Update code from https://github.com/unisonweb/unison/issues/822 to unison version: devel/M1l-171-g8b57038d

scratch.u

ability State s where
  Get : s
  Put : s -> ()

runState : s -> Request (State s) a -> (s, a)
runState s = cases
  {State.Get -> k} ->
    handle k s with runState s
  {State.Put s -> k} ->
    handle !k with runState s
  {a} ->
    (s, a)

ability Error e where
  Throw : e -> a
  Catch : (e -> {Error e, t} a) -> '{Error e, t} a -> {Error e} a

runError : Request (Error e) a -> Either e a
runError = cases
  {Error.Throw e -> _} -> Left e
  {Error.Catch catch try -> k} ->
    let z = handle !try with runError
        z2 = match z with
                Left e -> handle (catch e) with runError
                Right a -> Right a
        match z2 with
          Left e -> Left e
          Right a -> handle k a with runError
  {x} -> Right x

catchThrownError : '{State Nat, Error ()} Nat
catchThrownError = 'let
  Error.Catch
    (err -> State.Get)
    ('let
        State.Put 1337
        Error.Throw ()
    )
  State.Get

> handle (handle !catchThrownError with runError) with runState 5

> handle (handle !catchThrownError with runState 5) with runError