def,ast_multi_assign,
+    ast_continue,ast_break,ast_ret
 };
 
-std::string ast_name(int type)
+const char* ast_name[]=
 {
-    switch(type)
-    {
-        case ast_null:         return "null";
-        case ast_root:         return "root";
-        case ast_block:        return "block";
-        case ast_nil:          return "nil";
-        case ast_num:          return "number";
-        case ast_str:          return "string";
-        case ast_id:           return "id";
-        case ast_func:         return "function";
-        case ast_hash:         return "hash";
-        case ast_vec:          return "vector";
-        case ast_hashmember:   return "hashmember";
-        case ast_call:         return "call";
-        case ast_callh:        return "callh";
-        case ast_callv:        return "callv";
-        case ast_callf:        return "callf";
-        case ast_subvec:       return "subvec";
-        case ast_args:         return "args";
-        case ast_default_arg:  return "deflt_arg";
-        case ast_dynamic_id:   return "dyn_id";
-        case ast_and:          return "and";
-        case ast_or:           return "or";
-        case ast_equal:        return "=";
-        case ast_addeq:        return "+=";
-        case ast_subeq:        return "-=";
-        case ast_multeq:       return "*=";
-        case ast_diveq:        return "/=";
-        case ast_lnkeq:        return "~=";
-        case ast_cmpeq:        return "==";
-        case ast_neq:          return "!=";
-        case ast_less:         return "<";
-        case ast_leq:          return "<=";
-        case ast_grt:          return ">";
-        case ast_geq:          return ">=";
-        case ast_add:          return "+";
-        case ast_sub:          return "-";
-        case ast_mult:         return "*";
-        case ast_div:          return "/";
-        case ast_link:         return "~";
-        case ast_neg:          return "unary-";
-        case ast_not:          return "unary!";
-        case ast_trino:        return "trino";
-        case ast_for:          return "for";
-        case ast_forindex:     return "forindex";
-        case ast_foreach:      return "foreach";
-        case ast_while:        return "while";
-        case ast_new_iter:     return "iter";
-        case ast_conditional:  return "conditional";
-        case ast_if:           return "if";
-        case ast_elsif:        return "elsif";
-        case ast_else:         return "else";
-        case ast_multi_id:     return "multi_id";
-        case ast_multi_scalar: return "multi_scalar";
-        case ast_definition:   return "def";
-        case ast_multi_assign: return "multi_assign";
-        case ast_continue:     return "continue";
-        case ast_break:        return "break";
-        case ast_return:       return "return";
-    }
-    return "null";
-}
+    "null",
+    "root","block",
+    "nil","num","str","id","func","hash","vec",
+    "hashmember","call","callh","callv","callf","subvec",
+    "args","deflt_arg","dyn_id",
+    "and","or",
+    "=","+=","-=","*=","/=","~=",
+    "==","!=",
+    "<","<=",
+    ">",">=",
+    "+","-","*","/","~",
+    "unary-","unary!",
+    "trino",
+    "for","forindex","foreach","while","iter",
+    "conditional","if","elsif","else",
+    "multi_id","multi_scalar",
+    "def","multi_assign",
+    "continue","break","return"
+};
 
 class nasal_ast
 {
@@ -98,39 +54,24 @@ private:
     std::string str;
     std::vector<nasal_ast> children;
 public:
-    nasal_ast();
-    nasal_ast(int,int);
+    nasal_ast(){line=0;type=ast_null;}
+    nasal_ast(int l,int t){line=l;type=t;}
     nasal_ast(const nasal_ast&);
-    ~nasal_ast();
     nasal_ast&  operator=(const nasal_ast&);
     void   print_ast(int);
     void   clear();
-    void   set_line(int);
-    void   set_type(int);
-    void   set_str(std::string&);
-    void   set_num(double);
-    void   add_child(nasal_ast);
-    int    get_line();
-    int    get_type();
-    double get_num();
-    std::string& get_str();
-    std::vector<nasal_ast>& get_children();
+    void   add_child(nasal_ast ast){children.push_back(ast);}
+    void   set_line(int l){line=l;}
+    void   set_type(int t){type=t;}
+    void   set_str(std::string& s){str=s;}
+    void   set_num(double n){num=n;}
+    int    get_line(){return line;}
+    int    get_type(){return type;}
+    double get_num(){return num;}
+    std::string& get_str(){return str;}
+    std::vector<nasal_ast>& get_children(){return children;}
 };
 
-nasal_ast::nasal_ast()
-{
-    line=0;
-    type=ast_null;
-    return;
-}
-
-nasal_ast::nasal_ast(int _line,int _type)
-{
-    line=_line;
-    type=_type;
-    return;
-}
-
 nasal_ast::nasal_ast(const nasal_ast& tmp)
 {
     line=tmp.line;
@@ -141,12 +82,6 @@ nasal_ast::nasal_ast(const nasal_ast& tmp)
     return;
 }
 
