state.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 h s in k s
      { State.put s -> k } ->
        handle h s in k ()
      { a } -> a
    handle h s in !c

  stackProgram : '{State [Nat]} ()
  stackProgram =
    '( a = pop
       if a != 5
         then push 5
         else
           push 3
           push 8 )

  runStack : [Nat]
  runStack = state [5,4,3,2,1] stackProgram