diff --git a/.github/workflows/c-cpp.yml b/.github/workflows/c-cpp.yml
index 419659b1..db27e23c 100644
--- a/.github/workflows/c-cpp.yml
+++ b/.github/workflows/c-cpp.yml
@@ -4,7 +4,7 @@ on:
   schedule:
    - cron: "0 16 * * *"
   push:
-    branches: [ master ]
+    branches: [ master,develop ]
   pull_request:
     branches: [ master ]
   workflow_dispatch:
diff --git a/ast/nasal_new_ast.cpp b/ast/nasal_new_ast.cpp
new file mode 100644
index 00000000..266ed2d2
--- /dev/null
+++ b/ast/nasal_new_ast.cpp
@@ -0,0 +1,19 @@
+#include "nasal_new_ast.h"
+
+bool stmt::accept(ast_visitor* visitor) {
+    visitor->visit_stmt(this);
+    return true;
+}
+
+bool expr::accept(ast_visitor* visitor) {
+    visitor->visit_expr(this);
+    return true;
+}
+
+void ast_visitor::visit_stmt(stmt* node) {
+    node->accept(this);
+}
+
+void ast_visitor::visit_expr(expr* node) {
+    node->accept(this);
+}
\ No newline at end of file
diff --git a/ast/nasal_new_ast.h b/ast/nasal_new_ast.h
new file mode 100644
index 00000000..154d9017
--- /dev/null
+++ b/ast/nasal_new_ast.h
@@ -0,0 +1,112 @@
+#pragma once
+
+#include "nasal.h"
+#include "nasal_err.h"
+
+enum class ast_node_type:u32 {
+    ast_null=0,      // null node
+    ast_stmt,
+    ast_expr,
+    ast_root,        // mark the root node of ast
+    ast_block,       // expression block 
+    ast_file,        // used to store which file the sub-tree is on, only used in main block
+    ast_nil,         // nil keyword
+    ast_num,         // number, basic value type
+    ast_str,         // string, basic value type
+    ast_id,          // identifier
+    ast_bool,        // bools
+    ast_func,        // func keyword
+    ast_hash,        // hash, basic value type
+    ast_vec,         // vector, basic value type
+    ast_pair,        // pair of key and value in hashmap
+    ast_call,        // mark a sub-tree of calling an identifier
+    ast_callh,       // id.name
+    ast_callv,       // id[index]
+    ast_callf,       // id()
+    ast_subvec,      // id[index:index]
+    ast_params,      // mark a sub-tree of function parameters
+    ast_default,     // default parameter
+    ast_dynamic,     // dynamic parameter
+    ast_and,         // and keyword
+    ast_or,          // or keyword
+    ast_equal,       // =
+    ast_addeq,       // +=
+    ast_subeq,       // -=
+    ast_multeq,      // *=
+    ast_diveq,       // /=
+    ast_lnkeq,       // ~=
+    ast_btandeq,     // &=
+    ast_btoreq,      // |=
+    ast_btxoreq,     // ^=
+    ast_cmpeq,       // ==
+    ast_neq,         // !=
+    ast_less,        // <
+    ast_leq,         // <=
+    ast_grt,         // >
+    ast_geq,         // >=
+    ast_add,         // +
+    ast_sub,         // -
+    ast_mult,        // *
+    ast_div,         // /
+    ast_link,        // ~
+    ast_neg,         // unary -
+    ast_lnot,        // unary !
+    ast_bnot,        // unary ~ bitwise not
+    ast_bitor,       // bitwise or
+    ast_bitxor,      // bitwise xor
+    ast_bitand,      // bitwise and
+    ast_trino,       // ?:
+    ast_for,         // for keyword
+    ast_forindex,    // forindex keyword
+    ast_foreach,     // foreach keyword
+    ast_while,       // while
+    ast_iter,        // iterator, used in forindex/foreach
+    ast_cond,        // mark a sub-tree of conditional expression
+    ast_if,          // if keyword
+    ast_elsif,       // elsif keyword
+    ast_else,        // else keyword
+    ast_multi_id,    // multi identifiers sub-tree
+    ast_tuple,       // tuple, only used in multiple assignment
+    ast_def,         // definition
+    ast_multi_assign,// multi assignment sub-tree
+    ast_continue,    // continue keyword, only used in loop
+    ast_break,       // break keyword, only used in loop
+    ast_ret          // return keyword, only used in function block
+};
+
+class ast_visitor;
+
+class ast_node {
+private:
+    span nd_loc;
+    ast_node_type nd_type;
+public:
+    ast_node(const span& location,ast_node_type node_type):
+        nd_loc(location), nd_type(node_type) {}
+    ~ast_node() = default;
+    virtual bool accept(ast_visitor*) = 0;
+};
+
+class stmt:public ast_node {
+public:
+    stmt(const span& location,ast_node_type node_type):
+        ast_node(location, node_type) {}
+    ~stmt() = default;
+    virtual bool accept(ast_visitor*);
+};
+
+class expr:public ast_node {
+private:
+
+public:
+    expr(const span& location,ast_node_type node_type):
+        ast_node(location, node_type) {}
+    ~expr() = default;
+    virtual bool accept(ast_visitor*);
+};
+
+class ast_visitor {
+public:
+    virtual void visit_stmt(stmt*);
+    virtual void visit_expr(expr*);
+};
\ No newline at end of file
diff --git a/makefile b/makefile
index 6376f168..01e09995 100644
--- a/makefile
+++ b/makefile
@@ -18,11 +18,14 @@ SRC=\
 
