diff --git a/README.md b/README.md
index 2e4a5a5..e62ca96 100644
--- a/README.md
+++ b/README.md
@@ -633,7 +633,7 @@ var fib(var* args,usize size,gc* ngc){
     // ok, you must know that vm_num now is not managed by gc
     // if want to return a gc object, use ngc->alloc(type)
     // usage of gc is the same as adding a native function
-    return {vm_num,fibonaci(num.tonum())};
+    return var::num(fibonaci(num.tonum()));
 }
 
 // then put function name and address into this table
diff --git a/doc/README_zh.md b/doc/README_zh.md
index a8863b7..582e089 100644
--- a/doc/README_zh.md
+++ b/doc/README_zh.md
@@ -606,7 +606,7 @@ var fib(var* args,usize size,gc* ngc){
         return nas_err("extern_fib","\"num\" must be number");
     // vm_num作为普通的数字类型，不是内存管理的对象，所以无需申请
     // 如果需要返回内存管理的对象，请使用ngc->alloc(type)
-    return {vm_num,fibonaci(num.tonum())};
+    return var::num(fibonaci(num.tonum()));
 }
 
 // 然后将函数名字和函数地址放到一个表里，一定要记住表尾是{nullptr,nullptr}
diff --git a/module/fib.cpp b/module/fib.cpp
index f55d7f8..43f26fa 100644
--- a/module/fib.cpp
+++ b/module/fib.cpp
@@ -13,7 +13,7 @@ var fib(var* args,usize size,gc* ngc) {
         return nas_err("fib","lack arguments");
     }
     var num=args[0];
-    return {vm_num,fibonaci(num.tonum())};
+    return var::num(fibonaci(num.tonum()));
 }
 
 var quick_fib(var* args,usize size,gc* ngc) {
@@ -22,7 +22,7 @@ var quick_fib(var* args,usize size,gc* ngc) {
     }
     double num=args[0].tonum();
     if (num<2) {
-        return {vm_num,num};
+        return var::num(num);
     }
     double a=1,b=1,res=0;
     for(double i=1;i<num;i+=1) {
@@ -30,7 +30,7 @@ var quick_fib(var* args,usize size,gc* ngc) {
         a=b;
         b=res;
     }
-    return {vm_num,res};
+    return var::num(res);
 }
 
 mod_func func_tbl[]={
diff --git a/module/keyboard.cpp b/module/keyboard.cpp
index d48ea2c..38bed54 100644
--- a/module/keyboard.cpp
+++ b/module/keyboard.cpp
@@ -73,14 +73,14 @@ public:
 
 noecho_input this_window;
 var nas_getch(var* args,usize size,gc* ngc) {
-    return {vm_num,(double)this_window.noecho_getch()};
+    return var::num((double)this_window.noecho_getch());
 }
 var nas_kbhit(var* args,usize size,gc* ngc) {
-    return {vm_num,(double)this_window.noecho_kbhit()};
+    return var::num((double)this_window.noecho_kbhit());
 }
 var nas_noblock(var* args,usize size,gc* ngc) {
     if (this_window.noecho_kbhit()) {
-        return {vm_num,(double)this_window.noecho_getch()};
+        return var::num((double)this_window.noecho_getch());
     }
     return nil;
 }
diff --git a/module/matrix.cpp b/module/matrix.cpp
index 7f9a1a1..27d9ab6 100644
--- a/module/matrix.cpp
+++ b/module/matrix.cpp
@@ -24,8 +24,8 @@ var nas_vec2_add(var* args,usize size,gc* ngc) {
     if (v0.size()!=2 || v1.size()!=2)
         return nil;
     var res=ngc->alloc(vm_vec);
-    res.vec().elems.push_back({vm_num,v0[0].num()+v1[0].num()});
-    res.vec().elems.push_back({vm_num,v0[1].num()+v1[1].num()});
+    res.vec().elems.push_back(var::num(v0[0].num()+v1[0].num()));
+    res.vec().elems.push_back(var::num(v0[1].num()+v1[1].num()));
     return res;
 }
 
@@ -37,8 +37,8 @@ var nas_vec2_sub(var* args,usize size,gc* ngc) {
     if (v0.size()!=2 || v1.size()!=2)
         return nil;
     var res=ngc->alloc(vm_vec);
-    res.vec().elems.push_back({vm_num,v0[0].num()-v1[0].num()});
-    res.vec().elems.push_back({vm_num,v0[1].num()-v1[1].num()});
+    res.vec().elems.push_back(var::num(v0[0].num()-v1[0].num()));
+    res.vec().elems.push_back(var::num(v0[1].num()-v1[1].num()));
     return res;
 }
 
@@ -50,8 +50,8 @@ var nas_vec2_mult(var* args,usize size,gc* ngc) {
     if (v0.size()!=2 || v1.size()!=2)
         return nil;
     var res=ngc->alloc(vm_vec);
-    res.vec().elems.push_back({vm_num,v0[0].num()*v1[0].num()});
-    res.vec().elems.push_back({vm_num,v0[1].num()*v1[1].num()});
+    res.vec().elems.push_back(var::num(v0[0].num()*v1[0].num()));
+    res.vec().elems.push_back(var::num(v0[1].num()*v1[1].num()));
     return res;
 }
 
@@ -63,8 +63,8 @@ var nas_vec2_div(var* args,usize size,gc* ngc) {
     if (v0.size()!=2 || v1.size()!=2)
         return nil;
     var res=ngc->alloc(vm_vec);
-    res.vec().elems.push_back({vm_num,v0[0].num()/v1[0].num()});
-    res.vec().elems.push_back({vm_num,v0[1].num()/v1[1].num()});
+    res.vec().elems.push_back(var::num(v0[0].num()/v1[0].num()));
+    res.vec().elems.push_back(var::num(v0[1].num()/v1[1].num()));
     return res;
 }
 
@@ -75,8 +75,8 @@ var nas_vec2_neg(var* args,usize size,gc* ngc) {
     if (v0.size()!=2)
         return nil;
     var res=ngc->alloc(vm_vec);
-    res.vec().elems.push_back({vm_num,-v0[0].num()});
-    res.vec().elems.push_back({vm_num,-v0[1].num()});
+    res.vec().elems.push_back(var::num(-v0[0].num()));
+    res.vec().elems.push_back(var::num(-v0[1].num()));
     return res;
 }
 
@@ -90,8 +90,8 @@ var nas_vec2_norm(var* args,usize size,gc* ngc) {
     auto y=v0[1].num();
     auto t=std::sqrt(x*x+y*y);
     var res=ngc->alloc(vm_vec);
-    res.vec().elems.push_back({vm_num,x/t});
-    res.vec().elems.push_back({vm_num,y/t});
+    res.vec().elems.push_back(var::num(x/t));
+    res.vec().elems.push_back(var::num(y/t));
     return res;
 }
 
