DCE reports only on inner value or an example with recursion.

[cristianoc]

let rec subList = (b, e, l) =>
  switch (l) {
  | [] => failwith("subList")
  | [h, ...t] =>
    let tail =
      if (e == 0) {
        [];
      } else {
        subList(b - 1, e - 1, t);
      };
    if (b > 0) {
      tail;
    } else {
      [h, ...tail];
    };
  };

This reports on tail but not on subList.

[cristianoc]

With this variation, it reports on subList:

let rec subList = (b, e, l) =>
  switch (l) {
  | [] => failwith("subList")
  | [h, ...t] =>
    let tail = () =>
      if (e == 0) {
        [];
      } else {
        subList(b - 1, e - 1, t);
      };
    if (b > 0) {
      tail();
    } else {
      [h, ...tail()];
    };
  };

[cristianoc]

There's a heuristic not to report on inner function if there's a report on the outer function. It could be the heuristic misfires in this case and considers subList to be inside tail.

[cristianoc]

The difference between the two version is that tail has side effects in the first version, and no side effects in the second (because it is a function definition).