unison/unison-src/tests/state2b-min.u

--State2 effect
effect State s where
  put : s -> {State s} ()

state : s -> Effect (State s) a -> s
state s eff = case eff of
  { State.put snew -> k } -> handle (state snew) in k ()
  { a } -> s

handle (state 10) in
  State.put (11 + 1)
  State.put (5 + 15)
  ()
  -- should be 20