diff --git a/include/pocketpy/compiler.h b/include/pocketpy/compiler.h
index 0b6d1134..4065c4c4 100644
--- a/include/pocketpy/compiler.h
+++ b/include/pocketpy/compiler.h
@@ -121,6 +121,7 @@ class Compiler {
 
 public:
     Compiler(VM* vm, std::string_view source, const Str& filename, CompileMode mode, bool unknown_global_scope=false);
+    void precompile();
     CodeObject_ compile();
 };
 
diff --git a/include/pocketpy/lexer.h b/include/pocketpy/lexer.h
index f5428346..29c2b676 100644
--- a/include/pocketpy/lexer.h
+++ b/include/pocketpy/lexer.h
@@ -59,14 +59,6 @@ struct Token{
 
   Str str() const { return Str(start, length);}
   std::string_view sv() const { return std::string_view(start, length);}
-
-  // Str info() const {
-  //   SStream ss;
-  //   ss << line << ": " << TK_STR(type) << " '" << (
-  //       sv()=="\n" ? "\\n" : sv()
-  //   ) << "'";
-  //   return ss.str();
-  // }
 };
 
 // https://docs.python.org/3/reference/expressions.html#operator-precedence
diff --git a/src/compiler.cpp b/src/compiler.cpp
index 92cff48e..c6200ac2 100644
--- a/src/compiler.cpp
+++ b/src/compiler.cpp
@@ -1225,6 +1225,35 @@ __EAT_DOTS_END:
         init_pratt_rules();
     }
 
+    void Compiler::precompile(){
+        SStream ss;
+        ss << PK_VERSION << '\n';                   // L1: version string
+        ss << lexer.src->filename << '\n';          // L2: filename
+        ss << mode() << '\n';                       // L3: compile mode
+        ss << (int)unknown_global_scope << '\n';    // L4: unknown global scope
+        auto tokens = lexer.run();
+        ss << '=' << (int)tokens.size() << '\n';    // L5: token count
+        for(auto token: lexer.run()){
+            ss << (int)token.type << '\n';
+            int offset = token.start - lexer.src->source.c_str();
+            ss << offset << '\n';
+            ss << token.length << '\n';
+            ss << token.line << '\n';
+            ss << token.brackets_level << '\n';
+            // visit token value
+            std::visit([&ss](auto&& arg){
+                using T = std::decay_t<decltype(arg)>;
+                if constexpr(std::is_same_v<T, i64>){
+                    ss << 'i' << arg << '\n';
+                }else if constexpr(std::is_same_v<T, f64>){
+                    ss << 'f' << arg << '\n';
+                }else if constexpr(std::is_same_v<T, Str>){
+                    ss << 's' << arg.escape() << '\n';
+                }
+            }, token.value);
+        }
+        std::cout << ss.str() << std::endl;
+    }
 
     CodeObject_ Compiler::compile(){
         PK_ASSERT(i == 0)       // make sure it is the first time to compile
diff --git a/src/lexer.cpp b/src/lexer.cpp
index 4ecf9629..64384827 100644
--- a/src/lexer.cpp
+++ b/src/lexer.cpp
@@ -486,6 +486,7 @@ static bool is_unicode_Lo_char(uint32_t c) {
     }
 
     std::vector<Token> Lexer::run() {
+        PK_ASSERT(curr_char == src->source.c_str());
         while (lex_one_token());
         return std::move(nexts);
     }