@@ -103,7 +103,7 @@ var nas_vec2_len(var* args,usize size,gc* ngc) {
         return nil;
     auto x=v0[0].num();
     auto y=v0[1].num();
-    return {vm_num,std::sqrt(x*x+y*y)};
+    return var::num(std::sqrt(x*x+y*y));
 }
 
 var nas_vec2_dot(var* args,usize size,gc* ngc) {
@@ -113,7 +113,7 @@ var nas_vec2_dot(var* args,usize size,gc* ngc) {
     auto& v1=args[1].vec().elems;
     if (v0.size()!=2 || v1.size()!=2)
         return nil;
-    return {vm_num,v0[0].num()*v1[0].num()+v0[1].num()*v1[1].num()};
+    return var::num(v0[0].num()*v1[0].num()+v0[1].num()*v1[1].num());
 }
 
 var nas_vec3_add(var* args,usize size,gc* ngc) {
@@ -124,9 +124,9 @@ var nas_vec3_add(var* args,usize size,gc* ngc) {
     if (v0.size()!=3 || v1.size()!=3)
         return nil;
     var res=ngc->alloc(vm_vec);
-    res.vec().elems.push_back({vm_num,v0[0].num()+v1[0].num()});
-    res.vec().elems.push_back({vm_num,v0[1].num()+v1[1].num()});
-    res.vec().elems.push_back({vm_num,v0[2].num()+v1[2].num()});
+    res.vec().elems.push_back(var::num(v0[0].num()+v1[0].num()));
+    res.vec().elems.push_back(var::num(v0[1].num()+v1[1].num()));
+    res.vec().elems.push_back(var::num(v0[2].num()+v1[2].num()));
     return res;
 }
 
@@ -138,9 +138,9 @@ var nas_vec3_sub(var* args,usize size,gc* ngc) {
     if (v0.size()!=3 || v1.size()!=3)
         return nil;
     var res=ngc->alloc(vm_vec);
-    res.vec().elems.push_back({vm_num,v0[0].num()-v1[0].num()});
-    res.vec().elems.push_back({vm_num,v0[1].num()-v1[1].num()});
-    res.vec().elems.push_back({vm_num,v0[2].num()-v1[2].num()});
+    res.vec().elems.push_back(var::num(v0[0].num()-v1[0].num()));
+    res.vec().elems.push_back(var::num(v0[1].num()-v1[1].num()));
+    res.vec().elems.push_back(var::num(v0[2].num()-v1[2].num()));
     return res;
 }
 
@@ -152,9 +152,9 @@ var nas_vec3_mult(var* args,usize size,gc* ngc) {
     if (v0.size()!=3 || v1.size()!=3)
         return nil;
     var res=ngc->alloc(vm_vec);
-    res.vec().elems.push_back({vm_num,v0[0].num()*v1[0].num()});
-    res.vec().elems.push_back({vm_num,v0[1].num()*v1[1].num()});
-    res.vec().elems.push_back({vm_num,v0[2].num()*v1[2].num()});
+    res.vec().elems.push_back(var::num(v0[0].num()*v1[0].num()));
+    res.vec().elems.push_back(var::num(v0[1].num()*v1[1].num()));
+    res.vec().elems.push_back(var::num(v0[2].num()*v1[2].num()));
     return res;
 }
 
@@ -166,9 +166,9 @@ var nas_vec3_div(var* args,usize size,gc* ngc) {
     if (v0.size()!=3 || v1.size()!=3)
         return nil;
     var res=ngc->alloc(vm_vec);
-    res.vec().elems.push_back({vm_num,v0[0].num()/v1[0].num()});
-    res.vec().elems.push_back({vm_num,v0[1].num()/v1[1].num()});
-    res.vec().elems.push_back({vm_num,v0[2].num()/v1[2].num()});
+    res.vec().elems.push_back(var::num(v0[0].num()/v1[0].num()));
+    res.vec().elems.push_back(var::num(v0[1].num()/v1[1].num()));
+    res.vec().elems.push_back(var::num(v0[2].num()/v1[2].num()));
     return res;
 }
 
@@ -179,9 +179,9 @@ var nas_vec3_neg(var* args,usize size,gc* ngc) {
     if (v0.size()!=3)
         return nil;
     var res=ngc->alloc(vm_vec);
-    res.vec().elems.push_back({vm_num,-v0[0].num()});
-    res.vec().elems.push_back({vm_num,-v0[1].num()});
-    res.vec().elems.push_back({vm_num,-v0[2].num()});
+    res.vec().elems.push_back(var::num(-v0[0].num()));
+    res.vec().elems.push_back(var::num(-v0[1].num()));
+    res.vec().elems.push_back(var::num(-v0[2].num()));
     return res;
 }
 
@@ -196,9 +196,9 @@ var nas_vec3_norm(var* args,usize size,gc* ngc) {
     auto z=v0[2].num();
     auto t=std::sqrt(x*x+y*y+z*z);
     var res=ngc->alloc(vm_vec);
-    res.vec().elems.push_back({vm_num,x/t});
-    res.vec().elems.push_back({vm_num,y/t});
-    res.vec().elems.push_back({vm_num,z/t});
+    res.vec().elems.push_back(var::num(x/t));
+    res.vec().elems.push_back(var::num(y/t));
+    res.vec().elems.push_back(var::num(z/t));
     return res;
 }
 
@@ -211,7 +211,7 @@ var nas_vec3_len(var* args,usize size,gc* ngc) {
     auto x=v0[0].num();
     auto y=v0[1].num();
     auto z=v0[2].num();
-    return {vm_num,std::sqrt(x*x+y*y+z*z)};
+    return var::num(std::sqrt(x*x+y*y+z*z));
 }
 
 var nas_rotate_x(var* args,usize size,gc* ngc) {
@@ -222,9 +222,9 @@ var nas_rotate_x(var* args,usize size,gc* ngc) {
         return nil;
     auto angle=args[1].num();
     var res=ngc->alloc(vm_vec);
-    res.vec().elems.push_back({vm_num,v0[0].num()});
-    res.vec().elems.push_back({vm_num,v0[2].num()*std::sin(angle)+v0[1].num()*std::cos(angle)});
-    res.vec().elems.push_back({vm_num,v0[2].num()*std::cos(angle)-v0[1].num()*std::sin(angle)});
+    res.vec().elems.push_back(var::num(v0[0].num()));
+    res.vec().elems.push_back(var::num(v0[2].num()*std::sin(angle)+v0[1].num()*std::cos(angle)));
+    res.vec().elems.push_back(var::num(v0[2].num()*std::cos(angle)-v0[1].num()*std::sin(angle)));
     return res;
 }
 
@@ -236,9 +236,9 @@ var nas_rotate_y(var* args,usize size,gc* ngc) {
         return nil;
     auto angle=args[1].num();
     var res=ngc->alloc(vm_vec);
-    res.vec().elems.push_back({vm_num,v0[0].num()*std::cos(angle)-v0[2].num()*std::sin(angle)});
-    res.vec().elems.push_back({vm_num,v0[1].num()});
-    res.vec().elems.push_back({vm_num,v0[0].num()*std::sin(angle)+v0[2].num()*std::cos(angle)});
+    res.vec().elems.push_back(var::num(v0[0].num()*std::cos(angle)-v0[2].num()*std::sin(angle)));
+    res.vec().elems.push_back(var::num(v0[1].num()));
+    res.vec().elems.push_back(var::num(v0[0].num()*std::sin(angle)+v0[2].num()*std::cos(angle)));
     return res;
 }
 