-nasal_ast::~nasal_ast()
-{
-    children.clear();
-    return;
-}
-
 nasal_ast& nasal_ast::operator=(const nasal_ast& tmp)
 {
     line=tmp.line;
@@ -167,67 +102,12 @@ void nasal_ast::clear()
     return;
 }
 
-void nasal_ast::set_line(int l)
-{
-    line=l;
-    return;
-}
-
-void nasal_ast::set_type(int t)
-{
-    type=t;
-    return;
-}
-
-void nasal_ast::set_str(std::string& s)
-{
-    str=s;
-    return;
-}
-
-void nasal_ast::set_num(double n)
-{
-    num=n;
-    return;
-}
-
-void nasal_ast::add_child(nasal_ast ast)
-{
-    children.push_back(ast);
-    return;
-}
-
-int nasal_ast::get_line()
-{
-    return line;
-}
-
-int nasal_ast::get_type()
-{
-    return type;
-}
-
-std::string& nasal_ast::get_str()
-{
-    return str;
-}
-
-double nasal_ast::get_num()
-{
-    return num;
-}
-
-std::vector<nasal_ast>& nasal_ast::get_children()
-{
-    return children;
-}
-
 void nasal_ast::print_ast(int depth)
 {
     std::string indentation="";
     for(int i=0;i<depth;++i)
         indentation+="|  ";
-    std::cout<<indentation<<ast_name(type);
+    std::cout<<indentation<<ast_name[type];
     if(type==ast_str || type==ast_id || type==ast_dynamic_id || type==ast_callh)
         std::cout<<":"<<str;
     else if(type==ast_num)
diff --git a/nasal_builtin.h b/nasal_builtin.h
index 4eaa9e6..04dafe6 100644
--- a/nasal_builtin.h
+++ b/nasal_builtin.h
@@ -55,7 +55,7 @@ nasal_val* builtin_right(std::vector<nasal_val*>&,nasal_gc&);
 nasal_val* builtin_cmp(std::vector<nasal_val*>&,nasal_gc&);
 nasal_val* builtin_chr(std::vector<nasal_val*>&,nasal_gc&);
 
-void builtin_err(std::string func_name,std::string info)
+void builtin_err(const char* func_name,std::string info)
 {
     std::cout<<">> [vm] "<<func_name<<": "<<info<<".\n";
     return;
@@ -65,7 +65,7 @@ void builtin_err(std::string func_name,std::string info)
 // this table must end with {"",NULL}
 struct FUNC_TABLE
 {
-    std::string name;
+    const char* name;
     nasal_val* (*func)(std::vector<nasal_val*>&,nasal_gc&);
 } builtin_func[]=
 {
diff --git a/nasal_codegen.h b/nasal_codegen.h
index 56a933b..fe0e074 100644
--- a/nasal_codegen.h
+++ b/nasal_codegen.h
@@ -216,13 +216,13 @@ private:
     void ret_gen(nasal_ast&);
 public:
     nasal_codegen();
-    int                       get_error();
+    int                       get_error(){return error;}
     void                      main_progress(nasal_ast&);
     void                      print_op(int);
     void                      print_byte_code();
-    std::vector<std::string>& get_str_table();
-    std::vector<double>&      get_num_table();
-    std::vector<opcode>&      get_exec_code();
+    std::vector<std::string>& get_str_table(){return str_res_table;}
+    std::vector<double>&      get_num_table(){return num_res_table;}
+    std::vector<opcode>&      get_exec_code(){return exec_code;}
 };
 
 nasal_codegen::nasal_codegen()
@@ -420,7 +420,7 @@ void nasal_codegen::func_gen(nasal_ast& ast)
         exec_code[local_label].num+=i.size();
     local.pop_back();
 
-    if(!block.get_children().size() || block.get_children().back().get_type()!=ast_return)
+    if(!block.get_children().size() || block.get_children().back().get_type()!=ast_ret)
     {
         nil_gen();
         gen(op_ret,0);
@@ -708,10 +708,10 @@ void nasal_codegen::loop_gen(nasal_ast& ast)
 
