;; A kept if let with no else at the end of its chain gives an Option: Some of
;; the arm that ran, None when none did.

fn pick(a: Option(i32), k: i32) -> Option(i32)
  if let Some(x) = a then x
  elif k > 0 then k

fn only(a: Option(i32)) -> Option(i32)
  if let Some(x) = a then x * 2

fn lead(k: i32, b: Option(i32)) -> Option(i32)
  if k > 5
    k
  elif let Some(y) = b
    y

;; An arm that returns stays Never, and the rest of the chain decides.
fn early(a: Option(i32)) -> Option(i32)
  if let Some(x) = a
    return None
  elif true
    3

;; An arm that is already an Option is the whole, flattened (decision 140).
fn flat(a: Option(i32), o: Option(i32)) -> Option(i32)
  if let Some(x) = a then o

;; An Option of it wanted: the arm is Some of its value, and no match is the
;; outer None.
fn nest(a: Option(i32), o: Option(i32)) -> Option(Option(i32))
  if let Some(x) = a then o

fn level(oo: Option(Option(i32)))
  match oo
    Some(o) -> if o? then println(o) else println("some none")
    None -> println("none")

fn flat_chain(a: Option(i32), k: i32)
  let r =
    if let Some(x) = a
      x
    elif k > 0
      None
  show(r)

fn dyn_only(a: Option(i32)) -> dyn
  if let Some(x) = a then x

fn show(o: Option(i32))
  match o
    Some(v) -> println(v)
    None -> println("none")

fn main()
  show(pick(Some(1), 0))
  show(pick(None, 4))
  show(pick(None, 0))
  show(only(Some(3)))
  show(only(None))
  show(lead(9, None))
  show(lead(1, Some(2)))
  show(lead(1, None))
  show(early(None))
  show(early(Some(1)))
  show(flat(Some(1), Some(4)))
  show(flat(Some(1), None))
  show(flat(None, Some(4)))
  level(nest(Some(1), Some(4)))
  level(nest(Some(1), None))
  level(nest(None, Some(4)))
  flat_chain(Some(3), 0)
  flat_chain(None, 1)
  flat_chain(None, 0)
  println(dyn_only(Some(5)))
  println(dyn_only(None))
  ;; As a statement it is unchanged.
  if let Some(x) = Some(7)
    println(x)