@@ -250,9 +250,9 @@ var nas_rotate_z(var* args,usize size,gc* ngc) {
         return nil;
     auto angle=args[1].num();
     var res=ngc->alloc(vm_vec);
-    res.vec().elems.push_back({vm_num,v0[0].num()*std::cos(angle)-v0[1].num()*std::sin(angle)});
-    res.vec().elems.push_back({vm_num,v0[0].num()*std::sin(angle)+v0[1].num()*std::cos(angle)});
-    res.vec().elems.push_back({vm_num,v0[2].num()});
+    res.vec().elems.push_back(var::num(v0[0].num()*std::cos(angle)-v0[1].num()*std::sin(angle)));
+    res.vec().elems.push_back(var::num(v0[0].num()*std::sin(angle)+v0[1].num()*std::cos(angle)));
+    res.vec().elems.push_back(var::num(v0[2].num()));
     return res;
 }
 
@@ -263,7 +263,7 @@ var nas_vec3_dot(var* args,usize size,gc* ngc) {
     auto& v1=args[1].vec().elems;
     if (v0.size()!=3 || v1.size()!=3)
         return nil;
-    return {vm_num,v0[0].num()*v1[0].num()+v0[1].num()*v1[1].num()+v0[2].num()*v1[2].num()};
+    return var::num(v0[0].num()*v1[0].num()+v0[1].num()*v1[1].num()+v0[2].num()*v1[2].num());
 }
 
 mod_func func_tbl[]={
diff --git a/module/nasocket.cpp b/module/nasocket.cpp
index 6abd935..fc13837 100644
--- a/module/nasocket.cpp
+++ b/module/nasocket.cpp
@@ -29,16 +29,16 @@ var nas_socket(var* args,usize size,gc* ngc) {
     if (args[0].type!=vm_num || args[1].type!=vm_num || args[2].type!=vm_num)
         return nas_err("socket","\"af\", \"type\", \"protocol\" should be number");
     int sd=socket(args[0].num(),args[1].num(),args[2].num());
-    return {vm_num,(double)sd};
+    return var::num((double)sd);
 }
 
 var nas_closesocket(var* args,usize size,gc* ngc) {
     if (args[0].type!=vm_num)
         return nas_err("closesocket","\"\" should be number");
 #ifdef _WIN32
-    return {vm_num,(double)closesocket(args[0].num())};
+    return var::num((double)closesocket(args[0].num()));
 #else
-    return {vm_num,(double)close(args[0].num())};
+    return var::num((double)close(args[0].num()));
 #endif
 }
 
@@ -47,7 +47,7 @@ var nas_shutdown(var* args,usize size,gc* ngc) {
         return nas_err("shutdown","\"sd\" must be a number");
     if (args[1].type!=vm_num)
         return nas_err("shutdown","\"how\" must be a number");
-    return {vm_num,(double)shutdown(args[0].num(),args[1].num())};
+    return var::num((double)shutdown(args[0].num(),args[1].num()));
 }
 
 var nas_bind(var* args,usize size,gc* ngc) {
@@ -62,7 +62,7 @@ var nas_bind(var* args,usize size,gc* ngc) {
     server.sin_family=AF_INET;
     server.sin_addr.s_addr=inet_addr(args[1].str().c_str());
     server.sin_port=htons(args[2].num());
-    return {vm_num,(double)bind(args[0].num(),(sockaddr*)&server,sizeof(server))};
+    return var::num((double)bind(args[0].num(),(sockaddr*)&server,sizeof(server)));
 }
 
 var nas_listen(var* args,usize size,gc* ngc) {
@@ -70,7 +70,7 @@ var nas_listen(var* args,usize size,gc* ngc) {
         return nas_err("listen","\"sd\" must be a number");
     if (args[1].type!=vm_num)
         return nas_err("listen","\"backlog\" must be a number");
-    return{vm_num,(double)listen(args[0].num(),args[1].num())};
+    return var::num((double)listen(args[0].num(),args[1].num()));
 }
 
 var nas_connect(var* args,usize size,gc* ngc) {
@@ -86,7 +86,7 @@ var nas_connect(var* args,usize size,gc* ngc) {
     addr.sin_port=htons(args[2].num());
     hostent* entry=gethostbyname(args[1].str().c_str());
     memcpy(&addr.sin_addr,entry->h_addr,entry->h_length);
-    return {vm_num,(double)connect(args[0].num(),(sockaddr*)&addr,sizeof(sockaddr_in))};
+    return var::num((double)connect(args[0].num(),(sockaddr*)&addr,sizeof(sockaddr_in)));
 }
 
 var nas_accept(var* args,usize size,gc* ngc) {
@@ -101,7 +101,7 @@ var nas_accept(var* args,usize size,gc* ngc) {
 #endif
     var res=ngc->temp=ngc->alloc(vm_hash);
     auto& hash=res.hash().elems;
-    hash["sd"]={vm_num,(double)client_sd};
+    hash["sd"]=var::num((double)client_sd);
     hash["ip"]=ngc->newstr(inet_ntoa(client.sin_addr));
     ngc->temp=nil;
     return res;
@@ -114,7 +114,7 @@ var nas_send(var* args,usize size,gc* ngc) {
         return nas_err("send","\"buff\" must be a string");
     if (args[2].type!=vm_num)
         return nas_err("send","\"flags\" muse be a number");
-    return {vm_num,(double)send(args[0].num(),args[1].str().c_str(),args[1].str().length(),args[2].num())};
+    return var::num((double)send(args[0].num(),args[1].str().c_str(),args[1].str().length(),args[2].num()));
 }
 
 var nas_sendto(var* args,usize size,gc* ngc) {
@@ -134,7 +134,7 @@ var nas_sendto(var* args,usize size,gc* ngc) {
     addr.sin_port=htons(args[2].num());
     hostent* entry=gethostbyname(args[1].str().c_str());
     memcpy(&addr.sin_addr,entry->h_addr,entry->h_length);
-    return {vm_num,(double)sendto(args[0].num(),args[3].str().c_str(),args[3].str().length(),args[4].num(),(sockaddr*)&addr,sizeof(sockaddr_in))};
+    return var::num((double)sendto(args[0].num(),args[3].str().c_str(),args[3].str().length(),args[4].num(),(sockaddr*)&addr,sizeof(sockaddr_in)));
 }
 