 void nasal_codegen::load_continue_break(int continue_place,int break_place)
 {
-    for(int i=0;i<continue_ptr.front().size();++i)
-        exec_code[continue_ptr.front()[i]].num=continue_place;
-    for(int i=0;i<break_ptr.front().size();++i)
-        exec_code[break_ptr.front()[i]].num=break_place;
+    for(auto i:continue_ptr.front())
+        exec_code[i].num=continue_place;
+    for(auto i:break_ptr.front())
+        exec_code[i].num=break_place;
     continue_ptr.pop_front();
     break_ptr.pop_front();
     return;
@@ -737,7 +737,7 @@ void nasal_codegen::for_gen(nasal_ast& ast)
     switch(ast.get_children()[0].get_type())
     {
         case ast_null:break;
-        case ast_definition:def_gen(ast.get_children()[0]);break;
+        case ast_def:def_gen(ast.get_children()[0]);break;
         case ast_multi_assign:multi_assign_gen(ast.get_children()[0]);break;
         case ast_nil:
         case ast_num:
@@ -780,7 +780,7 @@ void nasal_codegen::for_gen(nasal_ast& ast)
     switch(ast.get_children()[2].get_type())
     {
         case ast_null:break;
-        case ast_definition:def_gen(ast.get_children()[2]);break;
+        case ast_def:def_gen(ast.get_children()[2]);break;
         case ast_multi_assign:multi_assign_gen(ast.get_children()[2]);break;
         case ast_nil:case ast_num:case ast_str:case ast_func:break;
         case ast_vec:case ast_hash:
@@ -955,7 +955,7 @@ void nasal_codegen::calc_gen(nasal_ast& ast)
             calc_gen(ast.get_children()[0]);
             gen(op_unot,0);
             break;
-        case ast_definition:
+        case ast_def:
             single_def(ast);
             call_id(ast.get_children()[0]);
             break;
@@ -969,7 +969,7 @@ void nasal_codegen::block_gen(nasal_ast& ast)
         switch(tmp.get_type())
         {
             case ast_null:case ast_nil:case ast_num:case ast_str:case ast_func:break;
-            case ast_definition:def_gen(tmp);break;
+            case ast_def:def_gen(tmp);break;
             case ast_multi_assign:multi_assign_gen(tmp);break;
             case ast_conditional:conditional_gen(tmp);break;
             case ast_continue:
@@ -1010,7 +1010,7 @@ void nasal_codegen::block_gen(nasal_ast& ast)
             case ast_or:
             case ast_and:
             case ast_trino:calc_gen(tmp);pop_gen();break;
-            case ast_return:ret_gen(tmp);break;
+            case ast_ret:ret_gen(tmp);break;
         }
     return;
 }
@@ -1045,14 +1045,12 @@ void nasal_codegen::main_progress(nasal_ast& ast)
     global.clear();
     local.clear();
     gen(op_intg,0);
-    int size=ast.get_children().size();
-    for(int i=0;i<size;++i)
+    for(auto& tmp:ast.get_children())
     {
-        nasal_ast& tmp=ast.get_children()[i];
         switch(tmp.get_type())
         {
             case ast_null:case ast_nil:case ast_num:case ast_str:case ast_func:break;
-            case ast_definition:def_gen(tmp);break;
+            case ast_def:def_gen(tmp);break;
             case ast_multi_assign:multi_assign_gen(tmp);break;
             case ast_conditional:conditional_gen(tmp);break;
             case ast_while:
@@ -1100,31 +1098,22 @@ void nasal_codegen::main_progress(nasal_ast& ast)
 
 void nasal_codegen::print_op(int index)
 {
-    // print opcode ptr
-    printf("0x%.8x: ",index);
-    // print opcode name
-    for(int i=0;code_table[i].name;++i)
-        if(exec_code[index].op==code_table[i].type)
-        {
-            std::cout<<code_table[i].name<<" ";
-            break;
-        }
-    // print opcode index
-    printf("0x%.8x ",exec_code[index].num);
+    // print opcode index,opcode name,opcode immediate number
+    printf("0x%.8x: %s 0x%.8x",index,code_table[exec_code[index].op].name,exec_code[index].num);
     // print detail info
     switch(exec_code[index].op)
     {
-        case op_pnum:std::cout<<'('<<num_res_table[exec_code[index].num]<<')';break;
-        case op_callb:std::cout<<'('<<builtin_func[exec_code[index].num].name<<')';break;
+        case op_pnum:printf(" (%lf)\n",num_res_table[exec_code[index].num]);break;
+        case op_callb:printf(" (%s)\n",builtin_func[exec_code[index].num].name);break;
         case op_happ:
         case op_pstr:
         case op_callh:
         case op_mcallh:
         case op_para:
         case op_defpara:
-        case op_dynpara:std::cout<<'('<<str_res_table[exec_code[index].num]<<')';break;
+        case op_dynpara:printf(" (%s)\n",str_res_table[exec_code[index].num].c_str());break;
+        default:printf("\n");break;
     }
-    std::cout<<'\n';
     return;
 }
 
@@ -1134,30 +1123,9 @@ void nasal_codegen::print_byte_code()
         std::cout<<".number "<<num<<'\n';
     for(auto& str:str_res_table)
         std::cout<<".symbol "<<str<<'\n';
-    int size=exec_code.size();
-    for(int i=0;i<size;++i)
+    for(int i=0;i<exec_code.size();++i)
         print_op(i);
     return;
 }
 
