Find the smallest free number in a list

SmallestFreeNumber.re

let rec minfrom = (a, n, xs) => {
  let b = a + 1 + n/2;
  let (us, vs) = List.partition(fun (x) => x < b, xs);
  let m = List.length(us);
	
  if(n == 0)
    a
  else if(m == b - a)
    minfrom(b, n - m, vs)
  else
    minfrom(a, m, us)
};

let minfree = xs => {
  minfrom(0, (List.length(xs)), xs)
};

[8, 23, 9, 0, 12, 11, 1, 10, 13, 7, 41, 4, 14, 21, 5, 17, 3, 19, 2, 6] |> minfree |> Js.log;