 var nas_recv(var* args,usize size,gc* ngc) {
@@ -149,7 +149,7 @@ var nas_recv(var* args,usize size,gc* ngc) {
     var res=ngc->temp=ngc->alloc(vm_hash);
     auto& hash=res.hash().elems;
     char* buf=new char[(int)args[1].num()];
-    hash["size"]={vm_num,(double)recv(args[0].num(),buf,args[1].num(),args[2].num())};
+    hash["size"]=var::num((double)recv(args[0].num(),buf,args[1].num(),args[2].num()));
     hash["str"]=ngc->newstr(buf);
     delete[] buf;
     ngc->temp=nil;
@@ -171,9 +171,9 @@ var nas_recvfrom(var* args,usize size,gc* ngc) {
     auto& hash=res.hash().elems;
     char* buf=new char[(int)args[1].num()+1];
 #ifdef _WIN32
-    hash["size"]={vm_num,(double)recvfrom(args[0].num(),buf,args[1].num(),args[2].num(),(sockaddr*)&addr,&socklen)};
+    hash["size"]=var::num((double)recvfrom(args[0].num(),buf,args[1].num(),args[2].num(),(sockaddr*)&addr,&socklen));
 #else
-    hash["size"]={vm_num,(double)recvfrom(args[0].num(),buf,args[1].num(),args[2].num(),(sockaddr*)&addr,(socklen_t*)&socklen)};
+    hash["size"]=var::num((double)recvfrom(args[0].num(),buf,args[1].num(),args[2].num(),(sockaddr*)&addr,(socklen_t*)&socklen));
 #endif
     buf[(int)hash["size"].num()]=0;
     hash["str"]=ngc->newstr(buf);
diff --git a/nasal_builtin.h b/nasal_builtin.h
index 8a3339a..8ee38cd 100644
--- a/nasal_builtin.h
+++ b/nasal_builtin.h
@@ -86,9 +86,9 @@ var builtin_setsize(var* local,gc& ngc) {
 var builtin_system(var* local,gc& ngc) {
     var str=local[1];
     if (str.type!=vm_str) {
-        return {vm_num,(f64)-1};
+        return var::num(-1);
     }
-    return {vm_num,(f64)system(str.str().c_str())};
+    return var::num((f64)system(str.str().c_str()));
 }
 
 var builtin_input(var* local,gc& ngc) {
@@ -180,7 +180,7 @@ var builtin_rand(var* local,gc& ngc) {
     for(u32 i=0;i<5;++i) {
         num=(num+rand())*(1.0/(RAND_MAX+1.0));
     }
-    return {vm_num,num};
+    return var::num(num);
 }
 
 var builtin_id(var* local,gc& ngc) {
@@ -198,12 +198,12 @@ var builtin_int(var* local,gc& ngc) {
     if (val.type!=vm_num && val.type!=vm_str) {
         return nil;
     }
-    return {vm_num,f64((i32)val.tonum())};
+    return var::num(f64((i32)val.tonum()));
 }
 
 var builtin_floor(var* local,gc& ngc) {
     var val=local[1];
-    return {vm_num,std::floor(val.num())};
+    return var::num(std::floor(val.num()));
 }
 
 var builtin_num(var* local,gc& ngc) {
@@ -218,7 +218,7 @@ var builtin_num(var* local,gc& ngc) {
     if (std::isnan(res)) {
         return nil;
     }
-    return {vm_num,res};
+    return var::num(res);
 }
 
 var builtin_pop(var* local,gc& ngc) {
@@ -248,100 +248,100 @@ var builtin_size(var* local,gc& ngc) {
         case vm_vec:  num=val.vec().size();  break;
         case vm_hash: num=val.hash().size(); break;
     }
-    return {vm_num,num};
+    return var::num(num);
 }
 
 var builtin_i32xor(var* local,gc& ngc) {
-    return {vm_num,(f64)(i32(local[1].num())^i32(local[2].num()))};
+    return var::num((f64)(i32(local[1].num())^i32(local[2].num())));
 }
 
 var builtin_i32and(var* local,gc& ngc) {
-    return {vm_num,(f64)(i32(local[1].num())&i32(local[2].num()))};
+    return var::num((f64)(i32(local[1].num())&i32(local[2].num())));
 }
 
 var builtin_i32or(var* local,gc& ngc) {
-    return {vm_num,(f64)(i32(local[1].num())|i32(local[2].num()))};
+    return var::num((f64)(i32(local[1].num())|i32(local[2].num())));
 }
 
 var builtin_i32nand(var* local,gc& ngc) {
-    return {vm_num,(f64)(~(i32(local[1].num())&i32(local[2].num())))};
+    return var::num((f64)(~(i32(local[1].num())&i32(local[2].num()))));
 }
 
 var builtin_i32not(var* local,gc& ngc) {
-    return {vm_num,(f64)(~i32(local[1].num()))};
+    return var::num((f64)(~i32(local[1].num())));
 }
 
 var builtin_u32xor(var* local,gc& ngc) {
-    return {vm_num,(f64)(u32(local[1].num())^u32(local[2].num()))};
+    return var::num((f64)(u32(local[1].num())^u32(local[2].num())));
 }
 
 var builtin_u32and(var* local,gc& ngc) {
-    return {vm_num,(f64)(u32(local[1].num())&u32(local[2].num()))};
+    return var::num((f64)(u32(local[1].num())&u32(local[2].num())));
 }
 
 var builtin_u32or(var* local,gc& ngc) {
-    return {vm_num,(f64)(u32(local[1].num())|u32(local[2].num()))};
+    return var::num((f64)(u32(local[1].num())|u32(local[2].num())));
 }
 
 var builtin_u32nand(var* local,gc& ngc) {
-    return {vm_num,(f64)(u32)(~(u32(local[1].num())&u32(local[2].num())))};
+    return var::num((f64)(u32)(~(u32(local[1].num())&u32(local[2].num()))));
 }
 
 var builtin_u32not(var* local,gc& ngc) {
-    return {vm_num,(f64)(u32)(~u32(local[1].num()))};
+    return var::num((f64)(u32)(~u32(local[1].num())));
 }
 
 var builtin_pow(var* local,gc& ngc) {
     var x=local[1];
     var y=local[2];
     if (x.type!=vm_num || y.type!=vm_num) {
-        return {vm_num,std::nan("")};
+        return var::num(std::nan(""));
     }
-    return {vm_num,std::pow(x.num(),y.num())};
+    return var::num(std::pow(x.num(),y.num()));
 }
 
 var builtin_sin(var* local,gc& ngc) {
     var val=local[1];
-    return {vm_num,val.type==vm_num?sin(val.num()):std::nan("")};
+    return var::num(val.type==vm_num?sin(val.num()):std::nan(""));
 }
 
 var builtin_cos(var* local,gc& ngc) {
     var val=local[1];
-    return {vm_num,val.type==vm_num?cos(val.num()):std::nan("")};
+    return var::num(val.type==vm_num?cos(val.num()):std::nan(""));
 }
 
 var builtin_tan(var* local,gc& ngc) {
     var val=local[1];
-    return {vm_num,val.type==vm_num?tan(val.num()):std::nan("")};
+    return var::num(val.type==vm_num?tan(val.num()):std::nan(""));
 }
 
 var builtin_exp(var* local,gc& ngc) {
     var val=local[1];
-    return {vm_num,val.type==vm_num?exp(val.num()):std::nan("")};
+    return var::num(val.type==vm_num?exp(val.num()):std::nan(""));
 }
 
 var builtin_lg(var* local,gc& ngc) {
     var val=local[1];
-    return {vm_num,val.type==vm_num?log(val.num())/log(10.0):std::nan("")};
+    return var::num(val.type==vm_num?log(val.num())/log(10.0):std::nan(""));
 }
 
 var builtin_ln(var* local,gc& ngc) {
     var val=local[1];
-    return {vm_num,val.type==vm_num?log(val.num()):std::nan("")};
+    return var::num(val.type==vm_num?log(val.num()):std::nan(""));
 }
 