-std::vector<std::string>& nasal_codegen::get_str_table()
-{
-    return str_res_table;
-}
-
-std::vector<double>& nasal_codegen::get_num_table()
-{
-    return num_res_table;
-}
-
-std::vector<opcode>& nasal_codegen::get_exec_code()
-{
-    return exec_code;
-}
-
-int nasal_codegen::get_error()
-{
-    return error;
-}
-
 #endif
\ No newline at end of file
diff --git a/nasal_gc.h b/nasal_gc.h
index 8217226..ce2f94f 100644
--- a/nasal_gc.h
+++ b/nasal_gc.h
@@ -242,11 +242,11 @@ std::string nasal_val::to_string()
 
 struct nasal_gc
 {
-#define STACK_MAX_DEPTH (65536<<4)
+#define STACK_MAX_DEPTH (65536<<2)
     nasal_val*              zero_addr;               // reserved address of nasal_val,type vm_num, 0
     nasal_val*              one_addr;                // reserved address of nasal_val,type vm_num, 1
     nasal_val*              nil_addr;                // reserved address of nasal_val,type vm_nil
-    nasal_val*              val_stack[STACK_MAX_DEPTH];
+    nasal_val*              val_stack[STACK_MAX_DEPTH+16];// 16 reserved to avoid stack overflow
     nasal_val**             stack_top;               // stack top
     std::vector<nasal_val*> num_addrs;               // reserved address for const vm_num
     std::vector<nasal_val*> str_addrs;               // reserved address for const vm_str
@@ -344,17 +344,15 @@ void nasal_gc::gc_init(std::vector<double>& nums,std::vector<std::string>& strs)
     num_addrs.resize(nums.size());
     for(int i=0;i<nums.size();++i)
     {
-        nasal_val* tmp=new nasal_val(vm_num);
-        tmp->ptr.num=nums[i];
-        num_addrs[i]=tmp;
+        num_addrs[i]=new nasal_val(vm_num);
+        num_addrs[i]->ptr.num=nums[i];
     }
     // init constant strings
     str_addrs.resize(strs.size());
     for(int i=0;i<strs.size();++i)
     {
-        nasal_val* tmp=new nasal_val(vm_str);
-        *tmp->ptr.str=strs[i];
-        str_addrs[i]=tmp;
+        str_addrs[i]=new nasal_val(vm_str);
+        *str_addrs[i]->ptr.str=strs[i];
     }
     return;
 }
diff --git a/nasal_import.h b/nasal_import.h
index 0009ecf..52f651b 100644
--- a/nasal_import.h
+++ b/nasal_import.h
@@ -4,24 +4,24 @@
 class nasal_import
 {
 private:
-    nasal_lexer    import_lex;
-    nasal_parse    import_par;
-    nasal_ast      import_ast;
-    std::vector<std::string> filename_table;
     int error;
-    void die(std::string,std::string);
+    nasal_lexer import_lex;
+    nasal_parse import_par;
+    nasal_ast   import_ast;
+    std::vector<std::string> filename_table;
+    void die(std::string&,const char*);
     bool check_import(nasal_ast&);
-    bool check_exist(std::string);
+    bool check_exist(std::string&);
     void linker(nasal_ast&,nasal_ast&);
     nasal_ast file_import(nasal_ast&);
     nasal_ast load(nasal_ast&);
 public:
-    int  get_error();
+    int  get_error(){return error;}
     void link(nasal_ast&);
-    nasal_ast& get_root();
+    nasal_ast& get_root(){return import_ast;}
 };
 
-void nasal_import::die(std::string filename,std::string error_stage)
+void nasal_import::die(std::string& filename,const char* error_stage)
 {
     ++error;
     std::cout<<">> [import] in <\""<<filename<<"\">: error(s) occurred in "<<error_stage<<".\n";
@@ -51,7 +51,7 @@ only this kind of node can be recognized as 'import':
     return true;
 }
 
-bool nasal_import::check_exist(std::string filename)
+bool nasal_import::check_exist(std::string& filename)
 {
     // avoid importing the same file
     for(auto& fname:filename_table)
@@ -75,7 +75,7 @@ nasal_ast nasal_import::file_import(nasal_ast& node)
     nasal_ast tmp(0,ast_root);
 
     // get filename and set node to ast_null
-    std::string filename=node.get_children()[1].get_children()[0].get_str();
+    std::string& filename=node.get_children()[1].get_children()[0].get_str();
     node.clear();
 
     // avoid infinite loading loop
@@ -131,14 +131,4 @@ void nasal_import::link(nasal_ast& root)
     return;
 }
 
