changed tokens from mutable to ref

so that when record is copied it's still the same field

lib/parser.ml

@@ -6,33 +6,33 @@ open Lexer
 open Stmt
 
 type parse_result = (stmt_node list, parser_error list) result
-type state = { mutable tokens : token list; is_in_loop : bool }
+type state = { tokens : token list ref; is_in_loop : bool }
 type stmt_result = (stmt_node, parser_error) result
 type expr_result = (expr_node, parser_error) result
 
 let with_is_in_loop (f : state -> 'a) (state : state) : 'a =
   let new_state = { state with is_in_loop = true } in
   let result = f new_state in
-  state.tokens <- new_state.tokens;
+  (* state.tokens <- new_state.tokens; *)
   result
 
 let make_state tokens = { tokens; is_in_loop = false }
 
 let is_at_end state =
-  assert (not (List.is_empty state.tokens));
-  (List.hd state.tokens).token_type = Eof
+  assert (not (List.is_empty !(state.tokens)));
+  (List.hd !(state.tokens)).token_type = Eof
 
-let advance state = state.tokens <- List.tl state.tokens
-
-let next state =
-  let token = List.hd state.tokens in
-  advance state;
-  token
-
-let peek state = List.hd state.tokens
+(* let advance state = state.tokens <- List.tl state.tokens *)
+let advance state = state.tokens := List.tl !(state.tokens)
+let peek state = List.hd !(state.tokens)
 let peek_tt (state : state) : token_type = (peek state).token_type
 let cur_pos state = (peek state).pos
 
+let next state =
+  let token = peek state in
+  advance state;
+  token
+
 let advance_if state tt =
   if peek_tt state = tt then (
     advance state;
@@ -384,5 +384,5 @@ let parse (tokens : token list) : parse_result =
   let tokens =
     List.filter (fun tok -> match tok.token_type with Comment _ -> false | _ -> true) tokens
   in
-  let state = make_state tokens in
+  let state = make_state (ref tokens) in
   parse_impl state