 var builtin_sqrt(var* local,gc& ngc) {
     var val=local[1];
-    return {vm_num,val.type==vm_num?sqrt(val.num()):std::nan("")};
+    return var::num(val.type==vm_num?sqrt(val.num()):std::nan(""));
 }
 
 var builtin_atan2(var* local,gc& ngc) {
     var x=local[1];
     var y=local[2];
     if (x.type!=vm_num || y.type!=vm_num) {
-        return {vm_num,std::nan("")};
+        return var::num(std::nan(""));
     }
-    return {vm_num,atan2(y.num(),x.num())};
+    return var::num(atan2(y.num(),x.num()));
 }
 
 var builtin_isnan(var* local,gc& ngc) {
@@ -355,7 +355,7 @@ var builtin_time(var* local,gc& ngc) {
         return nas_err("time","\"begin\" must be number");
     }
     time_t begin=(time_t)val.num();
-    return {vm_num,(f64)time(&begin)};
+    return var::num((f64)time(&begin));
 }
 
 var builtin_contains(var* local,gc& ngc) {
@@ -406,9 +406,9 @@ var builtin_find(var* local,gc& ngc) {
     var haystack=local[2];
     usize pos=haystack.tostr().find(needle.tostr());
     if (pos==string::npos) {
-        return {vm_num,(f64)-1};
+        return var::num((f64)-1);
     }
-    return {vm_num,(f64)pos};
+    return var::num((f64)pos);
 }
 
 var builtin_type(var* local,gc& ngc) {
@@ -450,7 +450,7 @@ var builtin_substr(var* local,gc& ngc) {
 var builtin_streq(var* local,gc& ngc) {
     var a=local[1];
     var b=local[2];
-    return {vm_num,f64((a.type!=vm_str || b.type!=vm_str)?0:(a.str()==b.str()))};
+    return var::num(f64((a.type!=vm_str || b.type!=vm_str)?0:(a.str()==b.str())));
 }
 
 var builtin_left(var* local,gc& ngc) {
@@ -494,7 +494,7 @@ var builtin_cmp(var* local,gc& ngc) {
     if (a.type!=vm_str || b.type!=vm_str) {
         return nas_err("cmp","\"a\" and \"b\" must be string");
     }
-    return {vm_num,(f64)strcmp(a.str().c_str(),b.str().c_str())};
+    return var::num((f64)strcmp(a.str().c_str(),b.str().c_str()));
 }
 
 var builtin_chr(var* local,gc& ngc) {
@@ -600,7 +600,7 @@ var builtin_read(var* local,gc& ngc) {
     buf.str()=buff;
     buf.val.gcobj->unmut=true;
     delete []buff;
-    return {vm_num,res};
+    return var::num(res);
 }
 
 var builtin_write(var* local,gc& ngc) {
@@ -612,7 +612,7 @@ var builtin_write(var* local,gc& ngc) {
     if (str.type!=vm_str) {
         return nas_err("write","\"str\" must be string");
     }
-    return {vm_num,(f64)fwrite(str.str().c_str(),1,str.str().length(),(FILE*)fd.obj().ptr)};
+    return var::num((f64)fwrite(str.str().c_str(),1,str.str().length(),(FILE*)fd.obj().ptr));
 }
 
 var builtin_seek(var* local,gc& ngc) {
@@ -622,7 +622,7 @@ var builtin_seek(var* local,gc& ngc) {
     if (!fd.objchk(nas_obj::file)) {
         return nas_err("seek","not a valid filehandle");
     }
-    return {vm_num,(f64)fseek((FILE*)fd.obj().ptr,pos.num(),whence.num())};
+    return var::num((f64)fseek((FILE*)fd.obj().ptr,pos.num(),whence.num()));
 }
 
 var builtin_tell(var* local,gc& ngc) {
@@ -630,7 +630,7 @@ var builtin_tell(var* local,gc& ngc) {
     if (!fd.objchk(nas_obj::file)) {
         return nas_err("tell","not a valid filehandle");
     }
-    return {vm_num,(f64)ftell((FILE*)fd.obj().ptr)};
+    return var::num((f64)ftell((FILE*)fd.obj().ptr));
 }
 
 var builtin_readln(var* local,gc& ngc) {
@@ -666,17 +666,17 @@ var builtin_stat(var* local,gc& ngc) {
     }
     var ret=ngc.alloc(vm_vec);
     ret.vec().elems={
-        {vm_num,(f64)buf.st_dev},
-        {vm_num,(f64)buf.st_ino},
-        {vm_num,(f64)buf.st_mode},
-        {vm_num,(f64)buf.st_nlink},
-        {vm_num,(f64)buf.st_uid},
-        {vm_num,(f64)buf.st_gid},
-        {vm_num,(f64)buf.st_rdev},
-        {vm_num,(f64)buf.st_size},
-        {vm_num,(f64)buf.st_atime},
-        {vm_num,(f64)buf.st_mtime},
-        {vm_num,(f64)buf.st_ctime}
+        var::num((f64)buf.st_dev),
+        var::num((f64)buf.st_ino),
+        var::num((f64)buf.st_mode),
+        var::num((f64)buf.st_nlink),
+        var::num((f64)buf.st_uid),
+        var::num((f64)buf.st_gid),
+        var::num((f64)buf.st_rdev),
+        var::num((f64)buf.st_size),
+        var::num((f64)buf.st_atime),
+        var::num((f64)buf.st_mtime),
+        var::num((f64)buf.st_ctime)
     };
     return ret;
 }
@@ -686,7 +686,7 @@ var builtin_eof(var* local,gc& ngc) {
     if (!fd.objchk(nas_obj::file)) {
         return nas_err("readln","not a valid filehandle");
     }
-    return {vm_num,(f64)feof((FILE*)fd.obj().ptr)};
+    return var::num((f64)feof((FILE*)fd.obj().ptr));
 }
 
 var builtin_fld(var* local,gc& ngc) {
@@ -714,7 +714,7 @@ var builtin_fld(var* local,gc& ngc) {
             res|=1<<(bit+len-i-1);
         }
     }
-    return {vm_num,(f64)res};
+    return var::num((f64)res);
 }
 
 var builtin_sfld(var* local,gc& ngc) {
@@ -746,7 +746,7 @@ var builtin_sfld(var* local,gc& ngc) {
     if (res&(1<<(len-1))) {
         res|=~((1<<len)-1);
     }
-    return {vm_num,(f64)((i32)res)};
+    return var::num((f64)((i32)res));
 }
 