-nasal_ast& nasal_import::get_root()
-{
-    return import_ast;
-}
-
-int nasal_import::get_error()
-{
-    return error;
-}
-
 #endif
\ No newline at end of file
diff --git a/nasal_lexer.h b/nasal_lexer.h
index 6a90739..7762b0d 100644
--- a/nasal_lexer.h
+++ b/nasal_lexer.h
@@ -103,20 +103,20 @@ private:
 	std::string line_code;
 	std::vector<char> res;
     std::vector<token> token_list;
-	int  get_tok_type(std::string);
-	void die(std::string,std::string,int,int);
+	int  get_tok_type(std::string&);
+	void die(std::string,int);
 	std::string id_gen();
     std::string num_gen();
     std::string str_gen();
 public:
-	void openfile(std::string);
+	void openfile(std::string&);
     void scanner();
     void print_token();
-    int  get_error();
-	std::vector<token>& get_token_list();
+    int  get_error(){return error;}
+	std::vector<token>& get_token_list(){return token_list;}
 };
 
-void nasal_lexer::openfile(std::string filename)
+void nasal_lexer::openfile(std::string& filename)
 {
 	error=0;
 	res.clear();
@@ -126,7 +126,6 @@ void nasal_lexer::openfile(std::string filename)
 		++error;
         std::cout<<">> [lexer] cannot open file \""<<filename<<"\".\n";
 		fin.close();
-		res_size=0;
         return;
     }
     while(!fin.eof())
@@ -137,11 +136,10 @@ void nasal_lexer::openfile(std::string filename)
         res.push_back(c);
     }
     fin.close();
-	res_size=res.size();
     return;
 }
 
-int nasal_lexer::get_tok_type(std::string tk_str)
+int nasal_lexer::get_tok_type(std::string& tk_str)
 {
 	for(int i=0;token_table[i].str;++i)
 		if(tk_str==token_table[i].str)
@@ -149,12 +147,12 @@ int nasal_lexer::get_tok_type(std::string tk_str)
 	return tok_null;
 }
 
-void nasal_lexer::die(std::string code,std::string error_info,int line=-1,int column=-1)
+void nasal_lexer::die(std::string error_info,int line)
 {
 	++error;
-	std::cout<<">> [lexer] line "<<line<<" column "<<column<<": \n"<<code<<"\n";
-	for(int i=0;i<column-1;++i)
-		std::cout<<(code[i]=='\t'?'\t':' ');
+	std::cout<<">> [lexer] line "<<line<<" column "<<line_code.length()<<": \n"<<line_code<<"\n";
+	for(auto i:line_code)
+		std::cout<<(i=='\t'?'\t':' ');
 	std::cout<<"^ "<<error_info<<'\n';
 	return;
 }
@@ -183,7 +181,7 @@ std::string nasal_lexer::num_gen()
 		line_code+=token_str;
 		if(token_str=="0x")
 		{
-			die(line_code,"incorrect number.",line,line_code.length());
+			die("incorrect number.",line);
 			return "0";
 		}
 		return token_str;
@@ -198,7 +196,7 @@ std::string nasal_lexer::num_gen()
 		line_code+=token_str;
 		if(token_str=="0o")
 		{
-			die(line_code,"incorrect number.",line,line_code.length());
+			die("incorrect number.",line);
 			return "0";
 		}
 		return token_str;
@@ -214,7 +212,7 @@ std::string nasal_lexer::num_gen()
 		if(ptr>=res_size)
 		{
 			line_code+=token_str;
-			die(line_code,"incorrect number.",line,line_code.length());
+			die("incorrect number.",line);
 			return "0";
 		}
 		while(ptr<res_size && IS_DIGIT(res[ptr]))
@@ -223,7 +221,7 @@ std::string nasal_lexer::num_gen()
 		if(token_str.back()=='.')
 		{
 			line_code+=token_str;
-			die(line_code,"incorrect number.",line,line_code.length());
+			die("incorrect number.",line);
 			return "0";
 		}
 	}
@@ -234,7 +232,7 @@ std::string nasal_lexer::num_gen()
 		if(ptr>=res_size)
 		{
 			line_code+=token_str;
-			die(line_code,"incorrect number.",line,line_code.length());
+			die("incorrect number.",line);
 			return "0";
 		}
 		if(ptr<res_size && (res[ptr]=='-' || res[ptr]=='+'))
@@ -242,7 +240,7 @@ std::string nasal_lexer::num_gen()
 		if(ptr>=res_size)
 		{
 			line_code+=token_str;
-			die(line_code,"incorrect number.",line,line_code.length());
+			die("incorrect number.",line);
 			return "0";
 		}
 		while(ptr<res_size && IS_DIGIT(res[ptr]))
@@ -251,7 +249,7 @@ std::string nasal_lexer::num_gen()
 		if(token_str.back()=='e' || token_str.back()=='E' || token_str.back()=='-' || token_str.back()=='+')
 		{
 			line_code+=token_str;
-			die(line_code,"incorrect number.",line,line_code.length());
+			die("incorrect number.",line);
 			return "0";
 		}
 	}
@@ -277,15 +275,15 @@ std::string nasal_lexer::str_gen()
 			line_code+=res[++ptr];
 			switch(res[ptr])
 			{
-				case 'a':token_str.push_back('\a');break;
-				case 'b':token_str.push_back('\b');break;
-				case 'f':token_str.push_back('\f');break;
-				case 'n':token_str.push_back('\n');break;
-				case 'r':token_str.push_back('\r');break;
-				case 't':token_str.push_back('\t');break;
-				case 'v':token_str.push_back('\v');break;
-				case '?':token_str.push_back('\?');break;
-				case '0':token_str.push_back('\0');break;
+				case 'a': token_str.push_back('\a');break;
+				case 'b': token_str.push_back('\b');break;
+				case 'f': token_str.push_back('\f');break;
+				case 'n': token_str.push_back('\n');break;
+				case 'r': token_str.push_back('\r');break;
+				case 't': token_str.push_back('\t');break;
+				case 'v': token_str.push_back('\v');break;
+				case '?': token_str.push_back('\?');break;
+				case '0': token_str.push_back('\0');break;
 				case '\\':token_str.push_back('\\');break;
 				case '\'':token_str.push_back('\'');break;
 				case '\"':token_str.push_back('\"');break;
@@ -297,9 +295,9 @@ std::string nasal_lexer::str_gen()
 	}
 	// check if this string ends with a " or '
 	if(ptr++>=res_size)
-		die(line_code,"get EOF when generating string.",line,line_code.length());
+		die("get EOF when generating string.",line);
 	if(str_begin=='`' && token_str.length()>1)
-		die(line_code,"\'`\' is used for string that includes one character.",line,line_code.length());
+		die("\'`\' is used for string that includes one character.",line);
 	return token_str;
 }
 
