;; if let: the block runs when the pattern matches, its names bound there only.

data Shape
  Rect(w: i32, h: i32)
  Dot

enum Dir
  north = 0
  south = 1

fn describe(o: Option(i32), k: i32) -> i32
  if let Some(g) = o
    g + 1
  elif k > 5
    100
  else
    0

fn first(xs: [3 i32]) -> i32
  if let Some(x) = get(xs, 0) then x else -1

;; elif let: the rest of the chain is the else of an if let.
fn pick(a: Option(i32), b: Option(i32), k: i32) -> i32
  if let Some(x) = a
    x
  elif let Some(y) = b
    y * 10
  elif k > 5
    100
  else
    0

;; Nested: a pattern tested inside the block of another.
fn area(s: Option(Shape)) -> i32
  if let Some(x) = s
    if let Rect(w, h) = x
      w * h
    else
      -1
  else
    -2

fn main()
  let g = 7
  println(describe(Some(4), 0))
  println(describe(None, 9))
  println(describe(None, 1))
  println(first([9, 8, 7]))
  println(pick(Some(1), None, 0))
  println(pick(None, Some(2), 0))
  println(pick(None, None, 9))
  println(pick(None, None, 1))
  ;; Shadowing: g is the payload inside the block and the outer g after it.
  if let Some(g) = Some(1)
    println(g)
  println(g)
  ;; Any match pattern.
  if let None = get([1, 2], 5)
    println("absent")
  if let :north = Dir.north
    println("north")
  println(area(Some(Shape.Rect{.w 2 .h 3})))
  println(area(Some(Shape.Dot)))
  println(area(None))
  ;; when on one line and with a block.
  let w = when g > 3 then g * 2
  match w
    Some(v) -> println(v)
    None -> println("none")
  when g > 1
    println("when block")