 var builtin_setfld(var* local,gc& ngc) {
@@ -808,8 +808,8 @@ var builtin_pipe(var* local,gc& ngc) {
     if (pipe(fd)==-1) {
         return nas_err("pipe","failed to create pipe");
     }
-    res.vec().elems.push_back({vm_num,(f64)fd[0]});
-    res.vec().elems.push_back({vm_num,(f64)fd[1]});
+    res.vec().elems.push_back(var::num((f64)fd[0]));
+    res.vec().elems.push_back(var::num((f64)fd[1]));
     return res;
 #endif
     return nas_err("pipe","not supported");
@@ -821,7 +821,7 @@ var builtin_fork(var* local,gc& ngc) {
     if (res<0) {
         return nas_err("fork","failed to fork a process");
     }
-    return {vm_num,(f64)res};
+    return var::num((f64)res);
 #endif
     return nas_err("fork","not supported");
 }
@@ -836,8 +836,8 @@ var builtin_waitpid(var* local,gc& ngc) {
     i32 ret_pid,status;
     ret_pid=waitpid(pid.num(),&status,nohang.num()==0?0:WNOHANG);
     var vec=ngc.alloc(vm_vec);
-    vec.vec().elems.push_back({vm_num,(f64)ret_pid});
-    vec.vec().elems.push_back({vm_num,(f64)status});
+    vec.vec().elems.push_back(var::num((f64)ret_pid));
+    vec.vec().elems.push_back(var::num((f64)status));
     return vec;
 #endif
     return nas_err("waitpid","not supported");
@@ -895,9 +895,9 @@ var builtin_closedir(var* local,gc& ngc) {
 var builtin_chdir(var* local,gc& ngc) {
     var path=local[1];
     if (path.type!=vm_str) {
-        return {vm_num,(f64)-1};
+        return var::num((f64)-1);
     }
-    return {vm_num,(f64)chdir(path.str().c_str())};
+    return var::num((f64)chdir(path.str().c_str()));
 }
 
 var builtin_environ(var* local,gc& ngc) {
@@ -1136,9 +1136,9 @@ var builtin_cocreate(var* local,gc& ngc) {
     cort.localr[0]=func.func().local[0];
     cort.top[0]=nil; // old upvalr
     cort.top++;
-    cort.top[0]={vm_addr,(var*)nullptr}; // old localr
+    cort.top[0]=var::addr((var*)nullptr); // old localr
     cort.top++;
-    cort.top[0]={vm_ret,(u32)0}; // old pc, set to zero to make op_ret recognizing this as coroutine function
+    cort.top[0]=var::ret(0); // old pc, set to zero to make op_ret recognizing this as coroutine function
 
     cort.funcr=func; // make sure the coroutine function can use correct upvalues
     cort.status=nas_co::suspended;
@@ -1216,7 +1216,7 @@ var builtin_millisec(var* local,gc& ngc) {
     f64 res=std::chrono::duration_cast<std::chrono::milliseconds>
             (std::chrono::high_resolution_clock::now().time_since_epoch())
             .count();
-    return {vm_num,res};
+    return var::num(res);
 }
 
 var builtin_sysargv(var* local,gc& ngc) {
diff --git a/nasal_gc.h b/nasal_gc.h
index 43bfd68..8a7f16d 100644
--- a/nasal_gc.h
+++ b/nasal_gc.h
@@ -39,6 +39,7 @@ enum vm_type:u8 {
     vm_obj,
     vm_co
 };
+
 const u32 gc_tsize=vm_co-vm_str+1;
 
 struct nas_vec;  // vector
@@ -59,27 +60,27 @@ struct var {
         nas_val* gcobj;
     } val;
 
-    // vm_none/vm_nil
-    var(const u8 t=vm_none):type(t) {}
-    // vm_ret
-    var(const u8 t,const u32 n):type(t) {val.ret=n;}
-    // vm_cnt
-    var(const u8 t,const i64 n):type(t) {val.cnt=n;}
-    // vm_num
-    var(const u8 t,const f64 n):type(t) {val.num=n;}
-    // nas_val
-    var(const u8 t,nas_val* n):type(t) {val.gcobj=n;}
-    // vm_addr
-    var(const u8 t,var* n):type(t) {val.addr=n;}
-    // copy
-    var(const var& nr):type(nr.type),val(nr.val) {}
+    var() = default;
+    var(const var&) = default;
     bool operator==(const var& nr) const {return type==nr.type && val.gcobj==nr.val.gcobj;}
     bool operator!=(const var& nr) const {return type!=nr.type || val.gcobj!=nr.val.gcobj;}
+
     // number and string can be translated to each other
-    f64    tonum();
+    f64 tonum();
     string tostr();
     friend std::ostream& operator<<(std::ostream&,var&);
-    bool   objchk(u32);
+    bool objchk(u32);
+
+    // create new var object
+    static var none();
+    static var nil();
+    static var ret(u32);
+    static var cnt(i64);
+    static var num(f64);
+    static var gcobj(nas_val*);
+    static var addr(var*);
+
+    // get content
     var*       addr();
     u32        ret ();
     i64&       cnt ();
@@ -95,21 +96,25 @@ struct var {
 
 struct nas_vec {
     std::vector<var> elems;
+
+    // mark if this is printed, avoid stackoverflow
     bool printed;
 
     nas_vec():printed(false) {}
     usize size() const {return elems.size();}
-    var  get_val(const i32);
+    var get_val(const i32);
     var* get_mem(const i32);
 };
 
 struct nas_hash {
     std::unordered_map<string,var> elems;
+
+    // mark if this is printed, avoid stackoverflow
     bool printed;
 
     nas_hash():printed(false) {}
     usize size() const {return elems.size();}
-    var  get_val(const string&);
+    var get_val(const string&);
     var* get_mem(const string&);
 };
 
@@ -127,13 +132,16 @@ struct nas_func {
 };
 
 struct nas_upval {
+    // if on stack, use these three variables
     bool onstk;
-    u32  size;
+    u32 size;
     var* stk;
+
+    // if not on stack, use this
     std::vector<var> elems;
 
     nas_upval() {onstk=true;stk=nullptr;size=0;}
-    var& operator[](usize n) {return onstk?stk[n]:elems[n];}
+    var& operator[](usize n) {return onstk? stk[n]:elems[n];}
     void clear() {onstk=true;elems.clear();size=0;}
 };
 
@@ -194,6 +202,8 @@ struct nas_co {
         running,
         dead
     };
+
+    // calculation stack
     var  stack[STACK_DEPTH];
     
     u32  pc;
@@ -205,29 +215,30 @@ struct nas_co {
     var  upvalr;
 
     u32 status;
-    nas_co() {
-        clear();
-    }
+    nas_co() {clear();}
     void clear() {
         for(u32 i=0;i<STACK_DEPTH;++i) {
-            stack[i]={vm_nil,(f64)0};
+            stack[i]=var::nil();
         }
         pc=0;
         localr=nullptr;
         memr=nullptr;
         top=stack;
         status=nas_co::suspended;
-        funcr={vm_nil,(f64)0};
-        upvalr={vm_nil,(f64)0};
+        funcr=var::nil();
+        upvalr=var::nil();
     }
 };
 