@@ -309,6 +307,7 @@ void nasal_lexer::scanner()
     line=1;
 	ptr=0;
 	line_code="";
+	res_size=res.size();
 
 	std::string token_str;
 	while(ptr<res_size)
@@ -351,7 +350,7 @@ void nasal_lexer::scanner()
 			line_code+=res[ptr];
 			token new_token(line,get_tok_type(token_str),token_str);
 			if(!new_token.type)
-				die(line_code,"incorrect operator.",line,line_code.length());
+				die("incorrect operator.",line);
 			token_list.push_back(new_token);
 			++ptr;
 		}
@@ -386,7 +385,7 @@ void nasal_lexer::scanner()
 		else
 		{
 			line_code+=res[ptr++];
-			die(line_code,"unknown character.",line,line_code.length());
+			die("unknown character.",line);
 		}
 	}
 	token tk(line,tok_eof,"");
@@ -397,19 +396,9 @@ void nasal_lexer::scanner()
 
 void nasal_lexer::print_token()
 {
-    int size=token_list.size();
-    for(int i=0;i<size;++i)
-        std::cout<<"("<<token_list[i].line<<" | "<<token_list[i].str<<")\n";
+    for(auto tok:token_list)
+        std::cout<<"("<<tok.line<<" | "<<tok.str<<")\n";
     return;
 }
 