 STD=c++14
 
-nasal:$(SRC)
+nasal:$(SRC) nasal_new_ast.o
 	$(CXX) -std=$(STD) -O3 main.cpp -o nasal -fno-exceptions -ldl -Wshadow -Wall
 nasal.exe:$(SRC)
 	$(CXX) -std=$(STD) -O3 main.cpp -o nasal.exe -fno-exceptions -Wshadow -Wall -static
 
+nasal_new_ast.o: ast/nasal_new_ast.h ast/nasal_new_ast.cpp nasal.h
+	$(CXX) -std=$(STD) -c -O3 ast/nasal_new_ast.cpp -fno-exceptions -fPIC -o nasal_new_ast.o -I .
+
 stable-release:$(SRC)
 	$(CXX) -std=$(STD) -O2 main.cpp -o nasal -fno-exceptions -ldl -Wshadow -Wall
 stable-release-mingw:$(SRC)
diff --git a/nasal_dbg.h b/nasal_dbg.h
index 495c5053..a10fc426 100644
--- a/nasal_dbg.h
+++ b/nasal_dbg.h
@@ -4,9 +4,61 @@
 #include "nasal_err.h"
 #include "nasal_opcode.h"
 #include "nasal_vm.h"
+
+#include <cstring>
 #include <algorithm>
+#include <unordered_map>
 
 class dbg:public vm {
+private:
+    enum class dbg_cmd {
+        cmd_error,
+        cmd_help,
+        cmd_backtrace,
+        cmd_continue,
+        cmd_list_file,
+        cmd_global,
+        cmd_local,
+        cmd_upval,
+        cmd_register,
+        cmd_show_all,
+        cmd_next,
+        cmd_break_point,
+        cmd_exit
+    };
+
+private:
+    const std::unordered_map<string, dbg_cmd> command_table = {
+        {"h", dbg_cmd::cmd_help},
+        {"help", dbg_cmd::cmd_help},
+        {"bt", dbg_cmd::cmd_backtrace},
+        {"backtrace", dbg_cmd::cmd_backtrace},
+        {"c", dbg_cmd::cmd_continue},
+        {"continue", dbg_cmd::cmd_continue},
+        {"f", dbg_cmd::cmd_list_file},
+        {"file", dbg_cmd::cmd_list_file},
+        {"g", dbg_cmd::cmd_global},
+        {"global", dbg_cmd::cmd_global},
+        {"l", dbg_cmd::cmd_local},
+        {"local", dbg_cmd::cmd_local},
+        {"u", dbg_cmd::cmd_upval},
+        {"upval", dbg_cmd::cmd_upval},
+        {"r", dbg_cmd::cmd_register},
+        {"register", dbg_cmd::cmd_register},
+        {"a", dbg_cmd::cmd_show_all},
+        {"all", dbg_cmd::cmd_show_all},
+        {"n", dbg_cmd::cmd_next},
+        {"next", dbg_cmd::cmd_next},
+        {"bk", dbg_cmd::cmd_break_point},
+        {"break", dbg_cmd::cmd_break_point},
+        {"q", dbg_cmd::cmd_exit},
+        {"exit", dbg_cmd::cmd_exit}
+    };
+    dbg_cmd get_cmd_type(const string& cmd) const {
+        return command_table.count(cmd)?
+            command_table.at(cmd):dbg_cmd::cmd_error;
+    }
+
 private:
     bool  next;
     usize fsize;
@@ -15,11 +67,12 @@ class dbg:public vm {
     error& src;
 
     std::vector<string> parse(const string&);
-    u16 fileindex(const string&);
+    u16 file_index(const string&) const;
     void err();
     void help();
-    void callsort(const u64*);
-    void stepinfo();
+    void list_file() const;
+    void call_sort(const u64*) const;
+    void step_info();
     void interact();
 public:
     dbg(error& err):
@@ -50,7 +103,7 @@ std::vector<string> dbg::parse(const string& cmd) {
     return res;
 }
 
