diff --git a/ast/ast_dumper.cpp b/ast/ast_dumper.cpp
index 0aaba4a..89293c6 100644
--- a/ast/ast_dumper.cpp
+++ b/ast/ast_dumper.cpp
@@ -2,367 +2,434 @@
 
 #include <iostream>
 
-bool ast_dumper::visit_expr(expr* node) {
-    std::cout << "expr\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
-    node->accept(this);
-    return true;
-}
-
 bool ast_dumper::visit_null_expr(null_expr* node) {
-    std::cout << "null\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "null" << format_location(node->get_location());
     return true;
 }
 
 bool ast_dumper::visit_nil_expr(nil_expr* node) {
-    std::cout << "nil\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "nil" << format_location(node->get_location());
     return true;
 }
 
 bool ast_dumper::visit_number_literal(number_literal* node) {
-    std::cout << "number " << node->get_number() << "\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "number " << node->get_number();
+    std::cout << format_location(node->get_location());
     return true;
 }
 
 bool ast_dumper::visit_string_literal(string_literal* node) {
-    std::cout << "string " << node->get_content() << "\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "string \"" << rawstr(node->get_content()) << "\"";
+    std::cout << format_location(node->get_location());
     return true;
 }
 
 bool ast_dumper::visit_identifier(identifier* node) {
-    std::cout << "identifier " << node->get_name() << "\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "identifier " << node->get_name();
+    std::cout << format_location(node->get_location());
     return true;
 }
 
 bool ast_dumper::visit_bool_literal(bool_literal* node) {
-    std::cout << "bool " << node->get_flag() << "\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "bool " << node->get_flag();
+    std::cout << format_location(node->get_location());
     return true;
 }
 
 bool ast_dumper::visit_vector_expr(vector_expr* node) {
-    std::cout << "vector\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "vector";
+    std::cout << format_location(node->get_location());
+    push_indent();
     for(auto i : node->get_elements()) {
+        if (i==node->get_elements().back()) {
+            set_last();
+        }
         i->accept(this);
     }
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_hash_expr(hash_expr* node) {
-    std::cout << "hash\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "hash";
+    std::cout << format_location(node->get_location());
+    push_indent();
     for(auto i : node->get_members()) {
+        if (i==node->get_members().back()) {
+            set_last();
+        }
         i->accept(this);
     }
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_hash_pair(hash_pair* node) {
-    std::cout << "pair\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "pair " << node->get_name();
+    std::cout << format_location(node->get_location());
     if (node->get_value()) {
+        push_indent();
+        set_last();
         node->get_value()->accept(this);
+        pop_indent();
     }
     return true;
 }
 
 bool ast_dumper::visit_function(function* node) {
-    std::cout << "function\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "function";
+    std::cout << format_location(node->get_location());
+    push_indent();
     for(auto i : node->get_parameter_list()) {
         i->accept(this);
     }
+    set_last();
     node->get_code_block()->accept(this);
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_code_block(code_block* node) {
-    std::cout << "block\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "block";
+    std::cout << format_location(node->get_location());
+    push_indent();
     for(auto i : node->get_expressions()) {
+        if (i==node->get_expressions().back()) {
+            set_last();
+        }
         i->accept(this);
     }
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_parameter(parameter* node) {
-    std::cout << "parameter\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "parameter";
+    std::cout << format_location(node->get_location());
+    push_indent();
+    if (!node->get_default_value()) {
+        set_last();
+    }
     node->get_parameter_name()->accept(this);
     if (node->get_default_value()) {
+        set_last();
         node->get_default_value()->accept(this);
     }
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_ternary_operator(ternary_operator* node) {
-    std::cout << "ternary\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "ternary";
+    std::cout << format_location(node->get_location());
+    push_indent();
     node->get_condition()->accept(this);
     node->get_left()->accept(this);
+    set_last();
     node->get_right()->accept(this);
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_binary_operator(binary_operator* node) {
-    std::cout << "binary\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "binary";
+    std::cout << format_location(node->get_location());
+    push_indent();
     node->get_left()->accept(this);
+    set_last();
     node->get_right()->accept(this);
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_unary_operator(unary_operator* node) {
-    std::cout << "unary\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "unary";
+    std::cout << format_location(node->get_location());
+    push_indent();
+    set_last();
     node->get_value()->accept(this);
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_call_expr(call_expr* node) {
-    std::cout << "call_expr\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "call_expr";
+    std::cout << format_location(node->get_location());
+    push_indent();
+    if (!node->get_calls().size()) {
+        set_last();
+    }
     node->get_first()->accept(this);
     for(auto i : node->get_calls()) {
+        if (i==node->get_calls().back()) {
+            set_last();
+        }
         i->accept(this);
     }
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_call_hash(call_hash* node) {
-    std::cout << "call_hash\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "call_hash" << node->get_field();
+    std::cout << format_location(node->get_location());
     return true;
 }
 
