diff --git a/lib/error.ml b/lib/error.ml
index 475030e..adab803 100644
--- a/lib/error.ml
+++ b/lib/error.ml
@@ -6,8 +6,8 @@ module LexerError = struct
 
   let make (pos : code_pos) (msg : string) : lexer_error = { pos; msg }
 
-  let print (e : lexer_error) =
-    Printf.printf "LexerError at line %d, column %d: %s\n" e.pos.line e.pos.col e.msg
+  let show (e : lexer_error) =
+    Printf.sprintf "LexerError at line %d, column %d: %s" e.pos.line e.pos.col e.msg
 end
 
 type parser_error = { pos : code_pos; msg : string }
@@ -17,8 +17,8 @@ module ParserError = struct
 
   let make (pos : code_pos) (msg : string) : parser_error = { pos; msg }
 
-  let print (e : parser_error) =
-    Printf.printf "ParserError at line %d, column %d: %s\n" e.pos.line e.pos.col e.msg
+  let show (e : parser_error) =
+    Printf.sprintf "ParserError at line %d, column %d: %s" e.pos.line e.pos.col e.msg
 end
 
 (* type runtime_error = { pos : code_pos; msg : string; type_ : runtime_error_type } *)
@@ -31,10 +31,10 @@ module RuntimeError = struct
   let break () : runtime_error = Break
   let continue () : runtime_error = Continue
 
-  let print (e : runtime_error) =
+  let show (e : runtime_error) =
     match e with
     | Error { pos; msg } ->
-        Printf.printf "RuntimeError at line %d, column %d: %s\n" pos.line pos.col msg
+        Printf.sprintf "RuntimeError at line %d, column %d: %s" pos.line pos.col msg
     | Break | Continue -> assert false
 end
 
@@ -48,16 +48,16 @@ let print_error (e : lox_error) =
   | LexerError es ->
       let num_errors = List.length es in
       assert (num_errors <> 0);
-      Printf.printf "found %d %s:\n" num_errors
+      Printf.fprintf stderr "found %d %s:\n" num_errors
         (if num_errors = 1 then "LexerError" else "LexerErrors");
-      List.iter LexerError.print es
+      List.iter (fun e -> LexerError.show e |> Printf.fprintf stderr "%s\n") es
   | ParserError es ->
       let num_errors = List.length es in
       assert (num_errors <> 0);
-      Printf.printf "found %d %s:\n" num_errors
+      Printf.fprintf stderr "found %d %s:\n" num_errors
         (if num_errors = 1 then "ParserError" else "ParserErrors");
-      List.iter ParserError.print es
-  | RuntimeError e -> RuntimeError.print e
+      List.iter (fun e -> ParserError.show e |> Printf.fprintf stderr "%s\n") es
+  | RuntimeError e -> RuntimeError.show e |> Printf.fprintf stderr "%s\n"
 
 let of_lexer_error e = Result.map_error (fun e -> LexerError e) e
 let of_parser_error e = Result.map_error (fun e -> ParserError e) e
diff --git a/lib/interpreter.ml b/lib/interpreter.ml
index a598516..a4014a1 100644
--- a/lib/interpreter.ml
+++ b/lib/interpreter.ml
@@ -83,10 +83,7 @@ let rec interpret_stmt (env : environment) (stmt_node : stmt_node) : (unit, runt
   | Expr expr ->
       let* _ = interpret_expr env expr in
       Ok ()
-  | Break ->
-      (* print_endline "break!";
-         Ok () (* TODO *) *)
-      RuntimeError.break () |> Result.error
+  | Break -> RuntimeError.break () |> Result.error
   | Continue -> RuntimeError.continue () |> Result.error
   | Print expr ->
       let* value = interpret_expr env expr in
diff --git a/lib/lox.ml b/lib/lox.ml
index 3e60171..5a43f1e 100644
--- a/lib/lox.ml
+++ b/lib/lox.ml
@@ -16,13 +16,13 @@ let run ?(env : Environment.environment option) ?(debug = false) (source : strin
   let () =
     if debug then
       let print_tokens () =
-        print_endline "--- Tokens ---";
-        let f token = Printf.printf "%s " (Lexer.show_token token) in
-        Printf.printf "Got %d tokens\n" (List.length tokens);
+        Printf.fprintf stderr "--- Tokens ---\n";
+        let f token = Printf.fprintf stderr "%s " (Lexer.show_token token) in
+        Printf.fprintf stderr "Got %d tokens\n" (List.length tokens);
         List.iter f tokens;
-        print_newline ();
-        print_endline "--------------";
-        print_newline ()
+        Printf.fprintf stderr "\n";
+        Printf.fprintf stderr "--------------\n";
+        Printf.fprintf stderr "\n"
       in
       print_tokens ()
     else ()
@@ -31,11 +31,11 @@ let run ?(env : Environment.environment option) ?(debug = false) (source : strin
   let () =
     if debug then
       let print_statements () =
-        print_endline "--- Statements ---";
-        let f (stmt : Stmt.stmt_node) = print_endline (Stmt.show_stmt stmt.stmt) in
+        Printf.fprintf stderr "--- Statements ---\n";
+        let f (stmt : Stmt.stmt_node) = Printf.fprintf stderr "%s\n" (Stmt.show_stmt stmt.stmt) in
         List.iter f stmts;
-        print_endline "------------------";
-        print_newline ()
+        Printf.fprintf stderr "------------------\n";
+        Printf.fprintf stderr "\n"
       in
       print_statements ()
     else ()