type Primality {
  Prime,
  Composite,
}

sieve(limit: Int): Array Primality =
  // Array memory is automatically managed statically,
  // without any refcounting overhead in this case.
  let init_arr =
    Array.fill(limit, Prime)
    |> Array.set(0, Composite)
    |> Array.set(1, Composite)
  in
  // Iterator logic compiles to a simple loop, with
  // no heap allocations or virtual dispatch.
  Iter.range(2, limit)
  |> Iter.foldl(init_arr, \(arr, n) ->
    match Array.get(arr, n) {
      Prime ->
        Iter.ints(2)
        |> Iter.map(\i -> i * n)
        |> Iter.take_while(\i -> i < limit)
        |> Iter.foldl(
          arr,
          // Array updates logically copy the array,
          // but are automatically performed in-place
          // when safe.
          \(new, i) -> Array.set(new, i, Composite)
        ),
      Composite -> arr,
    }
  )