-u16 dbg::fileindex(const string& filename) {
+u16 dbg::file_index(const string& filename) const {
     for(u16 i=0;i<fsize;++i) {
         if (filename==files[i]) {
             return i;
@@ -66,24 +119,30 @@ void dbg::err() {
 }
 
 void dbg::help() {
-    std::cout
+    std::clog
     <<"<option>\n"
-    <<"\th,   help      | get help\n"
-    <<"\tbt,  backtrace | get function call trace\n"
-    <<"\tc,   continue  | run program until break point or exit\n"
-    <<"\tf,   file      | see all the compiled files\n"
-    <<"\tg,   global    | see global values\n"
-    <<"\tl,   local     | see local values\n"
-    <<"\tu,   upval     | see upvalue\n"
-    <<"\tr,   register  | show vm register detail\n"
-    <<"\ta,   all       | show global,local and upvalue\n"
-    <<"\tn,   next      | execute next bytecode\n"
-    <<"\tq,   exit      | exit debugger\n"
+    <<"  h,   help      | get help\n"
+    <<"  bt,  backtrace | get function call trace\n"
+    <<"  c,   continue  | run program until break point or exit\n"
+    <<"  f,   file      | see all the compiled files\n"
+    <<"  g,   global    | see global values\n"
+    <<"  l,   local     | see local values\n"
+    <<"  u,   upval     | see upvalue\n"
+    <<"  r,   register  | show vm register detail\n"
+    <<"  a,   all       | show global,local and upvalue\n"
+    <<"  n,   next      | execute next bytecode\n"
+    <<"  q,   exit      | exit debugger\n"
     <<"<option> <filename> <line>\n"
-    <<"\tbk,  break     | set break point\n";
+    <<"  bk,  break     | set break point\n";
+}
+
+void dbg::list_file() const {
+    for(usize i=0; i<fsize; ++i) {
+        std::clog<<"["<<i<<"] "<<files[i]<<"\n";
+    }
 }
 
-void dbg::callsort(const u64* arr) {
+void dbg::call_sort(const u64* arr) const {
     typedef std::pair<u32,u64> op;
     std::vector<op> opcall;
     u64 total=0;
@@ -105,22 +164,22 @@ void dbg::callsort(const u64* arr) {
     std::clog<<" total  : "<<total<<'\n';
 }
 
-void dbg::stepinfo() {
+void dbg::step_info() {
     u32 line=bytecode[ctx.pc].line==0?0:bytecode[ctx.pc].line-1;
     u32 begin=(line>>3)==0?0:((line>>3)<<3);
     u32 end=(1+(line>>3))<<3;
     src.load(files[bytecode[ctx.pc].fidx]);
-    std::cout<<"\nsource code:\n";
+    std::clog<<"\nsource code:\n";
     for(u32 i=begin;i<end && i<src.size();++i) {
-        std::cout<<(i==line?back_white:reset)<<(i==line?"--> ":"    ")<<src[i]<<reset<<"\n";
+        std::clog<<(i==line?back_white:reset)<<(i==line?"--> ":"    ")<<src[i]<<reset<<"\n";
     }
 
     begin=(ctx.pc>>3)==0?0:((ctx.pc>>3)<<3);
     end=(1+(ctx.pc>>3))<<3;
     codestream::set(cnum, cstr, files);
-    std::cout<<"next bytecode:\n";
+    std::clog<<"next bytecode:\n";
     for(u32 i=begin;i<end && bytecode[i].op!=op_exit;++i) {
-        std::cout
+        std::clog
         <<(i==ctx.pc?back_white:reset)
         <<(i==ctx.pc?"--> ":"    ")
         <<codestream(bytecode[i], i)
@@ -130,65 +189,51 @@ void dbg::stepinfo() {
 }
 
 void dbg::interact() {
-    // special operand
+    // special operand, end execution
     if (bytecode[ctx.pc].op==op_exit) {
         return;
     }
     
-    if (
-        (bytecode[ctx.pc].fidx!=bk_fidx || bytecode[ctx.pc].line!=bk_line) && // break point
-        !next // next step
-    ) {
+    if ((bytecode[ctx.pc].fidx!=bk_fidx ||
+        bytecode[ctx.pc].line!=bk_line) && // break point
+        !next) {// next step
         return;
     }
 