-const u8 GC_UNCOLLECTED=0;   
-const u8 GC_COLLECTED  =1;
-const u8 GC_FOUND      =2;
 struct nas_val {
-    u8 mark;
-    u8 type;
+    enum status:u8 {
+        uncollected=0,   
+        collected,
+        found
+    };
+
+    u8 mark; // mark if it is visited by gc or collected by gc
+    u8 type; // value type
     u8 unmut; // used to mark if a string is unmutable
     union {
         string*    str;
@@ -247,7 +258,7 @@ struct nas_val {
 var nas_vec::get_val(const i32 n) {
     i32 size=elems.size();
     if (n<-size || n>=size) {
-        return {vm_none};
+        return var::none();
     }
     return elems[n>=0?n:n+size];
 }
@@ -279,7 +290,7 @@ var nas_hash::get_val(const string& key) {
     if (elems.count(key)) {
         return elems.at(key);
     } else if (elems.count("parents")) {
-        var ret(vm_none);
+        var ret=var::none();
         var val=elems["parents"];
         if (val.type==vm_vec) {
             for(auto& i:val.vec().elems) {
@@ -292,7 +303,7 @@ var nas_hash::get_val(const string& key) {
             }
         }
     }
-    return {vm_none};
+    return var::none();
 }
 
 var* nas_hash::get_mem(const string& key) {
@@ -338,7 +349,7 @@ void nas_func::clear() {
 }
 
 nas_val::nas_val(u8 val_type) {
-    mark=GC_COLLECTED;
+    mark=status::collected;
     type=val_type;
     unmut=0;
     switch(val_type) {
@@ -378,7 +389,7 @@ void nas_val::clear() {
 }
 
 f64 var::tonum() {
-    return type!=vm_str?val.num:str2num(str().c_str());
+    return type!=vm_str? val.num:str2num(str().c_str());
 }
 
 string var::tostr() {
@@ -412,6 +423,54 @@ bool var::objchk(u32 objtype) {
     return type==vm_obj && obj().type==objtype && obj().ptr;
 }
 
+var var::none() {
+    var t;
+    t.type=vm_none;
+    return t;
+}
+
+var var::nil() {
+    var t;
+    t.type=vm_nil;
+    t.val.num=0;
+    return t;
+}
+
+var var::ret(u32 pc) {
+    var t;
+    t.type=vm_ret;
+    t.val.ret=pc;
+    return t;
+}
+
+var var::cnt(i64 n) {
+    var t;
+    t.type=vm_cnt;
+    t.val.cnt=n;
+    return t;
+}
+
+var var::num(f64 n) {
+    var t;
+    t.type=vm_num;
+    t.val.num=n;
+    return t;
+}
+
+var var::gcobj(nas_val* p) {
+    var t;
+    t.type=p->type; // use nas_val::type directly
+    t.val.gcobj=p;
+    return t;
+}
+
+var var::addr(var* p) {
+    var t;
+    t.type=vm_addr;
+    t.val.addr=p;
+    return t;
+}
+
 var*       var::addr () {return val.addr;             }
 u32        var::ret  () {return val.ret;              }
 i64&       var::cnt  () {return val.cnt;              }
@@ -424,9 +483,9 @@ nas_upval& var::upval() {return *val.gcobj->ptr.upval;}
 nas_obj&   var::obj  () {return *val.gcobj->ptr.obj;  }
 nas_co&    var::co   () {return *val.gcobj->ptr.co;   }
 
-const var zero={vm_num,(f64)0};
-const var one ={vm_num,(f64)1};
-const var nil ={vm_nil,(f64)0};
+const var zero=var::num(0);
+const var one =var::num(1);
+const var nil =var::nil();
 
 struct gc {
     /* main context */
@@ -461,6 +520,7 @@ struct gc {
     std::vector<nas_val*> memory; // gc memory
     std::vector<nas_val*> unused[gc_tsize]; // gc free list
 
+    // heap increase size
     u32 incr[gc_tsize]={
         128, // vm_str
         128, // vm_vec
@@ -488,10 +548,10 @@ struct gc {
     void init(const std::vector<string>&,const std::vector<string>&);
     void clear();
     void info();
-    var  alloc(const u8);
-    var  newstr(char);
-    var  newstr(const char*);
-    var  newstr(const string&);
+    var alloc(const u8);
+    var newstr(char);
+    var newstr(const char*);
+    var newstr(const string&);
     void ctxchg(nas_co&);
     void ctxreserve();
 };
@@ -509,7 +569,9 @@ void gc::mark() {
     bfs.push_back(funcr);
     bfs.push_back(upvalr);
     bfs.push_back(temp);
-    if (cort) { // scan main process stack
+
+    // if coroutine is running, scan main process stack from mctx
+    if (cort) {
         for(var* i=mctx.stack;i<=mctx.top;++i) {
             bfs.push_back(*i);
         }
@@ -523,7 +585,7 @@ void gc::mark() {
         if (tmp.type<=vm_num || tmp.val.gcobj->mark) {
             continue;
         }
-        tmp.val.gcobj->mark=GC_FOUND;
+        tmp.val.gcobj->mark=nas_val::status::found;
         switch(tmp.type) {
             case vm_vec:
                 for(auto& i:tmp.vec().elems) {
@@ -561,12 +623,12 @@ void gc::mark() {
 
 void gc::sweep() {
     for(auto i:memory) {
-        if (i->mark==GC_UNCOLLECTED) {
+        if (i->mark==nas_val::status::uncollected) {
             i->clear();
             unused[i->type-vm_str].push_back(i);
-            i->mark=GC_COLLECTED;
-        } else if (i->mark==GC_FOUND) {
-            i->mark=GC_UNCOLLECTED;
+            i->mark=nas_val::status::collected;
+        } else if (i->mark==nas_val::status::found) {
+            i->mark=nas_val::status::uncollected;
         }
     }
 }
@@ -586,25 +648,30 @@ void gc::extend(u8 type) {
 }
 
 void gc::init(const std::vector<string>& s,const std::vector<string>& argv) {
-    // initiaize function register
+    // initialize function register
     funcr=nil;
     worktime=0;
 
+    // initialize counters
     for(u8 i=0;i<gc_tsize;++i) {
         size[i]=gcnt[i]=acnt[i]=0;
     }
+
+    // coroutine pointer set to nullpre
     cort=nullptr;
+
     // init constant strings
     strs.resize(s.size());
     for(u32 i=0;i<strs.size();++i) {
-        strs[i]={vm_str,new nas_val(vm_str)};
+        strs[i]=var::gcobj(new nas_val(vm_str));
         strs[i].val.gcobj->unmut=1;
         strs[i].str()=s[i];
     }
+
     // record arguments
     env_argv.resize(argv.size());
     for(usize i=0;i<argv.size();++i) {
-        env_argv[i]={vm_str,new nas_val(vm_str)};
+        env_argv[i]=var::gcobj(new nas_val(vm_str));
         env_argv[i].val.gcobj->unmut=1;
         env_argv[i].str()=argv[i];
     }
@@ -646,10 +713,10 @@ void gc::info() {
             std::clog<<" | "<<std::left<<std::setw(maxlen)<<std::setfill(' ')<<size[i]<<"\n";
         }
     }
-    double t=worktime*1.0/1000000000; // seconds
-    std::clog<<" time  | "<<(t<0.1? t*1000:t)<<(t<0.1? " ms\n":" s\n");
+    double sec=worktime*1.0/1000000000; // seconds
+    std::clog<<" time  | "<<(sec<0.1? sec*1000:sec)<<(sec<0.1? " ms\n":" s\n");
     if (total) {
-        std::clog<<" avg   | "<<t/total*1000<<" ms\n";
+        std::clog<<" avg   | "<<sec/total*1000<<" ms\n";
     }
 }
 
