state2.u

  use .base
  use List Optional

  List.head : [a] -> Optional a
  List.head a = List.at 0 a

  ability State s where
    put : s -> {State s} ()
    get : {State s} s

  pop : '{State [a]} (Optional a)
  pop = ' let stack = get
          put (drop 1 stack)
          head stack
  push : a -> {State [a]} ()
  push a = put (cons a get)

  > .base.head [1,2,3]

  state : s -> '({State s} a) -> a
  state s c =
    h s e = match e with
      { State.get -> k } ->
        handle k s with h s
      { State.put s -> k } ->
        handle k () with h s
      { a } -> a
    handle !c with h s