diff --git a/src/lox.cpp b/src/lox.cpp
index 969cff7..0e8212c 100644
--- a/src/lox.cpp
+++ b/src/lox.cpp
@@ -46,12 +46,12 @@ void run_prompt(void) {
 
   for (;;) {
     cout << ">";
-
     getline(cin, line);
-
     auto scan_result = run(line.substr(0, line.length()));
-    // print_tokens(&scan_result.token_list);
+
     if (is_ok(scan_result)) {
+      auto tokens = Ok(scan_result);
+      print_tokens(&tokens);
       auto expression = (new Parser())->parse(get<vector<Token>>(scan_result));
       if (is_ok(expression)) {
         cout << Ok(expression)->as_string() << "\n";
@@ -69,10 +69,9 @@ Result<vector<Token>> run(string source) {
   return scanner->scan_tokens();
 }
 
-void print_tokens(vector<Token> *list) {
-  for (vector<Token>::iterator token = list->begin(); token != list->end();
-       ++token) {
-    cout << token->as_string() << "(" << token->literal << "), ";
+void print_tokens(vector<Token> *const list) {
+  for (Token token : *list) {
+    cout << token.as_string() << "(" << token.literal << "), ";
   }
 
   cout << "\n";
diff --git a/src/parser.cpp b/src/parser.cpp
index b8b8212..21f1b11 100644
--- a/src/parser.cpp
+++ b/src/parser.cpp
@@ -9,7 +9,7 @@ Expression::~Expression() = default;
 
 // class Binary
 
-string Binary::as_string() {
+string Binary::as_string() const {
   return "(" + token_name(op->tokentype) + " " + left->as_string() + " " +
          right->as_string() + ")";
 }
@@ -21,7 +21,7 @@ Binary::~Binary() = default;
 
 // class Grouping
 
-string Grouping::as_string() { return "(" + expr->as_string() + ")"; }
+string Grouping::as_string() const { return "(" + expr->as_string() + ")"; }
 
 Grouping::Grouping(Expression *_expr) : expr(_expr){};
 
@@ -29,7 +29,7 @@ Grouping::~Grouping() = default;
 
 // class Unary
 
-string Unary::as_string() {
+string Unary::as_string() const {
   return token_name(op->tokentype) + right->as_string();
 }
 
@@ -39,7 +39,7 @@ Unary::Unary(Token *_operator, Expression *_right)
 Unary::~Unary() = default;
 
 // class Literal
-string Literal::as_string() {
+string Literal::as_string() const {
   string text;
   if (holds_alternative<string>(value)) {
     return "\"" + get<string>(value) + "\"";
diff --git a/src/parser.hpp b/src/parser.hpp
index d8835ed..a4804b6 100644
--- a/src/parser.hpp
+++ b/src/parser.hpp
@@ -11,7 +11,7 @@ enum class ExprType { Binary, Grouping, Unary, Literal, None };
 /// Base class for expressions
 class Expression {
 public:
-  virtual std::string as_string() = 0; // get string rep for debugging
+  virtual std::string as_string() const = 0; // get string rep for debugging
   virtual ~Expression();
 };
 
@@ -22,7 +22,7 @@ class Binary : public Expression {
   std::unique_ptr<Expression> right;
 
 public:
-  std::string as_string() override;
+  std::string as_string() const override;
   Binary(Expression *_left, Token *_operator, Expression *_right);
   ~Binary();
 };
@@ -32,7 +32,7 @@ class Grouping : public Expression {
   std::unique_ptr<Expression> expr;
 
 public:
-  std::string as_string() override;
+  std::string as_string() const override;
   Grouping(Expression *_expr);
   ~Grouping();
 };
@@ -43,7 +43,7 @@ class Unary : public Expression {
   std::unique_ptr<Expression> right;
 
 public:
-  std::string as_string() override;
+  std::string as_string() const override;
   Unary(Token *_operator, Expression *_right);
   ~Unary();
 };
@@ -64,7 +64,7 @@ public:
   Literal(std::string _str) : value(_str){};
   Literal(bool _boolean) : value(_boolean){};
 
-  std::string as_string() override;
+  std::string as_string() const override;
 };
 
 class Parser {
diff --git a/src/tokens.cpp b/src/tokens.cpp
index 037b6a1..5d338d8 100644
--- a/src/tokens.cpp
+++ b/src/tokens.cpp
@@ -16,4 +16,4 @@ string token_name(Token::Type tokentype) {
   return tokens[(int)tokentype];
 }
 
-std::string Token::as_string() { return token_name(tokentype); }
+std::string Token::as_string() const { return token_name(tokentype); }
diff --git a/src/tokens.hpp b/src/tokens.hpp
index 1cd9b69..4d796a0 100644
--- a/src/tokens.hpp
+++ b/src/tokens.hpp
@@ -49,7 +49,7 @@ public:
     WHILE = 38,
   } tokentype;
 
-  std::string as_string();
+  std::string as_string() const;
 
   Token(Token::Type _tokentype, std::string _lexeme, std::string _literal,
         int line);