 bool ast_dumper::visit_call_vector(call_vector* node) {
-    std::cout << "call_vector\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "call_vector";
+    std::cout << format_location(node->get_location());
+    push_indent();
     for(auto i : node->get_slices()) {
+        if (i==node->get_slices().back()) {
+            set_last();
+        }
         i->accept(this);
     }
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_call_function(call_function* node) {
-    std::cout << "call_function\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "call_function";
+    std::cout << format_location(node->get_location());
+    push_indent();
     for(auto i : node->get_argument()) {
+        if (i==node->get_argument().back()) {
+            set_last();
+        }
         i->accept(this);
     }
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_slice_vector(slice_vector* node) {
-    std::cout << "slice\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "slice";
+    std::cout << format_location(node->get_location());
+    push_indent();
+    if (!node->get_end()) {
+        set_last();
+    }
     node->get_begin()->accept(this);
     if (node->get_end()) {
+        set_last();
         node->get_end()->accept(this);
     }
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_definition_expr(definition_expr* node) {
-    std::cout << "definition\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "definition";
+    std::cout << format_location(node->get_location());
+    push_indent();
     if (node->get_variable_name()) {
         node->get_variable_name()->accept(this);
     } else {
         node->get_variables()->accept(this);
     }
+    set_last();
     node->get_value()->accept(this);
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_assignment_expr(assignment_expr* node) {
-    std::cout << "assignment\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "assignment";
+    std::cout << format_location(node->get_location());
+    push_indent();
     node->get_left()->accept(this);
+    set_last();
     node->get_right()->accept(this);
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_multi_identifier(multi_identifier* node) {
-    std::cout << "multi_define\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "multi_define";
+    std::cout << format_location(node->get_location());
+    push_indent();
     for(auto i : node->get_variables()) {
+        if (i==node->get_variables().back()) {
+            set_last();
+        }
         i->accept(this);
     }
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_tuple_expr(tuple_expr* node) {
-    std::cout << "tuple\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "tuple";
+    std::cout << format_location(node->get_location());
+    push_indent();
     for(auto i : node->get_elements()) {
+        if (i==node->get_elements().back()) {
+            set_last();
+        }
         i->accept(this);
     }
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_multi_assign(multi_assign* node) {
-    std::cout << "multi_assign\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "multi_assign";
+    std::cout << format_location(node->get_location());
+    push_indent();
     node->get_tuple()->accept(this);
+    set_last();
     node->get_value()->accept(this);
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_while_expr(while_expr* node) {
-    std::cout << "while\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "while";
+    std::cout << format_location(node->get_location());
+    push_indent();
     node->get_condition()->accept(this);
+    set_last();
     node->get_code_block()->accept(this);
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_for_expr(for_expr* node) {
-    std::cout << "for\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "for";
+    std::cout << format_location(node->get_location());
+    push_indent();
     node->get_initial()->accept(this);
     node->get_condition()->accept(this);
     node->get_step()->accept(this);
+    set_last();
     node->get_code_block()->accept(this);
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_iter_expr(iter_expr* node) {
-    std::cout << "iter\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "iter";
+    std::cout << format_location(node->get_location());
+    push_indent();
+    set_last();
     if (node->get_name()) {
         node->get_name()->accept(this);
     } else {
         node->get_call()->accept(this);
     }
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_forei_expr(forei_expr* node) {
-    std::cout << "forei\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    if (node->get_type()==forei_expr::forei_loop_type::foreach) {
+        std::cout << "foreach";
+    } else {
+        std::cout << "forindex";
+    }
+    std::cout << format_location(node->get_location());
+    push_indent();
     node->get_iterator()->accept(this);
     node->get_value()->accept(this);
+    set_last();
     node->get_code_block()->accept(this);
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_condition_expr(condition_expr* node) {
-    std::cout << "condition\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "condition";
+    std::cout << format_location(node->get_location());
+    push_indent();
+    if (!node->get_elsif_stataments().size() &&
+        !node->get_else_statement()) {
+        set_last();
+    }
     node->get_if_statement()->accept(this);
     for(auto i : node->get_elsif_stataments()) {
+        if (i==node->get_elsif_stataments().back() &&
+            !node->get_else_statement()) {
+            set_last();
+        }
         i->accept(this);
     }
     if (node->get_else_statement()) {
+        set_last();
         node->get_else_statement()->accept(this);
     }
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_if_expr(if_expr* node) {
-    std::cout << "if\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "if";
+    std::cout << format_location(node->get_location());
+    push_indent();
     if (node->get_condition()) {
         node->get_condition()->accept(this);
     }
+    set_last();
     node->get_code_block()->accept(this);
+    pop_indent();
     return true;
 }
 
