unison/unison-src/tests/state2b.u

--State2 ability

structural type Optional a = None | Some a

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

state : s -> Request (State s) a -> (s, a)
state s = cases
  { State.get -> k } -> handle k s with state s
  { State.put snew -> k } -> handle k () with state snew
  { a } -> (s, a)

modify : (s ->{} s) -> {State s} ()
modify f = State.put (f State.get)

increment : '{State Nat} ()
increment = '(modify ((+) 1))

second : (a, b) -> b
second = cases (_,b) -> b

first : (a, b) -> a
first = cases (a,_) -> a

ex : Nat
ex =
  result = handle
    State.put (11 + 1)
    State.put (5 + 15)
    ()
    with state 10

  first result

-- should return `20`, but actually returns `12`
-- seems like only one `put` is actually being run
> ex