Add associativity table

parser.ml

@@ -29,9 +29,20 @@ let precedence = [
 let precedence_of op =
   Hashtbl.find precedence op
 
-let op_is_right_to_left = function
-  | Exp -> true
-  | _ -> false
+type associativity =
+  | Left_to_right
+  | Right_to_left
+
+let oper_assoc = [
+  Exp, Right_to_left;
+] |> List.to_seq |> Hashtbl.of_seq
+
+let op_is_right_to_left op =
+  let a =
+    Hashtbl.find_opt oper_assoc op
+    |> Option.value ~default: Left_to_right
+  in
+  a = Right_to_left
 
 let operators = [
   Token.Plus, Add;