@@ -666,8 +733,8 @@ var gc::alloc(u8 type) {
     if (unused[index].empty()) {
         extend(type);
     }
-    var ret={type,unused[index].back()};
-    ret.val.gcobj->mark=GC_UNCOLLECTED;
+    var ret=var::gcobj(unused[index].back());
+    ret.val.gcobj->mark=nas_val::status::uncollected;
     unused[index].pop_back();
     return ret;
 }
@@ -691,6 +758,7 @@ var gc::newstr(const string& buff) {
 }
 
 void gc::ctxchg(nas_co& ctx) {
+    // store running state to main context
     mctx.pc=pc;
     mctx.top=top;
     mctx.localr=localr;
@@ -700,6 +768,7 @@ void gc::ctxchg(nas_co& ctx) {
     mctx.canary=canary;
     mctx.stack=stack;
 
+    // restore coroutine context state
     pc=ctx.pc;
     top=ctx.top;
     localr=ctx.localr;
@@ -710,12 +779,15 @@ void gc::ctxchg(nas_co& ctx) {
     stack=ctx.stack;
     cort=&ctx;
 
+    // set coroutine state to running
     cort->status=nas_co::running;
 }
 
 void gc::ctxreserve() {
     // pc=0 means this coroutine is finished
     cort->status=pc? nas_co::suspended:nas_co::dead;
+
+    // store running state to coroutine
     cort->pc=pc;
     cort->localr=localr;
     cort->memr=memr;
@@ -724,6 +796,7 @@ void gc::ctxreserve() {
     cort->canary=canary;
     cort->top=top;
 
+    // restore main context state
     pc=mctx.pc;
     localr=mctx.localr;
     memr=mctx.memr;
@@ -738,7 +811,7 @@ void gc::ctxreserve() {
 // use to print error log and return error value
 var nas_err(const string& err_f,const string& info) {
     std::cerr<<"[vm] "<<err_f<<": "<<info<<"\n";
-    return {vm_none};
+    return var::none();
 }
 
 typedef var (*mod)(var*,usize,gc*); // module function type
diff --git a/nasal_vm.h b/nasal_vm.h
index c05745e..b365310 100644
--- a/nasal_vm.h
+++ b/nasal_vm.h
@@ -366,7 +366,7 @@ void vm::o_loadu() {
 }
 
 void vm::o_pnum() {
-    (++top)[0]={vm_num,cnum[imm[pc]]};
+    (++top)[0]=var::num(cnum[imm[pc]]);
 }
 
 void vm::o_pnil() {
@@ -421,7 +421,7 @@ void vm::o_para() {
     nas_func& func=top[0].func();
     // func->size has 1 place reserved for "me"
     func.keys[imm[pc]]=func.psize;
-    func.local[func.psize++]={vm_none};
+    func.local[func.psize++]=var::none();
 }
 
 void vm::o_deft() {
@@ -444,7 +444,7 @@ void vm::o_unot() {
         case vm_str:{
             const f64 num=str2num(val.str().c_str());
             if (std::isnan(num)) {
-                top[0]={vm_num,(f64)val.str().empty()};
+                top[0]=var::num((f64)val.str().empty());
             } else {
                 top[0]=num?zero:one;
             }
@@ -457,11 +457,11 @@ void vm::o_unot() {
 }
 
 void vm::o_usub() {
-    top[0]={vm_num,-top[0].tonum()};
+    top[0]=var::num(-top[0].tonum());
 }
 
 #define op_calc(type)\
-    top[-1]={vm_num,top[-1].tonum() type top[0].tonum()};\
+    top[-1]=var::num(top[-1].tonum() type top[0].tonum());\
     --top;
 
 void vm::o_add() {op_calc(+);}
@@ -474,7 +474,7 @@ void vm::o_lnk() {
 }
 
 #define op_calc_const(type)\
-    top[0]={vm_num,top[0].tonum() type cnum[imm[pc]]};
+    top[0]=var::num(top[0].tonum() type cnum[imm[pc]]);
 
 void vm::o_addc() {op_calc_const(+);}
 void vm::o_subc() {op_calc_const(-);}
@@ -485,7 +485,7 @@ void vm::o_lnkc() {
 }
 
 #define op_calc_eq(type)\
-    top[-1]=memr[0]={vm_num,memr[0].tonum() type top[-1].tonum()};\
+    top[-1]=memr[0]=var::num(memr[0].tonum() type top[-1].tonum());\
     memr=nullptr;\
     top-=imm[pc]+1;
 
@@ -500,7 +500,7 @@ void vm::o_lnkeq() {
 }
 
 #define op_calc_eq_const(type)\
-    top[0]=memr[0]={vm_num,memr[0].tonum() type cnum[imm[pc]&0x7fffffff]};\
+    top[0]=memr[0]=var::num(memr[0].tonum() type cnum[imm[pc]&0x7fffffff]);\
     memr=nullptr;\
     top-=(imm[pc]>>31);
 
@@ -601,7 +601,7 @@ void vm::o_cnt() {
         die("must use vector in forindex/foreach");
         return;
     }
-    (++top)[0]={vm_cnt,(i64)-1};
+    (++top)[0]=var::cnt(-1);
 }
 
 void vm::o_findex() {
@@ -609,7 +609,7 @@ void vm::o_findex() {
         pc=imm[pc]-1;
         return;
     }
-    top[1]={vm_num,(f64)top[0].cnt()};
+    top[1]=var::num(top[0].cnt());
     ++top;
 }
 
@@ -666,7 +666,7 @@ void vm::o_callv() {
             die("out of range:"+std::to_string(val.tonum()));
             return;
         }
-        top[0]={vm_num,f64((u8)str[num>=0? num:num+len])};
+        top[0]=var::num(f64((u8)str[num>=0? num:num+len]));
     } else {
         die("must call a vector/hash/string");
         return;
@@ -753,8 +753,8 @@ void vm::o_callfv() {
     }
 
     top[0]=upvalr;
-    (++top)[0]={vm_addr,localr};
-    (++top)[0]={vm_ret,pc};
+    (++top)[0]=var::addr(localr);
+    (++top)[0]=var::ret(pc);
     pc=func.entry-1;
     localr=local;
     upvalr=nil;
@@ -797,8 +797,8 @@ void vm::o_callfh() {
     }
 
     top[0]=upvalr;
-    (++top)[0]={vm_addr,localr};
-    (++top)[0]={vm_ret,pc}; // rewrite top with vm_ret
+    (++top)[0]=var::addr(localr);
+    (++top)[0]=var::ret(pc); // rewrite top with vm_ret
     pc=func.entry-1;
     localr=local;
     upvalr=nil;
@@ -806,12 +806,18 @@ void vm::o_callfh() {
 
 void vm::o_callb() {
     // reserve place for builtin function return,
-    // in fact this code is changed because of coroutine
+    // this code is written for coroutine
     (++top)[0]=nil;
-    // this ++top should not be used like: (++top)[0] here
-    // because if running a builtin function about coroutine
+
+    // if running a builtin function about coroutine
     // (top) will be set to another context.top, instead of main_context.top
-    top[0]=(*builtin[imm[pc]].func)(localr,ngc);
+    var tmp=(*builtin[imm[pc]].func)(localr,ngc);
+
+    // so we use tmp variable to store this return value
+    // and set it to top[0] later
+    top[0]=tmp;
+
+    // if get none, this means errors occurred when calling this native function
     if (top[0].type==vm_none) {
         die("native function error");
         return;