;; Swift optionals over dyn values: nil is the absence. ??, !, a test's as,
;; and chaining through a map's field.

let calls = 0

fn fallback(v)
  calls += 1
  v

fn describe(o, p)
  if o? as g
    g + 100
  elif p? as h
    h + 200
  else
    0

;; A kept if let with no else: the value, or nil.
fn kept(o) -> dyn
  if o? as g then g * 2

fn power(car) = car.engine?.power

fn run(a, n)
  println(a ?? 7, n ?? 7)
  println(n ?? n ?? 9)
  println(a ?? fallback(1), n ?? fallback(2), calls)
  println(a!)
  println(describe(a, n), describe(n, a), describe(n, n))
  println(kept(a), kept(n))
  let car = {:name "a" :engine {:power 90}}
  let bare = {:name "b" :engine nil}
  println(power(car), power(bare))
  println(power(car) ?? -1, power(bare) ?? -1)
  println(car.engine?.power ?? -1)

fn main()
  run(3, nil)