-int nasal_lexer::get_error()
-{
-    return error;
-}
-
-std::vector<token>& nasal_lexer::get_token_list()
-{
-	return token_list;
-}
 #endif
\ No newline at end of file
diff --git a/nasal_parse.h b/nasal_parse.h
index 453579b..248e3ad 100644
--- a/nasal_parse.h
+++ b/nasal_parse.h
@@ -99,20 +99,11 @@ private:
     nasal_ast break_expr();
     nasal_ast ret_expr();
 public:
-    int get_error();
-    void set_toklist(std::vector<token>&);
+    int get_error(){return error;}
+    void set_toklist(std::vector<token>& toks){tok_list=toks;}
     void main_process();
-    nasal_ast& get_root();
+    nasal_ast& get_root(){return root;}
 };
-int nasal_parse::get_error()
-{
-    return error;
-}
-void nasal_parse::set_toklist(std::vector<token>& toks)
-{
-    tok_list=toks;
-    return;
-}
 void nasal_parse::main_process()
 {
     reset();
@@ -147,10 +138,6 @@ void nasal_parse::main_process()
     <<"please check \'(\',\'[\',\'{\',\')\',\']\',\'}\' match or not.\n";
     return;
 }
-nasal_ast& nasal_parse::get_root()
-{
-    return root;
-}
 void nasal_parse::reset()
 {
     ptr=in_function=in_loop=error=0;
@@ -237,7 +224,7 @@ bool nasal_parse::check_function_end(nasal_ast& node)
     if(
         node.get_children().empty() || 
         (
-            type!=ast_definition &&
+            type!=ast_def &&
             type!=ast_equal &&
             type!=ast_addeq &&
             type!=ast_subeq &&
@@ -695,7 +682,7 @@ nasal_ast nasal_parse::scalar()
     else if(tok_list[ptr].type==tok_var)
     {
         match(tok_var);
-        node.set_type(ast_definition);
+        node.set_type(ast_def);
         node.add_child(id_gen());
         match(tok_id);
         match(tok_eq);
@@ -795,7 +782,7 @@ nasal_ast nasal_parse::subvec()
 }
 nasal_ast nasal_parse::definition()
 {
-    nasal_ast node(tok_list[ptr].line,ast_definition);
+    nasal_ast node(tok_list[ptr].line,ast_def);
     if(tok_list[ptr].type==tok_var)
     {
         match(tok_var);
@@ -1058,7 +1045,7 @@ nasal_ast nasal_parse::break_expr()
 }
 nasal_ast nasal_parse::ret_expr()
 {
-    nasal_ast node(tok_list[ptr].line,ast_return);
+    nasal_ast node(tok_list[ptr].line,ast_ret);
     match(tok_ret);
     int type=tok_list[ptr].type;
     if(type==tok_nil || type==tok_num || type==tok_str || type==tok_id || type==tok_func ||
diff --git a/nasal_vm.h b/nasal_vm.h
index 4de1bb0..58d9807 100644
--- a/nasal_vm.h
+++ b/nasal_vm.h
@@ -82,7 +82,7 @@ private:
     void opr_mcallh();
     void opr_ret();
 public:
-    nasal_vm();
+    nasal_vm():stack_top(gc.stack_top){};
     void init(
         std::vector<std::string>&,
         std::vector<double>&,
@@ -91,11 +91,6 @@ public:
     void run();
 };
 