 bool ast_dumper::visit_continue_expr(continue_expr* node) {
-    std::cout << "continue\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "continue";
+    std::cout << format_location(node->get_location());
     return true;
 }
 
 bool ast_dumper::visit_break_expr(break_expr* node) {
-    std::cout << "break\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "break";
+    std::cout << format_location(node->get_location());
     return true;
 }
 
 bool ast_dumper::visit_return_expr(return_expr* node) {
-    std::cout << "return\n";
-    std::cout << " -> " << node->get_location().file << ":"
-    << node->get_location().begin_line << ":"
-    << node->get_location().begin_column << "\n";
+    dump_indent();
+    std::cout << "return";
+    std::cout << format_location(node->get_location());
     if (node->get_value()) {
+        push_indent();
+        set_last();
         node->get_value()->accept(this);
+        pop_indent();
     }
     return true;
 }
\ No newline at end of file
diff --git a/ast/ast_dumper.h b/ast/ast_dumper.h
index 4599537..e6fbb51 100644
--- a/ast/ast_dumper.h
+++ b/ast/ast_dumper.h
@@ -2,9 +2,44 @@
 
 #include "ast_visitor.h"
 
+#include <iostream>
+#include <cstring>
+#include <sstream>
+
 class ast_dumper:public ast_visitor {
+private:
+    std::vector<std::string> indent;
+    void push_indent() {
+        if (indent.size()) {
+            if (indent.back()=="|--") {
+                indent.back() = "|  ";
+            } else if (indent.back()=="+--") {
+                indent.back() = "   ";
+            }
+            // indent.back() = (indent.back()=="|--")? "|  ":"   ";
+        }
+        indent.push_back("|--");
+    }
+    void pop_indent() {indent.pop_back();}
+    void set_last() {indent.back() = "+--";}
+    void dump_indent() {
+        if (indent.size() && indent.back()=="|  ") {
+            indent.back() = "|--";
+        }
+        for(const auto& i : indent) {
+            std::cout << i;
+        }
+    }
+    std::string format_location(const span& location) {
+        std::stringstream ss;
+        ss << " -> ";
+        ss << location.file << ":";
+        ss << location.begin_line << ":" << location.begin_column;
+        ss << "\n";
+        return ss.str();
+    }
+
 public:
-    bool visit_expr(expr*) override;
     bool visit_null_expr(null_expr*) override;
     bool visit_nil_expr(nil_expr*) override;
     bool visit_number_literal(number_literal*) override;
diff --git a/ast/nasal_new_main.cpp b/ast/nasal_new_main.cpp
index 3f473b8..6ba6175 100644
--- a/ast/nasal_new_main.cpp
+++ b/ast/nasal_new_main.cpp
@@ -51,7 +51,7 @@ std::ostream& logo(std::ostream& out) {
     <<"  \\_\\ \\/ \\__,_|___/\\__,_|_|\n"
     <<"ver  : "<<__nasver<<" ("<<__DATE__<<" "<<__TIME__<<")\n"
     <<"std  : c++ "<<__cplusplus<<"\n"
-    <<"core : "<<std::thread::hardware_concurrency()<<" cores\n"
+    <<"core : "<<std::thread::hardware_concurrency()<<" core(s)\n"
     <<"repo : https://github.com/ValKmjolnir/Nasal-Interpreter\n"
     <<"repo : https://gitee.com/valkmjolnir/Nasal-Interpreter\n"
     <<"wiki : https://wiki.flightgear.org/Nasal_scripting_language\n"
@@ -89,7 +89,7 @@ void execute(
     parse.compile(lex).chkerr();
     if (cmd&VM_AST) {
         auto dumper = new ast_dumper();
-        dumper->visit_expr(parse.tree());
+        dumper->visit_code_block(parse.tree());
     }
 
