module SafeList = {
  type empty =
    | Empty;
  type nonEmpty =
    | NonEmpty;
  type t('a, 's) =
    | []: t('a, empty)
    | ::(('a, t('a, 's))): t('a, nonEmpty);
  let rec length: type s. t(_, s) => int =
    fun
    | [] => 0
    | [h, ...t] => 1 + length(t);
  let head: type a. t(a, nonEmpty) => a =
    fun
    | [x, ..._] => x;

   let tail: t('a, nonEmpty) => t('a, nonEmpty) =
     fun
     | [a] => [a]
     | [a, b, ...tail] => [b, ...tail]
};