-nasal_vm::nasal_vm():stack_top(gc.stack_top)
-{
-    return;
-}
-
 void nasal_vm::init(
     std::vector<std::string>& strs,
     std::vector<double>&      nums,
@@ -123,7 +118,7 @@ void nasal_vm::clear()
 }
 void nasal_vm::die(std::string str)
 {
-    printf(">> [vm] 0x%.8x: %s\n",pc,str.data());
+    printf(">> [vm] 0x%.8x: %s\n",pc,str.c_str());
     loop_mark=false;
     return;
 }
@@ -137,7 +132,7 @@ bool nasal_vm::condition(nasal_val* val_addr)
         std::string& str=*val_addr->ptr.str;
         double number=str2num(str.c_str());
         if(std::isnan(number))
-            return !str.empty();
+            return str.empty();
         return number;
     }
     return false;
@@ -262,20 +257,19 @@ void nasal_vm::opr_unot()
     nasal_val* new_val=nullptr;
     int type=val->type;
     if(type==vm_nil)
-        new_val=gc.one_addr;
+        *stack_top=gc.one_addr;
     else if(type==vm_num)
-        new_val=val->ptr.num?gc.zero_addr:gc.one_addr;
+        *stack_top=val->ptr.num?gc.zero_addr:gc.one_addr;
     else if(type==vm_str)
     {
         double number=str2num(val->ptr.str->c_str());
         if(std::isnan(number))
-            new_val=val->ptr.str->empty()?gc.one_addr:gc.zero_addr;
+            *stack_top=val->ptr.str->empty()?gc.one_addr:gc.zero_addr;
         else
-            new_val=number?gc.zero_addr:gc.one_addr;
+            *stack_top=number?gc.zero_addr:gc.one_addr;
     }
     else
         die("unot: incorrect value type");
-    *stack_top=new_val;
     return;
 }
 void nasal_vm::opr_usub()
@@ -433,9 +427,7 @@ void nasal_vm::opr_less()
 {
     nasal_val* val2=*stack_top--;
     nasal_val* val1=*stack_top;
-    int a_type=val1->type;
-    int b_type=val2->type;
-    if(a_type==vm_str && b_type==vm_str)
+    if(val1->type==vm_str && val2->type==vm_str)
         *stack_top=(*val1->ptr.str<*val2->ptr.str)?gc.one_addr:gc.zero_addr;
     else
         *stack_top=(val1->to_number()<val2->to_number())?gc.one_addr:gc.zero_addr;
@@ -445,9 +437,7 @@ void nasal_vm::opr_leq()
 {
     nasal_val* val2=*stack_top--;
     nasal_val* val1=*stack_top;
-    int a_type=val1->type;
-    int b_type=val2->type;
-    if(a_type==vm_str && b_type==vm_str)
+    if(val1->type==vm_str && val2->type==vm_str)
         *stack_top=(*val1->ptr.str<=*val2->ptr.str)?gc.one_addr:gc.zero_addr;
     else
         *stack_top=(val1->to_number()<=val2->to_number())?gc.one_addr:gc.zero_addr;
@@ -457,9 +447,7 @@ void nasal_vm::opr_grt()
 {
     nasal_val* val2=*stack_top--;
     nasal_val* val1=*stack_top;
-    int a_type=val1->type;
-    int b_type=val2->type;
-    if(a_type==vm_str && b_type==vm_str) 
+    if(val1->type==vm_str && val2->type==vm_str) 
         *stack_top=(*val1->ptr.str>*val2->ptr.str)?gc.one_addr:gc.zero_addr;
     else
         *stack_top=(val1->to_number()>val2->to_number())?gc.one_addr:gc.zero_addr;
@@ -469,9 +457,7 @@ void nasal_vm::opr_geq()
 {
     nasal_val* val2=*stack_top--;
     nasal_val* val1=*stack_top;
-    int a_type=val1->type;
-    int b_type=val2->type;
-    if(a_type==vm_str && b_type==vm_str)
+    if(val1->type==vm_str && val2->type==vm_str)
         *stack_top=(*val1->ptr.str>=*val2->ptr.str)?gc.one_addr:gc.zero_addr;
     else
         *stack_top=(val1->to_number()>=val2->to_number())?gc.one_addr:gc.zero_addr;
@@ -502,12 +488,9 @@ void nasal_vm::opr_jf()
 }
 void nasal_vm::opr_counter()
 {
-    if((*stack_top)->type!=vm_vec)
-    {
-        die("cnt: must use vector in forindex/foreach");
-        return;
-    }
     counter.push(-1);
+    if((*stack_top)->type!=vm_vec)
+        die("cnt: must use vector in forindex/foreach");
     return;
 }
 void nasal_vm::opr_cntpop()
@@ -556,15 +539,10 @@ void nasal_vm::opr_callv()
     int type=vec_addr->type;
     if(type==vm_vec)
     {
-        int num=val->to_number();
-        nasal_val* res=vec_addr->ptr.vec->get_val(num);
-        if(res)
-            *stack_top=res;
-        else
-        {
+        nasal_val* res=vec_addr->ptr.vec->get_val(val->to_number());
+        if(!res)
             die("callv: index out of range");
-            return;
-        }
+        *stack_top=res;
     }
     else if(type==vm_hash)
     {
@@ -612,10 +590,7 @@ void nasal_vm::opr_callvi()
     // cannot use operator[],because this may cause overflow
     nasal_val* res=val->ptr.vec->get_val(exec_code[pc].num);
     if(!res)
-    {
         die("callvi: index out of range");
-        return;
-    }
     *(++stack_top)=res;
     return;
 }
@@ -646,7 +621,7 @@ void nasal_vm::opr_callfv()
     nasal_val* func_addr=*(vec-1);
     if(func_addr->type!=vm_func)
     {
-        die("callfv: called a value that is not a function");
+        die("callfv: must call a function");
         return;
     }
     // push new local scope
@@ -690,7 +665,7 @@ void nasal_vm::opr_callfh()
     nasal_val* func_addr=*(stack_top-1);
     if(func_addr->type!=vm_func)
     {
-        die("callfh: called a value that is not a function");
+        die("callfh: must call a function");
         return;
     }
     // push new local scope
@@ -756,10 +731,9 @@ void nasal_vm::opr_slc()
         default:die("slc: error value type");break;
     }
     nasal_val* res=(*stack_top)->ptr.vec->get_val((int)num);
-    if(res)
-        gc.slice_stack.back()->ptr.vec->elems.push_back(res);
-    else
+    if(!res)
         die("slc: index out of range");
+    gc.slice_stack.back()->ptr.vec->elems.push_back(res);
     return;
 }
 void nasal_vm::opr_slc2()
@@ -824,10 +798,7 @@ void nasal_vm::opr_mcallv()
         }
         nasal_val** res=(*vec_addr)->ptr.vec->get_mem(num);
         if(!res)
-        {
             die("mcallv: index out of range");
-            return;
-        }
         addr_stack.push(res);
     }
     else if(type==vm_hash)
@@ -848,10 +819,7 @@ void nasal_vm::opr_mcallv()
         addr_stack.push(res);
     }
     else
-    {
         die("mcallv: cannot get memory space in a string");
-        return;
-    }
     return;
 }
 void nasal_vm::opr_mcallh()
@@ -957,6 +925,8 @@ void nasal_vm::run()
     for(pc=0;loop_mark&&!gc.val_stack[STACK_MAX_DEPTH-1];++pc)
         (this->*opr_table[exec_code[pc].op])();
     float total_time=((double)(clock()-begin_time))/CLOCKS_PER_SEC;
+    if(gc.val_stack[STACK_MAX_DEPTH-1])
+        die("stack overflow");
     std::cout<<">> [vm] process exited after "<<total_time<<"s.\n";
     return;
 }