     // linker gets parser's ast and load import files to this ast
diff --git a/ast/nasal_new_parse.cpp b/ast/nasal_new_parse.cpp
index 00e3727..e175910 100644
--- a/ast/nasal_new_parse.cpp
+++ b/ast/nasal_new_parse.cpp
@@ -610,7 +610,7 @@ expr* parse::scalar() {
     } else if (lookahead(tok::lbrace)) {
         node = hash();
     } else if (lookahead(tok::lcurve)) {
-        const auto& loc=toks[ptr].loc;
+        const auto& loc = toks[ptr].loc;
         match(tok::lcurve);
         node = calc();
         node->set_begin(loc.begin_line, loc.begin_column);
@@ -625,7 +625,7 @@ expr* parse::scalar() {
         node = def_node;
     } else {
         die(thisspan, "expected scalar");
-        return node;
+        return null();
     }
     // check call and avoid ambiguous syntax
     if (is_call(toks[ptr].type) && !(lookahead(tok::lcurve) && toks[ptr+1].type==tok::var)) {
@@ -839,7 +839,10 @@ expr* parse::loop() {
         case tok::rfor:    node = for_loop();   break;
         case tok::forindex:
         case tok::foreach: node = forei_loop(); break;
-        default: break;
+        default:
+            die(thisspan, "unreachable");
+            node = null();
+            break;
     }
     --in_loop;
     return node;
diff --git a/main.cpp b/main.cpp
index aef7268..084e28d 100644
--- a/main.cpp
+++ b/main.cpp
@@ -11,6 +11,7 @@
 #include "nasal_vm.h"
 #include "nasal_dbg.h"
 
+#include <thread>
 #include <unordered_map>
 
 const u32 VM_AST   =0x01;
@@ -55,6 +56,7 @@ std::ostream& logo(std::ostream& out) {
     <<"  \\_\\ \\/ \\__,_|___/\\__,_|_|\n"
     <<"ver  : "<<__nasver<<" ("<<__DATE__<<" "<<__TIME__<<")\n"
     <<"std  : c++ "<<__cplusplus<<"\n"
+    <<"core : "<<std::thread::hardware_concurrency()<<" core(s)\n"
     <<"repo : https://github.com/ValKmjolnir/Nasal-Interpreter\n"
     <<"repo : https://gitee.com/valkmjolnir/Nasal-Interpreter\n"
     <<"wiki : https://wiki.flightgear.org/Nasal_scripting_language\n"
@@ -90,15 +92,15 @@ void execute(
 
     // parser gets lexer's token list to compile
     parse.compile(lex).chkerr();
+    if (cmd&VM_AST) {
+        parse.tree().dump();
+    }
 
     // linker gets parser's ast and load import files to this ast
     ld.link(parse, file, cmd&VM_DETAIL).chkerr();
 
     // optimizer does simple optimization on ast
     optimize(parse.tree());
-    if (cmd&VM_AST) {
-        parse.tree().dump();
-    }
 
     // code generator gets parser's ast and import file list to generate code
     gen.compile(parse, ld).chkerr();
diff --git a/nasal.h b/nasal.h
index 9624c35..7290f62 100644
--- a/nasal.h
+++ b/nasal.h
@@ -105,7 +105,7 @@ using usize=std::size_t;
 using f64=double;
 using std::string;
 
-const u32 STACK_DEPTH=1024;
+const u32 STACK_DEPTH=4096;
 
 f64 hex2f(const char* str) {
     f64 ret=0;