diff --git a/build.sh b/build.sh
index 2798482..9894bb1 100755
--- a/build.sh
+++ b/build.sh
@@ -1,3 +1,3 @@
 #!/bin/bash
 
-g++ -std=c++17 -I. lispoo.cpp -o lispoo
+g++ -Werror -std=c++20 -Isrc src/lispoo.cpp -o lispoo
diff --git a/core.h b/src/core.h
similarity index 100%
rename from core.h
rename to src/core.h
diff --git a/lispoo.cpp b/src/lispoo.cpp
similarity index 94%
rename from lispoo.cpp
rename to src/lispoo.cpp
index 05ddcae..6b1a1f9 100644
--- a/lispoo.cpp
+++ b/src/lispoo.cpp
@@ -15,7 +15,7 @@ int main(int argc, char* argv[]) {
   ss << ifs.rdbuf();
 
   lispoo::init();
-  unsigned long cursor = 0;
+  long cursor = 0;
   std::vector<std::string> tokens;
   lispoo::tokenize(ss.str(), tokens);
   auto expr = lispoo::parse(tokens, cursor);
diff --git a/lispoo.h b/src/lispoo.h
similarity index 96%
rename from lispoo.h
rename to src/lispoo.h
index d7334fb..f391411 100644
--- a/lispoo.h
+++ b/src/lispoo.h
@@ -70,7 +70,7 @@ static const std::shared_ptr<Expr> nil = std::make_shared<Null>();
 class List: public Expr {
 public:
   Type type() override { return Type::List; }
-  std::vector<std::shared_ptr<Expr>>& value() { return value_; }
+  const std::vector<std::shared_ptr<Expr>>& value() const { return value_; }
   void append(const std::shared_ptr<Expr>& expr) { value_.emplace_back(expr); }
 
 private:
@@ -162,7 +162,7 @@ inline void assert_type(const std::shared_ptr<Expr>& expr) {
     oops("syntax error");
   }
 }
-inline void assert_len(const std::shared_ptr<Expr>& expr, unsigned long expect) {
+inline void assert_len(const std::shared_ptr<Expr>& expr, long expect) {
   if (!is_type<Type::List>(expr)) {
     oops("assert_len() failed, not List type");
   }
@@ -190,7 +190,7 @@ inline void tokenize(const std::string& str, std::vector<std::string>& tokens) {
   }
 }
 
-inline std::shared_ptr<Expr> parse_atom(const std::vector<std::string>& tokens, unsigned long& cursor) {
+inline std::shared_ptr<Expr> parse_atom(const std::vector<std::string>& tokens, long& cursor) {
   auto token = tokens[cursor];
   auto type = Type::Symbol;
   if (std::isdigit(token[0]) || token[0] == '-') {
@@ -221,7 +221,7 @@ inline std::shared_ptr<Expr> parse_atom(const std::vector<std::string>& tokens,
   return nil;
 }
 
-inline std::shared_ptr<Expr> parse(const std::vector<std::string>& tokens, unsigned long& cursor) {
+inline std::shared_ptr<Expr> parse(const std::vector<std::string>& tokens, long& cursor) {
   if (cursor >= tokens.size()) {
     oops("parse error");
   }