     next=false;
     string cmd;
-    stepinfo();
+    step_info();
     while(true) {
-        std::cout<<">> ";
+        std::clog<<">> ";
         std::getline(std::cin, cmd);
         auto res=parse(cmd);
         if (res.size()==0) {
-            stepinfo();
+            step_info();
         } else if (res.size()==1) {
-            if (res[0]=="h" || res[0]=="help") {
-                help();
-            } else if (res[0]=="bt" || res[0]=="backtrace") {
-                traceback();
-            } else if (res[0]=="c" || res[0]=="continue") {
-                return;
-            } else if (res[0]=="f" || res[0]=="file") {
-                for(usize i=0;i<fsize;++i) {
-                    std::cout<<"["<<i<<"] "<<files[i]<<"\n";
-                }
-            } else if (res[0]=="g" || res[0]=="global") {
-                gstate();
-            } else if (res[0]=="l" || res[0]=="local") {
-                lstate();
-            } else if (res[0]=="u" || res[0]=="upval") {
-                ustate();
-            } else if (res[0]=="r" || res[0]=="register") {
-                reginfo();
-            } else if (res[0]=="a" || res[0]=="all") {
-                detail();
-            } else if (res[0]=="n" || res[0]=="next") {
-                next=true;
-                return;
-            } else if (res[0]=="q" || res[0]=="exit")
-                std::exit(0);
-            else {
-                err();
+            switch(get_cmd_type(res[0])) {
+                case dbg_cmd::cmd_help: help(); break;
+                case dbg_cmd::cmd_backtrace: traceback(); break;
+                case dbg_cmd::cmd_continue: return;
+                case dbg_cmd::cmd_list_file: list_file(); break;
+                case dbg_cmd::cmd_global: gstate(); break;
+                case dbg_cmd::cmd_local: lstate(); break;
+                case dbg_cmd::cmd_upval: ustate(); break;
+                case dbg_cmd::cmd_register: reginfo(); break;
+                case dbg_cmd::cmd_show_all: detail(); break;
+                case dbg_cmd::cmd_next: next=true; return;
+                case dbg_cmd::cmd_exit: std::exit(0);
+                default: err(); break;
             }
-        } else if (res.size()==3 && (res[0]=="bk" || res[0]=="break")) {
-            bk_fidx=fileindex(res[1]);
+        } else if (res.size()==3 &&
+            get_cmd_type(res[0])==dbg_cmd::cmd_break_point) {
+            bk_fidx=file_index(res[1]);
             if (bk_fidx==65535) {
-                std::cout<<"cannot find file named `"<<res[1]<<"`\n";
-                bk_fidx=0;
+                std::clog<<"cannot find file named `"<<res[1]<<"`\n";
+                continue;
             }
             i32 tmp=atoi(res[2].c_str());
             if (tmp<=0) {
-                std::cout<<"incorrect line number `"<<res[2]<<"`\n";
+                std::clog<<"incorrect line number `"<<res[2]<<"`\n";
             } else {
                 bk_line=tmp;
             }
@@ -201,8 +246,7 @@ void dbg::interact() {
 void dbg::run(
     const codegen& gen,
     const linker& linker,
-    const std::vector<string>& argv
-) {
+    const std::vector<string>& argv) {
     verbose=true;
     fsize=linker.filelist().size();
     init(gen.strs(), gen.nums(), gen.codes(), linker.filelist(), argv);
@@ -269,7 +313,7 @@ void dbg::run(
         ++ctx.pc;
     }
 
-    callsort(count);
+    call_sort(count);
     ngc.info();
     ngc.clear();
     imm.clear();
diff --git a/nasal_gc.h b/nasal_gc.h
index baa673aa..a889d63f 100644
--- a/nasal_gc.h
+++ b/nasal_gc.h
@@ -249,7 +249,7 @@ struct ghost_register_table {
         if (mapper.count(name)) {
             std::cerr<<"nasal_gc.h: ghost_register_table::register_ghost_type: ";
             std::cerr<<"ghost type \""<<name<<"\" already exists.\n";
-            std::exit(-1);
+            std::exit(1);
         }
         auto res=destructors.size();
         mapper[name]=res;
@@ -754,7 +754,7 @@ void gc::extend(u8 type) {
         if (!tmp) {
             std::cerr<<"nasal_gc.h: gc::extend: ";
             std::cerr<<"failed to allocate memory\n";
-            std::exit(-1);
+            std::exit(1);
         }
 
         // add to heap
@@ -762,7 +762,7 @@ void gc::extend(u8 type) {
         unused[index].push_back(tmp);
     }
 
-    incr[index]*=2;
+    incr[index]=incr[index]+incr[index]/2;
 }
 
 void gc::init(const std::vector<string>& s, const std::vector<string>& argv) {
diff --git a/test/life.nas b/test/life.nas
index 626d68c7..bffbf7a1 100644
--- a/test/life.nas
+++ b/test/life.nas
@@ -11,7 +11,7 @@ var new_map=func(width,height){
 }
 
 var prt=func(map){
-    var s='\ec\e[1;1H';
+    var s='\e[H';
     foreach(var line;map){
         foreach(var elem;line)
             s~=elem~' ';