Refactored the entire code.

bin/octal.ml

@@ -31,6 +31,9 @@ let octal_reader (type a) (module I : IntType with type t = a) sin : a =
     let l = List.init (String.length s) (String.get s) in
     let rec aux a = function
       | c :: rest ->
+         if (Char.code c) > (Char.code '7') || (Char.code c) < (Char.code '0') then
+           I.of_int 0
+         else
          aux ((a * (I.of_int 8)) + (I.of_int (int_of_char c)) - (I.of_int (int_of_char '0'))) rest
       | [] -> a
     in aux (I.of_int 0) l